BASIC DENTAL MATERIALS 4th Edition PDF

BASIC DENTAL MATERIALS

BASIC DENTAL MATERIALS Fourth Edition

JOHN J MANAPPALLIL MDS

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Jaypee Brothers Medical Publishers (P) Ltd Bhotahity, Kathmandu Nepal Phone: +977-9741283608 Email: [emailprotected] Website: www.jaypeebrothers.com Website: www.jaypeedigital.com © 2016, Jaypee Brothers Medical Publishers The views and opinions expressed in this book are solely those of the original contributor(s)/author(s) and do not necessarily represent those of editor(s) of the book. All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission in writing of the publishers. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. Medical knowledge and practice change constantly. This book is designed to provide accurate, authoritative information about the subject matter in question. However, readers are advised to check the most current information available on procedures included and check information from the manufacturer of each product to be administered, to verify the recommended dose, formula, method and duration of administration, adverse effects and contraindications. It is the responsibility of the practitioner to take all appropriate safety precautions. Neither the publisher nor the author(s)/editor(s) assume any liability for any injury and/or damage to persons or property arising from or related to use of material in this book. This book is sold on the understanding that the publisher is not engaged in providing professional medical services. If such advice or services are required, the services of a competent medical professional should be sought. Every effort has been made where necessary to contact holders of copyright to obtain permission to reproduce copyright material. If any have been inadvertently overlooked, the publisher will be pleased to make the necessary arrangements at the first opportunity. Inquiries for bulk sales may be solicited at: [emailprotected] Basic Dental Materials First Edition: 1998 Second Edition: 2003 Reprint: 2004, 2007, 2008 Third Edition: 2010 Fourth Edition: 2016 ISBN: 978-93-5250-048-2 Printed at

Dedicated to The teachers who have inspired us

Contributors Akshay Bhargava Dean Faculty of Dental Sciences Shree Guru Gobind Singh Tricentenary (SGT) University, Gurgaon Haryana, India

Jacob Kurien Professor and Head Department of Conservative Dentistry and Endodontics Kannur Dental College Kannur, Kerala, India

G Vinaya Kumar Professor and Head Department of Prosthodontics College of Dental Sciences Davangere, Karnataka, India

Rajashekar Sangur Professor and Head Department of Prosthodontics Rama Dental College Hospital and Research Centre Kanpur, Uttar Pradesh, India

Preface A successful dentist has to combine technical skills along with sound clinical knowledge. Knowledge of dental materials is one of the keys to a successful dental practice. To the beginner, the task may appear formidable because of the wide array of materials available. This is quite normal and fortunately disappears with use and familiarity. Intimate knowledge of dental materials is required throughout one’s career for a successful practice. As with dentistry in general, the science of dental materials combines a wide array of disciplines, including chemistry, physics, mechanics, and biology with clinical sciences. Format Basic Dental Materials, first published in 1998, is now in its 18th year. Its publication was inspired by the desire to help students navigate the complex field of dental materials from the very first year of the course. Being the first published book on dental materials from India, it had set new standards, including moving away from traditional formats. Its unique student friendly format has contributed much to its popularity particularly among dental students from India and around the world, and has made the understanding of this subject within the grasp of the novice. Over the years, readers have contributed valuable information as well as suggestions, many of which have been incorporated in the current edition. Comments and suggestions are welcome and readers are encouraged to send in their feedback via e-mail ([emailprotected]). Challenges With each new edition, the challenges continue to grow, and revising the previous edition was certainly no exception. Dental material is a vibrant subject as new products and technology are constantly appearing in the market. A few of the materials have been eliminated from the book or have just briefly been mentioned as they are no longer marketed. Knowledge of the history of dental materials is useful to understand the evolution of materials and why newer materials were developed. Over the past decade, the field of ceramics has seen vast improvements. Current developments in CAD/CAM and 3D printing are opening new frontiers. Knowledge of values helps improve depth of understanding and is useful for making comparisons. Actual values of the various materials have been presented wherever possible. However, one must remember that values are not necessarily absolute, variations can occur over time, between brands and methods of testing. Climatic differences affect properties like working and setting times. New chapters Another challenge is defining dental materials. Traditionally, the subject of dental materials primarily included materials used in restorative dentistry and related auxiliary materials. Currently, the trend is to be more inclusive of materials from other specialties, which have traditionally been excluded. This has been partly addressed in this edition with the inclusion of two new chapters on endodontic materials. With succeeding editions, it is hoped to be even more inclusive and cover the entire spectrum of materials used in dentistry, including surgical and orthodontic materials. Materials such as anesthetics and drugs are not within the scope of this book. Metallurgy which was not included earlier, has been included in the current edition (Structure and properties of

x BASIC DENTAL MATERIALS

metals and alloys). Another new chapter is in the field of ‘additive manufacturing’, popularly known as 3D printing. Biomaterials is another exciting area of development with an explosion of new materials and technology. Differences in information One of the challenges faced by the readers is the wide variation in information between different books. Differences do exist between various books and the reader is often in a dilemma as to which information to follow. The best source is the original source, which includes original studies, information from the manufacturers and publications of the International Standards Organization (ISO). The ‘International Standards Organization’ is a significant reference source for manufacturers as well as authors and researchers. The technical committees in-charge of the specification constantly strive to keep pace with changes in knowledge and technology, through publication of new editions of the specifications periodically. The edition is indicated by the year attached to the specification. Most dental product manufacturers strive to keep pace with changes in standards. Significant changes have taken place in the specifications and classifications of many products. The fourth edition of Basic Dental Materials too has reflected these changes and therefore, differences will exist between the current edition and previous editions as well as other textbooks on the subjects, particularly in the area of classifications and technical details. Readers and teaching staff in particular are requested to look out for these changes and refer to the source when available. Explanatory footnotes have been provided wherever needed. International Standards Organization (ISO) Many nations, including India and the US are members of the ISO. Founded in 1947 with just 26 members, its membership has grown to 162, including 119 full members, 38 correspondent members and 5 subscriber members. India has not only been a full member since its inception, but has also assumed council positions and has been a part of technical committees at various times. Current ADA specifications have been adopted from the ISO. In its website, the ADA has stated that their specifications are identical to the relevant ISO standards. Therefore, use of both specifications for the same product is repetitive. The fourth and subsequent editions of the book will therefore gradually phase out the ADA specifications and replace them with those of the ISO. Other specifications, including the ADA will be used only if ISO standards are not available for the particular product. Organization of the book Other changes include the reorganization of the book into segments. The 30 chapters in the book have been organized into 7 parts for ease of reference. Another new feature is the chapter outline at the beginning of each chapter as requested by some readers. Many materials adversely affect the other’s property, and therefore, material interactions have been introduced where information is available. A familiar one to most readers is the effect of eugenol from ZOE-based products on resin-based composites. A relatively less familiar one is that occurring between provisional composites and elastomeric impression materials, which if not managed well, can introduce significant errors in the impression. Critical assessment of new products Today’s dentists in India are fortunate to have a wide choice of materials. The economic liberalization of the late 1980s saw the opening of the market to a range of high quality international products. Dental practitioners should have a good understanding of basic dental materials science to

Preface xi

enable them to select and critically assess the plethora of new materials that are constantly being introduced and aggressively marketed. It is also advisable to request long-term in vitro and in vivo independently acquired evidence of the performance of a material before deciding to use it. It is not possible to cover all aspects of the material in the book; therefore, the operator should read the information which comes with a particular product whenever available. Information exchange and update Students are encouraged to read from a wide source of materials for greater understanding and depth of knowledge. Thanks to journals, scientific conferences and the internet, there is exchange of information between individuals, transcending geographical barriers. Concepts are constantly changing with improved understanding and new research. It is encouraging to see a lot of new publications within the country and abroad. Encouragement from professional publishing houses and new regulations by the DCI have in no small measure, contributed to this increase. Indian professionals are now contributing significantly to international research, literature and education world over. Educational challenges Dental institutions today are facing innumerable challenges, and constant adaptation is required to reflect changing curricula around the world and higher expectations among the student community. The challenge now is in reorganizing and streamlining the courses to changing times. It is encouraging to see some leading institutions take bold new initiatives in instituting improved learning techniques and investing in infrastructure to raise standards of education. In this regard, the roles of regulatory bodies, including the Dental Council of India and various Dental Associations are critical to ensure that the profession continues to develop. It is my fervent hope that a new generation of young, highly trained and motivated dentists will emerge, providing improved patient care and upholding the dignity of the profession.

Acknowledgments Every book has its share of contributors and influences, and this book is certainly no exception. I am deeply indebted to professors Akshay Bhargava, Jacob Kurien, G Vinaya Kumar and Rajashekar Sangur who over the years have contributed their knowledge and experience to the various chapters in the book. It is my honor and privilege to have you all associated with this book. My deepest gratitude goes to all those who helped with the proofreading and corrections of the manuscripts. In this regard, I thank Rajanikant AV and Preeti Pachauri from Rama Dental College, Hospital and Research Centre, Kanpur, Uttar Pradesh, India; Ginu Philip, Vijayasree Sreekumar and Jojen Thomas from Bneid Al Gar Dental Center, Kuwait; and my wife Dr Divya Susan. The countless hours spent on the project meant hours away from my family. My deep appreciation goes to my family especially my wife Divya and kids Reuben and Jordan and my parents. Without their encouragement, support and tolerance, this project would not have been possible. This edition is dedicated to our respected teachers who have influenced all of us. This includes not only the professors and clinicians who taught us at dental school, but also those who shared their information at the continuing education programs and conferences. I wish to acknowledge the significant influence of the brilliant authors from around the world whose books have been a source of so much knowledge and inspiration. I pay tribute to these great individuals who have inspired us all. I wish to express my appreciation to those who contributed to the previous editions, in particular my former colleagues, Shubha Rao and Atley George from BDCH, Davangere. In spite of the significant modifications many of the chapters contain portions created by them. I also take this occasion to once again renew bonds of friendship and affection with all my students and readers. I thank all the readers who have given their feedback and suggestions. Your support is what gives me the inspiration to continue this book. It is a privilege to have you all on board. Last but not least, a special thanks to Shri Jitendar P Vij (Group Chairman), Mr Ankit Vij (Group President), Mr Tarun Duneja (Director-Publishing) and staff of M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India, for their great expertise in creating a truly remarkable book. John J Manappallil MDS

Undetailed Contents Section 1: Structure and Properties of Dental Materials

15. Elastic Impression Materials— Agar and Alginate 16. Elastomeric Impression Materials

1. Overview of Dentistry and Dental Materials 3 2. Structure and Properties of Dental Materials 9 3. Structure and Properties of Metals and Alloys 39 4. Electrochemical Properties of Materials 56 5. Biological Properties of Dental Materials 61

Section 5: Dental Laboratory— Materials and Processes

Section 2: Direct Restorative Materials 6. Introduction to Restorations, Luting and Pulp Therapy 7. Cavity Liners and Varnish 8. Dental Cements 9. Dental Amalgam 10. Direct Filling Gold 11. Resin-based Composites and Bonding Agents

17. 18. 19. 20. 21. 22. 23. 24.

73 80 84 128 156

258 277

Model, Cast and Die Materials 301 Gypsum Products 310 Waxes in Dentistry 327 Dental Investments and Refractory Materials 345 Dental Casting and Metal Fabrication Procedures 358 Abrasion and Polishing 378 Metal Joining and Dental Lasers 392 Additive Manufacturing in Dentistry 407

Section 6: Alloys in Dentistry 25. Dental Casting Alloys 26. Dental Implant Materials 27. Wrought Metals and Alloys

421 452 466

167

Section 7: Indirect Restorative Section 3: Endodontic Materials and Prosthetic Materials 12. Endodontic Medicaments and Irrigants 13. Endodontic Sealers and Obturating Materials

207 216

Section 4: Impression Materials 14. Rigid Impression Materials— Impression Compound and ZOE Paste

243

28. Dental Ceramics 29. Denture Resins and Polymers 30. Maxillofacial Prosthetic Materials

479 529 572

Appendices 579 Further Reading 589 Index 595

Contents in Detail Section 1: Structure and Properties of Dental Materials 1. Overview of Dentistry and Dental Materials

Dental Treatment 3 Maintenance Phase 5 The Dental Specialties 5 The Dental Laboratory 6 Classification of Dental Materials 6 International Standards Organization (ISO) 7 US Standards for Dental Materials 8

2. Structure and Properties of Dental Materials Structure of Matter 9 Interatomic Bonds 10 Crystal Structure 11 Noncrystalline Structure 12 Stress and Strain 12 Diffusion 12 Surface Tension 12 Wetting 13 Properties of Dental Materials 14 Mechanical Properties 14 Stress 14 Strain 15 Complex Stresses 15 Poisson’s Ratio 16 Proportional Limit 16 Elastic Limit 16 Yield Strength 16 Modulus of Elasticity 17 Flexibility 17 Resilience 17 Impact 18 Impact Strength 18 Permanent Deformation 19 Strength 19 Weibull Statistics 21 Stress Concentration 22 Fatigue 22 Toughness 23 Brittleness 23 Ductility 23 Malleability 23 Hardness 24 Abrasion Resistance 25

Rheology 25 Terms and Properties in Rheology 26 Physical Properties 27 Forms of Matter 27 Melting Point 28 Boiling Point 28 Density 28 Glass Transition Temperature (TG) 28 Solubility 29 Thermal Properties 29 Thermal Conductivity 29 Thermal Diffusivity 30 Thermal Expansion 30 Coefficient of Thermal Expansion (CTE) 30 Optical Properties and Color 31 Dimensions of Color 31 Measurement of Color 32 Metamerism 32 Tooth Esthetics 32 Transparence, Translucence, and Opacity 32 Fluorescence 33 Clinical Considerations 34 Radiological Properties 34 Magnetic Properties of Matter 36 Classification 36 Terms 36 Types 37 Uses 37

3. Structure and Properties of Metals and Alloys Uses in Dentistry 39 Metallurgy 42 Periodic Table 42 General Properties of Metals 42 Valence Electron 42 Metallic Bonding 43 Alloys 43 Phase 44 Solid Solutions 44 Dental Applications 46 Solidification and Microstructure of Metals and Alloys 46 Time-temperature Graph 48 Equilibrium Phase Diagrams 49 Eutectic Alloys 50 Peritectic Alloys 51 Solid State Reactions 52 Classification of Alloys 54 Functions of Alloying Elements 54

xvi BASIC DENTAL MATERIALS

4. Electrochemical Properties of Materials 56 Definitions 56 Electromotive Force Series (EMF) 57 Classification of Corrosion 58 Types of Electrolytic Corrosion 58 Factors Affecting Corrosion of Restorations in the Mouth 60 Protection against Corrosion 60

5. Biological Properties of Dental Materials 61 Biomaterials 61 Biological Requirements of Dental Materials 61 Classification of Materials from a Biological Perspective 62 Biohazards Related to the Dental Materials 62 Biological Considerations of Restoration Design 63 Physical Factors Affecting Pulp Health 64 Classification of Adverse Reactions from Dental Materials 65 Toxicity Evaluation 66 Therapeutic Effects of Dental Materials 67 Osseointegration 67 Effect of Pressure on Tissues 67 Effect of Pontic Design 68 Effect of Material – Porcelain versus Resin 68 Infection Control 69

Section 2: Direct Restorative Materials 6. Introduction to Restorations, Luting and Pulp Therapy

Restorations 73 Temporary Restorations 73 Intermediate Restorations 74 Requirements of a Temporary Filling Material 74 Permanent 74 Direct and Indirect Restorations 75 Esthetic and Nonesthetic 75 Luting 75 Types 75 General Requirements of Luting Materials 76 Pulp Capping 76 Criteria for Pulp Capping 76 Types of Pulp Capping 77 Bases 77 Types 78 Properties 78 Clinical Considerations 79 Liners and Varnish 79

7. Cavity Liners and Varnish

Cavity Liners 80 Supplied as 80 Composition 80 Properties 80 Manipulation 81 Other Liners 81 Cavity Varnish 81 Application 81 Supplied as 81 Composition 81 Properties 81 Manipulation 82 Precautions 82 Clinical Considerations 82 Contraindications 82 Fluoride Varnishes 82 Composition 83 Manipulation 83 Contraindications 83

8. Dental Cements Classification 84 General Structure 85 Uses of Cements 85 General Properties of Cements 86 Silicate Cements 88 Zinc Phosphate Cement 88 Applications 89 Classification 89 Available as 89 Composition 89 Manufacture 90 Setting Reaction 90 Properties 91 Manipulation 92 Advantages and Disadvantages of Zinc Phosphate 93 Copper Cements 93 Applications 94 Classification 94 Composition 94 Properties 94 Manipulation 94 Zinc Polycarboxylate Cement 94 Applications 95 Available as 95 Composition 95 Manufacture 96 Setting Reaction 96 Properties 96 Manipulation 97 Advantages and Disadvantages 98 Zinc Oxide Eugenol Cement 98 Classification (ISO 3107:2011) 99

Contents xvii Available as 99 Composition 100 Setting Reaction 100 General Properties of Zinc Oxide Eugenol Cements 101 Manipulation 102 Modified Zinc Oxide Eugenol Cements 102 Eba-alumina Modified Cements 102 Polymer Reinforced Zinc Oxide Eugenol Cement 104 Other Zinc Oxide Eugenol Products 105 Zinc Oxide/Zinc Sulphate Cements 105 Glass Ionomer Cements 106 Application 107 Classification 107 Available as 107 Composition 108 Manufacture 109 Setting Reaction 109 Properties 110 Manipulation 111 Packable Glass Ionomer for Posterior Restorations 114 Fissure Sealing (Special Applications) 115 Modified Glass Ionomers 115 Metal Modified Glass Ionomer Cement 115 Types 115 Uses 115 Properties 116 Resin-modified Glass Ionomer 116 Classification 116 Supplied as 117 Composition 117 Setting Reaction 118 Manipulation 118 Properties 118 Calcium Hydroxide Cement 118 Applications 119 Available as 119 Composition 119 Setting Reaction 120 Properties 120 Manipulation 120 Setting Time 120 Light Activated Calcium Hydroxide Cement 121 Calcium Hydroxide Root Canal Sealing Pastes 121 Resin Cements 121 Applications 122 Classification 122 Supplied as 122 Composition 123 Polymerization 123 Properties 123 Manipulation and Technical Considerations 123 Compomer (Polyacid-modified Composite Resins) 124 Applications 124

Supplied as 125 Composition 125 Setting Reaction 125 Bonding and Curing 126 Manipulation 126 Properties 126 Advantages and Disadvantages 127

9. Dental Amalgam

128

Indications 129 Contraindications 129 Classification of Amalgam Alloys 130 Manufacture of Alloy Powder 130 Supplied as 131 Composition 132 Comparison of Lathe Cut and Spherical Alloys 133 Low Copper Alloys 133 Composition 133 Available as 134 Setting Reaction 134 High Copper Alloys 135 Admixed Alloy Powder 135 Types 135 Composition 135 Setting Reaction 136 Single Composition Alloys 137 Composition 137 Setting Reaction 137 Advantages/Disadvantages of Spherical Highcopper Amalgam 137 Properties of Set Amalgam 138 Technical Considerations 143 Mulling 148 Condensation 148 Shaping and Finishing 150 Amalgam Bonding 151 Mercury Toxicity 152 Amalgam Disposal 153 Advantages and Disadvantages of Amalgam Restorations 154

10. Direct Filling Gold Applications 157 Contraindications 157 Types 157 Composition and Purity 158 Gold Foil 158 Manufacture 158 Supplied as 158 Electrolytic Precipitate 159 Available as 160 Mat Gold 160 Mat Foil 160 Alloyed Electrolytic Precipitates 160 Powdered Gold 160 Manufacture 160 Available as 160

156

xviii BASIC DENTAL MATERIALS Manipulation of Direct Filling Gold 161 Properties of Compacted Gold 164 Advantages and Disadvantages 165

11. Resin-based Composites and Bonding Agents

167

Composite Resins 168 Uses 168 Types 169 Restorative Composite Resins 171 Supplied as 171 Indications for Various Composite Resins 172 Composition and Structure 172 Polymerization (Setting) Mechanisms 177 Microfilled Composite 181 Composition 181 Clinical Considerations 181 Hybrid Composite Resins 182 Nano and Nanohybrid Composite Resins 182 Properties of Composite Resins 182 Problems in the Use of Composites for Posterior Restorations 186 Manipulation and Placement of Composite Resins 187 Techniques of Insertion 188 Finishing and Polishing 189 Bonding 189 Acid Etch Technique 190 Enamel Bond Agents 192 Enamel/Dentin Bond Systems 192 Repair of Composites 196 Sandwich Technique 197 Specialized Composite Resins 197 Advantages and Disadvantages of Restorative Composite Resins 203

Section 3: Endodontic Materials 12. Endodontic Medicaments and Irrigants

207

Root Canal Irrigants 208 Chemically Active Irrigants 209 Intracanal Medicaments 211 Phenol and Related Compounds 212 Antibiotics 213 Halogens 214 Quaternary Ammonium Compounds 215 Calcium Hydroxide 215 Chlorhexidine (CHX) Gluconate 215

13. Endodontic Sealers and Obturating Materials Root Canal Obturating Materials 217 Silver Points 217 Gutta-percha 218

Paste-type Obturating Materials 224 Root Canal Sealers 225 Zinc Oxide-eugenol-based Sealers 226 Epoxy Resin-based Sealers 228 Calcium Hydroxide based Sealers 229 Glass Ionomer-based Sealers 231 Silicon-based Sealers 231 Mineral Trioxide Aggregate (MTA) 232 Endodontic Solvents 237

Section 4: Impression Materials 14. Rigid Impression Materials— Impression Compound and ZOE Paste

Advantages of Using a Cast or Model 243 Desirable Properties of an Impression Material 244 Classification of Impression Materials 244 Rigid Impression Materials 245 Impression Compound 245 Classification 245 Supplied as 245 Applications 246 Requirements of Impression Compound 246 Composition 247 Properties of Impression Compound 247 Manipulation 248 Removal of Impression from the Mouth 249 Disinfection 250 Pouring the Cast and Cast Separation 250 Advantages and Disadvantages 250 Zinc Oxide Eugenol Impression Paste 250 Classification 251 Available as 251 Composition 252 Setting Reaction 252 Microstructure 252 Setting Time 252 Properties 253 Manipulation 254 Impression Tray 254 Disinfection 255 Pouring the Impression 255 Advantages and Disadvantages 255 Other Zinc Oxide Pastes 255

15. Elastic Impression Materials— Agar and Alginate

216

243

Hydrocolloids 259 Reversible Hydrocolloids—Agar 260 Classification Based on Viscosity (ISO 21563:2013) 260 Uses 260 Supplied as 261 Composition 261

258

Contents xix Gelation or Setting of Agar 262 Manipulation 262 Impression Trays 263 Making the Impression 263 Working and Setting Time 263 Removal of Impression 263 Storage of Agar Impression 263 Separation from Cast 264 Properties of Agar Hydrocolloids 264 Laminate Technique (Agar–Alginate Combination Technique) 265 Wet Field Technique 265 Cast Duplication 265 Impression Disinfection 266 Advantages and Disadvantages of Agar Hydrocolloid 266 Irreversible Hydrocolloid—Alginate 267 Types 267 Supplied as 267 Applications 268 Composition 268 Setting Reaction 269 Properties of Alginate Hydrocolloid 269 Shelf Life and Storage 271 Manipulation 271 Mixing Time 272 Working Time 272 Gelation Time (Setting Time) 272 Tray Selection 272 Loading the Tray 273 Seating the Tray 273 Time of Removal and Test for Set 273 Removal of the Impression 274 Impression Disinfection 274 Storage of Alginate Impression 275 Construction of Cast 275 Advantages and Disadvantages of Alginate 276

16. Elastomeric Impression Materials

277

Chemistry and Structure of Elastomeric Polymers 278 Types 279 Uses of Elastomeric Impression Materials 280 Supplied as 280 General Properties of Elastomeric Materials 280 Polysulfides 282 Supplied as 282 Composition 282 Chemistry and Setting Reactions 282 Properties 283 Silicone Rubber Impression Materials 283 Types 284 Condensation Silicone 284 Supplied as 284 Composition 285 Chemistry and Setting Reaction 285 Properties 285

Addition Silicones (Polyvinyl Siloxane) 286 Supplied as 286 Composition 286 Chemistry and Setting Reaction 286 Properties 287 Polyether Rubber Impression Material 287 Available as 287 Composition 288 Chemistry and Setting Reaction 288 Properties 289 Manipulation of Elastomeric Impression Materials 289 Impression Techniques 291 Removal of the Impression 294 Infection Control 294 Impression Errors 294 Specialized Materials 297

Section 5: Dental Laboratory— Materials and Processes 17. Model, Cast and Die Materials

301

Types of Die Materials 302 Ideal Requirements of Die Materials 302 Alternate Die Materials 303 Improved Dental Stone or Die Stone 303 Advantages 303 Disadvantages 303 Electroformed/Electroplated Casts and Dies 303 Advantages 304 Disadvantages 304 Electroforming 304 Components of an Electroplating Apparatus 304 Composition of the Electroplating Bath 305 Procedure 305 Polyurethane 306 Mode of Supply 306 Indications 306 Properties 306 Manipulation 306 Epoxy Resin Die Materials 307 Advantages 307 Disadvantages 307 Available as 307 Refractory Cast for Wax Patterns 307 Refractory Cast for Ceramics 308 Die Stone-investment Combination (Divestment) 308 Divestment Phosphate or DVP 308

18. Gypsum Products Applications 310 Classification 311

310

xx BASIC DENTAL MATERIALS Type 1 or Dental Plaster, Impression 311 Type 2 or Dental Plaster, Model, Mounting 312 Type 3 or Dental Stone, Model 312 Type 4 or Dental Stone, Die, High Strength, Low Expansion 313 Type 5 or Dental Stone, Die, High Strength, High Expansion 313 Manufacture of Gypsum Products 314 Setting Reaction 315 Theories of Setting 315 The Microstructure of Set Gypsum 316 Manipulation 317 Setting Time 318 Properties 321 Setting Expansion 321 Strength 322 Hardness and Abrasion Resistance 323 Flow 323 Reproduction of Detail 323 Specialized Gypsum Products 324 Care of Gypsum 325 Infection Control 326 Differences between Dental Plaster and Dental Stone 326

19. Waxes in Dentistry

327

Components of Dental Waxes 327 Chemical Nature of Waxes 327 Classification of Dental Waxes 330 General Properties 330 Pattern Waxes 331 Inlay Casting Wax 332 RPD Casting Wax 336 Milling Wax 337 Baseplate Wax 337 Processing Waxes 339 Boxing Wax and Beading Wax 339 Utility Wax 340 Sticky Wax 340 Carding Wax 340 Shellac 341 Impression Waxes 342 Corrective Impression Wax 342 Bite Registration Wax 343

20. Dental Investments and Refractory Materials 345 Requirements of an Investment Material 345 Classification of Refractory Materials in Dentistry (ISO 15912:2006) 346 General Composition of Investments 346 Gypsum Bonded Investments 347 Classification 347 Uses 347 Supplied as 347 Composition 347 Manipulation 348

Setting Reaction 348 Setting Time 348 Properties of Gypsum Investments 348 Hygroscopic Thermal Inlay Casting Investment 351 Investments for Casting High Melting Alloys 351 Phosphate Bonded Investment 351 Uses 351 Classification 351 Supplied as 352 Composition 352 Setting Reaction 352 Manipulation 352 Properties 353 Specialized Refractory Materials 354 Silica Bonded Investments 356 Types 356 Manipulation 356 Brazing (Soldering) Investment 356 Uses 357 Types 357 Composition 357 Properties 357 Procedure 357

21. Dental Casting and Metal Fabrication Procedures 358 Metal Restorations in Dentistry 358 Casting 360 Steps in Making a Small Cast Restoration 360 Tooth/Teeth Preparation 360 Die Preparation 360 Die Spacer 360 Wax Pattern 360 Sprue Former 361 Casting Ring Lining 361 Investing 362 Wax Elimination (Burnout) and Thermal Expansion 362 Casting-process and Equipment 363 Casting Defects 366 Types of Casting Defects 366 Other Methods of Fabricating Restorations and Prostheses 371 Capillary Casting Technique (Captek) 371 Mode of Supply 372 Capillary Casting 372 Overall Composition after Capillary Casting 373 Coping Thickness 373 Coping Microstructure 373 Technique 373 CAD/CAM Milling 373 Advantages of CAD/CAM 374 Copy Milling 374 Electroforming 374 Electrical Discharge Machining 375 Applications 375

Contents xxi Technique 375 Advantages 376 Disadvantages 376 Additive Manufacturing 376 Types of Metal in Additive Manufacturing Technologies 377 Applications 377 Metals Available for 3D Printing 377

22. Abrasion and Polishing

378

Abrasion 378 Defined as 378 Types of Abrasion 379 Supplied as 379 Mechanism of Abrasive Action 380 Stress, Strain and Heat Production during Abrasion 380 Rate of Abrasion 380 Classification 381 Types of Abrasives 381 Desirable Characteristics of an Abrasive 383 Grading of Abrasive and Polishing Agents 383 Binder 383 Diamond Burs 383 Polishing 384 Difference between Abrasion and Polishing 384 Nonabrasive Polishing 385 Technical Considerations (Procedure) 385 Dentifrices 387 Function 387 Available as 387 Classification of Dentifrices 387 Composition 388 Prophylactic Abrasives 390 Function 390 Prophyjet 390 Denture Cleansers 391 Brushing 391 Soaking 391

23. Metal Joining and Dental Lasers

392

Terms and Definitions 392 Ideal Requirements of a Brazing Material (Dental Solder) 393 Types of Solders or Brazing Materials 393 Applications of Soldering 394 Composition 394 Properties of Dental Brazing Materials 395 Fluxes 396 Antiflux 397 Technical Considerations 397 Technique of Soldering 398 Pitted Solder Joints 399 Advantages and Disadvantages 399 Welding 400 Indications 400 Types 400

Resistance Spot Welding 400 Tungsten Inert Gas (TIG) Welding and Plasma Arc Welding (PAW) 401 Procedure 401 Advantages and Disadvantages 401 Laser Welding 402 Commercial Names 404 Indications 404 Mechanism 405 Advantages of Laser Welding 405 Cast-joining 406 Radiographic Assessment of Joints 406

24. Additive Manufacturing in Dentistry 407 Applications 407 Fundamentals of 3D Printing 408 Classification of Additive Manufacturing (AM) Technologies 409 Description of Some Additive Manufacturing (AM) Technologies 410 3D Dental Printers 412 Support Structures for 3D Printed Objects 412 Raw Materials for 3D Printing 414 Post-manufacturing Processing 414 3D Printed Maxillofacial Prostheses 414 3D Printing Technology in Surgical Planning 415 Tissue Engineering 415 Bioink 416 Osteoink 417 Comparison of Additive and Subtractive Manufacturing 418 Advantages of 3D Printing 418

Section 6: Alloys in Dentistry 25. Dental Casting Alloys

421

Terminology 421 History and Classification of Dental Casting Alloys 422 Classification According to Use of Dental Casting Alloys 425 General Requirements of Casting Alloys 425 Alloys for All Metal Restorations 426 Classification (ANSI/ADA Sp. No. 5) 426 Uses 426 Types 427 Gold Alloys (for All-metal Restorations) 427 Gold Content 427 Composition of Gold Alloys 428 Properties of Gold Alloys 429 Heat Treatment of Gold Alloys 431 Low Gold Alloys 431 Silver-palladium Alloys 432 Nickel-chrome and Cobalt-chromium Alloys 433

xxii BASIC DENTAL MATERIALS Titanium and Titanium Alloys 433 Aluminum-bronze Alloy 433 Metal-ceramic Alloys 433 Evolution of Metal-ceramic Alloys 434 Requirements of Alloys for Porcelain Bonding 434 Uses of Metal-ceramic Alloys 434 Types (Classification) of Metal-ceramic Alloys 434 The High Noble (Gold-based) Metal-ceramic Alloys 435 Gold-palladium-platinum Alloys 436 Gold-palladium-silver Alloys 437 Gold-palladium Alloys 438 Noble (Palladium-based) Metal-ceramic Alloys 438 Common Features of Palladium-based (Noble) Alloys 438 Palladium-silver Alloys 439 Palladium-copper Alloys 439 Palladium-cobalt Alloys 440 Palladium-gallium Alloys 440 Base Metal Alloys for Metal-ceramic Restorations 440 Nickel-chromium Alloys 441 Titanium and Its Alloys for Metal-ceramic Applications 443 Removable Denture Alloys 446 Cobalt-chromium Alloys 447 Technical Considerations for Casting Alloys 449 Advantages and Disadvantages of Base Metal Alloys 451 Comparison of a Gold Alloy and a Base Metal Alloy 451

26. Dental Implant Materials

452

Definition 452 Types of Implants 453 Materials Used 454 Implant Parts 455 Implant Abutment Connection 458 Platform Switching 459 Biointegration and Osseointegration 459 Titanium Allergy 461 Zirconia Implants 461 Zirconia Anatomic Root-form Implants 461 Implant Surfaces and Coatings 462

27. Wrought Metals and Alloys Manufacture of Wrought Alloys 466 Structure of Wrought Alloys 467 Annealing 467 Uses of Wrought Alloys 468 Orthodontic Wires 468 General Properties of Orthodontic Wires 469 Types 469 Wrought Gold Alloys 469 Uses 469 Classification 470

466

Composition 470 Properties 470 Wrought Base-metal Alloys 470 Stainless Steel 470 Ferritic Stainless Steels 471 Martensitic Stainless Steels 471 Austenitic Stainless Steels 471 Braided and Twisted Wires 473 Solders for Stainless Steel 473 Fluxes 473 Wrought Cobalt-chromium-nickel Alloys 473 Composition 473 Heat Treatment 473 Physical Properties 474 Nickel-titanium Alloys 474 Properties of Nitinol Alloys 474 Titanium Alloys 475 Composition 476 Mechanical Properties 476

Section 7: Indirect Restorative and Prosthetic Materials 28. Dental Ceramics

479

Uses and Applications 479 Evolution of Dental Ceramics 480 Classification of Dental Porcelains 481 Basic Constituents and Manufacture of Feldspathic Porcelain 482 Manufacture 485 Porcelain/Ceramic Systems 485 Metal-ceramic Restorations 486 Types of Metal-ceramic Systems 487 Cast Metal-ceramic Restorations 487 Uses 487 Composition of Ceramic for Metal Bonding 487 Supplied as 488 Manipulation and Technical Considerations 488 Porcelain-metal Bond 492 Advantages and Disadvantages of Metal-ceramic Restorations 492 Other Metal-ceramic Systems 493 All-ceramic Restorations 494 Porcelain Jacket Crown 494 Castable Glass Ceramic 496 Heat Pressed (Hot-isostatically Pressed) Ceramics 498 Glass Infiltrated Ceramics 500 CAD/CAM Ceramics 503 Advantages and Disadvantages of CAD/CAM Ceramic Restorations 514 General Properties of Fused Ceramics 515 Cementing of Ceramic Restorations 520 Repair of Ceramic Restorations 522

Contents xxiii Porcelain Denture Teeth 523 Monolithic Restorations 524 Ceramic Posts 526 Pediatric Zirconia Crowns 526 Zirconia Implants and Abutments 526 Zirconia Anatomic Root-form Implants 527

29. Denture Resins and Polymers

529

Polymers 531 Nature of Polymers 531 Structure of Polymers (Spatial Structure) 532 Polymerization—Chemistry 533 Copolymerization 535 Cross-linking 536 Plasticizers 537 Classification of Denture Base Materials 537 Synthetic Resins 538 Classification of Resins 538 Ideal Requirements of Dental Resins 538 Uses of Resins in Dentistry 539 Acrylic Resins 540 Poly (Methyl Methacrylate) Resins 540 Heat Activated Denture Base Acrylic Resins 540 Chemically Activated Denture Base Acrylic Resins 548 Light Activated Denture Base Resins 552 Microwave Cured Denture Resins 553 Specialized Poly (Methyl Methacrylate) Materials 553 Properties of Methylmethacrylate Denture Resins 555

Processing Errors 559 Repair of Acrylic Resin Dentures 561 Infection Control for Dentures 561 Care of Acrylic Dentures 561 Denture Cleansers 562 CAD/CAM Dentures 562 Denture Reliners 564 Heat Cured Acrylic Resin (Hard Liner) 564 Chairside Reliners (Hard Short-term Liner) 564 Soft or Resilient Denture Liners 564 Long-term Soft Liners 564 Tissue Conditioners (Short-term Soft Liner) 566 Denture Adhesives 567 Supplied as 567 Composition 567 Properties 567 Rebasing of Dentures 568 Provisional Crown and FDP Materials 568

30. Maxillofacial Prosthetic Materials

572

Evolution of Maxillofacial Materials 573 3D Printed Maxillofacial Prostheses 577 3D Bioprinting 577

Appendices 579 Further Reading

589

Index 595

Section-1

Structure and Properties of Dental Materials Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5

Overview of Dentistry and Dental Materials, 3 Structure and Properties of Dental Materials, 9 Structure and Properties of Metals and Alloys, 39 Electrochemical Properties of Materials, 56 Biological Properties of Dental Materials, 61

Chapter

Overview of Dentistry and Dental Materials Chapter Outline • Prevention • Rehabilitation • The Dental Laboratory

• Classification of Dental Materials • International Standards • US Standards for Dental Materials

• Ada Certification • Summary

Dentistry over the years has evolved into a highly complex field and materials play a crucial role in every aspect of treatment. Dental treatment not only includes the practice of medicine and surgery but also restoration of missing or lost structures. Besides restorations, appliances for various functions are also constructed for use in the mouth. The oral cavity is a challenging environment and materials placed in the mouth have to withstand high masticatory forces as well as corrosion. Besides direct use in the oral cavity, many materials are also used in the laboratory to aid in the fabrication of appliances or prostheses. Thus dentistry incorporates the knowledge of various materials as well as principles of engineering.

Dental treatment For convenience of description, dental care may be divided into various phases. These include preventive, disease control and elimination, restorative, rehabilitative and maintenance phases.

Preventive The preventive phase is very important. It includes educating the patient on how to maintain his oral hygiene through regular brushing, flossing and periodic checkup at the dental office. Regular brushing with a suitable brush and paste has been shown to be very effective at controlling caries as well as gum (periodontal) problems. The role of fluorides and fluoride therapy in the control of dental caries has been known to us for a long time. Fluoridation of drinking water and fluoride therapy at the dental office has played a significant role in reducing dental caries especially in children. Caries often begins in deep fissures in teeth. Fissure sealants is another preventive measure especially in children to prevent caries.

Disease control and elimination The next stage in the progress of dental disease is the actual development of dental caries and periodontal disease. This phase of treatment focuses on eliminating or controlling diseases of the mouth to halt their destruction. Commonly, patients will come to a dental office because

4 Part 1 Structure and Properties of Dental Materials of pain caused by cavities or infection. This phase focuses on treating cavities (by placing fillings), eliminating infection (by root canal or tooth removal), and managing gum health (oral prophylaxis and other periodontal procedures). Caries involves the demineralization and destruction of tooth structure. The focus is to arrest the caries process. This involves removing the carious tooth structure and restoring the cavity with a suitable temporary or permanent filling material. The famous silver filling has been in use for more than a century and is currently the most widely used filling material. The silver amalgam restoration would certainly look unpleasant if used for the front (anterior) teeth. Therefore anterior teeth are restored with an esthetic (tooth colored) material. Other ways to restore teeth involve the use of gold inlays and ceramic inlays. As caries progresses, it gets closer to the pulp, which can lead to pain (pulpitis) and infection of the pulp. If the pulp is only mildly affected, pulp therapy is started using materials which have a therapeutic effect on the pulp. These materials can be soothing and promote healing by forming a new layer of dentin (secondary dentin). If the pulp is infected, it is removed (pulpectomy) and root canal treatment popularly known as RCT is initiated. After removing the pulp, the canal is made sterile and sealed using root canal filling materials. The root canal treated tooth is weak and is prone to fracture if not protected with a crown or onlay.

Restorative This phase of treatment focuses on restoring the function and/or form of the teeth and mouth following the destruction caused by the original disease process. Common treatments during this phase include prosthesis (implants, bridges, partials, and dentures) to replace missing teeth and crowns to protect teeth. Before the discovery of tooth colored crown materials, metallic crowns were given (the famous gold tooth). Modern dentists are able to provide crowns that are natural looking and pleasing. Many of these structures are processed outside the mouth, in the laboratory. The dental technician uses an accurate model of the teeth to fabricate these restorations. Models are made from a negative record of the mouth called an impression. This is sent to the laboratory where the technician pours a mix of plaster or stone into the impression. When the mix hardens, we obtain a model. If the coronal tooth structure is entirely gone or destructed, even a crown would not stay. In this case, the dentist has to place a post and core. The part placed into the root canal is known as post and the rest of it is known as the core. The crown is then constructed and cemented onto the core. Following extraction of teeth, the patient often desires that it be replaced with an artificial tooth. There are many ways of replacing the tooth. Today implants have become very popular. A titanium screw can be implanted into the jaw surgically followed by an artificial crown. Another next choice is the fixed partial denture (bridge). Usually the teeth by the side of the missing tooth is reduced in size (prepared) in order to receive the bridge. The bridge is then cemented onto these teeth. If too many teeth are missing, we might have to consider the removable partial denture which replaces the missing teeth but is not fixed in the mouth. It can be removed by the patient for cleaning and hygiene. The ideal removable partial denture is usually made of a combination of metal and plastic (cast partial denture). Interim or temporary partial dentures are made entirely of plastic also and are often referred to as treatment partial dentures. The final stage is when all the teeth have to be replaced. One is, of course, familiar with the complete denture which is often seen in elderly individuals. These artificial teeth replace the

Overview of Dentistry and Dental Materials Chapter 1

entire dentition and are usually of the removable type (fixed complete dentures are also available which are supported and retained by implants). The complete denture is usually made of a type of plastic called acrylic. The teeth used in the denture can be made of acrylic or porcelain. Besides all the materials mentioned above, different specialties in dentistry have their special materials. Some of these are not covered in this book. For example, endodontists use root canal files along with various irrigants to clean and debride the root canal. A variety of root canal sealing pastes and medicaments are also available. The periodontists use different types of graft material to restore lost periodontal bone and tissue. Unfortunately, not all the materials used in dentistry are within the scope of this book.

Maintenance Phase Once the treatment is completed, a maintenance phase focuses on keeping the dental work in functioning order through periodic recalls, maintaining health (oral prophylaxis), and screening for oral cancer at each six-month exam.

The DENTAL SPECIALTIES Currently nine specialties are recognized by the Dental Council of India. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Conservative Dentistry and Endodontics Periodontics Prosthodontics Public Health Dentistry Oral Medicine and Radiology Oral and Maxillofacial Surgery Orthodontics and Dentofacial Orthopedics Oral and Maxillofacial Pathology Pedodontics and Preventive Dentistry

Conservative dentistry That phase of dentistry concerned with restoration of parts of the teeth that are defective through disease, trauma, or abnormal development to a state of normal function, health, and esthetics, including preventive, diagnostic, biologic, mechanical, and therapeutic techniques, as well as material and instrument science and application.*1 Endodontics Endodontics is the branch of dentistry which is concerned with the morphology, physiology and pathology of the human dental pulp and periradicular tissues. Its study and practice encompass the basic and clinical sciences including biology of the normal pulp, the etiology, diagnosis, prevention and treatment of diseases and injuries of the pulp and associated periradicular conditions.*1 Periodontics Periodontics is that specialty of dentistry which encompasses the prevention, diagnosis and treatment of diseases of the supporting and surrounding tissues of the teeth or their substitutes and the maintenance of the health, function and esthetics of these structures and tissues. Prosthodontics Prosthodontics is the dental specialty pertaining to the diagnosis, treatment planning, rehabilitation and maintenance of the oral function, comfort, appearance and health of patients with clinical conditions associated with missing or deficient teeth and/or oral and maxillofacial tissues using biocompatible substitutes.*2 *1 Free Dictionary *2 Adapted from the Council on Dental Education and Licensure, American Dental Association

6 Part 1 Structure and Properties of Dental Materials Public health dentistry Dental Public Health is the science and art of preventing and controlling dental diseases and promoting dental health through organized community efforts. It is that form of dental practice which serves the community as a patient rather than the individual. It is concerned with the dental health education of the public, with applied dental research, and with the administration of group dental care programs as well as the prevention and control of dental diseases on a community basis.*2 Oral medicine and radiology Oral medicine is concerned with clinical diagnosis and nonsurgical management of nondental pathologies affecting the orofacial region (the mouth and the lower face). Oral and maxillofacial radiology is the specialty of dentistry and discipline of radiology concerned with the production and interpretation of images and data produced by all modalities of radiant energy that are used for the diagnosis and management of diseases, disorders and conditions of the oral and maxillofacial region.*2

Oral and maxillofacial surgery Oral and Maxillofacial Surgery is the specialty of dentistry which includes the diagnosis, surgical and adjunctive treatment of diseases, injuries and defects involving both the functional and esthetic aspects of the hard and soft tissues of the oral and maxillofacial region.*2 Orthodontics and dentofacial orthopedics Orthodontics and dentofacial orthopedics is the dental specialty that includes the diagnosis, prevention, interception, and correction of malocclusion, as well as neuromuscular and skeletal abnormalities of the developing or mature orofacial structures.*2 Oral and maxillofacial pathology Oral Pathology is the specialty of dentistry and discipline of pathology that deals with the nature, identification, and management of diseases affecting the oral and maxillofacial regions. It is a science that investigates the causes, processes, and effects of these diseases. The practice of oral pathology includes research and diagnosis of diseases using clinical, radiographic, microscopic, biochemical, or other examinations.*2 Pedodontics and preventive dentistry Pediatric Dentistry is an age-defined specialty that provides both primary and comprehensive preventive and therapeutic oral health care for infants and children through adolescence, including those with special health care needs.*2

The dental laboratory Many materials are used in the dental laboratory to aid in the fabrication of stents, prostheses, appliances and other structures used in and around the mouth. These include cutting, abrading and polishing materials. Investment materials are used in the creation of moulds in the casting of metal structures. Waxes are used in various stages of construction of different structures. Gypsum products are used to make casts, models, molds and to secure articulators.

Classification of Dental Materials Traditionally the subject of dental materials primarily included materials used in Restorative Dentistry including related auxiliary material. Currently, there is a trend to be more inclusive and include materials from specialties, which have traditionally been excluded like the Endodontic and Surgical specialties. There is no classification that satisfactorily categorizes all materials used in dentistry. This is because many of the materials have multiple utilities and overlapping functions. *1 Free Dictionary *2 Adapted from the Council on Dental Education and Licensure, American Dental Association

Overview of Dentistry and Dental Materials Chapter 1

General classification of all materials All materials can be classified into four classes 1. 2. 3. 4.

Metals Ceramics Polymers Composites

Classification of dental materials 1. Preventive materials 2. Restorative materials 3. Auxiliary materials 4. Prosthetic materials

5. Appliance materials 6. Biomaterials 7. Therapeutic agents

Preventive materials include pit and fissure sealants and other materials used to prevent the onset of dental diseases. Restorative materials include materials used to repair or replace tooth structure. This includes materials like amalgam, composites, ceramics, cast metal structures and denture materials. Auxiliary materials are substances that aid in the fabrication process but do not actually become part of the restoration, appliance or prosthesis. This includes materials like gypsum products, impression materials, casting investments, waxes, etching gels, custom tray materials, etc. Prosthetic materials are materials used to replace missing teeth and oral and maxillofacial structures. These include the alloys, ceramics and polymers used in fixed and removable partial denture construction and maxillofacial prostheses. A biomaterial is a biological or synthetic substance which can be introduced into body tissue as part of an implanted medical device or used to replace an organ, bodily function. Although many traditional materials qualify as biomaterials, this term has been introduced to include bone and tissue grafts. Therapeutic agents include various chemicals, medicaments, antimicrobials and other locally applied agents that are capable of producing a specific effect in the area to which it is applied. In reality, many materials have dual or multiple uses and so the above categorization is difficult to strictly apply.

International standards ORGANIZATION (ISO) The Federation Dentaire Internationale (FDI) and the International Organizations for Standardization (ISO) are two organizations working for the development of specifications and terminology on an international level. The FDI is restricted to dental products whereas the ISO covers all products. The ISO is a nongovernmental body composed of the national organizations of more than 80 countries including India (Bureau of Indian Standards). The ISO standards (Fig. 1.1A) (see also appendix) are formulated by a ‘technical committee’ (TC). Dental products are covered by TC 106. Various subgroups known as ‘subcommittees’ (SC) cater to specific areas. The subcommittees are further divided into ‘working groups’ (WG) to cover individual products or items. For example, TC 106/SC 1: WG 7 covers dental amalgam and mercury. Considering the worldwide supply and demand for dental products the benefits from the ISO are invaluable. Suppliers and consumers can be assured of impartial reliable data to assess the quality of products and equipment regardless of its country of origin and use. Standards are constantly revised; therefore, it is imperative for manufacturers and researchers alike to refer to the latest edition of the ISO specifications to stay abreast of changes in requirements and classifications.

8 Part 1 Structure and Properties of Dental Materials

Figures 1.1A and B Examples of standards: (A) International Standards Organization’s specification for zinc oxide eugenol cement (ISO). (B) ANSI/ ADA specification No. 122 for dental waxes.

US Standards for Dental Materials Standards are specifications by which the quality of a product can be gauged. Standards identify the requirements of physical and chemical properties of a material which ensures satisfactory performance for the function for which it is intended. The earliest standards in the US were developed by the National Bureau of Standards in 1919 on the request of the US Army for the purchase and use of dental amalgam. The task was assigned to a team led by Wilmer Souder. Souder’s report and testing methods were well received by the dental profession and test data were requested for other dental materials. By 1928, the responsibility for continued research into standards was assumed by the ADA.

ADA certification Currently the ADA under direction of the ANSI (American National Standards Institute) sponsors two committees. The ADA Standards Committee for Dental Products develops specifications for all dental products, instruments and equipment (excluding drugs and X-ray films). The ADA’s Council on Scientific Affairs is responsible for the evaluation of drugs, teeth cleaning agents, teeth whitening agents, therapeutic agents used in dentistry and dental X-ray films. After formulation of the specifications by the ADA, it is submitted to the ANSI. On approval, it becomes a national standard (Fig. 1.1B). Manufacturers can submit their product for the ADA seal of approval. This falls into three categories – Accepted, Provisionally Accepted, and Unaccepted. ADA certification is an important symbol of a dental product safety and effectiveness. ADA acceptance is effective for a period of 5 years. Currently, the ADA have adopted the ISO specifications. The ADA specification for a particular product is identical to its ISO counterpart.

Summary Materials used for dentistry are highly specialized. Each one is designed with a specific set of properties depending on what it is intended for. For example, materials used as tooth restorations should be able to withstand occlusal forces as well as bond to tooth structure. Impression materials should be highly accurate and stable in order to duplicate the original structure. Modern science, research and technology has provided dentistry with an everexpanding selection of unique combinations of materials and techniques to serve dental treatment needs.

Chapter

Structure and Properties of Dental Materials Chapter Outline • • • • • • • • • • • • • • • • • • • • • • • • •

Structure of Matter Forms of Matter Change of State Interatomic Bonds –– Primary Bonds –– Secondary Bonds Thermal Expansion Crystal Structure Noncrystalline Structure Stress and Strain Diffusion Surface Tension Wetting Contact Angle Physical Properties of Dental Materials Stress Complex Stresses Poisson’s Ratio Proportional Limit Elastic Limit Yield Strength Modulus of Elasticity Flexibility Resilience Impact Impact Strength Permanent Deformation

• Strength –– Tensile Strength –– Compressive Strength –– Shear Strength –– Transverse or Flexural Strength

• Fatigue –– Static Fatigue • Toughness • Brittleness • Ductility • Malleability • Hardness –– Brinell –– Rockwell Hardness Number

• • • • • • • •

(RHN)

–– Vickers Hardness Test (VHN)

–– Knoop Hardness Test (KHN) –– The Shore and the Barcol

• Rheology –– Viscosity –– Creep –– Flow –– Thixotropic –– Relaxation –– Shear Stress and Shear Strain Rate

• • • • • • • •

–– Newtonian –– Pseudoplastic –– Dilatant

Physical Properties Forms of Matter Melting Point Boiling Point Density Glass Transition Temperature (Tg) Solubility Thermal Properties –– Thermal Conductivity –– Thermal Diffusivity –– Thermal Expansion –– Coefficient of Thermal Expansion (CTE) Optical Properties and Color Dimensions of Color Measurement of Color Metamerism Translucence, Transparence and Opacity Fluorescence Radiopacity and Radiolucency –– Measurement of Radiopacity Magnetic Properties

All materials are made up of atoms. If the reaction of a material and its properties are to be predicted, a basic knowledge of matter is essential. All dental restorations, whether they be ceramic, plastic or metal, are built from atoms.

Structure of Matter Atom An atom is the smallest unit of matter that defines the chemical elements. Atoms are very small. The size of atoms is measured in picometers, which is trillionths (10–12) of a meter. Every atom is composed of a nucleus and one or more electrons that orbit the nucleus (Fig. 2.1).

10 Part 1 Structure and Properties of Dental Materials

Figure 2.1 Building blocks of matter.

Protons, neutrons and electrons The nucleus is made of one or more protons and neutrons. Over 99.94% of the atom’s mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has in excess or lesser number of electrons relative to protons, then it has an overall positive or negative charge, and is called an ion. Electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively-charged protons from one another. The number of protons in the nucleus defines to what chemical element the atom belongs, for example, all copper atoms contain 29 protons. Quarks Protons and neutrons are made up of subatomic particles called quarks. Quarks are believed to be the basic building blocks of matter.

INTERATOMIC BONDS Atoms are held together by some force. These interatomic bonding forces that hold atoms together are cohesive forces. Interatomic bonds may be classified as 1. Primary bonds, or 2. Secondary bonds

Primary Bonds These are chemical in nature. 1. Ionic

2. Covalent

3. Metallic

Ionic Bonds These are simple chemical bonds, resulting from mutual attraction of positive and negative charges. The classic example is sodium chloride Na+ Cl¯. Covalent Bonds In many chemical compounds, two valence electrons are shared. The hydrogen molecule (H2) is an example of covalent bonding. Another example is methane. The carbon atom has 4 valence electrons that can be stabilized by joining with hydrogen.

H H:C:H H :

: .

4H + C

Structure and Properties of Dental Materials Chapter 2

Metallic Bonds One of the chief characteristics of a metal is its ability to conduct heat and electricity. Such conduction is due to the mobility of the so-called free electrons present in the metals. The outer shield valence electrons can be removed easily from the metallic atom leaving the balance of the electrons tied to the nucleus, thus forming a positive ion. The free valence electrons are able to move about in the metal space lattice to form what is, sometimes, described as an electron ‘cloud’ or ‘gas’. The electrostatic attraction between this electron ‘cloud’ and the positive ions in the lattice bonds the metal atoms together as a solid.

Secondary Bonds (Van der waals forces) A second type of bond between molecules may be seen. They are also known as van der Waals forces (named after Dutch scientist Johannes Diderik van der Waals. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations (dipole) of nearby particles. They are defined as weak, short-range electrostatic attractive forces between uncharged molecules, arising from the interaction of permanent or transient electric dipole moments. Dipole van der Waals Forces are due to the formation of dipole. A dipole is formed when electrons shift to one side of the atoms or molecules resulting in the formation of a negative polarity on the side and on the other half a positive polarity. This attracts other similar dipoles. There are three kinds of van der Waals forces – 1. Keesom force (between two permanent dipoles), 2. Debye force (between a permanent dipole and an induced dipole) 3. London dispersion force (between two instantaneously induced dipoles). Van der Waals forces are relatively weak compared to covalent bonds, but play a fundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. Van der Waals forces define many properties of organic compounds. In nature geckos and spiders utilize van der Waals forces to climb and cling on to smooth surfaces (Fig. 2.2).

CRYSTAL STRUCTURE Space lattice or crystal can be defined as any arrangement of atoms in space such that every atom is situated similar to every atom. Space lattice may be the result of primary or secondary bonds. There are 14 possible lattice type forms, but many of the metals used in dentistry belong to the cubic system. The simplest cubic space lattice is shown in Figs. 2.3A to C. The solid circles represent the position of the atoms. Their positions are located at the points of intersection of three sets of parallel planes, each set being perpendicular to other planes. These planes are often referred to as crystal planes.

Simple cubic

A Figure 2.2 Geckos can stick to walls and ceilings because of van der Waals forces.

Body centered cubic

Figures 2.3A to C Crystal structure.

Face centered cubic

12 Part 1 Structure and Properties of Dental Materials NONCRYSTALLINE STRUCTURE In a crystalline structure, the arrangement of atoms in the lattice is orderly and follows a particular pattern. In noncrystalline structures or amorphous structures, e.g. waxes, the arrangement of atoms in the lattice is disorderly and distributed at random. There is, however, a tendency for the arrangement of atoms or molecules to be regular, for example, glass is considered to be a noncrystalline solid, yet its atoms bind to form a short range order rather than long range order lattice. In other words, the ordered arrangement of glass is localized with large number of disordered units between the ordered units. Since such an arrangement is also typical of liquids, such solids are, sometimes, called supercooled liquids.

STRESS AND STRAIN The distance between two atoms is known as interatomic distance. This interatomic distance depends upon the electrostatic fields of the electrons. If the atoms come too close to each other, they are repelled from each other by their electrons charges. On the other hand, forces of attraction keep them from separating. Thus the atoms are kept together at a position where these forces of repulsion and attraction become equal in magnitude (but opposite in direction). This is the normal equilibrium position of the atoms. The normal position of the atoms can be changed by application of mechanical force. For example, the interatomic distance can be increased by a force pulling them apart. If the displacing force is measured across a given area it is known as a stress and the change in dimension is called a strain. In simple words, stress is the force applied and strain is the resulting change in shape. Theoretically, a stress and a strain exist whenever the interatomic distance is changed from the equilibrium position. If the stress pulling the atoms apart exceeds the resultant force of attraction, the atoms may separate completely, and the bonds holding them together are broken. Strain can also occur under compression. However, in this case, the strain produced is limited because when the atoms come closer than their normal interatomic distance, a sudden increase in energy is seen.

DIFFUSION The diffusion of molecules in gases and liquids is well known. However, molecules or atoms diffuse in the solid state as well. Diffusion rates depend mainly on the temperature. The higher the temperature, the greater will be the rate of diffusion. The diffusion rate will, however, vary with the atom size, interatomic or intermolecular bonding lattice imperfections. Thus every material has its own diffusion rate. The diffusion rate in noncrystalline materials may occur at a rapid rate and often may be seen.

SURFACE TENSION Energy at the surface of a solid is greater than in its interior. For example, inside a lattice, all the atoms are equally attracted to each other. The interatomic distances are equal, and energy is minimal. However, at the surface of the lattice, the energy is greater because there are no atoms on the outside. Hence there is only a force from the inside of the lattice pulling the outermost atoms inwards. This creates a tension on the outer surface and energy is needed to pull the outermost atoms away. The increase in energy per unit area of surface is referred to as the surface energy or surface tension (Figs. 2.4A and B). The surface atoms of a solid tend to form bonds to any atom that comes close to the surface in order to reduce the surface energy of the solid. This attraction across the interface for unlike

Structure and Properties of Dental Materials Chapter 2

Figures 2.4A and B (A) Schematic representation of molecular view of surface tension. (B) Surface tension causes a paper clip to float on water despite the fact that metal in the paper clip has a higher density than water.

molecules is called adhesion. In summary, the greater the surface energy, the greater will be the capacity for adhesion.

WETTING It is very difficult to force two solid surfaces to adhere. However smooth their surfaces may appear, they are likely to be very rough at the atomic or molecular level. When they are placed together, only the ‘hills’ or high spots are in contact. Since these areas form only a small percentage of the total surface, no adhesion takes place. For proper adhesion, the distance between the surface molecules should not be greater than 0.0007 micrometer or micron (µm). One method of overcoming this difficulty is to use a fluid that will flow into these irregularities and thus provide contact over a great part of the surface of the solid. For A example, when two glass plates are placed one on top of the other, they do not usually adhere. However, if a film of water is placed in between them, it becomes difficult to separate the two plates. B To produce adhesion in this manner, the liquid must flow easily over the entire surface and adhere to the solid. This characteristic is referred to as wetting. The degree of wetting is indicated by the contact angle of the adhesive to the adherend.

Contact angle The contact angle is the angle formed by the adhesive (e.g. water) and the adherend (e.g. glass) at their interface. The extent to which an adhesive will wet the surface of an adherend may be determined by measuring the contact angle between the adhesive and the adherend. Based on the contact angle there are four classes of wetting (Figs. 2.5A to D):

Contact Angle = 0 Contact Angle = 0 < 3 µm Particle size < 3 µm Average particle size — 0.04 µm Nano range (5–100 nm or 0.005–0.01 µm)

Based on filler particle size Fillers play an important role in the composite performance. However, a universal system of classifying fillers has not been agreed upon. Differences exist on the nomenclature and the size range of the filler particles.

Macrofillers (10-100 µm) Midifillers (1-10 µm) Minifillers (0.1-1 µm) Microfillers (0.01-0.1 µm) a. hom*ogenous — contains only microfillers

170 PART 2 Direct Restorative Materials

b. Heterogenous — microfillers combined with prepolymerized fillers — splintered prepolymerized particles — spherical prepolymerized particles c. Agglomerated — microfiller sintered to form larger filler complexes Nanofillers (0.005–0.01 µm) Hybrid (range of sizes which usually includes micro or nanofillers with macro, midi or mini fillers

Based on viscosity Conventional Flowable Packable Viscosity determines the flow characteristics during placement. A flowable composite flows like liquid or a loose gel. A packable composite is firm and offers some resistance to condensation.

Based on applications and commercial availability 1. Restorative composites—direct intraoral restorations –– Hybrid composites (Fig. 11.2) -- Macrofilled hybrids -- Midifilled hybrids -- Minifilled hybrids -- Nanofilled hybrids –– Microfilled (Fig. 11.3) –– Nanofilled composites (Fig. 11.4) –– Flowable (Fig. 11.5) –– Packable –– Core build-up composites 2. Prosthodontic composites (for fabrication of inlays, veneers, crowns and FDPs) 3. Provisional composites (for temporary crowns, FDPs, etc.) 4. Luting composites 5. Repair composites (intraoral repair of fractured ceramic or acrylic or composite veneer)

FIGURE 11.2 Representative microhybrid composite.

FIGURE 11.3 A microfilled composite.

Resin-based Composites and Bonding Agents CHAPTER 11

FIGURE 11.4 Universal nano-hybrid composite.

171

FIGURE 11.5 Flowable composites are often identified by the syringe like dispensing tips.

RESTORATIVE COMPOSITE RESINS The first tooth colored restorative system were developed in the late 1950s and early 1960s by Bowen (also known as Bowen’s resin). The early composite resins were generally macrofilled resins and were referred to as conventional composites. These were chemically activated. This was followed by U-V light activated and later visible light activated systems. The early composites had poor wear resistance and stained easily. This was attributed to the large sized (macrofillers) filler particles used. The introduction of the microfilled composites improved wear resistance and better esthetics. However, they had poor mechanical properties. The hybrid composites attempted to combine the esthetics and wear resistance of the microfilled with the mechanical properties of the macrofilled composites. The latest entry into the field - the nanocomposites (Fig. 11.4) holds the promise of high polishability with further improvement in mechanical properties.

SUPPLIED AS Composites used for restoring teeth are usually supplied as a kit (Fig. 11.2) containing the following

Composite resin (either chemical or light cured) Etching liquid (37% phosphoric acid) Bonding agent Shade guide.

Chemically activated composite resins are available as Two paste (base and catalyst) system Supplied in small jars or syringes (Fig. 11.6). Powder-liquid systems Powder (inorganic phase plus the initiator) is supplied in jars. Liquid (BIG-GMA diluted with monomers) in bottles.

Light activated resins are available as Single paste form in dark or light proof syringes (Fig. 11.5). Trade names Various commercial products available are presented in Table 11.1.

FIGURE 11.6 Chemically cured composite.

172 PART 2 Direct Restorative Materials INDICATIONS FOR VARIOUS COMPOSITE RESINS The indications for different products are presented in Table 11.2.

COMPOSITION AND STRUCTURE The essentials components of a composite resin (Fig. 11.7) Resin matrix/binder—Bis-GMA or urethane dimethacrylate Filler—Quartz, colloidal silica or heavy metal glasses Coupling agent—Organosilanes.

In addition they contain A curing system—Chemical or light curing chemicals. Inhibitors (0.01%)—Prevents premature polymerization, e.g. butylated hydroxytoluene (BHT).

FIGURE 11.7 Essentials of a composite.

TABLE 11.1 Representative commercial products Type

Commercial name

Chemically Activated

Isopast, Alfa comp (voco), Brilliant (coltene), Medicept

Light Activated Hybrid (Universal)

Filtek Z250(3M), Herculite (Kerr), TPH spectrum, Venus,Charisma (Heraeus), Tetric Ceram (Ivoclar), Point-4 (Kerr)

Microfilled

Helio Progress, Durafil VS (Heraeus), A110 (3M-ESPE), Sculpt-it (Jeneric/Pentron), Amelogen Microfill (Ultra-dent), Renamel Microfill (Cosmedent).

Flowable

C- Fill Flow, Synergy D6 Flow (Coltene), Tetric flow, Venus Flow (Heraeus), Flow Plus (Medicept), etc.

Packable

Surefil (Dentsply), Heliomolar HB (Ivoclar), Solitaire 2 (Heraeus), Tetric Ceram HB, etc.

Nanocomposite

Grandio (Voco), Filtek Supreme (3 M), Venus Diamond, (Heraeus), Composite-nanohybrid (Medicept)

TABLE 11.2 Various restorative composites currently marketed and their uses Type

Indications

Hybrid or universal (microhybrids)

Anterior and posterior restorations in high stress areas requiring improved polishing (e.g. Classes I, II, III, IV)

Packable Hybrids

Class II cavities where greater packability is needed for improved contact with adjacent teeth

Flowable Hybrids

• • • • • •

Microfilled (hom*ogenous and heterogeneous) Hybrid or universal (nanohybrids)

Class V lesions subjected to flexing stresses Mini cavities Repair of composite In layered composite restorations as first layer (for better adaptation) Low stress areas Areas requiring high polish (to reduce plaque accumulation, e.g. subgingival areas, Class V lesions, etc.)

Anterior and posterior restorations in high stress areas requiring greater polishability, e.g. Classes I-IV and Class V cervical lesions

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UV absorbers—to improve color stability Opacifiers—(0.001 to 0.007%) e.g. titanium dioxide and aluminum Color pigments—to match tooth color.

RESIN MATRIX Dental composites use a blend of monomers that are aromatic or aliphatic dimethacrylates. Of these, bis-GMA (Bisphenol-A-glycidyl methacrylate), urethane dimethacrylate (UDMA) and bis-EMA (Bisphenol-A-polyethylene glycol diether dimethacrylate) are most commonly used. Triethylene glycol dimethacrylate (TEGDMA) is added to control the viscosity. Bis-GMA was developed by RL Bowen (Bowen’s resin) in the early 1960s. Its properties were superior to those of acrylic resins. However, it had a few limitations like

A high viscosity which required the use of diluent monomers Difficulty in synthesizing a pure composition Strong air inhibition to polymerization High water sorption because of diluents used Polymerization shrinkage and thermal dimensional changes still existed Like other resins it does not adhere to tooth structure

To make it clinically acceptable, diluent monomers are added to the resin matrix to reduce the viscosity of the resin. It also allows more fillers to be incorporated. Diluents allow extensive cross-linking between chains, thereby increasing the resistance of the matrix to solvents. The commonly used diluent monomer is TEGDMA (triethylene glycol dimethacrylate). Thus composite resins have to be blended with different monomers to optimize their properties. Drawbacks of TEGDMA include 1. High shrinkage. 2. Contributes to reduced shelf-life by migration in to the plastic walls. 3. Being hydrophilic it makes the composite susceptible to moisture leading to thickening or softening of the paste in certain climatic conditions. (Because of these drawbacks some manufacturers have replaced the majority of the TEGDMA with a blend of UDMA and bis-EMA). The refractive index is an important property for anterior restorative materials. To have acceptable esthetics composite resins must match the translucency of enamel. Bis-GMA and TEGDMA have a refractive index of 1.55 and 1.46 respectively which average to around 1.5 when combined together.

FILLER PARTICLES Fillers play a crucial role in the composite resin. Most of the important properties of the resin is determined by its filler content. Composite fillers are classified by material, shape and size. Many different classifications of fillers have been proposed. They are broadly classified into 3 groups - macrofillers, microfillers and nanofillers. A mixture of different particle sizes is referred to as a hybrid. For details of filler size and nomenclature see ‘classification of composite resins.’

Functions of fillers Addition of filler particles in to the resin significantly improves its properties. 1. Improves strength—Fillers reinforce the resin and improve mechanical properties like strength, stiffness, hardness, etc.

174 PART 2 Direct Restorative Materials 2. Reduces shrinkage—As less resin is present, the curing shrinkage is reduced thereby reducing marginal leakage. 3. Reduces Wear—Fillers play a crucial role in reducing the wearing of composite resins. The smaller the size and higher the concentration of fillers the better the wear resistance. 4. Surface smoothness and esthetics—Fillers affect the surface smoothness and subsequent esthetics of the composite. The smaller the particle size, the greater the polishability. Larger particle sizes in the early composites contributed to surface roughness and staining. 5. Reduces water sorption—Resins absorb water and makes it more prone to wear and staining. Filler reduce water sorption by reducing the overall resin content. 6. Reduces thermal expansion and contraction—Fillers have a lower CTE in comparison to the resin. 7. Improves clinical handling (increased viscosity makes them easier to handle clinically). 8. Imparts radiopacity—Radiopaque fillers help improve diagnostics (e.g. caries detection through roentgenograms, etc.). Important attributes of fillers Important attributes of fillers, that determine the properties and clinical application of composites are

Amount of filler added (filler loading) Size of particles and its distribution Shape of fillers Index of refraction Radiopacity Hardness

Filler size The size of the fillers affect the surface smoothness and the wear resistance. The smaller the fillers the greater the surface smoothness. Microfilled composites have the best surface smoothness and lowest wear. This is because the particles are removed at the same rate as the resin matrix if they are smaller. Larger particles result in a rougher surface. The introduction of the nanoparticles hold great promise of improved smoothness, good wear resistance as well as improved mechanical properties. Filler loading Filler loading refers to the amount of fillers that can be practically incorporated into the resin. The amount of filler that can be added depends on the type of filler and the purpose for which it is intended. Most hybrid composites have a filler loading ranging from 60 to 70% volume. The introduction of the newer nanofillers allow a greater filler loading of up to 79.5% vol. Microfillers thicken the resin quickly. Thus microfilled resins usually do not have the same filler loading as resins with larger particle sizes such as the hybrids. Particle size distribution Most modern hybrid composites have particles range in size from 0.01 to 10 µm. Microfillers are usually in the range of 0.01 – 0.07 (average 0.04 µm). In order to increase the filler amount in the resin, it is necessary to add the fillers in a range of particle sizes. If a single particle size is used, a space will exist between the particles. Smaller particles can then fill-up these spaces, thus increasing the filler content (Figs. 11.8A to E). Shape of fillers Based on shape 3 types of fillers are used—irregular, spherical and fibrous. The shape affects the filler loading and the handling characteristics of the composite. Refractive index For esthetics, the filler should have a translucency similar to tooth structure. To achieve this, the refractive index of the filler should closely match that of the resin. Most glass and quartz fillers have a refractive index of 1.5, which match that of bis-GMA and TEGDMA.

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FIGURES 11.8A TO E Effect on particle size on surface smoothness. (A) Traditional composite. (B) Hybrid composite. (C) Microfilled heterogenous type (showing prepolymerized fillers). (D) Microfilled hom*ogenous type. (E) Nanocomposite showing nanoparticles and nanoclusters.

Measurement of filler content—volume versus weight percentage Filler content is designated in percent volume (vol.%) or percent weight (wt.%). Weight percent is usually higher in value than percent volume. The volume percentage may be a more reliable indicator of filler content than the weight percentage. This is because of differences in density between different fillers. For example, composites can have a similar volume percentage of fillers yet different weight percentages. This is because the composite containing a larger fraction of heavy metal glass fillers will have a higher weight percentage.

FILLERS TYPES Composite resins may contain a variety of fillers

Ground quartz Glasses or ceramic containing heavy metals Boron silicates Lithium aluminum silicates Ytterbium trifluoride Colloidal silica

Quartz fillers They are obtained by grinding or milling quartz. They were mainly used in conventional composites. They are chemically inert and very hard. This makes the restoration more difficult to polish and can cause abrasion of the opposing teeth and restorations. Glasses/ceramics containing heavy metals These fillers provide radiopacity to the resin restoration. Increased radiopacity make composites detectable on radiographs which aid diagnostics. Examples are barium, zirconium, ytterbium fluoride, zinc and strontium glasses. The most commonly used is barium glass. It is not as inert as quartz. Some barium may leach out with time. Fluoride releasing fillers Some current composites have fluoride releasing capability. This includes fillers like ytterbium trifluoride and Ba-Al-fluorosilicate glass. In one commercial product (Tetric ceram) the YbF3 content is as high as 17 wt.%. The Ba-Al-fluorosilicate glass content was 5 wt.%. Colloidal silica They have the same composition and refractive index as quartz but not as hard or abrasive. This is due to their ‘amorphous’ (or noncrystalline) form. They are also

176 PART 2 Direct Restorative Materials referred to as ‘microfillers’. They are obtained by a pyrolytic or a precipitation process. Colloidal silica particles (Fig. 11.9) have a large surface area (50 to 400 m2/g), thus, even small amounts of microfillers thicken the resin. They are added in small amounts in hybrid composites (5 wt. %) to adjust the paste viscosity. The hybrid varieties have a microfiller loading of 10 to 15% weight. In microfilled composites it is the main filler used (20 to 59% volume). Since they cannot be added in large amounts the overall filler loading of microfilled composites is lower than conventional or hybrid varieties.

FIGURE 11.9 Electron microscopic picture of colloidal silica 0.06 µm.

METHODS TO INCREASE MICROFILLER LOADING Manufacturers are constantly on the look out for methods to increase the filler content of the microfilled composites. 1. One method is to sinter (fuse) the colloidal silica particles, thereby reducing surface area. These are known as agglomerated silica. 2. Addition of prepolymerized fillers This is the more common method. Also known as ‘organic fillers’. They are prepared by adding 60 to 70 wt.% of silane coated colloidal silica to the monomer, which is held at a slightly higher temperature to reduce its viscosity. It is then heat cured and ground. The composite is obtained by adding these prepolymerized fillers along with more silane coated microfillers into unpolymerized resin matrix (Fig. 11.13). Silica nanoparticles These are currently the smallest filler particles used in dental composites. Nanoparticles are defined as particles between 1 and 100 nanometers in size. In nanotechnology, a particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. Adoption of nanoparticle technology have ushered in the next generation of composite resins. Incorporation of silica nanoparticles into the composite resins have improved many of the properties of composite resins, particularly wear resistance and polishability. Nanoparticles in composites can be used in 2 forms.

Nanoparticle—a single nanoparticle (size ranges from 5-25 nm) Nanoclusters—a group of nanoparticles (forms larger sized particles).

Manufacture of fillers Filler particles can be generated: (1) by crushing, grinding, and sieving large blocks of ceramic, (2) by condensation of SiO2 from the vapor phase as small droplets of microfiller, or (3) by precipitation of filler particles from solution (sol-gel). The smallest fillers can only be manufactured in a practical way from the vapor phase or by sol-gel processes.

COUPLING AGENTS Coupling agents bond the filler particles to the resin matrix. The earliest composites did not use coupling agents. This resulted in microscopic defects between the filler and surrounding resin. Microleakage of fluids into these defects led to surface staining and failure. Typically the manufacturer treats the surface of the filler with a coupling agent to bond the filler to the resin matrix.

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Functions of coupling agents 1. They improve the properties of the resin through transfer of stresses from the more plastic resin matrix to the stiffer filler particles. 2. They prevent water from penetrating the filler-resin interface. 3. They bond the fillers to the resin matrix thereby reducing the wear. The most commonly used coupling agents are organosilanes (i.e. 3-methacryloxypropyltrimethoxysilane). O

OCH3

CH2==C—C—O—CH2CH2CH2—Si—OCH3 CH3

OCH3

The agent is a molecule with a methacrylate groups on one end and methoxy groups (OCH3) on the other end. In the presence of adsorbed water the methoxy groups hydrolyze to form silanol groups (-Si-OH) which then form ionic bonds with the silanol groups of the filler forming a siloxane bond (-Si-O-Si-). The other end has a methacrylate group which forms a covalent bond with the resin when it is polymerized. This completes the coupling process. Zirconates and titanates can also be used as coupling agents.

POLYMERIZATION (SETTING) MECHANISMS They polymerize by the addition mechanism that is initiated by free radicals as described in resins. The free radicals can be generated by chemical activation or external energy (heat, light or microwave). Based on the mode of activation of polymerization, there are three main types A. Chemically activated resins B. Light-activated resins C. Combination of the above (dual cure).

CHEMICALLY ACTIVATED COMPOSITE RESINS This is a two-paste system

Base paste contains—benzoyl peroxide initiator Catalyst paste—tertiary amine activator (i.e. N, N-dimethyl-p-toluidine).

Setting When the two pastes are spatulated, the amine reacts with the benzoyl peroxide to form the free radicals which starts the polymerization.

Disadvantages 1. No control of curing time. 2. No control of curing shrinkage. 3. Hand mixing increases possibility of voids.

LIGHT ACTIVATED COMPOSITE RESINS Under normal light they do not interact. However, when exposed to light of the correct wavelength the photoinitiator (camphorquinone) is activated and reacts with the amine to form free radicals which then start the polymerization.

178 PART 2 Direct Restorative Materials UV light activated systems The earliest systems used ultraviolet (UV) light for curing. Light activation put control of the working time in the hand of the dentist. Limitations of UV light curing were 1. Limited penetration of the light into the resin. Thus, it was difficult to polymerize thick sections. 2. Lack of penetration through tooth structure.

Visible-light activated resins These have totally replaced UV light systems. They are also more widely used than the chemically activated resins. These are single paste systems containing Photoinitiator: Camphorquinone 0.2 wt.% Amine accelerator: Dimethylaminoethyl-methacrylate (DMAEMA 0.15 wt.%). Camphorquinone has light absorption range between 400 and 500 nm. This is in the blue region of the visible light spectrum. In some cases inhibitors are added to enhance its stability to room light or dental operatory light.

DUAL CURE RESINS A combination of chemical and light curing is used to overcome some of the drawbacks of light curing. Dual cure resins are supplied as two pastes. When mixed together a slow setting reaction is initiated. These resins are used for cementing crowns or bulk restorations where there is limited or no light penetration. After the initial light cure, the remainder of the resin cures over a period of time by the chemical process.

CURING LAMPS A number of curing lights are manufactured. Most use visible light in the blue spectrum (between 400 and 500 nm). In some units the light source is remote and is transmitted to the site of restoration through a light guide which is a long, flexible fiber-optic cord. There are also hand held light curing devices which transmit the light through short light guides (Fig. 11.10).

Important features of curing lamps 1. Spectral range Light emitted by curing lamps should fall in the photoabsorbtion spectral range of camphorquinone. This is in the violet-blue region of the spectrum and is between 400-500 nm. Some curing lamps produce light within this spectrum whereas others produce more broad spectrum light which then needs to be filtered to emit only the light in the blue range. 2. Light intensity Light intensity should be high for a shorter curing time and greater depth of cure. Light intensity of lamps vary from 300 to 1200 milliwatts/cm2. For maximum curing a radiant energy of 16 J/cm2 is needed for a 2 mm thick section of composite.

Types of lamps Currently many forms of curing lights are available. These include QTH (Quartz-tungsten-halogen) These were the earliest visible light lamps. The light source is a tungsten-halogen-quartz bulb (Fig. 11.11). The white light generated passes through a filter that removes all wavelengths except those in the blue range. Heat is also generated

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FIGURE 11.10 A dental curing light in the visible blue range (QTH).

FIGURE 11.11 A wired curing light device (QTH).

179

FIGURE 11.12 A wireless LED lamp.

thus requiring a cooling fan. The intensity of light gradually reduces with time (aging) and so calibration is required at intervals. LED (Light emitting diodes) LED (Fig. 11.12) is increasingly popular as a light source in all spheres including dentistry ever since the discovery of the blue diode in the 1990s. It is similar in power to QTH lamps (700 mW/cm2). Research has shown that the curing depth and degree of conversion is significantly better with LEDs than with QTH. It emits light only in the blue part of the spectrum. Thus, it does not require filters. Its advantages also include low power consumption (can even be operated by batteries), no heat generation (eliminating cooling fan), and low noise (due to the absence of cooling fan). PAC (Plasma arc curing) These lamps generate an intense white light by ionizing xenon gas to produce a plasma. Filters are required to remove heat and the unwanted wavelengths. Their high power allows faster cures as well as greater depth of cures. Argon laser These produce light of the greatest intensity. They emit light of a single wavelength around 490 nm and therefore do not require filters. They do not produce little heat because of the limited infrared. However, these lamps are more expensive. They have a narrow light tip (spot size) requiring clinicians to increase number of overlaps in case of a larger restoration. The high intensity lights like the PAC and the laser provide a faster cure (as short as 5 seconds for a 2 mm section). Besides being expensive the accelerated curing can introduce substantial stresses. Further studies are needed.

EYE PROTECTION Staring into the curing lights for prolonged periods can cause retinal damage. It is best to look away while the curing is in progress. Various kinds of eye protection are available and should be used when working with composites.

DEGREE OF POLYMERIZATION AND DEPTH OF CURE The total amount of resin polymerized depends on several factors.

Transmission of light through the material This is controlled by absorption and scattering of light by the filler particles, as well as any tooth structure interposed between the light source and the resin. For this reason, microfilled composites with smaller and more numerous particles will not cure to as great a depth as conventional composites. Shade of resin Darker shades require longer exposure time.

180 PART 2 Direct Restorative Materials

Amount of photoinitiator and inhibitor present For polymerization to take place at any depth, a particular amount of photons must be available. This is directly related to the intensity of light and time of exposure. Curing time Manufacturers recommend curing times for each material and shade. This depends on the output of the particular curing device. Thus, 80 to 240 seconds is required with a low intensity light whereas to achieve the same result, a high intensity light requires only a 20 to 60 second exposure. Intensity of light Light intensity is measured in milliwatts/cm2. The time required for curing a 2 mm depth of resin by a QTH lamp is 40 seconds. The same thickness can be cured in 20 seconds if the light intensity is increased to 800 mW/cm2. Type of light High intensity lights like PAC and LASER cure faster and to a greater depth than the QTH and LED generated lights. Thickness of resin Thickness greater than 2–3 mm are difficult to cure because of the lack of light penetration. Distance from light Optimum distance is 1 mm with the light positioned 90 degrees from the surface of the resin.

BOX 11.1 Traditional composite Traditional composites are also referred to as ‘conventional’ or ‘macrofilled composite’ (because of the large size of the filler particles). Traditional composites are rarely used currently and have been largely replaced by other hybrids. However, their discussion will continue largely for comparison to the newer composites. Composition Ground quartz was most commonly used as filler. There is a wide distribution of particle sizes. Although average size is 8 to 12 µm, particles as large as 50 µm may also be present. Filler loading: 70-80 wt.% or 60-70 vol.%. Composition Ground quartz was most commonly used as filler. There is a wide distribution of particle sizes. Although average size is 8 to 12 µm, particles as large as 50 µm may also be present. Filler loading: 70-80 wt.% or 60-70 vol.%. Properties The conventional composites have significantly improved properties when compared to the unfilled restorative resins which preceded them. The improvement is the result of the improved resin, the filler loading and the strong bond between the filler and the resin matrix. Compressive Strength It is four to five times greater than that of unfilled resins (250 to 300 MPa). Tensile Strength It is double that of unfilled acrylic resins (50 to 65 MPa). Elastic Modulus It is four to six times greater than the unfilled resins (8 to 15 GPa). Hardness It is considerably greater (55 KHN) than that of unfilled resins. Water Sorption It is less than that of unfilled resins (0.5 to 0.7 mg/cm2). Coefficient of Thermal Expansion The high filler-to resin ratio reduced the CTE (25 to 35 × 10–6/°C) significantly. Esthetics Polishing of the conventional composite results in a rough surface. This is due to the selective wear of the softer resin matrix leaving the hard filler particles elevated. This resulted in a tendency to stain over a period of time. Radiopacity Radiopacity is measured by a photo densitometer. Radiopacity allows proper assessment of the restoration as well as future diagnosis of caries. The heavy metal fillers contribute to the radiopacity of composites. Aluminum is used as a standard reference for radiopacity. A 2 mm thickness of dentin and enamel is equivalent to 2.5 and 4 mm of aluminum respectively. Traditional composites have a radiopacity of 2-3 mm of aluminum equivalent. Adhesion Composites do not adhere to tooth structure and require special bonding techniques to provide adhesion to the tooth structure.

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Disadvantages Although the conventional composites were superior to unfilled resins, they had certain disadvantages • High surface roughness • Polishing was difficult • Poor resistance to occlusal wear • Tendency to discolor—the rough surface tends to stain. Because of these disadvantages as well as the introduction of improved composites this type was gradually phased out. It is probably no longer marketed.

MICROFILLED COMPOSITE The microfilled composites (Fig. 11.3) were introduced soon after the traditional composites. They were developed to overcome the problems of surface roughness of traditional composites. The resin achieved the smoothness of unfilled acrylic direct filling resins and yet had the advantage of having fillers. Unfortunately, they could not achieve high levels of filler loading and therefore had somewhat inferior mechanical properties when compared to the traditional composites. For this reason, these composites are primarily used for esthetic restorations in stress free areas and in areas close to the gingiva where a smooth finish is required for reduced plaque accumulation. Synonyms Also referred to as microfine composites.

COMPOSITION The smoother surface is due to the incorporation of microfillers. Colloidal silica is used as the microfiller and is the only type of filler present in this type. The problem with colloidal silica was that it had a large surface area that could not be adequately wetted by the matrix resin. Thus, addition of even small amounts of microfillers resulted in thickening of the resin matrix. Thus, it was not possible to achieve the same filler loading as conventional composites. Manufacturers tried to overcome this problem by 1. Using prepolymerized or organic fillers (see section on fillers). These composites were referred to as ‘heterogenous’. 2. Using silica in cluster or agglomerate form. These were referred to as ‘hom*ogenous’ microfilled composites. Filler size The colloidal silica is 200-300 times smaller than the quartz fillers of conventional composite. Size ranges from 0.04 to 0.4 µm. Filler content With the inclusion of prepolymerized (organic) fillers, the filler content is 70 wt.% or 60 vol.%. However, the actual inorganic filler content is only 50 wt.%.

CLINICAL CONSIDERATIONS With the exception of compressive strength their mechanical properties are inferior to the other types of composites. This is because of their higher resin content (50 vol.%). Their biggest advantage is their esthetics. The microfilled composite is the resin of choice for esthetic resto ration of anterior teeth, especially in non-stress bearing situations. For most applications, the decreased physical properties do not create problems. However, in stress bearing situations like Class IV and Class II restorations they have a greater potential for fracture. Sometimes, chipping occurs at the margins.

182 PART 2 Direct Restorative Materials HYBRID COMPOSITE RESINS The hybrid type forms the majority of the composites used in dentistry currently (Fig. 11.2). These were developed to obtain better surface smoothness than that of the conventional large particle composites, yet maintain the properties of the latter. Hybrid composites have a surface smoothness and esthetics competitive with microfilled composites for anterior restorations. The hybrids are generally considered as multipurpose composites suitable for both anterior and posterior use.

Filler volume The total filler content is 75–80 wt.% or 60–65 vol.%. The overall filler loading is not as high as small particle composites because of the higher microfiller content.

Filler type Two kinds of filler particles are employed verage particle size is 0.4 to 1 µm. 75% of the ground particles A are smaller than 1.0 µm. 2. Colloidal silica Size—0.04 µm. It is present in higher concentrations (10 to 20 wt.%) and therefore, contributes significantly to its properties. The hybrids are generally considered as multipurpose composites suitable for both anterior and posterior use. They are widely used for anterior restorations, including class IV because of its smooth surface and good strength. 1. Heavy metal glasses

The hybrids are also widely employed for stress bearing restorations.

NANO AND NANOHYBRID COMPOSITE RESINS Continued interest in the reduction of the size of fillers has led to the adaptation of nanotechnology to the field of composite resins. A new type of composite resin based on nanosized filler particles has been recently introduced. Nanocomposites (Fig. 11.6) are similar to the microfilled, comprising of uniformly sized nanofillers. Nanohybrids like the conventional hybrids, come in a range of filler sizes including nanofillers. Unfortunately conflicting reports exist on the efficacy of these relatively new materials. Initial reports indicate that these materials have the mechanical properties of the hybrid composites with the esthetics and polishability of the microfilled composites. Thus they can be used for both anterior and posterior restorations. Nanohybrids are generally stronger than the nanocomposites. However, nanocomposites have improved polishability. Continued development along these lines might eventually lead to the phasing out of conventional hybrid and microfilled composites. However, further research is required to establish the efficacy of the nano and the nanohybrid composites.

Filler volume and type The predominant fillers are zirconium/silica or nanosilica particles measuring approximately 5 to 25 nm and nanoaggregates of approximately 75 nm. The aggregates are treated with silane so that they bind to the resin. The aggregates and nanoparticles filler distribution gives a high load, up to 79.5%.

PROPERTIES OF COMPOSITE RESINS Composite resins were developed after amalgam and therefore it is a useful material with which to compare restorative composite resins. The mechanical properties of these materials have steadily improved over the years. Hovever, when compared to amalgam these

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materials are highly technique sensitive and therefore optimal properties can be achieved only if proper techniques of manipulation and insertion are followed. The common factor affecting most physical and mechanical properties of the composite is the filler content.

FLEXURAL STRENGTH Hybrid Microfilled

80–160 60–120

MPa MPa

Nanohybrids Amalgam

180 MPa 90–130 MPa

240–290 240–300

MPa MPa

Nanohybrids Amalgam

460 MPa 510 MPa

30–55 25–40

MPa MPa

Nanohybrids Amalgam

81 64

GPa GPa GPa

Amalgam Enamel Dentin

62 GPa 83 GPa 19 GPa

COMPRESSIVE STRENGTH Hybrid Microfilled

TENSILE STRENGTH Hybrid Microfilled

MPa MPa

MODULUS OF ELASTICITY Hybrid Microfilled Nano

8.8–13 4–6.9 18

HARDNESS Hardness determines the degree of deformation of a material and it is generally accepted as an important property and a valuable parameter for comparison with tooth structure. To assure an optimized clinical performance of restorations, it is of paramount importance to employ materials with hardness at least similar to that of the dentinal substrate, not only superficially, but also in depth, since an accentuated decrease in hardness would adversely affect their mechanical properties and marginal integrity. Enamel 343 KHN Dentin 70 KHN Composites generally show lower hardness than enamel. The hardness varies between different products and depends on the amount and type of filler used.

Hybrid 60–117 KHN Microfilled 22–80 KHN Amalgam 110 KHN

Factors affecting hardness Time period Composites show an increase in surface hardness with time due to continued polymerization. The best results are seen 7 days after polymerization. For those unable to wait for a subsequent appointment a 15 minute delay is recommended before start of polishing procedures. Polishing Polishing has been shown to increase the surface hardness of composites. Polishing removes the surface organic layer and exposes the harder fillers below. However, polishing is best delayed at least 24 hours after polymerization.

POLYMERIZATION SHRINKAGE Polymerization in composite resins is accompanied by a shrinkage which varies between different composites depending on the resin to filler ratio. Thus, the polymerization shrinkage

184 PART 2 Direct Restorative Materials ranges from 0.6–1.4% (in composites with higher filler content) to 2–3% (in composites with lower filler content like microfilled composites). This creates tensile stresses as high as 130 kg/cm2 which severely strains the bond and can lead to marginal leakage. Sometimes, it may also cause the enamel at the restoration margin to crack or fracture. The total polymerization shrinkage between light activated and chemically activated resins do not differ. However, the pattern of shrinkage is different (see differences). The polymerization shrinkage is highest in case of the microfilled composites because of the higher resin content. Clinical techniques to reduce polymerization shrinkage include 1. Ramped curing Polymerizing the composite resin in layers or ramps (Fig. 11.13). 2. Soft start In this technique polymerization is initiated slowly. The device automatically begins with a low intensity light, gradually increasing and ending with high intensity light. This gives time for stress relaxation. 3. Delayed curing The restoration is partially cured with a low intensity light. The operator continues working on the restoration and then follows it with a final high intensity exposure. 4. Fabricating and curing the restoration extraorally on a cast (indirect technique) and then cementing on to the tooth, thereby completing the polymerization before cementing.

FIGURE 11.13 Ramped curing.

AIR OR OXYGEN INHIBITION Polymerization is inhibited by air or oxygen. To avoid this the surface of the restoration should be protected by a transparent matrix strip or celluloid crown former. If the composite is unprotected during polymerization the surface of the composite remains tacky. This is known as the air or oxygen inhibited layer (OIL).

THERMAL PROPERTIES Thermal expansion coefficient (TEC) Thermal expansion and contraction is cyclic in nature in the mouth and this can place additional strain on the tooth-resin bond. Over time this can lead to material fatigue, bond failure and percolation of fluids into the gap. Ideally the TEC of a restorative material should be close to that of tooth structure. Dentin 8.3 × 10–6/°C Enamel 11.4 × 10–6/°C The TEC of composite resins is again related to the proportion of resin. Thus composites with higher resin content like microfilled will show a greater TEC.

Hybrid Microfilled

25–38 × 10–6/°C 55–68 × 10–6/°C

Thermal conductivity The thermal conductivity influences the rate at which heat or cold is transmitted through the restoration. Ideally restorative materials should have low thermal conductivity to reduce transfer of excessive thermal stimuli to the pulp.

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Hybrid 25–30 × 10–4 cal/sec/cm2 (°C/cm) Microfilled 12–15 × 10–4 cal/sec/cm2 (°C/cm)

WATER SORPTION Water sorption is related to the resin content. The water sorption of hybrid composites are comparatively lower than that of the microfilled resins. ISO 4049 requirements limit the water sorption to a maximum of 40 µg/mm2.

Hybrid 5–17 µg/mm2 Microfilled 26–30 µg/mm2

DIMENSIONAL STABILITY A slow expansion (hygroscopic expansion) is associated with water sorption. The expansion which starts 15 minutes after polymerization reaches equilibrium in about 7 days. Microfilled resins show more expansion than hybrid varieties.

RETENTION Composite resins do not adhere chemically to tooth structure. Micromechanical retention together with bonding agents have to be used to enhance adhesion to tooth structure.

ESTHETICS Composites are highly esthetic direct restorative materials. Composites are supplied in a variety of shades. Special composite stains and other effects are also available to create lifelike restorations. Ideally, with wear the silica filler should be removed along with the resin in which it is embedded. This is possible by using smaller filler sizes. In nanofilled and microfilled composites, the higher resin content and presence of microfillers is responsible for the increased surface smoothness. The inorganic filler particles are smaller than the abrasive particles used for finishing the restoration. Composites with larger fillers have a reduced surface smoothness which results in staining over a period of time. Age related effects also include stress cracks, a partial debonding of the filler-resin bond. This results in a loss of opacity/and or loss of shade match over time.

BIOCOMPATIBILITY OF COMPOSITE RESINS The resin components are cytotoxic in vitro. Composites release some resin components for weeks after insertion. The level of release depends on the type of composite and efficiency of the cure. Thus composites resins have biocompatibility issues from three aspects.

Inherent chemical toxicity of the material on the pulp. Pulpal involvement due to microleakage. Allergic potential on contact with the oral mucosa. Allergic potential for personnel handling the material. Chronic inflammatory response in the periodontium when compared to amalgam in animal studies (monkeys). More cytotoxic than amalgam in vitro studies. Resin composite components have been shown to cause immunosuppression or immunostimulation and to inhibit DNA and RNA synthesis. Concerns over estrogenicity of Bisphenol A and its dimethacrylate.

186 PART 2 Direct Restorative Materials In spite of the controversies properly polymerized composites appear to be relatively biocompatible as long as there is sufficient thickness of dentin. In cases where the pulp is exposed some form of pulp capping overlayed with a glass ionomer liner is recommended. When used in proximity to gingival tissues, proper care should be taken to ensure correct technique of placement to prevent inflammatory responses associated with overhangs and microleakage. Issues concerning estrogenicity have not been proven to be of sufficient concern under intraoral conditions.

Pulp protection Glass ionomer liners are applied as pulp protection in deep cavities. Zinc oxide-eugenol is contraindicated as it interferes with polymerization. Bacterial contamination should be avoided by using rubber dam isolation.

WEAR RATES AND LIFE EXPECTANCY OF COMPOSITES Composites are ideal as an anterior restorative material where wear rates are low. For posterior teeth amalgam has long been the standard direct filling material. Due to the increasing demand for esthetics, concern about mercury toxicity and aggressive marketing, there is an increasing interest in the use of composites for class I and II restorations. The older generation composites showed high attrition rates. Newer formulations have shown improvements in wear resistance. All types of composites have been used for posterior restorations. Current guidelines require posterior composites to show less than 50 µm wear over 18 months. For posterior use, the cavity preparation should be conservative, and the manipulation technique meticulous.

PROBLEMS IN THE USE OF COMPOSITES FOR POSTERIOR RESTORATIONS 1. In Class V restorations, when the gingival margin is located in cementum or dentin, the material shrinks away from the margin leading to a gap. 2. The placement technique is more time consuming and demanding. 3. Composites wear faster than amalgam. However, the newer materials like hybrids and nanocomposites have less wear (20 µm per year), which approaches that of amalgam (10 µm). In terms of years the average life expectancy of the composite resin is around 8 years, which is near to that of amalgam (10 years). The major indications of composites for posterior use are 1. When esthetics is the prime consideration. 2. When a patient is allergic to mercury.

ADHESION Composites do not adhere to tooth structure or any dental related surface. Acid etch technique and bonding agents have to be used to ensure adhesion. Adhesion to instruments Composite adhere to the sculpting and packing instruments which interfere with adaptation to the cavity walls, increase porosity and reduce operator comfort. Some clinicians use alcohol or the bonding agent as a release agent. However, both these techniques should be avoided as these materials are solvents that can weaken the resin.

RADIOPACITY Radiopacity is a useful feature for any restorative material. Posterior restorations must demonstrate adequate radiopacity to permit detection of secondary caries, excess or

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inadequate quantities of material, air bubbles and other imperfections. ISO norm states that the minimum radiopacity of a restorative material should be equal or greater than that of the 2 mm-thick aluminum step wedge. Composites containing heavy metal glass fillers are radiopaque (2–3 mm/Al). Enamel Dentin Amalgam

4 mm/Al 2.5 mm/Al 10 mm/Al

Hybrid Microfilled

2–5 mm/Al 2–3 mm/Al

MANIPULATION AND PLACEMENT OF COMPOSITE RESINS Steps in the manipulation of composite resin are illustrated in Fig. 11.14 Placement of rubber dam Composite resins are highly technique sensitive and contamination from saliva, gingival fluid or blood is best avoided with rubber dam isolation (Figs. 11.15A to C). Cavity preparation The cavity is prepared and margins bevelled. Cleaning The tooth is cleaned with a mild abrasive.

FIGURE 11.14 Technique for placing a light-cured composite restoration.

188 PART 2 Direct Restorative Materials

FIGURES 11.15A TO C (A) Rubber dam isolation is critical. (B) Preformed celluloid crown formers or matrix strips are used to shape and protect the restoration from air inhibition. (C) The completed restoration.

Etching The enamel at the cavity margins is acid etched. The acid is rinsed off and the area is dried thoroughly. Bonding agent An enamel or dentin bond agent is applied and polymerized. (Discussed in detail subsequently). The cavity is now ready for the composite.

TECHNIQUES OF INSERTION Resins are manipulated with plastic or plastic coated instruments. Metal instruments should be avoided as it may abrade and discolor the composite. Composites are tacky and stick to metal instruments. Some operators use alcohol or bonding liquid as a release agent to reduce tackiness. However, this should be avoided as it can interfere with the properties of the resin. This is especially true of the bonding agent as it can dissolve the resin matrix and cause dilution. It is inserted into the cavity using a plastic instrument or a special syringe. Some manufacturers supply it in the form of a capsule which can be injected directly into the cavity with a special extruding gun.

CHEMICALLY ACTIVATED COMPOSITES The correct proportions of base and catalyst pastes are dispensed onto a mixing pad and combined by rapid spatulation for 30 seconds. It is inserted while still plastic for better adaptation to cavity walls. Air inclusions can be avoided by swiping the material into one side of the cavity and filling the cavity from bottom outward. The cavity is slightly overfilled. A matrix strip is used to apply pressure and to avoid inhibition by air.

LIGHT ACTIVATED COMPOSITES The light activated composites are single component pastes and require no mixing. The working time is under the control of the operator. Effect of ambient light Light cured composites are vulnerable to prolonged exposure to ambient room light or the operatory light if they are left exposed and unprotected on the mixing pad. The composite begins slow polymerization as soon as it is exposed to ambient light and within 60 to 90 seconds it may lose its ability to flow. Therefore, some precautions to be observed when using light activated materials. The paste is dispensed just before use Avoid dispensing excessive quantities The depth of cure is limited, so in deep cavities the restorations must be built up in increments, each increment being cured before inserting the next Between cures any excess material is protected by covering with a light proof dark or orange tinted cover

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The material hardens rapidly, on exposure to the curing light. To ensure maximal polymerization a high intensity light unit should be used. The light tip should be held as close as possible to the restoration. The exposure time should be no less than 40 to 60 seconds. The resin should be no greater than 2.0 to 2.5 mm thick. Darker shades require longer exposure times, as do resins that are cured through enamel. Microfilled resins also require a longer exposure. Retinal damage The high intensity light can cause retinal damage if one looks at it directly. Avoiding looking at the light directly and use of protective eye glasses is recommended. Even greater care should be exercised when using laser as even a short exposure can cause damage. Control of polymerization shrinkage As mentioned earlier composites exhibit polymerization shrinkage and build-up of stresses. This can be controlled by 1. Incremental curing The restoration is built-up in increments each increment being cured before inserting the next. 2. Soft-start technique The curing is started with low intensity and finished with high intensity. This extends the time for stress relaxation. Some commercially available lamps have this feature built in. Ramped curing is a variation of this technique. 3. Delayed curing In delayed curing the restoration is partially cured at low intensity. The operator then completes the shaping and contouring and follows it with a second exposure for the final cure.

FINISHING AND POLISHING Finishing is best done after 24 hours during which time the polymerization is complete. However, if a subsequent appointment is not desired finishing procedures can be started 15 minutes after curing. The initial contouring can be done with a knife or diamond stone. The final finishing is done with rubber impregnated abrasives or rubber cup with polishing pastes or aluminium oxide disks. The best finish is obtained when the composite is allowed to set against a matrix or mylar strip. Special glazes and coatings are available. These are basically lightly filled resins. They are applied with a brush on the surface of the restoration and cured.

BONDING One of the initial problems when resin restoratives were introduced was microleakage which resulted from the shrinkage of the resin while curing. The problem was overcome to a great extent by the introduction of the ‘acid etch technique’ by Buonocore in 1955. The acid etch technique used a combination of acid to etch the tooth and a bonding agent to improve the retention of the composite resin to the tooth (Box 11.2). BOX 11.2 Essentials of current bonding systems Etchant The etchant is an acid which selectively dissolves the tooth structure to provide retention for the restoration. They are also known as conditioners. The most popular etchant is 37% phosphoric acid. Primer Primers are hydrophilic monomers usually carried in a solvent. Because of their hydrophilic nature they are able to penetrate the moist tooth structure especially the dentin and its collagen mesh thus improving the bond. Thus they serve as a bridge connecting the tooth structure to the adhesive. The solvent used are acetone, ethanol or water. Some are used without solvents. Adhesive Adhesives are generally hydrophobic monomers. Being hydrophobic they do not wet the tooth leading to air entrapment, air inhibition and thereby poor bonding. Thus they have to be used in combination with primers to form an effective bond to tooth structure. The adhesive bonds the resin to the primer which in turn penetrates and binds to the tooth structure thus completing the bonding sequence.

190 PART 2 Direct Restorative Materials ACID ETCH TECHNIQUE The acid etch technique was initially developed to improve retention to enamel. Initial bond agents did not appear to bond to the dentin. At the time it was widely believed that Dentin could not be etched as well as enamel Acid etching of dentin would cause injury to the pulp One reason for the low bond strength to dentin was because of the hydrophobic nature of the early adhesive resins. In 1979 Fusyama demonstrated that dentin could be etched without causing any significant harm to the pulp. This together with the development of hydrophilic bonding agents significantly improved the bond strength to dentin.

The acid etch technique together with the application of current bonding agents is one of the most effective ways of improving the bond and marginal seal between resin and tooth structure.

ETCHANT/CONDITIONER The etchants are acidic in nature. They may be grouped as Mineral (e.g. phosphoric, nitric acid, etc Organic (e.g. maleic, citric, ethylenediamine-tetracetic (EDTA), etc.) Polymeric (e.g. polyacrylic acid). The most frequently used etchant FIGURE 11.16 37% phosphoric acid in a syringe. is 37% phosphoric acid. The acid in concentrations greater than 50% results in the formation of monocalcium phosphate monohydrate that reduces further dissolution. It may be supplied as clear or colored gel or liquid. Brushes are used to apply or the acid is supplied in a syringe for direct application on to the enamel (Fig. 11.16).

Another acid used is 10% maleic acid.

MODE OF ACTION ON ENAMEL 1. It creates microporosities by discrete etching of the enamel, i.e., by selective dissolution of enamel rod centers (Fig. 11.17), or peripheries, or both. 2. Etching increases the surface area. 3. Etched enamel has a high surface energy, allowing the resin to wet the tooth surface better and penetrate into the microporosities. When polymerized, it forms resin ‘tags’ which forms a mechanical bond to the enamel (Fig. 11.18).

FIGURE 11.17 SEM of etched enamel showing dissolution of rod centers (Courtesy: Mario Fernando).

FIGURE 11.18 Diagrammatic representation showing mechanism of composite adhesion to etched enamel.

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MODE OF ACTION ON DENTIN 1. Removes smear layer and partially opens the dentinal tubules (Fig. 11.19). 2. Provides modest etching of the intertubular dentin.

PROCEDURE The tooth is cleaned and polished with pumice before etching. The phosphoric acid is then applied onto the enamel and then on to the dentin (also known as totaletch technique). Originally the length of application was set at 60 seconds but now it has been shown that 15 seconds is sufficient. The etching time also depends on the history of the tooth, e.g. a tooth with high fluoride content and primary teeth requires longer etching time (to produce a similar etch pattern and bond strength 10% maleic acid needed at least 60 seconds of etching time). The acid along with dissolved minerals should be rinsed off with a stream of water for 15 seconds and the enamel dried using compressed air. After drying the enamel should have a white, frosted appearance (Fig. 11.20).

FIGURE 11.19 SEM of etched dentin showing the open dentinal tubules (Courtesy: Mario Fernando).

This surface must be kept clean and dry until the resin is placed. Even momentary contact of saliva, or blood can prevent effective resin tag formation and severely reduce the bond strength.

Avoiding desiccation of dentin Desiccation (excessive drying) of the dentin should be avoided. Desiccation can result in the collapse of the collagen mesh (Fig. 11.21A) or network which forms a dense film that is difficult to penetrate by the bond agent. The collagen mesh is crucial in the formation of the hybrid layer (Fig. 11.21B). The monomers impregnate and became entangled with the collagen fibrils of surface demineralized dentin, creating a hybrid layer after their polymerization. The hybrid layer together with the resin tags forms the prime mechanism for the adhesion of the composite restoration in dentin. After drying the tooth the dentin may be lightly remoistened with cotton and then blotted dry.

FIGURE 11.20 Frosted appearance after a 15 second etch with 37% phosphoric acid.

FIGURES 11.21A AND B (A) SEM of etched non-desiccated dentin showing collagen mesh (x 5000). (B) SEM of resin-dentin interphase. RC - Resin composite, BA - Bond agent, HL - Hybrid layer, RT - Resin tags. (x 2000).

192 PART 2 Direct Restorative Materials ENAMEL BOND AGENTS These were the earliest bond agents. The more viscous composite did not bond well to the etched enamel. The enamel bond agent helped improve the bond by flowing into all the microporosities of the etched enamel and creating a mechanical retention.

COMPOSITION They are unfilled resins similar to that of the resin matrix of composite resin, diluted by other monomers to lower the viscosity. These materials have been replaced by agents that bond to both enamel and dentin.

BOND STRENGTH Bond strengths to etched enamel range from 16 MPa (230 Psi) to 22 MPa (3200 Psi). Drying the enamel with warm air or using an ethanol rinse can increase the bond strength.

ENAMEL/DENTIN BOND SYSTEMS The term dentin bond agent is no longer relevant as current bond agents bond to both enamel and dentin. The usage of the term is relevant only to discuss their evolution. Due to acid etching, microleakage or loss of retention is no longer a hazard at the resin-enamel interface. The problem lies at the resin-dentin/cementum interface. Thus agents that could bond to dentin were needed. Developing agents that will adhere to dentin was more difficult because It is heterogenous. The high water content interferes with bonding. Its tubular nature provides a variable area. Presence of a smear layer on the cut dentin surface (The smear layer is the layer of debris which adheres tightly to the dentin and fills the tubules after cavity cutting). Ideally, the bond agent should be hydrophilic to displace the water and thereby wet the surface, permitting it to penetrate the porosities in dentin as well as react with the organic/ inorganic components.

Restorative resins are hydrophobic, therefore, bonding agents should contain both hydrophilic and hydrophobic parts. The hydrophilic part bonds with either calcium in the hydroxyapatite crystals or with collagen. The hydrophobic part bonds with the restorative resin.

SUPPLIED AS Dentin bond systems are supplied in one or more bottles containing conditioners (etchant)/ primers/ and adhesive depending on the generation (see box below and also evolution of dentin bond agents - the various generations).

EVOLUTION OF DENTIN BOND AGENTS—THE VARIOUS GENERATIONS For ease of description the evolution of bonding agents for composite resins are described under various generations (see also Table 11.3). First generation (1950 to 1970) Mineral acids were used to etch enamel. Dentin etching was not recommended as it was believed it would harm the pulp. They used glycerophosphoric acid dimethacrylate to provide a bifunctional molecule. The hydrophilic phosphate part reacted with calcium ions of the hydroxyapatite. The hydrophobic methacrylate groups bonded to the acrylic restorative resin. These were generally self cured. The main disadvantage was their

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TABLE 11.3 Various generations of bond agents Generation

Enamel etchant

Dentin conditioner/primer

Adhesive

1st generation

37% phosphoric acid

(not recommended)

GPDM

2nd generation

37% phosphoric acid

(not recommended)

Phenyl-P BisGMA/TEGDMA MPPA

3rd generation

37% phosphoric acid

Citric acid (10%)/CaCl (20%) Oxalic acid/aluminium nitrate EDTA

NPG-GMA/BPDM BisGMA/TEGDMA HEMA/BPDM 4 META/MMA HEMA/GPDM

4th generation

37% phosphoric HEMA/ (total etch technique) BPDM

NPG-GMA/BPDM BisGMA/TEGDMA HEMA/BPDM 4 META/MMA HEMA/GPDM

5th generation

37% phosphoric

PENTA, Methacrylated phosphonates

(total etch technique)

6th generation

Methacrylated phosphates in water (acidic primer-adhesive)

7th generation

Methacrylated phosphates in water (acidic primer-adhesive)

Abbreviations BisGMA – BPDM – EDTA – GPDM – GA – HEMA – 4-META –

Bisphenol-A-glycidyl methacrylate Biphenyl dimethacrylate Ethylenediaminetetraacetic acid Gylcerophosphoric acid dimethacrylate Glutaraldehyde 2-Hydroxyethyl methacrylate 4-Methyloxyethyl trimellitic acid

MMA MPPA TEGDMA PENTA ester NPG-GMA NTG-GMA

– – – –

Methyl methacrylate 2-methacryloxyphenyl phosphoric acid Triethylene glycol dimethacrylate Dipentaerythritol pentacrylate phosphoric acid

– N-Phenyl glycine glycidylmethacrylate – N-Tolyl glycine glycidylmethacrylate

low bond strength (2 to 6 MPa) because of their high polymerization shrinkage and the high CTE. Leakage was a concern at the dentin-resin interphase. Second generation (1970s) Developed as adhesive agents for composite resins which had by then replaced acrylic restorations. One system used NPG-GMA. It was proposed that the NPG portion bonded to the calcium of the tooth by chelation. Other products included phenyl-P, 2-methacryloxy phenyl phosphoric acid. Bond strengths achieved were three times more than the earlier generations. Disadvantage Bond strengths were still low. The adhesion was short term and the bond eventually hydrolysed, e.g. Prisma, Universal Bond, Clearfil, Scotch Bond. Third generation (1980s) The third generation bond agents made a serious attempt to deal with the smear layer which is formed when dentin is cut. It was believed that the smear layer prevented proper bonding to the underlying dentin. Yet its complete removal by aggressive etching was contraindicated because it was believed that it protected the pulp by preventing direct contact with the monomer. The third generation bond agents had bond strengths comparable to that of resin to etched enamel. Thus bond strengths improved to 12 to 15 MPa. However, their use is more complex and requires two to three application steps.

Etching of enamel using 37% phosphoric acid Conditioning of dentin using mild acids Application of separate primer Application of polymerizable monomer Placement of the resin.

194 PART 2 Direct Restorative Materials Examples are Tenure, Scotch bond 2, Prisma, Universal bond, Mirage bond, etc. Fourth generation (early 1990s) The fourth generation systems were possible because of some important ideological breakthroughs - like the total etch technique and the development of the hybrid zone. Research showed that acid etching of dentin did not significantly harm the pulp as long as bacterial contamination and microleakage was avoided. Thus, the total-etch technique was introduced. The hybrid layer (Fig. 11.21 B) In 1982, Nakabayashi and Fusayama reported the formation of a hybrid layer. The hybrid layer is defined as “the structure formed in dental hard tissues (enamel, dentin, cementum) by demineralization of the surface and subsurface, followed by infiltration of monomers into the collagen mesh (Fig. 11.21) and subsequent polymerization. However, dealing with the collagen mesh was not easy. It is delicate and can be destroyed by desiccation. Kanca (1991) introduced the idea of wet bonding again breaking with the traditional belief that thorough drying was necessary to improve bonding. Examples are All Bond 2, Scotch bond multipurpose (Fig. 11.22), Optibond, etc. The All Bond consists of 2 primers (NPG-GMA and Biphenyl dimethacrylate (BPDM) and an unfilled resin adhesive (40% BIS-GMA, 30% UDMA, 30% HEMA). This system bonds composite not only to dentin but to most dental related surfaces like enamel, casting alloys, amalgam, porcelain and composite. Bond strengths were high but as with the earlier system, multiple application steps were required. Fifth generation (mid 1990s) Because of the clinical complexity and multiple steps of the fourth generation dentists began asking for more simple adhesives. The fifth generation combined the primer and adhesive in to one bottle (self priming adhesive). Examples of the fifth generation self-priming adhesives are Single Bond (3M) (Fig. 11.23), One Step (BISCO), Prime and Bond (Dentsply). The advantages claimed are 1. Reduced application steps. 2. Less technique sensitive as it can bond to moist dentin. 3. Less volatile liquid. 4. Pleasant odor. 5. Higher bond strength. Sixth generation (mid to late 1990s) A separate etchant is not required. These are 2 bottle systems. Two varieties are seen—Type I and Type 2.

FIGURE 11.22 4th generation bonding system consisting of the conditioner (etchant), primer and the adhesive.

FIGURE 11.23 A 5th generation self priming adhesive (3M single Bond 2).

FIGURE 11.24 A 6th generation. Type I - self etching primer (Adhese - Ivoclar).

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Type I

bottle 2 step system. Etchant and primer are combined in one bottle (called self 2 etching primer). Other bottle contains adhesive. Examples are Clearfil SE bond (Curare), Adhese (Ivoclar -Fig. 11.24), Optibond solo plus(Kerr), Nano bond (Pentron) etc.

Type II

bottle 1 step system. Liquid A contains the primer. Liquid B contains a phosphoric 2 acid modified resin (self etching adhesive). Both liquids are mixed just before application. For example, Xeno III (Dentsply - Fig. 11.25), Adper prompt L-pop (3 M), Tenure unibond (Dent Mat) etc.

Seventh generation (early 2000) Attempts to combine all three (etchant, primer and adhesive) into a single product. Thus, seventh generation adhesives may be characterized as - ‘no mix self etching adhesives’. Examples include iBond (Heraeus Kulzer - Fig. 11.26), G bond (GC), Xeno IV (Dentsply) (glass ionomer based), Clearfil S3 (Curare). Unfortunately, insufficient research exists of the efficacy of the newer systems. Composition (Table 11.4) and procedure (Box 11.3) for one such product is presented.

INDICATIONS FOR USE OF BOND AGENTS 1. For bonding composite to tooth structure. 2. Bonding composite to porcelain and various metals like amalgam, base metal and noble metal alloys. 3. Desensitization of exposed dentin or root surfaces. 4. Bonding of porcelain veneers. Contraindication Bonding should not be done immediately after bleaching a tooth. It is advisable to wait at least a week following the procedure.

FIGURE 11.25 6th generation Type II (Xeno-Dentsply).

FIGURE 11.26 7th generation iBond (Heraeus kulzer).

BOX 11.3 Procedure for iBond 1. 2. 3. 4. 5. 6. 7.

Isolate the tooth from saliva contamination during the adhesive procedure. Clean the preparation, removing all debris with water. Remove excess water. Saturate the microbrush with iBondTM liquid from either the bottle or single dose vial. Apply 3 consecutive coats of iBondTM to both the enamel and dentin followed by gentle rubbing for 30 seconds. Use gentle air pressure or vacuum to remove the acetone and water solvent. Cure for 20 seconds with a dental curing light of at least 500 mW/C2. Place composite.

196 PART 2 Direct Restorative Materials TABLE 11.4 Composition of a 7th generation bonding agent (iBond) Component

Function

UDMA

Matrix component Wetting of the surface Promotion of infiltration Bonding to collagen via hydrogen bonding Bonding to Ca2+ ions of the apatite via chelation complexes

4-Meta (pH = 2.2)

Matrix component Film-forming properties Cross-linking

Acetone

Solvent for monomers Facilitates solvent evaporation

Water

Solvent for monomers Hydrolysis of 4-Meta to 4-Meta (= acid) Provides water for etching process

Camphorquinone

Photoinitiator

Glutaraldehyde

Disinfectant/Desensitizer agent Cross-linking of collagen fibrils

Stabilizers

BONDING MECHANISMS Though chemical bonding schemes have been proposed, there is little evidence supporting it. The bonding is more probably micromechanical, due to the penetration of the polymerizable monomer into the finely textured primed dentin. A fine collagen mesh exists on the surface of the dentin which current bond agents are able to infiltrate because of their hydrophilic components. One more precaution is that the dentin should not be dried excessively as desiccation can cause the collapse of the fine collagen meshwork (See Fig. 11.19) thereby reducing the bond strength.

BOND STRENGTH OF DENTIN BOND AGENTS Current dentin bond agents generate bond strengths comparable to that of resin to etched enamel. Bond strength is difficult to measure because of the wide variations in the dentin itself, test methods, and other factors. Bond strength reduces with increased depth of dentin. Various studies have shown values ranging from 15 to 35 MPa. An increasing number of studies are using the microtensile test methodology. The size of the specimen is smaller (1 mm2 in cross section). Thus, a number of specimens can be prepared from the same tooth thereby increasing the uniformity of the study.

DYE PENETRATION TESTS These are important tests related to the performance of the bonding system. A test for microleakage is indicative of the success or failure of a bond. These tests are done using tracers and staining to determine the depth of penetration.

REPAIR OF COMPOSITES Composite resins may be repaired by adding new material over the old. This is useful in correcting defects or altering contours of an existing restoration. The procedure differs depending on whether the restoration is fresh or old.

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Freshly polymerized restoration still has an inhibited layer of resin on the surface. More than 50% of unreacted methacrylate groups are available to copolymerize with the newly added material. In older composites, the presence of fewer methacrylate groups and the greater crosslinking reduces the ability of fresh monomer to penetrate into the matrix.

Method Remove contaminated material from the surface and roughen it. Place fresh composite after applying bonding agent.

SANDWICH TECHNIQUE Composite does not bond adequately to dentin, therefore during polymerization, a gap may result if the cavity margin is situated in dentin. The bond to dentin can be improved by placing a glass ionomer liner between the composite restoration and dentin. The glass ionomer bonds to the dentin through chemical adhesion whereas the resin bonds mechanically to porosities and crazing present on the surface of the glass ionomer liner. The glass ionomer can also be etched with the help of phosphoric acid to improve retention. In addition it also provides an anticariogenic effect due to its fluoride release. When used in this context it is often referred to as ‘sandwich technique’.

Indications 1. Lesions where one or more margins are in dentin, e.g. cervical lesions. 2. Class II composite restorations.

Procedure Dentin is conditioned and a thin layer of GIC cement is placed. The cement must be exposed at the margins in order to achieve fluoride release. Phosphoric acid is used to etch the enamel portion. Some operators also etch the GIC surface with the same phosphoric acid for 15 to 20 seconds to increase surface roughness (light cured GIC is not etched). The surface is then washed for 25 to 30 seconds. After adequate drying, a bond agent is applied to the cement surface and to the etched enamel. The composite resin is then inserted in the usual manner.

SPECIALIZED COMPOSITE RESINS FLOWABLE COMPOSITES As suggested by the name these are hybrid composites modified to have an increased flow (Fig. 11.27). The increased flow is achieved by a reduction in the filler content (30–55 vol % or 40–60 wt.%). When placed in the cavity the material flows readily and intimately adapts to the cavity wall. The reduced filler content affects some of the properties. Thus these materials

Are more prone to wear Have half the stiffness (more flexible) of regular hybrids (4–8 GPa) Greater polymerization shrinkage (3–5 vol%)

FIGURE 11.27 Flowable resin.

198 PART 2 Direct Restorative Materials These materials are intended for specialized usage 1. 2. 3. 4.

As a preventive material (fissure sealant and small class I cavity). Cervical lesions and Class V restorations. As a base or liner. Areas of reduced access.

PACKABLE COMPOSITES These are composites that have very high viscosity and low surface tackiness. They have a high filler loading (66–70% vol) with porous or irregularly shaped particles. They are not condensable like classic amalgam, rather they can be compressed and forced to flow using flat faced instruments, hence the term packable (Fig. 11.28). They are considered as posterior composites. The distinguishing characteristics of all packable compositions are less stickiness or stiffer viscosity than conventional composites, which allow them FIGURE 11.28 Example of packable composite— to be placed in a manner that somewhat Sure Fil by Dentsply. resembles amalgam placement. They have a higher wear resistance. Claims also include a greater depth of cure and low polymerization shrinkage. Iin general, mechanical properties of packable composites are not substantially better than those of most conventional universal composites. Commercial examples Solitaire (Heraeus), ALERT (Jeneric), and SureFil (Dentsply).

Indications They are indicated for use in Classes I and II cavities. In class II cavities where improved contacts with adjacent teeth are desired.

PROSTHODONTIC VENEER COMPOSITES (LABORATORY COMPOSITES) Resin may be used as a veneer (a tooth colored layer used to hide the underlying metal) for crowns and fixed partial dentures. They are also known as composites for dental indirect restorations. The earliest recorded use was in the Hollywood film industry where they were used to temporarily mask the teeth of actors. The early materials were heat cured poly (methyl methacrylate) improved by fillers and cross-linking agents. Current veneer materials are hybrid, micro or nanofiller reinforced resins such as bis-GMA, urethane dimethacrylate or 4,8-di(methacryloxy methylene) tricyclo-(5.2.1.02,6) -decane (Fig. 11.29A to C). Some are fiber reinforced. The newer resins have superior physical properties and are polymerized by light or heat and pressure. The resins are mechanically bonded to the metal using wire loops or retention beads (Fig. 11.29B). Recent improvements, include micromechanical retention created by acid etching the base alloy and the use of chemical bonding systems such as 4-META, phosphorylated methacrylate, epoxy resin, or silicon dioxide that is flame sprayed to the metal surface followed by the application of a silane coupling agent (silicoating). Commercial examples SR Adoro (Ivoclar), GC Gradia (GC), Targis Vectris, etc. Available as A kit containing various materials which include—incisal, dentin and other specialized resins, masking resins (Fig. 11.29B).

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FIGURES 11.29A TO C (A) Prosthodontic resin kit (SR Adoro (Ivoclar Vivadent). (B) Opaquer resin is applied to mask the metal. Multiple tiny nodules provide retention for the resin. (C) The completed restoration.

Indications 1. Inlays, onlays, veneers and anterior crowns (metal free). 2. As veneer over metal supported crowns and FDPs. 3. Long term temporaries (alone or in combination with Kavo C-temp blanks).

The advantages of resin when compared to porcelain 1. Ease of fabrication. 2. Easily repairable intraorally. 3. Less wear of opposing teeth or restorations.

Disadvantages 1. Microleakage of oral fluids and staining under the veneers due to thermal cycling and water sorption. 2. Surface staining and intrinsic discoloration. 3. Susceptibility to toothbrush wear. 4. Cannot be used in crowns serving as abutments for removable partial dentures. The clasp arm will abrade the resin. 5. Not as durable as other prosthodontic materials like ceramics and PFM.

RESIN INLAY SYSTEMS These were introduced in an attempt to overcome some of the limitations of traditional posterior composite resin restoration. The resin inlay is completely polymerized outside the mouth by light, heat, pressure or combination and then luted to the tooth using a resin cement. They may be fabricated using the direct method or indirect method.

Direct inlay system (fabricated in the mouth) Hybrid or microfilled resins are used. A separating medium (agar solution or glycerine) is applied to the prepared tooth. The restoration is then formed, light-cured, and removed from the tooth. The rough inlay is subjected to additional polymerization by light (6 minutes) or heat (100 °C for 7 minutes). After this the prepared tooth is etched and the inlay luted to place with a dual-cure resin cement and then polished.

Indirect inlay system (fabricated on a die) The inlay is fabricated with prosthodontic resin (described earlier) in the laboratory on a die made from an impression of the prepared tooth. Conventional light and heat or heat and pressure may also be used for polymerization.

200 PART 2 Direct Restorative Materials Advantages of inlays 1. Improved physical properties and wear resistance due to the higher degree of polymerization attained. 2. Induced stresses and potential for microleakage is reduced as polymerization shrinkage occurs outside the mouth. 3. Being resins they do not abrade opposing teeth and are repairable in the mouth.

PREFORMED COMPOSITE RESIN LAMINATES Composite resins are used as preformed laminate veneers to mask tooth discoloration or malformation. These shells are adjusted by grinding and are bonded to teeth using acid-etch technique and resin cement.

CORE BUILD-UP COMPOSITE RESINS Modified highly filled resins are used as core materials in combination with prefabricated posts during the restorations of broken down teeth. They are highly colored opaque materials. They are usually chemically cured or dual cured with a longer working time and shorter setting time. Some concerns exist regarding the strength of composite cores when compared to cast post and cores. Failures are often seen when these materials form the bulk of the support FIGURE 11.30 Core composite resin. for crowns and FDPs. The lower stiffness (greater flexibility) can result in slightly more frequent debonding of crowns and other restoration. Therefore, when using composite cores shared support from remaining tooth structure is indicated. MultiCore (Ivoclar Vivadent) (Fig. 11.30) is an example of a core build-up composite. The product also contains fluoride.

RESIN CEMENTS Lower viscosity filled resins (e.g. Panavia Ex, Infinity) are used for the cementation of laminates, crowns and orthodontic brackets. Etching and bonding is done before cementing (described in detail in chapter on cements).

PROVISIONAL COMPOSITES A temporary restoration is necessary to protect the teeth after preparation and in the interim period while the definitive restoration is being constructed. Composite resins are available for making provisional inlays, onlays, crowns and FDPs. E.g. Protemp (ESPE) (Fig. 11.31), Structur (VOCO), Integrity (Dentsply), etc.

Advantages

FIGURE 11.31 Composite for provisional crowns and fixed partial dentures.

1. Can be made directly in the mouth because of its low exothermic heat 18 to 28 °C.

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2. It is easily ground and shaped using regular high speed diamond burs without melting and clogging the burs (unlike the conventional acrylic provisional resins). 3. Improved mechanical properties including low wear rates when compared to acrylic provisionals. 4. Good radiopacity.

Available as Syringe form It comes as base and catalyst. Currently, two shades are available. It is supplied in a syringe form and is dispensed by rotating the plunger until a clicking sound is heard, this represents one unit. The catalyst is a two component system and only a small amount is required. As the plunger is rotated the two components are dispensed simultaneously. The base and catalyst are mixed with a spatula and used. Setting occurs in approximately two to three minutes. Cartridge form The material is also available in the cartridge form for use with a static mixing device. It consists of a caulking gun which forces the materials through special static mixing tips. The material comes out mixed and ready to use when expressed through these tips. After curing in the mouth the hardening can be accelerated by placing in hot water.

Precautions 1. As with all materials expressed from cartridges, the material that comes initially should be discarded as it may not be in the correct proportions. 2. Oxygen inhibited layer This reaction is strongly inhibited by oxygen diffusing from the atmosphere into the curing resins is responsible for the formation of a soft, sticky, superficial layer on the surface of the prepared tooth. This layer can affect inhibit the setting of polyvinyl siloaxane impression materials In some techniques where a putty impression is used as a template, this layer can prevent the bonding

COMPOSITE RESIN BLANKS FOR CAD-CAM SYSTEMS Composite resin blanks are available for the fabrication of long term (up to 1 year) provisional restorations including crowns and fixed partial dentures. They are fabricated by a CAD/CAM process (see chapter on ceramics for further details of the process). One such product is the Everest C-Temp (Kavo) (Fig. 11.32). The whole restoration may be fabricated as one piece or it may be fabricated as a framework which is subsequently veneered with conventional prosthodontic resins. These resins are characterized by high flexural strength. FIGURE 11.32 Composite resin CAD/CAM blanks.

PORCELAIN REPAIR COMPOSITE RESINS Composite resins are occasionally used to repair fractured or chipped ceramic restorations. The kit consists of an silane based bonding agent, a metal masking agent (opaquer). Conventional composite is then used to carry out the final repair. The material and procedure is discussed in further detail (refer chapter on porcelain).

PIT AND FISSURE SEALANTS Deep pits and fissures on posterior teeth are susceptible to decay as they provide shelter for organisms. They are often too narrow making it difficult to clean. Various materials have

202 PART 2 Direct Restorative Materials been used to seal these areas, especially in the child patient. The objective is for the resin to penetrate into the pits and fissures, thereby, sealing these areas against oral flora and debris (Figs. 11.33A and B).

Indications Sealants are most effective in children with high risk of caries. Both deciduous molars and young permanent molars with deep pits and fissures are common candidates. Commonly surfaces that are free of caries should be selected. However, a recent study where the sealant was placed intentionally in pits and fissures having caries showed that the lesion did not progress.

Types 1. Based on filler content 2 types are available—Filled and Unfilled. 2. Based on curing mechanism—they may be light cured or chemical cured. 3. Color—The sealants are available as transparent, tooth colored, opaque, tinted or white materials. The color contrast helps to determine the efficacy of the application. Recent products include photosensitive color reversible sealants. These sealants are normally colorless but change to a pink or green when exposed to a curing light. The color change which lasts about 5–10 minutes is useful for diagnosis during periodic recalls.

Composition The most commonly and successfully used sealant is bis-GMA (Figs. 11.33B and C). It may be cured chemically (amine-peroxide system) or by light. The bis-GMA resin is mixed with a diluent to obtain a low viscosity sealant that flows readily. In filled sealants small amounts of filler (up to 40%) are added to improve its stiffness and wear resistance. Other resins systems include polyurethanes and cyanoacrylates.

Properties Important properties of sealants are flow, wear resistance, fluoride release and long term retention. Sealants must have low viscosity so that they will flow readily into the depths of the pits and fissures (Fig. 11.33A) and wet the tooth. Wettability is also important for proper adaptation and penetration. Acid etching improves the wettability. Proper retention of the sealant is important for caries prevention. Acid etching with bonding agents is necessary for the retention of the sealant. The sealant must have sufficient mechanical properties like strength, stiffness and wear resistance for effective function and durability. Sealants with fillers usually have better mechanical properties than unfilled resins.

FIGURES 11.33A TO C Pit and fissure sealants. (A) Diagrammatic representation of a sealant in a fissure. (B) A pit and fissure sealant on a deciduous molar. (C) A typical pit and fissure sealant kit.

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Current products have been formulated for fluoride release during service. Initial fluoride release is high especially during the first 24 hours but gradually tapers to low levels which may not be effective for long term protection. Air inhibition during polymerization is also a problem in sealants. The tacky air inhibited layer is removed using a pumice paste on a cotton pellet or rotary cup or brush. The operator must ensure that sufficient thickness of sealant is applied to compensate for this loss. The unpolymerized surface layer was once a health concern because of the presence of BPA (bisphenol A) which is chemical similar to estrogen. However, this has since been discredited and is no longer a major concern.

Efficacy of sealant therapy Early clinical studies showed a retention rate of 42% and a caries reduction rate of 35% over 5 years. Improvements in materials and technique over the years have dramatically improved the success rates. Current studies indicate a success rate of over 90% in caries reduction.

Periodic recall Evidence has shown that sealant therapy cannot be taken for granted. The sealant should be re-examined every six months. If the sealant is missing it should be reapplied. Improper case selection and application of sealant may actually enhance caries.

COMPOSITE RESIN DENTURE TEETH Over the years various types of materials have been used for denture teeth. Traditionally conventional acrylic (Biotone), cross-linked acrylics (SR-Postaris, Genios-P, etc.) and porcelain denture teeth are used. Since their introduction composite resins too have been tried as denture teeth. Some examples are nano-filled (Veracia) and micro-filled composites (SR-Orthosit, Endura, Duradent, Surpass). In one study, the composites and cross-linked acrylic resins did not show significant difference in wear resistance properties. However, both were significantly better than conventional resin acrylic teeth.

ADVANTAGES AND DISADVANTAGES OF RESTORATIVE COMPOSITE RESINS ADVANTAGES 1. 2. 3. 4. 5. 6.

Highly esthetic tooth colored restorations possible. Multiple curing systems allow choice of working time. Relatively ease of placement. Moderately strong and durable. Does not corrode when compared to amalgam. Easy to repair.

DISADVANTAGES 1. 2. 3. 4. 5.

Highly technique sensitive. High shrinkage. Does not bond to tooth structure; requires dentin bonding techniques. Sticks to instruments. Not condensable like amalgam.

204 PART 2 Direct Restorative Materials 6. 7. 8. 9.

Possibility of microleakage and recurrent caries if improperly placed. Some materials exhibit slumping. Not as wear resistant as other metallic restorative materials. Shorter life span when compared to other more durable restorative materials like ceramics, DFGs, amalgam and cast metal restorations. 10. Higher allergic, inflammatory response, cytotoxic and other biologically adverse effects when compared to amalgam. 11. Potential for color instability and staining over time.

Section-3

Endodontic Materials Chapter 12 Endodontic Medicaments and Irrigants, 207 Chapter 13 Endodontic Sealers and Obturating Materials, 216

12 CHAPTER

Endodontic Medicaments and Irrigants Jacob Kurien

Chapter Outline • Root Canal Irrigants –– Sodium Hypochlorite –– Chlorhexidine –– Hydrogen Peroxide –– MTAD –– EDTA –– RC-Prep • Intracanal Medicaments • Phenol and Related Compounds –– Phenolic Compounds –– Eugenol –– Parachlorophenol (PCP)

–– Formocresol –– Cresatin –– Camphorated

Monochlorophenol (CMCP)

–– Glutaraldehyde –– Formaldehyde based (N2)

• Antibiotics –– PBSC and PBSN

(Polyantibiotic Pastes)

–– Sulfonamides –– Grossman’s Paste –– Corticosteroids - Antibiotics

–– Combinations • Halogens • Aminoacridine • Quaternary Ammonium Compounds –– Chloramine –– Iodine –– Calcium Hydroxide –– Chlorhexidine (CHX) Gluconate

Endodontics is the specialty of dentistry that manages the prevention, diagnosis, and treatment of the dental pulp and the periradicular tissues that surround the root of the tooth. The main objectives of root canal therapy are 1. Removal of the pathological pulp. 2. Cleaning and shaping of the root canal system. 3. Three dimensional obturation to prevent reinfection (Fig. 12.1). Irrigation is an essential part of root canal debridement because it allows for cleaning beyond what might be achieved by root canal instrumentation alone. Disinfection of the root canal system is one of the primary aims of root canal treatment. This can be achieved through FIGURE 12.1 A root canal treated tooth. the use of various antimicrobial agents in the form of irrigants (only used for relatively short periods of time) and medicaments (days or several weeks). The main objective of root canal obturation is to achieve a three dimensional well filled root canal with fluid tight seal (Fig. 12.1). It serves to prevent percolation and microleakage of periapical exudate into the root canal space and create a favorable environment for the healing process.

208 PART 3 Endodontic Materials Materials used in endodontics The materials used in Endodontics may be grouped as follows. 1. 2. 3. 4.

Root canal irrigants Root canal medicaments Obturating materials Endodontic sealers

ROOT CANAL IRRIGANTS Several irrigants and irrigant delivery systems are available, all of which behave differently and have relative advantages and disadvantages. Root canal irrigants must not only be effective for dissolution of the organic portions of the dental pulp, but also effectively eliminate bacterial contamination and remove the smear layer (the organic and inorganic layer) that is created on the walls of the root canal during instrumentation.

Desirable properties 1. Irrigants should have a broad antimicrobial spectrum and high efficacy against anaerobic and facultative microorganisms organized in biofilms. 2. Dissolve necrotic pulp tissue remnants. 3. Inactivate endotoxin. 4. Should be nontoxic, noncarcinogenic and nonantigenic. 5. Should be able to disinfect and penetrate dentin and its tubules. 6. Prevent the formation of a smear layer during instrumentation or dissolve the latter once it has formed. 7. Should not have adverse effect on dentin. 8. Should not discolor dentin. 9. Not irritate or damage vital periapical tissue (caustic or cytotoxic effects). 10. Should not weaken tooth structure. 11. Should not affect the sealing ability of filling materials.

Function of irrigants Irrigants are used to clean the root canal and are used along with the shaping instruments. The functions of irrigants include 1. 2. 3. 4. 5. 6.

Rinsing and flushing (helps remove debris). Lubricant function to reduce instrument friction during preparation. Dissolve inorganic tissue (dentin). Dissolve organic matter (dentin collagen, pulp tissue, biofilm). Penetrate to the peripheries of the root canal system. Kill bacteria and yeasts (antimicrobial).

Classification Chemically inactive irrigants 1. Water 2. Saline 3. Local anesthetic solution

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Chemically active irrigants 1. Tissue dissolving agents [e.g. Sodium hypochlorite (NaOCl)] 2. Oxidizing agents [e.g. Hydrogen peroxide (H2O2), Sodium hypochlorite] 3. Antibacterial agents (e.g. Chlorhexidine, NaOCl, MTAD, Iodine) 4. Chelating agents (e.g. EDTA)

CHEMICALLY ACTIVE IRRIGANTS SODIUM HYPOCHLORITE Sodium hypochlorite is used widely as a household bleach and disinfectant. In dentistry, sodium hypochlorite (NaOCl) is a popular as an irrigating solution for Endodontic use.

Available as Solutions in various concentrations between 0.5% and 6% in bottles (Fig. 12.2).

Chemical reaction NaOCl ionizes in water into Na+ and the hypochlorite ion, OCl-. The OCl- ion in turn combines with water to form Hypochlorous acid (HOCl). NaOCl + H2O → Na+ + Cl− + 2 HO

In commercial NaOCl solutions, the following species are in equilibrium H+ + OCl− ↔ HOCl HOCl + Cl− + H+ ↔ Cl2 + H2O

Hypochlorous acid is responsible for the antibacterial activity; the OCl ion is less effective than the undissolved HOCl. Hypochlorous acid disrupts several vital functions of the microbial cell resulting in cell death. At acidic and neutral pH, chlorine exists predominantly as HOCl, whereas at high pH of 9 and above, OCl predominates.

FIGURE 12.2 Representative commercially available sodium hypochlorite solutions.

It is a potent antimicrobial agent, killing most bacteria instantly on direct contact. It also effectively dissolves pulpal remnants and collagen, the main organic components of dentin. Hypochlorite is the only root canal irrigant of those in general use that dissolves necrotic and vital organic tissue. Although hypochlorite alone does not remove the smear layer, it affects the organic part of the smear layer, making its complete removal possible by subsequent irrigation with EDTA or citric acid (CA). It is used as an unbuffered solution at pH 11 in the various concentrations mentioned earlier, or buffered with bicarbonate buffer (pH 9.0), usually as a 0.5% (Dakin solution) or 1% solution. Sodium hypochlorite is the most important irrigating solution and the only one capable of dissolving organic tissue, including biofilm and the organic part of the smear layer. It should be used throughout the instrumentation phase.

Biological considerations Extrusion of NaOCl into periapical tissues can cause severe injury to the patient (Fig. 12.3). To minimize NaOCl accidents, the irrigating needle should be placed short of the working

210 PART 3 Endodontic Materials length, fit loosely in the canal and the solution must be injected using a gentle flow rate. Constantly moving the needle up and down during irrigation prevents wedging of the needle in the canal and provides better irrigation. The use of irrigation tips with side venting reduces the possibility of forcing solutions into the periapical tissues. Treatment of NaOCl accidents is palliative and consists of observation of the patient as well as prescribing antibiotics and analgesics.

Storage

FIGURE 12.3 Sodium hypochlorite

Sodium hypochlorite is generally not utilized in its injury caused by irrigant leaking into the periapical areas. most active form in a clinical setting. For proper antimicrobial activity, it must be prepared freshly just before its use. It should be stored at room temperature and not exposed to oxygen for extended periods of time. Exposure of the solution to oxygen and light can inactivate it significantly.

Disadvantages 1. Unpleasant taste 2. Relative toxicity 3. Inability to remove smear layer (inorganic)

CHLORHEXIDINE Chlorhexidine (Fig. 12.4) has a broad-spectrum antimicrobial action and relatively little toxicity. Chlorhexidine (CHX) however, lacks the tissue-dissolving ability. It penetrates the cell wall and attacks the bacterial cytoplasmic or inner membrane or the yeast plasma membrane. It may be used in concentrations between 0.2% and 2%. Its activity is pH dependent and is greatly reduced in the presence of organic matter.

HYDROGEN PEROXIDE It is a clear, colorless liquid (Fig. 12.5) used in a variety of concentrations. 1%–30% H2O2 is active against viruses, bacteria, and yeasts.

FIGURE 12.4 Chlorhexidine irrigant.

FIGURE 12.5 Hydrogen peroxide irrigant.

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Functions and actions It produces hydroxyl free radicals (OH), which attack several cell components such as proteins and DNA. In endodontics, H2O2 has long been used because of its antimicrobial and cleansing properties. It has been particularly popular in cleaning the pulp chamber from blood and tissue remnants, but it has also been used in canal irrigation. FIGURE 12.6 MTAD stands for mixture of tetracycline isomer, acid and detergent.

MTAD The acronym MTAD (Fig. 12.6) stands for mixture of tetracycline isomer, acid and detergent (doxycycline, citric acid, and the detergent Tween-80). It has sustained antibacterial activity. It has a low pH of 2.15. MTAD solubilized dentine well (70%) and pulp tissue to a lesser extent (50%). It is an alternative to EDTA.

EDTA (ETHYLENEDIAMINETETRAACETIC ACID)

FIGURE 12.7 EDTA irrigant for root canal.

EDTA (Fig. 12.7) (17%, disodium salt, pH 7) has little if any antibacterial activity. It effectively removes smear layer by chelating the inorganic component of the dentine. It aids in the mechanical canal shaping.

RC-PREP RC-Prep (Fig. 12.8) is composed of EDTA and urea peroxide in a base of Carbowax. It is not water soluble. Interaction of the urea peroxide in RC-Prep with sodium hypochlorite, produces a bubbling action thought to loosen and float out dentinal debris.

INTRACANAL MEDICAMENTS

FIGURE 12.8 RC-Prep is composed of EDTA.

If root canal treatment cannot be finished in a single visit, root canals are dressed with medicaments. A medicament is an antimicrobial agent that is placed inside the root canal between treatment appointments in an attempt to destroy remaining microorganisms and prevent reinfection.

Functions of intracanal medicaments Primary functions 1. Antimicrobial activity 2. Antisepsis 3. Disinfection Secondary functions 1. Hard-tissue formation 2. Pain control 3. Exudation control 4. Resorption control

Desirable properties 1. Antibacterial 2. Penetrate dentinal tubules

212 PART 3 Endodontic Materials 3. 4. 5. 6. 7. 8. 9.

Control exudation or bleeding Biocompatible Eliminate pain Induce calcific barrier Have no effect on temporary restoration Should be radiopaque Should not stain tooth

Types of intracanal medicaments 1. 2. 3. 4. 5. 6. 7. 8.

Phenol and related volatile compounds PBSC paste Sulfonamide and sulfathiazole Corticosteroid-antibiotic combination Calcium hydroxide N2 Halogens Quaternary ammonium compounds

PHENOL AND RELATED COMPOUNDS 1. 2. 3. 4. 5.

Phenol Eugenol CMCP Cresatin Formocresol

6. Glutaraldehyde 7. Cresol 8. Beechwood 9. Creosote 10. Thymol

PHENOLIC COMPOUNDS Phenol is a protoplasm poison and produces necrosis of soft tissue. However, because it has a strong inflammatory potential, at present it is rarely used as an intracanal medicament.

EUGENOL Eugenol is the chemical essence of the oil of clove. It is both antiseptic and an anodyne. It is a constituent of most root canal sealers and used as a temporary sealing material. It inhibits intradental nerve impulses.

PARACHLOROPHENOL (PCP) It is a substitution product of phenol. The aqueous solution of PCP penetrates deeper into the dentinal tubules, than camphorated chlorophenol. A 1% solution of PCP is capable of killing many of the microorganisms in the root canal. It may, however, produce mild inflammation.

FORMOCRESOL This is a combination of formalin and cresol in the ratio of 1:2 or 1:1. Formalin is a strong disinfectant that combines with albumin to form an insoluble, nondecomposable substance and fixes the tissues. It is used as a dressing for pulpotomy to fix the retained pulp tissue. Formocresol (Fig. 12.9) is a nonspecific bacterial medicament most effective against aerobic and anaerobic organisms found in root canals. Formocresol is also mutagenic and carcinogenic.

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It is placed on a cotton pellet, squeeze dried and then placed in the pulp chamber of the tooth in treatment. The vapors will penetrate the entire canal preparation.

CRESATIN Cresatin (Fig. 12.10) or metacresylacetate has both antiseptic and obtundent properties. But the antimicrobial effect is less than that of formocresol and camphorated parachlorophenol. It is less irritating to the tissue.

CAMPHORATED MONOCHLOROPHENOL (CMCP) FIGURE 12.9 Formocresol.

Camphorated monochlorophenol (Fig. 12.11) consists of 2 parts of parachlorophenol and 3 parts of gum camphor (p-chlorophenol 35%, camphor 65%). Camphor serves as a vehicle and diluent and reduces the irritating effect of pure PCP. It also prolongs the antimicrobial effect. It is used in the form of vapour forming intracanal medicaments. The vapours can pass through the apical foramen.

GLUTARALDEHYDE Glutaraldehyde is a colorless oil slightly soluble in water. It is a strong disinfectant and fixative. The antimicrobial action of glutaraldehyde is bacteriostatic in nature. The recommended concentration is 2%. FIGURE 12.10 Cresatin.

FORMALDEHYDE BASED (N2) N2 (Fig. 12.12) was introduced by Sargenti. It is a compound containing paraformaldehyde as the primary ingredient. It also contains Phenylmercuric borate, eugenol, and additional ingredients like lead, corticosteroid and antibiotics. It is claimed to be both intracanal medicament and sealer. The antibacterial effect of N2 is short-lived.

ANTIBIOTICS POLYANTIBIOTIC PASTES - PBSC AND PBSN

FIGURE 12.11 Camphorated monochlorophenol.

Polyantibiotic pastes were first introduced in 1951 by Grossman who was considered as the father of Endodontics. Grossman’s paste was also known as PBSC which stood for penicillin, bacitracin, streptomycin and caprylate sodium suspended in silicone oil. Subsequently the antifungal agent caprylate sodium was replaced by Nystatin, i.e. PBSN.

Composition Ingredient

Proportions

Function

Penicillin G

1,000,000 units

Effective against gram positive organisms

Bacitracin

10000 units

For penicillin resistant organism

Streptomycin

1.0 g

Effective against gram negative organisms

Caprylate sodium

1.0 g

Antifungal

Silicone fluid

3 mL

Vehicle

Nystatin

10000 units

Antifungal

214 PART 3 Endodontic Materials

FIGURE 12.12 N2 contains paraformaldehyde as the main agent.

They are available in a paste form that may be injected into root canals impregnated on paper points.

Drawbacks and concerns Polyantibiotic paste showed therapeutic potential, but owing to the drawbacks including ineffectiveness against anaerobic species and allergic reactions, the Food and Drug Administration (FDA) prohibited PBSC for endodontic use in 1975. Fears of development of resistant organisms was another concern. Antibiotics for use in root canal therapy should be carefully justified in order to avoid development of bacterial resistance.

SULFONAMIDES Sulfonilanide and sulfanizole are used as medicaments by mixing with sterile distilled water or by placing a moistened paper point into a fluffed jar containing the powder. This medication is suggested for use as intracanal medicament in acute periapical abscess. Disadvantage Yellowish tooth discoloration has been reported after use.

CORTICOSTEROIDS - ANTIBIOTICS COMBINATIONS These medicaments are highly effective in the treatment of over instrumentation. They must be placed in the inflamed tissue by a paper point or reamer to be effective. They are more effective in vital pulps than the necrotic pulp tissue. The steroid constituent reduces the periapical inflammation and gives instant relief of pain. The antibiotic constituents are present so that overgrowth of microorganisms will be prevented. An example of this category is the Ledermix paste developed by Schroeder and Triadan in 1960 which contains an antibiotic demeclocycline— HCl (3.2%) and a corticosteroid, triamcinolone acetonide (1%), in a polyethylene glycol base. Another example is the Odontopaste released in February 2008 (zinc oxide-based root canal paste with 5% clindamycin hydrochloride and 1% triamcinolone acetonide).

HALOGENS Sodium hypochlorite is also used as an intracanal medicament (ICM). Chlorine is the active ingredient in NaOCl. Sodium hypochlorite reacts rapidly with organic matter, and hence the longevity of its antimicrobial effect is questionable. Disadvantage Toxic to the periapical tissues.

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AMINOACRIDINE It is a mild antiseptic. It works by inhibiting bacterial protein synthesis.

QUATERNARY AMMONIUM COMPOUNDS CHLORAMINE It is a chlorine compound (NH2Cl) used in concentration of 5%. It has good antimicrobial qualities. It remains stable for a long period of time.

IODINE Iodine is highly reactive combines with proteins and forms salts which probably destroys microorganisms. Iodine-potassium iodide has a relatively high antibacterial effect and relatively low toxicity.

Disadvantage 1. It may cause staining of the tooth. 2. Allergic reaction.

CALCIUM HYDROXIDE Calcium hydroxide was introduced by Herman in 1920. It is one of the commonly used ICMs (Intracanal medicaments). It is a broad spectrum antimicrobial agent. Its antiseptic action probably relates to its high pH and its leaching action on necrotic pulp tissue. It is best used in ‘weeping canals’, where there is a constant clear or reddish exudate associated with large periapical lesion. In such cases calcium hydroxide is an excellent medicament to be used.

CHLORHEXIDINE (CHX) GLUCONATE Chlorhexidine (CHX) (2% gel) (Fig. 12.13) is also shown to be an excellent intra canal medicament. It is broad spectrum antimicrobial agent. It can be used alone in gel form or mixed with Ca(OH)2. CHX gel provides antimicrobial activity for up to 21 days after contamination. When it is used in combination with Ca(OH)2, the antimicrobial activity of this mixture is greater than the combination of Ca(OH)2 and saline. FIGURE 12.13 Chlorhexidine gel.

13 Chapter

Endodontic Sealers and Obturating Materials Jacob Kurien

Chapter Outline • Root Canal Obturating Materials –– Gutta-Percha –– Silver Points –– Paste-type Filling Materials –– Epiphany Root Canal System • Root Canal Sealers • Zinc Oxide-Eugenol-Based Sealers

• Rickert’s Sealer –– Grossman’s Formulation (Roth’s Sealer)

• Formaldehyde Containing

Sealers • Resin-Based Sealers –– Diaket –– AH-26 –– AH- Plus –– Epiphany Root Canal Sealer • Calcium Hydroxide-Based Sealers –– CRCS (Calciobiotic Root Canal Sealer)

• • • • • •

Seal Apex Apexit Plus Glass Ionomer-Based Sealers Silicon-Based Sealers Mineral Trioxide Aggregate (MTA) Endodontic Solvents

Following the cleaning, debriding and enlarging of the root canals, the canals spaces are sealed or obturated (Fig. 13.1) to prevent reinfection and colonization by bacteria. Why obturate canals? Microorganisms and their byproducts are the major cause of pulpal and periapical disease. However, it is difficult to consistently and totally disinfect root canal systems. Therefore, the goal of three-dimensional obturation is to provide an impermeable fluid tight seal within the entire root canal system, to prevent oral and apical microleakage. Successful obturation requires the use of materials and techniques capable of densely filling the entire root canal system and providing a fluid tight seal from the apical segment of the canal to the cavosurface margin in order to prevent reinfection. This also implies that an adequate coronal filling or restoration be placed to prevent oral bacterial microleakage. It has been shown that endodontic treatment success is dependent both on the quality of the obturation and the final restoration. The quality of the endodontic obturation is evaluated using radiographic images upon completion. The objectives of modern nonsurgical endodontic treatment are 1. To provide a clean and bacteria free. 2. To provide an apical seal. This prevents the ingress of fluids that will provide nutrients for canal bacteria and also prevents irritants leaving the canal and entering the periapical tissues. 3. To provide a ‘coronal’ seal. This prevents recontamination due to the ingress of oral microorganisms from the coronal end.

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Figure 13.1 A root canal treated tooth.

ROOT CANAL Obturating materials Historically a wide variety of materials and techniques Related ISO Standards have been used in an attempt to produce an impervious seal of the tooth root apex ranging from ISO 6877:2006 Dentistry — Root-canal orange wood through to precious metals and dental obturation points cements. During the Civil War, a material called “Hill’s ISO 6876:2012 Dentistry — Root canal stopping” (Box 13.2) (which contained gutta-percha, sealing materials quick lime, quartz and feldspar) was used. The most widely used root-canal sealing technique is a combination of root obturating points and canal sealer cements. A root canal filling material should prevent infection/reinfection of treated root canals. Together with an acceptable level of biocompatibility (inert material) this will provide the basis for promoting healing of the periodontal tissues and for maintaining a healthy periapical environment.

Classification of Root canal obturating materials Root canal obturating materials may be classified as 1. Solid core, e.g. Silver 2. Plastic/Semi solid core, e.g. Gutta-percha, Resilon 3. Paste type, e.g. iRoot SP

Silver points Silver points or cones (Fig. 13.2) were introduced by Jasper in 1941. Silver cones were the most widely used solid-core metallic filling material between 1940 to 1960 because of their bactericidal effect (Fig. 13.3). Points of gold, iridioplatinum, and tantalum were also available. These have been largely replaced by gutta-percha and are rarely used currently. They are largely of historical interest.

218 Part 3 Endodontic Materials

Figure 13.2 Silver points.

Figure 13.3 Root canal obturated with silver point. Silver points are rarely used currently.

Advantages 1. They had a bactericidal effect. 2. Can be used in narrow and curved canals. 3. Silver has more rigidity than gutta-percha, and hence can be pushed into tightly fitting canals and around curves where it is difficult to force gutta-percha.

Disadvantages 1. Silver points/cones have a circular cross section unlike the canals which may be oval hence a poor lateral seal. 2. Could show high levels of corrosion especially due to the dissolution of the sealers. 3. Corrosion products are cytotoxic. 4. Retrievability may be difficult in cases where retreatment is desired. 5. Preparation of canal for post and core reconstruction difficult. The disadvantages of silver points far outweigh their advantages and their use has been largely discontinued. However, they present an important evolutionary stage in the development of root canal filling materials.

GUTTA-PERCHA Gutta-percha is a polymeric resin-like material obtained from the coagulation of latex produced by Palaquium gutta tree (commonly known as the Isonandra gutta tree). Gutta-percha is a name derived from the Malay words ‘getah’ meaning gum and pertja’ (name of the tree in Malay). Long before Gutta-percha was introduced into the western world, it was used in crude form by the natives of Malaysian archipelago for making knife handles, walking sticks and other purposes (Box 13.1).

Manufacture of gutta-percha (Obach’s technique) The obtained pulp is mixed with water and heated to 75 °C to release the Gutta-percha threads and then cooled to 45 °C. The flocculated Gutta-percha called “yellow Gutta” contains 60% poly isoprene and 40% contaminants (resin, protein, dirt and water). Yellow Gutta is mixed with cold industrial gasoline at below 0 °C temperature. This treatment not only flocculates the Guttapercha but also dissolves resins and denatures any residual proteins. After removal of cold gasoline, deresinated Gutta threads are dissolved in warm water at 75 °C and dirt particulate is allowed to precipitate. Residual greenish yellow solution is bleached with activated clay, filtered to remove any particulate and then steam distilled to remove the gasoline. “Final

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Box 13.1 History of gutta-percha

Gutta-percha trees are native to South East Asia and Australia. Their sap is similar to rubber. It’s actually a natural polymer-like rubber. Though very similar to it, unlike rubber, Gutta-percha is biologically inert - it doesn’t react with biological materials - and that was the key to its usefulness. It was discovered by Western explorers in the middle of the 17th century, though local Malay people already knew about it and used it. The first Westerner to discover this material was John Tradescant, who brought this material after his travels from far-east in 1656, he named this material as “Mazer wood”. But the honour of introduction of this material goes to Dr. William Montogmerie, who was a medical officer in Indian service. He was the first to appreciate the potential of this material in medicine and for which he was awarded the gold medal by the Royal Society of Arts, London in 1841. As soon as it was introduced, it found use as an insulating medium in the laying of underground seawater cables. The first Gutta-percha patent was taken by Alexander, Cabriol and Duclos for a laminate consisting of three layers called “Gutta-percha fabric”. In 1845 Hanco*ck and Bewley formed the Gutta-percha company in United Kingdom. There were even jewels and ornaments made of it as they were considered to be precious materials at that time. Gutta-percha and the Telecom Revolution Back in the 19th century, the telegraph was revolutionising the way people communicated. The solution was to lay down undersea telegraph cables. However, to carry electricity an undersea cable needs to be protected and no one had succeeded in doing that. Rubber had been tried as an insulating layer for the cables but marine animals and plants just attacked it, and once the cable was open to the sea it became useless for sending signals. Guttapercha on the other hand is a great insulator but it doesn’t degrade in seawater. As it was the only known material that worked, soon all marine cable used Gutta-percha and as a result the British businessmen who controlled its supply became very rich. Dimpled Balls Early golf balls were filled with feathers. In 1848 Robert Adams Paterson came up with the idea of making them out of Gutta-percha since it was much easier to make than the laborious process of sewing balls of feathers. It was quickly realised, that after they had been used a few times they would fly further. It turned out this was due to the dimples that were made in the balls each time they were hit. The dimples improved the aerodynamics of the ball. That’s why modern golf balls are intentionally covered in dimples. The era of Gutta-percha golf balls lasted from 1845-1903, till the introduction of natural rubber. Medical use of Gutta-percha In medicine, they were used as splints for holding fractured joints and manufacture of handles of forceps, catheters, etc. It was earlier used to control Antique gutta-percha golf balls hemorrhage in extracted socket wounds. They were also used for skin diseases by the dermatologists, particularly against Small pox, Erysipelas, Psoriasis and Eczema. Gutta-percha in India In India the species of this genus is very scanty. The species found are Palaquium obavatum, Palaquium polyanthum, Palaquium ellipticum and Palaquium gutta trees in Assam and Western ghats. Palaquium gutta was recently introduced and planted in Botanical gardens, Bangalore. Gutta-percha in Dentistry Gutta-percha made its appearance in dentistry in the mid-1800s, when it was first used for filling cavities (Box 13.2). In 1887, S.S. White Manufacturing Co. began producing “points” of rolled gutta-percha for stuffing root canals. The material was valued for its plasticity when heated, which permits it to be ‘stuffed into the odd nooks and crannies’. And that’s how

220 Part 3 Endodontic Materials its still done today, with a material derived from the sap of a tropical tree. The annual market for gutta-percha in the US is estimated to be $30 million to $40 million, and most of it comes from Brazil. Even though it is the closest thing to an ideal root-canal filling material, it is hard to work with, and sometimes it still leaks. Developing a better material is what’s on the horizon. Indeed, the big dental-supply firm Dentsply International, based in York, Pa., is working on a synthetic replacement for gutta-percha that it hopes to introduce in the near future. The final retirement of a historical material with a magnificent name seems too sad to contemplate. So gutta-percha revolutionised global communications, changed the game of golf and even helped people with rotting teeth. Not bad for a tree.

ultra pure” gutta-percha has a gasoline scent, before it is modified with fillers into its final commercial product formulation.

Chemical structure of gutta-percha

Gutta-percha is 1,4-trans-polyisoprene isomer (natural gutta-percha) CH2 groups are on opposite sides of the double bond for each successive monomer. Since its molecular structure is close to that of natural rubber, which is a cis-isomer of polyisoprene, it has a number of similarities but a difference in form makes it to behave more like crystalline polymers. Thus it not exhibit the classic elastic properties of rubber

Forms of gutta-percha Gutta-percha exists in three forms α - (or alpha form)—runny, tacky and sticky β - (or beta form)—solid, compactible and ductile γ - (or gamma form)—amorphous and unstable form The β form is used with mechanical condensation techniques. The α form is used with the thermomechanical and injectable techniques.

Box 13.2 Evolution into dentistry

Gutta-percha was first introduced to dentistry as a temporary filling material by Edwin Truman. 1847 - Hill Developed “Hill’s-stopping” a restorative material, a mixture of bleached Gutta-percha and carbonate of lime and quartz. 1867 - Bowman was the first to use Gutta-percha for root canal filling. 1883 - Perry used pointed gold wire wrapped with soft Gutta-percha, rolled and packed it into the canal. 1887 - S.S White Company was the first to start the commercial manufacture of Gutta-percha points. 1893 - Rollins used Gutta-percha with pure oxide of mercury into root canal filling. 1914 - Callahan introduced softening and dissolution of Gutta-percha with the use of rosins in obturation. 1959 – Ingle and Levine were the first persons to propose standardization of root canal instruments and filling materials and at their behest, standardized Gutta-percha was introduced to the profession in 1959 after the 2nd International Conference of Endodontics at Philadelphia. 1976 - A group was formed for the approval of specifications of root canal instruments and filling materials which subsequently evolved into the present day International Standards Organization (ISO).

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Supplied as 1. Solid core Gutta-percha points –– Standardized points (Figs. 13.4A and B) –– Nonstandardized points 2. Thermomechanical compactible Gutta-percha points 3. Thermoplasticized Gutta-percha –– Solid core system –– Injectable form (Figs. 13.7 to 13.9) 4. Medicated Gutta-percha –– Iodoform containing –– Calcium hydroxide containing –– Chlorhexidine containing –– Tetracycline containing Gutta-percha used for the above techniques are supplied in point or pellet form. Tapered points of varying sizes. The sizes range from 15 to 80 (Fig. 13.4B). The various sizes are usually color coded for easy identification. Pellet form is used for the injectable technique (Fig. 13.7)

Composition of commercial gutta-percha Ingredient

Wt %

Function

Gutta-percha Zinc oxide Waxes or resins Metal sulphates

8–22% 59–76% 1–4% 1–18%

Matrix Filler Plasticity Radiopacity (barium or strontium)

Figures 13.4A and B Commercially available Gutta-percha points of varying sizes. They are color coded for differentiation.

Figure 13.5 Obtura III.

Figure 13.6 Hot Shot is a wireless battery operated.

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Figure 13.7 Gutta-percha pellets for use in the thermoplastic technique.

Figure 13.8 Extrusion of gutta-percha.

Figure 13.9 Backfill technique.

Thermoplasticized Gutta-percha Gutta-percha may also be delivered to the canal in fluid form through a syringe (Fig. 13.8). The gutta-percha is melted in the chamber of the device and injected into the canal starting apically and proceeding coronally. This is known as “backfilling” (Fig. 13.9). There are many such devices available in the market. The Obtura III (Fig. 13.5), Calamus, Elements, HotShot (Fig. 13.6), and Ultrafil 3D are examples of such devices. The Obtura III system heats the gutta-percha to 160 °C, whereas the Ultrafil 3D system employs a low-temperature gutta-percha that is heated to 90 °C. The Obtura III system (Obtura Spartan) consists of a hand-held “gun” that contains a chamber surrounded by a heating element into which pellets of gutta-percha are loaded. Silver needles (varying gauges of 20, 23, and 25) are attached to deliver the thermoplasticized material to the canal. The control unit allows the operator to adjust the temperature and thus the viscosity of the gutta-percha.

Figure 13.10 Thermoplastic techniques are often used in cases with significant canal irregularities like internal resorption.

Thermoplastic techniques are often indicated in cases with irregularities in the canal or internal resorption (Fig. 13.10)

Drawbacks The difficulties with this system include lack of length control. Both overextension (Fig. 13.11) and underextension can occur. To overcome this drawback, a hybrid technique may be used, in which the clinician begins filling the canal by the lateral compaction technique. When the master cone and several accessory cones have been placed so that the mass is firmly lodged in the apical portion of the canal, a hot plugger is introduced, searing the points off approximately 4 to 5 mm from the apex. Light vertical compaction is applied to restore the integrity of the apical plug of gutta-percha. The remainder of the canal is then filled with thermoplasticized gutta-percha injected as previously described.

Figure 13.11 Thermoplastic backfilling techniques are often associated with extrusion of gutta-percha beyond the apex.

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Properties At raised temperatures, gutta-percha behaves like other plastic materials (thermoplastic), softening above 65 °C, melting at 100 °C, and in the a-form beyond 160 °C without decomposing while remaining soft and fluid.

Removal of gutta-percha Occasionally gutta-percha requires to be removed for retreatment or placement of a post for reconstruction of the tooth. This can be achieved with the aid of 1. Rotary instruments like NiTi (Protaper retreatment series) 2. Gutta-percha solvents like Chloroform or xylol 3. Thermosoftening.

Advantages and Disadvantages of Gutta-percha Advantages 1. It is compactible and adapts excellently to the irregularities and contour of the canal by the lateral and vertical condensation method. 2. It can be softened and made plastic by heat or by organic solvents (eucalyptol, chloroform, xylol, turpentine). 3. It is inert. 4. It is dimensional stable; when unaltered by organic solvents, it will not shrink. 5. It is tissue tolerant (nonallergenic). 6. It will not discolor the tooth structure. 7. It is radiopaque. 8. It can be easily removed from the canal when retreatment is indicated.

Disadvantages 1. It lacks rigidity. The smallest, standardized gutta-percha cones are relatively more difficult to use unless canals are enlarged above size no. 25. 2. It lacks adhesive quality. Gutta-percha does not adhere to the canal walls; consequently, a sealer is required. The necessary use of a cementing agent introduces the risk of tissueirritating sealers. 3. Gutta-percha does not bond to any sealers. 4. It can be easily displaced by pressure. 5. Gutta-percha is almost wholly dependent on a coronal seal to prevent the apical migration of bacteria if it is challenged by coronal leakage.

resilon-Epiphany Root canal obturating System This system is an alternative to gutta-percha. It consists of resin obturating points sealed with dual cure, hydrophilic resin sealer. The system consists of three parts (Figs. 13.12 and 13.13) A. Resilon – A thermoplastic synthetic polymer-based (polyester) root canal filling material, as the major component. B. Epiphany sealer – A resin-based composite that forms a bond to the dentin wall and the core material. It sets with a chemical reactions and halogen curing light. C. Primer - Which prepares the canal wall for contact with Resilon and the sealer.

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Figure 13.12 Epiphany root canal filling system consists of polyester points for insertion into the canal, a sealer and primer.

Figure 13.13 Resilon points for obturation.

Primer The primer is a self-etching system that is cured by the sealer. The primer penetrates all the dentinal tubules. The bonding procedure is preceded by irrigating with a 17% solution of EDTA. This last process is necessary for removing oxide radicals from the NaOCl and peroxides irrigants. Failure to do so can interfere with the curing of the dentin bonding agent. Sealer The sealer bonds to the primer thereby eliminating potential for microleakage. The sealer used in this system is a dual cure bis-GMA, ethoxylated bis-GMA, UDMA, hydrophilic difunctional methacrylate. Fillers like calcium hydroxide, barium sulphate and barium glass are present 7% by weight. It contains the catalyst that initiates curing of self-etching primer in the dentinal tubules.

Obturator The Resilon obturator is a thermoplastic polyester and contains the following components: (1) Bioactive glass, (2) barium sulfate and (3) bismuth oxychloride. The bioglass is a unique component that forms calcium/phosphate when in contact with body fluid. It does not dissolve in fluid but instead it releases ions to stimulate the formulation of osseous tissue. As already mentioned, its radiopacity is better than gutta-percha, condenses laterally and vertically like gutta-percha and softens at around 70 to 85 °C. The Resilon obturator bonds to the surface of the sealer which in turn bonds to the primer that has hybridized with the tubular surfaces. Thinning Resin In addition to the above, many systems include a thinning resin, which may be added to thin the sealer to the desired viscosity.

Paste-type obturating materials Various paste-type products have been tried as the sole obturating materials because they have sufficient volume stability to maintain a seal. These include zinc oxide-eugenol based pastes, epoxy resins (AH-26), AH-Plus, Ketac-Endo, polyvinyl resins (Diaket), calcium hydroxide, etc. Many of these are dual use, i.e. they may be used as sole obturating material or as sealer (when combined with gutta-percha).

Disadvantages 1. Removal of a hard set cement can be a challenge if retreatment is required. 2. Risk of microleakage when used as the sole obturating material. Because of these inherent problems, use of a hard setting non-thermoplastic, non-soluble cement type filling material as the sole obturating material is not routinely recommended.

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ROOT CANAL Sealers The sealer plays an important role in the obturation of root canal. Gutta-percha cannot entirely obliterate the spaces within the root canal because of its physical limitations. The sealer fills all the spaces, the gutta-percha is unable to fill. Factors like the shape of the canal, defects within the canal, internal resorption, iatrogenic damage, accessory canals, etc., make intimate sealing of the root canal system a challenging task. A total hermetic seal of the root canal system is necessary to prevent the ingress of bacteria and reinfection of the canal. The sealer also acts as a binding agent, to the dentin and to the core material, which usually is gutta-percha.

Ideal requirements of a root canal sealer 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

It should be tacky when mixed. It should adhere well to the gutta-percha and the canal wall when set. It should make a hermetic seal. It should be radiopaque so that it can be visualized in the radiograph. The particles should be very fine so that they can mix easily with the liquid. It should not shrink upon setting. It should not stain tooth structure. It should be bacteriostatic. It should set slowly to provide suitable working time. It should be insoluble in tissue fluids. It should be tissue tolerant, that is, nonirritating or toxic to periradicular tissue. It should be soluble in a solvent if it is necessary to remove the root canal filling. It should not provoke an immune response in periradicular tissue. It should be neither mutagenic nor carcinogenic.

Classification Endodontic sealers can be broadly classified based on the principal components that react and set to form the binding matrix (Table 13.1) Table 13.1 Showing classification of different root canal sealers Type

Sub-type

Zinc oxide-Eugenol Silver-containing (Rickert’s based formula based) Grossman’s formula based (Silver free) Therapeutic - Formaldehyde - Iodofor - Steroid

Pulp Canal Sealer (SybronEndo)

Resin based

Real Seal SE (SybronEndo), Acroseal (Septodont), Epiphany (Pentron), EndoREZ (Ultradent), Diaket (3M) Sealer 26 (Dentsply), AH Plus & AH-26 (Dentsply),

BisGMA UDMA based Epoxy resin based

Commercial examples

Calcium hydroxide based

Wach’s paste,Tubliseal (SybronEndo), Roths, Intrafill (SS White), Roth Root 801 (Roth) N2/RC2B, Endomethasone, SPAD, Riebler’s paste Zical (Prevest Denpro) Endomethasone N (Septodont), Endofill (Dentsply)

CRS (Hygienic), Sealapex (SybronEndo), Life Apexit (Ivoclar) Vitapex

Silicone based

GuttaFlow (Coltene), RoekoSeal (Coltene)

Glass ionomer based

Ketac-Endo (ESPE)

MTA based

MTA Fillapex (Angelus), Endo CPM Sealer (EGEO), MTA Obtura (Angelus), ProRoot Endo Sealer (Dentsply)

226 Part 3 Endodontic Materials Zinc Oxide-Eugenol-Based sealers Commercial names Tubli-Seal (SybronEndo), Pulp canal sealer (SybronEndo), Roth’s cement, Proco-sol

Rickert’s formula based (silver containing) The earliest sealers were made by dissolving gutta-percha in solvents like chloroform and was termed ‘chloropercha’. These sealers had problems resulting from shrinkage. Rickert’s formula was developed in 1931 as an alternative to the chloropercha technique.

Composition Powder

Zinc oxide

41.2

Precipitated silver

White resin

Thymol iodide

Liquid

Oil of clove

Canada balsam

12.8

The silver was added for its radiopacity and germicidal qualities. It has excellent lubricating and adhesive qualities, and sets in about half an hour. However the silver content caused discoloration of tooth structure. Pulp canal sealer (Fig. 13.14) is based on Rickert’s formula.

Grossman’s and Roth’s formula based (Silver free ZOE sealers) This includes most current ZOE sealers. In 1958, Grossman introduced a nonstaining ZOE cement as a substitute for Rickert’s formula. This formulation is considered standard by which other cements are measured because it reasonably meets most of Grossman’s requirements for sealers. Many current sealers are still based on Grossman’s formula (Fig. 13.16).

Composition Powder

wt %

Liquid

Zinc oxide, reagent

Eugenol

Staybelite resin

Bismuth subcarbonate

Barium sulphate

Sodium borate, anhydrous

Roth’s sealer (Fig. 13.15) was similar to Grossman’s except for the addition of Bismuth subnitrate instead of Bismuth subcarbonate.

Figure 13.14 Rickert’s sealer is sold under the trade name Pulp canal sealer.

Figure 13.15 Roth’s root canal sealer. It is a silver ZOE formulation.

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Figures 13.16A and B Two other representative zinc-oxide eugenol based sealers. (A) Proco-sol. (B) Tubli-seal.

Therapeutic sealers The first formaldehyde containing sealer was introduced by Sargenti in 1954 (also called Sargenti’s Paste). These sealers constantly release antimicrobial formalin. Formalin is highly cytotoxic. Therapeutic sealers contain antibiotics, bactericidal and anti-inflammatory agents. Bactericidal agents include formaldehyde and iodoform. Antiinflammatory agents include hydrocortisone, prednisolone, etc.

Commercial names Formaldehyde containing - N2/RC2B (Fig. 13.17A), SPAD, Riebler’s paste (drug combination consisting of zinc oxide, barium sulfate, formalin and resorcinol), etc. Corticosteroid containing - Endomethasone (Fig. 13.17B), Endofill, etc.

Composition Powder

Liquid

Zinc oxide

Bismuth subcarbonate

Eugenol

Barium sulphate

Hydrocortisone

Geraniol

Prednisolone

Paraformaldehyde

Titanium dioxide

Phenyl mercuric borate

Lead tetraoxide

Figures 13.17A and B Representative therapeutic sealers. (A) N2. (B) Endomethazone.

228 Part 3 Endodontic Materials Advantages 1. Good antibacterial effect. 2. Good anti-inflammatory effect.

Disadvantages 1. Irreversible damage to the nerve tissue. 2. Causes coagulation necrosis of the tissues.

EPOXY RESIN-BASED SEALERS Commercial names Diaket, AH-26, AH Plus (Dentsply), Adseal.

DIAKET Diaket (Fig. 13.18 A) was introduced by Schmidt in 1951. During setting, a resin-reinforced chelate is formed between zinc oxide and diketone. It has a high resistance to absorption.

Advantages 1. 2. 3. 4.

Good adhesion. Sets quickly in the root canal. Low solubility and good volume stability. Superior tensile strength.

Disadvantages 1. It is highly toxic. 2. It is nonresorbable and forms fibrous encapsulation if extruded into the periapical tissues.

AH-26 AH-26 (Fig. 13.18B) was introduced by Schroeder 1957. It is an epoxy resin based sealer. It is a powder-liquid system.

Composition Powder

Wt%

Liquid

Silver powder

10%

Bisphenol diglycidyl ether

Bismuth oxide

60%

Hexamethylene tetramine

25%

Titanium oxide

Figures 13.18A to C Resin-based sealers. (A) Diaket. (B) AH-26. (C) AH Plus.

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Manipulation and setting AH 26 powder and resin are mixed to produce a root canal filling material. As it sets traces of formaldehyde are temporarily released, which initially makes it antibacterial. It is not sensitive to moisture and will even set under water. However, it will not set in the presence of hydrogen peroxide. It sets slowly, in 24 to 36 hours. It has strong adhesive properties.

Disadvantages 1. Slight contraction while setting. 2. Delayed setting. 3. Staining.

AH- Plus AH Plus (Fig. 13.18C) is an epoxy-amine resin based two paste root canal sealer. Epoxy paste contains radiopaque fillers.

Composition Epoxide paste contains bisphenol-A and F as epoxy resin, calcium tungstate, zirconium oxide, silica and iron oxide pigments and Amine paste contains dibenzylediamine, aminoadmantace, tricyclodecane-diamine, calcium tungstate zirconium oxide, silica and silicone oil.

Advantages over AH-26 1. 2. 3. 4. 5. 6. 7.

Less toxic. New amines added to maintain the natural color of the tooth. Half the film thickness. Better flow. Four-hour working time. Eight-hour setting time allows for corrections of fillings. Increased radiopacity.

Epiphany Root canal Sealer Epiphany as described earlier is a dual cure, hydrophilic resin obturating material/sealer system.

CALCIUM HYDROXIDE BASED SEALERS Dentists have been using calcium-based chemicals in clinical practice for over a century. Calcium hydroxide was introduced to endodontics by Herman in 1920 for its pulp-repairing ability. In endodontics, it is mainly used for pulp-capping procedures, as an intracanal medicament, in some apexification techniques, and as a component of several root canal sealers. The two most important reasons for using calcium hydroxide as a root-filling material are stimulation of the periapical tissues in order to maintain health or promote healing and secondly for its antimicrobial effects. The exact mechanisms are unknown, but the following mechanisms of actions have been proposed. 1. Calcium hydroxide is antibacterial depending on the availability of free hydroxyl ions. It has a very high pH (hydroxyl group) that encourages repair and active calcification.

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3. 4. 5.

There is an initial degenerative response in the immediate vicinity followed rapidly by a mineralization and ossification response. The alkaline pH of calcium hydroxide neutralizes lactic acid from osteoclasts and prevents dissolution of mineralized components of teeth. This pH also activates alkaline phosphatase that plays an important role in hard tissue formation. Calcium hydroxide denatures proteins found in the root canal. Calcium hydroxide activates the calcium-dependent adenosine triphosphatase reaction associated with hard tissue formation. Calcium hydroxide diffuses through dentinal tubules and may communicate with the periodontal ligament space to arrest external root resorption and accelerate healing.

Setting of calcium hydroxide-based sealers in root canals The setting time of calcium hydroxide-based sealers the root canal is dependent upon the availability of moisture. The setting reaction can progress very quickly even in canals which have been inadequately dried. The amount of moisture required for the setting reaction reaches the root canal by means of the dentinal tubules. The material begins to set at the apex, as dentin is thinnest in this region and the apical foramen admits additional moisture.

CRCS (Calciobiotic Root Canal Sealer) CRCS (Fig. 13.19A) is essentially a ZOE/eucalyptol sealer to which calcium hydroxide has been added for its called osteogenic effect. CRCS takes 3 days to set fully in either dry or humid environments. It also shows very little water sorption. This means it is quite stable, which improves its sealant qualities, but brings into question its ability to stimulate cementum and/or bone formation. If the calcium hydroxide is not released from the cement, it cannot exert an osteogenic effect, and thus its intended role is negated

SEALAPEX Sealapex (Fig. 13.19B) is a zinc oxide based calcium hydroxide sealer containing polymeric resin. It is available as a two paste system.

Advantages 1. Biocompatible 2. Extruded material resorbs in 4 months 3. Good therapeutic effect.

Figures 13.19A to C Representative calcium hydroxide-based sealers. (A) CRCS. (B) Sealapex. (C) Apexit plus.

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Disadvantages 1. Long setting time. 2. Absorbs water while setting and expands. 3. Poor cohesive strength.

APEXIT PLUS Apexit Plus (Fig. 13.19C) is a radiopaque, non-shrinking root canal sealer paste that is based on calcium hydroxide. It is available as a two paste system. It is used for the permanent obturation of root canals and it is suitable for use in conjunction with all obturation techniques involving gutta-percha.

Working and setting characteristics 1. Long working time (over 3 hours at room temperature) 2. Setting Time - 3–5 hours in normal canals. Up to 10 hours in extremely dry canals.

Advantages 1. Excellent tissue tolerance. 2. Durable sealing of the root canal due to the slight setting expansion. 3. Its easy flowing composition allows the material to adapt well even to morphologically complicated canals. 4. Convenient application (static mix syringe and intracanal tip). 5. Better seal than that provided by Sealapex.

GLASS IONOMER-BASED SEALERS Commercial name Ketac-Endo (Fig. 13.20)

Advantages 1. Biocompatible. 2. Chemical bonding with the root dentine, hence strengthens the root. 3. Less solubility. 4. Dimensionally stable. 5. Less technique sensitive.

Figure 13.20 Glass ionomer-based sealers.

Disadvantages 1. Extruded sealer is highly resistant to resorption (delayed resorption). 2. Retrievability is difficult.

Silicon-BASED SEALERS The silicon-based sealers are based on the polydimethyl siloxane system. Commercial names RoekoSeal (Coltene) and Guttaflow (Langenau) (Figs. 13.21A and B).

Composition RoekoSeal - Polydimethylsiloxane, silicone oil, zirconium oxide. Gutta flow - Polydimethylsiloxane, silicone oil, zirconium oxide, gutta-percha.

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Figures 13.21A and B Silicon-based sealers. (A) Roekoseal. (B) GuttaFlow.

Properties 1. 2. 3. 4. 5. 6. 7.

Excellent flow properties and good spreadability. Contains nanosilver which prevent further spread of bacteria. Good adaptability and tight seal of the root canal. Flowable cold filling system. Solubility is virtually zero. Excellent radiopacity. The included nanosilver can also have a preserving effect in the canal. The chemical type and concentration of the silver does not cause corrosion or color changes in the GuttaFlow. 8. A Gutta-percha containing silicone sealer expands slightly and thus leakage was reported to be less than the AH-26 over a period of 12 months. 9. Very good biocompatibility with lower cytotoxicity than the AH Plus. 10. More easily removed from the canals than a resin-based sealer.

Disadvantages 1. Poor wettability of GuttaFlow 2. GuttaFlow does not adhere chemically to the canal wall. 3. Due to its viscosity, it is more likely to be extruded into the periapical tissue when placed under pressure.

Mineral trioxide aggregate (MTA) The first reported use of Portland cement in dental literature dates to 1878, when Dr. Witte in Germany published a case report on using Portland cement to fill root canals. At the time the material itself was relatively new as Portland cement was invented in 1824. Mineral trioxide aggregate (MTA) was first described in modern dental scientific literature in 1995. It was developed at Loma Linda University, California, USA, by Torabinejad and Dean White who subsequently obtained two US patents for this Portland cement-based endodontic material, which became known as mineral trioxide aggregate (MTA). Since then, over 20 patents have been issued in the USA and the EU for materials that include Portland cement for dentistry. The term mineral trioxide aggregate (MTA) was coined from the three oxides present in Portland cement namely, calcia, silica and alumina (CaO, SiO2 and Al2O3). Furthermore, the powder particles of cement are in aggregate form.

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Indications Mineral trioxide aggregate materials are indicated for various restorative, endodontic, and regenerative dental procedures. 1. 2. 3. 4. 5. 6. 7.

Vital pulp therapy (pulp capping and pulpotomy) Apexification Perforation repair (lateral and furcation) (Figs. 13.22A and B) Root-end filling Internal bleaching Resorption repair As sealer and as obturating material (partial or complete).

Commercial names The first commercially available product was a gray mineral trioxide aggregate, marketed as ProRoot® MTA (Dentsply) (Fig. 13.23A). Subsequently for esthetic reasons a tooth-colored or white formulation of MTA was introduced (Dentsply) in 2002. Currently Many MTA sealer formulations are available. These include Endo CPM Sealer (EGEO SRL, Argentina), MTA Obtura (Angelus, Brazil), MTA Fillapex (Angelus) (Fig. 13.23B), Endocem MTA (Maruchi, Korea) and ProRoot Endo Sealer (Dentsply Maillefer, Switzerland).

Figures 13.22A and B (A) Perforation in the floor of maxillary molar. (B) Perforation repaired with MTA.

Figures 13.23A and B MTA based sealers. (A) ProRoot MTA. (B) MTA Fillapex.

234 Part 3 Endodontic Materials Supplied as 1. Powder and liquid form (e.g. ProRoot MTA) 2. Two paste - base and catalyst in tubes (MTA Fillapex) 3. Two paste - in plunger tubes as static mixing system (MTA Fillapex).

Composition A wide variation in composition exists between the different products. ProRoot MTA is calcium silicate-based endodontic sealer. The major components of the powder of are tricalcium silicate and dicalcium silicate, with inclusion of calcium sulfate (gypsum) as setting retardant, bismuth oxide as radiopacifier, and a small amount of tricalcium aluminate. Tricalcium aluminate is necessary for the initial hydration reaction of the cement. Powder * Ingredient

Formula

Wt%

Function

Tricalcium silicate

(CaO)3.SiO2

45–75

Dicalcium silicate

(CaO)2.SiO2

7–32

Tricalcium aluminate

(CaO)2.Al2O3

0–13

Initial hydration

Bismuth or tantalum oxide

Bi2O3or Ta2O5

20–35

Radiopacity

Calcium sulphate dihydrate (gypsum)

CaSO4.2H2O

2–10

Retarder

Tetracalcium aluminoferrite

(CaO)4.Al2O3.Fe2O3

0–18

Impart gray color in MTA. Absent in white MTA.

* Adapted from Phillips Science of Dental Material. Ed. 12.

Liquid The liquid component consists of viscous aqueous solution of a water soluble polymer to improve the workability. MTA Fillapex (Fig. 13.23B) is a mineral trioxide aggregate-based, salicylate resin root canal sealer. It is designed to provide a high flow rate and a low film thickness for easy penetration of lateral and accessory canals. It contains 13% MTA and salicylate resin for their antimicrobial and biocompatibility properties. The working time is 23 minutes with a complete set time is approximately 2 hours. MTA Fillapex is a two-paste system and is provided in a 4 g static mixing syringe and 30 g tubes. CPM sealer The composition of CPM sealer after mixing is reported to be 50% MTA (SiO2, K2O, Al2O3, SO3, CaO, and Bi2O3), 7% SiO2, 10% CaCO3, 10% Bi2O3,10% BaSO4, 1% propylene glycol alginate, 1% propylene glycol, 1% sodium citrate, and 10% calcium chloride. MTA Obtura is a mixture of white MTA with a proprietary viscous liquid.

Difference between white and gray MTA The difference between the gray and the white materials is the presence of iron in the gray material, which makes up the phase tetracalcium alumino-ferrite.

Comparison of MTA with portland cement The similarity of MTA with Portland cement was reported in 2000. Further studies comparing the two showed the cements to have similar constituent elements. However, some differences were also noted. The prime difference between the two is the addition of radiopaque fillers to enable radiographic differentiation. Secondly, MTA manufactured for dental use have to pass FDA regulations to enable it to be used in humans, thus components considered harmful have to be eliminated or minimized. A comparison of the two are presented in Table 13.2.

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Table 13.2 Comparison of MTA and Portland cement MTA

Portland cement

Radiopaque fillers

Present

Absent

Tricalcium aluminate

Present

Absent

Tricalcium silicate

Lower levels

Higher level

Calcium sulphate hemihydrate

Absent

Present

Particle size

Fine

Coarser

Heavy metal content

Minimal or absent

Present

Biological properties When placed in the canal, it releases calcium activity and causes cell attachment and proliferation, increases the pH, modulates cytokines like interleukin (IL4, IL6, IL8, IL10), and hence causes proliferation, migration, and differentiation of hard tissue producing hydroxyapatite which aids in the formation of physical bond between sealer and MTA. The polymer did not seem to affect the biocompatibility of the materials and the hydration characteristics were similar to those reported for MTA. Sealers based on MTA have been reported to be biocompatible, stimulate mineralization, and encourage apatite-like crystalline deposits along the apical- and middle-thirds of canal walls. These materials exhibited higher push-out strengths after storage in simulated body fluid and had similar sealing properties to epoxy resin-based sealer when evaluated using the fluid filtration system. Fluoride-doped MTA demonstrated stable sealing up to 6 months, and was significantly better than conventional MTA sealers and comparable to AH Plus. The study supports the suitability of MTA sealers in association with warm GP for root filling.

Manipulation P/L Ratio The powder liquid ratio of MTA can vary according to its intended use. For use as a sealer a creamy consistency is preferred. For use in perforation repair a putty consistency may be preferred. P/L therefore ranges from 4 to 1 to 2 to 1. Open a pouch of ProRoot MTA root repair material and dispense the powder onto a mixing pad. Liquid from the ampoule squeeze out onto the mixing pad next to the powder. Gradually incorporate the liquid into the cement with a plastic spatula. Mixing time Mix the material with the liquid for about one minute to ensure all the powder particles are hydrated. If needed (one extra ampoule is provided, sterile water can also be used), one or two drops of liquid can be added to make the material into a thick, creamy consistency. Working time The ProRoot MTA root repair material will set over a period of four hours, but the working time is about five minutes. If more working time is needed, cover the mixed material with a moist gauze pad to prevent evaporation.

Setting time Traditionally these materials generally have long setting times. Newer products currently being marketed have shorter setting times. Examples include Endocem MTA and Biodentine. ProRoot MTA

Endocem MTA

Initial setting time

165 minutes

2 minutes

Final setting time

4 to 6 hours

4 minutes

Biodentine

9–12 minutes

236 Part 3 Endodontic Materials It has also been stated that the faster setting time is achieved by increasing particle size, adding calcium chloride to the liquid component, and decreasing the liquid content.

Chemistry and setting reaction MTA sets through a hydration reaction when mixed with water. MTA + water → calcium hydroxide + calcium silicate hydrate

When MTA is mixed with water a highly alkaline (pH 12) cement matrix comprising of calcium hydroxide and calcium silicate hydrate is formed. A setting expansion of 0.1% is seen which contributes to its sealing ability. An acidic environment does not interfere with the setting of the MTA.

Properties 1. Compressive strength It has been shown that once it is set, it has a compressive strength equal to IRM and Super EBA but less than amalgam. Compressive strength of MTA within 24 hours of mixing was about 40.0 MPa and increases to 67.3 MPa after 21 days. In comparison gray MTA exhibited greater compressive strength than white MTA. 2. Setting Expansion Set MTA exhibits a low setting expansion of less than 0.1%. 3. Radiopacity MTA is less radio opaque than IRM, amalgam or gutta-percha and has similar radiodensity as Zinc Oxide Eugenol. The mean radiopacity of MTA is 7.17 mm of equivalent thickness of aluminium, which is sufficient to make it easy to visualize radiographically. 4. Solubility Although the set MTA shows no signs of solubility, the solubility might increase if more water is used during mixing. The set MTA when exposed to water releases calcium hydroxide is responsible for its cementogenic property. 5. Marginal adaptation and sealing ability This property is most vital for any restorative material especially when used for root end filling, repair of perforations, pulp capping or pulpotomy procedures. Bates et al found MTA superior to the other traditional root-end filling materials. MTA expands during setting which may be the reason for its excellent sealing ability. According to Torabinejad et al MTA seals very superiorly and no gaps were found in any of the experimental specimen. However, amalgam, Super EBA and IRM exhibited gaps ranging from 3.8 to 14.9 microns. MTA has also proved itself to be superior in the bacterial leakage test by not allowing the entry of bacteria at the interface. MTA thickness of about 4 mm is sufficient to provide a good seal. 6. Antibacterial and antifungal property Torabinejad et al reported that MTA shows no antimicrobial activity against any of the anaerobes but have some effect on five (S. mitis, S. mutans, S. salivarius, Lactobacillus and S. epidermidis) of the nine facultative bacteria. Since most of the flora in the root canal are strict anaerobic bacteria with few facultative anaerobes, MTA may not be beneficial as a direct antibacterial in endodontic practice. However, it can be proclaimed as an antibacterial agent only by virtue of providing a good seal and preventing micro leakage. 7. Reaction with other dental materials MTA does not react or interfere with any other restorative material. Glass Ionomer cements or composite resins, used as permanent filling material do not affect the setting of MTA when placed over it. Residual calcium hydroxide may interfere with the adaptation of MTA to dentin thereby reducing its sealing ability either by acting as a mechanical obstacle or by chemically reacting with MTA. This may be important when calcium hydroxide is placed in the cavity in between the appointments prior to the placement of MTA. 8. Biocompatibility Kettering and Torabinejad studied MTA in detail and found that it is not mutagenic and is much less cytotoxic compared to Super EBA and IRM. This supports the superiority of MTA over formocresol as a pulpotomy medicament. Genotoxicity tests of

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cells after treatment of peripheral lymphocytes with MTA showed no DNA damage. On direct contact they produce minimal or no inflammatory reaction in soft tissues and in fact are capable of inducing tissue regeneration. 9. Tissue regeneration MTA is capable of activation of cementoblasts and production of cementum. It consistently allows for the overgrowth of cementum and also facilitates regeneration of the periodontal ligament. MTA allows bone healing and eliminates clinical symptoms in many cases. In animal studies, MTA produced cementum growth which was very unique compared to other root-end filling materials. Arens and Torabinejad reported osseous repair of furcation perforations treated with MTA. MTA showed good interaction with bone-forming cells. Investigations by Koh et al revealed that MTA offers a biologically active substrate for bone cells and stimulates interleukin production. MTA is also said to stimulate cytokine production in human osteoblasts. 10. Mineralization MTA, just like calcium hydroxide, induces dentin bridge formation and is believed to be due to its sealing property, biocompatibility, alkalinity and other associated properties. Tricalcium oxide in MTA reacts with tissue fluids to form calcium hydroxide, resulting in hard-tissue formation in a manner similar to that of calcium hydroxide cement. In comparison the dentin bridge formed with MTA is faster, thicker with good structural integrity and more complete than with calcium hydroxide. MTA also proves to be better at stimulating reparative dentin formation and maintaining the integrity of the pulp.

Storage Powder form MTA pouches must be kept tightly closed and stored in a dry area to avoid degradation by moisture. ProRoot MTA root repair material must be placed intraorally immediately after mixing with liquid, to prevent dehydration during setting. Excess water will retard curing process. Excess moisture in cotton pellets should be held to a minimum. The area should not be irrigated after placement of the material.

Placement technique Using a rubber dam, debride the root canal system using intracanal instruments, and irrigate with NaOCI. Dry the canal with paper points and isolate the perforation. Obturate all the canal space, apical to the perforation. The material is prepared according to the manufacturers instruction. Using the carrier, dispense the material into the perforation site. Condense the material into the perforation site using a small plugger, cotton pellets or paper points. Confirm placement material with a radiograph. If an adequate barrier has not been created, rinse the root repair material out of the canal and repeat the procedure. Following satisfactory obturation. Take a moist cotton pellet (remove excess moisture) and place in the canal. Seal the access preparation with a temporary restoration for a minimum of four hours. After four hours, or at a subsequent appointment, use a rubber dam and examine the MTA. This cement should be hard. If not, rinse and repeat the application. When the material is hardened, obturate the remaining canal space. The ProRoot MTA root repair material remains as a permanent part of the root canal filling.

Endodontic Solvents Endodontic treatment may not always be successful and failures may be seen on occasion. If retreatment is indicated, the old endodontic filling materials have to be removed to gain access to the canals. The various removal methods available are solvents, heat and mechanical instrumentation. Silver points are removed by grasping and pulling with a pair of pliers. Gutta-

238 Part 3 Endodontic Materials percha and resin-based obturating material removal is usually achieved by a combination of mechanical removal and chemical dissolution. However one must remember that the solvent must act on both the obturating material and sealer for effective cleansing of the canal of old material. Thus an endodontic solvent may be more effective for a particular material and not all. Materials often encountered in the canals include gutta-percha, resin-based sealers, silicone, zinc oxide eugenol and glass ionomer. Most solvents are cytotoxic and potentially carcinogenic and therefore has to be used with proper precautions. Commonly used solvents for gutta-percha and sealers are 1. 2. 3. 4. 5. 6.

Xylol or xylene Orange oil Chloroform Halothane Rectified turpentine Eucalyptol

Commercial names 1. For resin-based sealers: Endosolv R (septodont) 2. For zinc oxide based sealers: Endosolv E (septodont), DMS IV (Dentsply).

Xylol or xylene Xylene is a colorless, sweet smelling but flammable liquid. Xylene is a petrochemical product and a widely used industrial solvent. Chemically it is an aromatic hydrocarbon mixture consisting of a benzene ring with two methyl groups [dimethylbenzene or C6H4(CH3)2]. Xylene exists in three isomeric forms—ortho, meta and para. It is one of the most effective solvents for gutta-percha and resin-based sealers like resilon.

Chloroform Chloroform (trichloromethane - CHCl3) is a colorless, sweet-smelling, dense liquid and is considered hazardous (Fig. 13.24). It is an effective solvent for gutta-percha and resin based sealers like AH Plus. It is not effective for GIC based sealers like Ketac Endo. It has low solubility for CaOH based sealers (Apexit).

Figure 13.24 Endodontic solvents.

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Halothane Halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) is an inhalational general anesthetic. It is packaged in dark-colored bottles and contains 0.01% thymol as a stabilizing agent. It is an effective solvents for gutta-percha and resin-based sealers like AH Plus. It is not effective for GIC based sealers like Ketac Endo and ZOE sealers. It has low solubility for CaOH based sealers (Apexit).

Rectified Turpentine Spirit of turpentine (syn: oil of turpentine, wood turpentine) is a fluid obtained by the distillation of resin obtained from many trees other than the pines. It is an industrial solvent and paint thinner. Rectified turpentine is obtained by treating turpentine oil with sodium hydroxide, and redistilling. Medically it is used externally as a counterirritant. It is an effective solvents for gutta-percha especially if warmed to 70 °C.

Orange oil or d-limonene Orange oil is an essential oil extracted from the rind of an orange. It is composed of mostly (greater than 90%) d-limonene. D-limonene can be extracted from the oil by distillation. It was originally introduced as a general solvent and cleansing agent for the dental office. It is an effective solvent for zinc oxide eugenol sealers and an alternative solvent for thermoplastic gutta-percha. It has no effect on resin-based sealers like resilon.

Eucalyptol Eucalyptol (C10H18O) is a distillation product of eucalyptus oil. It is natural organic compound that is a colorless liquid with a fresh camphor-like smell and a spicy, cooling taste. Compared to the others it is a comparatively less effective solvent for gutta-percha. Its effect increases on heating.

Section-4

Impression Materials Chapter 14 Chapter 15 Chapter 16

Rigid Impression Materials, 243 – Impression compound and Zoe Paste – Zinc Oxide Eugenol Impression Paste Elastic Impression Materials, 258 – Agar – Alginate Elastomeric Impression Materials, 277 – Polysulfides – Silicone Impression Materials • Condensation Silicone • Addition Silicones – Polyether Rubber Impression

14 CHAPTER

Rigid Impression Materials—Impression Compound and ZOE Paste Chapter Outline • Advantages of Using Cast or • • • • •

Model Impression Material Desirable Properties Classification Rigid Impression Materials Impression Compound –– Properties –– Manipulation –– Removal of Impression –– Disinfection –– Pouring the Cast

–– Cast Separation –– Advantages –– Disadvantages

• Zinc Oxide Eugenol Paste –– Classification –– Available As –– Composition –– Setting Reaction –– Microstructure –– Setting Time –– Properties –– Manipulation

• • • •

–– –– –– –– ––

Impression Tray Disinfection Pouring the Impression Advantages Disadvantages Other Zinc Oxide Pastes Surgical Pastes (Periodontal Packs) Noneugenol Impression and Surgical Pastes Bite Registration Pastes

A dental impression is a negative record of the tissues of the mouth. It is used to reproduce the form of the teeth and surrounding tissues. The negative reproduction of the tissues given by the impression material is filled up with dental stone or other model materials to get a positive cast. The positive reproduction of a single tooth is described as a ‘die’, and when several teeth or a whole arch is reproduced, it is called a ‘cast’ or ‘model’. The application of dental impression compound has also decreased with the increased use of rubber impression materials, which can also be electroformed to produce metal dies. However, impression compound is useful for checking cavity preparations for undercuts and for making impressions of full crown.

ADVANTAGES OF USING A CAST OR MODEL 1. Models provide a three-dimensional view of the oral structures, thus aiding in diagnosis and treatment planning. 2. Many restorations or appliances are best constructed on casts. It may be inconvenient to both dentist and patient if these have to be made directly in the patient’s mouth. 3. Models can be used to educate the patient. 4. They serve as treatment records. 5. By using casts, technical work can be passed on to technicians, saving valuable clinical time.

244 PART 4 Impression Materials DESIRABLE PROPERTIES OF AN IMPRESSION MATERIAL 1. Should be nontoxic and nonirritant to dentist and patient. 2. Acceptable to the patient. a. Have a pleasant taste, odor, consistency and color. b. Should set quickly once placed in the mouth. 3. Should be accurate. a. Accurate surface detail. b. Elastic properties with freedom from permanent deformation after strain. c. Dimensionally stable. 4. Have adequate shelf life for storage and distribution. 5. Be economical. 6. Handling properties. a. Sufficient working time. b. Set quickly in mouth (saves chairside time). c. Be easy to use with the minimum equipment. d. Satisfactory consistency and texture. 7. Have adequate strength so that it will not break or tear while removing from the mouth. 8. Should be compatible with the die and cast materials. 9. Should be able to be electroplated.

CLASSIFICATION OF IMPRESSION MATERIALS There are several ways of classifying impression materials. 1. According to mode of setting and elasticity. 2. According to tissue displacement during impression procedure. 3. According to their uses in dentistry.

According to mode of setting and elasticity The terms thermoset, thermoplastic, rigid and elastic are used to describe these materials (Table 14.1).

According to tissue displacement Depending on whether tissues are displaced while making impressions a material may be 1. Mucostatic 2. Mucocompressive (Mucodisplacive)

TABLE 14.1 Classification of impression materials according to mode of setting and elasticity Mode of setting

Rigid

Elastic

Set by chemical reaction (irreversible Impression plaster Zinc oxide eugenol Alginate hydrocolloid Nonaqueous or thermoset) elastomers -e.g. polysulfide, silicone Set by temperature change (reversible/ Compound, Waxes thermoplastic)

Agar hydrocolloid

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Mucostatic materials produce minimal displacement of the tissue during impression, e.g. plaster, zinc oxide eugenol, low viscosity alginates, low viscosity elastomeric materials, etc. Mucocompressive materials are more viscous and displace the tissues while recording them, e.g. compound, high viscosity alginates, high viscosity elastomers, etc.

According to their uses in dentistry Impression materials used for complete denture prosthesis Impression plaster, impression compound and impression paste set to a hard rigid mass, and hence cannot be removed from undercuts without the impression being fractured or distorted. Therefore these materials are best suited for edentulous mouth. Impression materials used for dentulous mouths On the other hand alginates and rubber base impressions are sufficiently elastic to be withdrawn from undercut areas. Such elastic impression materials are suitable for impressions for fabrication of removable and fixed partial denture prostheses, where the impressions of the ridge and teeth are required.

RIGID IMPRESSION MATERIALS As mentioned earlier the rigid impression materials are 1. Impression plaster 2. Impression compound 3. Zinc oxide eugenol impression paste 4. Impression waxes (Impression plaster is described in the chapter on Gypsum Products).

IMPRESSION COMPOUND Impression compound is one of the oldest of the dental impression materials. It can be described as a rigid, reversible impression material which sets by physical change. On applying heat, it softens and on cooling it hardens. It is mainly used for making impressions of edentulous ridges. A more viscous variety of compound called tray compound is used to form a tray in which a second more fluid material is placed to make a more detailed impression compound. Synonyms Modeling compound or modeling plastic.

CLASSIFICATION Type I - Impression compound Type II - Tray compound Type II Tray compound is used to prepare a tray for making an impression. A second material is then carried in it in order to make an impression of oral tissues. Since reproduction of the fine details is not essential, it is generally stiffer and has less flow than regular impression compound. The use of dental tray compound decreased with the increased substitution of acrylic tray materials.

SUPPLIED AS Supplied as sheets, sticks, cakes and cones in a variety of colors (Fig. 14.1).

246 PART 4 Impression Materials

FIGURE 14.1 Impression compound cakes and sticks.

APPLICATIONS 1. For making a preliminary impression in an edentulous mouth (mouth without teeth). 2. For impressions of full crown preparations where gingival tissues must be displaced. 3. Peripheral tracing or border molding.

   Type I  

4. To check undercuts in inlay preparation. 5. To make a special tray.

  Type II 

Single tooth impression In conservative dentistry, an impression is made of a single tooth in which a cavity is prepared. The compound is softened and carried in a copper band. The filled band is pressed over the tooth and the compound flows into the prepared cavity. It is referred to as a tube impression. Tube impressions were also used to make electroformed dies.

Complete denture impressions In complete denture fabrication, it is common to make two sets of impressions—the preliminary and the final impression. The preliminary impression is made in a stock tray. A study cast made from this is used to construct a custom tray or special tray. The custom tray is used to make the final impression. The technique of making a preliminary and final impression greatly improves the accuracy of the complete denture.

REQUIREMENTS OF IMPRESSION COMPOUND An ideal impression material should 1. Harden at or little above mouth temperature. 2. Be plastic at a temperature not injurious or harmful to oral tissues.

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Not contain irritating or toxic ingredients. Harden uniformly when cooled without distortion. Have a consistency when softened which will allow it to reproduce fine details. Be cohesive but not adhesive. Not undergo permanent deformation or fracture while withdrawing the impression from the mouth. Be dimensionally stable after removal from the mouth and during storage. Exhibit a smooth glossy surface after flaming. Withstand trimming with sharp knife without flaking or chipping after hardening. Should not boil and lose volatile components on flaming. Should remain stable without losing soluble plasticizers when immersed in water for long periods.

COMPOSITION In general impression compound is a mixture of waxes, thermoplastic resins, organic acids, fillers and coloring agents. Ingredient

Parts

Resin

Copal resin

Carnauba wax

Stearic acid Talc

5 25

Coloring agent (e.g. rouge)

Plasticizers Compounds, such as shellac, stearic acid and gutta-percha are added to improve plasticity and workability. These substances are referred to as plasticizers. Synthetic resins are being used in increasing amounts. Waxes and resin give the material its characteristic thermoplastic properties. Fillers These are small particles of inert materials which strengthen or improve the physical properties of many materials. Fillers are chemically different from the principal ingredient. In such a case the filler particles are sometimes referred to as the core and the surrounding ingredients as the matrix. For example, the waxes and resins in impression compound impart high flow and low strength. Consequently, a filler such as talc is added to reduce the plasticity and increase strength of the matrix material. Other fillers used are diatomaceous earth, soap stone and French chalk.

PROPERTIES OF IMPRESSION COMPOUND FUSION TEMPERATURE When impression compound is heated in a hot water bath the material starts to soften at approximately 39 °C. However at this stage, it is still not plastic or soft enough for making an impression. This temperature at which the material looses its hardness or brittleness on heating or forms a rigid mass upon cooling is referred to as fusion temperature. Impression compound exhibits a fusion temperature range rather than a fixed point. On continued heating above 43.5 °C, the material continues to soften and flow to a plastic mass that can be manipulated. Thus all impressions with compound should be made above this temperature. Below this temperature an accurate impression cannot be expected.

248 PART 4 Impression Materials THERMAL PROPERTIES Thermal conductivity Impression compound has very low thermal conductivity, i.e. they are poor conductors of heat. Significance 1. During softening of the material, the outside will soften first and the inside last. So to ensure uniform softening the material should be kept immersed for a sufficient period in a water bath. Kneading of the material ensures further uniform softening. 2. The low thermal conductivity affects the cooling rate. The layer adjacent to the oral tissues cools faster than the inside. Removal of the impression at this stage can cause serious distortion. Thus it is important to wait for the compound to cool thoroughly before removing it from the mouth.

Coefficient of thermal expansion (CTE) The CTE of compound is comparatively high due to the presence of resins and waxes. The linear contraction from mouth temperature to room temperature is 0.3%. Errors from thermal distortion can be reduced 1. By obtaining an impression and then passing the impression over a flame until the surface is softened and then obtaining a second impression. During the second impression, the shrinkage is relatively lower, since only the surface layer has been softened. 2. Another way of reducing the thermal contraction is by spraying cold water on the metal tray just before it is inserted in the mouth. Thus the material adjacent to the tray will be hardened, while the surface layer is still soft. In both techniques, the impression is likely to be stressed considerably and so the stone cast should be constructed at the earliest.

FLOW Good flow is desirable during impression making. The softened material should flow into all the details of the tissue contour. Once the compound hardens, it should have minimum flow, otherwise it will get distorted.

Dimensional stability Since the release of strains is unavoidable, the safest way to prevent distortion is to pour the cast immediately or at least within the hour. Another cause of warpage is removal of the impression too early from the mouth before complete hardening.

Detail reproduction Surface detail reproduction is comparatively less because of its high viscosity and low flow. Because of the viscosity, pressure has to be used during impression, which compresses or distorts the tissues. Thus the tissues are recorded in a distorted state.

MANIPULATION STICKS Small amounts of compound (stick compound) can be softened over a flame (Fig. 14.2). When a direct flame is used, the compound should not be allowed to boil or ignite, otherwise, the plasticizers are volatilized.

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CAKES Larger amounts of compound are softened in warm water in a thermostatically controlled water bath (Fig. 14.3) usually in the range of 65 to 75 °C. After the compound is removed from the water bath, it is usually kneaded with the fingers in order to obtain uniform plasticity throughout the mass.

LOADING THE TRAY A slightly oversized tray is selected. The softened material is loaded onto the tray and quickly seated on to the tissues to be recorded (Figs. 14.4 to 14.6). Any delay can cause the impression to harden prematurely. If the compound is too hot, it may be tempered by briefly immersing in slightly cooler water. The lips are manipulated to mold the borders of the impression while it is still soft.

Precautions 1. Prolonged immersion in a water bath causes the compound to become brittle and grainy because some of the ingredients may be leached out. 2. Overheating in water makes the compound sticky and difficult to handle. 3. Avoid incorporating water while kneading.

REMOVAL OF IMPRESSION FROM THE MOUTH The impression is removed from the mouth only after it has completely cooled and hardened.

FIGURE 14.2 Manipulation of stick compound for border molding of a custom tray.

FIGURE 14.3 A thermostatically controlled water bath. This water bath maintains a steady softening temperature and is ideal for softening impression compound.

FIGURE 14.4 A slightly oversized stock metal tray.

250 PART 4 Impression Materials

FIGURE 14.5 Placing the material in the tray is known as loading the tray.

FIGURE 14.6 Positioning the loaded tray over the ridges for the impression.

DISINFECTION The recommended disinfectant is 2% glutaraldehyde.

POURING THE CAST AND CAST SEPARATION The cast should be poured without delay. The cast is separated from the impression by immersing it in warm water until it is soft enough. Excessively hot water is avoided as it can make the material sticky and difficult to remove from the cast.

ADVANTAGES AND DISADVANTAGES ADVANTAGES 1. 2. 3. 4.

The material can be reused a number of times (for the same patient only) in case of errors. Inaccurate portions can be remade without having to remake the entire impression. Accuracy can be improved by flaming the surface. The material has sufficient body to support itself especially in the peripheral portions. It does not collapse completely if unsupported by the tray.

DISADVANTAGES 1. 2. 3. 4. 5.

Records less detail because of its high viscosity. Compresses soft tissues during impression. Distortion due to its poor dimensional stability. Difficult to remove if there are severe undercuts. There is always the possibility of overextension especially in the peripheries.

ZINC OXIDE EUGENOL IMPRESSION PASTE Zinc oxide and eugenol based products are widely used in dentistry. 1. Cementing and insulating medium. 2. Temporary filling material. 3. Root canal filling material.

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4. Surgical pack in periodontal surgical procedures. 5. Bite registration paste. 6. Temporary relining material for dentures. 7. Impressions for edentulous patients (Fig. 14.7). In dentistry, zinc oxide eugenol is popular as an impression material for making impressions of edentulous arches for the construction of complete dentures. It is classified as a rigid, irreversible impression material. It cannot be used for recording impressions of dentate arches and in areas of severe undercuts.

CLASSIFICATION ADA specification No. 16.

Type I or Hard Type II or Soft

AVAILABLE AS In paste form in two tubes (Fig. 14.8)

Base paste (white in color) Accelerator or reactor or catalyst paste (red in color)

FIGURE 14.7 Impressions of the upper (right) and lower (left) edentulous arches made with zinc oxide eugenol impression paste in custom trays.

FIGURE 14.8 DPI (India) and SS white (USA) are examples of two commercially available zinc oxide eugenol impression pastes (Courtesy: KDC, Kannur).

252 PART 4 Impression Materials COMPOSITION Base Paste

Accelerator paste

Ingredient

Wt. %

Ingredient

Wt. %

Zinc oxide

87%

Oil of cloves or eugenol

12%

Vegetable or mineral oil

13%

Gum or polymerized rosin

50%

Filler (Silica type)

20%

Lanolin

Resinous balsam

Calcium chloride and color

Zinc oxide should be finely divided and should contain slight amount of water. Fixed vegetable or mineral oil acts as plasticizer and also aids in masking the action of eugenol as an irritant. Oil of cloves contains 70–85% eugenol. It is sometimes used in preference to eugenol because it reduces burning sensation. Gum or polymerized rosin speeds the reaction and improves hom*ogeneity. Canada and Peru balsam improves flow and mixing properties. Calcium chloride acts as an accelerator of setting reaction. Other accelerators are

1. Zinc acetate

2. Primary alcohols

3. Glacial acetic acid

SETTING REACTION The setting reaction is a typical acid-base reaction to form a chelate. This reaction is also known as chelation and the product is called zinc eugenolate. 1. ZnO

H2O

2. Zn(OH)2 + 2HE (Base)

Zn(OH)2 ZnE2

(Acid)

(Salt)

+ 2H2O + (Water)

(Eugenol) (Zinc eugenolate)

MICROSTRUCTURE The chelate (zinc eugenolate) forms a matrix surrounding a core of zinc oxide particles. The chelate is thought to form as an amorphous gel that tends to crystallize giving strength to the set mass. Formation of crystalline zinc eugenolate is greatly enhanced by zinc acetate dehydrate (accelerator) which is more soluble than Zn(OH)2 and can supply zinc ions more rapidly.

SETTING TIME Working time There should be sufficient time for mixing, loading onto the tray and seating the impression into the mouth.

Setting time Once the material is in place, it should set fast.

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Why should an impression material set quickly in the mouth? Any material which takes a long time to set in the mouth.

Would obviously be uncomfortable to the patient. Movement is bound to occur, resulting in stresses and errors in the impression. Result in a wastage of time for the dentist. In a busy practice, this is unacceptable.

Initial setting time is the period from the beginning of the mixing until the material ceases to pull away or string out when its surface is touched with a metal rod of specified dimensions. The impressions should be seated in the mouth before the initial set. The final set occurs when a needle of specified dimension fails to penetrate the surface of the specimen more than 0.2 mm under a load of 50 gm. Initial setting time

Final setting time

Type I

3–6 minutes

10 minutes

Type II

3–6 minutes

15 minutes

Factors controlling setting time 1. Particle size of zinc oxide powder If the particle size is small and if it is acid coated, the setting time is less. 2. By varying the lengths of the two pastes (not recommended). 3. Setting time can be decreased by adding zinc acetate or a drop of water or acetic acid (acetic acid is a more effective than water. It increases speed of formation of the zinc hydroxide). 4. Longer the mixing time, shorter is the setting time. 5. High atmospheric temperature and humidity accelerate setting. 6. Setting can be delayed by cooling the mixing slab, spatula or adding small amounts of retarder or oils or waxes.

PROPERTIES Consistency and flow These are clinically important properties. A paste of thick consistency can compress the tissues. A thin free flowing material copies the tissues without distorting them. According to ADA specification No. 16, the spread is

Type I pastes — 30 to 50 mm Type II pastes — 20 to 45 mm

Clinically, these materials have a very good flow. Poor quality impression pastes, thicken unduly and have a poor flow.

Detail reproduction It registers surface details quite accurately due to the good flow.

Rigidity and strength The impression should resist distortion and fracture when removed from the mouth after setting. The compressive strength of hardened ZOE is approximately 7 MPa two hours after mixing.

Dimensional stability The dimensional stability is quite satisfactory. A negligible shrinkage (less than 0.1%) may occur during hardening.

254 PART 4 Impression Materials

FIGURE 14.9 Proper dispensing is an important aspect of the manipulation of materials supplied in tubes. For zinc oxide eugenol both the ropes should be of equal length and width in order to ensure correct proportioning. One way of obtaining this is by ensuring the extruded paste has a uniform width and length.

FIGURES 14.10A AND B Manipulation of zinc oxide eugenol paste. (A) Equal lengths of base and reactor pastes are dispensed. (B) Mixing is done with a stainless steel spatula using circular motions until a streak free mix is obtained (Manufacturers usually provide such materials in contrasting colors to aid in visually ascertaining completion of mix).

Biological considerations Some patients experience a burning sensation in the mouth due to eugenol. It can also cause tissue irritation. Non-eugenol pastes can be substituted.

MANIPULATION The mixing is done on an oil-impervious paper or glass slab. Two ropes of paste of same length and width, one from each tube are squeezed onto the mixing slab (Fig. 14.9). A flexible stainless steel spatula is used. The two ropes are collected with the spatula and mixed until a uniform color is observed (Figs. 14.10A and B). Mixing time 1 minute. Mechanical mixing A rotary mixing device can also be used (Fig. 14.11). Special circular mixing pads are attached to the circular table of the device. After dispensing the material, the machine is switched on. As the table rotates, the operator first collects the material using the sides of the spatula. He then spreads the material by flattening the spatula. The process of collecting and flattening is repeated alternately until a uniform mix is obtained. Mechanical mixing gives a faster, uniform mix with less voids and bubbles.

FIGURE 14.11 Mechanical mixer.

IMPRESSION TRAY Custom impression tray made of stable resin is recommended for zinc oxide eugenol. The material adheres to the tray so no special adhesive is required. A primary compound impression can also be used as a tray. The material is loaded into the tray by swiping on to the sides of

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the tray and then spread in a smooth uniform motion. Loading and spreading through a patting motion can trap air.

DISINFECTION The impression is rinsed and placed in disinfectant solution. Rinsing removes saliva and other contaminants. The recommended disinfectant solution is 2% glutaraldehyde solution. Glutaraldehyde is an organic compound with the formula CH2(CH2CHO)2. A pungent colorless oily liquid, glutaraldehyde is used to sterilize medical and dental equipment.

POURING THE IMPRESSION As with most impression materials the pouring of the cast should not be delayed for too long. After setting, the impression is removed off the cast after softening it through immersion in hot water.

ADVANTAGES AND DISADVANTAGES ADVANTAGES It has sufficient body so as to make-up for any minor under extensions in the tray itself during impression making. 1. 2. 3. 4. 5. 6.

It has enough working time to complete border molding. It can be checked in the mouth repeatedly without deforming. It registers accurate surface details. It is dimensionally stable. Does not require separating media since it does not stick to the cast material. Minor defects can be corrected locally without discarding a good impression.

DISADVANTAGES 1. 2. 3. 4.

It requires a special tray for impression making. It is sticky in nature and adheres to tissues. Eugenol can cause burning sensation and tissue irritation. It cannot be used for making impression of teeth and undercut areas as it is inelastic in nature.

OTHER ZINC OXIDE PASTES SURGICAL PASTES (PERIODONTAL PACKS) After certain periodontal surgeries (e.g. gingivectomy, i.e. surgical removal of diseased gingival tissues) where sutures cannot be placed, a zinc oxide-based surgical paste (Figs. 14.12A and B) may be placed over the wound to aid in the retention of the medicament, to protect the wound and to promote healing (also known as periodontal pack). Earlier pastes were eugenol based and have been around since 1923 (Ward’s Wondrpak). Current surgical pastes avoid eugenol because of the potential of tissue irritation. These are called noneugenol pastes.

NONEUGENOL IMPRESSION AND SURGICAL PASTES The chief disadvantage of zinc oxide eugenol paste is the burning sensation caused by eugenol. Some patients find the taste of eugenol disagreeable and in cases where the surgical pack

256 PART 4 Impression Materials

FIGURES 14.12A AND B (A) Periodontal dressing. (B) Coe-Pak is a popular brand of periodontal dressing material.

is worn for several weeks chronic gastric disturbance may result. Hence, noneugenol pastes were developed. Noneugenol pastes consist of a base and reactor paste. The base paste contains ZnO, gum and lorothidol (fungicide). The reactor pastes contains coconut fatty acids, rosin (thickening), chlorothymol (bacteriostatic), etc. Antibiotics like tetracycline may be incorporated at the time of mixing.

Setting reaction Zinc oxide is reacted with a carboxylic acid. ZnO + 2RCOOH

(RCOO)2 Zn + H2O

The reaction is not greatly affected by temperature or humidity. Compared to impression pastes the surgical pastes are less brittle and weaker after hardening. The setting time is longer (around 15 minutes). They are available as a 2 paste system. The paste is mixed and formed into a rope that is packed over the gingival wounds (using wet fingers) and into the interproximal spaces to provide retention. The final product after setting should be sufficiently strong so that it is not readily displaced during mastication. Light cured periodontal dressing materials and single component pastes (that set by heat and moisture in the mouth are also available). An automixing cartridge version of Coe-Pak is also available (Fig. 14.13).

FIGURE 14.13 Cartridge dispensed static mixing version of Coe-Pak is also available.

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FIGURES 14.14A AND B (A) Bite registration paste. (B) Bite registration paste used for making a facebow transfer.

BITE REGISTRATION PASTES These are materials used for recording the occlusal relationship between two occluding surfaces, e.g. teeth, occlusion rims, etc. ZOE pastes (Figs. 14.14A and B) used for this purpose have slightly different properties Shorter setting time to prevent distortion. More plasticizers to prevent it from sticking to the teeth or occlusion rims. Other bite registration materials include wax and silicones. ZOE registrations are more rigid than registrations made in wax or silicones. They are more stable, and offer less resistance to the closing of the jaw than wax. Resistance free closure is often indicated in complete dentures where denture base movement or tissue displacement occurring from closure is not desired.

15 Chapter

Elastic Impression Materials—Agar and Alginate Chapter Outline • Hydrocolloids • Types of Hydrocolloids • Reversible Hydrocolloids—

Agar –– Classification (ISO 21563:2013) –– Uses –– Composition –– Functions of the Ingredients –– Gelation or Setting of Agar –– Manipulation –– The Hydrocolloid Conditioner –– Impression Trays –– Making the Impression –– Working and Setting Time –– Removal of Impression

–– Storage of Agar Impression –– Separation from Cast –– Properties of Agar Hydrocolloids

–– Gelation, Liquefaction and Hysteresis

–– Syneresis and Imbibition –– Laminate Technique

(Agar–Alginate Combination Technique) –– Advantages –– Wet Field Technique • Cast Duplication • Impression Disinfection • Advantages and Disadvantages of Agar Hydrocolloid

• IrreversibleHydrocolloid—

Alginate –– Types –– Supplied As –– Modified Alginates –– Applications –– Composition –– Setting Reaction –– Properties of Alginate –– Shelf Life and Storage –– Manipulation –– Impression Disinfection –– Storage of Alginate Impression –– Construction of Cast • Advantages and disadvantages • Improper Use of Alginates

The rigid impression materials described previously are best suited for recording edentulous areas. Teeth or severe undercuts if present, would make the removal of such impressions difficult. The impression could distort or fracture on removal. The ideal impression material for accurately reproducing tooth form and relationship would be an elastic substance which can be withdrawn from the undercut area and return to its original form without any distortion. By definition, an elastic impression material is one that can transform from a semisolid, nonelastic state to a highly elastic solid state.

TYPES OF ELASTIC IMPRESSION MATERIALS Two systems are used 1. Hydrocolloids 2. Elastomeric materials

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HYDROCOLLOIDS Solution and Suspension In a solution (e.g. sugar in water) one substance, usually a solid is dispersed in another, usually a liquid and the two phases are microscopically indistinguishable. Thus, a solution exists as a single phase because there is no separation between the solute and the solvent. A suspension on the other hand, consists of larger particles that can be seen under a microscope or even by the naked eye, dispersed in a medium. Similarly, liquid distributed in liquids are emulsions. Suspensions and emulsions are two phase systems.

Colloids They are often classed as the fourth state of matter known as colloidal state. A colloid is a two-phase system. The ‘colloidal solution’ or ‘colloidal sol’ is somewhere between the smaller molecules of a solution and the larger particles of a suspension. The two phases of the colloidal sol are

Dispersed phase or dispersed particle (the suspended particle). Dispersion phase or medium (the substance in which it is suspended).

Types of colloids Colloidal sols may be

Liquid or solid in air (Aerosol) Gas, liquid or solid in liquid (Lyosol) Gas, liquid or solid in solid.

Hydrocolloids They consist of gelatin particles suspended in water (Lyosol). Since water is the dispersion medium it is known as hydrocolloid. The particles are larger than those in solutions and size ranges from 1 to 200 nanometers (1 nm = 10–9 m). There is no clear demarcation between solutions, colloids, and suspensions (emulsions).

Gels, Sols, Gelation Colloids with a liquid as the dispersion medium can exist in two different forms known as ‘sol’ and ‘gel’. A sol has the appearance and many characteristics of a viscous liquid. A gel is a jelly like elastic semisolid and is produced from a sol by a process called gelation by the formation of fibrils or chains or micelles of the dispersed phase which become interlocked. Gelation is thus the conversion of a sol to gel. The dispersion medium is held in the interstices between the fibrils by capillary attraction or adhesion. Gelation may be brought about in one of the two ways 1. Lowering the temperature, e.g. agar. 2. By a chemical reaction, e.g. alginate.

Gel strength The gel strength depends on 1. Density of the fibrillar structure Greater the concentration, greater will be the number of micelles and hence greater the brush heap density.

260 Part 4 Impression Materials 2. Filler particles trapped in the fibrillar network. Their size, shape and density determine their effectiveness. Fillers also increases the viscosity of the sol. 3. In reversible hydrocolloids, the lower the temperature, the greater is the strength, as gelation is more complete. 4. Types of hydrocolloids. Based on the mode of gelation, they are classified as

Reversible hydrocolloids They are called reversible because their physical state can be reversed. This makes them reusable. Irreversible hydrocolloids Once these set, it is usually permanent, and so are known as irreversible.

Reversible hydrocolloids—agar In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type of impression material. It is said that Poller’s objective was to develop a material that could be sterilized and applied without pressure to the exposed surface of the dura mater for perfectly recording its convulsion and the bony margins of the skull. Later Poller’s Negacol was modified and introduced to the dental profession as Dentacol in 1928. Agar hydrocolloid was the first successful elastic impression material to be used in dentistry. It is an organic hydrophilic colloid (polysaccharide) extracted from a type of seaweed (Gelidium, Gracilaria, etc. Box 15.1 and Fig. 15.1). China and South America are major sources of farmed seaweed. Agar is a sulfuric ester of a linear polymer of galactose. Although it is an excellent impression material and yields accurate impressions, presently it has been largely replaced by alginate hydrocolloid and rubber impression materials.

Classification BASED ON VISCOSITY (ISO 21563:2013) Type 1 — Heavy bodied (for use as tray material) Type 2 — Medium bodied (for use as tray or syringe material) Type 3 — Light bodied (for syringe use only) Type 3A — Light bodied for agar-alginate combination technique

USES 1. Widely used at present for cast duplication (e.g. during the fabrication of cast metal removable partial dentures, etc.). Box 15.1 AGAR AGAR Agar has been used for centuries in Asia where it is called ‘kanten’ by the Japanese and ‘dongfen’ by the Chinese. It was brought to Malaysia by Chinese immigrants where it came to be known as agar. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture media for microbiological work. Agar (agar-agar) can be used as a laxative, an appetite suppressant, a vegetarian substitute for gelatin, a thickener for soups, in fruit preserves, ice cream, and other desserts, as a clarifying agent in brewing, and for sizing paper and fabrics Figure 15.1 Gelidium seaweed.

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Figure 15.2 Agar impression syringe and syringe material.

2. For full mouth impressions without deep undercuts. 3. It was used extensively for FPD impressions prior to elastomers. 4. As a tissue conditioner.

SUPPLIED AS Gel in collapsible tubes (for impressions). As cartridges or gel sticks (syringe material, Fig. 15.2). In bulk containers (for duplication, Figs. 15.3A and B). Commercial names Syringe materials include— herculloid, cartriloids (Van R), etc. Duplicating materials include Wirogel (Bego), Dubliform (Dentaurum).

COMPOSITION

B Figures 15.3A and B (A) Agar duplication gel samples. (B) Bulk packing.

Ingredient

Wt. %

Agar

13–17%

Borates

0.2–0.5%

Potassium sulphate

1–2%

Wax, hard

0.5–1%

Thixotropic materials

0.3–0.5%

Alkylbenzoates

0.1 %

Coloring and flavoring agents

Traces

Water

Balance (around 84%)

Functions of the Ingredients Agar Basic constituent 13–17% for tray material 6–8% for syringe material. Borates Improves the strength of the gel (it also retards the setting of plaster or stone cast when poured into the finished impression—a disadvantage). Potassium sulfate It counters retarding effect of borates, thereby ensures proper setting of the cast or die. Hard wax It acts as a filler. Fillers affect the strength, viscosity and rigidity of the gel. Other fillers are zinc oxide, diatomaceous earth, silica, rubber, etc.

262 Part 4 Impression Materials Thixotropic materials It acts as plasticizer. Examples are glycerine, and thymol. Thymol acts as bactericide also. Alkylbenzoates It acts as preservative. Coloring and flavoring For patient comfort and acceptance. Water It acts as the dispersion medium.

Gelation or setting of agar Agar changes from the sol to the gel state (and vice versa) by a physical process. As the agar sol cools the dispersed phase groups to form fibrils called micelles. The fibrils branch and intermesh together to form a brush-heap structure. The fibrils form weak covalent bonds with each other which break easily at higher temperatures resulting in gel turning to sol. The process of converting gel to sol is known as liquefaction which occurs at a temperature between 70 and 100 °C. On cooling agar reverses to the gel state and the process is called gelation. Gelation occurs at or near mouth temperature which is necessary to avoid injury to oral tissues. The gelling property of agar-agar is due to the three equatorial hydrogen atoms on the 3,6-anhydro-L-galactose residues, which constrain the molecule to form a helix. The interaction of the helixes causes the formation of the gel.

MANIPULATION The equipment and material required for an agar impression are

Hydrocolloid conditioner (Fig. 15.4F) Water cooled rim lock trays (Figs. 15.4E and 15.5) Impression syringes (Figs. 15.3 and 15.4C) Connecting water hose (Fig. 15.4D) Agar tray material in tubes (Fig. 15.4B) Agar syringe material (Figs. 15.2 and 15.4A)

Figures 15.4A to F Typical equipment and material for an agar impression procedure.

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Hydrocolloid conditioner 46 °C for about two minutes with the material loaded in the tray. This reduces the temperature so that it is tolerated by the sensitive oral tissues. It also makes the material viscous. Boiling section or Liquefaction section

Ten minutes in boiling water (100 °C). The sol should be hom*ogeneous and free of lumps. Every time the material is liquefied, three minutes should be added. After every use the agar brush heap structure gets more difficult to break.

Storage section

65–68 °C temperature is ideal. It can be stored in the sol condition.

Tempering section

46 °C for about two minutes with the material loaded in the tray. This reduces the temperature so that it is tolerated by the sensitive oral tissues. It also makes the material viscous.

Impression Trays Rim lock trays with water circulating devices are used. The rim lock is a beading on the inside edge of the tray border which helps to retain the material (as agar does not adhere to the tray). It also has an inlet and outlet for connecting the water tubes (Fig. 15.5). The tray should allow a space of 3 mm occlusally and laterally and extend distally to cover all teeth.

Figure 15.5 A water cooled tray.

Making the Impression The tray containing the tempered material is removed from the bath. The outer surface of the agar sol is scraped off, then the water hoses are connected, and the tray is positioned in the mouth by the dentist. Water is circulated at 18–21 °C through the tray until gelation occurs. Rapid cooling (e.g. ice cold water) is not recommended as it can induce distortion. To guide the tray into position, three stops of compound are prepared on non-involved teeth. A post dam is constructed with compound to prevent distal flow of the impression material. In a deep palate case, compound is placed on the palatal aspect of the tray in order to provide a uniform thickness of the hydrocolloid. The mandibular tray is prepared by placing compound on the distal aspect to limit the impression material. Black tray compound is used as it is not affected in the tempering bath.

Working and Setting Time The working time ranges between 7 minutes and 15 minutes and the setting time is about 5 minutes. Both can be controlled by regulating the flow of water through the cooling tubes. Since the cooling tubes are on the periphery, the material sets from the periphery towards the teeth surfaces.

Removal of Impression When the agar has gelled, the peripheral seal is broken, and the impression is removed from the mouth rapidly. The impression is rinsed thoroughly with water and the excess water is removed by shaking the impression.

Storage of Agar Impression Storage of agar impression is to be avoided at all costs. The cast should be poured immediately. Storage in air results in dehydration, and storage in water results in swelling of the impression.

264 Part 4 Impression Materials Storage in 100% relative humidity results in shrinkage as a result of continued formation of the agar network agglomeration. If storage is unavoidable, it should be limited to one hour in 100% relative humidity.

Separation from Cast When the gypsum product has set, the agar impression must be removed promptly since the impression will dehydrate, become stiff and difficult to remove. Weaker portions of the model may fracture. In addition, prolonged contact will result in a rougher surface on the model.

PROPERTIES OF AGAR HYDROCOLLOIDS The ISO 21563 (2013) sets the standard for properties required of agar–hydrocolloid impression materials.

Gelation, liquefaction and hysteresis Most materials melt as well as resolidify at the same temperature. However, in agar, this does not coincide. Gelation (solidification) occurs at 37 °C approximately, whereas liquefaction (melting) occurs at a higher temperature, i.e. 60–70 °C higher than the gelation temperature. This temperature lag between liquefaction and gelation is known as hysteresis.

Syneresis and imbibition (dimensional stability) Since hydrocolloids use water as the dispersion medium, they are prone for dimensional change due to either loss or gain of water. If left in a dry atmosphere, water is lost by syneresis and evaporation, and if it is immersed in water, it absorbs water by a process known as imbibition. The exuding of fluid from the gel is known as syneresis. Some of the more soluble constituents are also lost. During syneresis small droplets of exudate are formed on the surface of the hydrocolloid and the process occurs irrespective of the humidity of the surrounding atmosphere. Agar exhibits the properties of syneresis and imbibition. However, when immersed in water, they do not imbibe more than original content which was lost by evaporation (unlike alginates). Importance Syneresis and imbibition can result in dimensional changes and therefore inaccurate casts. To avoid this hydrocolloid impressions should be poured immediately.

Flexibility ISO 21532:2013 requires flexibility ranging between 4% and 15%, when a stress of 12.2 N is applied. A few set materials, however, have a flexibility of 20%. On an average a flexibility of 11% is desirable.

Elasticity and elastic recovery They are highly elastic, and elastic recovery occurs to the extent of 98.8% (ISO 21532:2013 min. 96.5 %).

Gel strength including tear and compressive strengths The gel can withstand great stresses particularly shear stress, without flow, provided the stress is applied rapidly. Thus, the impression should be removed as rapidly as possible in order to avoid distortion. Agar has a tear strength of 0.8–0.9 kN/m and compressive strength of 0.5–0.9 g/cm2. (ISO 21532:2013, minimum tear strength for Type 1 and 2 is 0.75 N/mm; for Type 3 is 0.50 N/mm.) The above values are for tray materials. The syringe materials have poorer mechanical properties.

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Factors affecting strength 1. The composition—agar concentration, borate and filler content, etc. 2. The temperature—the lower the temperature the greater the strength.

Flow The material is sufficiently fluid to record the fine details if correctly manipulated.

Reproduction of detail A reproduction of a groove of 25 µm (micrometers) is achievable with agar.

Accuracy and dimensional change Some contraction takes place during gelation. If the material is retained well in the tray, the material contracts towards the tray resulting in larger dies. Agar impressions are highly accurate at the time of removal from the mouth, but shrink when stored in air or 100% relative humidity and expand when stored in water. The least dimensional change occurs when the impressions are stored in 100% humidity (for not more than one hour). However, prompt pouring of plaster or stone models is recommended.

LAMINATE TECHNIQUE (AGAR–ALGINATE COMBINATION TECHNIQUE) After injecting the syringe agar on to the area to be recorded, an impression tray containing a mix of chilled alginate that will bond with the agar is positioned over it. The alginate gels by a chemical reaction, whereas the agar gels through contact with the cool alginate (Fig. 15.6), rather than the water circulating through the tray.

Advantages 1. 2. 3. 4.

The syringe agar gives better details than alginate. Figure 15.6 Agar–alginate combination or laminate technique. Less air bubbles. Water cooled trays are not required and therefore more convenient. It sets faster than the regular agar technique.

WET FIELD TECHNIQUE In this technique the areas to be recorded are actually flooded with warm water. Then the syringe material is introduced quickly, liberally, and in bulk to cover the occlusal and/or incisal areas only. While the syringe material is still liquid, the tray material is seated. The hydraulic pressure of the viscous tray materials forces the fluid syringe hydrocolloid down into the areas to be recorded. This motion displaces the syringe materials as well as blood and debris throughout the sulcus.

CAST DUPLICATION With the introduction of alginate, agar slowly lost its appeal as an impression material. However, it is still popular as a duplicating material primarily because

When liquefied it flows readily (like a fluid) over the cast to be duplicated. This makes it an ideal mould material. Large quantities can be prepared relatively easily. It is economical, because it can be reused.

266 Part 4 Impression Materials

Figures 15.7A and E (A) The agar hydrocolloid duplicating machine liquefies the agar using heat. Rotating blades in the machine further break up the agar. (B) Duplicating flask (left) with cast inside (right). (C) Liquefied agar is poured into the duplicating flask virtually surrounding the cast. (D) The completed mould. (E) Investment material is poured into the agar mould to create a duplicate in refractory material.

In the construction of cast removable partial dentures (RPD) the relieved and blocked master cast is duplicated in investment material. This is known as a refractory cast. The master cast to be duplicated is placed in a duplicating flask or mould former (Fig. 15.7C). The agar is broken into small chunks and loaded into the liquefying machine (Fig. 15.7A) where it is liquefied and stored. The liquid agar is poured into a mould former (Figs. 15.7B and C) to create a mould (Fig. 15.7 D). Later, investment is poured into this to create a refractory cast (Fig. 15.7E) which is used in the fabrication of the cast partial denture framework.

IMPRESSION DISINFECTION Since the impression has to be sent to the laboratory, the need to disinfect it is very important. Most manufacturers recommend a specific disinfectant. The agent may be iodophor, bleach or glutaraldehyde. Apparently little distortion occurs if the recommended immersion time is followed and if impression is poured promptly.

Advantages and Disadvantages of Agar Hydrocolloid Advantages 1. 2. 3. 4. 5. 6. 7.

Accurate dies can be prepared, if the material is properly handled. Good elastic properties help reproduce most undercut areas. It has good recovery from distortion. Hydrophilic, moist mouth not a problem. It also gives a good model surface. It is palatable and well tolerated by the patient. It is economical when compared to synthetic elastic materials. It can be reused when used as a duplicating material (reuse is not recommended when used as impression material). 8. Low cost because it can be reused.

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Disadvantages 1. 2. 3. 4. 5. 6. 7. 8. 9.

Does not flow well when compared to newly available materials. It cannot be electroplated. During insertion or gelation the patient may experience thermal discomfort. Tears relatively easily. Greater gingival retraction is required for providing adequate thickness of the material. Only one model can be poured. Has to be poured immediately. Cannot be stored for too long. Requires special and expensive equipment. A soft surface of the gypsum cast results unless a plaster hardener is used. Although it can be reused, it is impossible to sterilize this material. Also with repeated use there may be contamination of the materials and a deterioration in its properties.

IRREVERSIBLE HYDROCOLLOID—ALGINATE The word alginate comes from ‘alginic acid’ (anhydro-β-d-mannuronic acid) which is a mucous extract yielded by species of brown seaweed (Phaeophyceae). Alginic acid is a naturally occurring hydrophilic colloidal polysaccharide. Alginate was developed as a substitute for agar when it became scarce due to World War II (Japan was a prime source of agar). Currently alginate is more popular than agar for dental impressions, because it is simpler to use. Alginate is perhaps the most widely used impression material in the world.

TYPES Type I — Fast setting.

Figure 15.8 Representative commercially available bulk packed alginate.

Type II — Normal setting

SUPPLIED AS A powder that is packed

Commonly in bulk packing (tins, bins or sachets) (Fig. 15.8). In preweighed packets for individual impression (Fig. 15.9). A plastic scoop is supplied for dispensing the bulk powder and a plastic cylinder is supplied for measuring the water.

Modified alginates

In the form of a sol, containing the water. A reactor of plaster of Paris is supplied separately.

Figure 15.9 Preweighed sachet for individual impressions are also available. The one displayed in the illustration is a special low viscosity alginate for use with syringe. (Courtesy: The dental center, Chennai).

268 Part 4 Impression Materials

Figure 15.10 Other commercially available alginates including dust free (extreme left) and chromatic alginate (extreme right).

As a two paste system One contains the alginate sol, while the second contains the calcium reactor. These materials are said to contain silicone and have superior resistance to tearing when compared to unmodified alginates. They may be supplied in both tray and syringe viscosity. One product is supplied in low density for use with syringe (Fig. 15.9) Dust free alginates Concern over the inhalation of alginate dust have prompted manufacturers to introduce ‘dust free alginates’ (Fig. 15.10). Chromatic alginates Alginates which change color on setting (Fig. 15.10). Commercial Names Zelgan (DPI), Jeltrate (Dentsply), Hydrogum (Zhermack), etc.

APPLICATIONS 1. It is used for impression making –– When there are undercuts. –– In mouths with excessive flow of saliva. –– For partial dentures with clasps. 2. For making preliminary impressions for complete dentures. 3. For impressions to make study models and working casts. 4. For duplicating models.

COMPOSITION Ingredients

% wt.

Function

Sodium or potassium or triethanolamine 15% alginate

Dissolves in water and reacts with calcium ions

Calcium sulfate (reactor)

16%

Reacts with potassium alginate and forms insoluble calcium alginate

Zinc oxide

Acts as a filler

Potassium titanium fluoride

Gypsum hardener

Diatomaceous earth

60%

Acts as a filler

Sodium phosphate (retarder)

Reacts preferentially with calcium sulfate

Coloring and flavoring agent

Traces

e.g. wintergreen, peppermint, anice, orange, etc.

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SETTING REACTION When alginate powder is mixed with water a sol is formed which later sets to a gel by a chemical reaction. The final gel, i.e. insoluble calcium alginate is produced when soluble sodium alginate reacts with calcium sulfate (reactor). However, this reaction proceeds too fast. There is not enough working time. So the reaction is delayed by addition of a retarder (trisodium phosphate) by the manufacturer. Calcium sulfate prefers to react with the retarder first. Only after the supply of the retarder is over does calcium sulfate react with sodium alginate. This delays the reaction and ensures adequate working time for the dentist. In other words, two main reactions occur during setting Reaction 1 2Na3PO4 + 3CaSO4 Reaction 2

Ca3(PO4)2 + 3Na2SO4

Sodium alginate + CaSO4 + H2O

(Powder)

Ca alginate + Na2SO4 (Gel)

Initially the sodium phosphate reacts with the calcium sulfate to provide adequate working time. Next after the sodium phosphate is used up, the remaining calcium sulfate reacts with sodium alginate to form insoluble calcium alginate which forms a gel with water.

Gel structure The final gel consists of a brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles and reaction byproducts. It is a cross-linked structure (i.e. each fiber is tied to each other at certain points). Calcium is responsible for cross-linking.

PROPERTIES OF ALGINATE HYDROCOLLOID Taste and Odor Alginate has a pleasant taste and smell. Over the years, manufacturers have added a variety of colors, odors and tastes to make it as pleasant as possible to the patient. Flavors include strawberry, orange, mint, vanilla, etc.

Flexibility It is about 14% at a stress of 12.2 N. However, some of the hard set materials have lower values (5–8%). Lower W/P ratio (thick mixes) results in lower flexibility (ISO 21563:2013—minimum requirement ranges from 5 to 20%).

Elasticity and Elastic Recovery Alginate hydrocolloids are highly elastic (but less when compared to agar) and about 98.2% elastic recovery occurs. Thus, permanent deformation is more for alginate (about 1.8%). Permanent deformation is less if the set impression is removed from the mouth quickly.

Reproduction of Tissue Detail Detail reproduction is also lower when compared to agar hydrocolloid. ISO 21563:2013 requires the material to reproduce a line that is 20 µm in width. A number of products exceed this minimum value.

270 Part 4 Impression Materials Strength Compressive strengths

Ranges from 0.5 to 0.9 MPa.

Tear strength This is an important property for alginates. Values range from 0.4 to 0.7 kN/m Factors affecting strength are

Water/powder ratio Too much or too little water reduces gel strength. Mixing time Over and under mixing both reduce strength. Time of removal of impression Strength increases if the time of removal is delayed for few minutes after setting.

Syneresis and Imbibition Like agar–agar alginate also exhibits the properties of synerisis and imbibition. When placed in contact with water alginates absorb water and swell (Figs. 15.11A and B). Continued immersion in water results in the total disintegration of the alginate.

Dimensional Stability Set alginates have poor dimensional stability due to evaporation, syneresis and imbibition. Therefore, the cast should be poured immediately. If storage is unavoidable, keeping in a humid atmosphere of 100% relative humidity (humidor) results in the least dimensional change. Alginates can also be stored in sealed plastic bags. Modern alginates, both regular and extended pour varieties have shown to have good clinically acceptable dimensional stability for periods ranging form 1 to 5 days according to some studies (Box 15.2).

Biological Properties No known chemical or allergic reaction have been identified for alginate. Silica particles present in the dust which rises from the can after fluffing alginate powder, are a possible health hazard. Avoid breathing the dust. Some manufacturers supply ‘dust free’ alginates. Dustless alginates contain glycol. It acts by coating the powder.

Adhesion Alginate does not adhere well to the tray. Good adhesion is important for the accuracy of the impression. Retention to the tray is achieved by mechanical locking features in the tray or by applying an adhesive.

Figures 15.11A and B Demonstration of imbibition. (A) Alginate shortly after setting. (B) The dimensional change is evident after a 48-hour storage in water.

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Shelf Life and Storage Alginate material deteriorates rapidly at elevated temperatures and humid environment.

The material should be stored in a cool, dry environment (not above 37 °C). The lid of bulk package can, must be replaced after every use, so as to minimize moisture contamination. Stock only for one year.

MANIPULATION Fluff or aerate the powder by inverting the can several times. This ensures uniform distribution of the filler before mixing. The top of the can should be taken off carefully to prevent the very fine silica particles from being inhaled. Mixing equipment includes –– A clean flexible plastic bowl and –– A clean wide bladed, reasonably stiff metal spatula. Note It is better to use separate bowls for plaster and alginate as plaster contamination can accelerate setting.

The proper W/P ratio as specified by the manufacturer should be used (usually one measure water with two level scoops of powder. The water measure and scoop are supplied by the manufacturer). The water is taken first. The powder is sprinkled in to the water in the rubber mixing bowl and the lid of the metal can is replaced immediately. The mixing is started with a stirring motion to wet the powder with water. Once the powder has been moistened, rapid spatulation by swiping or stropping against the side of the bowl is done (Fig. 15.12A). A vigorous figure-eight motion can also be used. This helps 1. Remove most of the air bubbles. 2. Wipe dissolved algin from the surface of the yet undissolved algin thereby promoting complete dissolution. Mechanical devices (Fig. 15.13) are available for spatulating alginate. Their main advantages are 1. Speed 2. Convenience

Figures 15.12A and B (A) Alginate is mixed by stropping or swiping the material against the sides of the bowl. (B) The loaded tray.

272 Part 4 Impression Materials 3. Elimination of the human variable. A proper mix is smooth and creamy with minimum voids and does not drip off the spatula when it is raised from the bowl.

Mixing Time For fast set alginate—45 seconds. For normal set alginate—60 seconds. Over mixing results in Reduction in final strength as the gel fibrils are destroyed. Reduction in working time.

Figure 15.13 Alginate mixing device.

Under mixing results in

Inadequate wetting, lack of hom*ogeneity and reduced strength. The mix being grainy and poor recording of detail.

Working Time Fast set alginate—1¼ minutes. Normal set alginate—2 minutes.

Gelation Time (Setting Time) Type I (fast set)—1.5–2.0 minutes Type II (normal)—3–4.5 minutes Control of gelation time Ideal gelation time is 3–4 minutes (at 20 °C room temperature). Gelation time is best controlled by adding retarders (which is in manufacturer’s hands). The dentist can best control the setting time by altering the temperature of the water for mixing alginate material. –– Colder the water, the longer is the gelation time. –– Warmer the water, the shorter is the gelation time. –– Even the mixing bowl and spatula can be cooled. Note Control of setting by changing W/P ratio is not recommended.

Tray Selection Since alginate has poor adhesion, tray selection is very important. Alginate can be retained by

Mechanical locking features in the tray –– A rim lock (a beading round the edges of the tray) (Fig. 15.14) –– Perforations (holes or slits) in the tray (Fig. 15.14) Applying adhesive (available as liquid or sprays) (Fig. 15.15) A combination of the above.

Figure 15.14 Tray with rim lock and perforations is recommended for alginate.

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The tray should cover the entire impression area and provide a space of at least 3 mm on all sides.

LOADING THE TRAY The mixed alginate is pressed and swiped (Fig. 15.16) into the perforated rim lock tray so that the material is forced out through the holes in the tray, thereby locking itself mechanically into the tray (Fig. 15.17). A loaded tray is shown in Fig. 15.12B. The surface of the alginate in the tray may be smoothened out using a moist finger. However, this is not mandatory. A small amount of material may be taken on the index finger and applied on the occlusal surfaces of the teeth and on the rugae area. This help to reduce voids and improve accuracy.

F i g u r e 1 5 . 1 5 Tr a y adhesive improves the retention to the tray.

SEATING THE TRAY Since the material sets from tissues towards periphery any movement during gelation may result in distortion. So once the tray is seated, it must be held in place firmly without any movement.

TIME OF REMOVAL and test for set The alginate impression should be left in the mouth for at least 2–3 minutes after initial gelation. The strength and elasticity of the alginate gel continues to increase for several minutes after initial gelation.

Figure 15.16 Alginate is loaded using the sides of the tray to force the material in to the tray and through the perforations.

Figure 15.17 Alginate extruding through the holes helps in retention of the material to the tray.

274 Part 4 Impression Materials Test for Set The material loses its tackiness when set. It should rebound fully when prodded with a blunt instrument. Color indicators Although chromatic alginates indicate a color change after setting, it is still best to test for set by prodding the material at the periphery with a blunt instrument.

REMOVAL OF THE IMPRESSION An alginate impression when set, develops a very effective peripheral seal. This seal should be freed by running the finger round the periphery. In addition to holding the tray handles, additional displacing force may be applied with a finger on the buccal flange of the set material and tray. A completed impression is shown in Fig. 15.18. The impression must be removed as quickly as possible. The brush-heap structure of a gel responds more favourably to a sudden force. A gentle, long, continued pull will frequently cause the alginate to tear or separate away from the tray (Fig. 15.19). It also causes higher permanent deformation.

Figure 15.18 The completed impression.

After removal from the mouth, the impression should be

Washed with cold water to remove saliva. Disinfected by immersion in a suitable disinfectant. Covered with a damp napkin to prevent drying. Cast should be poured as soon as possible, preferably within 15 minutes after making the impression.

IMPRESSION DISINFECTION Disinfection of impression is a concern because of viral diseases such as hepatitis B, AIDS and herpes simplex. The viruses can contaminate the gypsum models and present a risk to dental laboratory and operating personnel. Recommended disinfectants include phenol, iodophor, bleach or glutaraldehyde. Irreversible hydrocolloids may be disinfected by immersion in, or spraying. Current protocol recommended

Figure 15.19 Separation from tray is a serious error resulting in distortion. The impression should be repeated.

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Box 15.2 Extended pour alginates Traditionally alginates were considered to be dimensional unstable. For decades, dental professionals were taught that casts should be generated from alginate impressions immediately or within 60 minutes after the impression is removed from the patient’s mouth. Studies have challenged these assumptions for currently available alginates (J Am Dent Assoc 2010;141;32-39). Many alginates today are marketed with manufacturers claiming good dimensional stability for up to 5 days (Hydrogum 5). These are known as extended pour alginates. Dimensional stability over a longer period is desirable for transportation of alginate impressions to distant labs. Studies have shown that both conventional and extended pour alginates meet the requirements for dimensionally stability and accuracy for periods ranging from 2 to 5 days under recommended storage conditions.

by the Center for Disease Control and Prevention is to spray the impression with disinfectant. The impression is then wrapped in disinfectant soaked paper towel. Immersion disinfection if used should not exceed 10 minutes to reduce dimensional change.

STORAGE OF ALGINATE IMPRESSION Alginate impressions must be poured as soon as possible. If it becomes necessary to store the impression, the following methods may be used Wrap the impression lightly with a moist paper towel and cover with a rubber bowl or Keep the impression in a sealed plastic bag. Note Even under these conditions storage should not be done for more than one hour (Box 15.2). Care should be taken not to use a soaking wet paper towel or gauze as it can cause imbibition of water.

CONSTRUCTION OF CAST The early alginates required immersion in a gypsum hardening solution, such as potassium sulfate, zinc sulfate, manganese sulfate, and potash alum (most effective is 2% potassium sulfate solution). However, the formulas of presently available alginates have been adjusted so that no hardening solution is required. Alginate is a hydrophilic material and wets easily reducing the entrapment of air. After rinsing (Fig. 15.20) the excess water is shaken off. The impression is held against a vibrator to reduce the trapping off air. Freshly mixed stone is placed at one end of the impression. The impression is rotated to facilitate the flow of the stone around the arch. The stone displaces water and wets the surface of the impression as it flows. It is then allowed to flow out through the other side and discarded (Figs. 15.21A to C).

Figure 15.20 Rinsing removes traces of saliva, bacteria and other contaminants.

This helps to

Reduce the trapping of air bubbles. Removes the water rich surface layer which can result in a weaker cast surface. The impression is filled with the remaining stone and placed aside to set. The stone cast should not be separated for at least 30 minutes. For alginate, best results are obtained

276 Part 4 Impression Materials

Figures 15.21A to C Pouring an alginate impression. (A) The first portion of stone is placed in one corner of the impression. (B) With the help of a vibrator, the stone is flowed along the surface of the impression and round to the other side and allowed to drip off. This improves the wetting of the impression and reduces air entrapment. (C) The rest of the stone is poured to complete the cast.

if the cast is removed in one hour. The cast should not be left in the impression for too long a period either because it can result in a rough and chalky surface. Alginate, dries and stiffens. Removal can break the teeth and other thin portions of the cast.

Advantages and disadvantages of alginate Advantages 1. 2. 3. 4. 5. 6. 7. 8.

It is easy to mix and manipulate. Minimum requirement of equipment. Flexibility of the set impression. Accuracy if properly handled. Low cost. Comfortable to the patient. It is hygienic, as fresh material must be used for each impression. It gives a good surface detail even in presence of saliva.

Disadvantages 1. Cannot be electroplated so metal dies are not possible. 2. It cannot be corrected. 3. Distortion may occur without it being obvious if the material is not held steady while it is setting. 4. Poor dimensional stability—it cannot be stored for long time. 5. Poor tear strength. 6. Because of these drawbacks and the availability of better materials, it is not recommended where a higher degree of accuracy is required, e.g. cast RPD, crowns and FDPs, etc. (Box 15.3). Box 15.3 Improper use of alginates Because of their low cost alginates are often used to make final impressions during the fabrication of high precision restorations like RPDs, crowns and FPDs. Although they are reasonably accurate, they have a few properties which make them inferior to elastomeric materials. These include lower dimensional stability, poor tear strength, lower detail reproduction, etc. Some clinics process their impressions in distant laboratories which then involves a period of delay before it can be cast in stone. Alginate impressions are inaccurate if pouring is delayed. For these reason alginates are not recommended where high levels of accuracy is required, especially where it has to be withdrawn over severe undercuts (as in dentate subjects) and where very thin areas which can tear easily are present (gingival margins).

16 Chapter

Elastomeric Impression Materials Chapter Outline • Chemistry of Elastomers • Types • Uses of Elastomeric Impression Materials

• Supplied as • General Properties of

Elastomeric Materials

• Polysulfides –– Supplied as –– Composition –– Chemistry and Setting –– Properties • Silicone Impression Materials • Condensation Silicone –– Available as –– Commercial Names –– Composition –– Chemistry and Setting Reaction

–– Properties

• Addition Silicones (Poly Vinyl

Siloxane) –– Supplied as –– Composition –– Chemistry and Setting Reaction –– Properties • Polyether Rubber Impression Material –– Available as –– Commercial Names –– Composition –– Chemistry and Setting Reaction –– Properties • Manipulation of Elastomers –– Hand Mixing –– Kneading –– Rotary Table Assisted Mixing –– Static Mixing –– Dynamic Mechanical Mixing

• Impression Techniques –– Monophase) Technique –– Dual-phase) Technique –– Two-stage Putty-wash Technique)

–– (One-stage Putty-wash Technique

• Removal of the Impression • Infection Control • Impression Errors –– Air Entrapment –– Seating Voids –– Fluid Entrapment and Fluid Trails

–– Effect of Provisional Crown Materials

• Specialized Elastomers –– Bite Registration Silicones –– Fit Checking Silicones –– Duplicating Silicones

The first elastomeric materials to be introduced to dentistry were the natural rubbers introduced as denture base materials in the 1850s. These were called vulcanite as they were converted into rubber from their natural latex by a process called vulcanization. The first elastomeric or rubber-based ‘impression material’ to be introduced was polysulfide which was introduced in 1950. Interestingly, they were originally developed as an industrial sealant for gaps between concrete structures. This was followed by condensation silicone in 1955, polyether in 1965 and the addition silicones in 1975. Introduction of the elastomers were a considerable technological advance in the quality of dental services. Elastomers are soft and rubber-like and far more stronger and stable than the hydrocolloids. They are known as synthetic rubbers. The ADA Sp. No. 19 referred to them as nonaqueous elastomeric dental impression materials. The term nonaqueous was used to differentiate them from agar and alginate (considered aqueous or water containing materials). Currently ISO 4823 simply refers to them as ‘elastomeric impression materials’. These materials are the most accurate and dimensionally stable impression materials available in dentistry.

278 Part 4 Impression Materials

Figures 16.1A and B Diagrammatic representation of an elastomer. (A) An unstressed elastomer. (B) The same elastomer under stress. When the stress is removed, it will return to configuration A. The dots represent cross-links.

Synonyms Initially they were called rubber-base or rubber impression materials. Currently, they referred to simply as elastomeric impression materials.

Chemistry and structure of Elastomeric polymers The term ‘elastomer’ is derived from the words elastic polymers. Thus elastomers are essentially polymers with elastic or rubber-like properties. Other polymers used in dentistry are the denture and composite resins. Gutta-percha is also a polymer (cis-polyisoprene) which is closely related to natural latex (trans-polyisoprene). Natural latex is currently used in dentistry to manufacture examination gloves and rubber dams. Gutta-percha is used as an endodontic obturation material. Elastomeric materials contain large molecules with weak interaction between them. They are tied together at certain points to form a three-dimensional network. On stretching, the chains uncoil, and on removal of the stress they snap back to their relaxed entangled state (Figs. 16.1A and B). Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. As a result of this extreme flexibility, elastomers can reversibly extend from 5% to 700%, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.

Polymerization Polymers are long chains of large high-molecular weight macromolecules. They are formed by a chemical reaction where a large number of smaller units or monomers join to form polymer macromolecules, a process called polymerization. Elastomers are liquid polymers which can be converted to solid rubber at room temperature. By mixing with a suitable catalyst, they undergo polymerization and/or crosslinking (by condensation or addition) reaction to produce a firm elastic solid.

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Types of polymerization reactions In elastomers 3 types of polymerization reactions are seen. 1. Addition polymerization 2. Condensation polymerization 3. Ring opening polymerization

Glass transition temperature in elastomers The glass–liquid transition (or glass transition for short) is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state.

TYPES According to their chemistry Chemically, there are four kinds of elastomers. 1. 2. 3. 4.

Polysulfide Condensation polymerizing silicones Addition polymerizing silicones Polyether

Classification based on viscosity (ISO 4823:2015) Each type may be further divided into four viscosity classes (Fig. 16.2) based on consistencies determined immediately after completion of mixing. Type 0—Putty consistency (very heavy) Type 1—Heavy-bodied consistency (tray consistency) Type 2—Medium-bodied consistency (regular bodied) Type 3—Light-bodied (syringe consistency)

Figure 16.2 Elastomers are available in different viscosities and forms.

280 Part 4 Impression Materials According to wettability or contact angle Impression materials are also classified as 1. Hydrophilic, if their contact angle is from 80 to 105°. 2. Hydrophobic, if their contact angle is from 40 to 70°.

USES OF ELASTOMERIC IMPRESSION MATERIALS 1. 2. 3. 4. 5. 6.

In fixed partial dentures for impressions of prepared teeth. Impressions of dentulous mouths for removable partial dentures. Impressions of edentulous mouths for complete dentures. Polyether is used for border molding of edentulous custom trays. For bite registration. Silicone duplicating material is used for making refractory casts during cast partial denture construction.

SUPPLIED AS Regardless of type all elastomeric impression materials are supplied as two component (base and catalyst) systems. Collapsible tubes Cartridges—light and regular body material are also supplied in cartridges to be used with static mixing tips and dispensers Putty consistency is supplied in jars The various forms of elastomers are shown in Figure 16.2.

GENERAL PROPERTIES OF ELASTOMERIC MATERIALS 1. Excellent reproduction of surface details. The low viscosity is capable of producing very fine details (Table 16.1 for minimum requirements as specified by ISO 4323:2015). 2. They are generally hydrophobic (except polyether which is hydrophilic), so the oral tissues in the area of impression should be absolutely dry for better flow of the impression material. Because of their hydrophobic (water hating) nature, care must be taken while pouring stone in the impression. The poor wetting and high contact angle can result in air entrapment. Commercial surfactants sprays are available which improve wetting. 3. Elastic properties of elastomers is good with near complete elastic recovery. Repeated pouring of impression is possible (though not recommended when high accuracy is critical). Table 16.1 Property requirements (ISO 4823:2015)* Type

Consistency (Test disc diameter) mm

Detail Linear dimensional Compatibility with reproduction (Line change gypsum (line width width reproduced)a % reprodcued)a

Elastic recovery %

Strain-incompression %

min

max

μm

max

μm

min

max

–

1.5

96.5

0.8

–

1.5

96.5

0.8

1.5

96.5

2.0

–

1.5

96.5

2.0

* Adapted from ISO 4823:2015

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4. Coefficient of thermal expansion of elastomers is high. Thermal contraction occurs when impression is transferred from mouth to room temperature. 5. In general dimensional changes and inaccuracies occur due to –– Curing shrinkage. –– Loss of by-products of reaction, e.g. condensation silicones lose alcohol and shrink. Polysulfides (hydroperoxide type) lose volatile accelerators causing contraction. –– Polyether being hydrophilic absorbs water and loses soluble plasticizers causing change in dimension (e.g. when immersed in disinfectant). –– Thermal contraction when transferred from mouth to room temperature. –– Removing impression before complete setting can cause serious distortion. –– Incomplete recovery after deformation during removal. –– Amount of filler When filler content is increased, the polymer content is reduced and shrinkage is less. Thus, less shrinkage is seen in putty, and higher shrinkage is observed in light bodied. –– Uniform thickness of material gives more accurate impression as the shrinkage is uniform. –– Good adhesion of impression to the tray (using adhesives) minimizes dimensional changes as the shrinkage is directed towards the tray. In the absence of adhesion between the tray and impression, the shrinkage is directed centrally and the model prepared will be smaller in size. –– Time of pouring Impression should be poured after elastic recovery but before dimensional changes set in. 6. The tear strength of these materials are excellent, thus making it more resistant to tearing even when the impression is in thin sections. 7. Electroplating Elastomers can be copper and/or silver plated. 8. Radiopacity Radiopacity of impression materials is important for radiographic identification of excess material which may be accidentally swallowed, aspirated or left in gingival tissues. Presently, only the polysulfide materials exhibit significant radiopacity due to their lead dioxide content. 9. Retention to tray Elastomeric materials do not adhere well to the impression tray. They may be retained by –– Mechanically by using perforated trays (only in case of putty). –– Tray adhesives These are tacky liquids that are applied with a brush. Each elastomer type has a specific adhesive which is not interchangeable. 10. The shelf life is about two years. The silicones have a slightly lower shelf life. Storage under cool conditions increases shelf life. 11. Color They come in a variety of colors (Box 16.1).

Box 16.1 Color Color is an important feature of elastomeric impression materials. Elastomeric impression materials come in a variety of colors. This helps to differentiate between the various consistencies while making impressions. The base and catalyst are in contrasting colors to visually determine completion of mixing.

282 Part 4 Impression Materials POLYSULFIDES This was the first elastomeric impression material to be introduced (1950). It is also known as Mercaptan or Thiokol. Interestingly, they were first developed as an industrial sealant for gaps between sectional concrete structures.

SUPPLIED AS They are supplied as a two-paste system in collapsible tubes. The base paste is white colored. The accelerator may be brown or gray.

Available in three viscosities

Light bodied Medium bodied Heavy bodied

Commercial names Permlastic (Kerr) (Fig. 16.3) Coe-flex Lead dioxide system Omni flex Copper hydroxide system

Composition Base paste Ingredient

Wt. percent

Liquid polysulfide polymer

80–85%

Inert fillers (Titanium dioxide, zinc sulfate, 16–18% copper carbonate or silica)

Figure 16.3 A representative polysulfide impression material.

Reactor paste Ingredient

Wt. percent

Lead dioxide

60–68%

Dibutyl phthalate

30–35%

Sulfur

Other substances like magnesium, stearate 2% (retarder) and deodorants

Tray adhesive The adhesive cement should be compatible with the polysulfide impression material. Butyl rubber or styrene/acrylonitrile dissolved in a volatile solvent, such as chloroform or a ketone is used with polysulfide.

CHEMISTRY AND SETTING REACTIONS When the base and accelerator pastes are mixed, it undergoes a chemical reaction, whereby the liquid polymer sets to form a solid, but highly elastic and flexible rubber like material.

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The lead dioxide reacts with the polysulfide polymer causing Chain lengthening by oxidation of terminal—SH groups Cross-linking by oxidation of the pendant—SH groups The reaction is exothermic with a 3–4 °C rise in temperature. It is accelerated by heat and moisture.

HS – R – SH

PbO2 + S

HS – R – S – S – R – SH + H2O

Mercaptan + Lead dioxide

Polysulfide + Water

As an alternative to lead dioxide, an organic hydroperoxide can be used (e.g. t-butyl hydroperoxide). However, these compounds are volatile and so are dimensionally unstable. The other cross-linking system successfully used are certain complex inorganic hydroxides (e.g. copper).

PROPERTIES 1. Unpleasant odor and color. It stains linen and is messy to work with. 2. These materials are extremely viscous and sticky. Mixing is difficult. However, they exhibit pseudoplasticity, i.e. if sufficient speed and force is used for spatulation, the material will seem easier to handle. The mixing time is 45 seconds. 3. It has a long setting time of 12.5 minutes (at 37 °C). In colder climates setting can take as longer. This adds to the patient’s discomfort. Heat and moisture accelerate the setting time (sets faster in the mouth). 4. Excellent reproduction of surface detail. 5. Dimensional stability The curing shrinkage is high (0.45%) and continues even after setting. It has the highest permanent deformation (3–5%) among the elastomers. Elastic recovery improves with time and so pouring of the model should be delayed by half an hour. Further delay is avoided to minimize curing shrinkage. Loss of the by-product (water) also causes shrinkage. 6. It has high tear strength (4000 g/cm) (Box 16.2). 7. It has good flexibility (7%) and low hardness. A 2 mm spacing in the tray is sufficient for making an impression. 8. It is hydrophobic so the mouth should be dried thoroughly before making an impression. Care should also be taken while pouring the stone to avoid air pockets. 9. It can be electroplated. More with silver than copper. 10. The shelf life is good (2 years).

SILICONE RUBBER IMPRESSION MATERIALS These materials were developed to overcome some of the disadvantages of polysulfide materials, such as their objectionable odor, the staining of linen and clothing by the lead dioxide,

Box 16.2 Tear strength In fixed prosthodontics the impression material is often extruded into the sulcus of the prepared tooth. When the impression is removed, the material in the sulcus being very thin, can tear away and remain in the sulcus. Therefore, a high tear strength is advantageous.

284 Part 4 Impression Materials the amount of effort required to mix the base with the accelerator, the rather long setting times, the moderately high shrinkage on setting, and the fairly high permanent deformation.

TYPES Two types of silicone impression materials are available based on the type of polymerization reaction occurring during its setting. 1. Condensation silicones 2. Addition silicones Both silicones are available in a variety of colors, such as pastel pinks, purples, blues, greens, oranges, etc. Different viscosities may be identified by their color.

CONDENSATION SILICONE This was the earlier of the two silicone impression materials. It is also referred to as conventional silicones.

SUPPLIED AS Paste Supplied as two pastes in unequal sized collapsible tubes. The base paste comes in a larger tube while the catalyst paste is supplied in a much smaller tube (Fig. 16.4B). Putty The putty is supplied in a single large plastic jar (Fig. 16.4A). The catalyst may be in paste form or sometimes it may be supplied as a liquid. They come in a variety of colors. The base and accelerator are typically in contrasting colors (which aids mixing).

Available in three viscosities Light bodied Medium bodied Putty Commercial names Sil 21, Coltex, Dent-a-scon, etc.

FIgures 16.4A and B Condensation silicone. (A) Putty. (B) Regular body base and catalyst. Notice the smaller size of the catalyst paste. Note also quantity of activator dispensed is less.

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COMPOSITION Base Ingredient

Wt. percent

Polydimethyl siloxane (hydroxy—terminated)

80–85%

Colloidal silica or microsized metal oxide filler

35–75% (depending on viscosity)

Color pigments

16–18%

Reactor paste/accelerator Ingredient

Action

Orthoethyl silicate

crosslinking agent

Stannous octoate

catalyst

CHEMISTRY AND SETTING REACTION It is a condensation reaction. Polymerization occurs as a result of crosslinking between the orthoethyl silicate and the terminal hydroxy group of the dimethyl siloxane, to form a threedimensional network. Stannous octoate acts as the catalyst. The reaction is exothermic (1 °C rise). CH3 OC2H5 | | Stannous OH – Si – OH + C2H5O – Si – OC2H5 Silicone rubber + CH3CH2OH | | octoate CH3 OC2H5 or Dimethyl Orthoethyl Stannous Silicone Ethyl + + siloxane silicate alcohol octoate rubber

The ethyl alcohol formed as a by-product evaporates gradually from the set rubber leading to shrinkage. Tray adhesive The adhesive for silicones contain poly (dimethyl siloxane) or a similar reactive silicone, and ethyl silicate. Hydrated silica forms from the ethyl silicate to create a physical bond with the tray, and poly (dimethyl siloxane) bonds with the rubber.

PROPERTIES 1. Pleasant color and odor. Although nontoxic, direct skin contact should be avoided to prevent any allergic reactions. 2. Setting time is 6–9 minutes. Mixing time is 45 seconds. 3. Excellent reproduction of surface details. 4. Dimensional stability is comparatively less because of the high curing shrinkage (0.4–0.6%), and shrinkage due to evaporation of the ethyl alcohol by-products. To avoid this the cast should be poured immediately. The permanent deformation is also high (1–3%). 5. Tear strength (Box 16.2) (3000) g/cm is lower than the polysulfides. 6. It is stiffer and harder than polysulfide. The hardness increases with time. The spacing in the tray is increased to 3 mm to compensate for the stiffness. 7. It is hydrophobic. The field should be well-dried before making an impression. Care should also be taken while pouring the cast to avoid air entrapment.

286 Part 4 Impression Materials 8. Can be plated with silver/copper. Silver-plating is preferred. 9. Shelf life is slightly less than polysulfides due to the unstable nature of the orthoethyl silicates.

ADDITION SILICONES (POLYVINYL SILOXANE) These materials were introduced subsequent to the introduction of the condensation silicones. These new materials had better properties when compared to the condensation silicones. It is also known as polyvinyl siloxane. Currently, the addition silicones are very popular and is perhaps the most widely used elastomeric impression material worldwide.

SUPPLIED AS 1. Tubes The base and catalyst pastes come in equal sized tubes (unlike condensation silicones). The different viscosities usually come in different colors like orange, blue, green, etc. 2. Cartridge form with static mixing tips For use with a dispensing gun. 3. Putty jars Two equal sized plastic jars—containing the base and catalyst. 4. A larger electric driven autodispenser and mixing device is also available (Pentamix— ESPE). This machine stores larger quantities. At the press of the button, it dispenses and mixes the material.

Available in four viscosities (Fig. 16.2) Light bodied Medium bodied Heavy bodied Putty Representative Commercial Products Reprosil (Dentsply—Fig. 16.1), Provil, President (Coltene), etc.

COMPOSITION Base paste

Reactor paste

Poly (methyl hydrogen siloxane)

Divinyl polysiloxane

Other siloxane prepolymers

Fillers (amorphous silica or fluorocarbons)

Platinum salt - catalyst (chloroplatinic acid)

Palladium - hydrogen absorber

Fillers

Retarders Coloring agents

Chemistry and setting reaction It is an addition reaction. In this case, the base polymer is terminated with vinyl groups and is crosslinked with silane (hydride groups). The reaction is activated by the platinum salt. CH3 CH3 CH3 CH3 | | | | Pt Si – CH2 – CH2 – Si Si – H + CH2 = CH – Si | | | | Salt CH3 CH3 CH3 CH3

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Box 16.3 Effect of sulfur compounds Sulfur compounds retarded the setting of the early silicones. One source of sulfur contamination is from latex gloves worn by the operator when mixing putty. Vinyl gloves should be used. Some brands of latex gloves and silicones appear to be less affected. Pt Silicone Vinyl Silane + silozane siloxane Salt rubber

There are no by-products as long as, there is balance between the vinyl siloxane and the silane siloxane. If unbalanced, hydrogen gas is produced causing air bubbles in the stone models. To avoid this, palladium is added to absorb the hydrogen.

PROPERTIES 1. Pleasant odor and color. 2. This may also cause allergic reaction so direct skin contact should be avoided. 3. Excellent reproduction of surface details. Polyvinyl siloxanes are currently considered to reproduce the greatest detail of all the impression materials. The international standard for dental elastomeric impression materials states that a type 3 (light bodied) impression material must reproduce a line 20 μm in width. With the exception of the very high viscosity putty materials, all polyvinyl siloxanes (light, medium and heavy body) achieve this. Very low viscosity materials can reproduce lines 1–2 µm wide. 4. Setting time ranges from 5 to 9 minutes. Mixing time is 45 seconds. Working time may be extended by chilling the tubes. Gains of up to 90 seconds have been reported when the materials are chilled to 2 °C. 5. It has the best dimensional stability among the elastomers. It has a low curing shrinkage (0.17%) and the lowest permanent deformation (0.05–0.3%). 6. Early materials had the problem of hydrogen gas formation. If hydrogen gas is liberated pouring of stone is delayed by 1–2 hours to prevent formation of air bubbles in the stone cast. Current materials do not have this problem because of the addition of palladium. 7. It has good tear strength (3000 g/cm). 8. It is extremely hydrophobic, so similar care should be taken while making the impression and pouring the wet stone. Some manufacturers add a surfactant (detergent) to make it more hydrophilic. 9. It can be electroplated with silver or copper. However, hydrophilic silicones are more difficult to electroplate because of the surfactant added. 10. It has low flexibility and is harder than polysulfides. Extra spacing (3 mm) should be provided in the impression tray. Care should also be taken while removing the stone cast from the impression to avoid any breakage. 11. Shelf life ranges from 1 to 2 years.

POLYETHER RUBBER IMPRESSION MATERIAL Polyether was introduced in Germany in the late 1960s. It has good mechanical properties and dimensional stability. Its disadvantage was that the working time was short and the material was very stiff. It is also expensive.

AVAILABLE AS Available as base and accelerator in collapsible tubes, cartridges for static mixing and dynamic mechanical mixing devices. The accelerator tube is usually smaller (Fig. 16.5). Originally, it was supplied in a single viscosity. A third tube containing a thinner was provided.

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Figures 16.5A and B Representative polyether impression pastes. Notice the smaller size of the reactor tubes.

Currently, it is available in three viscosities. Light bodied Medium bodied Heavy bodied Commercial examples Impregum (3M ESPE), Ramitec, Polyjel (Dentsply), Permadyne (ESPE).

Composition Base Ingredient

Wt. Percent/Function

Polyether polymer

80–85%

Colloidal silica

Filler

Glycolether or phthalate

Plasticizer

Reactor/accelerator paste Ingredient

Function

Aromatic sulfonate ester

Crosslinking agent

Colloidal silica

Filler

Phthalate or glycolether

Plasticizer

CHEMISTRY AND SETTING REACTION It is cured by the reaction between aziridine rings which are at the end of branched polyether molecule. The main chain is a copolymer of ethylene oxide and tetrahydrofuran. Crosslinking is brought about by the aromatic sulfonate ester via the imine end groups. The reaction is exothermic (4 to 5 °C). H O O H | | | | CH3 – C – CH2 – C – O – R – O – C – CH2 – C – CH + ester | | N N | | CH2 – CH2 CH2 – CH2

Crosslinked rubber

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or Polyether + Sulfonic ester

Crosslinked rubber

PROPERTIES 1. Pleasant odor and taste. 2. The sulfonic ester can cause skin reactions. Thorough mixing is recommended before making an impression and direct skin contact should be avoided. 3. Setting time is around 6–8 minutes. Mixing should be done quickly that is 30 seconds. Heat decreases the setting time. 4. Dimensional stability is very good. Curing shrinkage is low (0.24%). The permanent deformation is also low (0.8–1.6%). However, polyethers absorb water and can change dimension. Therefore, prolonged storage in water or in humid climates is not recommended. 5. It is extremely stiff (flexibility 3%). It is harder than polysulfides and increases with time. Removing it from undercuts can be difficult, so additional spacing (4 mm) is recommended. Care should also be taken while removing the cast from the impression to avoid any breakage. 6. Tear strength is good (3000 g/cm). 7. It is hydrophilic, so moisture in the impression field is not so critical. It has the best compatibility with stone among the elastomers. 8. It can be electroplated with silver or copper. 9. The shelf life is excellent — more than 2 years. 10. It has excellent detail reproduction (20 microns). 11. Many medicaments, such as aluminum sulfate and ferric sulfate, used on gingival retraction cords have been accused of causing inhibition of set of polyvinyl siloxane materials. However, studies have not found not any inhibitory effect. 12. Material interactions Composite based provisional crown materials like Protemp 4 have been observed to have an inhibitory effect on the setting of polyvinyl siloxane materials. When a provisional crown is made directly in the mouth using a putty impression as a template, an oily residue called the oxygen inhibited layer (OIL) remains on the tooth and in the impression after separation of the provisional crown. Failure to adequately remove the OIL can result in impaired setting.

MANIPULATION OF ELASTOMERIC IMPRESSION MATERIALS There are many methods of mixing and using elastomeric impression materials depending on whether it is supplied in tube, cartridge or putty form. The 5 main mixing techniques are 1. 2. 3. 4. 5.

Hand or manual spatulation Manual kneading Rotary table assisted mixing Static or extrusion mixing Dynamic mechanical mixing

HAND MIXING - Pastes in tubes Hand or manual spatulation and is primarily used elastomers supplied in tubes.

290 Part 4 Impression Materials Polysulfides and addition silicones Equal lengths of base and accelerator pastes are extruded on to the mixing pad alongside each other without touching. The accelerator paste is then incorporated into the base paste. Mixing is done using a tapered stiff bladed metal or plastic spatula. Just before loading the tray the material should be spread in a thin layer to release the trapped air bubbles. A streak free mix is obtained in 45 seconds.

Condensation silicone Unlike addition silicone, the quantity of catalyst paste needed is very little. The manufacturer usually marks the length required on the mixing pad. The two pastes therefore are of unequal length and diameter (Fig. 16.4B).

For polyether The required amount of thinner (when supplied) may be added to the base and accelerator depending on the viscosity needed. Again, like condensation silicone, the quantity of accelerator needed is very little. The ratio is usually displayed on the mixing pad. The mixing should be done quickly. The mixing time is 30 seconds.

Kneading - putty Kneading is primarily employed for very heavy or putty consistency elastomers. In case of addition silicones, equal scoops of base and accelerator are dispensed. With condensation silicones, the required number of scoops of base and recommended amount of liquid or paste accelerator is taken. In either case mixing is done by kneading between the fingers. Mixing is continued until a streak free mix is obtained.

Rotary table-assisted mixing The technique is similar to that described for zinc oxide impression pastes. The pastes are dispensed on to a rotating table (Fig. 16.7). The spatula is used to scoop and flatten the pastes alternately and continuously as the table rotates until a uniform mix is obtained.

Static OR EXTRUSION Mixing Static mixing also known as ‘extrusion mixing’ (ISO 4823:2015), has grown in popularity over the years, primarily because of its high accuracy and convenience. Extrusion mixing is a method by which two or more material components are extruded simultaneously from their separate primary containers through a special mixing tip from which the material components emerge as a hom*ogeneous mixture (Figs. 16.6A to D).

Advantages of static mixing 1. Shorter mixing time. 2. More uniform proportioning and mixing. A

B Figure 16.6 (A) Static mixing tip showing internal helical mixers. The used tip show set material inside the tip. (B) Internal helix demonstrating flow division. (C) Accessory tips for direct delivery to impression site. (D) Static mixing device.

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3. Less voids. 4. Mix can be delivered directly to the tray or impression site. The system consists of a gun with a dual plunger (Fig. 16.6D). The cartridges are loaded onto this device. A static mixing tip is then attached to the cartridge. The tip contains helical mixing blades on the inside (Fig. 16.6B). Forcing of the base and accelerator through the tip results in its mixing.

Precautions 1. The initial portion should be discarded as material from the right and left tubes may not have extruded evenly. This can affect its setting characteristics. 2. The material should be ejected in a continuous stream, avoiding lifting or skipping across segments. This reduces the chances of air entrapment and voids.

Dynamic Mechanical Mixing Another device is an electrically operated dynamic mechanical mixer (ESPE Pentamix— Figs. 16.8A and B). The base and catalyst are supplied in large plastic bags which are loaded in to the machine. On pressing a button, the material is mixed and extruded through the tip, directly into the impression tray.

IMPRESSION techniques Impressions may be made in custom or stock trays. Elastomers do not adhere well to the tray. An adhesive (Fig. 16.9A) should be applied to the tray and allowed to dry before making impressions. The adhesive cements provided with the various elastomers are not interchangeable. A slightly roughened tray surface will increase the adhesion. For putty impressions, a perforated stock tray is used. The perforations help retain the putty in the tray (Fig. 16.9B). In case of elastomers, the bulk of the impression should be made with a heavier consistency (to reduce shrinkage). Light bodied should only be used in a thin layer as a wash impression.

Classification of elastomeric impression techniques Based on the viscosity used 1. Single viscosity technique 2. Dual viscosity techniques a. Dual viscosity technique using light body-heavy bodied b. Putty-wash technique

Figure 16.7 Rotary mixing device.

Figures 16.8A and B Two representative dynamic mechanical mixing devices.

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Figures 16.9A and B Retention of elastomers. (A) Tray adhesive. (B) Mechanical retention through perforations in tray.

Based on the number of stages used 1. One stage technique—Both viscosities are dispensed and allowed to set simultaneously. 2. Two stage technique—This is usually employed with putty. In this technique a preliminary impression is made first with the ultra-heavy or putty viscosity. This is relined later by the lighter viscosity called as wash impression.

One-stage single viscosity (MONOPHASE) Technique Tray used

Resin custom tray with 2–4 mm spacing.

Viscosity

Medium only.

Method The paste is mixed and part of it is loaded on to the tray and part into a syringe. The syringe material is then injected on to the prepared area of impression. The tray with material is seated over it. The material is allowed to set. This technique utilizes the principle of shear thinning. The same material when ejected under pressure through the syringe tip it exhibits pseudoplasticity and behaves like a material with lower viscosity. Shear thinning allows as single material to be used as both syringe and tray material.

One-stage dual viscosity (dual-PHASE) Technique Tray used Resin custom tray with 2–4 mm spacing. Viscosity used (a) Heavy bodied and (b) light bodied. Method The two viscosities are mixed simultaneously on separate pads. The heavy body is loaded into the tray while the light bodied is loaded into the syringe. The syringe material is injected over the preparation. The tray containing the heavy body if then seated over it. Both materials set together to produce a single impression.

Two-stage putty Reline (Two-stage Putty-wash Technique) Tray used Perforated stock tray. Viscosity used (a) Putty and (b) Light body. Method (Fig. 16.10 A to J) First a preliminary impression is made with putty in the stock tray. Before seating the tray in the mouth, a thin plastic sheet is placed over the putty (it acts as a spacer). After setting it is removed and kept aside. Mixed light bodied is loaded into a syringe and injected over the preparation. Static mixing tips fitted with special direct delivery tips can also be used for this purpose. Light viscosity material is also loaded into the putty impression. The preliminary impression is then seated over the injected material and held till it sets.

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Figures 16.10A to J Two-stage Putty-wash technique. (A) Equal quantities of base and catalyst is dispensed. (B) Mixing by kneading until uniform color is achieved. (C) A plastic sheet spacer is placed to provide space for the final impression material. (D) Making the preliminary impression. (E) The completed preliminary impression. (F) The final impression material dispensed in to the preliminary impression. (G) Simultaneously some material is loaded in to the syringe. (H) The syringe material is injected around the prepared tooth/teeth. (I) The loaded tray is seated in the mouth. (J) The completed impression.

Advantages 1. No special tray required. 2. Simple and quick as custom tray is not required. 3. Accurate.

Precautions Some clinicians use the preliminary impression as a template to construct a provisional restoration (temporary crown). The same preliminary impression is then relined with the final wash material to make the final impression. If a composite based provisional restorative material [e.g. Protemp 4 (ESPE), Structur (VOCO)] has been used to construct the provisionals, a shiny oily layer called the air or oxygen inhibited layer (OIL) from this material forms a coating over the putty in this region. The OIL can affect both setting as well as bonding of the wash material. This can result in delayed setting, distortion or separation of the wash impression from the preliminary impression. This can be eliminated by rigorously wiping off the OIL or physically removing of a layer of putty with a trimmer.

294 Part 4 Impression Materials One-stage putty RELINE (one-stage PUTTY-WASH technique) Tray used Perforated stock tray. Viscosity used (a) Putty and (b) Light body. Method Unlike the previous technique, the putty and light body are dispensed and mixed simultaneously. The putty is loaded into a perforated stock tray whereas the light body is injected on to the prepared tooth. The tray is then taken to the mouth and pressed into position. The heavier putty forces the lighter material into the details. Both material set simultaneously to produce an accurate impression.

Advantage No special tray required. The technique is simple and quick.

REMOVAL OF THE IMPRESSION The material is checked for set by prodding with blunt instrument. When set, it should be firm and return completely to its original contour. The impression is dislodged from the mouth as quickly as possible for the following reasons Elastic recovery is better Tear resistance is higher. However, rapid removal may be difficult as well as uncomfortable to the patient. Removal is facilitated by breaking the air-seal. This can be done by teasing the borders of the tray parallel to the path of insertion until the air leaks into the tray. Compressed air through an air syringe may also be used. In addition to the holding the tray handle, a finger on the buccal portion of the tray may be used to apply additional pressure to dislodge the tray.

INFECTION CONTROL Rubber impression materials are disinfected by immersing in disinfectant solutions. 10 minutes in 2% glutaraldehyde or 3 minutes in chlorine dioxide solutions have been found to be satisfactory. Because of its tendency to absorb water, a spray of chlorine dioxide is preferred in case of polyether. Other disinfectants used are phenol and iodophor.

Impression Errors Errors in impressions do occur and can result in inaccurate casts and prostheses. Most of the errors occur due to poor technique and a failure to understand the properties of the material. Some of the impression errors that can occur are 1. 2. 3. 4. 5. 6. 7.

Air entrapment Fluid entrapment Seating trails Contamination from provisional crowns Contamination from latex gloves Rough or uneven surface Distortion

Air entrapment Voids in the impression mostly result from a trapping of air.

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Figures 16.11A and B (A) Gingival bleeding following teeth preparation. (B) Impression errors (fluid trails and loss of detail) caused by active bleeding and inadequate fluid control.

Voids occur from a variety of causes. 1. Air entrapment due to faulty loading of the tray. Care should be taken when loading the tray. Material should be loaded in a continuous motion from one end of the tray to the other pushing the material ahead as it is ejected from the static mixing device. The tip should be in close proximity to the surface of the tray. Lifting the tip away from the tray and moving it from place to place in a discontinuous motion can result in air entrapment. 2. Air entrapment from faulty placement of material around the prepared tooth. When placing the material on the prepared tooth using a syringe or a static mixing tip, the material should be extruded in close proximity to the sulcus and prepared tooth in a continuous motion around the prepared tooth starting from the sulcus and finish line through to the occlusal surface. Discontinuous and erratic motion with lifting of the tip from place to place can result in air entrapment.

Fluid entrapment and fluid trails Elastomers with the exception of polyether are generally being hydrophobic require the impression field to be reasonably clean and dry. Fluid entrapment and fluid trails can occur due to

Failure to adequately remove water or control saliva in impression field .

Failure to control bleeding and exudate. Continuos bleeding from injured or inflamed soft tissues near the prepared tooth can result in loss of detail and fluid trails as the blood continues to flow (Figs. 16.11A and B).

Seating voids or trails Seating voids or trails are usually seen when using a ‘single stage putty-wash technique’. The putty tends to displace the light bodied resulting in voids or ‘seating trails’ (Figs. 16.12A and B). Seating trails usually correspond to prominent cusps. The void caused by the penetration of the cusp fails to get filled by the wash material because of the displacement. This can cause inaccuracies in the impression especially if it is related to the prepared tooth. Seating trails correspond to the direction of seating. For this reason the single stage putty reline technique is not advocated for procedures requiring a high degree of accuracy. The two-stage technique is superior in this regard.

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FIGURES 16.12A and B (A) Seating trails (arrows) corresponding to the palatal cusps of the maxillary molars result in a significant loss of detail in the impression and the corresponding area of the cast (B).

EFFECT OF PROVISIONAL CROWN MATERIALS (Material interaction) A knowledge of material interaction are important to avoid impression errors. According to one study direct contact of polyvinyl siloxane impression materials to some brands of resin based provisional interim fixed prosthodontic materials resulted in polymerization inhibition (delayed setting). The brands tested included Trim Plus, Unifast, Integrity, Systemp C&B, TuffTemp, Protemp IV. Among the elastomers polyether was least affected. Composite based provisional crown materials like Protemp 4 are used to make direct intraoral provisional (temporary crowns). The material is usually placed on to the tooth in a clear vacuum-formed template (suck-down). Some clinicians use a preoperative putty impression as a template. When a provisional crown is made directly in the mouth using a putty impression as a template. An oily coating of resin also known as the oxygen inhibited layer (OIL) remains on the teeth as well as the impression surfaces after separation of the provisional crown. If the operator continues with the final impression by relining the putty impression in which the provisional crown was made without adequate cleansing of the OIL can result in 2 problems. 1. Impaired setting in the area resulting in an inaccurate impression. 2. Failure of the wash impression material to bond to the underlying putty resulting in physically separation from the putty (Figs. 16.13A and B).

Figures 16.13A and B Impression error caused by oxygen inhibited layer (OIL) contamination of the putty as well as the prepared tooth. In this instance the putty impression had been used earlier as a template to make the provisional crown using Protemp 4. The material has totally separated from the impression (A) and remained on the prepared tooth (B). Areas of inhibited setting can be seen on the adjacent second molar (arrow).

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Figure 16.14 Bite registration procedure using bite registration silicone.

297

Figure 16.15 Bite registration silicone.

To ensure accuracy of the impression some precautions which may be observed are

Thorough physical removal of the OIL from the tooth surface by rubbing with gauze. According to one study 3% H2O2 was found to be effective in cleaning the OIL from the tooth. Avoid reusing the putty impression template in which provisional was made. If the putty template is reused, ensure thorough removal of the OIL by vigorous rubbing with gauze or scraping with putty knife or bur. Holding the impression for a slight longer period to ensure complete set.

Specialized materials Bite Registration silicones Registering the three-dimensional relationship between two articulating surfaces is known as bite registration (Fig. 16.14). Many materials are used for this purpose in dentistry. The earliest materials were wax and plaster. A specialized addition type of silicone is increasingly popular as a bite registration material. Unlike the regular impression silicone these materials show greater stiffness and greater hardness (32–45 Shore D), when set. A faster setting time is also important to reduce errors caused by movement and to reduce discomfort to the patient. Setting time ranges from as low as 20 seconds to a minute depending on the type. Other important properties required of these materials is that they should not slump or drip when initially placed. A scannable version has also been introduced for use in CAD CAM (Virtual CADbite Registration, Ivoclar). Most are supplied in cartridge form for use with a caulking gun. Some are supplied in collapsible tubes. One product (Colorbite D, Zhermack—Fig. 16.15) has thermochromic indicators to help the clinician ascertain setting in the mouth.

Fit checking silicones Another specialized addition type silicone is used for detecting errors in the internal surface of crowns and fixed partial dentures. They are available as a two-paste system (Fig. 16.16A). Small but equal lengths of the two pastes are mixed and applied to the internal surface of the crown. The crown is seated on the tooth and the material allowed to set. Areas of premature contact are revealed as bare areas or areas where the internal surface of the crown is showing through (Fig. 16.16B). These areas are marked and reduced. The material can also be used to assess the fit of complete and partial dentures.

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Figures 16.16A and B (A) Addition silicone for locating high spots, interferences, fulcrum points and pressure spots on the fitting surfaces of restorations and prostheses (fit checking). (B) Crown with fit checker. The high spots show as areas of metal exposure.

Duplicating silicones Duplicating silicones are primarily used in the fabrication process of cast removable partial dentures for constructing duplicate of the master cast in a refractory material (refractory cast) (Fig. 16.17B). The duplicating silicones were introduced as an alternative to agar duplicating material. The material is supplied as base and catalyst in the fluid consistency (Fig. 16.17A). They are usually supplied in bulk containers ranging from 250 g to 10 kg. They are mixed in a ratio of 1:1. The working time ranges from 2 to 5 minutes. The setting time of these materials are comparatively longer ranging from 10 to 30 minutes. Shore hardness of these materials range from 17 to 26. Like conventional silicones they exhibit a low shrinkage usually in the range of –0.03 to 0.05%. Because of their high dimensional stability and elastic recovery they may be used to create multiple casts.

Figures 16.17A and B (A) Duplicating silicone (base and catalyst) including flasks. (B) Duplicating silicone being poured into the mould former.

Section-5

Dental Laboratory— Materials and Processes Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24

Model, Cast and Die Materials, 301 Gypsum Products, 310 Waxes in Dentistry, 327 Dental Casting Investments, 345 Dental Casting and Metal Fabrication Procedures, 358 Abrasion and Polishing, 378 Metal Joining and Dental Lasers, 392 Additive Manufacturing in Dentistry, 407

17 CHAPTER

Model, Cast and Die Materials Chapter Outline • • • • • • • • • • •

Models Casts Dies Types of Die Materials Gypsum Metal and Metal-coated Dies Polymers Cements Refractory Materials Ideal Requirements of Die Materials Alternate Die Materials

• Improved Dental Stone or Die Stone –– Advantages –– Disadvantages • Electroformed/ Electroplated Casts and Dies –– Advantages –– Disadvantages • Electroforming –– Components of an Electroplating Apparatus

–– Composition of the Electroplating Bath

• • • • •

–– Procedure

Polyurethane Epoxy Resin Die Materials Refractory Cast for Wax Patterns Refractory Cast for Ceramics Die Stone-Investment Combination (Divestment) • Divestment Phosphate or DVP • Difference between Divestment Cast and Refractory Investment Cast

Casts and models are an important part of dental services. Plaster and stone are the usual materials used to prepare casts and models. However, it must be remembered that other materials can also be used for this purpose.n

MODELS Models are used primarily for observation, diagnosis and patient education, e.g. orthodontic study models (Fig. 17.1), diagnostic casts, etc.

CASTS A working model or master cast is the positive replica on which restorations or appliances are fabricated, e.g. complete denture, removable partial denture (Fig. 17.2), orthodontic appliances. Casts should be made with a high level of accuracy. They should be handled with great care, taking care not to scratch or damage its surface.

DIES A positive replica of a prepared tooth or teeth in a suitable hard substance on which inlays, crowns and other restorations are made (Fig. 17.3). Similar care should be taken in ensuring its accuracy as well as handling.

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FIGURE 17.1 A study model.

FIGURE 17.2 Casts are used to fabricate dental restorations.

FIGURE 17.3 Dies are used to fabricate dental restorations.

TYPES OF DIE MATERIALS Gypsum Type IV dental stone Type V dental stone, high strength, high expansion Type V dental stone + lighosulfonates (this wetting agent reduces the water requirement of a stone and thus enables the production of a hard, stronger and more dense set gypsum) Metal and metal-coated dies Electroformed Sprayed metals Amalgam Polymers Metal or inorganic filled resins Polyurethane Epoxy Cements Silicophosphate or polyacrylic acid bonded cement. These are no longer commonly used currently. Refractory materials This includes investments and divestments. Investment casts are used to make patterns for RPD frames. Divestment dies are used in direct baking of porcelain crowns or preparation of wax patterns.

IDEAL REQUIREMENTS OF DIE MATERIALS An ideal die material should 1. Be dimensionally accurate. 2. Have good abrasion resistance, strength and toughness to allow burnishing of foil and resist breakage. 3. Have a smooth surface. 4. Be able to reproduce all fine details in the impression. 5. Be compatible with all impression materials. 6. Have a color contrast with wax, porcelain and alloys.

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7. Be easy to manipulate and quick to fabricate. 8. Be noninjurious to health by touch or inhalation. 9. Be economical.

ALTERNATE DIE MATERIALS Polymers

They shrink during polymerization and so tend to produce an undersized die.

Cements

All cements shrink slightly and exhibit brittleness and have a tendency to crack due to dehydration.

Metal-sprayed

The bismuth-tin alloy is rather soft; care is needed to prevent abrasion of the die.

IMPROVED DENTAL STONE OR DIE STONE The most commonly used die materials are still alpha hemihydrate type IV and type V gypsum products. Type IV gypsum products have cuboidal-shaped particles and the reduced surface area produce the required properties of strength, hardness and minimal setting expansion. The most recent gypsum product, having an even higher compressive strength than the type IV is the high strength, high expansion type V stone. The setting expansion has been increased from 0.01 to 0.3%. This higher setting expansion is required in the stone used for the die to aid in compensation for the base metal alloy solidification shrinkage.

ADVANTAGES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Good strength. Minimal shrinkage. Easy manipulation. Good working time. Sets quickly. Compatible with impression materials. Has smooth, hard surface. Can be easily trimmed. Has good color contrast. Is economical.

DISADVANTAGES 1. Brittle. 2. Not as abrasion resistant as the epoxy and electroformed dies. Edges and occlusal surface may be rubbed off.

ELECTROFORMED/ELECTROPLATED CASTS AND DIES Electrodeposition of copper or silver on the impression gives a hard metallic surface to the cast. Electroformed dies are not used currently; however, they will be described for historical reasons.

304 PART 5 Dental Laboratory—Materials and Processes ADVANTAGES 1. 2. 3. 4. 5. 6. 7.

Dimensional accuracy. Hard and abrasion resistant. Imparts a smooth surface to the wax pattern in contact. Not very expensive. Better marginal definition. Does not absorb oil or water. Prevents cuspal wear due to repeated contact with opposing cast.

DISADVANTAGES 1. 2. 3. 4. 5.

Difficult to trim. Silver bath is a potential health hazard. Not compatible with all impression materials. Color contrast not as good as die stone. Adaptation of wax not as good, pattern tends to lift from margins.

ELECTROFORMING Electroforming (also known as electroplating or electrodeposition) is a process by which a thin coating of metal is deposited on the impression, after which a gypsum cast is poured. The cast thus obtained will have a metallic surface layer. Metals used for electroforming are

Copper Silver

Plating can be done for

Individual tooth impression Full arch impression

Plating is done on Compound impression (usually copper plated) Polysulfide impression (usually silver plated) Silicone impression Other impression materials show dimensional changes when plated.

COMPONENTS OF AN ELECTROPLATING APPARATUS A commercially available apparatus for electroplating is displayed in Figure 17.4. A typical electroplating unit consists of (Fig. 17.5)

Cathode The impression to be coated is made the cathode. Anode is the metal to be deposited, i.e. copper or silver. Anode holder, cathode holder. Electrolyte is a solution through which the electric current is passed. Ions are deposited from anode to cathode, e.g. silver cyanide or copper sulfate.

Model, Cast and Die Materials CHAPTER 17

FIGURE 17.4 Electroplating unit.

305

FIGURE 17.5 Diagrammatic representation of electroplating unit.

Ammeter registers the current in milliamperes (0–500 mA). The current passed is 10 mA per tooth area, for 12 hours. Plating tank is made of glass or hard rubber with a well-fitting cover to prevent evaporation. Temperature 77 to 80 °F (room temperature).

COMPOSITION OF THE ELECTROPLATING BATH Copper forming

Silver forming

Copper sulfate crystals

200 g Silver cyanide

36 g

Sulfuric acid (concentrated)

30 mL Potassium cyanide

60 g

Phenol sulfonic acid Water (distilled)

2 mL Potassium carbonate 1000 mL Water (distilled)

45 g 1000 mL

PROCEDURE

Wash and dry the impression. Metallizing Most impression materials do not conduct electricity. They are made conductive by applying a metallizing solution or powder with a brush. The metallizing agents are –– Bronzing powder suspended in almond oil. –– Aqueous suspension of silver powder. –– Powdered graphite. The surface of the impression tray is covered with wax 2 mm beyond the margin of the impression. This protects the tray and prevents its plating. With a dropper, the impression is filled with electrolyte, avoiding air-bubbles. The impression is attached to the cathode holder with an insulated wire. The electrode is attached to the cathode and the impression is immersed in the electrolyte bath. Distance between the cathode (impression) and anode (metal) should be at least 4 inches. Initially, current should not exceed 5 mA. Later the current is increased to 10 mA per tooth for 12–15 hours, to get a deposit of 0.5 mm (If a high current is used the surface will be granular, uneven and weak. With low currents the deposit is smooth and hard).

306 PART 5 Dental Laboratory—Materials and Processes

FIGURE 17.6 Polyurethane die material kit.

The current is disconnected. The impression is washed. The die is completed by pouring resin or dental stone to form the cast and base.

POLYURETHANE Resin die materials were developed for applications where increased hardness and abrasion resistance is desired. One such material developed for this purpose is a polyurethane resin (Fig. 17.6).

MODE OF SUPPLY Supplied in glass bottles containing

Base material (200 mL) Hardener (100 mL) Filler (400 g)

INDICATIONS Indicated for use with elastomers. Die separator must be applied when casting polyether impression.

Contraindication Not indicated for use with alginates and hydrocolloid impression materials.

PROPERTIES It is flowable, accurate in detail and dimensionally stable. It has high edge strength and abrasion resistance and is easy to trim and saw.

MANIPULATION Briefly shake the bottles containing both the base material and hardener prior to mixing. Close the glass bottles carefully immediately after use. Do not allow the material to come into contact with water (foam). Fill the required amount of base material in the dispensing and mixing container supplied. Then add the correct quantity of filler and thoroughly spatulate the mixture. Add the correct quantity of hardener and spatulate the mixture again thoroughly.

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FIGURE 17.7 Epoxy resin die.

Mixing ratio Base:hardener = 2:1. (10 g:5 g). Approximately 15 grams filler is required for a full dental arch. Mixing time Approximately 30 seconds. Pouring After mixing the resin is poured in a thin stream into the cleaned and dried impression. The material remains flowable for approximately 2 minutes at 20 °C. Curing In order to prevent air voids the die may be hardened for 15 minutes, after pouring, in a dry pressure vessel at 2–4 bars. The die is sufficiently hard after 1 hour to permit trimming and grinding.

EPOXY RESIN DIE MATERIALS Epoxy is another resin material that has been developed for die construction. They are most effective with rubber impression materials (Fig. 17.7).

ADVANTAGES Tougher and more abrasion resistant than die stone.

DISADVANTAGES 1. Slight shrinkage (0.1%). 2. Viscous, does not flow readily. 3. Setting may take up to 24 hours.

AVAILABLE AS Two components—resin paste and hardener.

REFRACTORY CAST FOR WAX PATTERNS A refractory cast is a special cast made from a heat resistant (investment) material. Such casts are used in the fabrication of certain large metal structures, e.g. cast removable partial dentures. Small wax structures like inlays, crowns and small FPDs can be constructed on a regular die as it can be removed from the die without significant distortion and invested separately. However, larger wax structures like that for the cast RPD, would distort if removed

308 PART 5 Dental Laboratory—Materials and Processes from the cast. RPD patterns are best constructed on a refractory cast. The pattern is invested together with the refractory cast. Why not invest an ordinary gypsum cast? The conventional (nonrefractory) gypsum cast cannot withstand the high temperatures involved in the casting of metal and would disintegrate under these conditions.

REFRACTORY CAST FOR CERAMICS Refractory dies are also available for ceramic restorations [e.g. polyvest (Fig. 17.8) and VHT— Whipmix]. The all-porcelain restoration is directly built up on these refractory dies and fired (further detail in chapter on investments).

DIE STONE-INVESTMENT COMBINATION (DIVESTMENT) This is a combination of die material and investing medium. A gypsum-bonded material called divestment is mixed with a colloidal silica liquid. A die is prepared from the mix and a wax pattern is constructed on it. Then the wax pattern together with die is invested in divestment. The setting expansion of divestment is 0.9% and thermal expansion 0.6%, when heated to 677 °C. As it is a gypsum-bonded material it is not recommended for high fusing alloys, e.g. metalceramic alloys.

ADVANTAGE It is a highly accurate technique for conventional gold alloys, especially for extracoronal preparations. In this technique, removal of the wax pattern from the die is not required. Thus, possibility of distortion of wax pattern during removal from the die or during setting of the investment is minimized.

DIVESTMENT PHOSPHATE OR DVP This is a phosphate-bonded investment that is similar to the divestment and is suitable for use with high fusing alloys.

FIGURE 17.8 Refractory die material for use in the fabrication of ceramic restorations.

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DIFFERENCE BETWEEN DIVESTMENT CAST AND REFRACTORY INVESTMENT CAST Though both are quite similar, there are some fundamental differences. The investment casts are not as strong and abrasion resistant as the divestment cast. In fact, they are quite fragile and can disintegrate easily. Manufacturers have provided certain hardening solutions to compensate for this. Divestment is generally used for smaller castings, whereas investment refractory casts are used during the fabrication of larger structures, such as partial dentures frames and complete denture bases.

18 Chapter

Gypsum Products Chapter Outline • Applications –– Supplied As • Classification • Type 1 or Impression Plaster • Type 2 or Dental Plaster, Model • Type 3 or Dental Stone, Model • Type 4 or Dental Stone, Die, High Strength, Low Expansion • Type 5 or Dental Stone, Die, High Strength, High Expansion • Manufacture of Gypsum Products • Setting Reaction

• Theories of Setting • The Microstructure of Set • • • •

•

Gypsum Manipulation Setting Time –– Penetrometers Properties Setting Expansion –– Normal Setting Expansion (0.05 to 0.5%) –– Hygroscopic Setting Expansion Strength –– Tensile Strength

• Hardness and Abrasion Resistance

• Flow • Reproduction of Detail • Specialized Gypsum Products –– Dental Casting Investments –– Divestment –– Synthetic Gypsum –– Orthodontic Stone –– Resin Modified Stones –– Mounting Plaster –– Fast Setting Stone • Care of Gypsum • Infection Control

Products of gypsum are used extensively in dentistry. Gypsum was found in mines around the city of Paris, so it is also called plaster of Paris. This is a misnomer as gypsum is found in most countries. The mineral gypsum CaSO4. 2H2O is usually white to yellowish white in color and is found as a compact mass. Gypsum is also an industrial byproduct. For centuries gypsum has been used for construction purposes and making statues. Alabaster, a form of gypsum which is white in color, was used for building in ancient times. Besides dentistry, gypsum is also used in orthopedics for splinting fractured bones.

APPLICATIONS 1. Impression plaster was used extensively in the past for impressions of the mouth and face. 2. Various types of plasters are used to make moulds, casts and dies over which dental prostheses and restorations are made (Figs. 18.1A to E). 3. To attach casts to an articulator (Fig. 18.1D). 4. For bite registration (e.g., to record centric jaw relation). 5. Dental investments Plaster mixed with silica is known as dental investment. They are used to form refractory moulds into which molten metal is cast.

Supplied As Powders of various colors in small preweighed sachets, in medium-sized bags or containers or in large bags, sacks or bins (bulk) (Figs. 18.2A to C).

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Classification ISO 6873:2013 Type 1—Dental plaster for impressions Type 2—Dental plaster Class 1 - for mounting Class 2 - for models Type 3—Dental stone for models Type 4—Dental stone (high strength, low expansion) for dies Type 5—Dental stone (high strength, high expansion) for dies

TYPE 1 OR Dental plaster, IMPRESSION Impression plaster (Fig. 18.3A) was one of the earliest impression materials in dentistry. Because of its rigidity (not elastic), it often had to be fractured to remove it from undercut areas in the mouth. The fractured pieces were then reassembled outside and a cast poured. Since the introduction of better materials, it is rarely used as an impression material. Currently, it is more useful as a bite registration material. Impression plaster may be flavored to make it more acceptable by the patient. It is colored to help the dentist and technician distinguish between the cast material and the impression. Impression plaster, sometimes, contains potato

Figures 18.1A to E Gypsum products are widely used in dentistry. (A) Orthodontic models. (B) A cast with removable die made from die stone. (C) A plaster mould used in denture construction. (D) Mounting plaster for mounting casts on an articulator. (E) Dental restoration constructed on a stone working cast.

Figures 18.2A to C Gypsum products are supplied in a variety of forms. as preweighed sachets, in medium sized containers or in large bags or sacks (bulk packing). (A) Mounting plaster. (B) High strength stone (die stone) in 1 to 3 kg container. (C) Dental stone (can range from 5 to 25 kg bulk pack).

312 Part 5 Dental Laboratory—Materials and Processes starch to make it soluble. After the cast has hardened, the impression and cast are put in hot water. The starch swells and the impression disintegrates, making it easy to separate the cast. This type is often called ‘soluble plaster’.

Uses 1. For making impressions in complete denture and maxillofacial prosthetics (not used currently for this purpose). 2. Bite registration material.

Ideal requirements 1. The setting time should be under accurate control. The dentist must have sufficient time to mix, load the impression tray, carry the loaded tray to the patient’s mouth and place it in position. However, once in position the plaster should harden promptly, so that there is minimum discomfort to the patient. The setting time desirable is 3 to 5 minutes. 2. For better accuracy the setting expansion should be low. Both setting time and expansion are controlled by modifiers (accelerators and retarders) added by the manufacturers. 3. The plaster should have enough strength to fracture cleanly without crumbling to facilitate removal from undercuts.

Composition Dental plaster + K2SO4 + Borax + Coloring and flavoring agents.

TYPE 2 OR DENTAL PLASTER, modeL, MOUNTING Synonyms Model plaster, laboratory plaster, mounting plaster (Fig. 18.3B). The International Standards Organizations (ISO 6873:2013) has classified Type 2 plaster is into 2 subtypes—Class I (for mounting) and Class 2 (for models).

Uses 1. For making study casts and models. 2. To make molds for curing dentures. 3. For mounting casts on articulator.

Requirements of an ideal cast material 1. 2. 3. 4. 5. 6.

It should set rapidly but give adequate time for manipulation. It should set to a very hard and strong mass. It should flow into all parts of the impression and reproduce all the minute details. It should neither contract nor expand while setting. After setting, it should not warp or change shape. It should not lose its strength when subjected to moulding and curing procedures.

Composition Contains beta hemihydrate and modifiers.

TYPE 3 OR DENTAL STONE, model Synonym Class I stone or Hydrocal (Fig. 18.3C).

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Uses For preparing master casts and to make molds.

Composition Ingredient

Action

Alphahemihydrate Coloring matter

2 to 3%

Potassium Sulphate (K2SO)

Accelerator

Borax

Retarder

Some commercial dental stones contain a small amount of beta hemihydrate to provide a mix of smoother consistency. A stone with a setting time established by the addition of proper quantities of both accelerator and retarder is called ‘balanced stone’. Typical accelerators are potassium sulfate and potassium sodium tartrate (Rochelle Salts). Typical retarders are sodium citrate and sodium tetraborate decahydrate (Borax).

The compressive strength varies from 3000 to 5000 psi. The setting expansion of dental stone is 0.06% to 0.12%. Hardness: 82 RHN.

TYPE 4 OR Dental stone, die, high strength, low expansion Synonyms Class II stone, die stone, densite, improved stone.

Uses Die stone (Fig. 18.3D) is the strongest and hardest variety of gypsum product. It is used when high strength and surface hardness is required. Uses include model bases, CAD/CAM dies and dies for fabricating inlay, crown and bridge wax patterns. A thick mix is prepared as per manufacturer’s instruction and vibrated into a rubber base impression. The base for such a model is poured in dental stone or dental plaster. Die stone should be left for twenty four hours to gain maximum hardness and the cast should be separated one hour after pouring. The abrasion resistance of die stone is not high as other die materials like epoxy resin. Recent revision of the ISO (2013) have included additional requirements for Type 4 stone to reflecting the introduction of new technologies like CAD/CAM (Table 18.1).

TYPE 5 OR Dental stone, die, high strength, high expansion It is the most recent gypsum product (Fig. 18.3E) having a higher compressive strength than Type 4 stone. Improved strength is attained by making it possible to lower the w/p ratio even further. Setting expansion has been increased from a maximum of 0.10 to 0.30%. This is to compensate for the shrinkage of base metal alloys, during solidification (see Casting Alloys). Hard Rock, Jade Rock and Resinrock XL5 (by Whipmix) and Denflo-HX are examples of Type 5 stone.

Uses To prepare dies with increased expansion.

314 Part 5 Dental Laboratory—Materials and Processes

Figures 18.3A to E The 5 types of gypsum products in dentistry.

Manufacture of Gypsum Products The process of heating gypsum for the manufacture of plaster is known as calcination. Mined gypsum is ground and heated. When heated, gypsum (calcium sulphate dihydrate) loses part of its water of crystallization and changes to calcium sulphate hemihydrate. On further heating, the remaining water of crystallization is lost. First, hexagonal anhydrite (soluble anhydrite) is formed. Later, orthorhombic anhydrite (insoluble anhydrite) is formed. CaSO4.2H2O

110-130 °C 130-200 °C 200-1000 °C CaSO4.½H2O CaSO4 CaSO4

(Calcium sulphate dihydrate)

(Calcium sulphate hemihydrate)

(Hexagonal anhydrite)

(Orthorhombic anhydrite)

Alpha and beta hemihydrate Depending on the method of calcination, there are two forms of hemihydrates.

Beta hemihydrate (plaster) Alpha hemihydrate (stone) Alpha modified hemihydrate (die stone)

Manufacture of dental plaster Gypsum is ground and heated in an open kettle on kiln at a temperature of 110 to 130 °C. The process is called dry-calcination. β type of crystals are formed.

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Box 18.1 Chemically both α and β types are the same. They differ mainly in crystal size and form. The β type is spongy and irregular. Whereas, α crystals are more dense and prismatic. Some authors suggest that the use of α and β prefixes should be discontinued.

Microscopically Fibrous aggregate of fine crystals with capillary pores. They are then ground to breakup the needlelike crystals. This improves packing. CaSO4.2H2O

Heat

CaSO4.1/2H2O 110-130 °C (β hemihydrate)

Manufacture of dental stone Gypsum is calcined under steam pressure in an autoclave at 120 to 130 °C at 17 lbs/sq. inch for 5 to 7 hours. Thus, the product obtained is much stronger and harder than β hemihydrate. 120-130 °C CaSO4.2H2O CaSO4.½H2O 17 lbs/sq. inch pressure (α hemihydrate)

Microscopically Cleavage fragments and crystals in the form of rods and prisms.

Manufacture of high strength (α modified) stone The gypsum is calcined by boiling it in 30% calcium chloride solution. The chlorides are then washed away or autoclaved in presence of sodium succinate 0.5%. These particles are the densest of all three types. After controlled grinding, these powders have an even higher apparent density and yield a stronger set. Microscopically cuboidal in shape.

Setting Reaction When plaster is mixed with water it takes up one and a half molecules of water, i.e., it regains its water of crystallization and becomes calcium sulphate dihydrate. (CaSO4)2 .H2O + 3H2O Hemihydrate + Water

2 CaSO4.2H2O + unreacted (CaSO4)2 . ½H2O + Heat Dihydrate

+ Unreacted hemihydrate

+ Heat

The reaction is exothermic and is the same for all gypsum products. The amount of water required to produce a workable mix varies between the products. As evident from the above reaction not all of the hemihydrate converts to dihydrate. The amount of conversion is dependent on the type of stone. The highest conversion rate is seen in plaster (90%). In Type 4 and 5 stone the dihydrate content is about 50%.

THEORIES OF SETTING Three theories have been proposed. 1. Colloidal theory 2. Hydration theory 3. Dissolution - precipitation theory

316 Part 5 Dental Laboratory—Materials and Processes Colloidal theory The theory proposes that when mixed with water, plaster enters into a colloidal state through a sol-gel mechanism. In the sol state, hemihydrate combines with water (hydrates) to form dihydrate. As the water is consumed, the mass turns to a ‘solid gel’.

Hydration theory The hydration theory suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.

Dissolution–precipitation theory (Crystalline theory) This theory is more widely accepted. According to the theory, the plaster dissolves and reacts to form gypsum crystals which interlock to form the set solid. The setting reaction is explained on the basis of difference in solubility of hemihydrate and dihydrate. Hemihydrate is four times more soluble than dihydrate. When hemihydrate is mixed in water, it forms a fluid workable suspension. Hemihydrate dissolves until it forms a saturated solution. Some dihydrate is formed due to the reaction. Solubility of dihydrate is much less than hemihydrate, the saturated hemihydrate is supersaturated with respect to the dihydrate. All supersaturated solutions are unstable. So the dihydrate crystals precipitate out. As the dihydrate precipitates out, the solution is no longer saturated with hemihydrate and so it continues to dissolve. The process continues until no further dihydrate precipitates out of the solution. Initially there is little reaction and thus little or no rise in temperature. This time is referred to as induction period. As the reaction proceeds, gypsum is formed in the form of needle-like clusters, called spherulites (Figs. 18.4A and B). Continued growth and intermeshing of crystals of gypsum leads to thickening and hardening of the mass into a strong solid structure.

The Microstructure of Set Gypsum The set material consists of an entangled aggregate of gypsum crystals (Figs. 18.4A and B) having lengths of 5 to 10 µm. Two distinct types microscopic porosity can be seen in the mass.

Microporosity caused by residual unreacted water. These voids are spherical and occur between clumps of gypsum crystals.

Figures 18.4A and B (A) SEM of set gypsum showing needle-like clusters (x1550). (B) A single crystal is called a spherulite.

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Box 18.2 Excess water The actual amount of water necessary to mix the calcium sulphate hemihydrate is greater than the amount required for the chemical reaction (18.61 gm of water per 100 gm of hemihydrate). This is called excess water. The excess water itself does not react with the hemihydrate crystals. It is eventually lost by evaporation once the gypsum is set. The excess water serves only to aid in mixing the powder particles and is replaced by voids.

Microporosity resulting from growth of gypsum crystals. These voids are associated with setting expansion and are smaller than the first type. They appear as angular spaces between individual crystals in the aggregate.

MANIPULATION Proportioning To secure maximum strength a low water/powder ratio should be used. The water should be measured and the powder weighed.

Water/powder ratio The W/P ratio is a very important factor in deciding the physical and chemical properties of the final product. Example The higher the water-powder ratio, the longer is the setting time and weaker will be the gypsum product. Therefore, water/powder ratio should be kept as low as possible but at the same time sufficient to produce a workable mix.

Water requirement of a product is affected by 1. Shape and compactness of crystals Thus, irregular, spongy plaster particles need more water than the denser stone. 2. Small amounts of surface active materials like gum arabic plus lime markedly reduce water requirement of all gypsum products. 3. Particle size distribution Grinding of the powder breaks up needle like crystals. This improves packing characteristics and reduces the water needed.

Recommended w/p ratio Impression plaster : 0.50 to 0.75 Dental plaster

: 0.45 to 0.50

Dental stone

: 0.28 to 0.30

Die stone, Type 4

: 0.22 to 0.24

Die stone, Type 5

: 0.18 to 0.22

Instruments Flexible rubber/plastic bowl, stiff bladed spatula.

Procedure for Hand-Mixing

Water is taken first to prevent adherence of dry powder to the sides of the bowl. Water and powder are dispensed according to the recommended W/P ratio. The powder is sifted into water in the rubber bowl. Plaster/stone dispensers are also available (Fig. 18.5).

318 Part 5 Dental Laboratory—Materials and Processes It is allowed to settle for 30 seconds to minimize air entrapment. The mix is stirred vigorously. Periodically wipe the inside of the bowl with a spatula to ensure wetting of the powder and breaking up of lumps. Continue till a smooth creamy mix is obtained. Spatulation should be completed in 45 to 60 seconds. Vibrate the mix (using a mechanical vibrator (Fig. 18.6) or by repeated tapping against a bench) and pour it into the impression, taking care not to entrap air (Fig. 18.7). The mixing equipment must be meticulously clean. There should be no particles of set plaster from a previous mix sticking to the bowl or spatula. These if present will act as additional nuclei of crystallization and cause faster setting. No air must be trapped in the mixed mass. It causes loss of surface detail and weakens the cast.

Mechanical Mixing Mechanical mixing under vacuum gives stronger and denser casts. However, the equipment is expensive.

Setting Time The time elapsing from the beginning of mixing until the material hardens is called setting time. Mixing time It is the time from the addition the powder to the water until mixing is complete. A mixing time of 1 minute is usually sufficient. Working time It is the time available to work with the mix for the intended purpose, i.e., one that maintains an even consistency. At the end of the working period the material thickens

Figure 18.5 Stone/plaster dispenser.

Figure 18.6 Stone/plaster vibrator.

Figure 18.7 A vibrator improves the flow and reduces voids, thereby improving strength and accuracy.

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and is no longer workable. The freshly mixed mass is semifluid in consistency and quite free flowing. A working time of 3 minutes is usually sufficient. Initial setting time As the reaction proceeds, more hemihydrate crystals react to form dihydrate crystals. The viscosity of the mass is increased and it can no longer be poured. The material becomes rigid (but not hard). It can be carved but not moulded. This is known as initial setting time. Final setting time The time at which the material can be separated from the impression without distortion or fracture.

Measurement of setting time Usually by some type of penetration tests. Occasionally, other tests are used. 1. Loss of gloss method As reaction proceeds the gloss disappears from the surface of plaster mix (sometimes used to indicate initial set). 2. Exothermic reaction The temperature rise of the mass may also be used for measurement of setting time as the setting reaction is exothermic. 3. Penetration tests By using penetrometers.

Types of penetrometers Vicat needle Gillmore needles Vicat needle (Fig. 18.8) It weighs 300 gm and the needle diameter is 1 mm. The time elapsing from the start of mixing till the needle does not penetrate to the bottom of the plaster is the setting time. The setting time obtained with the Vicat needle is similar to the initial Gillmore.

Gillmore needles Two types—small and large (Fig. 18.9). The small Gillmore needle has a 1/4 lb weight and a diameter of 1/12” (2.12 mm) while the large Gillmore has a 1 lb wt and diameter of 1/24” (1.06 mm). Initial Gillmore The time elapsing from the start of mixing until the time when the point of the 1/4 lb Gillmore needle no longer penetrates the surface is the initial setting time

Figure 18.8 Vicat needle.

Figure 18.9 Gillmore apparatus. Besides dentistry, it is also used in general industry to determine initial and final set times of Portland cement, masonry cement, hydrated lime, mortars, etc.

320 Part 5 Dental Laboratory—Materials and Processes Final Gillmore Similarly the time elapsing from the start of mixing until the point of the 1 lb Gillmore needle leaves only a barely visible mark on the surface of the set plaster is known as the final setting time.

Factors affecting setting time 1. 2. 3. 4. 5.

Manufacturing process Mixing and spatulation (time and rate) Water/Powder ratio Temperature Modifiers

Manufacturing process 1. If calcination is incomplete and excess gypsum (dihydrate) is left in the final product, the resulting plaster will set faster. 2. If soluble anhydrite is in excess, plaster will set faster. 3. If natural anhydrite is in excess, plaster will set slow. 4. Fineness Finer the hemihydrate particle size, the faster the set, because –– Hemihydrate dissolves faster, and –– The gypsum nuclei are more numerous and therefore, crystallization is faster. Mixing and spatulation Within limits the longer and faster the plaster is mixed, the faster it will set because nuclei of crystallization are broken and well-distributed within the mass. Water/Powder ratio More the water used for mixing, the fewer the nuclei per unit volume. Thus setting time will be prolonged. Temperature On increasing from a room temperature of 20 °C to a body temperature of 37 °C, the rate of the reaction increases slightly and the setting time is shortened. As the temperature is raised above 37 °C the rate of reaction decreases and the setting time is lengthened. At 100 °C the solubilities of hemihydrate and dihydrate are equal, in which case no reaction can occur and the gypsum will not set. Modifiers (Accelerators and Retarders) Modifiers are chemicals added in order to alter some of the properties and make it more acceptable to the dentist. If the chemical added decreases the setting time it is called an accelerator, whereas if it increases the setting time it is called a retarder. Accelerators and retarders not only modify setting time, they also affect other properties like setting expansion and strength.

Accelerators

Finely powdered gypsum (up to 1%) is added by manufacturers to accelerate setting time. Acts by providing additional nuclei of crystallization. One source of gypsum is slurry water. In low concentrations, salts like sodium or potassium sulphate (2 to 3%) and sodium chloride (up to 2%) are accelerators. They act by making the hemihydrate more soluble.

Retarders Retarders generally act by forming a layer on the hemihydrate to reduce its solubility. It also inhibits the growth of gypsum crystals.

Borax (1–2%) is the most effective retarder. During setting, it forms a coating of calcium borate around the hemihydrate. Thus, the water cannot come in contact with the hemihydrate.

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In higher concentrations, sodium chloride (3.4% to 20%) and sodium sulphate act as retarders. In higher concentrations, the salt precipitates and poisons the nuclei of crystallization. Acetates, borates, citrates, tartrates and salts like ferric sulphate, chromic sulphate, aluminium sulphate, etc., are retarders, which act by nuclei poisoning by reducing the rate of solution of hemihydrate or by inhibiting growth of dihydrate crystals. Some additives react with hemihydrate, e.g., soluble tartrates and citrates precipitate calcium tartrate and citrate, respectively. Colloids such as gelatin, glue, agar, coagulated blood, etc. are effective retarders, presumably acting by nuclei poisoning. Contact with the gypsum during setting results in a soft, easily abraded surface. To avoid The impression should be thoroughly rinsed in cold water to remove blood and saliva before pouring.

PROPERTIES The important properties of gypsum products are

Setting expansion Strength Hardness and abrasion resistance Reproduction of detail

Setting Expansion Setting expansion is measured using an extensometer. Setting expansion is of two types 1. Normal setting expansion 2. Hygroscopic setting expansion

Normal Setting Expansion (0.05 to 0.5%) All gypsum products show a linear expansion during setting, due to the outward thrust of the growing crystals during setting. Crystals growing from the nuclei not only intermesh but also intercept each other during growth. Importance of setting expansion In dentistry, setting expansion may be both desirable and undesirable depending on the use. It is undesirable in impression plaster, dental plaster and stone as it will result in an inaccurate cast or change in the occlusal relation if used for mounting. ISO requirements for setting expansion for the various types is given in Table 18.1. Table 18.1 Properties of various gypsum products * % Setting Expansion at * Comp str. (1 hr) (MPa) Hardness (Dry) (RHN) 2 hrs.

* (µm) Detail Reproduction

Type 1

0.15 max

4 min 8 max

75 ± 8

Type 2 (Class 1)

0.05

75 ± 8

Type 2 (Class 2)

0.06 min, 0.30 max

75 ± 8

Type 3

0.20 max

50 ± 8

Type 4

0.15 max (At 24 hr max 0.18) 35

50 ± 8

Type 5

0.16 min 0.30 max

*Minimum requirement ISO 6873:2013

50 ± 8

322 Part 5 Dental Laboratory—Materials and Processes Increased setting expansion is desired in case of investment materials as it helps to compensate the shrinkage of the metal during casting. Control of setting expansion 1. Mechanical mixing reduces setting expansion when compared to hand mixed stone. 2. Increase in W/P ratio reduces the setting expansion. 3. Modifiers generally reduce the setting expansion. 4. Potassium sulphate 4% solution reduces setting expansion from 0.5 to 0.06 %. 5. Sodium chloride and borax also decrease setting expansion. For accuracy in dental procedures, the setting expansion has to be minimized. The manufacturers achieve this by addition of K2SO4. This, however, reduces the setting time. To counteract this, retarders like borax are also added (borax also reduces setting expansion).

Hygroscopic Setting Expansion When a gypsum product is placed under water before the initial set stage, a greater expansion is seen. This is due to hygroscopic expansion. When expansion begins, externally available water is drawn into pores forming in the setting mass and this maintains a continuous aqueous phase in which crystal growth takes place freely. Under dry conditions this additional water is not available and as expansion occurs, the aqueous phase in the mix is reduced to a film over the growing crystals. It is greater in magnitude than normal setting expansion. Importance Used to expand some gypsum bonded investments.

Strength The strength increases rapidly as the material hardens after the initial setting. Minimum strength requirements (ISO) for various gypsum products are presented in Table 18.1.

Factors affecting strength The free water content (excess water) The greater the amount of free water in the set stone, the less the strength. Wet strength It is the strength when excess free water (more than is necessary for reaction) is present in the set gypsum. The wet strength (1 hour compressive strength) for model plaster, dental stone, and die stone are 12.5, 31 and 45 MPa respectively. Dry strength It is the strength of gypsum when the excess free water is lost due to evaporation. It is two or more times greater than the wet strength. Excess water may be removed from gypsum cast by low-temperature drying. But there is no strength increase until the last 2% of free water (Fig. 18.10) is removed. This strength increases on drying is reversible, thus soaking a dry cast in water reduces its strength to the original level. Many products have strength values in excess of the ISO requirements. One Type 4 product claims a wet strength (1 hr) of 67 MPa and a dry strength of 121.6 MPa. Temperature Gypsum is stable only below Figure 18.10 Effect of drying on the strength about 40 °C. Drying at higher temperatures must of dental stone.

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be carefully controlled. Loss of water of crystallization occurs rapidly at 100 °C or higher and causes shrinkage and a reduction in strength. Other factors affecting strength W/P ratio The more the water, the greater the porosity and less the strength. Spatulation Within limits, strength increases with increased spatulation. Addition of accelerators and retarders Lowers strength.

Tensile Strength Gypsum is a brittle material, thus weaker in tension than in compression. The one hour tensile strength of model plaster is approximately 2.3 MPa. When dry, the tensile strength doubles. The tensile strength of dental stone is twice than that of plaster. Significance Teeth on a cast may fracture while separating from the impression. Since in practice fracture of gypsum typically occurs in tension, tensile strength is a better guide to fracture resistance. Time at which cast can be used The cast cannot be used as soon as it reaches its final setting (as defined by the Vicat and Gillmore tests). This is because the cast has not reached its full strength. Technically the cast can be used when it has attained at least 80% of its one hour strength. Current products are ready for use in 30 minutes.

Hardness and Abrasion Resistance Dies and casts are often used to construct restorations and prostheses. A good surface hardness and abrasion resistance is therefore essential. Hardness of some gypsum products are presented in Table 18.1. Hardness is related to the compressive strength. The higher the compressive strength of the hardening mass, the higher the surface hardness. After the final setting occurs, the surface hardness remains practically constant until most of the excess water is dried, after which it increases. The surface hardness increases at a faster rate than the compressive strength since the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass. Commercial hardening solutions are available to increase the surface hardness of stone. However, surface hardness and abrasion resistance are not always related, for example, epoxy resin is more abrasion resistant than die stone, even though die stone is harder of the two.

Flow The flow of freshly mixed gypsum depends on the amount of water used (W/P ratio). The greater the amount of water used, the greater would be the flow. However, a correctly proportioned mix has sufficient flow. Vibrating the mix greatly improves the flow. The flow reduces as it approaches its initial set.

Reproduction of Detail Gypsum products reproduce detail accurately (Table 18.1). Significance

Impression plaster has to accurately record oral tissues. Cast material has to duplicate all the detail recorded by the impression.

324 Part 5 Dental Laboratory—Materials and Processes Factors which affect detail reproduction include compatibility with the impression material, trapped air bubbles in the mix and surface contaminants like saliva. Use of a mechanical vibrator and proper technique considerably improve detail reproduction.

Specialized GYPSUM PRODUCTS Some gypsum products are manufactured for specific uses in dentistry. Each type is developed with specific physical properties suitable for the particular purpose.

DENTAL CASTING INVESTMENTS Uses To prepare refractory molds for casting dental alloys. Adding a refractory material like silica or quartz or cristobalite to dental plaster or stone permits it to withstand high temperatures. These are called dental casting investments (Fig. 18.11) (detailed in Chapter on investments).

Figure 18.11 Gypsum bonded investment (Courtesy: MCODS, Manipal).

DIVESTMENT Uses To make refractory dies. It is a combination of die stone and gypsum-bonded investment mixed with colloidal silica. A die is made and the wax pattern constructed on it. Then the entire assembly (die and pattern) is invested in the divestment (normally the wax pattern is removed from the die and invested separately). The setting expansion of the material is 0.9% and thermal expansion is 0.6% when heated to 677 °C. The advantage of divestment is that the wax pattern does not have to be removed from the die, thus distortion of the pattern can be avoided.

SYNTHETIC GYPSUM It is possible to make alpha and beta hemihydrate from the byproducts during the manufacture of phosphoric acid. The synthetic product is usually more expensive than that made from natural gypsum, but when the product is properly made, its properties are equal to or exceed the latter. However, manufacture is difficult and a few have succeeded (e.g., Japan and Germany).

Orthodontic stone For orthodontic study models, many orthodontists prefer to used white stone or plaster (Fig. 18.12). These products have a longer working time for pouring of multiple models. To produce a glossy surface, finished models may be treated with ‘model glow’ model soap.

Resin modified stones A new resin fortified die stone (e.g., ResinRock, Whipmix corporation) is available. It is a blend of synthetic resin and alpha gypsum. These stones are less brittle, have improved surface

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smoothness and increased resistance to abrasion. When mixed, it forms a creamy, thixotropic mix which flows more easily under vibration. Their compressive strength can be as high as 79 MPa.

Mounting plaster Plaster used for attaching the cast to the articulator (Fig. 18.1D) is known as mounting plaster (Fig. 18.13). Regular plaster (type II, class 1) has higher setting expansion and should be avoided for mounting. However, plasters with lower setting expansion (described by ISO 6873:2013 as Type II, Class 1) specialized for this purpose are available commercially. Important properties for these products include a low setting expansion (0 to 0.05 %) which is important for the accuracy of the mounting, low strength (12 MPa) which allows easy separation from the cast and fast setting time (3 minutes).

Fast setting Stone These are exceptionally fast setting stones (2 minutes) with an early high compressive strength (1 hour - 41 MPa) which allows separation of the cast from the impression in 5 minutes. An example includes Snap stone (Whipmix).

Care of Gypsum Care of the Cast If the gypsum cast has to be soaked in water it must be placed in a water bath in which plaster debris is allowed to remain constantly on the bottom of the container to provide a saturated solution of calcium sulfate at all times. This is known as ‘slurry water’. If the cast is washed in ordinary water, surface layer may dissolve, hence slurry water is used to preserve surface details. Such a procedure also causes a negligible expansion. All gypsum casts must be handled carefully as any departure from the expected accuracy may result in a poorly fitting appliance.

Storage of the Powder 1. As plaster is hygroscopic, it should be kept in air-tight containers. When the relative humidity is more than 70%, plaster starts taking up moisture initiating a setting reaction. This produces small crystals of gypsum which act as nuclei of crystallization. Thus in the early stages, moisture contaminated plaster sets faster. In later stages, as the hygroscopic action continues, the entire hemihydrate mass is covered by more crystals of gypsum. The water penetrates the mass with difficulty, thereby delaying setting. Thus heavily moisture

Figure 18.12 Orthodontic stone (Kalabhai) and model.

Figure 18.13 Mounting plaster.

326 Part 5 Dental Laboratory—Materials and Processes contaminated stone or plaster sets slower. The humidity factor is a major consideration in parts of India with high atmospheric humidity. 2. It should be kept clean with no dirt or other foreign bodies.

INFECTION CONTROL There has been an increased interest over possible cross-contamination to dental office personnel through dental impressions. If an impression has not been disinfected, it is wise to disinfect the stone cast. Gypsum products may be disinfected by 1. Immersing cast in a disinfection solution. 2. Addition of disinfectant into the stone. 3. Overnight gas sterilization while treating patients known to have an infection (impractical for routine use).

Differences between Dental Plaster and Dental Stone Though chemically identical, their differences are detailed in Table 18.2. Table 18.2 Comparison of plaster and stone Plaster

Stone

Manufacture

Dry calcination

Wet calcination

Particle size/shape

Larger, irregular porous

Smaller, regular and dense

W/P ratio

Requires more water

Requires less water

Porosity

Porous

More dense

Properties

Lower strength and hardness

Greater strength and hardness

Application

Used when strength is not of primary Used when greater strength and hardness is importance (e.g., diagnostic casts) required (e.g., dies, master casts)

19 Chapter

Waxes in Dentistry Chapter Outline • Components of Dental Waxes • Chemical Nature of Waxes –– Mineral Waxes –– Plant Waxes –– Insect Wax –– Animal Wax –– Synthetic Waxes –– Wax Additives • Classification of Dental Waxes • General Properties • Pattern Waxes • Inlay Casting Wax • Uses

–– Ideal Requirements of Inlay –– –– –– –– –– –– –– –– –– ––

Casting Waxes Classification Supplied As Composition Properties of Inlay Wax Flow Thermal Properties Wax Distortion Residue on Ignition Manipulation of Inlay Wax Direct Technique

• • • • • • • • • • •

–– Indirect Technique –– Polishing

Rpd Casting Wax Milling Wax Baseplate Wax Processing Waxes Boxing Wax and Beading Wax Utility Wax Sticky Wax Carding Wax Shellac Corrective Impression Wax Bite Registration Wax

During construction of a denture and many other appliances, wax is used as a modeling material. Different types of waxes are used to prepare patterns for alloy castings. There are many varieties of waxes used, both in the clinic and laboratory. Each has particular properties depending on what it is used for. Their basic constituents are essentially similar, their exact proportion is different.

COMPONENTS OF DENTAL WAXES Dental waxes contain natural waxes, synthetic waxes and additives (Table 19.1).

CHEMICAL NATURE OF WAXES Natural waxes are long chain, complex combinations of organic compounds of reasonably high molecular weight. The two principal groups of organic compounds contained in waxes are— Hydrocarbons, e.g. saturated alkanes, and Esters, e.g. myricyl palmitate (bees wax). Some waxes, in addition, contain free alcohol and acids. Ester—formed from union of higher fatty acids (e.g. carboxylic acid) with higher aliphatic alcohol with elimination of water.

Alcohol + Fatty acid

Ester + Water

328 Part 5 Dental Laboratory—Materials and Processes Table 19.1 Wax components Minerals

Plants

Natural waxes

Synthetic waxes

Additives

Paraffin

Acrawax C

Fats

Microcrystalline

Aerosol, OT

- Stearic acid

Barnsdall

Castorwax

- Glyceryl tristearate

Ozokerite

Flexowax C

Ceresin

Epolene N-10

Montan

Albacer

Oils

Aldo 33

Turpentine

Durawax 1032

Color

Carnauba

Natural resins

Ouricury

Rosin

Candelilla

- Copal

Japan wax

- Dammar

Cocoa butter

- Sandarac - Mastic - Shellac - Kauri

Insect

Beeswax

Animal

Spermaceti

Synthetic resins

Lanolin

- Elvax - Polyethylene - Polystyrene

Mineral Waxes Paraffin and microcrystalline waxes These are distillation products of petroleum. They are both hydrocarbons. Paraffin (melts 40–70 °C) tends to be brittle. Microcrystalline (60–90 °C) is more flexible and tougher.

Plant Waxes Carnauba Carnauba, also called Brazil wax and palm wax, is a wax of the leaves of the palm Copernicia prunifera, a plant native to and grown only in the northeastern Brazil. In its pure state it usually comes in the form of hard yellow-brown flakes. Melting range is 84–91 °C. Ouricury Ouricury wax is a brown-colored wax obtained from the leaves of a Brazilian Feather Palm Syagrus coronata or Cocos coronata by scraping the leaf surface. It melts between 79–84 °C. Both Carnauba and Ouricury raise melting range and hardness of paraffin. Candelilla It is a wax derived from the leaves of the small Candelilla shrub native to northern Mexico and the southwestern United States, Euphorbia cerifera and Euphorbia antisyphilitica. It is yellowish-brown, hard, brittle, aromatic, and opaque to translucent. Melting range is 68 to 75 °C. Mainly hardens paraffin wax.

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Japan wax and cocoa butter These are not true waxes but are chiefly fats. Japan wax is also known as sumac wax, China green tallow, Japan tallow, etc. It is obtained from the lacquer tree and the Japanese wax tree which are native to Japan and China. It is pale yellow, sticky, tough, malleable, has a gummy feel and melts at 51 °C. Cocoa butter is brittle. Japan wax improves tackiness and emulsifying ability of paraffin.

Insect Wax Beeswax (63–73 °C) Brittle at room temperature, plastic at body temperature. Its addition reduces brittleness. Shellac wax From the lac insect Kerria lacca.

Animal Wax Spermaceti is found in the spermaceti organ inside the sperm whale’s head. It is not widely used. Mainly used as a coating for dental floss. Lanolin is a wax obtained from wool, consisting of esters of sterols.

Synthetic Waxes The natural waxes are not consistent in their composition, and thus their properties. To overcome this, synthetic waxes are used. These are carefully prepared under controlled conditions to give standardized reliable results. They are highly refined unlike natural waxes which are frequently contaminated. Their use is still limited. Ozokerite It is an earth wax found in western US and central Europe. It improves the physical characteristics of paraffin. Montan Montan wax is a fossilized wax extracted from coal and lignite. It is very hard, reflecting the high concentration of saturated fatty acids and alcohols. It is hard, brittle and lustrous. Although dark brown and smelly, they can be purified and bleached. It can be substituted for plant waxes. Ceresin It is obtained from petroleum and lignite refining. They are harder and are used to raise melting range of paraffin. Barnsdall It raises melting range and hardness, reduces flow of paraffin.

Wax Additives Gums They are viscous, amorphous exudates from plants that harden when exposed to air. They are complex substances mainly made of carbohydrates. They either dissolve in water or form sticky, viscous liquids, e.g. gum Arabic and tragacanth. Fats They are tasteless, odorless and colorless substances. They are similar to wax but have lower melting temperatures and are softer. Chemically they are composed of glycerides, e.g. beef tallow and butter. They can be used to increase melting range and hardness of waxes. Oils They lower the melting point of paraffin. Hydrocarbon oils soften waxes. Silicone oils improve ease of polishing of waxes. Resins are exudates of certain trees and plants (except shellac which is from insects). They are complex, amorphous mixtures of organic substances. They are insoluble in water. They improve toughness. They are also used to make varnishes (by dissolving in an organic solvent). Synthetic resins They are also used.

330 Part 5 Dental Laboratory—Materials and Processes CLASSIFICATION OF DENTAL WAXES According to Origin (Described Earlier)

Mineral Plant Insect Animal

According to Use Pattern waxes

Processing waxes

Impression waxes

Inlay casting

Boxing

Corrective

RPD casting

Utility

Bite registration

Base plate

Sticky Carding Shellac

ISO classification (ISO 15854: 2005) for casting inlay and baseplate wax The ISO recognizes 2 types of waxes which are further sub-classified according to their flow characteristics that represent their hardness. Type I (Casting wax) - for cast metal restorations

Class 1 - Soft

Class 2 - Hard

Type II (Baseplate wax) - for denture bases and occlusion rims

Class 1 - Soft

Class 2 - Hard

Class 3 - Extra hard

GENERAL PROPERTIES Waxes have a number of important properties in relation to their dental use. Different uses require different properties. Waxes for patterns probably require most careful balance. Some of the important properties are 1. 2. 3. 4. 5. 6.

Melting range Thermal expansion Mechanical properties Flow Residual stresses Ductility

Melting Range Waxes have melting ranges rather than melting points. Mixing of waxes may change their melting range. Melting range varies depending on its use.

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Thermal Expansion Waxes expand when subjected to a rise in temperature and contract as the temperature is decreased. Coefficient of thermal expansion and its importance Dental waxes and their components have the largest CTE among the materials used in restorative dentistry. Temperature changes in wax patterns after removal from the mouth can produce inaccuracies in the finished restoration.

Mechanical Properties The elastic modulus, proportional limit and compressive strength of waxes are low compared to other dental materials. These properties are strongly dependent on the temperature. As temperature decreases, the properties improve.

Flow Flow is an important property, especially in inlay waxes. When melted, the wax should flow readily into all the parts of the die. Flow is dependent on 1. Temperature of the wax 2. Force applied 3. The length of time the force is applied. Flow increases as the melting point of the wax is approached.

Residual Stress Regardless of the method used to make a wax pattern, residual stresses will exist in the completed pattern. The stress may be compressive or tensile in nature. Example A When a specimen is held under compression during cooling, the atoms and molecules are forced closer together. After the specimen is cooled to room temperature and the load is removed, the motion of the molecules is restricted. This restriction results in residual stress (hidden stresses) in the specimen. When the specimen is heated, release of the residual stress is added to the normal thermal expansion, and the total expansion is greater than normal. Example B When a specimen is cooled while under tension, the release of the residual tensile stress results in a dimensional change that is opposite to thermal expansion, i.e., it can result in overall contraction of the specimen.

Ductility Like flow, the ductility increases as the temperature of the wax is increased. In general, waxes with low melting points have greater ductility than those with high melting points.

PATTERN WAXES Many dental restorations or prostheses are first made with pattern waxes. The wax is later replaced with the permanent material, e.g. cast gold alloys, cobalt-chromium-nickel alloys, or polymethyl methacrylate resin. All pattern waxes have two major qualities which cause serious problems in their use—thermal change in dimension and tendency to warp or distort on standing, e.g. inlay casting wax, RPD casting wax and baseplate wax.

332 Part 5 Dental Laboratory—Materials and Processes Types 1. Casting waxes –– Inlay –– Removable partial denture (the metal frame) –– Milling wax 2. Baseplate wax (used in the construction of complete and partial denture).

INLAY CASTING WAX The inlay casting wax is among the oldest waxes in dentistry.

Uses The pattern for inlays, crowns and FPDs is first made in wax (Figs. 19.1A to D), and then replaced by metal during casting. Direct and indirect techniques If the pattern is made directly in the tooth (in the mouth), it is said to be prepared by direct technique (Class 2 wax). If it is prepared on a replica of the tooth (die), it is called indirect technique (Class 1 wax).

Ideal Requirements of Inlay Casting Waxes 1. When softened, the wax should be uniform, there should be no graininess or hard spots in the plastic material. 2. The color should contrast with the die. A definite color contrast helps in identifying and finishing of margins. 3. The wax should not flake or crumble when the wax is softened. 4. The wax should not chip, flake or tear during carving. 5. During burnout (500 °C), it should vaporize completely without residue.

Figures 19.1A to D (A) 2 forms of inlay casting waxes. (B) Wax bath used for the dipping technique. (C) Wax patterns of crowns made from inlay wax made on a die. (D) An inlay pattern.

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TABLE 19.2 Flow requirements for inlay casting wax (Adapted from ISO 15854:2005) Flow in % At 45 °C

At 30 °C

At 37 °C

At 40 °C

Class 1

Max 1%

Min 50%

Min 70%

Max 90%

Class 2

Max 1%

Max 20%

Min 70%

Max 90%

6. The wax pattern should be completely rigid and dimensionally stable at all times until it is eliminated. 7. It should be sufficiently plastic slightly above mouth temperature and become rigid when cooled to mouth temperature (for class I waxes). 8. The wax should have good flow when heated and set rigidly when cooled (at the recommended temperature for each type see Table 19.2.

Classification* According to ISO 15854:2005, inlay casting waxes are classified as

Class 1 Soft—Extraoral or laboratory use Class 2 Hard—Intraoral use

Supplied As Blue, green or purple sticks or cakes (Fig. 19.1A). Also available as small pellets and cones. The waxes are also available in preformed shapes. Commercial Names Harvard, Kerr, etc.

Composition Paraffin wax, gum damar, carnauba or candelilla and coloring agents. Paraffin wax (40–60%) This is the main ingredient. It is used to establish the melting point. Different varieties, with different melting points can be produced. Paraffin wax flakes trimmed do not give a smooth surface, so other waxes are added to modify. Ceresin (10%) Partially replaces paraffin. Increases toughness. Easy to carve. Gum damar (1%) Damar resin (a natural derivative from pine tree) improves the smoothness during molding and makes it more resistant to cracking and flaking. It also increases toughness of the wax and enhances the luster of the surface. Carnauba wax (25%) This wax is quite hard and has a high melting point. It is combined with paraffin to decrease the flow at mouth temperature. It has an agreeable odor and gives glossiness to the wax surface. Candelilla wax This wax can be added to replace carnauba wax. It contributes the same qualities as carnauba wax, but its melting point is lower and is not as hard as carnauba wax. Synthetic waxes In modern inlay waxes, carnauba wax is often replaced partly by certain synthetic waxes (Montan). Because of their high melting point, more paraffin can be incorporated and the general working qualities are improved.

Properties of Inlay Wax Class 1 inlay wax is meant for use in the laboratory whereas, Class 2 wax is used in the mouth (indirect technique). Obviously, both would have slightly different properties. * One popular US reference text has described this classification in reverse. However this could not be verified in current or previous ADA or ISO sources. Staff and students are advised to the use the version provided in the this text book as it is sourced from the original document (ISO 1584:2005).

334 Part 5 Dental Laboratory—Materials and Processes Flow Requirements according to ISO 15854:2005 (Table 19.2)

At 45 °C – Both Class 1 and Class 2 should have a flow between 70 to 90%.

At 37 °C – Class 2 should not flow more than 1%.

At 30 °C – Class 1 should not flow more than 1%.

It is clear that Class 1 inlay wax This type melts and flows, when heated to around 45 °C. This temperature is tolerated by the patient. Good flow at this temperature ensures good reproduction of the inlay cavity. The wax cools down and hardens at 37 °C (mouth temperature), allowing the operator to carve and shape it in the mouth. Class 2 inlay wax This type on the other hand hardens at 30 °C (room temperature). This wax is more suitable for the laboratory. The flow characteristics are not suitable for use in the mouth.

Thermal properties Thermal conductivity The thermal conductivity of these waxes is low. It takes time to heat the wax uniformly and to cool it to body or room temperature. Coefficient of thermal expansion Inlay wax has a high CTE. It has a linear expansion of 0.7% with increase in temperature of 20 °C. Its thermal changes are higher than any other dental material. Importance This property is more significant in direct technique because contraction of the pattern can occur when it is taken from mouth to room temperature (especially in air conditioned rooms or in cold climates). Factors affecting If the wax is allowed to cool under pressure, its thermal properties are changed. When reheated, the linear CTE is increased. The temperature of the die and the method used to apply pressure on the wax as it solidifies also influences the CTE.

Wax distortion Wax distortion is the most serious problem in inlay wax. It is due to release of stresses in the pattern caused due to 1. Contraction on cooling 2. Occluded gas bubbles 3. Change of shape of the wax during molding 4. From manipulation—carving, pooling, removal, etc. Thus the amount of residual stress is dependent on The method of forming the pattern Its handling, and Length of time and temperature of storage of the wax pattern Causes of distortion Distortion is due to any method of manipulation that creates inhom*ogeneity of wax involving the intermolecular distance (Figs. 19.2A and B).

Figures 19.2A and B Demonstration of wax distortion. (A) Bent stick of wax kept in water at room temperature. (B) Straightened appreciably after 24 hours.

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Factors causing distortion under control of the operator cannot be totally eliminated. Distortion of the wax can occur

If wax is not at uniform temperature when inserted in the cavity, some parts of the wax pattern may thermally contract more than others when stresses are introduced. If wax is not held under uniform pressure during cooling. If fresh wax is melted and added in an area of deficiency, the added wax will introduce stresses during cooling. During carving, some molecules of wax will be disturbed and stresses will result.

To avoid 1. Minimal carving and change in temperature. 2. Minimal storage of pattern. Invest immediately. 3. Store it in a refrigerator if necessary. Some relaxation and distortion of pattern occurs regardless of the method used. It cannot be totally eliminated. It can only be reduced to a point which is not of clinical importance.

Residue on ignition Waxes vaporize during burnout. ISO 15854:2005 limits the nonvaporizable residue to a maximum of 0.1%. Excess residue can result in an incomplete casting.

Manipulation of Inlay Wax Direct technique Hold the stick of wax over the visible flame and rotate it rapidly until it becomes plastic taking care not to volatilize the wax. The softened wax is shaped approximately to the form of the prepared cavity. After the wax is inserted into the cavity, it is held under finger pressure while it solidifies. The wax should be allowed to cool gradually to mouth temperature. Cooling rapidly by application of cold water results in differential contraction and development of internal stresses. Localized reheating of wax with warm carving instruments has a similar effect and more distortion may occur. A cold carving instrument should be used for direct wax pattern. Withdraw the wax pattern carefully in the long axis of the preparation. The pattern should be touched as little as possible with the hands to avoid temperature changes.

Indirect technique Inlay pattern is prepared over a lubricated die. If molten wax is used, very little residual stresses occur.

Dipping method In case of full crowns, the die can be dipped repeatedly, into hot liquid wax. The wax is allowed to cool, carved, and removed from the die. Softening in warm water This technique is not recommended because –– Soluble constituents may leach out and the properties of wax will change –– Water gets into the wax causing splattering on the flame, interference with the softening of the wax surface and distortion of the pattern on thermal changes. Addition The wax is melted and added in layers using a spatula or a brush.

Polishing Polishing is done by rubbing with a silk cloth.

336 Part 5 Dental Laboratory—Materials and Processes Note 1. Invest all wax patterns as soon as possible to avoid distortion. 2. Waxes oxidize on heating. Prolonged heating causes it to evaporate. There will also be darkening and precipitation of gummy deposits. To avoid this, use the lowest temperature needed for melting.

RPD CASTING WAX The partial denture casting waxes are quite unlike the inlay casting waxes in appearance and handling properties. Currently, no ADA or ISO specification have been formulated for these waxes. However, a US federal specification (U-W-140) has been formulated to cover these waxes. These specifications are different from those of inlay waxes.

Uses To make patterns of the metallic framework and sprues of removable partial dentures.

Supplied As It is available in different forms (Fig. 19.3A).

Sheets 0.40 and 0.32 mm thickness Preformed shapes –– Round (10 cm), half round and half pear-shaped rods –– Reticular, grid or mesh form –– Clasp shapes –– Other forms Bulk wax as blocks or in containers Rolls or coils of various diameters ranging for 2 to 5 mm for forming sprues.

Properties These waxes are tacky and highly ductile as they must adapt easily and stick onto the refractory cast. They should copy accurately the surface against which they are pressed. The pattern for the RPD frame is made on a special cast known as the refractory cast (Fig. 19.3B). Since the wax comes in preformed shapes, it is quite easy to assemble. The wax forms are sticky and

Figures 19.3A and B RPD casting wax. (A) Preformed casting waxes save valuable laboratory time and give more consistent results. (B) RPD pattern formed from preformed waxes are used in the construction of removable partial dentures.

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Figures 19.4A to C Machinable wax. (A) CAD/CAM milling. (B) Milling wax in cake form. (C) Milling with handpiece.

pliable and can be adapted easily onto the cast. After the pattern is completed, it is invested and ignited. Like inlay wax, they too must vaporize with little residue during burnout.

Milling wax Synonyms Machinable wax Milling or machinable wax is wax that can be shaped by milling or machining using CAD/ CAM (Fig. 19.4A) or dental drills (Fig. 19.4C). Machinable wax is an extremely hard wax with high melting temperature that is formulated to deliver machining properties including high resolution detail. The wax pattern formed after machining is invested and cast like regular casting waxes.

Available as Machinable wax is available as

Blocks Cylinders Discs (Fig. 19.4A) Cakes in containers (Fig. 19.4B)

Properties It is harder and has a higher melting temperature than most other waxes. It powders or flakes on milling. Hardness

: 53 (Shore “D” Scale)

Specific Gravity

: 0.92

Melting Point

: 115 ºC

Burnout Residue

: 0.0066%

Flexural Modulus

: 45,250 PSI

Coefficient of Thermal Expansion: 7.5 x 10–5 (cm/cm/ºC)

BASEPLATE WAX Most students would be familiar with this wax. It is sometimes referred to as modeling or Type 2 (ISO 15854) wax. They are classified under pattern waxes because they are used to create the form of dentures and appliances made of acrylic and like materials. Ideally, these waxes should be easy to carve, should not chip and break at try-in and should boil out without leaving any oily residue. Flow requirements as per ISO specifications are given in Table 19.3.

338 Part 5 Dental Laboratory—Materials and Processes Table 19.3 Flow requirements of Baseplate wax* Temperature

Type 2 Baseplate wax Class 1 Min.

°C

Class 2 Max.

Min.

Class 3 Max.

Min.

Max.

23,0 ± 0,1

–

1,0

–

0,6

–

0,2

30,0 ± 0,1

–

37,0 ± 0,1

5,0

90,0

–

10,0

–

1,2

40,0 ± 0,1

–

45,0 ± 0,1

–

50,0

90,0

5,0

50,0

* ISO 15854:2005

Figures 19.5A and B (A) Baseplate wax. (B) Occlusion rims.

Uses These waxes are used for the following 1. To make occlusion rims (Fig. 19.5B). 2. To form the contour of the denture after teeth are set. 3. To make patterns for orthodontic appliances and other prostheses which are to be constructed of plastics.

Classification (ISO 15854:2005) Type I Type II Type III

Soft Hard Extra-hard

— — —

for building veneers to use in mouths in normal climates for use in tropical climates

Supplied As Sheets of pink or red color (Fig. 19.5A).

Composition Component

Percent

Paraffin or ceresin

80.0

Beeswax

12.0

Carnauba

2.5

Natural or synthetic resins

3.0

Microcrystalline

2.5

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PROCESSING WAXES These are those waxes used mainly as accessory aids in the construction of a variety of restorations and appliances, either clinically or in the laboratory, e.g. boxing wax, beading wax, utility wax, blockout wax, carding wax and sticky wax.

BOXING WAX AND BEADING WAX Uses Used to build up vertical walls around the impression, in order to pour the stone and make a cast. The procedure is known as boxing (Fig. 19.6).

Supplied As Boxing wax as sheets, beading wax as strips (Figs. 19.7A and B).

Advantages of Beading and Boxing 1. 2. 3. 4.

Preserves the extensions and landmarks. Controls the thickness of the borders. Controls the form and thickness of the base of the cast. Conserves the artificial stone.

Properties They are pliable and can be adapted easily. A slight tackiness allows it to stick to the impression. Note The terms carding wax and boxing wax have been used interchangeably. Carding wax was the original material on which porcelain teeth were fixed when received from the manufacturer. Boxing wax is a more acceptable term.

Figure 19.6 Boxed impression ready for pouring stone.

A Figures 19.7A and B (A) Beading wax. (B) Boxing wax.

340 Part 5 Dental Laboratory—Materials and Processes

Figures 19.8A and B (A) Utility wax is often used for tray extension (B).

Technique Beading wax is adapted around the periphery. This wax should be approximately 4 mm wide and 3–4 mm below the borders of the impression. The height is adjusted until a boxing wax strip extends approximately 13 mm above the highest point on the impression. Stone is vibrated into the boxed impression.

UTILITY WAX Composition Consists mainly of beeswax, petrolatum, and other soft waxes in varying proportions.

Supplied As It is available in the form of sticks and sheets (Figs. 19.8A and B).

Uses It is used to adjust contour of perforated tray for use with hydrocolloids (e.g. to raise flange height, to extend the tray posteriorly, to raise palatal portion of the tray in cases of deep palate, etc.). It is pliable and can be easily molded. It is adhesive and can stick to the tray.

STICKY WAX Composition It consists mainly of yellow beeswax, rosin, and natural resins such as gum dammar.

Properties It is sticky when melted and adheres closely to the surfaces to which it is applied. At room temperature, it is firm, free from tackiness, and brittle (Fig. 19.9).

Uses Used for joining (assembling) metal parts before soldering and for joining fragments of broken dentures before repair procedure. A variety of other uses, mainly joining, are possible with this wax.

Carding Wax Carding wax is used by manufacturers for the packaging of acrylic or porcelain teeth (Fig. 19.10). They are soft, tacky and pliable at room temperatures. The are available as sheets or strips.

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Figure 19.9 Different forms of sticky wax.

Figure 19.10 Carding wax.

Shellac Shellac was once extensively used in dentistry to fabricate temporary denture bases and custom trays. It is also used for bite registration.

Composition It contains shellac wax which is a wax from the lac insect Kerria lacca, plasticizers like stearin and stearic acid and fillers like mica (strength), talc. Some contain aluminum which is also used as a filler to adjust viscosity. It may be white (bleached), brown (natural color) and pink or bronze (dye) (Figs. 19.11A and B). Heating of the shellac in water above 70 °C causes leaching of the plasticizers. Heating over flame above 100 °C results in polymerization with release of water (characterized by bubbling). This results in a marked increase in its viscosity (becomes stiffer).

Manipulation Being a thermoplastic material, it is manipulated by softening with heat to adapt, cut and shape it (Fig. 19.11C).

Drawbacks Again being a thermoplastic material, it is affected by heat and is, therefore, potentially unstable and subject to distortion. It is now largely replaced by resins which are more stable.

342 Part 5 Dental Laboratory—Materials and Processes

Figures 19.11A to C (A) White shellac. (B) Brown shellac. (C) Fabrication of shellac baseplate.

IMPRESSION WAXES These are used to record non-undercut edentulous portions of the mouth, and are generally used in combination with other impression materials such as polysulfide rubber, ZOE, or dental impression compound, e.g. corrective impression wax, bite registration wax.

CORRECTIVE IMPRESSION WAX Waxes were used widely in the past for making dental impressions. Waxes are highly unstable and susceptible to distortion and are, therefore, not particularly suited for conventional impressions. However, they may be used in certain situations.

Types 1. 2. 3. 4. 5.

Aluwax (Fig. 19.12A) Korecta wax (No. 4) (extra soft - orange) (Fig. 19.12B) Iowa wax—Available as 6 inch sticks or in a small container (Fig. 19.12C) H-L physiologic paste (yellow-white) Adaptol (green) (Fig. 19.12D)

Uses It is used as a wax veneer over an original impression to contact and register the details of the soft tissues. 1. To make functional impression of free end saddles (Class I and II removable partial dentures). 2. To record the posterior palatal seal in dentures. 3. Functional impression for obturators.

Composition and Properties They consist of paraffin, ceresin and beeswax. It may also contain metal particles like copper or aluminium. One product (Aluwax, Fig. 19.12A) uses aluminum particles. The flow at 37 °C is 100%. These waxes are subject to distortion during removal from the mouth. They should be poured immediately. Each grade is designed for a specific purpose.

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Figures 19.12A to D Impression waxes. (A) Aluwax. (B) Korecta wax orange. (C) Iowa wax. (D) Adaptol Green.

BITE REGISTRATION WAX Uses It is used to record the relationship between the upper and lower teeth. This is necessary in order to mount the casts correctly in the articulator.

Supplied As U-shaped rods or wafers (Figs. 19.13 and 19.14). A thin metallic foil may be present on the undersurface or between the wax layers.

Composition Beeswax or paraffin or ceresin. Some contain aluminum or copper particles.

Procedure The wax is softened in warm water. The soft wax is then placed between the teeth and the patient is asked to bite. After the wax hardens, it is then taken out and placed in chilled water.

344 Part 5 Dental Laboratory—Materials and Processes

Figure 19.13 Bite registration wax.

Figure 19.14 Bite registration process.

It is replaced back in the mouth and the patient asked to occlude for a final check. The casts of the patient is placed in the indentations formed by the teeth in the wax. It is then mounted with plaster on the articulator. Mounting should not be delayed as wax distortion can lead to inaccurate results. Bite registration can be done with other materials like zinc oxide, eugenol and silicones.

20 Chapter

Dental Investments and Refractory Materials Chapter Outline • Requirements of an Investment • Classification of refractory Materials

• General Composition • Gypsum Bonded Investments • Properties of Gypsum

Investments –– Thermal Behavior of Gypsum –– Thermal Behavior of Silica

–– Expansion –– Normal Setting Expansion –– Hygroscopic Setting

Expansion –– Factors Affecting Hygroscopic Setting Expansion –– Thermal Expansion • Phosphate Bonded Investments

• Specialized Refractory Materials • Phosphate Bonded Refractory Casts for Rpd (Type II)

• Investments for Ceramics • Investments for Titanium Castings

• Silica Bonded Investments • Brazing (Soldering) Investments

A refractory is a nonmetallic material that can withstand high temperatures without degrading, softening, or losing its strength. An investment can be described as a ceramic material which is suitable for forming a mold into which molten metal or alloy is cast (Fig. 20.1). The procedure for forming the mold is described as ‘investing’. These materials can withstand high temperatures. For this reason, they are also known as refractory materials. Investment materials are covered by ISO 15912:2006. This standard also covers, brazing investments and refractory die materials.

REQUIREMENTS OF AN INVESTMENT MATERIAL 1. The investment mold must expand to compensate for the alloy shrinkage, which occurs during the cooling of the molten alloy. 2. The powder should have a fine particle size to give a smooth surface to the casting. 3. The manipulation should be easy. It should have a suitable setting time. 4. The material should have a smooth consistency when mixed. 5. The set material should be porous enough to permit air in the mold cavity to escape easily during casting. 6. At higher temperatures, the investment must not decompose to give off gases that may corrode the surface of the alloy. 7. It must have adequate strength at room temperature to permit handling, and enough strength at higher temperatures to withstand Figure 20.1 Cross-section through a mould. the impact force of the molten metal.

346 Part 5 Dental Laboratory—Materials and Processes 8. Casting temperatures should not be critical. 9. After casting, it should break away readily from the surface of the metal and should not react chemically with it. 10. The material should be economical.

CLASSIFICATION OF REFRACTORY MATERIALS IN DENTISTRY (ISO 15912:2006) The classification covers all refractory materials in dentistry including casting investment, brazing investments and refractory dies.

A. Classification based on application (ISO 15912:2006) Type 1: for the construction of inlays, crowns and other fixed restorations Type 2: for the construction of complete or partial dentures or other removable appliances Type 3: for the construction of casts used in brazing procedures Type 4: for the construction of refractory dies

B. Sub-classification based on method of burnout (ISO 15912:2006) Class 1: recommended for burn-out by a slow- or step-heating method Class 2: recommended for burn-out by a quick-heating method

C. Classification based on type of binder used There are three types of investment materials based on the binder used. They all contain silica as the refractory material. The type of binder used is different. 1. Gypsum bonded investments They are used for casting gold alloys. They can withstand temperature up to 700 °C. 2. Phosphate bonded investments For metal ceramic and cobalt-chromium alloys. They can withstand higher temperatures. 3. Ethyl silica bonded investments They are an alternative to the phosphate bonded investments, for high temperature casting. They are principally used in the casting of base metal alloy partial dentures.

General COMPOSITION OF INVESTMENTS All investment materials contain a refractory, a binder and modifiers.

Refractory A refractory is a material that will withstand high temperatures without decomposing or disintegrating, e.g. silica. Allotropic forms Silica exists in at least four allotropic forms.

Quartz Tridymite Cristobalite Fused quartz

They serve two functions 1. Act as a material that can withstand high temperatures. 2. Regulate the thermal expansion.

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Figure 20.2 Representative gypsum bonded investments.

Binder A material which will set and bind together the particles of refractory substance, e.g. gypsum, phosphate and silicate. The common binder used for gold alloys is dental stone (alphahemihydrate). The investments for casting cobalt chromium alloys use ethyl silicate, ammonium sulphate or sodium phosphate.

Chemical modifiers Chemicals such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide are added in small quantities to modify properties.

GYPSUM BONDED INVESTMENTS CLASSIFICATION The ISO (15912:2006) classification for refractory materials applies to gypsum bonded investments and phosphate bonded investments. Thus, there are four types based on application—Type 1, 2, 3 and 4 and two subclasses—Class 1 and 2 based on method of burnout. (See ISO classification at beginning of chapter for details).

Uses For casting of inlays, fixed partial dentures, removable partial denture frameworks using gold alloys and other low-fusing alloys.

Supplied as Powder in bulk or preweighed packs (Fig. 20.2). Representative commercial products Cristobalite, Novocast (Whipmix), etc.

Composition Component

Proportion

Silica Alpha-hemihydrate (dental stone) Chemical modifiers

60 to 65% 30 to 35% 5%

* The original name for the standard was ‘gypsum bonded investments for gold alloy casting investments.’ In the 2001 revision, the limitation to ‘gold alloys’ was removed.

348 Part 5 Dental Laboratory—Materials and Processes Functions of Constituents Alpha hemihydrate

It binds and holds the silica particles together. Permits pouring of the mix into the mold. It imparts strength to the mold. Contributes to mold expansion (by setting expansion).

Silica quartz or cristobalite

Acts as a refractory during heating. Regulates thermal expansion. Increases setting expansion of stone. Silica in the investment eliminates contraction of gypsum and changes it to an expansion during heating.

Modifiers

Coloring matter Reducing agents They reduce any oxides formed on the metal by providing a nonoxidizing atmosphere in the mold when the mold alloy enters, e.g. carbon or copper powder. Modifying chemicals They regulate setting expansion and setting time and also prevent shrinkage of gypsum when heated above 300 °C, e.g. boric acid and sodium chloride.

MANIPULATION The measured quantity of powder and water is mixed manually using a flexible rubber bowl and spatula or in a vacuum investment mixing machine.

SETTING REACTION The setting reaction is similar to dental stone. When the water is mixed, the hemihydrate reacts to form dihydrate which sets to form a solid mass which binds the silica particles together.

Setting Time According to ADA Sp. No. 2 for inlay investments, setting time should not be less than 5 minutes and not more than 25 minutes. The modern inlay investments set initially in 9 to 18 minutes. This provides sufficient time for mixing and investing the pattern.

Factors controlling setting time 1. 2. 3. 4. 5.

Manufacturing process Mixing time and rate Water-powder ratio Temperature Modifiers—accelerators and retarders

PROPERTIES OF GYPSUM INVESTMENTS Thermal Behavior of Gypsum When gypsum is heated to a high temperature, it shrinks and fractures. At 700 °C, it shows slight expansion and then great amount of contraction. The shrinkage is due to decomposition

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and release of sulfur dioxide. It contaminates the casting with the sulfides of silver and copper. So the gypsum bonded investments should not be heated above 700 °C.

Thermal Behavior of Silica When heated, quartz or cristobalite changes its crystalline form. This occurs at a transition temperature, characteristic of the particular form of silica. Quartz when heated, inverts from a ‘low’ form known as alpha-quartz to a ‘high’ form called as beta-quartz at a temperature of 375 °C. Cristobalite similarly when heated, inverts from ‘low’ or alpha-cristobalite to ‘high’ or betacristobalite at a temperature between 200 °C and 270 °C. The beta forms are stable only above the transition temperature. It changes back to the low or alpha-form occurs upon cooling in each case. The density changes (decreases) as alphaform changes to beta-form, with a resulting increase in volume and a rapid increase in linear expansion.

Expansion Expansion aids in enlarging the mold to compensate for the casting shrinkage of the gold alloys. Three types of expansions may be seen 1. Normal setting expansion 2. Hygroscopic setting expansion 3. Thermal expansion

Normal setting expansion A mixture of silica and dental stone results in a setting expansion which is greater than when the gypsum product is used alone. The silica particles probably interfere with the intermeshing of the crystals as they form. Thus, the thrust of the crystals is outward during growth. ADA Sp. No. 2 for Type-I investment permits a maximum setting expansion in air of 0.5%. Modern investments show setting expansion of 0.4%. It is regulated by retarders and accelerators.

Hygroscopic setting expansion (HSE) When gypsum products are allowed to set in contact with water, the amount of expansion exhibited is much greater than the normal setting expansion. The increased amount of expansion is because water helps the outward growth of crystals. This expansion is known as hygroscopic setting expansion. The investment should be immersed in water before initial set is complete. ADA Sp. No. 2 for Type-II investments requires a minimal 1.2% and maximum 2.2% expansion. Factors affecting hygroscopic setting expansion 1. Composition The finer the particle size of the silica, the greater is the HSE. Alphahemihydrate produces a greater expansion than beta-hemihydrate. Higher the silica content, greater is the expansion. 2. W-P ratio The higher the W-P ratio of the original investment water mixture, the less is the HSE. 3. Temperature Higher the temperature of the immersion water, less is the surface tension and hence, greater is the expansion. 4. Effect of time of immersion Immersion before the initial set results in greater expansion.

350 Part 5 Dental Laboratory—Materials and Processes 5. Spatulation Shorter the mixing time, the less is the HSE. 6. Effect of shelf-life of the investment The older the investment, the less is the hygroscopic expansion. 7. Confinement of the investment by the walls of the container or the wax pattern reduces HSE. This effect is much more pronounced on the HSE than on the normal setting expansion. 8. Effect of the amount of added water More amount of water added during the setting period, more is the expansion.

Thermal expansion In case of gypsum investments, thermal expansion (TE) is achiev ed by placing the mold in a furnace at a temperature not greater than 700 °C (the investment breaks down if it exceeds this temperature releasing gases which can contaminate the gold alloys). The thermal expansion behavior of one investment is shown in Fig. 20.3. The amount of thermal expansion required depends on which method is used for casting shrinkage compensation. If hygroscopic expansion technique is used, then TE of 0.5 to 0.6% is sufficient. But if the compensation is by TE together with normal setting expansion, then the TE should be 1 to 2%.

Figure 20.3 Thermal expansion of a gypsum bonded investment (Novocast, Courtesy: Whipmix Corporation).

Type-l investments should have a TE not less than 1 nor greater than 1.6%. Factors affecting thermal expansion 1. TE is related to the amount and type of silica used. 2. Effect of the W-P ratio more the water, less the TE. 3. Effect of chemical modifiers Small amounts of sodium chloride, potassium chloride and lithium chloride increases TE and eliminates the contraction caused by gypsum.

Strength According to ISO 15912:2006, the compressive strength for investments should not be less than 2 MPa when tested 2 hours after setting.

Factors affecting strength 1. 2. 3. 4.

Use of alpha-hemihydrate increases compressive strength (than beta-hemihydrate). Use of chemical modifiers increases the strength. More water used during mixing, less is the strength. Heating the investment to 700 °C may increase or decrease strength as much as 65% depending on the composition. The greatest reduction in strength upon heating is found in investments containing sodium chloride. 5. After the investment has cooled to room temperature, its strength decreases considerably because of fine cracks that form during cooling.

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Porosity The more the gypsum crystals present in the set investment, the less is its porosity. The less the hemihydrate content and greater the amount of gauging water, the more is its porosity. A mixture of coarse and fine particles exhibits less porosity than an investment composed of a uniform particle size (a certain amount of porosity is essential in the mold in order to allow escape of gases during casting).

Fineness A fine particle size is preferable to a coarse one. The finer the investment, the smaller will be the surface irregularities on the casting.

Storage Investments should be stored in airtight and moisture proof containers. Purchase in small quantities.

Hygroscopic thermal inlay casting investment Investment that can be used as a hygroscopic or thermal type is available (e.g. Beauty cast - Whipmix). The investment contains a blend of quartz and cristobalite as the refractory. For the hygroscopic expansion technique, the investment is heated only up to 482 °C. When the thermal casting technique is used the investment (is not immersed in water but) is heated to 649 °C to achieve expansion.

INVESTMENTS FOR CASTING HIGH MELTING ALLOYS The metal-ceramic alloys and the cobalt-chromium alloys have high melting temperatures. They are cast in molds at 850 to 1100 °C. At these temperatures, the gypsum bonded investments disintegrates. Hence, investments which can withstand higher temperatures are required. The binders used (phosphate and silicate) in these investments can withstand these high temperatures. The investments used for this purpose are

Phosphate bonded investments Silica bonded investments Magnesia/alumina/zirconia based investments for titanium

Phosphate bonded investment Phosphate bonded investments are perhaps the most widely utilized investment in dentistry. This is because a substantial amount of cast dental structures today use high fusing noble or base metal alloys.

Uses For casting high fusing alloys, e.g. high fusing noble metal alloys, metal ceramic alloys and base metal alloys like nickel-chromium and cobalt-chromium.

Classification The ISO (15912:2006) classification for refractory materials applies to phosphate bonded investments also. Thus, there are four types based on application—Type 1, 2, 3 and 4 and two subclasses—Class 1 and 2 based on method of burnout. (See ISO classification at beginning of chapter for details).

352 Part 5 Dental Laboratory—Materials and Processes Supplied as Powder in packets of varying weight with special liquid (Fig. 20.4).

Composition Powder Ammonium diacid phosphate NH4H2PO4 It gives strength at room temperature. It is soluble in water and provides Figure 20.4 Representative phosphate bonded phosphate ions. investment. They are mixed with a special liquid. It reacts with silica at high temperatures to increase strength at casting temperatures. Silica in the form of quartz or cristobalite (80%) functions as refractory.

Magnesium oxide Reacts with phosphate ions. Carbon Some investments contain carbon while others are carbon free. Carbon helps to produce clean castings and helps in easier divestment from the mold. For noncompatible alloys carbon free investments are preferred.

Liquid The phosphate bonded investments are mixed with a special liquid supplied by the manufacturer. This liquid is a form of silica sol in water, which gives higher thermal expansion.

Setting Reaction At room temperature ammonium diacid phosphate reacts with magnesium oxide to give the investment green strength or room temperature strength. NH4H2PO4 + MgO + 5H2O

NH4MgPO4.H2O

The ammonium diacid phosphate is used in a greater amount than is necessary for this reaction, so that the additional amount can react with silica at an elevated temperature. At higher temperatures, there is probably a superficial reaction between P2O5 and SiO2 to form silicophosphate, which increases the strength of investment at higher temperature.

Manipulation Powder/liquid ratio - 16 to 23 ml/100 gm. (The liquid is usually diluted with water. The amount of liquid to water ratio varies with the particular brand of investment and type of alloy used. The amount of water used ranges from 0 to 50% depending on the expansion required). The powder is mixed with a measured amount of liquid using a bowl and spatula. Following hand mixing for 20 seconds mechanical mixing under vacuum is done for a further 90 seconds (Fig. 20.5). Working time is around 8-9 minutes. The mixed material is vibrated into the casting ring or agar mold (RPD framework). The material is allowed to bench set for a minimum 30-45 minutes depending

Figure 20.5 Vacuum investment mixer.

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on the particular investment. Following this the glaze on top of the investment is scraped to allow air escape and reduce back pressure porosity during casting. Factors affecting setting time 1. Temperature of the mix and environment. Warmer temperatures accelerate the setting. Cooling the liquid prolongs the working time. 2. Increasing the mixing time accelerates the set. 3. An increased L-P ratio delays setting and gives more working time.

Properties Expansion As mentioned earlier, expansion of the mold is desirable to compensate for casting shrinkage. Phosphate investments get their expansion from three sources. 1. Wax pattern expansion The heat during setting allows a significant expansion of the wax pattern. 2. Setting expansion This is around 0.7 to 1%. 3. Thermal expansion Ranges from around 1 to 1.5%. The amount of expansion is adjusted by the manufacturer for each product depending on the alloy it is intended for. Factors affecting expansion 1. Special liquid to water ratio The liquid has a considerable influence on the setting and thermal expansion of the investment. The greater the concentration of special liquid to water, the greater the thermal and setting expansions (Fig. 20.6). 2. Powder to liquid ratio A greater powder to liquid ratio increases expansion.

Strength Regular investments are generally materials of low strength. Wet strength ranges from 4–10 MPa. Wet strength is important for handling the set material prior to casting. Dry strength is the strength of the investment under high temperatures. The investment should have sufficient strength to withstand the casting force of the molten alloy at high temperatures. Studies have shown that there is no correlation between wet and dry strength of phosphate bonded investments. One study indicates that investments exhibit plastic behavior at high

Figure 20.6 Influence of liquid to water concentration on the setting and thermal expansion of a phosphate bonded investment (Courtesy: Whip-mix Corporation).

354 Part 5 Dental Laboratory—Materials and Processes temperatures which, under casting pressure, may be a source of inaccurate casting, a hitherto unrecognized source of error.

Thermal reactions Phosphate bonded investments undergo thermochemical reactions when heated to high temperatures. The silica portion remains essentially unchanged. However the binder goes through various phases. On heating, the material initially dehydrates to (NH4MgPO4.H2O)n. Subsequently, it degrades into polymeric (Mg2P2O7)n, crystalline Mg2P2O7; then the latter reacts with excess MgO present to form the final product, Mg3(PO4)2.

Flow Investments appear to have low flow when mixed. However, they flow readily and envelope the pattern when poured into the mold under vibration. Therefore, use of a vibrator is recommended. Surface tension reducing agents are available and should be used on the wax pattern to improve wetting.

Surface smoothness Early phosphate investments produced rough castings when compared to gypsum based investments. Current investments have improved and now approach surface smoothness comparable to that of gypsum bonded investments.

Specialized refractory materials Phosphate bonded Refractory casts for RPDs (Type 2) A refractory cast is a special cast made from a heat resistant (investment) material. Such casts are used in the fabrication of certain large metal structures like cast removable partial dentures. Small wax structures like inlays, crowns and small fixed dental prostheses (FDP) can be constructed on a regular die as it can be removed from the die without significant distortion and invested separately. However, larger wax structures like that for the cast RPD, would distort if removed from the cast. RPD patterns are best constructed on a refractory cast (Fig. 20.7). The pattern is invested together with the refractory cast.

Investments for ceramics

Figure 20.7 Refractory casts with pattern and metal casting of the same.

Phosphate based investments are also available for ceramic restorations (e.g. Polyvest and VHT - Whipmix, Fig. 20.8).

Types Two types are available 1. Those used as refractory dies or casts to construct all-porcelain restorations like porcelain veneers and porcelain jacket crowns (ISO 11245). Two varieties are seen based on expansion.

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Figure 20.8 Representative phosphate bonded investment used for construction of ceramic restorations.

––

Medium expansion—Originally used for the cast glass technique. Currently used with medium expanding porcelains like Finesse, Ceramco Veneer, Duceram (Dentsply), Vita-VMK 68, Halo (3M), Noritake EX 3, etc. Example Polyvest. –– High expansion—The second type is used with high expansion porcelains like OptecHSP/VP (Jeneric/Pentron Inc.), Wil-Ceram (Williams Dental Co.), Excelco (Ney), Creation (Jensen). 2. The second type is used with heat pressed ceramics (Figs. 20.9A to C). It is not used as a die, rather, it is used to surround the wax pattern for the heat pressing process.

Properties These are fine-grained phosphate investment with a working time of 2–5 minutes. Unlike regular refractory materials, these can withstand repeated firings at furnace temperatures of up to 1200 °C without disintegrating. They can be used with high-expanding porcelains because of their compatible CTEs.

Investments for titanium castings Conventional silica (SiO2) based dental casting investments are used for the casting of pure titanium using casting machines specifically developed for this metal. Highly reactive molten

Figures 20.9A to C (A) Phosphate bonded investment for heat pressed ceramics. (B) Wax pattern attached to sprue former. (C) Wax pattern is placed inside the silicone mould former into which the investment is poured.

356 Part 5 Dental Laboratory—Materials and Processes titanium reduces SiO2 and titanium is, in turn, oxidized. For this reason possible alternatives to SiO2 have been studied in the past decade and MgO (magnesia) and Al2O3 (alumina) are the most common in current commercial investments released for titanium casting. The surface of titanium castings presents a layered structure and its evaluation in relation to clinical performance requires further study especially in relation to the setting and thermal behavior of newly developed investments for successful compensation of metal shrinkage.

SILICA BONDED INVESTMENTS The silica is the binder. It is derived from ethyl silicate or aqueous dispersion of colloidal silica or sodium silicate. These are less commonly used.

Types Based on the binder used two types may be seen. One such investment consists of silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid. The product of the hydrolysis is the formation of a colloidal solution of silicic acid and ethyl alcohol.

Si(OC2H5)4 + 4H2

HCl

Si(OH)4 + 4C2H5OH

Ethyl silicate has the disadvantage of containing inflammable components which are required for manufacture. Sodium silicate and colloidal silica are more commonly used as binders because of the above disadvantage. These investments are supplied along with two bottles of special liquid. One bottle contains dilute water-soluble silicate solution such as sodium silicate. The other bottle usually contains diluted acid solution such as hydrochloric acid (Fig. 20.10).

Manipulation The content of each bottle can be stored indefinitely. Before use, equal volume of each bottle is mixed so that hydrolysis can take place and freshly prepared silicic acid is formed. The powder/liquid ratio is according to manufacturer’s instruction.

Brazing (SOLDERING) INVESTMENT In the process of assembling the parts of a restoration by soldering (Fig. 20.11), such as clasps on a removable partial denture, it is necessary to surround the parts with a suitable ceramic or investment (Figs. 20.12A and B).

Figure 20.10 Ethyl silicate bonded investment by Nobilium.

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A Figure 20.11 Soldering procedure.

357

Figures 20.12A and B Soldering investments. (A) Gypsum bonded. (B) Phosphate bonded.

Uses 1. Joining segments of fixed partial dentures (Fig 20.11). 2. Fixing clasps on cast RPDs. 3. Attaching precision attachments.

Types Based on the type of binder used brazing investments are of two types. 1. Gypsum-bonded (for low melting alloys, e.g. Hi Heat, Whipmix, etc.) (Fig. 20.12A) 2. Phosphate-bonded (for high melting alloys), e.g. Bellatherm (Bego) (Fig. 20.12B).

Composition The investment for soldering purpose is similar to casting investments containing quartz and a calcium sulfate hemihydrate/or phosphate binder.

Properties Soldering investments are designed to have lower setting (0.2%) and thermal expansions (0.6–1%) than casting investments, a feature that is desirable so that the assembled parts do not shift position. Soldering investments do not have as fine a particle size as the casting investment, since the smoothness of the mass is less important. The compressive strength is generally low (between 2 to 10 MPa). Water-powder ratio ranges from 0.24 to 0.28. Setting time ranges from 15 to 20 minutes.

Procedure The parts are temporarily held together with sticky wax until they are surrounded with the appropriate investment material, after which the wax is removed. The portion to be soldered is left exposed and free from investment to permit removal of the wax and effective heating before being joined with solder (Fig. 20.11). After setting, the material must be completely dry before soldering. Recommended drying temperature varies between 400 to 450 °C.

21 CHAPTER

Dental Casting and Metal Fabrication Procedures Chapter Outline • Steps in Making a Cast • • • • • • • • •

•

Restoration Tooth/Teeth Preparation Die Preparation Wax Pattern Sprue Former Casting Ring Lining Investing Wax Elimination and Thermal Expansion Casting—Process and Equipment Casting Machines –– Torch Melting –– Induction Melting –– Arc Melting –– Crucibles Casting

• • • • • • •

Quenching (For Gold Alloys) Recovery of Casting Sandblasting Pickling Trimming Polishing Defects in Casting –– Classification –– Distortion –– Minimization of Wax Distortion –– Surface Roughness –– Porosity –– Shrink-spot or Localized Shrinkage Porosity –– Suck Back Porosity –– Microporosity –– Pin Hole Porosity

–– –– –– –– ––

• • • • • • •

Gas Inclusion Porosities Back Pressure Porosity Casting with Gas Blow Holes Incomplete Casting Too Bright and Shiny Casting with Short and Rounded Margins –– Small Casting –– Contamination –– Black Casting Other Methods of Fabricating Restorations and Prostheses Capillary Casting Technique Cad-Cam Milling Copy Milling Electroforming Electrical Discharge Machining Additive Manufacturing

The process of casting has been known since ancient times (Box 21.1). Casting is the most commonly used method for the fabrication of metal structures (inlays, crowns, partial denture frames, etc.) outside the mouth. A pattern of the structure is first made in wax. This is then surrounded by an investment material. After the investment hardens, the wax is removed (burnt out) leaving a space or mould. Molten alloy is forced into this mould. The resulting structure is an accurate duplication of the original wax pattern. However, casting is not the only way of fabricating restorations and prostheses in dentistry.

METAL RESTORATIONS IN DENTISTRY There are many ways of fabricating a metallic restoration in dentistry. 1. 2. 3. 4.

Direct restorations (e.g. direct filling gold and amalgam) Casting (e.g. cast crowns, posts, inlays, partial denture frames, etc.) Foil adaptation and sintering * (e.g. CAPTEK crowns) Electroforming

* Refer chapter on dental ceramics also

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BOX 21.1 Evolution of casting process Since its discovery, metal casting has played a critical role in the development and advancement of human cultures and civilization. The art of casting metal has been known to man since 4000 BC. Mesopotamia is generally accepted as the birthplace of castings. The first metal to be cast was copper because of its low melting point. Furnaces The earliest furnace were simple and easy to operate, with beeswax used for patterns and bellows for blowing air into the furnace. In the iron age probably ceramic ovens were used to melt the metals. Crucible and later flame ovens were available for the melting of copper, tin and lead alloys. Molds Different types of molds made from sand, stone, limestone and sun-baked clay were known from the early times. Patterns The lost form technique was also prevalently used from the early times. The first patterns of casting were made probably 4000 years in Mesopotamia from beeswax. The oldest known examples of the lost-wax technique are the objects discovered in the Cave of the Treasure (Nahal Mishmar) hoard in southern Israel, and which belong to the Chalcolithic period (4500–3500 BCE).

Objects from Cave of Treasure (Nahal Mishmar), Southern Israel, c. 3700 BCE.

The lost-wax method is well documented in ancient Indian literary sources. The Shilpa shastras, a text from the Gupta Period (c. 320-550 CE), contains detailed information about casting images in metal. 9000 BCE 5000–3000 BCE 3000–1500 BCE 2400–2200 BCE 2000 BCE 1500 BCE 1100 BCE 600 BCE 500 CE 400 CE

Earliest metal objects of wrought native copper Near East Chalcolithic period: melting of copper; experimentation with smelting in the Near East Bronze Age: Copper and tin bronze alloys—Near East and India Copper statue of Pharoah Pepi I Egypt Bronze Age Far East Iron Age (wrought iron) Ganga valley, India Discovery of wrought steel 11th century BCE Iron cast in China Steel casting in India Zinc extraction in India. Distillation technique developed in 1200 CE in India (Zawar, Rajasthan.)

A 19th century hand powered centrifugal casting machine

5. Machining (subtractive fabrication) –– Prefabricated posts –– CAD/CAM –– Copy milling –– Electrical discharge machining. 6. Three-dimensional printing (additive fabrication).

Bronze dancing girl, Indus valley, (c. 3300–1300 BCE)

Oldest iron pillar (circa 400 BCE) Ashoka’s Pillar, New Delhi, India remains unrusted for 24 centuries

360 PART 5 Dental Laboratory—Materials and Processes CASTING Many dental restorations are made by casting, e.g., inlays, crowns, removable partial denture frameworks, etc. Casting can be defined as the act of forming an object in a mold (GPT-8). The object formed is also referred to as ‘a casting’.

STEPS IN MAKING A SMALL CAST RESTORATION Casting is a complex process involving a number of steps and equipment. A restoration having a perfect fit is possible only if we have a good understanding of the techniques and materials used in casting. Given below are the series of steps involved in the fabrication of a simple full metal crown.

Tooth/teeth preparation Impression Die preparation Wax pattern fabrication Attachment of sprue former Ring liner placement Assembly of casting ring Investing Burn out or wax elimination Casting Sand blasting and recovery Finishing and polishing.

The procedures vary slightly depending on the type of restoration. Construction of larger structures like a removable partial denture frame involve additional steps like duplication.

TOOTH/TEETH PREPARATION The teeth are prepared by the dentist to receive a cast restoration. Care is taken to avoid undercuts in the preparation that may prevent seating. An accurate impression of the tooth/ teeth is made, usually with elastomers.

DIE PREPARATION A die is prepared from die stone or a suitable die material or the impression is electroformed.

DIE SPACER A die spacer is coated or painted over the die which provides space for the luting cement (Figs. 21.1A and B). The relief provided also improves seating of the casting.

WAX PATTERN A pattern of the final restoration is made with type II inlay wax (Fig. 21.2A) or other casting waxes with all precautions to avoid distortion. Before making the pattern, a die lubricant is applied to help separate the wax pattern from the die.

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FIGURES 21.1A AND B (A) Application of die spacer. (B) Close-up view of the spacer coated die. Two types of spacers are visible.

SPRUE FORMER A sprue former is made of wax, plastic or metal. The thickness is in proportion to the wax pattern. A reservoir is attached to the sprue or the attachment of the sprue to the wax pattern is flared. The length of the sprue is adjusted so that the wax pattern is approximately 1/4” from the other end of the ring (Fig. 21.3).

FUNCTIONS OF SPRUE FORMER/SPRUE 1. 2. 3. 4.

To form a mount for the wax pattern. To create a channel for the elimination of wax during burnout. Forms a channel for entry of molten alloy during casting. Provides a reservoir of molten metal which compensates for alloy shrinkage during solidification.

CASTING RING LINING A ring liner is placed inside of the casting ring. It should be short at one end. Earlier asbestos liners were used. Its use has been discontinued due to health hazard from breathing its dust.

TYPES OF NONASBESTOS RING LINERS 1. Fibrous ceramic aluminous silicate 2. Cellulose (paper) 3. Ceramic-cellulose combination (Fig. 21.2B).

FIGURES 21.2A AND B (A) A rubber crucible former with attached wax pattern and casting rings. (B) Ring liner.

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FIGURE 21.3 An assembled casting ring.

FIGURE 21.4 A vacuum investment mixer.

FUNCTIONS OF THE RING LINER 1. Allows for mold expansion (cushion effect). 2. When the ring is transferred from the furnace to the casting machine it reduces heat loss as it is a thermal insulator. 3. Permits easy removal of the investment after casting.

INVESTING Apply wetting agent (to reduce air bubbles) on the wax pattern. Seat the casting ring into the crucible former taking care that it is located near the center of the ring (Fig. 21.3). Mix the investment (in a vacuum mixer, Fig. 21.4) and vibrate. Some investment is applied on the wax pattern with a brush to reduce trapping air bubbles. The ring is reseated on the crucible former and placed on the vibrator and gradually filled with the remaining investment mix. It is allowed to set for 1 hour.

WAX ELIMINATION (BURNOUT) AND THERMAL EXPANSION The purpose of burnout is 1. To eliminate the wax (pattern) from the mold. 2. To expand the mold (thermal expansion). The crucible former is separated from the ring. If a metallic sprue former is used, it should be removed before burnout. Burnout is started when the mold is wet. If burnout has to be delayed the mold is stored in a humidor. The heating should be gradual. Rapid heating produces steam which causes the walls of the mold cavity to flake. In extreme cases an explosion may occur. Rapid heating can also cause cracks in the investment due to uneven expansion. It is very important to follow the investment manufacturer’s technique regarding time and temperature for burnout and expansion. Two stage burnout and expansion technique This technique may be used for wax but is particularly indicated if the patterns or sprue formers contain plastic.

The ring is placed in a burnout furnace (Fig. 21.5) and heated gradually to 400 °C in 20 minutes.

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Maintain it for 30 minutes. Over the next 30 minutes, the temperature is raised to 700 °C and maintained for a further 30 minutes.

Single stage burnout and expansion (Rapid technique) This technique is followed only if patterns and sprues are wax

Place molds directly into preheated oven at 700–850 °C (if higher temperature is warranted, place mold in preheated oven at 370 °C and then raise to final temperature). Hold for 30–40 minutes and cast.

FIGURE 21.5 Wax elimination and thermal expansion of mold

The casting should be completed as soon as the ring is ready. If casting is delayed the ring cools and the investment contracts. The crown becomes smaller.

CASTING-PROCESS AND EQUIPMENT It is the process by which molten alloy is forced into the heated investment mold.

CASTING MACHINES Based on method of casting the machines are 1. Centrifugal force type 2. Air pressure type Centrifugal machines may be spring driven or motor driven (Fig. 21.6). The main advantage of the centrifugal machines is the simplicity of design and operation, with the opportunity to cast both large and small castings on the same machine.

FIGURE 21.6 An induction casting machine. The molten metal is driven into the mold by centrifugal force. One arm of the machine has a counter weight (CW) which balances the weight of the arm carrying the crucible and mold as it rotates. The red hot crucible (C) and the casting ring is visible in the machine. The induction coil (IC—copper colored) is half visible and is used to melt the metal.

In air pressure type of machine, either compressed air or gases like carbon dioxide or nitrogen, can be used to force the molten metal into the mold. This type of machine is satisfactory for making small castings. This machine does not have vibration and high noise levels owing to the pressure casting and water cooling method. Some systems use argon gas to protect the alloy from oxidation (especially useful for melting titanium, Fig. 21.10). Attached vacuum system Casting machines (both centrifugal and gas pressure type) with attached vacuum system are available. The vacuum creates a negative pressure within the mold, which helps to draw the alloy into the mold. Casting machines can also be grouped based on heating system employed 1. Torch melted (Fig. 21.7) 2. Induction melted (Fig. 21.6) 3. Arc melted. Numerous combinations of these principles are employed in different machines.

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FIGURE 21.7 Flame melting.

FIGURE 21.8 Parts of the flame (A) Mixing zone. (B) Combustion zone. (C) Reducing zone. (D) Oxidizing zone.

Torch melting The fuel used is a combination of Natural or artificial gas and air, or Oxygen and acetylene gas (high fusion alloys). The flame has four zones (Fig. 21.8)

A. Mixing zone Air and gas are mixed here. No heat is present. It is dark in color. B. Combustion zone This surrounds the inner zone. It is green in color. It is a zone of partial combustion and has an oxidizing nature. C. Reducing zone It is a blue zone just beyond the green zone. It is the hottest part of the flame. This zone is used for the fusion of the casting alloy. D. Oxidizing zone Outermost zone in which final combustion between the gas and surrounding air occurs. This zone is not used for fusion. The air and gas mixture is adjusted to get a reducing flame, which is used to melt the alloy (Fig. 21.7). A reducing flame is preferred as it does not contaminate the alloy and is the hottest part of the flame.

Induction melting Heating through induction is a common method of melting dental alloys today (Figs. 21.6 and 21.9). Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater consists of an electromagnet, through which a high-frequency alternating current (AC) is passed. Induction melting is useful for melting high fusing alloys like metal-ceramic and base metal alloys.

Arc melting Alloys may also be melted by a process known as arc melting. Arc melting is used to melt industrial alloys like steel. Direct current is passed between two electrodes—a tungsten electrode and the alloy. Arc melting produces very high temperatures and is used to melt high fusion metals like titanium (Fig. 21.10). Arc melting may be done under vacuum or in an inert atmosphere like argon.

FIGURE 21.9 Induction melting. White hot molten alloy in crucible surrounded by the induction coil.

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FIGURE 21.10 Titanium casting machine (Dentaurum).

FIGURE 21.11 Casting crucible.

365

FIGURE 21.12 Casting flux.

Crucibles The crucible is a heat resistant container (Fig. 21.11) in which the alloy is melted prior to casting. Four types of casting crucibles are available. These are clay, carbon, quartz and ceramic. In dentistry, quartz or ceramic crucibles are commonly preferred as some alloys may be sensitive to carbon contamination. These include palladium-silver and nickel or cobalt based alloys.

Casting The alloy is melted with the suitable heat source. Flux powder (Fig. 21.12) may be sprinkled over the molten metal to reduce the oxides and increase fluidity for casting. When the alloy is molten it has a mirror-like appearance and shifts like a ball of mercury. The hot casting ring is shifted from the burnout furnace to the casting machine. The ring is placed in the casting cradle so that the sprue hole adjoins the crucible. The crucible is solid and placed against the ring to avoid spilling of molten metal. The arm is released and allowed to rotate. This creates a centrifugal force which forces the liquid metal into the mold cavity. The arm is allowed to rotate till it comes to rest. The ring is allowed to cool for 10 minutes till the glow of the metal disappears.

QUENCHING (FOR GOLD ALLOYS) The ring is then immersed into water. This leaves the cast metal in an annealed (softened) condition and also helps to fragment the investment. Metal-ceramic alloys and base metal alloys are not quenched.

RECOVERY OF CASTING The investment is removed and the casting recovered. A pneumatic (compressed air driven) chisel may be used to remove the investment. Final bits of investment is removed by sandblasting.

SANDBLASTING Sandblasting is the process by which particles of an abrasive (usually aluminum oxide) is projected at high velocity using compressed air in a continuous stream. The casting is held

366 PART 5 Dental Laboratory—Materials and Processes

FIGURES 21.13A AND B (A) Sandblasting in progress. (B) Close-up of sandblasting.

in a sandblasting machine (Figs. 21.13A and B) to clean the remaining investment from its surface.

PICKLING Surface oxides (e.g. black castings) from the casting are removed by pickling in 50% hydrochloric acid. HCl is heated but not boiled with the casting in it (done for gold alloys). Pickling is not a routine procedure and is performed only when indicated. Care should be exercised when handling strong acids.

FIGURE 21.14 Casting trimmed with a carbide bur.

TRIMMING The sprue is sectioned off with a cutting disc. The casting is trimmed, shaped and smoothed with suitable burs or stones (Fig. 21.14).

POLISHING Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously (see abrasives chapter).

CASTING DEFECTS A casting defect is an irregularity in the metal casting process that is very undesired. Errors in the procedure often results in defective castings. The casting in such a case may not fit or may have poor esthetic and mechanical properties.

TYPES OF CASTING DEFECTS The casting should be a replica of the pattern created in size, texture and form. Casting failures usually result from a failure to observe proper technique. Casting defects are difficult to classify because the causes and effects can often overlap. The casting defects may be classified as A. Metal excess (nodules, fins, larger castings, etc.) B. Metal deficiency (smaller casting, incomplete casting, porosity, etc.)

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C. Distortion of the casting D. Chemical contamination of the casting Trapped bubbles on the mold surface or cracks in the investment usually result in nodules or fins on the casting. Trapped air or shrinkage within the metal result in various types of porosity or voids in the metal. Casting defects may be described as follows 1. 2. 3. 4. 5. 6. 7. 8.

Casting size mismatch Distortion Surface roughness Nodules Fins Porosity Incomplete casting Contaminated casting

CASTING SIZE MISMATCH The restoration should retain its dimensions after casting. Thus the casting may be 1. Too small 2. Too large Casting size problems affect the fit of the restoration. Dimension related problems are usually related to improper technique and a failure to understand the properties of the materials involved in fabricating the restoration. Dimensional changes can occur at almost every stage of the restorative process starting with the impression procedure itself. Metal inherently shrinks on cooling and should be compensated for by proper matching of the alloy to the investment and technique.

DISTORTION Distortion of the casting is usually due to distortion of wax pattern.

Some distortion of wax occurs when the investment hardens or due to hygroscopic and setting expansion. It does not cause serious problems. Some distortion occurs during manipulation due to the release of stresses.

Wax distortion is minimized by

Manipulation of wax at high temperature. Investing pattern within one hour after finishing. If storage is necessary, store in refrigerator.

SURFACE ROUGHNESS Surface irregularities can range from surface roughness (Fig. 21.15) to larger nodules and fins.

Causes of surface roughness 1. Type of investment Phosphate bonded investments tend to have greater surface roughness when compared to gypsum bonded investments.

FIGURE 21.15 Surface roughness.

368 PART 5 Dental Laboratory—Materials and Processes 2. Composition of the investment Proportion of the quartz and binder influences the surface texture of casting. Coarse silica produces coarse castings. 3. Particle size of investment Larger particle size of investment produces coarse castings. 4. Improper W-P ratio A higher W-P ratio gives rougher casting. – Minimized by using correct W/P ratio and investment of correct particle size. 5. Prolonged heating causes disintegration of the mold cavity. – Minimized by complete the casting as soon as the ring is heated and ready. 6. Overheating of gold alloy has the same effect. It disintegrates the investment. 7. Too high or too low casting pressure. – Minimized by using 15 lbs/sq inch of air pressure or three to four turns of centrifugal casting machine. 8. Foreign body inclusion shows sharp, well-defined deficiencies. Inclusion of flux shows as bright concavities.

SURFACE NODULES Nodules (Fig. 21.16) on the inner surface of a casting can affect the fit of the restoration. They are usually caused by air or gas bubbles trapped on the wax pattern. Minimized by

Proper mixing of investment Vibration of mix Vacuum investing Painting of a think layer of investment on the pattern Application of wetting agent

FIGURE 21.16 Nodules on the inner surface of a crown.

FIN Fins are narrow raised areas on a casting usually corresponding to a crack in the investment (Fig. 21.17). Molten alloy fills and solidifies in these cracks resulting in fins. Cracks are usually caused by weak investment or too rapid a heating of the investment. Minimized by

Proper water powder ratio for improved strength of investment. FIGURE 21.17 Fins in casting. Avoid prolonged and rapid heating of the mold. Heat the ring gradually to 700 °C (in at least 1 hour). Proper spruing so as to prevent direct impact of the molten metal at an angle of 90º. Allow the investment adequate time to set properly. Avoid premature use. Careful handling of the mold to prevent it from dropping or impacting.

POROSITY Porosity may be internal or external. External porosity can cause discoloration of the casting. Severe porosity at the tooth restoration interphase can even cause secondary caries. Internal porosity weakens the restoration.

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Types of porosities 1. Those caused by solidification shrinkage –– Localized shrinkage porosity –– Suck back porosity (Irregular in shape) –– Microporosities 2. Those caused by gas –– Pin hole porosity –– Gas inclusions (Usually spherical in shape) –– Subsurface porosity 3. Those caused by air trapped in the mold (back pressure porosity) Shrink-spot or localized shrinkage porosity These are large irregular voids usually found near the sprue-casting junction (Fig. 21.18). It occurs when the cooling sequence is incorrect and the sprue freezes before the rest of the casting. During a correct cooling sequence, the sprue should freeze last. This allows more molten metal to flow into the mold to compensate for the shrinkage of the casting as it solidifies. If the sprue solidifies before the rest of the casting no more molten metal can be supplied from the sprue. The subsequent shrinkage produces voids or pits known as shrink-spot porosity. Avoid by

FIGURE 21.18 Localized shrink-spot porosity.

Using sprue of correct thickness. Attach sprue to thickest portion of wax pattern. Flaring the sprue at the point of attachment or placing a reservoir close to the wax pattern.

Suck back porosity It is a variation of the shrink spot porosity. This is an external void usually seen in the inside of a crown opposite the sprue. A hot spot is created by the hot metal impinging on the mold wall near the sprue which causes this region to freeze last. Since the sprue has already solidified, no more molten material is available and the resulting shrinkage causes a type of shrinkage called suck back porosity (Figs. 21.19A to C). It is avoided by reducing the tempe rature difference between the mold and the molten alloy.

FIGURES 21.19A TO C Suck back porosity. (A) Correct sequence of hardening. (B) Suck back porosity caused by incorrect sequence of solidification. (C) Suck back porosity in a casting.

370 PART 5 Dental Laboratory—Materials and Processes Microporosity These are fine irregular voids within the casting. It is seen when the casting cools too rapidly. Rapid solidification occurs when the mold or casting temperature is too low. Pin hole porosity Many metals dissolve gases when molten. Upon solidification the dissolved gases are expelled causing tiny voids, e.g. platinum and palladium absorb hydrogen. Copper and silver dissolve oxygen. Gas inclusion porosities Gas inclusion porosities are also spherical voids but are larger than the pin hole type. They may also be due to dissolved gases, but are more likely due to gases carried in or trapped by the molten metal. A poorly adjusted blow torch can also occlude gases. Back pressure porosity This is caused by inadequate venting of the mold. Air is trapped in the mold and is unable to escape. The sprue pattern length should be adjusted so that there is not more than 1/4” thickness of the investment between the bottom of the casting ring and the wax pattern. When the molten metal enters the mold, the air inside is pushed out through the porous investment at the bottom. If the bulk of the investment is too great, the escape of air becomes difficult causing increased pressure in the mold. The gold will then solidify before the mold is completely filled resulting in a porous casting with rounded short margins. Avoided by

Using adequate casting force. Use investment of adequate porosity. Place pattern not more than 6 to 8 mm away from the end of the ring. Providing vents in large castings.

Casting with gas blow holes If there is any wax residue remaining in the mold, it gives off a large volume of gas as the molten alloy enters the mold cavity. This gas can cause deficiencies in the casting and blow holes in the residue button. To help eliminate wax completely from the mold, the burnout should be done with the sprue hole facing downwards for the wax to run down.

INCOMPLETE CASTING An incomplete casting (Fig. 21.20) may result when 1. 2. 3. 4. 5. 6. 7. 8.

Insufficient alloy used. Alloy not sufficiently molten or fluid. Alloy not able to enter thinner areas of mold. Mold is not heated to proper temperature. Premature solidification of alloy. Sprue blocked with foreign bodies. Back pressure due to gases in mold cavity. Low casting pressure.

FIGURE 21.20 Incomplete casting.

Too bright and shiny casting with short and rounded margins When the wax is not completely eliminated, it combines with oxygen or air in the mold cavity forming carbon monoxide which is a reducing agent. The gas prevents the oxidation

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of the surface of the casting gold with the result that the casting which comes out from the investment is bright and shiny. The formation of gas in the mold is so rapid that it also has a back pressure effect. Small casting If compensation for shrinkage of alloy is not done by adequate expansion of mold cavity, then a small casting will result. Another reason is the shrinkage of the impression material.

CONTAMINATION A casting can be contaminated due to 1. Oxidation, caused by –– Overheating the alloy –– Use of oxidizing zone of flame –– Failure to use flux 2. Sulfur compounds, formed by the breakdown of the investment when the ring is overheated (see black casting below). Avoid by –– Not overheating alloy –– Use reducing zone of the flame –– Use flux Black casting Can be due to two reasons. 1. Overheating the investment above 700 °C causes it to decompose liberating sulfur or sulfur compounds. They readily combine with the metals in gold alloy forming a sulfide film. This gives a dark casting which cannot be cleaned by pickling. 2. A black casting can be also due to incomplete elimination of the wax pattern, as a result of heating the mold at too low temperature. A carbonized wax remains which sticks to the surface of the casting. It can be removed by heating over a flame.

OTHER METHODS OF FABRICATING RESTORATIONS AND PROSTHESES 1. 2. 3. 4. 5. 6. 7.

Capillary casting (foil adaptation and sintering) Electroforming Machining (subtractive fabrication) CAD/CAM Copy milling Electrical discharge machining Three-dimensional printing (additive fabrication).

CAPILLARY CASTING TECHNIQUE (CAPTEK) Adapting and sintering gold alloy foils (Renaissance and Captek) is a novel way of making a metal frame without having to cast it. The system was developed by Shoher and Whiteman and introduced to the dental community in 1993. Captek is an acronym for ‘capillary casting technique’.

372 PART 5 Dental Laboratory—Materials and Processes MODE OF SUPPLY They are supplied as thin metal impregnated wax like elastic strips in two forms called Captek P and Captek G (Figs. 21.21A to E). Captek P is platinum-colored strips. Captek G contains 97.5% Gold and 2.5% Silver. Captek P contains platinum, palladium and gold powder.

CAPILLARY CASTING Captek P (Platinum/Palladium/Gold) has a porous structure and serves as the internal reinforcing skeleton. On heating in a furnace, the Captek P acts like a metal sponge and draws in (capillary action) the hot liquid gold completely into it. Captek G provides the characteristic gold color of this system. The final coping can be described as a composite structure (Figs. 21.22A and B).

FIGURES 21.21A TO E Fabrication of Captek restorations. (A) Captek P and G. (B) Adapting captek P. (C) Adapting captek G. (D) Completed coping. (E) Completed crowns.

FIGURES 21.22A AND B Captek crown. (A) Cross-section through a Captek crown showing the composite structure. (B) Closeup of composite structure of Captek coping.

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OVERALL COMPOSITION AFTER CAPILLARY CASTING Captek—88.2% Gold, 4% platinum, 5% palladium, 2.8% silver

COPING THICKNESS Anterior

—

0.25 mm

Posterior

—

0.35 mm.

COPING MICROSTRUCTURE After sintering each coping is structured of three layers (Figs. 21.22 A and B).

Inner and outer layers are 25 µm thick with a bright gold color. Intermediate layer is a gold-platinum composite structure.

TECHNIQUE (FIG. 21.21) 1. A refractory die is made after duplicating the original die. 2. An adhesive is painted on to the die. 3. Strips of Captek P are cut and adapted to the die by hand instruments and light pressure. Care should be taken while adapting as the material tears and breaks easily. 4. The Captek P layer is fused in a furnace at 1075°C. This eliminates the adhesive and binders and fuses the platinum and palladium to form a porous interconnected structure. 5. Next the Captek G layer is adapted and again heated in the furnace to induce melting and infusion. 6. The composite coping is divested and trimmed. 7. Capfil and Capcon are used to form the connector when making Captek bridges. 8. A thin layer of gold slurry called Capbond - (composition similar to Captek G) is coated on to the coping. Capbond is a ceramometal bonder. It improves bonding to ceramic and also replenishes areas of the coping that have been trimmed away. 9. Opaquer and the various layers of porcelain are then condensed and fired to form the final crown.

Advantages 1. The thinner foil alloy coping allows a greater thickness of ceramic, thereby, improving the esthetics. 2. The gold color of the alloy improves the esthetics of the restoration. 3. Less reduction of tooth structure. 4. The nonesthetic high intensity high value opaquer layer seen with conventional metal ceramics is eliminated.

CAD/CAM MILLING Dental copings, crowns and FDP frameworks also can be machined from metal blanks (Fig. 21.23 and 21.24) via computer-aided designing and computer-aided machining (CAD/CAM). The process is similar to that described for ceramics.

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B FIGURE 21.24 CAD/CAM milling process.

FIGURES 21.23A AND B CAD/CAM blanks.

ADVANTAGES OF CAD/CAM 1. Improved fit. 2. Possibility of one visit restorations especially in case of smaller all restorations like inlays and crowns. 3. Complex castings like full arch FDPs, overdenture frames, partial dentures frames, etc. can be fabricated with greater ease and accuracy as compared to lost wax based castings. Reassembling the casting and soldering processes can be eliminated for these castings. 4. Structures are hom*ogenous with minimum porosity and defects. An image (scan) is taken of the prepared tooth and the surrounding teeth. This image, called a digital impression, draws the data into a computer. Proprietary software then creates a virtual restoration or structure. The data is sent to a milling machine where the part is milled out of a solid block of the metal (Fig. 21.24). Multiple units can be milled out of a single block. Metal blanks of different compositions, shapes and sizes are available (Fig. 21.23). Milling machines currently available are capable of multiaxis milling and multiple tool changes.

COPY MILLING Copy milling of metal structures is similar to that described for ceramics. It is based on the principle of scanning or tracing of a resin or wax pattern of the restoration milling a replica out of the metal blank. (Refer chapter on dental ceramics for additional information).

ELECTROFORMING Electroforming is another method of forming a metal coping for metal ceramic systems (Figs. 21.25A to D). Some examples of electroforming systems are Preciano (Heraeus Kulzer) and Microvision (Weiland). A die spacer is applied on to the die of the prepared tooth/teeth. The dies are duplicated with gypsum product having a slight expansion of 0.1 to 0.2 %. The duplicated die is coated with a conducting silver lacquer metallizing powder (Fig. 21.25C) and then connected to the electrodes and immersed in to the electroplating bath (Fig. 21.25A). The bath is a nontoxic cyanide-free gold-sulfite solution. Electroforming time varies according to the thickness desired and the current. 0.2 mm layer thickness—240 min. 0.3 mm layer thickness—329 min. The coping formed (Fig. 21.25D) is separated and used to fabricate a metal-ceramic restoration.

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FIGURES 21.25A TO D Electroforming. (A) Electroforming device. (B) Gold electroplate. (C) Silver conductor. (D) Electroformed coping.

Electroforming is also used to replenish the worn-out telescopic prostheses that have lost their friction fit.

ELECTRICAL DISCHARGE MACHINING The electrical discharge machining (EDM) unit (Fig. 21.26) was invented by the Russian Lazerenko brothers in 1943. Electrical discharge machining (EDM), sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking, wire burning or wire erosion, is a subtractive manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). One of the electrodes is called the tool-electrode or tool, while the other is called the workpiece-electrode, or workpiece. Material is removed from the workpiece by a series of rapidly recurring current discharges between two FIGURE 21.26 Electrical discharge machine. electrodes, separated by a dielectric liquid and subject to an electric voltage. EDM is useful for difficult to machine alloys such as tungsten and titanium. The advent of computer-aided EDM in the early seventies, helped it gain significance in manufacturing processes. In 1982, it was introduced into dentistry by Rubeling to fabricate precision attachments. Since 1990 it has been used widely in implant prostheses.

APPLICATIONS EDM is used in dentistry for the precise and accurate fabrication of precision attachment removable partial dentures, fixed-removable implant prostheses, and titanium-ceramic crowns. By improving the fit, a passive seating of the restoration on the implant is obtained thereby minimizing stresses to the bone.

TECHNIQUE The tool electrode is connected to the implant abutments. The workpiece-electrode is connected to the restoration framework (Fig. 21.27). Both electrode and workpiece are maintained in a liquid

FIGURE 21.27 Assembly being readied for spark erosion.

376 PART 5 Dental Laboratory—Materials and Processes medium (called dielectric fluid). A space is maintained between the electrode and work piece through out the machining process which is known as the cutting gap. The electrode moves towards and away from the work piece assisted by a hydraulic ram connected to it during the process. The dielectric fluid functions as a conductor and coolant during the procedure. This whole unit has a power source that maintains a direct current. The power level selection is dictated by the alloy properties used, size of object and amount of erosion required. When the cutting gap is sufficiently small, the fluid ionizes allowing electric discharges to occur. These electric discharges occur at regular intervals and such cycles takes place about 250,000 times a second. The sparks gradually erode the inner surface of the restoration in a precise controlled manner thereby achieving a passive and improved fit.

ADVANTAGES 1. Passive fit of restorations is achieved. 2. Complex structures can be shaped regardless of metal hardness. 3. Extremely thin work piece can be machined without distortion as it does not involve mechanical forces. 4. There is decreased stress on the work piece due to the cooling action of the dielectric fluid. 5. Smooth finish of final restoration is ensured. 6. There is decreased oxidation of metals during the procedure (especially useful in titanium to porcelain bonding). 7. It is rapid, efficient and accurate (within 0.0254 mm). 8. Frameworks with porcelain can be spark eroded without any stress on the porcelain due to the cooling action of the dielectric fluid.

DISADVANTAGES 1. Eroding tends to affect the corrosion resistance of titanium. 2. Skilled personnel and specialized lab equipment is mandatory. 3. The high cost of the technique limits its usage.

ADDITIVE MANUFACTURING Additive manufacturing (AM) or three-dimensional printing is finding increasing use in dentistry. It is an additive manufacturing process in contrast to milling which is a subtractive process. Metal powder is sequentially layered on to a gradually descending platform and fused using laser (Fig. 21.28) in a CAM machine (Fig. 21.29). This deposition and sintering process continues until the desired object is created. (Refer chapter on additive manufacturing for additional information).

FIGURE 21.28 Laser sintering in progress.

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TYPES OF METAL IN ADDITIVE MANUFACTURING TECHNOLOGIES A. Powder bed technologies 1. Laser beam melting -- Selective laser melting -- Laser melting -- Direct metal laser sintering (DMLS) 2. Electron beam melting (EBM) 3. Material jetting process B. Powder deposition technologies 1. Laser engineered net shaping (LENS) 2. Direct metal deposition (DMD) 3. Laser cladding

FIGURE 21.29 3D laser sintering machine.

APPLICATIONS Three-dimensional printing is used in the fabrication of metal structures in dentistry both directly and indirectly. 1. Indirect - machine prints patterns which are then cast to form metal structures. 2. Direct fabrication of metallic fixed and removable partial denture frames by laser sintering or melting (Figs. 21.28 to 21.31).

METALS AVAILABLE FOR 3D PRINTING All the metals generally available for fixed and removable dental prostheses are available in powder form for 3D printing. This includes cobalt-chromium, titanium and gold alloys.

FIGURE 21.30 Dental prostheses including crowns, RPDs and FDPs made via direct metal sintering using 3D Systems’ ProX 3D printer. (Courtesy: 3D Systems).

FIGURE 21.31 Representation of direct metal laser sintering process from a metal powder bed.

22 Chapter

Abrasion and Polishing Chapter Outline • • • • • •

Abrasion Defined as Types of Abrasion Supplied as Mechanism of Abrasive Action Stress, Strain and Heat Production during Abrasion • Rate of Abrasion • Classification • Types of Abrasives –– Emery –– Aluminum Oxide –– Garnet –– Pumice –– Kieselgurh –– Tripoli –– Rouge

• • • • •

–– –– –– –– –– –– –– ––

Tin Oxide Chalk Chromic Oxide Sand Carbides Diamond Zirconium Silicate Zinc Oxide Desirable Characteristics of an Abrasive Grading of Abrasive and Polishing Agents Binder Diamond Burs Polishing –– Difference between Abrasion and Polishing

–– Nonabrasive Polishing –– Glazing –– Electrolytic Polishing

• Technical Considerations • Dentifrices –– Function –– Classification –– Composition –– Whitening and Bleaching Herbal Tooth Pastes

–– Desensitizing Pastes

• Prophylactic Abrasives –– Prophyjet • Denture Cleansers

Before any restoration or appliance is placed permanently in the mouth it should be highly polished. In spite of all the care taken during processing, many restorations and prostheses usually require further trimming, smoothing and finally polishing. A rough or unpolished surface may

Be uncomfortable to the patient Cause food and other debris cling to it and makes it unhygienic Lead to tarnish and corrosion.

ABRASION It occurs when a hard, rough surface slides along a softer surface and cuts a series of grooves.

Defined As The wearing away of a substance or structure through a mechanical process, such as grinding, rubbing or scraping (GPT-8).

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Figures 22.1A to C Abrasive and polishing materials come in a wide variety of shapes and forms. (A) Abrasive polishing strip; (B) Electroplated diamond; (C) Bonded stone.

TYPES OF ABRASION Abrasion may be

A two body process, e.g. action of a diamond bur on enamel. A three body process, e.g. pumice applied with a bristle brush.

SUPPLIED AS In dentistry the abrasive is applied to the work by a variety of tools (Figs. 22.1A to C).

Paper/plastic coated The abrasive particles may be glued on to a paper or plastic disk that can be attached to a handpiece. Sand paper falls in this group. Coated strips The abrasive (e.g. diamond) may be attached to stainless steel or plastic strips (Fig. 22.1A) (used for proximal stripping of teeth). This category is similar to the above. Electroplating bonding In case of diamond rotary instruments the diamond chips are attached to steel wheels, disks and cylinders by electroplating with nickel based matrix. Bonded stones In grinding wheels and dental stones (Figs. 22.1B and C), the abrasive particles are mixed with a bonding agent that holds the particles together. Before hardening, the matrix material with the abrasive is moulded to form tools of desired shapes. Powder form An abrasive may also be mixed with water or glycerine to form a paste or slurry. It is applied with felt cone, rubber cup or brush and used for smoothening irregularities, e.g. pumice powder (Fig. 22.2A). Cake form They are also available in the form of cakes (Fig. 22.2B). Rubber impregnated Abrasives can be incorporated into rubber or shellac disks or cups for ‘soft grade’ abrasion. Paste form The abrasive is made into a paste and supplied in a tube, e.g. Ivoclar polishing paste, tooth paste, etc.

Figures 22.2A and B Forms of abrasives. (A) Pumice powder; (B) Polishing abrasive in cake form.

380 Part 5 Dental Laboratory—Materials and Processes Abrasion is affected by the properties of the abrasive as well as the target material. The important properties include hardness, strength, ductility, thermal conductivity, etc.

Mechanism of ABRASIVE ACTION In a cutting tool, e.g. carbide or steel bur (Fig. 22.3), the blades or cutting edges are regularly arranged and the removal of material corresponds to this regular arrangement. An abrasive tool on the other hand has many abrasive points that are not arranged in an ordered pattern. Thus, innumerable random scratches are produced. The action of an abrasive is essentially a cutting action. Each tiny particle presents a sharp edge that cuts through the surface similar to a chisel. A shaving is formed which crushes to a fine powder. This powder clogs the abrasive tool and frequent cleaning is required.

Figure 22.3 Steel bur. Unlike diamond these burs remove material by cutting or shaving.

Stress, strain and heat production during abrasion While abrading metals, the crystalline structure of the metal is disturbed to depth of 10 µm. The grains become disoriented and strain hardening may occur. Thus, the surface hardness increases. In denture resins too, rigorous abrasion introduces stresses. The generation of heat during abrasion partially relieves such stresses but if it is too great, it may relieve processing stresses and a warpage may result. The resin surface may even melt. Similarly high speed cutting of tooth structure generates excessive heat which can lead to pulpal damage. Therefore, it is very important to control the heat by air/water spray and intermittent cutting (rather than continuous cutting).

RATE OF ABRASION The rate of abrasion of a given material by a given abrasive is determined primarily by three factors. 1. Size of the abrasive particle. 2. The pressure of the abrasive agent. 3. Speed at which the abrasive particle moves across the surface being abraded.

Size of the Particles Larger particles cause deeper scratches in the material and wear away the surface at a faster rate. The use of course abrasive is indicated on a surface with many rough spots or large nodules. The scratches caused by the coarse abrasive must then be removed by finer ones.

Pressure Heavy pressure applied by the abrasive will cause deeper scratches and more rapid removal of material. However, heavy pressure is not advisable as it can fracture or dislodge the abrasive from the grinding wheel, thus reducing the cutting efficiency. In addition, operator has less control of the abrasion process because removal of material is not uniform.

Speed The higher the speed, the greater the frequency per unit of time the particles contacts the surface. Thus increasing the speed increases the rate of abrasion. In a clinical situation it is

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easier to control speed than pressure to vary the rate of abrasion. Varying the speed has the additional advantage that low pressure can be used to maintain a high cutting efficiency.

Rotational Speed and Linear Speed Rotational speed and linear speed should be differentiated. The speed with which the particles pass over the work is its linear speed or it is the velocity with which the particles pass over the work. Rotational speed is measured in revolutions per minute (RPM or r/min), whereas, linear speed is measured in meters per second Linear speed is related to rotational speed according to the following formula

V = CR, where

V = linear speed

C = circumference of the bur or disc

R = revolutions per minute

classification A. Finishing abrasives B. Polishing abrasives C. Cleansing abrasives

Finishing Abrasives Finishing abrasives are hard, coarse abrasives which are used initially to develop contour and remove gross irregularities, e.g. coarse stones.

Polishing Abrasives Polishing abrasives have finer particle size and are less hard than abrasives used for finishing. They are used for smoothening surfaces that have been roughened by finishing abrasives, e.g. polishing cakes, pumice, etc.

Cleansing Abrasives Cleansing abrasives are soft materials with small particle sizes and are intended to remove soft deposits that adhere to enamel or a restorative material.

TYPES OF ABRASIVES Emery Emery consists of a natural oxide of aluminum called corundum. There are various impurities present in it, such as iron oxide, which may also act as an abrasive. The greater the content of alumina, the finer the grade of emery.

Aluminum Oxide Pure alumina is manufactured from bauxite, an impure aluminum oxide. It can be produced in fine grain sizes and has partially replaced emery for abrasive purpose. Pure alumina is also used as a polishing agent. Alumina is used in sandblasting machines (Figs. 22.7 and 22.8).

Garnet It is composed of different minerals which possess similar physical properties and crystalline form. The mineral comprises of silicates of aluminium, cobalt, magnesium, iron and manganese. Garnet is coated on paper or cloth with glue. It is used on disks, which are operated on handpieces.

382 Part 5 Dental Laboratory—Materials and Processes Pumice It is a highly siliceous material of volcanic origin and is used either as an abrasive or polishing agent depending on particle size. Its use ranges from smoothening dentures to polishing teeth in the mouth.

Kieselguhr It consists of siliceous remains of minute aquatic plants known as diatoms. The coarser form (diatomaceous earth) is used as a filler in many dental materials. It is excellent as a mild abrasive and polishing agent.

Tripoli This mild abrasive and polishing agent is often confused with kieselguhr. True tripoli originates from certain porous rocks, first found in North Africa near Tripoli.

Rouge Rouge is a fine red powder composed of iron oxide. It is used in cake form. It may be impregnated on paper or cloth known as ‘crocus cloth’. It is an excellent polishing agent for gold and noble metal alloys but is likely to be dirty to handle.

Tin Oxide Putty powder used as polishing agent for teeth and metallic restorations in the mouth. It is mixed with water, alcohol or glycerin and used as paste.

Chalk It is calcium carbonate prepared by precipitation method. There are various grades and physical forms available for different polishing techniques. It is sometimes used in dentifrices.

Chromic Oxide A relatively hard abrasive capable of polishing a variety of metals. It is used as a polishing agent for stainless steel.

Sand Sand as well as other forms of quartz is used as sand paper or as powder in sandblasting equipment.

Carbides Silicon carbide and boron carbide are manufactured by heating silicon and boron at a very high temperature to effect their union with carbon. The silicon carbide is sintered or pressed with a binder into grinding wheels or disks. Most of the stone burs used for cutting tooth structure are made of silicon carbide.

Diamond It is the hardest and most effective abrasive for tooth enamel. The chips are impregnated in a binder or plated on to a metal shank to form the diamond ‘stones’ and disks so popular with the dental profession.

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Zirconium Silicate The mineral zircon is ground to various particle sizes and used as a polishing agent. It is used in dental prophylactic pastes and in abrasive impregnated polishing strips and disks.

Zinc Oxide Zinc oxide in alcohol can be used for polishing amalgam restorations.

DESIRABLE CHARACTERISTICS OF AN ABRASIVE 1. It should be irregular in shape so that it presents a sharp edge. Round smooth particles of sand are poor abrasives. Sand paper with cubical particles present flat faces to the work, and so they are not effective. Irregular and jagged particles are more effective. 2. Abrasive should be harder than the work it abrades. If it cannot indent the surface to be abraded, it cannot cut it and the abrasive dulls or wears out. 3. The abrasive should possess a high impact strength. The abrasive particle should fracture rather than dull out, so that a sharp edge is always present. Fracture of an abrasive is also helpful in shedding the debris accumulated from the work. A diamond does not fracture, it loses substances from the tip. They become clogged when used on ductile metals. They are most effective when used on brittle tooth enamel. 4. They should have attrition resistance, so that it does not wear.

GRADING OF ABRASIVE AND POLISHING AGENTS Abrasives are graded according to the number of the last sieve it passed through. Examples silicon carbide is graded as 8, 10, 12, 14, 16, 20, 24, etc. Finer abrasives are designated as powder or flours and are graded in increasing fineness as F, FF, FFF, etc. and impregnated papers as 0,00,000, etc.

BINDER The abrasives on a disk and wheel are held together by a binder. Commonly used binders in dentistry are

Ceramic bonding is used for silicon carbide or corundum in a mounted abrasive point. Electroplating with nickel is often used to bind the diamond chips on to the diamond rotary instruments (Fig. 22.4). For soft grade abrasion, rubber (Fig. 22.5) or shellac may be used. These wear rapidly, but they are useful in some dental operations in which delicate abrasion is required.

The type of binder is related to the life of the tool in use. In most abrasives the binder is impregnated throughout with an abrasive of a certain grade so that, as a particle is removed from the binder during use another takes its place as the binder wears. Furthermore, the abrasive should be so distributed that the surface of the tool wears evenly, particularly if the disk or wheel is used for cutting along its periphery.

DIAMOND BURS Either synthetic or natural diamond chips are employed for dental rotary instruments (Fig. 22.4). The cutting efficiency of diamond rotary instruments depends on whether the diamonds used are natural or synthetic, the grit size, the distribution and the extent of plating that

384 Part 5 Dental Laboratory—Materials and Processes

FIGure 22.4 Diamond burs of various grits. The color indicates grit size. Black - extra coarse, Green—course, Blue—medium, Red—fine, Yellow—extra fine, and White—ultra fine.

Figure 22.5 Rubber-bonded abrasives.

attaches the particles to the instrument shank. The larger the grit size the greater the abrasion. Some companies indicate the grit size by color coding (Fig. 22.6). Adequate water spray is essential not only to minimize heat but also to reduce clogging.

POLISHING It is the production of a smooth mirror like surface without much loss of any external form. If the particle size of an abrasive is reduced sufficiently, the scratches become extremely fine and may disappear entirely. The surface then acquires a smooth shiny layer known as a polish. The polishing agents remove material from the surface molecule by molecule. In the process fine Figure 22.6 Color coding and corresponding scratches and irregularities are filled in by powdered grit size. particulates being removed from the surface. The microcrystalline layer is referred to as polish layer or Beilby layer. A polishing agent is employed only after an abrasive obliterates or eliminates most of the fine scratches, leaving a smooth finish.

DIFFERENCE BETWEEN ABRASION AND POLISHING The difference between an abrasive agent and a polishing agent is difficult to define. The terms are generally interchangeable.

Particle size A given agent having a large particle size acts as an abrasive, producing scratches. The same abrasive with a smaller particle size is a polishing agent. Material removed Very little of the surface is removed during polishing—not more than 0.005 mm (0.002 inch). Speed The optimum speed for polishing is higher than that for abrading. Linear speed as high as 10000 ft/min may be used. It varies with the polishing agent. Average speed is approximately 7500 ft/min.

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Nonabrasive Polishing Polishing is usually achieved by an abrasive process. However a smooth shiny surface can also be achieved through nonabrasive means. These include 1. Application of a glaze layer, e.g. –– Glazing of composites –– Glazing of ceramics 2. Electrolytic polishing 3. Burnishing.

Composite Glazing A layer of glaze or gloss (a clear transparent liquid made of unfilled resin) is applied over the restoration and cured. This results in a smooth glossy surface.

Glazing Ceramics Ceramics are difficult to polish conventionally. The finished restoration is subjected to high temperatures. At this temperature the surface layer melts and flows to produce a smooth glass-like surface (refer ceramics chapter for more details). Alternatively a glaze layer can be applied and fired to obtain a shiny surface.

Electrolytic Polishing Electrolytic polishing (Fig. 22.7) is not true abrasion. Although material is removed, it is removed through an electrochemical process rather than abrasive process. This is the reverse of electroplating. The alloy to be polished is made the anode of an electrolytic cell. As the current is passed, some of the anode is dissolved leaving a bright surface. This is an excellent method for polishing the fitting surface of a cobalt–chromium alloy denture. So little material is removed, that the fit of the denture is virtually unaltered.

BURNISHING

Figure 22.7 Electrolytic polishing unit.

It is related to polishing in that the surface is drawn or moved. Instead of using many tiny particles, only one large point is used. If a round steel point is rubbed over the margins of a gold inlay, the metal is moved so that any gap between the inlay and the tooth can be closed. A special blunt bur revolving at high speed can also be used. Note The burnishing instrument should not adhere to or dissolve in the surface of the burnished metal, e.g. brass instruments impregnate copper into the surface of a gold inlay.

TECHNICAL CONSIDERATIONS (PROCEDURE) Methods of Abrasion Abrasion may be carried out 1. Manually, e.g. proximal stripping of enamel using abrasive strips. 2. Rotary instruments, e.g. burs, wheels, cups, discs, cones, etc.

386 Part 5 Dental Laboratory—Materials and Processes

Figure 22.8 Sandblasting.

Figure 22.9 The term sandblasting is misleading. The process actually uses 250 micron alumina (Al2O3).

Figures 22.10A and B Sequence of finishing is important. One should proceed from coarse to fine.

3. Blasting The object is blasted with a steady stream of abrasive, e.g. prophy-jet polishing of enamel, sandblasting (Figs. 22.8 and 22.9) to remove investment of castings, shell blasting, etc. –– Smoothen work with a coarse abrasive or bur (Fig. 22.10A) which leaves large scratches. These scratches are removed with finer abrasives but the difference in fineness should not be too great (Fig. 22.10B). The use of too fine an abrasive after a relatively coarse abrasive wastes time and may cause streaking. Figure 22.11 Canvas buff wheel with pumice is –– After changing to a finer abrasive, used for polishing complete dentures. the direction of abrasion should be changed each time if possible, so that new scratches appear at right angles to the coarser scratches, for uniform abrasion. –– When the scratches are no longer visible to the eye, the preliminary polishing can be accomplished with pumice flour applied with a canvas buff wheel [used for resin dentures (Fig. 22.11)]. –– The work is cleaned to remove all traces of abrasives and the particles of the material removed by the abrasive.

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–– A paste is formed of pumice and water to a sticky ‘muddy’ consistency. The buff wheel is turned at high speed. Apply the paste to the work and carry it to the buff. Hold the work firmly but without excessive pressure. Repeat this over the entire surface till the surface is bright and well polished. –– Clean the work with soap and water. Change to a flannel buff wheel. Rotate at high speed. A polishing cake with grease is held against the buff wheel to impregnate with the agent. The work is held against the wheel and turned, so that all of the surfaces are polished uniformly. A light pressure is used to avoid excess heat generation (especially in resins).

Dentifrices Popularly known as toothpastes these are agents used with a toothbrush to cleanse and polish natural teeth. They should have maximum cleansing efficiency with minimum tooth abrasion. Highly abrasive dentifrices should not be used especially when dentin (abrades 25 times faster) or cementum (35 times faster) is exposed.

Function 1. Assists the toothbrush to mechanically remove stains, debris and soft deposits from the teeth. 2. To impart a polished surface to the tooth. Thus, they help reduce caries, maintain healthy gingiva, improve esthetics and reduce mouth odors.

Available As Pastes, powders, liquid and gels (Fig. 22.12).

Classification of Dentifrices

Figure 22.12 Some popular dentifrices.

Dentifrices may classified based on their primary function. 1. Caries prevention and treatment –– Fluoride concentrations up to 1000 ppm –– Fluoride concentrations from 1000 to 2000 ppm –– Fluoride concentrations from 2500 to 5000 ppm 2. Periodontal disease prevention and treatment –– Natural antibacterial and antiseptic agents –– Synthetic antibacterial and antiseptic agent 3. Desensitizing pastes –– Analgesic –– Dentine tubules blocking 4. Whitening pastes –– Abrasive –– Bleaching

388 Part 5 Dental Laboratory—Materials and Processes 5. Pastes for specific purposes –– Toothpastes for xerostomia (e.g. olive oil, betaine and xylitol) –– Antiviral pastes (e.g. larifan for herpetic infection or aphthae)

Composition The basic component of a dentifrices is the abrasive. However, pastes today contain numerous other components. Specialized dentifrices contain medicaments for various problems, such as sensitivity and gum diseases. Composition of a typical dentifrices is presented in Table 22.1.

Herbal toothpastes Some toothpastes are marketed as natural or herbal-based toothpastes (Fig. 22.13). These contain extracts which are claimed to be beneficial to the gums and teeth. Some of these include extracts from the neem tree, miswak tree (salvadore persica), zanthoxylum alatum (Tomar beej - Dabur red), propolis (resinous extract collected by bees), etc.

Table 22.1 Components of toothpaste Ingredient

Abrasives

Action

Dibasic calcium phosphate (dihydrate) Calcium pyrophosphate Insoluble sodium metaphosphate Hydrated silica (most efficient) Alumina, titanium oxide Calcium carbonate, sodium bicarbonate Perlite (70–75% silica dioxide) Magnesium oxide, etc.

Water

Physically removes matter

Vehicle and moisturizing

Humectants

Glycerin, sorbitol, propylene glycol

Prevents drying of paste

Detergents and foaming agents

Sodium lauryl sulfate

Decreases surface tension

Binders

Carboxymethyl cellulose Xanthan gum

Artificial sweeteners

Sorbitol, saccharin

Therapeutic

Sodium monofluorophosphate (Na2PO3F) Anticariogenic agents Stannous fluoride (SnF2) Sodium fluoride (NaF) Aminofluorides

Coloring and flavoring

Triclosan, chlorhexidine, hydrogen peroxide, baking soda, povidone iodine, zinc citrate.

Bacteriostatic and bactericidal

Dibasic ammonium phosphate

Acid neutralizing

Potassium nitrate, Potassium citrate Strontium chloride, Strontium acetate Arginine, Hydroxyapatite, Calcium sodium phosphosilicate.

Desensitizing and dentin occluding

Mint, menthol Various fruit flavors

Flavoring

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Figure 22.13 Dentifrices containing herbal and other natural extracts.

Whitening and bleaching toothpastes

Box 22.1 RDA table

Although in several studies whitening toothpastes have A0 - 70 Low abrasive shown the ability to improve tooth color, they have 70 - 100 Medium abrasive several side effects. The most significant is enamel and dentin abrasion, which in turn leads to increased tooth 100 - 150 Highly abrasive sensitivity. A key indicator of toothpaste abrasiveness 150 - 250 Regarded as harmful limit is relative dentin abrasion (RDA) (Box 22.1). The larger the number, the greater the potential of dentin abrasiveness. Dentin abrasion significantly increases when the concentration of abrasive substances in toothpaste is increased. Teeth have a natural defence mechanism against abrasion, called the pellicle*. Its presence on tooth surfaces reduces the abrasive effect of toothpaste on enamel. Therefore, it is best to avoid mechanical tooth cleaning after consuming acidic foods or drinks, as they may dissolve the pellicle and can combine abrasive and erosive defects. Bleaching toothpastes Bleaching toothpastes contain chemicals, most commonly hydrogen peroxide or calcium peroxide. When peroxides touch the tooth surface, they break down the stain molecule, providing a bleaching effect. Various bleaching systems for home or professional use also contain these substances. When adding peroxides to a toothpaste it should be noted that the concentration is small (usually 1% hydrogen peroxide or 0.5–0.7% calcium peroxide), and the exposure time short. Therefore, there is a lack of evidence about whether such toothpastes can improve the internal tooth color. They certainly bleach the pellicle on the tooth surface.

Desensitizing pastes Desensitizing pastes work through two methods—analgesic action and blocking of dentinal tubules. Analgesic toothpastes Toothpastes containing potassium salts maintain a high K+ extracellular level, thus preventing repolarization of the nerve cell membrane and inhibiting the transmission of impulses without causing changes in the pulp. Toothpaste containing 5% or 10% potassium nitrate, can decrease tooth sensitivity for up to 4 weeks. * Dental pellicle is a protein film that forms on the surface enamel by selective binding of glycoproteins from saliva that prevents continuous deposition of salivary calcium phosphate. It forms in seconds after a tooth is cleaned or after chewing. It protects the tooth from the acids produced by oral microorganisms after consuming carbohydrates.

390 Part 5 Dental Laboratory—Materials and Processes Dentin tubule blocking toothpastes Also called tubular occludents. Examples include fluoride compounds (in high concentrations), strontium salts [10% strontium chloride (SrCl2) and strontium acetate], arginine (with calcium carbonate), hydroxyapatite and calcium sodium phosphosilicate (Novamin). Fluoride compounds increase dentin resistance against acids by forming a protective layer on the tooth surface, increasing the microhardness and precipitating fluoride compounds that block dentin tubules.

PROPHYLACTIC ABRASIVES Oral prophylaxis is a widely used procedure in the dental office. Prophylactic polishing agents may be available commercially or can be made in the dental office. They are usually employed in paste form by mixing with a suitable vehicle.

Function 1. They remove extrinsic stains, pellicle, materia-alba and oral debris. 2. Impart a polished and esthetic appearance. Different types of abrasives may be employed, e.g. zirconium silicate, silica, pumice, etc. In addition, some may contain fluoride in order to reduce caries. They are applied onto the teeth with the help of rubber cups or bristle brushes (Fig. 22.14) which are attached to a rotary instrument.

Figure 22.14 Prophylaxis in a dental office.

Prophyjet The Prophyjet (Fig. 22.15A) is a relatively new dental prophylaxis system of removing intraoral stains. An abrasive blasting process (Fig. 22.15C) is used to mechanically remove extrinsic (tobacco) stains as well as light supragingival adherent plaque and calculus. The powder is loaded into the device, which then directs the abrasive through a small nozzle in a steady stream of air and water.

Figures 22.15A to C (A) Prophyjet unit (Cavitron). (B) Prophyjet powder. (C) High pressure abrasive jet blasting through the nozzle.

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Composition It contains sodium carbonate, hydrophobic modified silica and a flavoring agent. It is supplied as powder in sachets or containers (Fig. 22.15B). Clinical considerations The Prophyjet is directed at 45º angles to the tooth surface. For obvious reasons, it is less effective in proximal areas. The chances of soft tissue injury exist especially if the tissue is inflamed and friable.

DENTURE CLEANSERS Dentures collect deposits in the same manner as natural teeth during their use. These can be removed by two ways.

Professional repolishing in the laboratory or clinic. Soaking or brushing the dentures daily at home.

Brushing The dentures may be brushed using a soft bristle brush (Fig. 22.16) and gentle abrasive or cream. Hard abrasives and stiff bristles should be avoided, because they may produce scratches on the denture surface.

Soaking Chemical cleaners (Fig. 22.17) are an alternative to brushing especially among very old or handicapped persons. Alkaline perborates Alkaline peroxides Alkaline hypochlorites Dilute acids The dentures are soaked in these chemical solutions for a prescribed period of time. Their main requirements are that they should be nontoxic, non-irritant and should not attack the materials used in denture construction.

Figure 22.16 A home cleansing kit for dentures.

Figure 22.17 A commercially available chemical cleanser for soaking.

23 Chapter

Metal Joining and Dental Lasers Chapter Outline • • • • • • • • • • • •

Terms and Definitions Welding Brazing and Soldering Substrate Metal Soft Solders Hard Solders Presoldering and Postsoldering Applications of Soldering Composition Gold Solders Silver Solders Properties of Dental Brazing Materials –– Fusion Temperature –– Flow

–– Factors Affecting Flow –– Color –– Tarnish and Corrosion • • • • • • • • •

Resistance Microstructure of Joints Fluxes Boric and Borate Compounds Fluorides Antiflux Free Hand Soldering Investment Soldering Steps in Soldering Procedure Technical Considerations –– Gap –– Pitted Solder Joints

• Welding –– Indications –– Types • Resistance Spot Welding • Plasma Arc and Tig Welding • Laser Welding • Laser Use in Dentistry • Dental Applications of Laser –– Commercial Names –– Indications –– Advantages of Laser Welding

• Cast-Joining • Radiographic Assessment of Joints

It is often necessary to construct a dental appliance as separate parts and then join them together either by a soldering or welding process. Dental brazing is covered by ISO 9333.

TERMS AND DEFINITIONS Metal joining operations are usually divided into four categories: welding, brazing, soldering and cast-joining.

Welding The term welding is used if two pieces of similar metal are joined together without the addition of another metal that is, the metal pieces are heated to a high enough temperature so they join together by melting and flowing.

Brazing and Soldering The words soldering and brazing are used if two pieces of metal are joined by means of a third metal called as filler.

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Brazing During soldering, metal parts are joined together by melting a filler metal between them at a temperature below the solidus temperature of the metal being joined and below 450 °C. In dentistry, the joining of metal parts are done at temperatures above 450 °C, and therefore the operation should ideally be called brazing. This is also the term used by the ISO. However, most dentists still prefer to use the word soldering. The term ‘brazing material’ is often used interchangeably with the term ‘solder’.

Substrate metal Substrate metal or parent metal refers to the metal parts to be joined. In dentistry alloys that can be soldered or welded include alloys of gold, silver, palladium, nickel, cobalt, titanium, etc.

IDEAL REQUIREMENTS OF a brazing material (DENTAL SOLDER) 1. 2. 3. 4. 5.

It should melt at temperatures below the solidus temperature of the parent metal. When melted, it should wet and flow freely over the parent metal. Its color should match that of metal being joined. It should be resistant to tarnish and corrosion. It should resist pitting during heating and application.

TYPES OF SOLDERS or brazing materials They may be divided into two major groups. 1. Soft solders 2. Hard solders

Soft Solders Soft solders have low melting range of about 260 °C. They can be applied by simple means like hot soldering iron. They lack corrosion resistance and so are not suitable for dental use, e.g. lead-tin alloys (plumbers solders).

Hard Solders These have a higher melting temperature and greater strength and hardness. They are melted with the help of gas blowtorches or occasionally in an electric furnace. Hard solders are more commonly used in dentistry. They are also used for industrial purposes and in the jewelry trade, e.g. gold solders and silver solders (Fig. 23.1).

Presoldering and Postsoldering The term presoldering (prebrazing) refers to soldering operation performed on metalceramic alloys prior to ceramic firing. Postsoldering (postbrazing) refers to soldering performed on the alloy after ceramic firing. Obviously properties required of the two solders would be different. Solders used in presoldering would be required to permit ceramic bonding as well as withstand the high porcelain firing temperatures.

394 Part 5 Dental Laboratory—Materials and Processes APPLICATIONS of soldering In dentistry they are used as follows. 1. For soldering various types of wires in orthodontics. 2. In pedodontics, to construct various types of space maintainers. 3. In fixed prosthodontics –– For joining of various components of fixed partial prostheses (Box 23.1). –– Repair of perforations in crowns and bridges. –– To develop contact points in crowns. –– For cutting and rejoining an ill-fitting distorted bridge. 4. In removable partial prosthodontics for soldering of clasps.

COMPOSITION Gold Solders In the past solders were referred to by a karat number. The numbers did not describe the gold content of the solder but rather the carat of gold alloys for which the solder was to be used. In recent years the term fineness has been substituted for karat. The composition of gold solders (Fig. 23.1A) vary considerably depending on its fineness. Component

Percent

Gold

45 to 81%

Silver

8 to 30%

Copper

7 to 20%

Tin

2 to 4%

Zinc

2 to 4%

Silver Solders Silver solders (Fig. 23.1B) are less commonly used in dentistry. They are used when a low fusing solder is required for soldering operations on stainless steel or other base metal alloys.

Figures 23.1A and B Dental brazing metals. (A) Gold solder. (B) Silver solder.

Box 23.1 Improving the fit of a fixed partial denture (FPD) through soldering The fit of a FDP is often improved when it is cast as two separate pieces. Long span FDPs are especially prone to poor fit because of distortion. The two parts of the prosthesis are tried separately in the mouth. After the operator is satisfied with the individual fit of the castings, the two pieces are assembled in the mouth and their relationship is recorded and transferred with the help of a suitable index material (impression plaster or zinc oxide eugenol or elastomers). The pieces are reassembled in the laboratory and invested using soldering investment. The parts are then joined with solder. If done correctly this technique can give superior fitting fixed partial dentures.

Metal Joining and Dental Lasers Chapter 23 Component

Percent

Silver

10 to 80%

Copper

15 to 50%

Zinc

4 to 35%

Cadmium or phosphorous

may be present in small amounts

395

PROPERTIES OF DENTAL brazing materials Fusion Temperature The fusion temperature of the solder should be at least 50 °C lower than the parent metal.

Gold solders – 690 to 870 °C Silver solders – 620 to 700 °C.

Flow A good flow and wetting (low wetting angle) of the parent metal by the solder is essential to produce a good bond.

Factors affecting flow 1. Melting range Solders with short melting ranges have better flow. 2. Composition of parent metal Gold and silver based alloys have better flow than nickel based alloys. 3. Oxides Presence of an oxide layer on the parent metal reduces the flow. 4. Surface tension of solder.

Color The color of gold solders varies from deep yellow to light yellow to white. In practice, most dental solders are able to produce an inconspicuous joint.

Tarnish and Corrosion Resistance Tarnish resistance increases as the gold content increases. However, lower fineness gold alloys also perform well clinically without any serious tendency to discolor. Silver solders have reduced tarnish resistance when compared to gold alloy solders.

Mechanical Properties Gold solders have adequate strength and hardness and are comparable to dental cast gold alloys having a similar gold content. Silver solders also have adequate strength and are similar to the gold solders.

Tensile strength of brazed joint According to ISO 9333:2006, the tensile strength of the brazed joint should exceed 250 MPa. If the 0,2 % proof strength of either one or both of the metallic materials to be joined by the brazing material is below 250 MPa, the tensile strength should exceed the lower of the two.

Microstructure of Soldered Joints Microscopic examination of an ideal well-formed soldered joint shows that the solder alloy does not combine excessively with the parts being soldered. There is a well-defined boundary

396 Part 5 Dental Laboratory—Materials and Processes between the solder and the soldered parts. If the heating is prolonged a diffusion takes place and the new alloy formed has inferior properties.

FLUXES The Latin word ‘flux’ means flow. For a solder to wet and flow properly, the parent metal must be free of oxides. This is accomplished with the help of a flux.

FUNCTION OF FLUX 1. To remove any oxide coating on the parent metal. 2. To protect the metal surface from oxidation during soldering.

TYPES Fluxes may be divided into three activity types.

Protective This type covers the metal surface and prevents access to oxygen so no oxide can form.

Reducing This reduces any oxide present to free metal and oxygen.

Solvent This type dissolves any oxide present and carries it away. Most fluxes are usually combination of two or more of the above.

Commonly Used Dental Fluxes The commonly used fluxes are 1. Boric and borate compounds 2. Fluoride fluxes

Boric and Borate Compounds Boric acid and borax are used with noble metal alloys. They act as protective and reducing fluxes.

Fluorides Fluoride fluxes (Fig. 23.2) like potassium fluorides are used on base metal alloys and are usually combined with borates. They help to dissolve the more stable chromium, nickel and cobalt oxides. Fluoride fluxes should be used carefully around porcelain as it can attack the porcelain. Note Excess flux should be avoided as it can get entrapped within the filler metal and result in a weak joint.

FLUXES MAY BE SUPPLIED AS

Liquid (applied by painting) Paste

Figure 23.2 A fluoride flux for base metal alloys.

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Powder Fused onto the solder Prefluxed solder in tube form.

ANTIFLUX There are times when the operator desires that the solder should not flow out of a specific area. The flow can be controlled by use of an antiflux material. Solder will not flow into an area where antiflux has been applied. It is applied before the flux or solder is applied. Examples of antiflux are graphite (soft lead pencil), rouge (iron oxide) or whiting (calcium carbonate) in an alcohol and water suspension.

Technical considerations HEAT SOURCE The heat source is a very important part of brazing. In dentistry, two heat sources may be used 1. Flame 2. Oven

Flame Brazing or soldering The most commonly used heat source is a gas-air or gas-oxygen torch. The flame must provide enough heat not only to melt the filler metal but also to compensate for heat loss to the surroundings. Thus, the flame should not only have a high temperature but also a high heat content. Low heat content of fuels lead to longer soldering time and more danger of oxidation. Heat content is measured in Btu per cubic foot of gas.

Types of gas fuel 1. Hydrogen It has the lowest heat content (275 Btu) and therefore heating would be slow (Figs. 23.3A and B). It is not indicated for soldering of large FDPs. 2. Natural gas It has a temperature of 2680 °C and heat content is four times that of hydrogen (1000 Btu). However, normally available gas is nonuniform in composition and frequently contains water vapor.

Figures 23.3A and B (A) A microflame soldering unit such as this uses hydrolysis to split water into hydrogen and oxygen which is then used as fuel for the flame. (B) Micro flame soldering.

398 Part 5 Dental Laboratory—Materials and Processes 3. Acetylene It has the highest flame temperature (3140 °C) and a higher heat content than H2 or natural gas. However, it has certain problems. Temperature from one part of its flame to another may vary by more than 100 °C. Therefore, positioning the torch is critical and proper part of the flame should be used. It is chemically unstable and decomposes readily to carbon and hydrogen. The carbon may get incorporated into the Ni and Pd alloys, and hydrogen may be absorbed by the Pd alloys. 4. Propane It is the best choice. It has the highest heat content (2385 Btu) and a good flame temperature (2850 °C). 5. Butane It is more readily available in some parts of the world and is similar to propane. Both propane and butane are uniform in quality and water-free.

Oven Brazing (Furnace Brazing) An electric furnace with heating coils may be used for brazing. The furnace also provides heated surroundings, so less heat is lost to other parts of the fixed partial denture and the atmosphere.

TECHNIQUE OF SOLDERING Two techniques of dental soldering are employed to assemble dental appliances A. Free hand soldering B. Investment soldering

Free Hand Soldering In free hand soldering, the parts are assembled and held in contact manually while the heat and solder are applied.

Investment Soldering In investment soldering, the parts to be joined are mounted in a soldering type of investment. The hardened investment holds it in position while the heat and solder are applied (Fig. 23.4).

Steps in Soldering Procedure 1. 2. 3. 4. 5. 6. 7. 8.

Selection of solder Figure 23.4 Investment soldering. Cleaning and polishing of components Assembly of the prosthesis in soldering investment Application of flux Preheating the bridge assembly Placement of solder Application of hot gas flame to joint and solder Cooling of assembly followed by quenching in water

Technical considerations for Successful Soldering

Cleanliness - Metal should be free of oxides Gap between parts Selection of solder - Proper color, fusion temperature and flow

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Flux-type and amount Flame - Neutral or reducing in nature Temperature Time

Gap The liquid solder is drawn into the joint through capillary action. Therefore, an optimum gap is necessary for proper flow, strength of the joint and to avoid distortion of the assembly. Gap width ranging from 0.13 to 0.3 have been suggested. If the gap is too narrow, strength is limited due to Porosity caused by incomplete flow Flux inclusion If the gap is too great

The joint strength will be the strength of the solder. There is a tendency for the parts to draw together as the solder solidifies.

Flame The flame has multiple zones (Fig. 23.5). The portion of the flame that is used should be neutral or slightly reducing. An improperly adjusted or positioned flame can lead to oxidation and/or carbon inclusion. Once the flame has been applied to the joint area, it should be not removed until brazing is complete. Due to its reducing nature, the flame gives protection from oxidation.

Figure 23.5 A neutral or reducing flame is used.

Temperature The temperature used should be the minimum required to complete the brazing operation. Prior to the placement of the solder, the parent metal is heated till it is hot enough to melt the filler metal as soon as it touches. A lower temperature will not allow the filler to wet the parent metal. A higher temperature increases the possibility of diffusion between parent and filler metal. Time The flame is held until the filler metal has flowed completely into the connection and a moment longer to allow the flux or oxide to separate from the fluid solder. Insufficient time increases chances of incomplete filling of joint and possibility of flux inclusion in the joint. Excessive time increases possibility of diffusion. Both conditions cause a weaker joint.

PITTED SOLDER JOINTS Pits or porosities in the solder joint often become evident during finishing. They are due to

Volatilization of the lower melting components due to heating at higher temperatures and for longer time. Improperly melted or excess flux that is trapped in the solder joint. To avoid such pitting, less flux is applied and the heating should be discontinued as soon as the flux and solder are well melted and flowed into position.

Advantages and Disadvantages Advantages 1. Low cost. 2. Relative effectiveness.

400 Part 5 Dental Laboratory—Materials and Processes Disadvantages 1. Problems such as oxidation of the parts joined by weld. 2. Joint porosity and overheating of the union during the welding process can promote small structural defects and failure of the rehabilitation treatment.

WELDING The term welding is used if two pieces of similar metal are joined together without the addition of another metal. It is usually used to join flat structures such as bands and brackets.

Indications 1. In orthodontics, to join flat structures like bands and brackets. 2. In pedodontics, to weld bands and other appliances. 3. In prosthodontics, to join wrought wire clasps and repair of broken metal partial dentures.

Types Welding processes used in dentistry are 1. 2. 3. 4.

Resistance spot welding Laser welding Plasma arc welding (PAW) Tungsten inert gas (TIG) welding

Resistance spot welding Welding is done by passing an electric current through the pieces to be joined. These pieces are also simultaneously pressed together. The resistance of the metal to flow of current causes intense localized heating and fusion of the metal. The combined heat and pressure fuses the metals into a single piece. Welding is done in an electric spot welding apparatus (Figs. 23.6A and B). The wires or the band to be welded is placed between the two copper electrodes of the welder. A flexible spring attached to the electrode helps to apply pressure on the metals. A hand controlled switch is

Figures 23.6A and B (A) An electric orthodontic spot welder. (B) Close up of the spot welder.

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used to operate the welder. On pressing the switch a large current passes through the wires or band between the copper electrodes. The combined heat and pressure fuses the metal pieces at that point and joins them. This kind of welding may also be referred to as ‘spot welding’. Prosthodontic appliances are welded in a larger machine. The parts to be joined are held together in a clamp. A hand or foot controlled switch controls the current. Weld joints are susceptible to corrosion because of precipitation of chromium carbide and consequent loss of passivation.

TUNGSTEN INERT GAS (TIG) WELDING AND PLASMA ARC WELDING (PAW) Plasma Arc Welding (PAW) (Fig. 23.7) and Tungsten Inert Gas (TIG) welding (Fig. 23.8) are techniques in which a union is obtained by heating materials by an arc established between a nonconsumable tungsten electrode and the part to be welded. The electrode and the area to be welded are protected by using an inert gas, usually argon or a mixture of inert gases (argon and helium). The basic equipment consists of a power supply, a torch with a tungsten electrode, a shielding gas source, and an opening system for the arc. The main difference between TIG and plasma welding is the use of a constrictor torch that concentrates the electric arc in plasma welding.

Figure 23.7 A dental plasma arc welding machine.

Procedure The equipment allows for the adjustment of both the pulse and current which is required for welding. After adjusting the machine, screw into one of the claws a structure without use and position the parts to be welded with hands or through specific equipment, which position the parts to be joined. The argon activation is done by a foot pedal. The foot pedal is pressed until the argon flows. The buzzer will indicate when contact is made. Quickly release the pedal. The weld will be made, and the flow of argon will continue for a few Figrue 23.8 Tungsten inert gas (TIG) welding. seconds. It is possible that in the first few attempts, the electrode will stick to the piece making it necessary to regrind the same.

Advantages and Disadvantages Advantages 1. This allows execution of welds of high quality and excellent finishing, particularly in small joints. 2. The thickness of the joint allows for welding in any position, e.g. repairing removable partial prosthesis. 3. Excellent control of the weld pool, i.e. the region being welded.

402 Part 5 Dental Laboratory—Materials and Processes 4. 5. 6. 7. 8.

Less time needed. It can be executed directly in the working model. The equipment is affordable compared to that of laser welding. Allows welding in regions near the resins and porcelains. Allows welding with the frameworks in close contact or with minimal space for welding, using filler metal.

Disadvantages 1. Large amount of heat to achieve fusion can cause microstructure transformations. These transformations occurring in the ‘heat affected zone’ can cause material distortion, residual stresses, generation of fragile microstructures, grain growth, cracks, fissures, and changes in mechanical, physical, and chemical properties, among others. 2. Insufficient weld penetration in butt type joints. 3. Porosities caused by inclusion of argon gas shield may occur in the weld region. These bubbles can initiate fractures leading to failure of welded structures.

LASER WELDING A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term “laser” originated as an acronym for “light amplification by stimulated emission of radiation”. The first laser was built in 1960 by Theodore H. Maiman at Hughes Laboratories. Laser is finding increasing applications in dentistry (Box 23.2) including welding. The laser used is a pulsed neodymium laser with a very high power density. Among their many applications, lasers are used in optical disc drives, laser printers, and barcode scanners; fiber-optic and free-space optical communication; laser surgery and skin treatments; laser pointers; cutting and welding materials; military and law enforcement devices for marking targets and measuring range and speed; and laser lighting displays in entertainment. Box 23.2 Laser use in dentistry Lasers have been used in dentistry since 1994 to treat a number of dental problems. But, despite FDA approval, no laser system has received the American Dental Association’s (ADA) Seal of Acceptance. That seal assures dentists that the product or device meets ADA standards of safety and efficacy, among other things. The ADA, however, states that it is cautiously optimistic about the role of laser technology in the field of dentistry. These lasers are different from the cold lasers used in phototherapy for the relief of headaches, pain, and inflammation. Dental Applications of Laser Tooth decay Lasers are used to remove decay within a tooth and prepare the surrounding enamel to receive the filling. Curing Lasers are also used to “cure” or harden composite fillings. Gum disease Lasers are used to reshape gums and eliminate pockets. Pulpectomy Remove bacteria during root canal procedures. Biopsy or lesion removal Lasers can be used to remove a small piece of tissue (called a biopsy) so that it can be examined for cancer. Laser are also used to remove lesions in the mouth. Pain relief Used to relieve the pain of canker sores. Surgical procedures Like frenectomy, gingivectomy (Fig. 23.10), healing abutment exposure, incisions, etc. Teeth whitening Lasers are used to speed up the in-office teeth whitening procedures. A peroxide bleaching solution, applied to the tooth surface, is “activated” by laser energy (Fig. 23.9C), which speeds up of the whitening process.

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Gum lightening Laser is used to remove the surface layer of mucosa containing the dark melanin pigment in patient with dark gums. On healing the new mucosa is lighter in color. Welding For joining of metal parts. How Do Lasers Work in Dentistry? All lasers work by delivering energy in the form of light (Fig. 23.9B and 23.10). When used for surgical and dental procedures, the laser acts as a cutting instrument or a vaporizer of tissue that it comes in contact with. When used for “curing” a filling, the laser takes the role of an intense curing light. When used in teeth whitening procedures, the laser acts as a heat source and enhances the effect of tooth bleaching agents. Advantages Compared to the traditional dental drill, lasers have certain advantages 1. They cause less pain in some instances, therefore, reducing the need for anesthesia. 2. They may reduce anxiety in patients uncomfortable with the use of the dental drill. 3. They minimize bleeding and swelling during soft tissue treatments. 4. They may preserve more healthy tooth during cavity removal. Disadvantages 1.

Lasers cannot be used on teeth with fillings already in place.

Lasers cannot be used in many commonly performed dental procedures. For example, lasers cannot be used to fill cavities located between teeth, around old fillings, and large cavities that need to be prepared for a crown. In addition, lasers cannot be used to remove defective crowns or silver fillings, or prepare teeth for fixed prostheses.

3. Traditional drills may still be needed to shape the filling, adjust the bite and polish the filling, even when a laser is used. 4.

Lasers do not fully eliminate the need for anesthesia.

5. Laser treatment is more expensive since the cost of the laser is much higher than a dental drill. Lasers can cost between 6 to 7 times the cost of a standard drill. Different Types of Dental Lasers Used Many different types of lasers are used in dentistry (Fig. 23.9A to D). They can be used in a wide range of power, ranging, from a fraction of a watt to 50 watts or even more. 1. The Erbium YAG laser (Fig. 23.9A) possesses the potential of replacing the drill. This laser is also used to alter pigmentation in the gingival tissues, providing the patient with pink gums. This laser is commonly used to prepare the patient for a cavity filling. 2. The carbon dioxide laser can be used to perform gingivectomy and to remove small tumors. As a laser that does not require local anesthesia, it poses no discomfort for the patient and is practically a bloodless procedure. 3. The argon laser is used in minor surgery. Its gas laser releases blue-green light through a fiberoptic cable to a handpiece or microscope. 4. The Nd:YAG laser is used in tissue retraction, endodontics and oral surgery. This laser usually does not require anesthesia. For procedures regarding the gingival pockets, the fiber is inserted between the gingiva and the tooth to sterilize and stimulate the tissue, causing the gingiva to adhere to the neck. 5. The Diode laser (Fig. 23.9D) introduced in the late 1990s has been effective for oral surgery and endodontic treatment. This laser also helps treat oral diseases and correct esthetic flaws. The diode is a compact laser. 6. Low level lasers are less well known, smaller and less expensive. Sometimes referred to as “soft lasers” the therapy performed by these lasers is called “low level laser therapy.” Low level lasers improve blood circulation and regenerate tissues. Waterlase (from Biolase) The Waterlase combines a laser with an ultra-fine stream of water, which is capable of cutting into tooth, bone and soft tissues. As the stream of water flows into the laser beam, the water molecules become laser energized and create tiny explosions on impact with teeth or soft tissue.

404 Part 5 Dental Laboratory—Materials and Processes

Figures 23.9A to D (A) Elexxion duros Er:YAG hard tissue laser. (B) Laser handpiece. (C) Laser handpiece for tooth whitening. (D) A 15 watt table top diode laser unit.

Figure 23.10 Laser gingivectomy.

Commercial Names Elexion Duros, Claros, Nano, Pico (Elexxion AG, Germany) Dental laser DL 2002 (Dentaurum, Germany) Haas laser LKS (Haas Laser GmbH, Germany) Heraeus Haas laser 44P (Heraeus Kulzer GmbH, Germany) Bego laserstar (Bego, Germany) The unit (Figs. 23.11A and B) consists of a small box that contains the laser tip, an argon gas source and a stereo microscope with lens crosshairs for correct alignment of the laser beam with the components. The maximum depth the laser can penetrate is 2.5 mm. The heat generated is small, so the parts can be hand held during welding and it can be done close to the ceramic or even resin facings without damaging it.

Indications Laser welding is used mainly to join titanium components. This is because the commercially pure titanium (cpTi) used in dentistry for fixed and removable partial denture frameworks

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Figures 23.11A and B (A) A Laser welding unit in operation. (B) Close-up of laser welding of a RPD clasp.

is highly reactive in air. Ordinary soldering procedures result in a weak joint because of the formation of thick layer of titanium oxide (especially when heated above 850 °C). Laser welding or plasma welding can be done at lower temperatures.

Mechanism When the light beam reaches the surface of the metal, the metal absorbs its energy, converting it into heat that penetrates into the interior of the metal by conduction. Owing to a high concentration of heat, the metal is taken to its melting point, and a series of events culminates in the formation of a keyhole or spots that will be filled with the melted metal.

Advantages of Laser Welding Advantages 1. Lower heat generation. 2. It can be executed directly in the working model. 3. Allows welding in regions near the resins and porcelain portions without fear of damage to these materials. 4. No oxide formation because of the inert argon atmosphere. 5. Joint made of the same pure titanium as the components, thus reducing the risk of galvanic corrosion. 6. It produces a keyhole that concentrates the energy absorbed in a small region, resulting in high penetration and formation of a narrow heat affected zone (HAZ) that results in less distortion compared to conventional welding methods; 7. Less time expended 8. Allows welding with the structures in close contact or with minimal space for brazing using filler metal.

Disadvantages 1. Residual stress introduced into welding joints is a consequence of heating and cooling cycles of the welding process. This affects the mechanical behavior of laser-welded structures. 2. Argon gas can cause porosities which can lead to the failure of welded structures. 3. Insufficient penetration of the laser beam can cause a big defect or internal failure if equipment not adjusted properly. 4. High cost of the equipment.

406 Part 5 Dental Laboratory—Materials and Processes CAST-JOINING Cast-joining is an alternative method of joining metals parts that are difficult to solder such as base metal alloys. The two parts are joined by a third metal which is cast into the space between the two. The two parts are held together purely by mechanical retention which is achieved by proper flow of the new metal during casting. Therefore, if the cast metal is poorly adapted it can result in a weak joint.

Figure 23.12 A mechanical interlocking design between two parts joined by cast-metal.

The joint area is ground to make a space of at least 1 mm. Mechanical undercuts are prepared at the interphase (Fig. 23.12). The parts to be joined are assembled with the help of an index. Hard inlay casting wax is flowed into the space and a sprue is attached. The structure is then invested in a casting ring using suitable casting investment. The wax is burnt out and molten metal is cast into the space.

Radiographic assessment of joints Most prosthodontic structures are usually fabricated in commercial laboratories. The quality of the structure especially that of the joints can be assessed through a radiograph. The structure is placed on a film and exposed. It is turned 90 degrees and exposed a second time on a new film. The structure is assessed for defects like porosity and cracks especially in the joints. Porosity and other defects in the joints can weaken the restoration causing it to fail in the mouth during function. The best joints are those that are cast as one piece, followed by soldered and cast-joined.

24 Chapter

Additive Manufacturing in Dentistry Chapter Outline • Classification of 3D Printing • • • •

Technologies Applications Fundamentals of 3D Printing Classification of Additive Manufacturing (AM) Technologies Description of Some Additive Manufacturing (AM) Technologies –– Material Jetting –– Binder Jetting

• • • •

–– –– –– –– –– –– ––

Digital Light Processing Fused Deposition Modelling Robocasting Electron Beam Melting (EBM) Laser Sintering Technique Sheet Lamination Directed Energy Deposition 3D Dental Printers Support Structures for 3D Printed Objects Materials for 3D Printing Bioink

• 3D Printed Maxillofacial Prostheses

• 3D Bioprinting • 3D Printing Technology in

Surgical Planning 3D Bioprinting Bioink Osteoink Comparison of Additive and Subtractive Manufacturing • Advantages of 3D Printing

• • • •

Additive manufacturing is finding increasing use in dentistry. More popularly known as ‘threedimensional printing’, it is an additive manufacturing process in contrast to conventional CAD/ CAM which is a subtractive process. The first 3D printer was invented by Hideo Kodama of Nagoya, Japan. In 1984, Chuck Hull of 3D Systems Corporation further refined the process and named it stereolithography. The process involved the sequential laying down of photocured polymer to produce a three-dimensional plastic object. As the technology developed it encompassed a wider variety of technologies and materials including metals, waxes, polymers, paper, ceramics, etc. By 2000, the umbrella term additive manufacturing (AM) technologies was used to describe all processes involving the CAD based production of objects through sequential layering. Synonyms Other terms include three-dimensional printing, desktop manufacturing, rapid manufacturing, additive fabrication, additive layer manufacturing, layer manufacturing, and freeform fabrication, rapid prototyping, etc.

Applications Using data from oral scans and CAD designs, 3D printing can be used to produce 1. 2. 3. 4.

Surgical guides (Fig. 24.1). Veneers for try-in. Study models (Fig. 24.2). Orthodontic appliances (aligners).

408 Part 5 Dental Laboratory—Materials and Processes 5. Surgical planning and mock surgeries using models designed with the aid of CT or MRI scan data. 6. Wax patterns (Fig. 24.3) for casting dental restorations like inlays, crowns and FDPs (Fig. 24.4). 7. Restorations and removable denture frames can be directly fabricated from the raw metal (Co-Cr, titanium or gold alloy in powder form). 8. Maxillofacial prostheses. 9. Bioprinting can potentially engineer living organs, bone, skin and other tissues for plastic and reconstructive surgery, drug testing, etc.

Fundamentals of 3d printing The fundamentals of additive manufacturing includes 1. Scan data input 2. Computer-aided design 3. Computer aided (additive) manufacturing 3D printable models may be created with a computer-aided design (CAD) package or via a 3D scan of the mouth, impression or model. CT or MRI data can also be used. The computer corrects errors in the scan data called fix-up. The 3D model which is in .skp, .dae, .3Ds or some other format that needs to be converted to either a .STL or a .OBJ format, to allow the printers software to be able to read it. Once that is done, the .STL file needs to be processed by a piece of software called a “slicer” which converts the model into a series of thin layers and produces

Figure 24.1 3D printed surgical guide for implants.

Figure 24.3 3D printed wax patterns of crowns, FDPs and removable partial dentures.

Figure 24.2 Dental models made from light cured photopolymerized resin.

Figure 24.4 3D printed metal crowns and FDPs.

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a G-code file containing instructions for the specific type of 3D printer used. The 3D printer follows the G-code instructions to lay down successive layers of liquid, powder, binder, paper or sheet material to build the model from a series of cross sections.

Classification of additive manufacturing (AM) Technologies The different types of 3D printers each employ a different technology that processes different materials in different ways. It is important to understand that one of the most basic limitations of 3D printing, in terms of materials and applications is that there is a no ‘one solution fits all’. For example some 3D printers process powdered materials (nylon, plastic, ceramic, metal), which utilize a heat source (laser, electron beam) to sinter/melt/fuse layers of the powder together in the defined shape. Others process liquid resin and again utilize a light/laser to solidify the resin layer by layer. Jetting of fine droplets is another 3D printing process, reminiscent of 2D inkjet printing, but with a binder to fix the layers. Perhaps the most common and easily recognized process is deposition, and this is the process employed by the majority of entry-level 3D printers. This process extrudes plastics, commonly PLA or ABS, in filament form through a heated extruder to form layers and create the predetermined shape. The various types of additive manufacturing technologies currently available are summarized in Table 24.1 and Boxes 24.1 to 24.8.

Table 24.1 Overview of additive manufacturing technologies Type

Technologies

Materials

Material extrusion

Fused filament fabrication (FFF) also called Thermoplastics (e.g. PLA, ABS, HIPS, fused deposition modelling (FDM) nylon), HDPE, metals, edible materials, rubber (Sugru), modelling clay, plasticine, RTV silicone, porcelain, metal clay, etc Robocasting

Ceramics, metal alloy, cermet, metal matrix composite, ceramic matrix composite

Direct metal laser sintering (DMLS)

Almost any metal alloy

Electron beam melting (EBM)

Almost any metal alloy including titanium

Selective laser melting (SLM)

Titanium, cobalt chromium, stainless steel,

Selective laser sintering (SLS)

Thermoplastics, metals, ceramics, glass

Selective heat sintering (SHS)

Thermoplastics

Binder jetting

Polymer, ceramic materials, metals

Plaster-based 3D printing (PP)

Plaster

Material jetting

Material jetting (U-V)

Wax, plastics (PMMA)

Sheet Lamination

Laminated object manufact-uring (LOM)

Paper, metal foil, plastic film

Powder bed fusion

Binder jetting

VAT-based photopolymerization Stereolithography (SLA)

Directed Energy Deposition

Photopolymer

Digital light processing (DLP)

Photopolymer

Continuous liquid interface production

Photopolymer

Electron beam free form fabrication (EBF3)

Almost any metal alloy in wire form

Laser cladding

Almost any metal alloy in powder form

Laser engineered net shaping (LENS)

Almost any metal alloy in powder form

Direct metal deposition (DMD)

Almost any metal alloy in powder form

410 Part 5 Dental Laboratory—Materials and Processes Description of some Additive manufacturing (AM) technologies Box 24.1 Material jetting Material jetting is a 3D printing process whereby the actual build materials (in liquid or molten state) are selectively jetted through multiple jet heads (with others simultaneously jetting support materials). However, the materials tend to be liquid photopolymers, which are cured with a pass of UV light as each layer is deposited. Material jetting is the only additive manufacturing technology that can combine different print materials within the same 3D printed model in the same print job.

Box 24.2 Binder jetting In binder jetting, the material being jetted is a binder. It is selectively sprayed into a powder bed to fuse it a layer at a time to create the required part. As is the case with other powder bed systems, once a layer is completed, the powder bed drops incrementally and a roller or blade smooths the powder over the surface of the bed, prior to the next pass of the jet heads. Advantages of this process like with SLS, include the fact that the need for supports is negated because the powder bed itself provides this functionality. Furthermore, a range of different materials can be used, including ceramics and food. A further distinctive advantage of the process is the ability to easily add a full color palette which can be added to the binder. The parts resulting directly from the machine, however, are not strong and may require post-processing to ensure durability.

Box 24.3 Digital light processing (DLP) DLP or digital light processing is a similar process to stereolithography in that it is a 3D printing process that works with photopolymers. The major difference is the light source. DLP uses a more conventional light source, such as an arc lamp, with a liquid crystal display panel or a deformable mirror device, which is applied to the entire surface of the of photopolymer resin in a single pass, generally making it faster than SL. However, one advantage of DLP over SL is that only a shallow vat of resin is required to facilitate the process, which generally results in less waste and lower running costs.

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Box 24.4 Fused deposition modeling 3D pr inting utilizing the ex trusion of thermoplastic material is probably the most common and recognizable 3D printing process. The popular name for the process is fused deposition modeling (FDM), however this is a trade name, registered by Stratasys, the company that originally developed it. Other manufacturers generally referred to it as Fused Filament Fabrication (FFF). The process works by melting plastic filament that is deposited, via a heated extruder, a layer at a time, onto a build platform. Each layer hardens as it is deposited and bonds to the previous layer. Systems have evolved and improved to incorporate dual extrusion heads.

Box 24.5 Robocasting Robocasting or Direct Ink Writing (DIW) is an additive manufacturing technique in which a filament of ‘ink’ is extruded from a nozzle, forming an object layer by layer. The technique was first developed in the United States in 1996 as a method to allow geometrically complex ceramic green bodies to be produced. A fluid (typically a ceramic slurry), referred to as an ‘ink’, is extruded through a small nozzle, drawing out the shape of each layer of the CAD model. The ink exits the nozzle in a liquid-like state but retains its shape immediately, exploiting the rheological property of shear thinning. It is distinct from fused deposition modeling (FDM) as it does not rely on the solidification or drying to retain its shape after extrusion. To date the most researched application for robocasting is in the production of biologically compatible tissue implants (bioprinting). Lattice structures can be formed quite easily which allow bone and other tissues in the human body to grow and eventually replace the transplant. With various medical scanning techniques the precise shape of the missing tissue is established and input into 3D modeling software and printed.

Box 24.6 Electron beam melting (EBM) The Electron Beam Melting 3D printing technique is a proprietary process developed by Swedish company Arcam. This metal printing method is very similar to the Direct Metal Laser Sintering (DMLS) process in terms of the formation of parts from metal powder. The key difference is the heat source, which, as the name suggests is an electron beam, rather than a laser, which necessitates that the procedure is carried out under vacuum conditions. EBM has the capability of creating fully-dense parts in a variety of metal alloys, even to medical grade, and as a result the technique has been particularly successful for a range of production applications in the medical industry, particularly for implants. However, other hi-tech sectors, such as aerospace and automotive have also looked to EBM technology for manufacturing fulfilment.

1. High voltage cable 2. Incandescent cathode 3. Bias cup 4. Primary anode 5. Electron beam 6. Focusing coil 7. Deflection coil 8. Weld bead 9. Work piece 10. Vacuum chamber

412 Part 5 Dental Laboratory—Materials and Processes Box 24.7 Laser sintering technique Laser sintering and laser melting are interchangeable terms that refer to a laser-based 3D printing process that works with powdered materials. The laser is traced across a powder bed of tightly compacted powdered material. As the laser interacts with the surface of the powdered material it sinters, or fuses, the particles to each other forming a solid. As each layer is completed the powder bed drops incrementally and a roller smooths the powder over the surface of the bed prior to the next pass of the laser for the subsequent layer to be formed and fused with the previous layer. The build chamber is completely sealed as it is necessary to maintain a precise temperature during the process. Once finished, the excess powder is removed to leave the ‘printed’ parts. One of the key advantages of this process is that the powder bed serves as an in-process support structure for overhangs and undercuts, and therefore complex shapes that could not be manufactured in any other way are possible with this process. However, on the downside, because of the high temperatures required for laser sintering, cooling times can be considerable. Furthermore, porosity has been a issue with this process, and while there have been significant improvements towards fully dense parts, some applications still necessitate infiltration with another material to improve mechanical characteristics.

Box 24.8 Sheet lamination and Directed energy deposition Sheet Lamination Sheet lamination processes include ultrasonic additive manufacturing (UAM) and laminated object manufacturing (LOM). The Ultrasonic Additive Manufacturing process uses sheets or ribbons of metal, which are bound together using ultrasonic welding. Directed Energy Deposition Directed energy deposition (DED) covers a range of terminology like laser engineered net shaping, directed light fabrication, direct metal deposition, 3D laser cladding’ It is a more complex printing process commonly used to repair or add additional material to existing components. In this class of technology metal or alloy powder is actually deposited on the work surface and sintered or melted using various means.

3D dental printers 3D printers for dental applications are manufactured by a range of companies (Figs. 24.5A to D). These include Invision wax printer, Objet Eden 260V, Varseo Dental 3D Printing System (BEGO), Projet 3510DPPro, 3Z LAB 3D wax printer (solidscape), Stratasys Crownworx and Frameworx, Renishaw AM 250 (for metal), etc. They range in size from small desktop models to larger floor machines. The technology they employ also varies according to the objects they fabricate.

Support structures for 3D printed objects Some printing techniques require external or internal supports to be built for undercut areas, cantilevers and other overhanging features for greater stability and strength during the manufacturing process. These supports are later mechanically removed or dissolved upon completion of the print.

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Figures 24.5A to D 3D printers for dental applications. (A) A 3D wax printer (FrameWorx by Stratasys). (B) A 3D printer for dental models and casts. (C) A 3D metal printer dental for alloys.. (D) The Bego (Varseo) uses LED based stereolithography technology to make RPD and other pattens for casting.

Figures 24.6A and B Support structures. (A) 3D printed metal crowns and FDPs showing support structures. (B) 3D printed wax crowns showing support structures.

1. In metal and plastic printing, multiple supports are printed connecting the restoration to the base (Fig. 24.6A). 2. Some 3D printers print wax patterns along with a second water soluble support material (Fig. 24.6B). This material is removed after completion by dissolving in water (Figs. 24.7A and B). 3. In powder bed systems, the unused powder surrounding the object functions as a support structure (Box 24.7).

414 Part 5 Dental Laboratory—Materials and Processes RAW Materials for 3D printing A wide variety of materials are available for 3D printing. These include thermoplastics, granulated alloys (titanium alloys, metal alloys), foil, paper, photopolymers, liquid resins, rubber-like materials, silicones, glass, ceramic slurry, foods, medication, bioink make of cells and stem cells for tissue generation. etc. These materials are in various forms like thermoplastic filaments, wire, granules, powder, photopolymerizing liquid resin, paste, liquid, etc (Figs. 24.8A to C).

Post-manufacturing processing Following the fabrication, the objects produced may require some form of processing for improved properties and finish. Objects manufactured may have layer lines and other artefacts of the printing process. Some of the products are produced in the green state and require to be sintered. Metals may require various forms of heat treatment. Thus postmanufacturing processing include support material removal, finishing and polishing, sealing, chemical treatment, sintering, heat treatment, etc.

3D printed maxillofacial prostheses British company Fripp Design and Research has developed 3D-printed prosthetic eyes that could be produced much faster than existing handmade versions, reducing the cost by 97 percent. The company which is also working on 3D-printed ears and noses for patients with facial disfigurements, has collaborated with Manchester Metropolitan University to

Figures 24.7A and B (A) Dissolution of support material (white) by immersing in water. (B) Same object after removal of support material.

Figures 24.8A to C Materials for Additive manufacturing. (A) Thermoplastic filaments for FFD. (B) Metal granules for laser sintering. (C) Photopolymerizing liquid resin for stereolithography.

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develop ocular prosthetics that are 3D-printed in batches, with intricate coloured details including the iris and blood vessels already included. Currently, prosthetic eyes are moulded in acrylic and painted by hand to match the patient’s eye color. This process is timeconsuming and expensive, whereas producing the eyes using a 3D printer (Fig. 24.9) enables up to 150 eyes to be made in an hour. All of the components are printed from powder in full Figure 24.9 3D printed eyes. color using a Z-Corp 510 machine before the resulting form is encased in resin. Compared to the existing handmade production method, this helps to remove any variation in quality and significantly reduces the cost of each eye. (Refer also chapter on maxillofacial prosthetic materials).

3D printing technology in surgical planning Hospitals around the world are increasing turning to 3D technology to plan complex surgeries. In January 2015, doctors at London’s St Thomas’ Hospital had used images obtained from a magnetic resonance imaging (MRI) scan to create a 3D printing replica of the heart of a two-year-old girl with a complex hole in it. They were then able to tailor a GoreTex patch to effect a cure. 3D printing means surgeons can go into an operation with a much better idea of what they would find.

Figure 24.10 3D printed skull of a patient with hypertelorism.

Similarly plastic and maxillofacial surgeons in Bengaluru, Karnataka, India, used a 3D printed model (Fig. 24.10) for a surgical procedure to plan and correct orbital hypertelorism using both a box osteotomy as well as a facial bipartition technique with the aid of Osteo 3D, a company that is focused on 3D printing for the healthcare industry.

Similar processes are now being used in hospitals and universities to plan complex surgeries, for patient education and as teaching aids for medical students.

Tissue engineering Several terms have been used to refer to this field of research including organ printing, bioprinting, computer-aided tissue engineering, etc. The term regenerative medicine is often used synonymously with tissue engineering, although those involved in regenerative medicine place more emphasis on the use of stem cells or progenitor cells to produce tissues. San Diego based Organovo, a regenerative medicine company, was the first company to commercialize 3D bioprinting technology. The company utilizes its NovoGen MMX Bioprinter (Fig. 24.11) for 3D bioprinting. The printer is optimized to be able to print skin tissue, cartilage, heart tissue and blood vessels among other basic tissues that could be suitable for surgical therapy and transplantation. It is hoped bioprinting technology will

416 Part 5 Dental Laboratory—Materials and Processes eventually be used to create fully functional human organs for transplants. They could be used in drug research and perhaps eliminate the need for testing in animals. The possibility of using 3D tissue printing to create soft tissue architectures for reconstructive and plastic surgery is also being explored. Though scientists have been able to engineer artificial organs such as livers and kidneys, these organs lack crucial elements required for full and independent functioning, such as working blood vessels, tubules for collecting urine, etc. Without these the body has no way to get the essential nutrients and oxygen deep within the tissues. Fully functional printed organs may yet be possibility in the future.

How bioprinting works In the bioprinting process, layers of living cells are deposited on to a gel medium or sugar matrix and slowly built up to form three-dimensional structures or scaffolds including vascular systems (Box 24.9). Another process uses an extracellular matrix (ECM) protein as scaffold. As Organovo have demonstrated, it is not necessary to print all of the details of an organ with a bioprinter, as once the relevant cells are placed in roughly the right place nature completes the job. This point is illustrated by the fact that the cells contained in a bioink spheroid are capable of rearranging themselves after printing. For example, experimental blood vessels bioprinted using bioink spheroids comprising of an aggregate mix of endothelial, smooth muscle and fibroblast cells, once placed in position by the bioprint head demonstrated migration and reorganization. With no technological intervention, the endothelial cells migrate to the inside of the bioprinted blood vessel, the smooth muscle cells move to the middle, and the fibroblasts migrate to the outside.

Incubating the new tissue Scientists hope to be able one day to print some types of replacement parts directly into patients’ bodies. Currently, tissues must spend a few days to few weeks maturing in a type of incubator called a bioreactor.

Bioink BioInk (Fig. 24.12) is a chemically defined hydrogel used to print 3D tissue models in bioprinters. It supports growth of different cell types. It allows cell adhesion, migration, and

Figure 24.11 Organovo’s NovoGen MMX bioprinter.

Figure 24.12 Bioink.

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Box 24.9 How bioprinting works

differentiation. It mimics the natural extracellular matrix and is biodegradable. This material was developed by researchers at the University of Missouri, Columbia. To make bioink, scientists create a slurry of cells that can be loaded into a cartridge and inserted into a specially designed printer, along with another cartridge containing a gel known as biopaper. After inputting the standards for the tissue, the printer lays down alternate layers to build a three-dimensional structure, with the biopaper creating a supportive matrix that the ink can thrive on. The individual droplets fuse together, eventually latticing upwards through the biopaper to create a solid structure. One potential use for bioink is in skin grafting. By creating grafts derived from the patient’s own cells, it could reduce the risk of rejection and scarring. Although initial bioink developments began with skin regeneration, the technology has advanced to also incorporate bone and muscle. This makes bioinks a total regenerative technology. Bioink could also be used to make replacements for vascular material removed during surgeries, allowing people to receive new veins and arteries. Eventually, entire organs could be constructed from this material. Bioprinted tissue could potentially remove the anxiety of finding donors as well as also allay fears about contaminated organ supplies or unscrupulous organ acquisition methods. These will enable medical researchers to test drugs on bioprinted models of the liver and other organs, thereby reducing the need for animal tests.

Osteoink Osteoink is a ready-to-use calcium phosphate paste for structural engineering dedicated to regenHU’s BioFactory and 3D Discovery printers. Osteoink is a highly osteoconductive biomaterial close to the chemical composition of human bone. Dedicated for hard tissue engineering, such as bone, cartilage or structural scaffold manufacturing, Osteoink can be combined with regenHU’s biomaterial product portfolio (e.g. BioInk) to create complex 3D tissue mimetic models.

418 Part 5 Dental Laboratory—Materials and Processes Table 24.2 Comparison of additive and subtractive manufacturing 3D printing

CAD milling

Computer aided

Creates objects by adding material

Creates objects by removing material

Does not require burs

Requires burs

Can produce complex shapes without requiring special Special strategies and parameters required to strategies or changing tools compensate for the size of the cutting tool

Can print multiple parts at the same time

Can combine multiple raw materials in a single object Can use only one type of raw material at a time (depending on the system)

Little waste produced

Machining produces considerable waste

There is nothing to break or change

Burs require changing as they break or wear

Can machine only one object at a time

Comparison of additive and subtractive manufacturing As dental laboratories become increasingly automated, the dental profession has to keep pace with the fast moving technology. Understanding these emerging technologies is the key to making choices on what is best suited for the individual dentist or laboratory professional. A comparison of the two CAD-based manufacturing techniques is presented in Table 24.2.

Advantages of 3D printing 1. 2. 3. 4. 5. 6. 7. 8. 9.

Reduction fabrication times. Reduced fabrication costs. Clean, safe and efficient process. Less waste as only the required amount of material is used (unlike CAD milling). No need to store bulky models as they can be stored digitally and reproduced on demand. Hollow objects can be created with greater ease. Possible to print complex shapes and structures. Possibility of using multiple materials in a single object. Less stresses introduced in the object (compared to machining which may introduce microcracks in the object). 10. Rapid prototyping.

Section-6

Alloys in Dentistry Chapter 25 Dental Casting Alloys, 421 Chapter 26 Dental Implant Materials, 452 Chapter 27 Wrought Alloys, 466

25 CHAPTER

Dental Casting Alloys Chapter Outline • Metal Restorations in Dentistry • Terminology • History and Classification of Dental Casting Alloys

• Classification • General Requirements • Alloys for All Metal Restorations –– Uses –– Types • Gold Alloys (for All Metal

Restorations) –– Gold Content –– Composition –– Functions of Constituents –– Properties –– Heat Treatment of Gold Alloys –– Low Gold Alloys • Silver-palladium Alloys • Nickel-chrome and Cobaltchromium Alloys

• Titanium and Titanium Alloys • Aluminum Bronze Alloy • Metal-ceramic Alloys –– Evolution of Metal-ceramic Alloys

•

Porcelain Bonding

• •

Metal-ceramic Alloys

•

–– Requirements of Alloys for –– Uses of Metal-ceramic Alloys –– Types (Classification) of • High Noble (Gold-based) Metal-

ceramic Alloys –– Gold-palladium-platinum Alloys –– Gold-palladium-silver Alloys –– Gold-palladium Alloys • The Noble (Palladium-based) Metal-ceramic Alloys –– Common Features of Palladium Based (Noble) Alloys

• • •

–– –– –– ––

Palladium-silver Alloys Palladium-copper Alloys Palladium-cobalt Alloys Palladium-gallium Alloys Base Metal Alloys for Metalceramic Restorations Nickel-chromium Alloys Titanium and its Alloys for Metal-ceramic Applications Removable Denture Alloys –– Additional Requirements for Partial Denture Alloys –– Types Cobalt-chromium Alloys Advantages and Disadvantages of Base Metal Alloys Comparison of a Gold Alloy and a Base Metal Alloy

Metal restorations and prostheses are an integral part of dentistry. Metals are among the strongest materials and provide strength and durability to any structure. There are two ways of constructing a metal restoration—direct and indirect. Direct techniques have been used in modern dentistry since the introduction of direct filling gold and amalgam in the 19th century. Indirect dental restorations were introduced into the dental profession with the patenting of the centrifugal casting machine and the lost wax technique by William H. Taggart in 1907.

TERMINOLOGY ALLOY An alloy is defined as a metal containing two or more elements, at least one of which is metal and all of which are mutually soluble in the molten state.

422 PART 6 Alloys in Dentistry NOBLE METALS Noble metals have been used for inlays, crowns and FDPs because of their resistance to corrosion in the mouth. Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are the eight noble metals. However, in the oral cavity, silver can tarnish and therefore is not considered a noble metal.

PRECIOUS METALS The term precious indicates the intrinsic value of the metal. The eight noble metals are also precious metals and are defined so by major metallurgical societies and federal government agencies, e.g. National Institute of Standards and Technology and National Material Advisory Board. All noble metals are precious but all precious metals are not noble. Of the eight noble metals, four are very important in dental casting alloys, i.e. gold, platinum, palladium and silver. All four have a face-centered cubic crystal structure and all are white colored except for gold. Gold Pure gold is a soft and ductile metal with a yellow ‘gold’ hue. It has a density of 19.3 g/cm3 and a melting point of 1063 °C. Gold has a good luster and takes up a high polish. It has good chemical stability and does not tarnish and corrode under normal circ*mstances. Silver Sometimes described as the ‘whitest’ of all metals. It has the lowest density (10.4 g/ cm3) and melting point (961°C) among the precious casting alloys. Its CTE is 15.7 × 10-6/°C which is comparatively high. Palladium Density is 12.02 g/cm3. Palladium has a higher melting point (1552°C) and lower CTE (11.1 × 10–6/°C) when compared to gold. Platinum It has the highest density (21.65 g/cm3) highest melting point (1769°C) and the lowest CTE among the four precious metals.

SEMIPRECIOUS METALS There is no accepted composition which differentiates ‘precious’ from ‘semiprecious’. Therefore, the term semiprecious should be avoided.

BASE METALS These are non-noble metals. They are important components of dental casting alloys because of their influence on physical properties, control of the amount and type of oxidation and their strengthening effect. Such metals are reactive with their environment and are referred to as ‘base metals’. Some of the base metals can be used to protect an alloy from corrosion by a property known as passivation. Although they are frequently referred to as nonprecious, the preferred term is base metal. Examples Chromium, cobalt, nickel, iron, copper, manganese, etc.

HISTORY AND CLASSIFICATION OF DENTAL CASTING ALLOYS At the beginning of the twentieth century when dental casting techniques were evolving, the alloys were predominantly gold based. Taggart in 1907 was the first to describe the lost wax technique in dentistry. The existing jewelry alloys were quickly adopted for dental purposes. Initially, copper, silver and platinum were the main alloying elements. As the alloys evolved it was felt that a classification was needed. In 1932, the National Bureau of Standards classified the alloys according to their hardness (Type I, Type II, etc.).

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At that time it was felt that gold alloy with less than 65% gold, tarnished too easily in the oral cavity. By 1948, metallurgists experimenting with various alloys were able to decrease the gold content while maintaining their resistance to tarnish. This breakthrough was due to palladium. It counteracted the tarnish potential of silver. The main requirements of the original dental casting alloys were simple 1. They should not tarnish in the mouth. 2. They should be strong (for use as bridges). This soon changed with the introduction of special alloys (metal-ceramic alloys) that could bond to porcelain in the late 1950s. The composition and requirements of these alloys became more complex. For example, they had to contain elements that could enhance bond to porcelain, they had to have a higher melting temperature (because porcelain had high fusion temperatures), etc. Another important development were the rapid increase in gold prices in the 1970s. As gold became more expensive, people began to look for less expensive metals for dental castings. Manufacturers began experimenting with base metal alloys like nickelchromium and cobalt-chromium. These alloys were already in use since the 1930s for the construction of cast partial denture frameworks. Prior to this, the Type IV gold alloys were used extensively for this purpose. These base metals soon replaced the Type IV gold alloys for partial denture use because of their light weight, lower cost and tarnish resistance. When the gold prices shot up, these base metal partial denture alloys were quickly adapted for use in fixed prosthodontics. Subsequently, newer formulations allowed their use as metal-ceramic alloys. Today there is such a wide variety of alloys in the market that classifying them is not easy. A number of different classifications are mentioned below.

ACCORDING TO USE A. Alloys for all metal and resin veneer restorations* (e.g. inlays, posts, resin and composite veneered crowns and FDPs). B. Alloys for metal-ceramics restorations (e.g. PFM crowns and FDPs). C. Alloys for removable dentures ** (e.g. RPD frames and complete denture bases).

BASED ON YIELD STRENGTH AND PERCENT ELONGATION (ADA SP. 5 ) Type I

Soft

Type II

Medium

Type III

Hard

Type IV

Extra-hard

(This 1934 classification was originally intended for gold alloys and were based on hardness. Since 1989, it was relaxed to include any dental alloy as long as they met the new yield strength and percentage elongation criteria. Types I and II are known as ‘inlay alloys’ and Types III and IV are known as ‘crown and bridge alloys’. Type IV is occasionally used for RPD frames).

*Some authors classify this as crown and bridge alloys. Unfortunately, this can create confusion; for example, metal-ceramic alloys are also crown and bridge alloys. **Also known as RPD alloys, which again unfortunately is not fully accurate as they can be used for other structures. However, until a more suitable terminology is found, this classification will be continued.

424 PART 6 Alloys in Dentistry ACCORDING TO NOBILITY (ADA 1984) A. High noble metal alloys (HN) Contains > 40 wt% Au and > 60 wt% noble metals B. Noble metal alloys (N) Contains > 25 wt% of noble metals C. Predominantly base metal alloys (PB) Contains < 25 wt% of noble metals D. Base metal This classification is popular among manufacturers.

BASED ON MECHANICAL PROPERTIES (ISO 22674:2006) The current ISO classification supersedes all previous classifications and covers all metals used in dentistry for restorations and prostheses. It makes no distinction between noble and base metal. ISO 22674:2006 classifies all metallic materials into six types according to its mechanical properties. Type 0 - Intended for low stress bearing single-tooth fixed restorations, e.g. small veneered one-surface inlays, veneered crowns.* Type 1 - Intended for low stress bearing single-tooth fixed restorations, e.g. veneered or unveneered one-surface inlays, veneered crowns. Type 2 - Intended for single tooth fixed restorations, e.g. crowns or inlays without restriction on the number of surfaces. Type 3 - Intended for multiple unit fixed restorations, e.g. bridges. Type 4 - Intended for appliances with thin sections that are subject to very high forces, e.g. removable partial dentures, clasps, thin veneered crowns, wide-span bridges or bridges with small cross-sections, bars, attachments, implant retained superstructures. Type 5 - Intended for appliances in which parts require the combination of high stiffness and strength, e.g. thin removable partial dentures, parts with thin cross-sections, clasps.

ACCORDING TO MAJOR ELEMENTS A. B. C. D. E. F.

Gold alloys Silver alloys Palladium alloys Nickel alloys Cobalt alloys Titanium alloys

ACCORDING TO THE THREE MAJOR ELEMENTS A. Gold-palladium-silver B. Palladium-silver-tin C. Nickel-chromium-molybdenum D. Cobalt-chromium-molybdenum E. Iron-nickel-chromium F. Titanium-aluminum-vanadium

ACCORDING TO THE NUMBER OF ALLOYS PRESENT A. Binary—two elements * also includes metal-ceramic restorations produced by electroforming or sintering.

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B. Ternary—three elements C. Quaternary (and so forth)—four elements

CLASSIFICATION ACCORDING TO USE OF DENTAL CASTING ALLOYS The huge choice of alloys in the market makes the process of identification a difficult task. They are similar in some aspects, but yet, each have their own distinct features. These alloys vary not only in the type of metal but also the percentage of each within the alloy. In spite of their wide variation in composition, they must meet the requirements of their intended use. For example, all metal-ceramic alloys regardless of whether they are noble or base must meet the requirements of porcelain bonding. For this reason, the classification according to use is recommended and will be the basis of the subsequent discussion of alloys. A. Alloys for all metal and resin veneer restorations –– High noble –– Noble –– Predominantly base metal –– Base metal B. Alloys for metal-ceramics restorations –– High noble –– Noble –– Predominantly base metal –– Base metal C. Alloys for casting large structures –– High noble –– Noble –– Predominantly base metal –– Base metal

GENERAL REQUIREMENTS OF CASTING ALLOYS All cast metals in dentistry have some basic common requirements. 1. 2. 3. 4. 5. 6. 7.

They must not tarnish and corrode in the mouth. They must be sufficiently strong for the intended purpose. They must be biocompatible (nontoxic and nonallergenic). They must be easy to melt, cast, cut and grind (easy to fabricate). They must flow well and duplicate fine details during casting. They must have minimal shrinkage on cooling after casting. They must be easy to solder.

Not all of them meet all the requirements. Some have shown a potential for allergic reactions (nickel containing alloys) and other side effects when used without proper precautions. Some are quite difficult to cast. Some are so hard (base metal alloys) that they are difficult to cut, grind and polish. All alloys shrink on cooling. Some (base metal alloys) show more shrinkage than others. The shrinkage cannot be eliminated, but it can be compensated for (see investments). Besides these general requirements, alloys intended for a certain specific use must meet requirements for that. For example, metal-ceramic alloys must have additional requirements in order to be compatible with porcelain. The requirements for metal-ceramic alloys will be described later.

426 PART 6 Alloys in Dentistry ALLOYS FOR ALL METAL RESTORATIONS These alloys were among the earliest alloys available to dentistry. The early alloys were mostly gold alloys. Since they were intended for all-metallic and later for resin veneered restorations, they just had to meet the basic requirements (see general requirements). No special requirements are needed for veneering with resin. Currently, the use of these alloys are slowly declining because of

Increased esthetic awareness has reduced the trend for metal display. Increasing popularity of all-ceramic and metal-ceramic restorations. Reducing popularity of resin and composite as veneering material. Resin facings have a number of disadvantages. –– They wear rapidly (poor wear resistance). –– They may change color (color instability and stain absorption). –– They are porous. They tend to absorb food material and bacteria. This makes it unhygienic and gives it a bad odor.

CLASSIFICATION (ANSI/ADA SP. NO. 5) (As mentioned before this 1934 classification was originally intended for gold alloys and was based on hardness. In 1989, it was relaxed to include any dental alloy as long as they met the new yield strength and percentage elongation criteria).

TYPE I SOFT Small inlays, Class III and Class V cavities which are not subjected to great stress. They are easily burnished.

TYPE II MEDIUM Inlays subject to moderate stress, thick 3/4 crowns, abutments, pontics, full crowns, and sometimes soft saddles.

TYPE III HARD Inlays, crowns and bridges, situations where there may be great stresses involved. They usually can be age hardened.

TYPE IV EXTRA-HARD Inlays subjected to very high stresses, partial denture frameworks and long span bridges. They can be age hardened. Types I and II are generally called ‘inlay alloys’ and Types III and IV are known as ‘crown and bridge alloys’. Because of the increased use of composite and ceramic inlays, the Type I and II inlay alloys are rarely used currently. Most of the discussion will focus on the Types III and IV alloys.

USES These alloys are not intended for porcelain bonding. They may be used as an all-metal restoration or with a resin veneer. 1. Inlays and onlays (Figs 25.1A) 2. Crowns and FDPs (Fig. 25.1B)

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FIGURES 25.1A AND B (A) Gold onlays. (B) A gold crown.

3. Partial denture frames (only the Type IV) 4. Post-cores (Fig. 25.2)

TYPES These alloys will be discussed under the following categories. High noble — Gold alloys Noble

— Silver palladium alloys

Base metal — Nickel-chrome alloys

Cobalt-chrome alloys

Titanium and its alloys

Aluminum-bronze alloys

FIGURE 25.2 Post-core.

GOLD ALLOYS (FOR ALL-METAL RESTORATIONS) Synonyms Traditional gold alloys, Au-Ag-Cu alloys. Why do we alloy gold? Pure gold is a soft and ductile metal and so is not used for casting dental restorations and appliances in its pure state. It is alloyed commonly with copper, silver, platinum, nickel and zinc. Alloying gold with these metals not only improves its physical and mechanical properties but also reduces its cost. The display of metal particularly gold was once acceptable and probably was even a symbol of social status. The current trend is to avoid the display of metal. At the same time, increase in the platinum, palladium and silver content of modern alloys have resulted in whiter colored gold alloys. Thus, there are ‘yellow gold alloys’ and ‘white gold alloys’. The rise in gold prices have also led to the availability of alloys with low gold content. These are the ‘low golds’. The gold alloys discussed here are high noble alloys because of their high noble metal content (see classification according to nobility).

GOLD CONTENT Traditionally, gold content of dental casting alloys was called

Karat Fineness

428 PART 6 Alloys in Dentistry KARAT It refers to the parts of pure gold present in 24 parts of alloy, e.g. 24 karat gold is pure gold. 22 karat gold is 22 parts pure gold and 2 parts of other metal. Note In current dental alloys, the term karat is rarely used.

FINENESS Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fine. Thus, if 3/4 of the gold alloy is pure gold, it is said to be 750 fine. Note The term fineness also is rarely used to describe gold content in current alloys (however, it is often used to describe gold alloy solders).

PERCENTAGE COMPOSITION The percentage composition of gold alloys is preferred over karat and fineness. Since 1977, ADA requires manufacturers to specify the percentage composition of gold, palladium and platinum on all their dental alloy packaging. Karat × 100 = %gold 24

Fineness is 10 times the percentage gold composition, i.e. fineness × 10 = %gold.

COMPOSITION OF GOLD ALLOYS Type

% Au

% Cu

% Ag

% Pd

% Pt

% In, Sn, Fe, Zn, Ga

0.5

—

Balance

—

Balance

III

3.5

—

Balance

12.5

3.5

Balance

FUNCTIONS OF CONSTITUENTS Gold It provides tarnish and corrosion resistance and has a desirable appearance. It also provides ductility and malleability.

Copper It is the principal hardener. It reduces the melting point and density of gold. If present in sufficient quantity, it gives the alloy a reddish color. It also helps to age harden gold alloys. In greater amounts, it reduces resistance to tarnish and corrosion of the gold alloy. Therefore, the maximum content should not exceed 16 percent.

Silver It whitens the alloy, thus helping to counteract the reddish color of copper. It increases strength and hardness slightly. In larger amounts, however, it reduces tarnish resistance.

Platinum It increases strength and corrosion resistance. It also increases melting point and has a whitening effect on the alloy. It helps reduce the grain size.

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Palladium It is similar to platinum in its effect. It hardens and whitens the alloy. It also raises the fusion temperature and provides tarnish resistance. It is less expensive than platinum, thus reducing the cost of the alloy. The minor additions are

Zinc It acts as a scavenger for oxygen. Without zinc, the silver in the alloy causes absorption of oxygen during melting. Later during solidification, the oxygen is rejected producing gas porosities in the casting.

Indium, tin and iron They help to harden ceramic gold-palladium alloys, iron being the most effective.

Calcium It is added to compensate for the decreased CTE that results when the alloy is made silver free (the elimination of silver is done to reduce the tendency for green stain at the metalporcelain margin).

Iridium, ruthenium, rhenium They help to decrease the grain size. They are added in small quantities (about 100–150 ppm). Note All modern noble metal alloys are fine grained. Smaller the grain size of the metal, the more ductile and stronger it is. It also produces a more hom*ogeneous casting and improves the tarnish resistance. A large grain size reduces the strength and increases the brittleness of the metal. Factors controlling the grain size are the rate of cooling, shape of the mold and composition of the alloy.

PROPERTIES OF GOLD ALLOYS COLOR Traditionally, the gold alloys were gold colored. The color of modern gold alloys can vary from gold to white. It depends on the amount of whitening elements (silver, platinum, palladium, etc.) present in the alloy.

MELTING RANGE Ranges between 920–960 °C. The melting range of an alloy is important. It indicates the type of investment required and the type of heating source needed to melt the alloy.

DENSITY It gives an indication of the number of dental castings that can be made from a unit weight of the metal. In other words, more number of cast restorations per unit weight can be made from an alloy having a lower density, than one having a higher density. Gold alloys are lighter than pure gold (19.3 g/cm3). Type III — 15.5 g/cm3 Type IV — 15.2 g/cm3 The castability of an alloy is also affected by density. Higher density alloys cast better than lower density alloys.

430 PART 6 Alloys in Dentistry YIELD STRENGTH The yield strength is Type III — 207 MPa

Type IV — 275 MPa

HARDNESS The hardness indicates the ease with which these alloys can be cut, ground or polished. Gold alloys are generally more user friendly than the base metal alloys which are extremely hard. The hardness values Type III — 121 MPa

Type IV — 149 MPa

ELONGATION It indicates the ductility of the alloy. A reasonable amount is required especially if the alloy is to be deformed during clinical use, e.g. clasp adjustment for removable partial dentures, margin adjustment and burnishing of crowns and inlays. Type I alloys are easily furnished. Alloys with low elongation are very brittle. Age hardening decreases ductility.

Type III—30–40%

Type IV—30–35%.

MODULUS OF ELASTICITY This indicates the stiffness/flexibility of the metal. Gold alloys are more flexible than base metal alloys (Type IV—90 × 103 MPa).

TARNISH AND CORROSION RESISTANCE Gold alloys are resistant to tarnish and corrosion under normal oral conditions. This is due to their high noble content. Noble metals are less reactive.

CASTING SHRINKAGE All alloys shrink when they change from liquid to solid. The casting shrinkage in gold alloys is less (1.25–1.65%) when compared to base metal alloys. The shrinkage occurs in three stages: 1. Thermal contraction of the liquid metal. 2. Contraction of the metal while changing from liquid to solid state. 3. Thermal contraction of solid metal as it cools to room temperature. Shrinkage affects the fit of the restoration. Therefore, it must be controlled and compensated for in the casting technique.

BIOCOMPATIBILITY Gold alloys are relatively biocompatible.

CASTING INVESTMENT Gypsum-bonded investments may be used for low fusing gold alloys.

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HEAT TREATMENT OF GOLD ALLOYS Heat treatment of alloys is done in order to alter its mechanical properties. Gold alloys can be heat treated if it contains sufficient amount of copper. Only Type III and Type IV gold alloys can be heat treated. There are two types of heat treatment. 1. Softening heat treatment (solution heat treatment). 2. Hardening heat treatment (age hardening).

SOFTENING HEAT TREATMENT Softening heat treatment increases ductility, but reduces strength, proportional limit and hardness.

Indications It is indicated for appliances that are to be ground, shaped or otherwise cold worked in or outside the mouth.

Method The casting is placed in an electric furnace for 10 minutes at 700 °C and then it is quenched in water. During this period, all intermediate phases are changed to a disordered solid solution and the rapid quenching prevents ordering from occurring during cooling. Each alloy has its optimum temperature. The manufacturer should specify the most favorable temperature and time.

HARDENING HEAT TREATMENT (OR AGING) Hardening heat treatment increases strength, proportional limit and hardness but decreases ductility. It is the copper present in gold alloys which helps in the age hardening process.

Indications For strengthening metallic dentures, saddles, FDPs and other similar structures before use in the mouth. It is not employed for smaller structures, such as inlays.

Method It is done by ‘soaking’ or aging the casting at a specific temperature for a definite time, usually 15–30 minutes. It is then water quenched or cooled slowly. The ageing temperature depends on the alloy composition but is generally between 200 and 450 °C. During this period, the intermediate phases are changed to an ordered solid solution (the proper time and temperature for age hardening an alloy is specified by its manufacturer). Ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment in order to relieve all strain hardening (stresses which occurs during finishing). Starting the hardening treatment when the alloy is in a disordered solid solution allows better control of the aging process.

LOW GOLD ALLOYS Also known as ‘economy golds’. They are crown and FDP alloys having gold content below 60% (generally in the 42–55% range). However, gold must be the major element.

432 PART 6 Alloys in Dentistry BOX 25.1 Technic alloy This is a gold colored base metal alloy which was frequently misused in India to make all-metal crowns and FDPs since many years. They are also sometimes referred to as Japanese gold or K-metal. These alloys do not contain any gold or precious metal. The alloy is absolutely contraindicated for any intraoral dental use because of its low strength, low wear resistance and tendency to tarnish. It has a high initial gold-like luster and patients are deliberately misled by unscrupulous practitioners into believing it was gold. Thanks to the availability of better materials its use has declined considerably. Unfortunately, one does come across restorations made from this alloy even to this day. Some practitioners still offer this material as a lower cost alternative, in addition to the regular alloys.

The low gold alloys were developed because of the increase in gold prices. However, reducing gold content increased tarnish and corrosion. This problem was overcome by two discoveries.

Palladium made the silver in gold alloy more tarnish resistant. 1% palladium was required for every 3% of silver. The silver-copper ratio had to be carefully balanced to yield a low silver rich phase in the microstructure.

ADVANTAGES Because of this research numerous low gold alloys were introduced into the market. Thus, these alloys were not only less expensive but also had good tarnish and corrosion resistance. Their properties are comparable to Types III and IV gold alloys.

SILVER-PALLADIUM ALLOYS These alloys were introduced as a cheaper alternative to gold alloys. It is predominantly silver in composition. Palladium (at least 25%) is added to provide nobility and resistance to tarnish. They may or may not contain copper and gold. They are white in color. Type

Component

Proportion

Ag-Pd (non-copper)

0–72%

25%

60%

25%

15%

Ag-Pd-Cu

Properties are like Type III gold alloys Properties are like Type IV gold alloys

The properties of the silver-palladium alloys are similar to the Types III and IV gold alloys. However, they have lower ductility and corrosion resistance. They also have a significantly lower density than gold alloy. This may affect its castability. A major difference between Types III and IV Ag-Pd alloys is that the latter can be significantly age hardened because of its gold and copper content.

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NICKEL-CHROME AND COBALT-CHROMIUM ALLOYS These are known as base metal alloys and are extensively used in many of the developing countries. In India, because of their relatively low cost many of the laboratories use these alloys along with resin facings. These metals are very strong and hard. Because of this, they are generally difficult to work with (cutting, grinding, polishing, etc.). They are dealt in more detail in subsequent sections.

TITANIUM AND TITANIUM ALLOYS These metals can be used for all-metal and metal-ceramic restorations, as well as partial dentures. They are described later under metal-ceramic restorations.

ALUMINUM-BRONZE ALLOY Bronze is an alloy known to man since ancient times. Traditional bronze is copper alloyed with tin. The ADA approved bronze (Fig. 25.3) does not contain tin. The composition is as follows: Component

Proportion

Copper

81–88%

Aluminum

8–10%

Nickel

2–4%

Iron

1–4%

Being relatively new, the information on these alloys is relatively scanty.

PROPERTIES* Color

Yellow gold

Melting range

1012–1068 °C

Density

7.8 g/cm3

Brinell harness number

104

Yield strength

30,000 psi

Elongation 29%

FIGURE 25.3 Aluminum bronze alloys (Courtesy: BDCH, Davangere).

METAL-CERAMIC ALLOYS Metal-ceramic alloys are those alloys that are compatible with porcelain and capable of bonding to it. A layer of porcelain is fused to the alloy to give it a natural tooth-like appearance. Porcelain being a brittle material fractures easily, so these alloys are used to reinforce the porcelain. Several types of alloys are used to cast substructures for porcelain-fused-to-metal crowns and FDPs. They may be noble metal alloys or base metal alloys (see classification). All have coefficient of thermal expansion (CTE) values which match that of porcelain. Note CTE has a reciprocal relationship with melting point, i.e. the higher the melting point of a metal, lower is its CTE. * Properties (as provided by the manufacturer)

434 PART 6 Alloys in Dentistry Synonyms Porcelain-fused-to-metal (PFM), ceramometal alloys, porcelain-bonded-to-metal (PBM). The preferred term, however, is metal ceramic or PFM.

EVOLUTION OF METAL-CERAMIC ALLOYS The metal-ceramic alloys evolved from resin-veneered crown and bridge alloys. Resin facing faced the problem of gradual wear and had to be replaced over time. Besides resin could not be used on the occlusal surface. To retain a resin veneered restoration undercuts had to be provided. The early metal-ceramic alloys were high gold alloys (88% gold). They were not strong enough for FDP use. In the early days before porcelain-metal bonding was clearly understood, porcelain had to be retained by mechanical means with the help of undercuts. Later, it was discovered that adding 1% of base metals like iron, tin, indium, etc. induced chemical bonding by the formation of an oxide layer. This significantly improved the bond strength between porcelain and metal.

REQUIREMENTS OF ALLOYS FOR PORCELAIN BONDING In addition to the general requirements of alloys mentioned earlier, metal-ceramic alloys have certain specific requirements in order to be compatible with porcelain veneering. 1. 2. 3. 4. 5.

Its melting temperature should be higher than porcelain firing temperatures. It should be able to resist creep or sag at these temperatures. Its CTE should be compatible with that of porcelain. They should be able to bond with porcelain. It should have a high stiffness (modulus of elasticity). Any flexing of the metal framework may cause porcelain to fracture or delaminate. 6. It should not stain or discolor porcelain.

USES OF METAL-CERAMIC ALLOYS 1. As the name implies these alloys are intended for porcelain veneered restorations (crowns and FDPs—Fig. 25.4). 2. They can also be used for all-metal restorations.

TYPES (CLASSIFICATION) OF METAL-CERAMIC ALLOYS Alloys for metal ceramics restorations may be categorized as 1. High noble (commonly referred to as gold alloys) (Fig. 25.5) –– Gold-palladium-platinum alloys –– Gold-palladium-silver alloys –– Gold-palladium alloys 2. Noble (commonly referred to as palladium alloys) –– Palladium-silver alloys –– Palladium-gallium-silver alloys –– Palladium-gold alloys –– Palladium-gold-silver alloys –– Palladium-copper alloys –– Palladium-cobalt alloys

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FIGURE 25.4 Metal-ceramic fixed partial denture.

435

FIGURE 25.5 Gold-based metal-ceramic alloys (1 g). Most are white gold alloys. V supra gold (Bottom row middle) has a light gold color. SF denotes silver free.

3. Base metal –– Nickel-chromium alloys –– Nickel-chromium-beryllium alloys –– Cobalt-chromium alloys –– Pure titanium –– Titanium-aluminum-vanadium

THE HIGH NOBLE (GOLD-BASED) METAL-CERAMIC ALLOYS The high noble alloys contain more than 40 wt% gold and are therefore also referred to as gold alloys or gold based alloys (Fig. 25.5).

COMMON FEATURES OF HIGH NOBLE (GOLD BASED) ALLOYS Cost These are the most expensive crown and bridge alloys. However, in spite of the cost, these alloys are user friendly and are preferred in practices where the clientele can afford the cost. Color The color can range from white to gold depending on the gold content. The whitening alloys are palladium and platinum. The gold color when present can enhance the vitality of the porcelain thus improving the esthetics. Melting range Porcelain is fired at a temperature of 900–960 °C. Thus obviously these alloys must have melting temperatures much higher than the temperatures at which porcelain is fired. Pure gold has a melting temperature of 1063 °C. The melting temperature is raised by the addition of platinum (1769 °C) and palladium (1552 °C). The melting temperatures of these alloys range from 1149–1304 °C. Density Ranges from 13.5 to 18.3 g/cm³ (depending on the gold content). Because of the high gold and noble metal content, these alloys have a high density. The density reduces as more palladium is added. Castability The high density of these alloys make them easy to cast. If done well one can expect most of the fine features to be accurately duplicated. Yield strength Ranges from 450 to 572 MPa. Hardness and workability Ranges from 182 to 220 VHN. These alloys are relatively softer when compared to the base metal alloys and so are extremely easy to work with. They are easy to cut, grind and polish.

436 PART 6 Alloys in Dentistry Percent elongation Ranges from 5 to 20%. This gives an indication of the ductility of the alloy. The higher the value, the more ductile it is. Porcelain bonding The presence of an oxide layer on the surface of metal ceramic alloys assists in chemical bonding of porcelain to the alloy. Pure noble metal alloys rarely form an oxide layer. To induce the formation of an oxide layer, 1% of base metals like tin, indium, iron, etc. are added to the alloy. This significantly improved the bond strength between the porcelain and the metal. Sag resistance During porcelain firing, the metal frame has to withstand temperatures as high as 950 °C. At these temperatures, there is a danger of the metal substructure sagging under its own weight, thereby deforming. The longer the span, the greater the risk. The ability of a metal to resist sag is known as sag resistance. Compared to base metal alloys, gold alloys are less sag resistant. Tarnish and corrosion Because of their high noble metal content, these alloys are extremely stable in the oral environment. Noble metals have low reactivity to oxygen and therefore do not tarnish easily. Biocompatibility High noble alloys have had a good and safe track record. They are not known to cause any problems in the mouth. Reusability These alloys are stable and so scrap from these alloys can be recast at least two or three times. However, the more volatile base metals like zinc, indium, tin, etc., may be lost. To compensate for this, equal amounts of new alloys should be mixed. The scrap should be cleaned by sandblasting and ultrasonic cleaning before use. Alloys from different manufacturers should not be mixed as it may change its composition and properties. Scrap value The high noble alloys have good scrap value. Many suppliers and manufacturers accept used alloy scrap. Soldering Gold-based alloys are quite easy to solder.

TYPES The following three types will be briefly described: 1. Gold-palladium-platinum alloys 2. Gold-palladium-silver alloys 3. Gold-palladium alloys Commercial names Some of the available alloys are presented in Table 25.1.

GOLD-PALLADIUM-PLATINUM ALLOYS COMPOSITION Component

Proportion

Gold

80–88 wt%

Palladium

5–11 wt%

Platinum

6–8 wt%

Silver

0–4.9 wt% (rarely present)

Base metals

Balance (around 1%)

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TABLE 25.1 Commercial names of some noble and high noble alloys High noble alloys (Au > 40%)

Noble alloys (Au < 40%)

Gold-palladium-platinum

Jelenko‘O’ (Jelenko) SMG-3

Gold-platinum

Willbond Bio 88 PF (Willkinson)*

Gold-platinum-palladium

Degudent H (Degussa)

Gold-palladium-silver

Wilbond 75 (Willkinson)** Cameo (Jelenko) RxWCG (Jeneric/Pentron) Special white (Degussa)

Gold-palladium

Olympia (Jelenko) Orion (Ney) Deva 4 (Degussa) Willbond 65SF(Willkinson)***

Palladium-gold

Nobilium 30 NS

Palladium-gold-gallium

Olympia II (Jelenko)

Palladium-gold-silver

Rx SWCG (Jeneric) Regent (Sterngold) Shasta (Willkinson)

Palladium-silver-gallium-gold

Wilpal 76 (Willkinson) Integrity (Jensen) Protocol (Williams)

Palladium-silver

Jelstar (Jelenko) Pors On (Degussa) Will-Ceram W-1 (Williams)

Palladium-copper-gallium-gold

Spirit (Jensen) Wilpal 76SF (Willkinson)

Palladium-gallium-cobalt

PTM-88 (Jelenko)

Palladium-cobalt-gallium

APF (Jeneric)

Palladium-cobalt

Bond-on (Aderer)

* Rich yellow colored ** Yellow colored *** SF denotes silver free

Sag resistance These alloys have a slightly lower sag resistance. Therefore, long span FDPs should be avoided with this alloy.

GOLD-PALLADIUM-SILVER ALLOYS COMPOSITION Component

Proportion

Gold

39–77 wt%

Palladium

10–40 wt%

Silver

9–22 wt%

Base metals

Balance (around 1%)

The silver has a tendency to discolor some porcelains.

438 PART 6 Alloys in Dentistry GOLD-PALLADIUM ALLOYS COMPOSITION Component

Proportion

Gold

44–55 wt%

Palladium

35–45 wt%

Base metals

Balance (around 1%)

The absence of silver eliminates the discoloration problem.

NOBLE (PALLADIUM-BASED) METAL-CERAMIC ALLOYS By definition, these alloys must contain at least 25% of noble metal alloy. Currently, the noble metal-ceramic alloys are mostly palladium-based. The high cost of gold prompted the development of the cheaper base metal alloys. Unfortunately, many soon became disillusioned because of the difficulty to work with these alloys (poor castability and high hardness). The palladium based alloys were developed during this period. Their properties were between that of the high noble alloys and the base metal alloys. They also had good scrap value.

COMMON FEATURES OF PALLADIUM-BASED (NOBLE) ALLOYS Cost Their cost range between that of the gold alloys and the base metal alloys. Color They are white in color. Density They are less denser than the gold alloys (10.5–11.5 g/cm³). Castability These alloys have a lower density than the gold alloys and so do not cast as well. However, they are better than the base metal alloys in this regard. Workability Like the gold alloys these alloys are extremely easy to work with. They are easy to cut, grind and polish. Melting range A typical melting range is 1155–1304 °C. The melting range of these alloys like the gold ceramic alloys are high. This is desirable to ensure that these alloys do not melt or sag during porcelain firing. Yield strength Ranges from 462 to 685 MPa. These compare favorably with the high noble ceramic alloys which in turn compare favorably to the Type IV alloys. Hardness Ranges from 189 to 270 VHN. They tend to be slightly harder than the high noble metal-ceramic alloys. Percent elongation Ranges from 10 to 34%. This gives an indication of the ductility of the alloy. The higher the value the more ductile it is. Porcelain bonding Like the gold alloys, base metals like tin, indium, etc. are added to enhance porcelain bonding. Tarnish and corrosion Because of their high noble metal content, these alloys are extremely stable in the oral environment. Scrap value The palladium based alloys have good scrap value. Many suppliers and manufacturers accept used alloy scrap. Biological considerations These alloys are very safe and biocompatible. Some concerns have been expressed over the copper content.

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TYPES The following are the palladium-based alloys. Palladium-silver alloys Palladium-copper alloys Palladium-cobalt alloys Palladium-gallium-silver alloys Palladium-gold alloys Palladium-gold-silver alloys Brand names The representative alloys are presented in Table 25.1.

PALLADIUM-SILVER ALLOYS These alloys were introduced in the 1970s as an alternative to gold and base metal alloys. Their popularity has declined a little because of the greening problem.

COMPOSITION Component

Proportion

Palladium

53–60 wt%

Silver

28–40 wt%

Base metals

Balance (1–8%)

Esthetics (greening) The high silver content causes the most severe greening (greenishyellow discoloration) problem among all the metal-ceramic alloys. This must be kept in mind when using it for anterior teeth. Some manufacturers have provided special agents to minimize this effect (gold metal conditioners and coating agents). Another alternative is to use special non-greening porcelain.

PALLADIUM-COPPER ALLOYS These are relatively new alloys. Little information is available regarding their properties.

COMPOSITION Component

Proportion

Palladium

74–80 wt%

Copper

5–10 wt%

Gallium

4–9 wt%

Gold

1–2 wt% (in some brands)

Base metals

around 1 wt%

Esthetics Copper can cause a slight discoloration of the porcelain, but is not a major problem. During the oxidation firing the metal acquires a dark brown almost black oxide layer. Care should be taken to mask this completely with opaquer. Also of concern is the dark line which develops at the margins. Castability These alloys are technique sensitive. Slight errors can lead to faulty castings.

440 PART 6 Alloys in Dentistry PALLADIUM-COBALT ALLOYS COMPOSITION Component

Proportion

Palladium

78–88 wt%

Cobalt

4–10 wt%

Gallium

up to 9 wt% (in some brands)

Base metals

around 1 wt%

Esthetics Cobalt can cause some insignificant discoloration. However, more care should be taken for masking the dark oxide layer with opaquer. Sag resistance They are the most sag resistant of all the noble alloys.

PALLADIUM-GALLIUM ALLOYS There are two groups of these alloys, viz. the palladium-gallium-silver and the palladiumgallium-silver-gold.

COMPOSITION Component

Proportion

Palladium

75 wt%

Gallium

6 wt%

Silver

5–8 wt%

Gold

6 wt% (when present)

Base metals

around 1 wt%

Esthetics The oxide layer though dark is still somewhat lighter than the palladium copper and palladium cobalt alloys. The silver content does not cause any greening.

BASE METAL ALLOYS FOR METAL-CERAMIC RESTORATIONS Alloys which contain little or no noble metals are known as base metal alloys. As mentioned earlier, these alloys were introduced as a cheaper alternative to the more expensive noble metalceramic alloys. In countries like the USA, Western Europe and the oil rich Middle-Eastern states, there is a preference for noble and high noble metal-ceramic alloys. In contrast, developing countries have shown a preference for base metal-ceramic alloys. This is because the economic concerns far outweigh the advantages of the more user-friendly high noble alloys. The first base metal alloys were the cobalt-chromium alloys primarily used for removable partial denture alloys. The nickel-chrome alloys were introduced later. The latest in the series are titanium and its alloys. Like the gold alloys, the base metal alloys can be used for many purposes. However, one must differentiate between the ones used for all-metal and the metal-ceramic restorations. Obviously, the metal-ceramic alloys would be formulated with specific properties since they are to be used with ceramics.

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Base metal alloys used for metal-ceramics include Nickel-chromium (nickel based) alloys Cobalt-chromium (cobalt based) alloys Pure titanium Titanium-aluminum-vanadium alloys Commercial names Trade names of some metal-ceramic alloys are presented in Table 25.2.

NICKEL-CHROMIUM ALLOYS Although cobalt chromium alloys are used for metal-ceramic crowns and FDPs, many laboratories prefer to use nickel-chromium alloys. For this reason, the discussion will focus mostly on these alloys. Cobalt-chromium will be discussed later under alloys for removable dentures. Representative commercial alloys are shown in Figure 25.6.

COMPOSITION Basic elements Component

Proportion

Nickel

61–81 wt%

Chromium

11–27 wt%

Molybdenum

2–9 wt%

(Some alloys occasionally contain one or more minor elements). TABLE 25.2 Commercial names of some base metal-ceramic alloys Nickel-based alloys

Cobalt-based alloys

Ni-Cr-Mo

Wiron 99 (Bego) Wirocer (Bego)

Ni-Cr-Mo-Be

Litecast B (Williams) Rexillium III (Pentron)

Co-Cr-Mo

Wirobond C (Bego)

Co-Cr

Remanium LFC

FIGURE 25.6 Representative nickel chromium alloys for metal-ceramic restorations (Courtesy: CODS, Davangere).

442 PART 6 Alloys in Dentistry The minor additions include Component

Proportion

Beryllium

0.5–2.0 wt%

Aluminum

0.2–4.2 wt%

Iron

0.1–0.5 wt%

Silicon

0.2–2.8 wt%

Copper

0.1–1.6 wt%

Manganese

0.1–3.0 wt%

Cobalt

0.4–0.5 wt%

Tin

1.25 wt%

(Function of the ingredients are described under removable partial denture alloys).

GENERAL PROPERTIES OF NICKEL-BASED ALLOYS Cost They are the cheapest of the casting alloys. Color They are white in color. Melting range A typical melting range is 1155–1304 °C. The melting range of these alloys like the gold ceramic alloys are high. Density Ranges from 7.8 to 8.4 g/cm³. They have just half the density of the gold alloys making them much lighter. One can get more castings per gram compared to the gold alloys. Castability They are extremely technique sensitive. One reason may be their lower density compared to the gold alloys. Hardness and workability Ranges from 175 to 360 VHN. They tend to be much harder than the high noble metal ceramic alloys. Unlike the gold alloys these alloys are extremely difficult to work with in the laboratory. Their high hardness makes them very difficult to cut (sprue cutting), grind and polish. In the mouth, more chair time may be needed to adjust the occlusion. Cutting and removing a defective crown or FDP can be quite demanding. The high hardness results in rapid wear of carbide and diamond burs. Yield strength Ranges from 310 to 828 MPa. These alloys are stronger than the gold and palladium based alloys. Modulus of elasticity Ranges from 150 to 218 GPa. This property denotes the stiffness of the alloy. Base metal alloys are twice as stiff as the gold ceramic alloys. Practically, this means that we can make thinner, lighter castings or use it in long span FDPs where other metals are likely to fail because of flexing. Gold alloys require a minimum thickness of at least 0.3–0.5 mm, whereas base metal alloys copings can be reduced to 0.3 mm (some even claim 0.1 mm). Percent elongation Ranges from 10 to 28%. This gives an indication of the ductility of the alloy. Though they may appear to be ductile, these alloys, however, are not easily burnishable. This may be related to additional factors like the high hardness and yield strength. Porcelain bonding These alloys form an adequate oxide layer which is essential for successful porcelain bonding. However, occasionally the porcelain may delaminate from the underlying metal. This has been blamed on a poorly adherent oxide layer which occurs under certain circ*mstances which have not been fully understood. Sag resistance These materials are far more stable at porcelain firing temperatures than the gold based alloys. They have a higher sag resistance. Esthetics A dark oxide layer may be seen at the porcelain metal junction.

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Scrap value As may be expected these alloys have poor scrap value because of the low intrinsic value of the elements. Tarnish and corrosion resistance These alloys are highly resistant to tarnish and corrosion. This is due to the property known as passivation. Passivation is the property by which a resistant oxide layer forms on the surface of chrome containing alloys. This oxide layer protects the alloy from further oxidation and corrosion. These alloys can maintain their polish for years. Other self passivating alloys are titanium and aluminum. Soldering Soldering is necessary to join bridge parts. Long span bridges are often cast in two parts to improve the fit and accuracy. The parts are assembled correctly in the mouth and an index made. The parts are then reassembled in the laboratory and joined together using solder. Base metal alloys are much more difficult to solder than gold alloys. Casting shrinkage These alloys have a higher casting shrinkage than the gold alloys. Greater mould expansion is needed to compensate for this. Inadequate compensation for casting shrinkage can lead to a poorly fitting casting. Etching Etching is necessary for resin-bonded restorations (e.g. Maryland bridges) to improve the retention of the cement to the restoration. Etching of base metal alloys is done in a electrolytic etching bath. Biological considerations Nickel may produce allergic reactions in some individuals. It is also a potential carcinogen. Beryllium which is present in many base metal alloys is a potentially toxic substance. Inhalation of beryllium containing dust or fumes is the main route of exposure. It causes a condition known as ‘berylliosis’. It is characterized by flu-like symptoms and granulomas of the lungs. Precautions Adequate precautions must be taken while working with base metal alloys. Fumes from melting and dust from grinding alloys should be avoided (wear mask). The work area should be well-ventilated. Good exhaust systems should be installed to remove the fumes during melting.

CASTING INVESTMENTS FOR METAL CERAMIC ALLOYS Due to the high melting temperature of these alloys, only phosphate-bonded or silica-bonded investments are used. However, in case of gold-based metal-ceramic alloys, carbon containing phosphate-bonded investments are preferred.

TITANIUM AND ITS ALLOYS FOR METAL-CERAMIC APPLICATIONS Titanium in the form of the oxide rutile, is abundant in the earth’s crust. The ore can be refined to metallic titanium using a method called the Kroll’s process. Titanium and its alloys have been available to the dental profession since the 1970s. Historically, titanium has been used extensively in aerospace, aeronautical and marine applications, because of its high strength and rigidity, its low density and corresponding low weight, its ability to withstand high temperatures and its resistance to corrosion. The use of titanium for medical and dental applications has increased dramatically in recent years. Over the past three decades, the development of new processing methods-like computer-aided machining and electric discharge machining, has expanded titanium’s useful range of applications in biomedical devices. Titanium has become available for use in metal-ceramics. It is also used for removable partial denture alloy frames and of course commercial implants. It has been adopted in dentistry, because of its excellent biocompatibility, light weight, good strength and ability to passivate.

444 PART 6 Alloys in Dentistry USES In dentistry 1. 2. 3. 4. 5. 6.

Metal-ceramic restorations. Dental implants. Partial denture frames (Fig. 25.7). Complete denture bases. Bar connectors. Titanium mesh membranes (Tiomesh) are used in bone augmentation. (In dentistry, it is especially useful as an alternative alloy to those who are allergic to nickel).

In surgery 1. 2. 3. 4. 5.

Artificial hip joints. Bone splints. Artificial heart pumps. Artificial heart valves parts. Pacemaker cases.

FIGURE 25.7 A titanium removable partial denture frame.

ASTM GRADES OF TITANIUM ASTM International (the American Society for Testing and Materials) recognizes four grades of commercially pure titanium (CpTi) and three titanium alloys (Ti-6Al-4V, Ti-6Al-4V extra-low interstitial [low components] and Ti-Al-Nb).

SUPPLIED AS Ingots weighing 18–40 g (height of 11.8–16.8 mm) in 1 kg boxes (Fig. 25.8). Representative products Rematitan M (Dentaurum) — Grade 4

Tritan (Dentaurum)

— Grade 1

PROPERTIES OF COMMERCIALLY PURE TITANIUM Phases In its metallic form at ambient temperature, titanium has a hexagonal, close-packed crystal lattice (α phase), which transforms into a body-centered cubic form (β-phase) at 883 °C. The phase is susceptible to oxidation. Color It is a white color metal.

FIGURE 25.8 Grade 4 titanium (Rematitan by Dentaurum).

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Density It is a light weight metal (density 4.5 g/cm3) when compared to nickel chrome (8 g/cm3) and gold alloys (15 g/cm3). Modulus of elasticity Its modulus of elasticity is 110 Gpa which makes it only half as rigid as base metal alloys. However, this appears to be sufficient for most dental uses. Melting point Its melting point is quite high (1668 °C). Special equipment is needed for casting titanium. Yield strength Varies from 460 to 600 MPa. Tensile strength Varies from 560 to 680 MPa. Coefficient of thermal expansion This is an important property when it is used as a metal ceramic alloy. When used as a metal ceramic alloy the CTE (8.4 × 10–6/ °C) is far too low to be compatible with porcelain (12.7 to 14.2 × 10–6/ °C). For this reason special low fusing porcelains have been developed to get around this problem. Tarnish and corrosion Titanium has the ability to self-passivate. The metal oxidizes almost instantaneously in air to form a tenacious and stable oxide layer approximately 10 nanometers thick. The oxide layer protects the metal from further oxidation. In addition, the oxide layer allows for bonding of fused porcelains, adhesive polymers or, in the case of endosseous implants, plasma-sprayed or surface-nucleated apatite coatings. Biocompatibility It is nontoxic and has excellent biocompatibility with both hard and soft tissues.

FABRICATION OF TITANIUM RESTORATIONS Titanium structures can be made by 1. Casting or 2. Machining.

Casting Casting of titanium is a challenge because of its high melting temperature, low density and high reactivity to atmospheric air. Machines for casting titanium are generally more expensive than that for other dental casting alloys. Dental castings are made via pressure-vacuum or centrifugal casting methods. The metal is melted using an electric plasma arc or inductive heating in a melting chamber filled with inert gas or held in a vacuum. The inert gas prevents surface reaction with the molten metal. Investments with high setting expansion are used to compensate for the high casting shrinkage of titanium.

Machining Dental implants generally are machined from billet stock of pure metal or alloy. Dental crowns and FDP frameworks also can be machined from metal blanks (Fig. 25.9) via CAD/CAM. Abrasive machining of titanium, however, is slow and inefficient, which greatly limits this approach. Another method for fabricating dental appliances is electric discharge machining, which uses a graphite die (often reproduced from the working die) to erode the metal to shape via spark erosion.

FIGURE 25.9 Titanium blanks for CAD-CAM.

446 PART 6 Alloys in Dentistry CERAMIC VENEERING Special low fusing porcelains with fusing temperatures below 800 °C are used with titanium. This is because titanium changes to the β-form (at 883 °C) which is susceptible to oxidation.

ADVANTAGES AND DISADVANTAGES OF TITANIUM Advantages 1. 2. 3. 4. 5. 6.

High strength. Light weight. Binary. Low tarnish and corrosion because of ability to passivate. Can be laser welded. Limited thermal conductivity.

Disadvantages 1. 2. 3. 4.

Poor castability. Highly technique sensitive. Requires expensive machines for casting and machining. Low fusing porcelains (below 800 °C) required to prevent β phase transformation.

REMOVABLE DENTURE ALLOYS Larger structures like complete denture bases and partial denture frames are also made from dental alloys. Being larger structures they require more quantities of alloy, which can make them quite heavy and expensive (if gold were to be used). Thus it became necessary to develop lighter and more economical alloys. Most of the large castings today are made from base metal alloys, occasionally Type IV gold alloys are used.

ADDITIONAL REQUIREMENTS FOR PARTIAL DENTURE ALLOYS Besides all the earlier mentioned general requirements of casting alloys, RPD alloys have a few special requirements. 1. They should be light in weight. Being much larger structures, the lighter weight aids in retention in the mouth. 2. They should have high stiffness. This aids in making the casting more thinner. This is important especially in the palate region, where having a thin palatal portion makes it more comfortable to the patient. The high stiffness prevents the frame from bending under occlusal forces. 3. They should have good fatigue resistance. This property is important for clasps. Clasps have to flex when inserted or removed from the mouth. If they do not have good fatigue resistance they may break after repeated insertion and removal. 4. They should be economical. Large structures would require more metal and therefore the cost of the alloy should be low. 5. They should not react to commercial denture cleansers.

TYPES The alloys for removable denture use are 1. Cobalt-chromium alloys

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447

Nickel-chromium alloys Aluminum and its alloys Type IV noble alloys Titanium

COBALT-CHROMIUM ALLOYS Cobalt-chromium alloys have been available since the 1920s. They possess high strength. Their excellent corrosion resistance especially at high temperatures, makes them useful for a number of applications. These alloys are also known as ‘stellite’ because of their shiny, star-like appearance. They are bright lustrous, hard, strong and possess nontarnishing qualities.

SUPPLIED AS Small ingots (cuboidal, cylindrical shapes) in 1 kg boxes (Fig. 25.10). Representative products Wironium plus (Bego), Sheralit imperial (Shera).

APPLICATIONS 1. Denture base 2. Cast removable partial denture framework (Fig. 25.11) 3. Crowns and fixed partial dentures 4. Bar connectors.

FIGURE 25.10 Cobalt-chromium alloy.

COMPOSITION Component

Proportion

Cobalt

35–65%

Chromium

23–30%

Nickel

0–20%

Molybdenum

0–7%

Iron

0–5%

Carbon

up to 0.4%

Tungsten, manganese, silicon and platinum

traces

According to ADA Sp. No. 14 a minimum of 85% by weight of chromium, cobalt, and nickel is required.

FIGURE 25.11 Cast RPD frame can be made from cobalt-chromium.

448 PART 6 Alloys in Dentistry FUNCTIONS OF ALLOYING ELEMENTS Cobalt Imparts hardness, strength and rigidity to the alloy. It has a high melting point.

Chromium Its passivating effect ensures corrosion resistance. The chromium content is directly proportional to tarnish and corrosion resistance. It reduces the melting point. Along with other elements, it also acts in solid solution hardening. 30% chromium is the upper limit for attaining maximum mechanical properties.

Nickel Cobalt and nickel are interchangeable. It decreases strength, hardness, MOE and fusion temperature. It increases ductility.

Molybdenum or tungsten They are effective hardeners. Molybdenum is preferred as it reduces ductility to a lesser extent than tungsten. Molybdenum refines grain structure.

Iron, copper and beryllium They are hardeners. In addition, beryllium reduces fusion temperature and refines grain structure.

Manganese and silicon Primarily oxide scavengers to prevent oxidation of other elements during melting. They are also hardeners.

Boron Deoxidizer and hardener, but reduces ductility.

Carbon Carbon content is most critical. Small amounts may have a pronounced effect on strength, hardness and ductility. Carbon forms carbides with the metallic constituents which is an important factor in strengthening the alloy. However, excess carbon increases brittleness. Thus, control of carbon content in the alloy is important.

PROPERTIES The cobalt-chromium alloys have replaced Type IV gold alloys, especially for making RPDs, because of their lower cost and good mechanical properties.

Density The density is half that of gold alloys, they are lighter in weight (8 to 9 g/cm3).

Fusion temperature Thus casting temperature of this alloy is considerably higher than that of gold alloys (1250 °C to 1480 °C). ADA Sp. No. 14 divides it into two types, based on fusion temperature, which is defined as the liquidus temperature.

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Type-l (high fusing)—liquidus temperature greater than 1300 °C. Type-ll (low fusing)—liquidus temperature not greater than 1300 °C.

Yield strength It is higher than that of gold alloys (710 MPa).

Elongation Their ductility is lower than that of gold alloys. It depends on composition, rate of cooling and the fusion and mould temperature employed. The elongation value is 1–12%. Caution These alloys work harden very easily, so care must be taken while adjusting the clasp arms of the partial denture. They may break if bent too many times.

Modulus of elasticity They are twice as stiff as gold alloys (225 × 103 MPa). Thus, casting can be made thinner, thereby, decreasing the weight of the RPD.

Hardness These alloys are 50% harder than gold alloys (432 VHN). Thus, cutting, grinding and finishing are difficult. It wears off the cutting instrument. Special hard, high speed finishing tools are needed.

Tarnish and corrosion resistance (passivation) Formation of a layer of chromium oxide on the surface of these alloys prevents tarnish and corrosion in the oral cavity. This is called ‘passivating effect’. Caution Hypochlorite and other chlorine containing compounds that are present in some denture cleaning solutions will cause corrosion in base metal alloys. Even the oxygenating denture cleansers will stain such alloys. Therefore, these solutions should not be used to clean chromium based alloys.

Casting shrinkage The casting shrinkage is much greater (2.3%) than that of gold alloys. The high shrinkage is due to their high fusion temperature.

Porosity As in gold alloys, porosity is due to shrinkage of the alloy and release of dissolved gases. Porosity is affected by the composition of the alloys and its manipulation.

TECHNICAL CONSIDERATIONS FOR CASTING ALLOYS Based on the melting temperatures of the alloys, we can divide the alloys into high fusing and low fusing alloys.

LOW-FUSING ALLOYS The gold alloys used for all-metal restorations may be considered as low fusing. Obviously, the technical requirements of these alloys would be different from the high-fusing alloys. Investment material Gypsum bonded investments are usually sufficient for the low-fusing gold alloys. Melting The regular gas-air torch is usually sufficient to melt these alloys.

450 PART 6 Alloys in Dentistry HIGH-FUSING ALLOYS The high-fusing alloys include noble metal-ceramic alloys (gold and palladium alloys) as well as the base-metal alloys (all-metal, metal-ceramic alloys and partial denture alloys). Investment material for noble metal alloys The high melting temperatures prevent the use of gypsum-bonded investments. Phosphate bonded or silica bonded investments are used for these alloys. Investment material for base-metal alloys Phosphate-bonded or silica-bonded investments are also used for these alloys. However, there is one difference. These alloys are very sensitive to a change in their carbon content. Therefore, carbon containing investments should be avoided when casting base-metal alloys. Burnout A slow burnout is done at a temperature of 732–982 °C. It is done two hours after investing. Melting The high fusion temperature also prevents the use of gas-air torches for melting these alloys. Oxygen-acetylene torches are usually employed. Electrical sources of melting such as carbon arcs, argon arcs, high frequency induction, or silicon-carbide resistance furnaces may also be used.

TECHNIQUE FOR SMALL CASTINGS The wax pattern is usually constructed on a die stone model. The wax pattern is removed and then invested (for more details see chapter on casting techniques).

TECHNIQUE FOR LARGE CASTINGS The procedure for large castings like RPD frames is slightly more complex. Unlike the crown or FDP pattern, the RPD pattern is difficult to remove from the model without distortion and damage. Therefore, a modification in the technique is required. A duplicate of the model is made using investment material (this is called refractory cast). The wax pattern is constructed on the refractory cast (Fig. 25.12A). The pattern is not separated from the refractory cast, instead the refractory cast is invested along with the pattern (Fig. 25.12C).

FIGURES 25.12A TO C (A) Partial denture wax patterns are constructed directly on the refractory cast. (B) The whole cast together with the pattern is invested to form a mold. The completed casting is also shown (C).

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ADVANTAGES AND DISADVANTAGES OF BASE METAL ALLOYS ADVANTAGES OF BASE METAL ALLOYS 1. 2. 3. 4.

Lighter in weight. Better mechanical properties (exceptions are present). As corrosion resistant as gold alloys (due to passivating effect). Less expensive than gold alloys.

DISADVANTAGES 1. 2. 3. 4. 5.

More technique sensitive. Complexity in production of dental appliance. High fusing temperatures. Extremely hard, so requires special tools for finishing. The high hardness can cause excessive wear of restorations and natural teeth contacting the restorations.

COMPARISON OF A GOLD ALLOY AND A BASE METAL ALLOY A comparison of the 2 alloys are shown in Table 25.3. TABLE 25.3 Comparison of cobalt-chromium and type IV gold alloy Properties

Cobalt-chromium

Gold Type - IV

Strength

Adequate

Density (g/cm)

8 (lighter)

15 (heavier)

Hardness

Harder than enamel

Softer than enamel

Stiffness

Stiff

More flexible

Melting temperature

1300°C

900°C

Casting shrinkage

2.25%

1.25–1.65%

Heat treatment

Complicated

Simple

Tarnish resistance

Adequate

Cost

Economical

High for large castings

Workability

Difficult to cut, grind and polish

Cutting and polishing easy

Investment

Phosphate bonded (non-carbon)

Gypsum bonded

Heat source for melting

Oxyacetylene torches

Gas-air torch

Solderability

Difficult

Easier

26 Chapter

Dental Implant Materials Chapter Outline • Definition • History and Development of • • • • • • • • •

Implants Types of Implants Materials Used Titanium Surface Coated Titanium Ceramics Stainless Steel Polymers and Composites Other Materials Implant Parts –– Basic Implant Design –– Implant Fixture

• • • • • •

–– –– –– –– ––

Implant Abutments Implant Motor Implant Drills Cover Screw Healing Abutment Implant Abutment Connection Platform Switching Biointegration and Osseointegration Titanium Allergy Zirconia Implants Zirconia Anatomic Root-form Implants

• Implant Surfaces and Coatings –– Grit Blasting –– Acid Etching –– Anodization –– Shot/Laser Peening –– Hydroxyapatite Coated Implants

–– Plasma Sprayed HA –– Electrophoretic Deposition (EPD) of HA

–– Biomimetic Coating Technique

Implanting a foreign material directly into the bone in order to replace missing teeth has been a goal sought since ancient times. Though many materials have been tried, currently, the vast majority of implant systems use titanium in some form.

DEFINITION A dental implant is a material or device placed in and/or on oral tissues to support an oral prosthesis (GPT-8). History and Development of Implants Man has been searching for ways to replace missing teeth for thousands of years. Ancient Egyptians used tooth shaped shells and ivory to replace teeth. The Etruscans living in what is now modern Italy, replaced missing teeth with artificial teeth carved from the bones of oxen. Further evidence of tooth replacement was found in 1931 by an archeological team excavating in Honduras. A mandible of Mayan origin was discovered that had toothshaped pieces of shells placed in the sockets of three missing lower incisor teeth. Modern implant dentistry began in the early 19th century. A lot of experiments were conducted on what material would work best. Attempts were first made

Mayan mandible with shell implant

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at implanting natural teeth from another person’s mouth, but these implants failed due to infection or were rejected by the host tissue. Implants made of gold, porcelain, silver and even lead were being tried, with only a fair measure of success and little or no predictability. As early as 1918, Greenfield devised the Iridoplatinum root form basket implant. Other early implants were those of Chercheve, Formiggini and others. An interesting design was the Tripodal pin implant of Scialom. Interestingly some of these early designs were ahead of their times. Their failure to gain widespread popularity could probably be attributed to the fact that prosthetic techniques, antibiotic use, infection control, instrumentation and impression materials had not yet advanced far enough. One of the early pioneers in this field Dr. AE Strock in 1931, suggested using Vitallium, a Leonard Linkow metal alloy for dental implants. In 1947, Manlio Formiggini of Italy developed an implant made of tantalum. At the same time, Raphael Chercheve was using implants made of a chrome-cobalt alloy. By 1964, commercially pure titanium was accepted as the material of choice for dental implants. Ever since almost all dental implants are made of titanium. The body does not recognize titanium as a foreign material, resulting in less host rejection of the implant. Other areas of medicine recognize this fact and use titanium for other implants such as joint replacements and heart valves. In the 1950s an important discovery was made which had great implications for tooth replacement therapy. During an experiment involving the study of blood circulation in animals, Dr. Per-Ingvar Branemark discovered that the hollow titanium rod used in the study was not retrievable when the experiment was complete. Further studies showed that the animals’ bone had directly attached to the titanium surface. This phenomenon was called osseointegration, defined by the American Academy of Implant Dentistry as “the firm, direct and lasting biological attachment of a metallic implant to vital bone with no intervening connective tissue.” This firm anchor is what makes dental implants a wonderful option for replacing teeth. Experimentation with implant designs, not just those that were shaped like the tooth root, was also being done. In 1941, Dr. Gustav Dahl of Sweden provided a retentive mechanism for jaws that were completely edentulous. This was the introduction of the subperiosteal implant. Dr. Leonard Linkow of New York introduced the blade form implant (Fig. 26.1) in 1967. These blades came in a variety of sizes and forms and were the most widely used type of implant until the 1980s. Implants are no longer restricted to the mouth. They have been successfully used all over the body for various roles. Whether implants are here to stay is no longer a question, but research into perfecting materials, procedures and training will continue in this exciting Per-Ingvar Branemark field of dentistry.

TYPES OF IMPLANTS A. Subperiosteal—a framework that rests upon the bony ridge but does not penetrate it (Fig. 26.1). B. Transosteal—penetrates completely through the mandible (Fig. 26.2). C. Endosseous—partially submerged and anchored within the bone (Figs. 26.3 and 26.4).

Figure 26.1 Subperiosteal implant radiograph (left). Intraoral view (right).

454 Part 6 Alloys in Dentistry

Figure 26.2 Transosteal implant.

Figure 26.3 Linkow’s blade vent endosseous implants were widely used prior to the era of cylindrical implants.

Figure 26.4 Radiograph showing an endosteal implant.

MATERIALS USED 1. Metals — Stainless steel — Cobalt-chromium-molybdenum based — Titanium and its alloys — Surface coated titanium 2. Ceramics — Hydroxyapatite — Bioglass — Aluminum oxide 3. Polymers and composites 4. Others — Gold, tantalum, carbon, etc.

TITANIUM Commercially pure titanium (cp Ti) is currently the most widely used material for implants (Fig. 26.5). It has become the material of choice because of its Low density (4.5 gm/cm2) but high strength. Minimal biocorrosion due to its passivating effect. Excellent biocompatibility. Titanium also has good stiffness. Although its stiffness is only half that of steel, it is still 5 to 10 times higher than that of bone.

Titanium alloys Alloyed forms of titanium are also used. Its alloyed form contains 6 wt.% aluminum and 4 wt.% vanadium.

Figure 26.5 Four titanium screw implants in the maxillary edentulous jaw are used to support a screw retained fixed prosthesis.

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Surface coated titanium The newer implant designs use titanium that is coated with a material that bonds and promotes bone growth (bioactive). The implant is coated with a thin layer of tricalcium phosphate or hydroxyapatite that has been plasma sprayed.

CERAMICS These may be bioactive on bioinert. Their applications are still limited because of their low tensile strength and ductility. Currently they are primarily used as surface coats on titanium implants. Bioactive, e.g., Hydroxyapatite Bioglass (CaO, NaO, P2O5 and SiO2) Bioinert, e.g., aluminum oxide is used either in the polycrystalline form or as a single crystal (sapphire). It is well-tolerated by bone but does not promote bone formation. They are available in screw or blade form and are used as abutments in partially edentulous mouths.

STAINLESS STEEL 18-8 or Austenitic steel had been tried as an implant material. It has high strength and ductility. Currently these materials are rarely used.

Precautions Since it contains nickel, it should be avoided in nickel sensitive patients. It is most susceptible to pit and crevice corrosion so the passivating layer must be preserved. Direct contact of the implant with a dissimilar metal crown is avoided to prevent galvanism.

POLYMERS AND COMPOSITES Polymers have been fabricated in porous and solid forms for tissue attachment and replacement augmentation. However, in some implants they are mainly used within the implants as connectors for stress distribution (shock absorption).

OTHER MATERIALS In the past, gold, palladium, tantalum, platinum and alloys of these metals have been used. More recently, zirconium and tungsten have been tried. Titanium has replaced most of these materials. Carbon compounds were used for root replacement in the 1970s. They are also marketed as coatings for metallic and ceramic devices.

Implant parts Basic implant design Since endosseous cylindrical root form implants are the most widely used design, subsequent discussions will focus on these. Implants can range from complex, having multiple components to more simple designs. Most endosseous implants can be divided into two basic parts (Fig. 26.6).

Fixture—embedded in bone Abutment—supports the crown

456 Part 6 Alloys in Dentistry The implant may be

One piece–Implant and abutment are joined together (Fig. 26.7A) Two-piece–Implant and abutment are separate. The abutment is secured to the implant by means of an abutment screw (Fig. 26.7B)

Implant fixture Over the years various implant designs have been developed and used. Currently, the most favored form is the cylindrical screw or the Figure 26.6 Components of endosseous implants. tapered screw (Figs. 26.8A to D). The implant is inserted through a surgical procedure. The abutment is usually screwed onto the implant at a later date. The crown is then constructed and either screwed on or cemented onto the abutment thus completing the restoration. Implants are usually designed as a system and depending on the company various accessory components are also available. The components are usually specific for the particular system and are usually not interchangeable. Some of them become part of the implant itself while others aid in the various stages of implant placement and tooth restoration. These include the drills, healing caps, impression copings, implant analogue, laboratory accessories, etc.

Implant abutments Definition Implant abutment is that portion of a dental implant that serves to support and/or retain any fixed or removable dental prosthesis (GPT-8).

Classification of abutments A. According to fabrication procedure 1. Stock abutment 2. Custom B. According to material used 1. Titanium 2. Zirconia

Figures 26.7A and B (A) One piece implant-abutment. (B) Two-piece implant-abutment.

3. CAD/CAM 3. Gold

4. Steel

Figures 26.8A to D Endosseous root form implants (A) Titanium screw. (B) Hydroxyapatite screw. (C) Hydroxyapatite. (D) Titanium plasma sprayed.

Dental Implant Materials Chapter 26 C. According to angulation 1. Angled

457

2. Nonangled

Stock abutments Also known as the preparable, easy, or direct abutments. They are factory produced and provide the most accurate fit to the implant. They can be modified at the chairside or in the laboratory. They come in varying sizes and emergence profiles. Some come with scalloped margins for improved marginal esthetics. Stock abutments are of two varieties Hollow abutments These are abutments with a channel through which the connecting screw attaches it to the implant fixture. Solid abutment The abutment comes with its own built-in screw. The abutment itself is screwed onto the implant. Custom Cast custom abutments have long been a workhorse in implant dentistry. They were first popularized as the “UCLA” abutment design and provided a means for waxing custom emergence profiles of the subgingival portion of the abutment, flexibility in margin level placement, and for correction of angulation problems. CAD/CAM State of the art software and milling machines utilize the scan data from the patient’s dental casts and the computer generated abutment design to enable production of an abutment specific for the patient. Angled abutments Angled abutments are used to correct the implant-crown alignment, e.g., upper central incisors often need angled abutments. Zirconia abutments Zirconia is a strong tooth colored ceramic material. These are indicated for patients demanding an increased level of esthetics. One-piece implant-abutment Some manufacturers do not provide separate abutments, rather the abutment is combined with the implant and is, therefore, inserted along with the implant.

Equipment and parts associated with implant surgery Implant motor These are high torque motors with foot control and saline stand (Fig. 26.9). The handpiece is contraangled and has a nozzle for saline irrigation. Torque ranges from 50–70 Ncm. Speed ranges from 300 to 40,000 rpm. Having low speed but high torque is important for implant drilling.

Implant drills The pilot drill is the first drill used. It helps to establish direction of the subsequent drilling and implant placement. Subsequent drills are introduced in series of increasing size (Fig. 26.10). Drills are usually made of stainless steel.

Figure 26.9 Implant motor with saline irrigation.

Cover screw Implant surgery is usually done in two stages. In the first stage the implant is inserted in the bone and left to integrate for a period of 2 months. After the

Figure 26.10 Implant drills.

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Figures 26.12A toC (A) Regular healing abutments. (B) Anatomic healing abutment. (C) Figure 26.11 Cover screw. Healing abutment in position.

implant is placed, a cover screw (Fig. 26.11) is used to cover the coronal end of the abutment. The cover screw prevents bony ingrowth and keeps the abutment chamber patent while the implant is integrating.

Healing abutment The healing abutment is usually placed 2–3 months later at the second stage. The cover screw of the buried implant is exposed. The cover screw is removed and a healing abutment placed instead. The healing abutment exposes the implant to the oral cavity (Fig. 26.12C) and prepares the space for the future abutment. The healing abutment may be cylindrical or anatomic in design (Figs. 26.12A and B). Function of healing abutment 1. 2. 3. 4.

Allows the punched out tissue to heal. Promotes the development of sulcus epithelium. Helps to develop papilla and marginal gingiva, thus improving the soft tissue esthetics. Helps to develop a proper emergence profile from the implant platform to the crown, thus improving the restoration esthetics (transition from the narrower implant to the broader artificial tooth).

Implant abutment connection The implant abutment connection is an important part of the implant design. It is one of the factors which help in proper functioning, stability and longevity of the implant. The abutment is secured to the implant with the help of a screw. During occlusal function considerable stress may be transferred to the screw and can lead to its early loosening or even fracture. Manufacturers have, therefore, incorporated certain design features to reduce the stress on the abutment screw.

Function of the implant abutment connection The implant/abutment interface determines joint strength, stability, and lateral and rotational stability. It also reduces stress on the abutment screw.

Types of implant abutment connections There are two ways the implant and abutment connect or interface. 1. External 2. Internal

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Figure 26.13 Various types of implant abutment connections.

External hex The original Brånemark protocol required several externally hexed implants to restore fully edentulous arches (Fig. 26.13).

Internal hex One of the first internally connected implants was designed with a 1.7-mm-deep hex below a 0.5 mm wide, 45° bevel. Its features were intended to distribute intraoral forces deeper within the implant to protect the retention screw from excess loading, and to reduce the potential of microleakage. Internally connected implants also provide superior strength for the implant/ abutment connection. Since the introduction of the internal connection concept, further design enhancements have been made in an attempt to enhance the implant/abutment connection. Included in such efforts is the “Morse” taper, wherein a tapered abutment post is inserted into the nonthreaded shaft of a dental implant with the same taper. Other internal connection designs have followed, frequently with variations in the numbers of hexes, connection length, angulation, etc. Some of the internal implant abutment connections are (Fig. 26.13)

3 point internal tripod — Replace select (Noble Biocare) 5 point internal pentagon — Tiologic (Dentaurum) 6 point internal hex — Frialit 2 (Friadent) 8 point internal morse taper — (ITI) Straumann 12 point conical seal — Astra (Astra Tech)

Platform switching Platform switching is a method used to preserve alveolar bone levels around dental implants. The concept refers to placing an abutment of narrower diameter on implants of wider diameter (Figs. 26.14A and B), rather than placing abutments of similar diameters (referred to as platform matching - Fig. 26.15).

Biointegration and Osseointegration For an implant to function, it must integrate with the oral tissues. The term osseointegration was first described by Per Ingvar Branemark and refers to the fusion of the bone with the implant.

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Figures 26.14A and B Platform switching.

Figure 26.15 Platform matching.

Defined as An apparent direct connection of an implant surface and host bone without intervening connective tissue [GPT-8] (Fig. 26.16). Thus a direct structural and functional connection between the bone and implant allows the implant to transmit functional stresses directly to the bone. To achieve osseointegration, the bone must be viable, space between the implant and bone should be less than 10 nm and contain no fibrous tissue. Presence of fibrous tissue usually signifies failure.

Figure 26.16 Representation of osseointegration.

Factors favoring osseointegration 1. 2. 3. 4. 5. 6. 7. 8.

Proper treatment planning Atraumatic drilling of bone Selection of proper implant material Implant design Favorable occlusal forces Bone quality Good oral hygiene No contraindicating local or systemic factors

Other factors include the nature of the surface coating and surface configuration. Recently there has been interest in coating titanium with certain materials that actively promote a favorable bone response. These are referred to as ‘bioactive’. Examples of bioactive materials are hydroxyapatite, tricalcium phosphate and bioactive glasses. Commercially available bioactive glasses include Bioglass, Ceravital, Biogran and glass ceramic A-W. These materials are generally too weak and brittle to stand alone. However, when used as a coating (50 to 75 µm) on the surface of titanium, it combines the strength of titanium with its bioactive qualities. If successful, the ceramic coating becomes completely fused with the surrounding bone. In this case the interphase is termed ‘biointegration’ because there is no intervening space between the bone and the implant. Implant material and design is continually evolving. With every passing year the failure rates are gradually reducing. Current implants enjoy a 95 to 98% clinical success rate. More advances in both design, material and technique may be expected in the future.

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Titanium Allergy In the last couple of year, the question if titanium allergy really exists has been raised in scientific literature. The reactions are not necessarily local, but appear in other parts of the body. One of the reasons why the existence of titanium allergy has been debated might be that the golden standard for metal allergy testing, patch testing, has not been properly developed for titanium. The patch test is a skin test, where salts of the metals tested are placed on the skin of the back under occlusion. 24–72 hours later a dermatologist evaluates the reaction and the presence of a rash is taken as evidence of a positive reaction. Unfortunately, titanium dioxide, a salt of titanium used for patch testing, does not penetrate the skin under the conditions of patch test. This is one of the reasons why patch test in its current form often gives false negative results in patients with titanium-induced inflammation in the body. The latest available research from Europe and Japan shows that between 2–4% of all patients with titanium implants develop an allergic reaction to either titanium or to one or more of the metals used in the titanium alloy. The symptoms most often observed after implantation with titanium-containing implants are varied, so they will be different in different patients. In addition to symptoms shown adjacent to implants on the mucosa in the mouth, or on the skin on the body, there may be other systemic symptoms. These symptoms are akin as those described after the exposure to other allergens, like nickel or mercury, in sensitised individuals. Symptoms arise because stimulated lymphocytes produce cytokines, which in turn, affect the HPA-axis. The result is multiple nonspecific symptoms such as profound fatigue, pain, cognitive dysfunction, headache, sleep problems, etc. Allergy due to titanium might be accountable for the failure of implants in some cases (known as cluster patients). It has been documented that the risk of titanium allergy is more prevalent in patients having sensitivity to other metals. In such types of cases, an allergy assessment is suggested to exclude problems related with titanium implants.

Zirconia implants Although titanium is the preferred choice for dental implants as it is an inert material, in rare circ*mstances it may encourage toxic or allergic type I or IV reactions. Its high strength, fracture toughness, and its white color make zirconia an interesting material for the construction of implant abutments and superstructures especially in the anterior zone.

Indications for zirconia implant 1. Esthetic considerations 2. Sensitivity or allergy to titanium Zirconia implants (Fig. 26.17) should be considered in patients with known allergy to titanium. (See chapter on Dental ceramics for more information on Zirconia implants)

Zirconia anatomic Root-form implants

Figure 26.17 Zirconia onepiece implant with abutment.

Synonyms Root analogue implants, anatomic implants, custom implants, bioimplant Zirconia-based anatomic root-form implants (Figs. 26.18A and B) were introduced into dental implantology as an alternative to conventional cylindrical implants. Owing to its ability to be milled into the shape of the natural tooth root and be placed immediately following

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Figures 26.18A and B (A) Zirconia root form implants. (B) Periapical view.

extraction, its excellent biomechanical characteristics, biocompatibility, and bright tooth-like color, zirconia has the potential to become a substitute for titanium as dental implant material. Immediate placement of implant similar in shape and size to the extracted root has its own advantages. By adapting the root to the extraction socket instead of adapting the bone to a preformed standardized implant they reduced the bone and soft tissue trauma. Immediate custom-made root analogue implants were possible because of advances in material and CAD/CAM technology. (See chapter on Dental Ceramics for more information on Zirconia anatomic root-form implants).

Advantages 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Esthetic Biocompatible Immediate placement No additional drilling or surgery required No complications of implant surgery Shorter waiting period Fits into original socket Immediate placement preserves bone and soft tissue (less resorption) Preserves root eminences of the alveolar bone Sinus lift not required Bone graft not required Little or no swelling or pain Faster recovery time

Implant surfaces and coatings The implant surface is critical to the proper biointegration of the implant. For this purpose various techniques have been used to modify the implant surfaces (Fig. 26.19) to improve its integration to the surrounding tissues. The techniques may be classified as 1. Ablative procedures Ablation is removal of material from the surface of an object by blasting, vaporization, chipping, or other erosive processes. –– Grit blasting –– Acid etching –– Anodizing –– Shot/ laser peening

Dental Implant Materials Chapter 26 2. Additive procedures It is a process of creating a layer by addition of material. –– Plasma spraying –– Electrophoretic deposition –– Sputter deposition –– Soluble gel coating –– Soluble blast media –– Pulsed laser deposition –– Biomimetic precipitation

Grit blasting

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Figure 26.19 On closer look many implants have a rough surface. Implant surface is critical to the proper biointegration of the implant.

A rough surface is created by grit-blasting the machined titanium implant surface with hard particles. The particles are projected through a nozzle at high velocity by means of compressed air. This is followed by washing in nonetching acid and distilled water to remove residual blasting material. The microtextured surfaces has been shown to allow for increased bone apposition compared to machined surfaces by increasing (Fig. 26.20A) the surface area. Some manufacturers combine both blasting and etching (Fig. 26.20B).

Grit blasting materials 1. 2. 3. 4.

Calcium phosphates like hydroxyapatite (HA) and beta-tricalcium phosphate Titanium oxide Zirconia Alumina

Disadvantages In the case of alumina, residual blasting media is often embedded into the implant surface (Fig. 26.21) and can interfere with integration.

Figures 26.20A to E Various implant surfaces with higher magnification.

464 Part 6 Alloys in Dentistry Acid etching The implant surface can be roughened through acid etching (Fig. 26.20C) with sulphuric or hydrochloric acids.

Advantages of acid etching 1. Produces a cleaner surface as there is no possibility of an external agent embedding onto the implant surface. 2. No possibility of loss of the layer through debonding, dissolution or wear thus avoiding concerns of long term fixation.

Figure 26.21 Embedded blast material (TiO2) following grit blasting.

Anodization Anodizing is an electrochemical process used to increase the thickness of the natural oxide layer on the surface of the titanium (to more than 1000 nm). Anodization modifies the microstructure and crystallinity of the titanium oxide layer (Fig. 26.20D). Anodization is done with the help of strong acids (e.g., sulphuric, phosphoric, nitric acids, etc.) at high current density and voltage (200 A/m2 at 100 V).

Shot/laser peening Shot peening is similar to sand blasting, where the surface is bombarded with small spherical particles causing small indentations to form. Laser peening involves the use of high intensity pulses of laser to create a regular honeycomb pattern with small pores.

Hydroxyapatite coated implants Hydroxyapatite (HA) (also called hydroxylapatite or calcium hydroxide phosphate), is a naturally occurring mineral form of calcium apatite with the formula Ca10(PO4)6(OH)2. Bone is a specialized mineralized connective tissue consisting by weight of 33% organic matrix, permeated by HA, which makes up the remaining 67% of bone. For this reason, HA is being investigated as a possible material for coatings or composites.

Rationale The use of hydroxyapatite implants have been reported to stimulate bone healing, resulting in an improvement in the rate and strength of initial implant integration. Hence, the dense HA layer on the top of titanium substrate is mainly for biointegration to bone tissue and enhanced implant stability. Thus, it is believed that the use of HA coatings on metallic implants would speed rehabilitation of patients by decreasing the time from implant insertion to final reconstruction.

Plasma sprayed HydroxyApatite Numerous methods of depositing HA on metallic implants have been reported. The current deposition process is plasma spraying or arc plasma spraying. A gas stream (pure argon or a mix of argon/hydrogen) is used to carry HA powder, which is then passed through an electrical plasma produced by a low-voltage, high-current electrical discharge. The plasma heats the hydroxyapatite, partially melting it. The semimolten HA powder is sprayed onto the titanium where it solidify. It has been reported that plasma spraying of HA results in coatings with a

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thickness greater than 30 µm. The bonding of the plasma-sprayed HA coatings appears to be entirely mechanical in nature. Besides HA, other materials like titanium can also be plasma sprayed onto the titanium surface (Fig. 26.20E).

Problems with plasma-sprayed coatings Problems cited with the plasma-sprayed coatings include 1. Variation in bond strength between the coatings and the metallic substrates 2. Alterations in HA structure due to the coating process 3. Poor adhesion between the coatings and metallic substrates

Electrophoretic Deposition (EPD) of HA EPD is a process in which colloidal particles of HA suspended in a liquid medium is deposited onto the implant under the influence of an electric field. EPD can produce coatings ranging from 1,500. A post-deposition heat treatment is required to densify the coating. A major disadvantage is the possibility of delamination of the layer in clinical use.

Biomimetic coating technique Biomimetic coating technique for nucleation and growth of bone-like crystals on a pretreated substrate by immersing it in a supersaturated solution of calcium phosphate under physiological conditions (37 °C and pH = 7.4). This method can be modified for the incorporation of drugs or growth factors onto the implant surface thereby making the implants osteoinductive and osteoconductive. The various brands and their implant surface types are outlined in Table 26.1. Table 26.1 Different surfaces in commercially available implants Implant

Surface modification

Ankylos Plus, XiVE, Frialit—Dentsply Friadent, GmbH

Sandblasted, large grit blasted, acid-etched

NobelActive—Nobel Biocare, Zurich, Switzerland

Phosphate enriched titanium oxide—TiUnite

GSIII—Osstem, South Korea

Resorbable blast media (RBM)—calcium phosphate hydroxyapatite

NanoTite—Biomet 3i, Palm Beach Gardens, Florida, USA

Calcium phosphate by discrete crystal deposition

Straumann Bone Level - Institute Straumann AG, Switzerland

SLActive—Sandblasted, large grit blasted followed by acid etching

Laserlok Surface—Biohorizon, Birmingham, Alabama

Laser peening

Pitt-Easy—Sybron Implant Solutions GmbH, Germany

Vacuum titanium plasma spray (V-TPS)

Tioblast—AstraTech Dental, MOIndal, Sweden Zimmer Screw Grit blasted with titanium oxide Vent Microtextured hydroxyapatite surface

27 Chapter

Wrought Metals and Alloys Chapter Outline • Manufacture of Wrought Alloys • Structure of Wrought Alloys –– Dislocations –– Twinning –– Fracture • Annealing –– Stages of Annealing –– Recovery –– Recrystallization –– Grain Growth • Uses of Wrought Alloys • General Properties of Orthodontic Wires

• • • • • • • •

Types Wrought Gold Alloys Wrought Base Metal Alloys Stainless Steel –– Passivation –– Types Ferritic Stainless Steels Martensitic Stainless Steels Austenitic Stainless Steels Annealed and Partially Annealed Wires –– Sensitization –– Stabilization

• • • • • • • •

Braided and Twisted Wires Solders for Stainless Steel Fluxes Wrought Cobalt-Chromiumnickel Alloys –– Physical Properties Nickel-Titanium Alloys Properties of Nitinol alloys Shape Memory and Superelasticity Titanium Alloys

Wrought metal is obtained from cast metal. A wrought metal or alloy is one that has been worked, drawn or shaped into a serviceable form, e.g. plates, band materials, bars, and wires. The process of forming wrought metal objects has been known since ancient times. For example, swords used in warfare were formed by subjecting a hot piece of metal to a beating process. Other things used in daily life like farming equipment and kitchen utensils are also made by a similar process.

Manufacture of wrought alloys Wrought metal is usually derived from cast metal or alloy. Wrought metal is formed when the parent metal is subject to various deformative processes like drawing, extruding, machining, beating, rolling, forging, etc. Examples of some of these processes are 1. Round wires are obtained by drawing a cast alloy through a series of dies. 2. Rolling process is used to form sheets and rods. 3. Forging is a process by which an object is formed by compressing the parent metal between two dies. Stainless steel crowns are made by this process. Manufacture of wrought alloys results in a tremendous amount of stresses (known as work hardening). These stresses are relieved by heat treatment during or after manufacture.

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Structure of Wrought Alloys All alloys are initially formed by casting. When a cast metal is subject to any deformation, it is considered a wrought metal. Wrought alloys have a fibrous structure which result from the cold working applied during the drawing operation to shape the wire. At the atomic level the deformative processes involved in the manufacture of wrought alloys results in various types of atomic deformations and disruptions. These include dislocations, twinning and fracture.

Dislocations On application of a shear force, dislocation of the atoms occurs along a plane called as the slip plane. The simplest type of dislocation is known as edge dislocation. The dominant slip planes are characteristic for each type of crystal structure. For example, face-centered cubic (fcc) structures have the greatest number of slip planes. Therefore, metals with a fcc structure like gold, copper, nickel, palladium, silver, platinum, etc. are highly ductile and easy to draw. Body-centered cubic (bcc) metal have intermediate levels of ductility. Hexagonal close-packed structures (hcp) have the least amount of slip systems and therefore are relatively brittle, e.g. zinc. Dislocations occur only in materials having a crystalline structure. Dislocations cannot exist in materials with a noncrystalline structure like dental ceramics and polymeric materials.

Twinning Another type of permanent deformation is known as twinning. The deformation occurs along either side of a plane in such a way that it mirrors each other. Twinning is favored over dislocation in metals that have relatively few slip systems.

Fracture Continuation of cold working in a heavily deformed metal eventually leads to fracture. The fracture initiates from microcracks that occur at points where there is an accumulation of dislocations or at boundaries between different microstructural phases. Alloys can undergo brittle or ductile fracture depending on a variety of factors, such as composition, microstructure and strain rate. When a ductile alloy fractures under tension, there is a reduction in the diameter of the metal (necking down) at the fracture site prior to fracture. Ductile fracture sites are characterized by a dimpled morphology. Microvoids or porosities may be seen at the fracture site. Fracture due to cold working is a cause for concern in dentistry. Examples are fractures of endodontic instruments like root canal files and reamers within the canal. Retrieval of such instruments can often be difficult. That is why it is necessary to use these instruments in the correct sequence and manner and to change these instruments at regular intervals rather than use them till it breaks.

Annealing The effects of cold working like strain hardening, susceptibility to corrosion and loss of ductility can be neutralized by a heating process called annealing.

Stages of annealing Annealing takes place in three stages 1. Recovery

468 Part 6 Alloys in Dentistry 2. Recrystallization 3. Grain growth The time and temperature for annealing is dependent on the melting temperature of the alloy. A commonly observed rule is to use a temperature that is approximately half the melting point of the metal or alloy on the absolute scale (K).

Recovery In the recovery stage, there is a slight decrease in tensile strength with no change in ductility. The most important beneficial changes occur during the recovery phase. As mentioned earlier, cold worked metal contains a lot of residual stresses. The purpose of annealing heat treatment is to relieve these stresses. Maximum stress relief occurs during the recovery stage.

Recrystallization On further heating, changes in the microstructure begin to take place. The deformed grains begin to recrystallize forming new stress free grains. The metal essentially regains its old soft and ductile condition. The metal loses its properties of resilience rendering it useless for its intended purpose. Thus recrystallization must be avoided.

Grain growth In this phase the recrystallized grains continue to grow with larger grains consuming smaller grains. Grain growth does not proceed indefinitely, but rather ceases until a coarse grain structure is formed. There is no significant difference in ductility and tensile strength from that observed in the previous stage. Significance It is clear from the above that annealing should be done only until the recovery stage. Uncontrolled heating of dental related appliances can result in unintended changes within the structure.

Uses of wrought alloys 1. 2. 3. 4. 5. 6.

Orthodontic wires Prosthodontic clasps Root canal instruments like files and reamers Steel bands and brackets for orthodontic and pedodontic use Stainless steel crowns Dental instruments

ORTHODONTIC WIRES Various types of wires are used in fixed and removable orthodontics for tooth movement and stabilization.

Classification (ISO 15841:2014) Wires are classified on the basis of their elastic behavior.

Type 1 wires: Wires displaying linear elastic behavior during unloading at temperatures up to 50 °C. Type 2 wires: Wires displaying nonlinear elastic behavior during unloading at temperatures up to 50 °C.

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General properties of orthodontic wires Orthodontic wires are formed into various configurations or incorporated into appliances. When activated, these wires apply forces to the teeth and move them to the desired alignment. The force is determined by the appliance design and the material properties of the wire. The following properties are important in orthodontic treatment.

Force generated The force generated by the wire on the tooth is dependent on its composition and design. For a given design, the force generated is proportional to the wire’s stiffness. Elastic deflection and working range Biologically, low constant forces are less damaging. This is best achieved by a large elastic deflection because it produces a more constant force and has a greater ‘working range’. Proportional limit (PL) Maximum elastic deflection = Moduluss of elasticity (MOE) Springiness It is a measure of how far a wire can be deflected without causing permanent deformation. Stiffness Amount of force required to produce a specific deformation. Stiffness = 1/springiness Resilience It is the energy storage capacity of the wires which is a combination of strength and springiness. Formability It represents the amount of permanent bending; the wire will tolerate before it breaks. Ductility of the wire. Ease of joining Most wires can be soldered or welded together. Corrosion resistance and stability in the oral environment is important for the appliance durability as well as biocompatibility. Biocompatibility Most orthodontic wires are biocompatible. People generally allergic to nickel may get allergic reactions from nickel containing orthodontic wires. Cost is a factor in orthodontics. The titanium alloy wires are more expensive than the stainless steel or the cobalt chromium nickel wires.

TYPES

Wrought gold alloys Wrought base-metal alloys Stainless steel Cobalt-chromium-nickel Nickel-titanium Beta-titanium

WROUGHT GOLD ALLOYS USES Primarily to make clasps in partial dentures.

470 Part 6 Alloys in Dentistry Classification Type I—High precious metal alloys Type II—Low precious metal alloys

COMPOSITION The composition varies widely. Gold Platinum Palladium Silver

— 25 to 70% Copper — 7 to 18% — 5 to 50% Nickel — 1 to 3% — 5 to 44% Zinc — 1 to 2% — 5 to 41%

PROPERTIES They generally resemble Type IV casting gold alloys. Because of the cold working, wires and other wrought forms have improved mechanical properties like hardness and tensile strength when compared to cast structures. However, care should be taken during soldering. Prolonged heating at higher temperatures can cause it to recrystallize. Recrystallization changes the properties and makes the wire brittle.

WROUGHT BASE-METAL ALLOYS A number of wrought base-metal alloys are used in dentistry, mainly as wires for orthodontic treatment. The alloys are

Stainless steel (iron-chromium-nickel) Cobalt-chromium-nickel Nickel-titanium Beta-titanium

STAINLESS STEEL Steel is an iron-based alloy which contains less than 1.2% carbon. When chromium (12 to 30%) is added to steel, the alloy is called as stainless steel. Elements other than iron, carbon and chromium may also be present, resulting in a wide variation in composition and properties of the stainless steels.

Passivation Stainless steels are resistant to tarnish and corrosion, because of the passivating effect of the chromium. A thin, transparent but tough and impervious oxide layer forms on the surface of the alloy when it is exposed to air, which protects it against tarnish and corrosion. It loses its protection if the oxide layer is ruptured by mechanical or chemical factors.

TYPES There are three types of stainless steel based on the lattice arrangement of iron. 1. Ferritic 2. Martensitic

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3. Austenitic 4. Duplex 5. Precipitation Hardening

FERRITIC STAINLESS STEELS Pure iron at room temperature has body-centered cubic (BCC) structure and is referred to as ferrite, which is stable up to 912 °C.

Properties and Use The ferric alloys have good corrosion resistance, but less strength and hardness. So they find little application in dentistry.

MARTENSITIC STAINLESS STEELS When austenite (face-centered cubic structure) is cooled very rapidly (quenched), it will undergo a spontaneous, diffusionless transformation to a body-centered tetragonal (BCT) structure called martensite. This is a highly distorted and strained lattice, which results in a very hard and strong but brittle alloy.

Properties and Uses Corrosion resistance of the martensitic stainless steel is less than that of the other types. Because of their high strength and hardness, martensitic stainless steels are used for surgical and cutting instruments. Bur shanks are also made from this steel.

AUSTENITIC STAINLESS STEELS At temperatures between 912 °C and 1394 °C, the stable form of iron is a face-centered cubic (FCC) structure called austenite. The austenitic stainless steel alloys are the most corrosion resistant of the stainless steels.

Austenite-finish temperature It is the temperature at which the metallurgical transformation from the low-temperature martensite phase to the high-temperature austenite phase is completed.

Composition Chromium — 18% Nickel — 8% Carbon — 0.08-0.15%

Uses This alloy is also known as 18-8 stainless steel. They are commonly used by orthodontists and pedodontists in the form of bands and wires (Figs. 27.1 and 27.2). Type 316 L (contains carbon-0.03% maximum) is the type usually used for implants.

Available as They are available as annealed and partially annealed wires. They are usually supplied as rolls of varying thickness.

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Figure 27.1 A stainless steel wire roll (left) and some of the appliances made from the wires.

Advantages Austenitic steel is preferred to ferritic alloys because of some desirable properties 1. Greater ductility and ability to undergo more cold work without breaking. 2. Substantial strengthening during cold working. 3. Greater ease of welding. 4. The abilit y to readily overcome sensitization. 5. Less critical grain growth. 6. Comparative ease in forming.

Figure 27.2 High tensile stainless steel wire.

Properties Sensitization The 18-8 stainless steel may lose its resistance to corrosion if it is heated between 400 and 900 °C (temperature used during soldering and welding). The reason for a decrease in corrosion resistance is the precipitation of chromium carbide at the grain boundaries at these high temperatures. The small, rapidly diffusing carbon atoms migrate to the grain boundaries from all parts of the crystal to combine with the large, slowly diffusing chromium atoms at the periphery of the grain. When the chromium combines with the carbon in this manner, its passivating qualities are lost and the corrosion resistance of the steel is reduced.

Stabilization (methods to minimize sensitization) 1. From a theoretical point, the carbon content of the steel can be reduced to such an extent that carbide precipitation cannot occur. However, this is not economically practical. 2. By stabilization, i.e., some element is introduced that precipitates as a carbide in preference to chromium. Titanium is commonly used. Titanium at six times the carbon content, inhibits the precipitation of chromium carbide at soldering temperatures. These are known as stabilized stainless steels.

Annealed and partially annealed wires When stainless steel wires are fully annealed, they become soft and highly formable. When it is partially annealed, the yield strength is increased and formability decreased. Stainless steel

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is available in different grades depending on their yield strength. Both the fully annealed and partially annealed wires are used as orthodontic wires.

Mechanical properties In orthodontic wires, strength and hardness may increase with a decrease in the diameter because of the amount of cold working in forming the wire.

Tensile strength — 2100 MPa Yield strength — 1400 MPa Hardness — 600 KHN

Braided and Twisted Wires Very small diameter stainless steel wires (about 0.15 mm) can be braided or twisted together to form either round or rectangular shaped (about 0.4 to 0.6 mm in cross-section) wires (Fig. 27.3). These wires are available as straight lengths or as formed archwires in the form of 3 strands or in increasing number of strands.

Figure 27.3 A braided wire.

These braided or twisted wires are able to sustain large elastic deflections in bending, and apply low forces for a given deflection when compared with solid stainless steel wire.

Solders for Stainless Steel Silver solders are used. The soldering temperatures for orthodontic silver solders are in the range of 620 to 665 °C.

Fluxes It is similar to that recommended for gold soldering with the exception of–

The addition of the potassium fluoride. Fluoride helps to dissolve the passivating film supplied by the chromium. A higher boric acid to borax ratio lowers the fusion temperature.

WROUGHT COBALT-CHROMIUM-NICKEL ALLOYS These wrought alloys were originally developed for use as watch springs (Elgiloy). Their properties are excellent also for orthodontic purposes.

COMPOSITION Co — 40% Cr — 20% Ni — 15% Mo — 7%

Mn — 2% C — 0.15% Be — 0.04% Fe — 15.8%

HEAT TREATMENT Softening heat treatment 1100 to 1200 °C followed by a rapid quench. Hardening heat treatment 260 to 650 °C, e.g. 482 °C for 5 hours.

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Figure 27.4 Nitinol arch wires (left) are used extensively in orthodontic treatment (above).

The wires are usually heat treated and supplied in several degrees of hardness (soft, ductile, semispring temper and spring temper).

PHYSICAL PROPERTIES Tarnish and corrosion resistance is excellent. Hardness, yield, and tensile strength similar to those of 18-8 stainless steel.

NICKEL-TITANIUM ALLOYS Nickel-titanium shape memory alloys were first discovered by Buehler in the early 1960s. He was working at the Naval Ordinance Laboratory (NOL) at the time, hence the name Nitinol. His discovery formed the basis of the first commercial shape memory alloy. These nickel-titanium alloy (also called nitinol) wires have large elastic deflections or working range and limited formability, because of their low stiffness and moderately high strength. They are used extensively as arch wires in fixed orthodontic treatment (Fig. 27.4). They are also used to manufacture endodontic instruments (Fig. 27.5).

Available as

Figure 27.5 Nickeltitanium files.

Nickel titanium alloy wires are available as springs in addition to formed arch wires. Nickel titanium wires are commercially available in martensitic (M-Niti) and austenitic (A-Niti) depending on their use in different phases of orthodontic treatment.

Composition The primary elements are nickel and titanium. Addition of copper to nickel and titanium alloy improves the thermal reactive properties of the wire, which help in consistent and efficient orthodontic tooth movement. Other additions made to alter the phase transformation temperature are elements such as iron and chromium which lower the temperature.

Properties of Nitinol alloys Shape Memory and Superelasticity This alloy exists in various crystallographic forms. At high temperature, a stable bodycentered cubic lattice (austenitic phase) exists. On appropriate cooling or an application of stress, this transforms to a close-packed hexagonal martensitic lattice with associated

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volumetric change. This behavior of the alloy (austenite to martensite phase transition) results in two features of clinical significance called as ‘shape memory’ and ‘superelasticity’, or ‘pseudoelasticity’. The ‘memory’ effect is achieved by first establishing a shape at temperatures near 482 °C. The appliance, e.g. archwire is then cooled and formed into a second shape. Subsequent heating through a lower transition temperature (37 °C - mouth temperature) causes the wire to return to its original shape. The phenomenon of superelasticity is produced by transition of austenite to martensite by stress due to the volume change which results from the change in crystal structure. Stressing an alloy initially results in standard proportional stress-strain behavior. However, at the stress where it induces the phase transformation, there is an increase in strain, referred to as superelasticity. At the completion of the phase, it reverts to standard proportional stressstrain behavior. Unloading results in the reverse transition and recovery. This characteristic is useful in some orthodontic situations because it results in low forces and a very large working range or springback. These wires are useful because it is possible to achieve phase transformation at room temperature when force is applied. Wires with different transformation temperatures are now available, which enable the clinician to select the precise wires for different needs. Density Their density is approximately 6.5 g/cm3. Melting range Melting temperature in the range 1240 to 1310 °C.

TITANIUM ALLOYS Like stainless steel and nitinol, pure titanium has different crystallographic forms at high and low temperatures. At temperatures below 885°C the hexagonal close packed (HCP) or alpha lattice is stable, whereas at higher temperatures the metal rearranges into a body-centered cubic (BCC) form called β-titanium. α-titanium is not used in orthodontic applications. The β-form is more useful in orthodontics. However, to retain the β-form as it cools to room temperature elements like molybdenum are added. This stabilizes the β-form and prevents its transformation to the α-form. For orthodontic use the titanium alloys are supplied as precut arch wires (Fig. 27.6) usually in a rectangular cross-sectional form (Fig. 27.7).

Figure 27.6 Titanium (Beta III by 3M) arch wire.

Figure 27.7 Wires come in different cross-sectional shapes including round, square and rectangular.

476 Part 6 Alloys in Dentistry COMPOSITION Ti

— 79%

Mo — 11% Zr — 6% Sn — 4%

MECHANICAL PROPERTIES 1. Modulus of elasticity - 70 GPa. 2. Yield strength - 860 to 1200 MPa. 3. The high ratio of yield strength to modulus produces orthodontic appliances that can undergo large elastic activations when compared with stainless steel. 4. Beta-titanium can be highly cold-worked. It can be bent into various configurations and has formability comparable to that of austenitic steel. 5. Welding Clinically satisfactory joints can be made by electrical resistance welding of beta-titanium. 6. Corrosion resistance Both forms have excellent corrosion resistance and environmental stability. 7. Heat treatment can alter its properties, therefore, heat treatment of these wires is not recommended.

Section-7

Indirect Restorative and Prosthetic Materials Chapter 28 Dental Ceramics, 479 Chapter 29 Denture Resins and Polymers, 529 Chapter 30 Maxillofacial Prosthetic Materials, 572

28 CHAPTER

Dental Ceramics Chapter Outline • Uses and Applications • Evolution of Dental Ceramics • Classification of Dental

• Glass Infiltrated Alumina Core

•

• • • • • • • • • • • • • •

Porcelains Basic Constituents and Manufacture Porcelain/Ceramic Systems Classification and Description of Ceramic Systems Metal-Ceramic Restorations Porcelain-Metal Bond Swaged Gold Alloy Foil Capillary Casting Technique Bonded Platinum Foil-Ceramic All-Ceramic Restorations Porcelain Jacket Crown Leucite Reinforced Porcelain Castable Glass Ceramic Glass Ceramics Pressable Ceramics Glass Infiltrated Ceramics

•

• • • • • • • • • • •

(In-Ceram Alumina) Glass Infiltrated Spinell Core (In-Ceram Spinell) Glass Infiltrated Zirconia (In-Ceram Zirconia) CAD/CAM Ceramics Combining Various Porcelains and Processing Techniques Essentials of a CAD/CAM Scanner or Digitizer Computer (CAD Process) Milling Station Ceramic Blanks Classification of Machinable Ceramic Blanks Sintering Furnaces Copy Milled (CAM) Systems Ceramill System Networked CAD/CAM

• Yttria Stabilized Zirconia –– Transformation Toughening • Advantages and Disadvantages of CAD/CAM

• Properties of Fused Porcelain • Methods of Strengthening • Low Fusing and Ultralow Fusing Ceramics

• Cementing of Ceramic • • • • • • •

Restorations –– Etching of Porcelain Repair of Ceramic Restorations Porcelain Denture Teeth Monolithic Zirconia Restorations Ceramic Posts Pediatric Zirconia Zirconia Implants and Abutments Crowns Zirconia Anatomic Root-Form Implants

Dental ceramics holds the promise of a restorative material, that can realistically duplicate teeth, to the extent that the layperson may find it difficult to differentiate (Fig. 28.1). One might argue that composite resins have a similar esthetic potential. However, there is a difference— dental ceramics are far more stronger, durable, wear resistant, and virtually indestructible in the oral environment. They are impervious to oral fluids and absolutely biocompatible. They do have some drawbacks which will be discussed subsequently. Because of their huge potential, it is still a fast growing area in terms of research and development. Thanks to the continuing research, these materials once restricted to restoring single crowns have now expanded to include long span fixed partial dentures.

USES AND APPLICATIONS 1. Inlays and onlays. 2. Esthetic laminates (veneers) over natural teeth. 3. Single (all ceramic) crowns.

480 PART 7 Indirect Restorative and Prosthetic Materials 4. 5. 6. 7. 8.

Short and long span (all ceramic) FDP. As veneer for cast metal crowns and bridges (metal ceramics). Artificial denture teeth (for complete denture and partial denture use). Ceramic post and cores. Ceramic orthodontic brackets.

EVOLUTION OF DENTAL CERAMICS Ceramics are among the oldest materials known to man. Ceramic objects dating back to 20,000 years have been found in China. The history of glass dates back to 3500 BC in Mesopotamia. The term ‘glass’ was first developed during the late Roman empire. Ceramics comes from Greek word keramikos, which means pottery and keramos, which means potter’s clay. An esthetic and durable material that could accurately reproduce missing teeth or teeth structure had always been a dream. Prior to the use of porcelain, crowns were made entirely of gold or other alloys. As demands for esthetics increased, tooth colored resin was used as a veneer over the metal in the esthetic areas. Around the early 1900s, porcelain crowns were introduced to dentistry by Charles Land (grandfather of aviator Charles Lindbergh) who coined the term porcelain jacket crowns (PJC). The restoration was extensively used after improvements were made by EB Spaulding and publicized by WA Capon. While not known for its strength due to internal microcracking, the porcelain “jacket” crown (PJC) was used extensively until the 1950s. These early crowns were made of feldspathic porcelains which generally were materials of poor strength. They were also very difficult to fabricate and did not fit well (poor margins).

“The worlds oldest ceramic object, the Venus of Dolni Vestonice, from the C z e c h R e p u b l i c, 26,000 years old.”

To reduce the risk of internal microcracking during the cooling phase of fabrication, the porcelain-fused-to-metal (PFM) crown was developed in the late 1950s by Abraham Weinstein. The bond between the metal and porcelain prevented stress cracks from forming. This led to the era of the metal-ceramics (Fig. 28.1). Prior to this, metal FDPs were veneered (covered) with tooth colored acrylic in order to hide the metal. These veneers did not last very long and had to be replaced often. Besides they could not be used to cover the occlusal surface because of their poor wear resistance. The metal-ceramic crowns and fixed dental prostheses were instantly accepted because of their superior esthetics, wear resistance and strength. The ceramic could be used to veneer the occlusal surface as well. Since the margins were in metal, the marginal fit was highly accurate. In spite of the success of the metal-ceramic restorations, they did not represent the final solution. The underlying opaquer covered metal did not allow the natural passage and reflection of light as in natural teeth. Under certain lighting conditions these crowns appeared dense, dark and opaque. The margin of the restoration appeared to be dark, even when hidden below the gums as it sometimes showed through the gums (the gums developed a bluish discoloration).

FIGURE 28.1 Porcelain-fused-to-metal fixed partial denture.

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Some manufacturers did attempt to solve this problem by introducing ‘shoulder porcelains’. A portion of the metal was removed from the labial margin (metal free margin) and replaced with shoulder porcelain. However, this still did not entirely solve the problem of translucency. The first breakthrough at developing a stronger all-ceramic restoration came in 1965. McLean and Hughes introduced an alumina reinforced core material which improved the strength of the porcelain. However, they were still not strong enough for posterior use and of course the problem of marginal adaptation still remained. The 1990s saw the reemergence of the all-ceramic crown (Fig. 28.2) as well as small fixed partial dentures. The strength of the restorations had been improved through the introduction of newer porcelains and fabrication techniques. The marginal adaptation and fit had also improved considerably when compared to the first generation all-porcelain crowns. The new generation ceramics included castable glass ceramics, injection molded ceramics, glass infiltrated core ceramics, CAD/CAM (computer-aided design, computer-aided machining) ceramics, etc. With the increase in strength the use of FIGURE 28.2 All porcelain (metal free) crowns. all ceramic restorations gradually expanded to include posterior crowns and bridges. A major reason for this was the introduction of stabilized zirconia and CAD/CAM. Ceramic technology continue to evolve because of the high demand for esthetic tooth colored restorations.

Definition Compounds of one or more metals with a nonmetallic element, usually oxygen. They are formed of chemical and biochemical stable substances that are strong, hard, brittle, and inert nonconductors of thermal and electrical energy (GPT- 8).

CLASSIFICATION OF DENTAL PORCELAINS The wide variety of ceramic systems available in the market make classification of ceramics a challenging task. The manufacturer provides equipment and material compatible for the particular system. They are usually not interchangeable.

ACCORDING TO FIRING TEMPERATURE

High fusing Medium fusing Low fusing Ultra low fusing

1300 °C or above 1101 °C to 1300 °C 850 °C to 1100 °C less than 850 °C

ACCORDING TO TYPE

Feldspathic porcelains Leucite reinforced glass ceramics Tetrasilicic fluormica based glass ceramics Lithia disilicate based ceramics Alumina reinforced ceramics Spinel reinforced ceramics Zirconia reinforced ceramics

Porcelain vs. Ceramic The terms porcelain and ceramics are often used interchangeably. However there are subtle differences. Ceramic is a broad term that includes earthenware, bone ware, pottery, brick, stoneware and of course porcelain and any other article made of clay and hardened by heat. Porcelain is a subcategory of ceramics and is characterized by its hard, glass-like and translucent qualities. All porcelains are ceramics, but all ceramics are not porcelains.

482 PART 7 Indirect Restorative and Prosthetic Materials ACCORDING TO ITS FUNCTION WITHIN THE RESTORATION

Core ceramics – Supports and reinforces the restoration in all-ceramic restorations Opaquer ceramics – Masks or hides the metal or underlying core ceramic. Bonds ceramic to underlying metal Veneering ceramics –– Body or dentin – Simulates the dentin portion of natural teeth –– Incisal – Simulates the enamel portion of natural teeth –– Gingival – Simulates the darker gingival portion of teeth –– Translucent – Simulates translucent incisal enamel seen sometimes in natural teeth Stains – Used to color ceramics to improve esthetics Glaze – Imparts a smooth glossy surface to the restoration

ACCORDING TO MICROSTRUCTURE

Glass ceramics Crystalline ceramics Crystal containing glasses

ACCORDING TO FABRICATION PROCESS

Condensable ceramics Slip-cast glass-infiltrated ceramics Heat pressed (hot isostatic) ceramics Castable ceramics Machinable ceramics Various combinations of the above

BASIC CONSTITUENTS AND MANUFACTURE OF FELDSPATHIC PORCELAIN The wide variety of ceramic products in the market, makes it virtually impossible to provide a single composition. Traditionally, porcelains were manufactured from a mineral called feldspar. These porcelains are referred to as feldspathic porcelains. As technology improved other ceramic systems were introduced, like core porcelains, glass ceramics, etc. The composition of these differ from the traditional feldspathic porcelains.

BASIC STRUCTURE Most current ceramics consist of two phases.

Glassy phase—acts as the matrix Crystalline phase—dispersed within the matrix and improves strength and other properties of the porcelain, e.g. quartz, alumina, spinel, zirconia, etc.

The structure of porcelain is similar to that of glass (Box 28.1). The basic structure therefore consists of a three dimensional network of silica (silica tetrahedra). Pure glass melts at too high a temperature for dental use. Adding certain chemicals lowers the melting temperature by disrupting the silica network. The glass obtains porcelain-like qualities when the silica network is broken by alkalies like sodium and potassium. This also lowers the fusion temperature. These chemicals are known as glass modifiers or fluxes. Other substances which act as glass modifiers are alumina (Al2O3) and boric oxide (B2O3). Boric oxide forms its own separate network between

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BOX 28.1 Comparison of glass and porcelain Glass

Porcelains

Random amorphous (noncrystalline) structure

Ordered crystalline structure in glassy matrix

Transparent

Opaque

Composed mainly of silica

Contains silica and other crystalline phases

the silica network. Adding certain opacifiers reduces the transparency and completes the transformation to dental porcelain.

BASIC CONSTITUENTS The basic constituents of feldspathic porcelains are

Feldspar – Basic glass former/matrix Kaolin – Green stage binder Quartz – Filler and opacifier Alumina – Additional glass former and flux Alkalies – Glass modifiers (flux) Color pigments – Modifies color Opacifiers – Reduces transparency

Feldspar It is a naturally occurring mineral and forms the basic constituent of feldspathic porcelains. Most of the components needed to make dental Feldspathic glass porcelain are found in feldspar. It thus contains potash (K2O), soda (Na2O), alumina (Al2O3) and silica (SiO2). It is the basic glass former. When fused at high temperatures (during manufacture) it forms a feldspathic glass containing potash feldspar (K2O.Al2O3.6SiO2) or soda feldspar (Na2O.Al2O3.6SiO2). Pure feldspathic glass is quite colorless and transparent. As explained earlier, various glass modifiers and opacifiers are added to alter its sintering temperature, viscosity, thermal coefficient of expansion (CTE) and appearance.

Kaolin Kaolin also called china clay, is a white clay-like material (hydrated aluminum silicate). Kaolin is named after the hill in China (Kao-ling) from which it was mined for centuries. It acts as a binder when wet, helping to shape the green porcelain. It also gives opacity to the mass. Some manufacturers use sugar or starch instead of kaolin.

Quartz Quartz is a form of silica. Ground quartz acts as a refractory skeleton, providing strength and hardness to porcelain during firing. It remains relatively unchanged during and after firing.

Alumina Aluminum oxide (alumina) replaces some of the silica in the glass network. It gives strength and opacity to the porcelain. It alters the softening point and increases the viscosity of porcelain during firing.

484 PART 7 Indirect Restorative and Prosthetic Materials Another glass former is boric oxide (B2O3) which forms its own glass network (also called lattice) interspersed between the silica network (lattice).

Glass modifiers Alkalies such as sodium, potassium and calcium are called glass modifiers. Glass modifiers lower the fusion temperature and increase the flow of porcelain during firing. They also raise the CTE (important in metal-ceramics). However, too high a concentration of glass modifiers is not good for the ceramic because

It reduces the chemical durability of the ceramic It may cause the glass to devitrify (crystallize)

Opacifiers Since pure feldspathic porcelain is quite colorless, opacifiers are added to increase its opacity in order to simulate natural teeth. Oxides of zirconium, titanium and tin are commonly used opacifiers.

Color modifiers Natural teeth come in a variety of shades. In addition, it acquires external stains from the environment. Thus, color modifiers are required to adjust the shades of the dental ceramic. Various metallic oxides provide a variety of color, e.g. titanium oxide (yellow-brown), nickel oxide (brown), copper oxide (green), manganese oxide (lavender), cobalt oxide (blue), etc. They are fused together with regular feldspar and then reground and blended to produce a variety of colors.

OTHER SPECIALIZED PORCELAINS Glazes It is a special type of colorless porcelain applied to the surface of the completed ceramic restoration to give it a smooth finish as well as increase the life of the restoration. Obviously they do not contain opacifiers. They must also have a lower fusion temperature and therefore must contain a lot of glass modifiers. This also makes them somewhat less chemically durable.

Stains They are porcelain powders containing a high concentration of color modifiers (as described previously). They too have a lower fusion temperature made possible by an increased content of glass modifiers. Stains are used to provide individual color variation in the finished restoration (Fig. 28.2).

Opaquer porcelains It is a specialized type of porcelain which is used to conceal the metal core in PFM (metalceramic) restorations. It is the first layer applied before the addition of the regular porcelain. Obviously, it contains a high concentration of opacifiers. Some amount of color modifiers are also added.

Reinforced core ceramics The low strength of traditional feldspathic porcelain prompted research into methods of reinforcing ceramics. The first reinforced ceramic (alumina reinforced) was introduced by McLean and Hughes in 1965. Subsequently other materials and techniques were introduced. Among the strongest of the core ceramics currently available are the machined zirconia cores.

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MANUFACTURE Traditionally, porcelain powders are manufactured by a process called fritting. Various components are mixed together and fused. While it is still hot, it is quenched in water. This causes the mass to crack and fracture, making it easier to powder it. The frit is ground to a fine powder and supplied to the consumer in bottles. Most of the chemical reaction takes place during the manufacture (pyrochemical reaction). During subsequent firing in the dental laboratory, there is not much of chemical reaction). The porcelain powder simply fuses together to form the desired restoration.

PORCELAIN/CERAMIC SYSTEMS Currently, various ceramic systems exist which can be quite confusing to the dental student. The entire restoration may be made of just one type of porcelain (e.g. an inlay machined from a single block of ceramic) or it may be layered with different types of porcelains. Many crowns and FDPs are fabricated as layered restorations. A layered restoration can be divided into 2 basic parts (Fig. 28.3).

Core (or substructure) Veneer (outer layer). FIGURE 28.3 Simplified structure of a ceramic crown.

Core The core provides support and Core is for support and may be made of metal in case of strength for the crown. Early crowns were a metal ceramic crown or a dense strong ceramic in case of an all ceramic crown. constructed entirely of a single type of feldspathic porcelain (e.g. PJC). In 1965 McLean demonstrated improved strength in crowns layered over an aluminous core. Since then other core materials and techniques have been introduced. The core functions as a supporting frame. Freshly mixed porcelain is like wet sand. It needs to be supported while it is being condensed and built up. The core is therefore usually constructed first. The rest of the restoration is built up on to the core. With metal-ceramic crowns the metal coping or frame takes the role of the ceramic core. They provide the support and reinforcement. Examples of core materials currently available are alumina, spinel, zirconia, etc. Veneer The core is usually dense and opaque and generally unesthetic. The esthetics is improved by firing additional layers of ceramic known as veneer porcelains. The core is veneered with various layers of specialized porcelains called dentin, enamel, cervical and translucent. It can also be internally and externally (surface) stained to mimic natural teeth color and finally glazed.

CLASSIFICATION AND DESCRIPTION OF CERAMIC SYSTEMS The ceramic restorations available today may be metal bonded or made completely of ceramic. Based on the substructure or core material used there are two basic groups. They are further divided based on the fabrication method. A. Metal-ceramic (metal bonded or PFM) restorations 1. Cast metal-ceramic restorations -- Cast noble metal alloys

486 PART 7 Indirect Restorative and Prosthetic Materials -- Cast base metal alloys -- Cast titanium (ultra low fusing porcelain). 2. Burnished foil metal ceramic restorations -- Capillary casting [sintered gold alloy foil coping Renaissance, Captek)] -- Bonded platinum foil coping. B. All ceramic restorations 1. Platinum foil matrix condensed porcelain restorations -- Conventional feldspathic porcelain restorations -- Porcelain restorations with aluminous core -- Ceramic jacket crown with leucite reinforced core (Optec HSP) 2. Castable glass ceramics (Dicor) 3. Pressable glass-ceramics -- Leucite reinforced glass-ceramics (IPS Empress) -- Lithia disilicate reinforced glass-ceramics (IPS Empress 2) 4. Glass infiltrated core porcelains -- Glass infiltrated aluminous core (In-Ceram) -- Glass infiltrated spinel core (In-Ceram Spinell) -- Glass infiltrated zirconia core (In-Ceram Zirconia) 5. Ceramic restorations from CAD/CAM ceramic blanks -- Feldspathic porcelain blanks (Vitablocs Mark II) -- Lithia disilicate glass ceramic blanks (IPS e max CAD, Kavo) -- Glass infiltrated blanks (Alumina, Spinell, Zirconia) -- Partially sintered zirconia blanks (Vita In-Ceram YZ) -- Sintered zirconia blanks (Everest ZH blanks) 6. Ceramic restorations from copy milled ceramic blanks -- Alumina blocks (Celay In-Ceram) -- MgAl2O4 blocks (In-Ceram spinell).

METAL-CERAMIC RESTORATIONS Synonyms Porcelain-fused-to-metal (PFM), metal-bonded restorations, ceramo metal, etc. The early porcelain jacket crowns (PJC) did not use reinforcing cores and were therefore weak. The metal-ceramic restorations (Fig. 28.4) were developed around the same time Mclean introduced the aluminous core porcelains (1965). The cast metal core (called coping) or framework (Fig. 28.5) significantly strengthened the porcelain restoration and this soon became the most widely used ceramic restoration. According to a 1994 survey, 90% of all ceramic restorations were porcelain-fused-to-metal. The metal-ceramic systems are covered by ISO 9693. The metal-ceramic system was possible because of some important developments. Development of a metal and porcelain that could bond to each other Raising of the CTE of the ceramic in order to make it more compatible to that of the metal. This obviously meant that a lot of research had to go into both porcelain and metal composition before they could be used for metal-ceramics.

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FIGURE 28.4 Cross section through a metal ceramic crown fused to nickel chromium alloy.

487

FIGURE 28.5 Parts of a metal ceramic crown. Transparent is used to duplicate the thin translucent enamel seen in some natural teeth.

TYPES OF METAL-CERAMIC SYSTEMS As previously mentioned the metal-ceramic systems can be divided into 1. Cast metal ceramic restorations –– Cast noble metal alloys (feldspathic porcelain) –– Cast base metal alloys (feldspathic porcelain) –– Cast titanium (ultra low fusing porcelain). 2. Swaged metal ceramic restorations –– Capillary cast [sintered gold alloy foil (Renaissance, Captek)] –– Bonded platinum foil coping.

CAST METAL-CERAMIC RESTORATIONS The cast metal-ceramic restoration is hugely popular. Because of the strong metal frame it is possible to make long span fixed partial dentures. It can also be used in difficult situations where an all-ceramic restoration cannot be given because of high stresses and reduced preparation depth.

USES 1. Single anterior and posterior crowns. 2. Short and long span anterior and posterior FDPs.

COMPOSITION OF CERAMIC FOR METAL BONDING Feldspathic porcelains are used for metal bonding. The basic composition is quite similar to that of feldspathic porcelain described earlier except for the higher alkali content (soda and potash). The higher alkali content was necessary in order to raise the CTE. Unfortunately this also increased the tendency of the ceramic to devitrify and appear cloudy. A typical composition is shown in Table 28.1. A special opaquer porcelain is needed to mask the underlying metal so that it does not show through the ceramic (Fig. 28.8). The opaquer has a high content of opacifiers. Similarly, the

488 PART 7 Indirect Restorative and Prosthetic Materials composition of glazes would be different. Glazes have a higher concentration of glass modifiers like soda, potash and boric oxide.

SUPPLIED AS 1. 2. 3. 4. 5. 6. 7. 8. 9.

Enamel porcelain powders in various shades (in bottles) (Fig. 28.6) Dentin porcelain powders in various shades (in bottles) Liquid for mixing enamel, dentin, gingival and transparent Opaquer powders in various shades/ and liquid for mixing (Fig. 28.7) Gingival porcelain powder in various shades Transparent porcelain powder A variety of stain (color) powders Glaze powder Special liquid for mixing stains and glaze

MANIPULATION AND TECHNICAL CONSIDERATIONS CONSTRUCTION OF THE CAST METAL COPING OR FRAMEWORK A wax pattern of the restoration is constructed and cast in metal. Metals used for the frame or coping include noble metal alloys, base metal alloys and recently titanium (see chapter on casting alloys and casting procedures). TABLE 28.1 A sample percentage composition of porcelain powder for metal ceramics Dentin porcelain

Enamel porcelain

Silica (SiO2)

59.2

63.5

Alumina (Al2O3)

18.5

18.9

Soda (Na2O)

4.8

5.0

Potash (K2O)

11.8

2.3

Boric oxide (B2O3)

4.6

0.12

Zinc oxide (ZnO)

0.58

0.11

Zirconium oxide (ZrO2)

0.39

0.13

FIGURE 28.6 Enamel and dentin powders with the modelling liquid.

FIGURE 28.7 Opaquer powder is mixed and applied to hide the metal. It is mixed with the liquid to produce a sandy mix. A glass spatula is used for mixing as metal might abrade and contaminate the porcelain.

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METAL PREPARATION A clean metal surface is essential for good bonding. Oil and other impurities from the fingers can contaminate. The surface is finished with ceramic bonded stones or sintered diamonds. Final texturing is done by sandblasting with an alumina air abrasive, which aids in the bonding. Finally, it is cleaned ultrasonically, washed and dried.

DEGASSING AND OXIDIZING The casting (gold porcelain systems) is heated to a high temperature (980°C) to burn off the impurities and to form an oxide layer which help in the bonding. Degassing is done in the porcelain furnace.

FIGURE 28.8 Application of opaquer.

OPAQUER The opaquer is a dense yellowish white powder supplied along with a special liquid. The opaquer has two important functions. It is used to cover (mask) the metal frame and prevent it from being visible. It also aids in bonding the veneering porcelains to the underlying frame. The metal framework is held with a pair of locking forceps. Opaquer powder is dispensed on to a ceramic palette and mixed with the special liquid to a paste like consistency (Fig. 28.7). It is applied on to the metal frame with a brush and condensed (Fig. 28.8). The excess liquid is blotted with a tissue paper. The opaquer is built up to a thickness of 0.2 mm. The casting with the opaquer is placed in a porcelain furnace (Fig. 28.9) and fired at the appropriate temperature (Box 28.2). Opaquer may be completed in two steps.

FIGURE 28.9 Ceramic furnace.

CONDENSATION The process of packing the powder particles together and removing the excess water is known as condensation.

Purpose Proper condensation packs the particles together. This helps minimize porosity, improve strength and reduce firing shrinkage. It also helps remove the excess water.

FIGURE 28.10 Condensing with mild vibration.

Condensation techniques Vibration Mild vibration by tapping or running a serrated instrument (Fig. 28.10) on the forceps holding the metal frame helps to pack the particles together and bring out the excess water which is then blotted by an absorbent paper (Fig. 28.11). An ultrasonic vibrator is also available for this purpose.

FIGURE 28.11 Blotting to remove excess water.

490 PART 7 Indirect Restorative and Prosthetic Materials BOX 28.2 Firing of porcelains The process of sintering and fusing the particles of the condensed mass is known as firing. The powder particles flow and fuse together during firing. Making the restoration dense and strong. Firing is done in a porcelain furnace. The Porcelain Furnace Firing is carried out in a porcelain furnace. There are many companies which manufacture furnaces. Modern furnaces are computer controlled and have built-in programs to control the firing cycle. The programs can also be modified by the operator. Firing Cycle The entire program of preheating, firing, subjecting to vacuum, subjecting to increased pressure, holding and controlled cooling is known as a firing cycle. The firing cycles vary depending on the stage - opaquer firing, dentin firing, glaze firing, etc. The firing temperature is lowered gradually for each subsequent firing cycle. The opaquer has the highest temperature and the glaze has the lowest. Preheating The condensed mass should not be placed directly into the hot furnace. This can cause a rapid formation of steam which can break up the mass. Modern furnaces have a mechanism whereby the work is gradually raised into the furnace. This is known as preheating. Vacuum Firing During firing of the porcelain, a vacuum (negative pressure) is created in the furnace. This helps to reduce the porosity in the ceramic. The vacuum is later released raising the pressure in the furnace. The increased pressure helps to further reduce the size of any residual air bubbles not eliminated by the vacuum. The vacuum is not activated during the glaze firing. Cooling The cooling of the fired porcelain should be well controlled. Rapid cooling can cause the porcelain to crack or it can induce stresses inside which weaken the porcelain. Cooling is done slowly and uniformly and is usually computer controlled.

Spatulation A small spatula is used to apply and smoothen the wet porcelain. This helps to bring out the excess water. Dry powder Dry powder is placed on the side opposite a wet increment. The water moves towards the dry powder pulling the wet particles together.

DENTIN AND ENAMEL

FIGURE 28.12 Building the restoration.

The dentin powder (pink powder) is mixed with distilled water or the supplied liquid. A glass spatula should be used (ceramic powder is abrasive and can abrade the metal and contaminate the porcelain). The bulk of the tooth is built up with dentin. A portion of the dentin in the incisal area is cut back and enamel porcelain (white powder) can be added (Fig. 28.12) building the restoration. After the build-up and condensation is over (Fig. 28.13), it is returned to the furnace for sintering.

ADDITIONS It is not necessary to build up the restoration in one step. Large or difficult restorations may be built up and fired in two or more stages. After each firing (Box 28.2) the porcelain may be shaped by grinding and additional porcelain is placed in deficient areas. Each additional firing is done at a lower temperature. FIGURE 28.13 The built up crown.

Caution The restoration should not be subject to too many firings. Excessive firings can give rise to a over translucent, lifeless restoration.

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FIGURES 28.14A AND B Stained porcelain crown. Staining improves the vitality of the crown. (A) The lateral incisor before staining appears white and artificial. (B) The same tooth after applying yellow brown cervical stains and white fluorosis streaks and patches.

GINGIVAL AND TRANSPARENT PORCELAIN The enamel of some natural teeth may appear transparent. This is usually seen near the incisal edges. If present it can be duplicated using transparent porcelain. The cervical portions of natural teeth may appear more darker (e.g. more yellow) than the rest of the tooth. When indicated cervical porcelains are used to duplicate this effect (they are also referred to as gingival or neck dentin).

SURFACE STAINING, CHARACTERIZATION AND EFFECTS Natural teeth come in variety of hues and colors. Some of them are present at the time of eruption (intrinsic, e.g. white fluorosis stains), while others are acquired over a period of time from the environment (extrinsic, e.g. coffee, tobacco, etc.). Staining and characterization helps make the restoration look natural and helps it to blend in with the adjacent teeth (Figs. 28.14A and B). The stain powders (Fig. 28.15) are mixed with a special liquid, applied and blended with a brush. With more and more emphasis on recreating the FIGURE 28.15 Porcelain stains and glazes. natural look, effects are created using special techniques. This includes defects, cracks or other anomalies within the enamel.

GLAZING Before final glazing, the restoration is tried in the mouth by the dentist. The occlusion is checked and adjusted by grinding. Final alterations can be made to improve the shape of the restoration. After all changes have been completed the restoration is ready for glazing. The restoration is smoothened with a fine stone prior to glazing to remove gross scratch marks. Glazing provides a smooth glossy surface to the restoration.

Objectives of glazing 1. Glazing enhances esthetics. 2. Enhances hygiene. 3. Improves the strength. Glazed porcelain is much stronger than unglazed ceramic. The glaze inhibits crack propagation.

492 PART 7 Indirect Restorative and Prosthetic Materials BOX 28.3 Glazing versus conventional polishing Porcelain can be polished using special abrasives. Porcelain is an extremely hard material and is quite difficult to polish. Glazing is considered by some to be superior to conventional polishing.

4. Reduces the wear of opposing teeth. The rough surface on unglazed porcelain can accelerate wear of the opposing natural teeth.

Types Over glaze The glaze powder is mixed with the special liquid and applied on to the restoration. The firing temperature is lower than that of the body porcelain. The firing cycle does not usually include a vacuum. Chemical durability of over glaze is lower because of the high flux content. Self glaze A separate glaze layer is not applied. Instead the restoration is subject to a controlled heating at its fusion temperature. This causes only the surface layer to melt and flow to form a vitreous layer resembling glaze.

PORCELAIN-METAL BOND Falls into two groups

Chemical bonding across the porcelain-metal interface. Mechanical interlocking between porcelain and metal.

CHEMICAL BONDING Currently regarded as the primary bonding mechanism. An adherent oxide layer is essential for good bonding. In base metal alloys, chromic oxide is responsible for the bond. In noble metal alloys, indium and tin oxide and possibly iridium oxide does this role. Both inadequate oxide formation and excessive oxide build up can lead to a weak bond resulting in delamination of the overlying porcelain (Fig. 28.16).

MECHANICAL INTERLOCKING

FIGURE 28.16 A failed metal-ceramic FDP. The ceramic veneer (canine) has delaminated leaving the metal exposed. In this case it was because of a poorly adherent metal oxide layer.

In some systems mechanical interlocking provides the principal bond. Sandblasting is often used to prepare the metal surface. Presence of surface roughness on the metal oxide surface improves retention, especially if undercuts are present. Wettability is important for bonding.

ADVANTAGES AND DISADVANTAGES OF METAL-CERAMIC RESTORATIONS ADVANTAGES 1. Better fracture resistance because of the metal reinforcement. 2. Better marginal fit because of the metal frame.

DISADVANTAGES 1. Poor esthetics when compared to all-ceramic restorations because the underlying metal and opaquer reduces the overall translucency of the tooth.

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2. The metal frame and the lack of translucency sometimes shows through the gingiva resulting in the characteristic dark margins.

OTHER METAL-CERAMIC SYSTEMS CAPILLARY CAST (SINTERED GOLD ALLOY FOIL-CERAMIC) RESTORATIONS Adapting and sintering gold alloy foils (Renaissance and Captek) is a novel way of making a metal frame without having to cast it. The system was developed by Shoher and Whiteman and introduced to the dental community in 1993. Captek is an acronym for ‘capillary casting technique’. The technique is used to make crowns and fixed prostheses using proprietary materials and techniques (Figs. 28.17A to C). (Refer chapter on ‘casting procedures’ for additional information).

Composition, mode of supply and capillary casting They are supplied as thin strips in two forms called Captek P and Captek G. Captek P (Platinum/ Palladium/ Gold) has a porous structure and serves as the internal reinforcing skeleton. Captek G is 97.5% Gold and 2.5% Silver. On heating in a furnace, the Captek P acts like a metal sponge and draws in (capillary action) the hot liquid gold completely into it. Captek G provides the characteristic gold color of this system. The final coping can be described as a composite structure.

Technique The technique for fabrication is described in the chapter ‘Casting procedures’.

BONDED PLATINUM FOIL—CERAMIC CROWNS A platinum foil coping is adapted on to the die. To improve the bonding of the ceramic to the platinum foil coping, an electrodeposition technique is used. A thin layer of tin is

FIGURES 28.17A TO C Sintered gold alloy foil-ceramic restorations. (A) A Captek coping. (B) Cross-section through a Captek crown. (C) A Captek FDP.

494 PART 7 Indirect Restorative and Prosthetic Materials electrodeposited on to the foil and then oxidized in a furnace. The advantages of using bonded platinum foil is similar to that for swaged gold alloy foil.

The electrodeposition technique This is a technique used to improve both esthetics and bonding. A layer of pure gold is electrodeposited on to the metal. This is followed by a quick minimal deposition of tin over the gold. Advantages 1. The gold color enhances the vitality of the porcelain, thereby, enhancing esthetics (the normal technique requires a heavy unesthetic opaque layer to cover the dark metal oxide surface). 2. The tin helps in chemical bonding (through formation of tin oxide). 3. Improves wetting at the gold-porcelain interface thereby reducing porosity. 4. The electrodeposition technique can be used on metals, such as stainless steel, cobalt chromium, titanium and other non-gold and low gold alloys.

ALL-CERAMIC RESTORATIONS The all-ceramic restorations are made without a metallic core or sub-structure. This makes them esthetically superior to the metal-ceramic restoration. Unfortunately, all-ceramic restorations had lower strength, thus, metal-ceramics continued to be the restoration of choice for the majority of restorations till the 1990s. Continued research have led to improved all-ceramic systems with greater strength and fracture resistance. Manufacturers today claim the new generation all-ceramic materials are capable of producing not only single crowns but anterior and even posterior all-ceramic FDPs as well. Long span FDPs have also been attempted. The all-ceramic restorations are grouped according to their type and method of fabrication 1. Condensed sintered –– Traditional feldspathic porcelain jacket crown –– Porcelain jacket crown with aluminous core (Hi-Ceram) –– Ceramic jacket crown with leucite reinforced core (Optec HSP). 2. Cast glass ceramics (Dicor). 3. Injection molded (leucite reinforced) glass ceramic (IPS Empress). 4. Slip cast-glass infiltrated ceramics –– Glass infiltrated aluminous core restorations (In-Ceram) –– Glass infiltrated spinell core restorations (In-Ceram Spinell) –– Glass infiltrated zirconia core (In-Ceram Zirconia). 5. Milled ceramic restoration or cores –– CAD/CAM restorations –– Copy milled restorations (Blocks or blanks of various ceramics are machined to form the restoration. Examples are alumina, zirconia, lithia disilicate, etc. The various types are detailed in a subsequent section - see classification of machinable ceramics).

PORCELAIN JACKET CROWN These are crowns made entirely of feldspathic porcelain. They are constructed on a platinum foil matrix which is subsequently removed.

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TYPES 1. Porcelain jacket crown (traditional). 2. Porcelain jacket crown with aluminous core. 3. Porcelain jacket crown with leucite reinforced core (Optek HSP). Note The above two are generally referred to as ‘porcelain jacket crowns’ or PJCs. The subsequently introduced ceramics are referred to as ‘ceramic jacket crowns’ or CJCs’ and ‘glass ceramic crowns’.

TRADITIONAL PORCELAIN JACKET CROWN The all-porcelain crown (PJC) has been around since a century (early 1900s). These early crowns are also referred to as traditional or conventional PJCs. They were made from conventional high fusing feldspathic porcelains. As mentioned before these were very brittle and fractured easily (half moon fractures). The marginal adaptation was also quite poor. Because of these problems they gradually lost popularity and are no longer used presently.

PORCELAIN JACKET CROWN WITH ALUMINOUS CORE The problems associated with traditional PJCs led to the development of the PJC with an alumina reinforced core (McLean and Hughes, 1965) (Fig. 28.18). The increased content of alumina crystals (40 to 50%) in the core strengthened the porcelain by interruption of crack propagation. In spite of the increased strength they were still brittle and therefore not indicated for posterior teeth and their use was restricted to anterior teeth. The composition of the alumina reinforced PJC is shown in Table 28.2.

Technical considerations The porcelain jacket crowns are made using the platinum foil matrix technique.

FIGURE 28.18 The porcelain jacket crown with aluminous core.

Platinum foil matrix A platinum foil is adapted to the die (Figs. 28.19A and B) with a wooden point. The platinum foil functions as matrix. It supports the porcelain during condensation and firing. TABLE 28.2 A sample percentage composition of aluminous porcelain Silica (SiO2)

Aluminous core

Dentin

Enamel

35.0

66.5

64.7

Alumina (Al2O3)

53.8

13.5

13.9

Calcium oxide (CaO)

1.12

—

1.78

Soda (Na2O)

2.8

4.2

4.8

Potash (K2O)

4.2

7.1

7.5

Boric oxide (B2O3)

3.2

6.6

7.3

Zirconium oxide (ZrO2)

496 PART 7 Indirect Restorative and Prosthetic Materials

FIGURE 28.19A TO C The porcelain jacket crown with aluminous core.

Condensation and firing The core porcelain is carefully condensed on to the foil (Fig. 28.19C). The foil with the condensed porcelain is carefully removed from the die. It is then placed in the furnace and fired. After cooling, the rest of the crown is built up with conventional feldspathic porcelain. Removing the foil After completion of the restoration the platinum foil is gently teased out and discarded. This can be quite difficult.

LEUCITE REINFORCED PORCELAIN (OPTEC HSP) Optec HSP is a feldspathic porcelain with a higher leucite crystal content (leucite reinforced). Its manipulation, condensation and firing is quite similar to the alumina reinforced porcelain jacket crowns (using platinum foil matrix).

Uses Inlays, onlays, veneers and low stress crowns.

Advantages 1. They are more esthetic because, the core is less opaque (more translucent) when compared to the aluminous porcelain. 2. Higher strength. 3. No need of special laboratory equipment.

Disadvantages 1. Fit is not as good as metal ceramic crowns. 2. Potential marginal inaccuracy. 3. Not strong enough for posterior use.

CASTABLE GLASS CERAMIC The castable glass ceramic is quite unlike the previously mentioned porcelains. Its properties are more closer to that of glass (Box 28.4) and its construction is quite different. This is the only porcelain restoration made by a centrifugal casting technique. The subsequent ‘ceramming’ process is also quite unique to this porcelain. Ceramming enhances the growth of mica crystals within the ceramic.

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BOX 28.4 Glass ceramics Glass ceramics are materials that are formed initially as glass, and then transformed into ceramic usually by a controlled heat treatment. The heat induces partial devitrification (crystallization within the glass) which increases the strength as well as improves esthetics by making it less transparent and more tooth-like. The glass-ceramics used in dentistry include the castable, machinable and hot-isostatically pressed glass-ceramics.

SUPPLIED AS The first commercially available castable glass-ceramic for dental use was ‘Dicor’ developed by Corning glass works and marketed by Dentsply. They are supplied as glass ingots. A precrystallized form called Dicor MGC is also available as machinable blanks for CAD/CAM.

COMPOSITION Dicor glass-ceramic contains 55 vol% of tetrasilicic fluormica crystals.

FEATURES The Dicor glass-ceramic crown is very esthetic. This is because of its greater translucency (unlike some other porcelains which have more opaque core). It also picks up some of the color from the adjacent teeth (chameleon effect) as well as from the underlying cement. Thus the color of the bonding cement plays an important role.

USES Inlays, onlays, veneers and low stress crowns.

FABRICATION OF A DICOR CROWN To understand the salient features of this material, the step-by-step construction of a crown will be described 1. The pattern is first constructed in wax (Fig. 28.20A) and then invested in refractory material like a regular cast metal crown. 2. After burning out the wax, nuggets of Dicor glass are melted and cast into the mold in a centrifugal casting machine. 3. The glass casting (Fig. 28.20B) is carefully recovered from the investment by sandblasting and the sprues are gently cut away. 4. The glass restoration is then covered with an embedment material to prepare it for the next stage called ceramming. 5. Ceramming is a heat treatment process by which the glass is strengthened. Ceramming results in the development of microscopic crystals of mica, which –– Improves the strength and toughness of the glass

B FIGURES 28.20A AND B Castable glass ceramics (Dicor). (A) Wax pattern. (B) Cast glass.

498 PART 7 Indirect Restorative and Prosthetic Materials –– Improves the esthetics of the restoration (it reduces the transparency of the glass making it more opaque and less glass-like). 6. The cerammed glass can be built up with special veneering porcelain and fired to complete the restoration. Surface stains may be applied to improve the esthetics.

ADVANTAGES 1. 2. 3. 4. 5. 6.

Ease of fabrication. Good esthetics (greater translucency and chameleon effect). Improved strength and fracture toughness. Good marginal fit. Very low processing shrinkage. Low abrasion of opposing teeth.

DISADVANTAGES 1. Inadequate strength for posterior use. 2. Internal characterization not possible. Has to be stained externally to improve esthetics.

HEAT PRESSED (HOT-ISOSTATICALLY PRESSED) CERAMICS This is another ceramic material which again is quite unlike the previous ceramics because of its unique way of fabrication (injection molding). It is a precerammed glass-ceramic having a high concentration of reinforcing crystals. The material supplied in the form of ingots is softened under high temperatures and forced into a mold created by a lost wax process. Synonyms

Injection moulded or Heat-pressed glass-ceramics.

TYPES AND MODE OF SUPPLY Heat pressed ceramics are supplied as ingots (Figs. 28.21 and 28.22) of various compositions. These include 1. Heat pressed glass ceramics –– Leucite or KAlSi2O6 reinforced (IPS Empress, Finesse, Optimal, Cerpress, etc). –– Lithium disilicate reinforced (IPS empress 2, OPC 3G). 2. Heat pressed veneering ceramics [e.g. IPS ZirPress (Fig. 28.22), Vita PM9] are available for use as a pressed layer over machined zirconia cores. Compatible veneering ceramics in powder-liquid form may be provided along with the ingots or acquired separately.

FIGURE 28.21 IPS Empress ingots.

FIGURE 28.22 IPS Zir Press.

FIGURE 28.23 The pressing furnace.

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USES Inlays, onlays, veneers (Fig. 28.25D) and low stress crowns. Small 3 unit FDPs may be constructed with IPS Empress 2.

MICROSTRUCTURE IPS Empress—contains 35 to 40% vol of leucite crystals (Table 28.3). IPS Empress 2—consists of 65 to 70% by volume (Table 28.3) of interlocked elongated lithia disilicate crystals. The crystal size varies from 0.5 to 4 µm in length. The crystals within the structure improve the fracture resistance by reducing crack propagation.

FABRICATION 1. The wax (Fig. 28.25B) patterns of the restorations are invested in refractory material and heated to 850 °C in a furnace to burn off the wax and create the mold space.

FIGURE 28.24 Schematic representation of the pressing process.

TABLE 28.3 Composition of two popular hot-isostatically pressed ceramics IPS Empress 1

IPS Empress 2

Silica (SiO2) Alumina (Al2O3) Soda (Na2O) Potash (K2O)

63 17.7 4.6 11.2

Boric oxide (B2O3) Calcium oxide (CaO2) Titanium dioxide (TiO2) Barium oxide (BaO)

0.6 1.6 0.2 0.7

Cerium oxide (CeO2) Pigments

0.4

Silica (SiO2) Alumina (Al2O3) Potash (K2O) Phosphorous pentoxide (P2O5) Lithium (Li2O) Zinc oxide (ZnO) Magnesium MgO Lanthanum oxide (La2O3) Pigments

57-80 0-5 0-13 0-11 11-19 0-8 0-5 0.1-6 0-8

D FIGURES 28.25A TO D IPS empress. (A) Teeth prepared for veneers. (B) Wax patterns. (C) Pressed ceramic still attached to the sprue. (D) The completed restorations.

500 PART 7 Indirect Restorative and Prosthetic Materials 2. It is then transferred to the pressing furnace (Fig. 28.23). A ceramic ingot and an alumina plunger is inserted in to the sprue (Fig. 28.24). 3. Pressing temperature for IPS Empress—1075 to 1180 °C 4. Pressing temperature for IPS Empress 2—920 °C 5. The pressing is done under air pressure of 1,500 psi. 6. The core or restoration (Fig. 28.25C) is retrieved from the flask. 7. Compatible veneering porcelains are added to the core to build up the final restoration (Fig. 28.25D). 8. It can also be directly fabricated as a crown in which case, the crown is stained and glazed directly.

ADVANTAGES 1. Better fit (because of lower firing shrinkage). 2. Better esthetics due to the absence of metal or an opaque core.

DISADVANTAGES 1. Need for costly equipment. 2. Potential of fracture in posterior areas.

GLASS INFILTRATED CERAMICS These are specialized core ceramics reinforced by an unique glass infiltration process. They are also sometimes referred to as slip-cast ceramics.

Types Currently there are three types depending on the core material used. 1. Glass infiltrated alumina core (In-Ceram Alumina). 2. Glass infiltrated spinell core (In-Ceram Spinell). 3. Glass infiltrated zirconia core (In-Ceram Zirconia).

Supplied as Oxide powder (alumina, spinell or zirconia) with mixing liquids, glass powder and veneering ceramics (Figs. 28.26 to 28.28).

FIGURE 28.27 Glass powder.

FIGURE 28.26 Alumina powder and other accessories used to make the core.

FIGURE 28.28 Vita VM7 is a veneering ceramic designed for In-Ceram.

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GLASS INFILTRATED ALUMINA CORE (IN-CERAM ALUMINA) This ceramic system has a unique glass infiltration process and the first of its kind claimed for anterior FDP fabrication. The glass infiltration process compensates for firing shrinkage. The final core after completion of the glass infiltration is made up of about 70% alumina and 30% (sodium lanthanum) glass.

Indications 1. Anterior and posterior crowns, and 2. Short span anterior fixed dental prostheses.

Composition Infiltration glass powder

Alumina powder

Al2O3

99.7

La2O3

49.6

MgO

0.03

SiO2

19.1

TiO2

6.16

CaO

3.14

Others

2.0

Fabrication (Figs. 28.29A to I) 1. Two dies are required. One in stone and the other in refractory die material. 2. Preparing the slip—Measured quantity (38 g) of alumina powder is added slowly into a beaker containing 1 ampoule of mixing liquid and a drop of additive liquid. Mixing is done with the help of a special ultrasonic unit (Vitasonic). The water in the Vitasonic should be chilled using ice cubes. The prepared slip should be smooth and hom*ogenous. The slip is applied on to the refractory die using the slip cast method (the water from the slurry is absorbed by the porous die leaving a dense layer of alumina on the surface). Once started the slip should not be allowed to dry out before the coping is completed. The process is continued until an alumina coping of sufficient thickness is obtained. 3. The fragile slip cast alumina coping is dried at 120 °C for 2 hours. 4. The coping is sintered (Inceramat furnace) for 10 hours at 1120 °C. 5. After sintering the copings are tested for cracks using a special dye. 6. The next step is glass infiltration. Glass powder is mixed with distilled water. One or two thick coats (1-2 mm) is applied on to the sintered alumina coping (outer surface only) and fired for 2-3 hours at 1110 °C on a platinum foil. The glass melts and infiltrates into the porous alumina coping through capillary action. 7. The excess glass forms a glassy layer on the surface which is trimmed off using special diamond burs, followed by sandblasting. A glass control firing (1000 °C) is carried out. 8. The coping is then built up using special veneering ceramics (Vita VM 7).

Advantages 1. Good fit and marginal adaptation. 2. Good strength when compared to the earlier all ceramic crowns. Claimed to be strong enough for posterior single crowns and anterior FDP use.

Disadvantages 1. Comparatively less esthetic because of the opacity of the alumina core. 2. Quite tedious to fabricate. 3. Not all the FDPs were successful, a few of them did fracture occasionally.

502 PART 7 Indirect Restorative and Prosthetic Materials

FIGURES 28.29A TO I Fabrication of an In-Ceram restoration. (A) Slip casting. (B) In-ceram furnace. (C) Sintering of the slip. (D) Sintered coping. (E) Glass slurry applicaion. (F) Embedded in investment. (G) Glass infiltration furnace. (H) Glass infiltrated coping. (I) The completed restoration.

Uses 1. In addition to inlays, onlays, veneers and low stress (anterior and posterior) crowns, this material can be used to construct low stress anterior FDPs. Because of its occasional tendency to fracture when used for FDP construction its use should be carefully selected. 2. For people allergic to metal based restorations. 3. Where esthetics is absolutely critical.

GLASS INFILTRATED SPINELL CORE (IN-CERAM SPINELL) In-Ceram Spinell is an offshoot of In-Ceram Alumina. Because of the comparatively high opacity of the alumina core, a new material was introduced known as In-Ceram spinell. It uses spinel (MgAl2O4) instead of alumina. The fabrication process is quite similar to that for In-Ceram Alumina.

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The In-Ceram Spinell is more translucent and therefore more esthetic compared to the InCeram Alumina core. Since the strength is lower, its use is limited to low stress situations.

Indications Its high translucency makes it a material of choice for crowns and restorations in esthetic (anterior crowns) and stress free zones.

Contraindications The high translucency contraindicates it in situations where the underlying tooth structure is severely discolored and needs to be masked. Its low strength also contraindicates it for posterior situations and FDPs.

GLASS INFILTRATED ZIRCONIA (IN-CERAM ZIRCONIA) Zirconia (ZrO2) is a naturally occurring mineral. Crystals of Zirconia are used as a substitute for diamond. In-Ceram Zirconia is the strongest of the three glass infiltrated core materials. The final glass infiltrated ICZ cores contains around 30 wt% zirconia and 70 wt% alumina.

Indications Its high strength makes it a material of choice for posterior crowns and short span fixed partial dentures in high stress areas (posterior FDPs). It is not particularly suited for esthetic zones because of its greater opacity. However, in cases where there is severe discoloration, In-Ceram Zirconia helps mask the discolored tooth structure because of its greater opacity.

CAD/CAM CERAMICS Constructing a dental ceramic restoration is technique sensitive, labor intensive and time consuming. Machined ceramics were introduced to overcome some of these problems. They are also known as milled or machined ceramics. Machinable ceramic systems can be divided into two categories 1. CAD/CAM systems 2. Copy milled systems

CAD/CAM SYSTEMS These are systems that can design and produce restorations out of blocks or blanks of ceramics with the aid of a computer. CAD/CAM is acronym for computer aided design-computer aided manufacturing.

HISTORY OF CAD/CAM The major development in the field of dental CAD/CAM took place in the 1980s. They were influenced by three important pioneers. The first was Duret who fabricated crowns through a series of processes starting with an optical impression of the prepared tooth. The milling was done by a numerically controlled milling machine (the precursor of modern CAM/CAM). The second pioneer was Mörmann, developer of the CEREC system at the University of Zurich. A compact chair-side machine milled the crown from measurements of the preparation taken by an intraoral camera. At the time, the system was innovative as it allowed ‘same-day restorations’. With the announcement of this system, the term CAD/CAM spread rapidly to the dental profession. The third was Andersson, the developer of the Procera system in the 1980s.

504 PART 7 Indirect Restorative and Prosthetic Materials The Japanese also developed many systems in the 1980s but these were not commercially successful because of the resistance from health insurance companies. The early systems had to overcome many problems including limited computing power, poor marginal accuracy, etc. Current CAD/CAM systems have come a long way. With improvements in technology, material and software, restoration fabrication is considerably more accurate and operator friendly as well. CAD/CAM systems are now part of everyday dentistry.

Commercially available CAD/CAM systems Many systems are currently available using a variety of techniques and materials (Fig. 28.30). Some examples of commercially available CAD/CAM systems are - Cerec (Sirona), Sirona InLab, Everest (Kavo), Cercon (Dentsply), Lava (3M ESPE), Zeno (Weiland), 5-tec (Zirkonzahn), etc.

ESSENTIALS OF A CAD/CAM SYSTEM The CAD/CAM system consists of 5 essentials 1. 2. 3. 4. 5.

Scanner or digitizer Computer Milling station Ceramic blanks Furnace

– – – – –

Virtual impression Virtual design (CAD) Produces the restoration or framework Raw material for the restoration For postsintering, ceramming etc.

FIGURE 28.30 Schematic representation of CAD/CAM production.

BOX 28.5 Combining various porcelains and processing techniques

•

In some systems the entire restoration can be made entirely from the same material. Example - An inlay or laminate may be constructed entirely with pressable ceramics or from a machined feldspathic block. • In many systems at least 2 or more processing techniques and materials have to be combined to produce the final restoration for a variety of reasons like esthetics, ease of fabrication, need for correction, etc. Most reinforced core ceramics are too opaque to be used to construct the entire restoration with it. These cores have to be built up with veneering ceramics, characterized with stains and then glazed to produce the final restoration. Example 1—Glass infiltrated cores are too opaque and have to be layered with condensing type veneering porcelains to produce the final restoration. Example 2—Machined alumina cores are frequently built up with condensable ceramics. Example 3—In one system machined zirconia blocks are overlayed with a pressable veneering material (Zir Cad and Zir Pres). An important point to remember is that various ceramics should be compatible with each other when used together. Non- compatible products may have difference in CTE which can cause failure of the restoration. The manufacturers usually specify the veneering materials compatible with their products.

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Scanner or digitizer The dimensions of the prepared tooth (or die or wax pattern) are picked up and digitized in order to create a 3 dimensional image of the prepared tooth in the computer. This is achieved by scanning of the preparation or the die. The 2 types of digitizers currently employed are 1. Contact probes Physically contacts the die as it moves along its surface while transmitting the information to the computer. E.g. Procera Forte contact scanner. 2. Scanners Unlike contact probes, scanners are optical devices. These include –– Intraoral hand-held wands (Fig. 28.31A) These are chairside scanners. The intraoral scanner reflects light (visible light, laser or LED) and captures it with a camera to create an optical impression of the prepared tooth and adjacent structures. Multiple images have to be captured to stitch together a composite 3D image in the computer. In some systems a special powder is dusted to reduce reflection and improve readability. –– Laboratory scanners These are larger devices that scan the cast or die using different technologies. Some use a camera to capture multiple images similar to the intraoral scanner (white light optical scanner). Others use 2 cameras to capture the object from multiple angles using white light (e.g. Kavo Everest - Figs. 28.31B and 28.32) or laser planes projected in a grid pattern. The Procera optical scanner uses a laser beam to measure distances (conoscopic holography).

FIGURES 28.31 A TO I CAD/CAM systems (A) Tooth preparation may be scanned directly in the mouth with a hand held scanner. (B) The preparation may also be scanned from a cast (Kavo scanner). (C) A computer aids in designing the final restoration or coping. (D) The design is transferred to the milling station and a restoration is milled. (E) A milled laminate. (F) cerec 3 (Sirona) inlab. (G) Zeno 2100 (Weiland). (H) Lava (3 M Espe). (I) Everest (Kavo) milling station.

506 PART 7 Indirect Restorative and Prosthetic Materials

FIGURE 28.32A AND B (A) Light beam scanner (Kavo Everest). (B) Laser conoscopic holography (new Procera optical scanner).

Computer (CAD process) The restoration or the core is designed in the computer (Fig. 28.31C). Most manufacturers have their own software for the CAD process. The CAD process aids in designing either the restoration, coping or the FDP substructure. The computer can automatically detect the finish line. Some use a library of tooth shapes that is stored on the computer to suggest the shape of the proposed restoration. A recording of the bite registration (the imprint of the opposing or antagonist tooth in a wax-like or rubbery material) is also added to the data. The combined information together with the 3D optical impression of the prepared tooth establishes the approximate zone in which the new restoration can exist. The proposed restoration can then be morphed to fit into this zone in an anatomically and functionally correct position. The dentist can make corrections or modify the design if required and then send it to the milling unit for completion.

Milling station Milling stations have evolved considerably since they were first introduced into the market (Figs. 28.31E to I). The earlier models ground only the internal surface. The external surface had to be manually ground. Current CAD/CAM machines can grind the external surface also. Signals from the computer control the milling tool which shapes the ceramic block according to the computer generated design. To begin the process the ceramic block is attached to the machine via a frame or built-in handle(s). The enlargement factor (see presintered zirconia) is also calculated where applicable. Milling is performed by a diamond or carbide milling tool. The Cerec station (Fig. 28.31F) uses 2 diamond burs to grind the internal and external surface simultaneously (Fig. 28.31D). Other machines use a single tool that moves along multiple axis (3 to 5 axis) and performs the milling action. The Everest (Kavo) Engine (Fig. 28.31I) is an example of a 5 axis milling action. Some machines (Kavo Everest) can mill both ceramic and titanium.

Ceramic blanks A variety of ceramic blanks in various sizes, shades and shapes are available for milling. Multiple units can be produced from the larger blocks. The smaller blanks may produce only a single coping or restoration. The blank is attached via a frame to the machine or by one or more handles on the blank itself. Classification of machinable ceramic blanks 1. Feldspathic porcelain blanks [Vitablocs Mark II (Vita)]. 2. Glass ceramic blanks – Tetrasilicic fluormica based glass ceramic [Dicor MGC (Dentsply)]

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– Leucite based [ProCad (Ivoclar), Everest G (Kavo)] – Lithia disilicate glass ceramic [IPS e max CAD (Kavo)]. 3. Glass infiltrated blanks – Alumina (Vita In-Ceram Alumina) – Spinell (Vita In-Ceram Spinell) – Zirconia (Vita In-Ceram Zirconia). 4. Presintered blanks – Alumina (Vita In-Ceram AL) – Ytrria stabilized Zirconia (Vita In-Ceram YZ). 5. Sintered blanks – Ytrria stabilized Zirconia (Everest ZH blanks).

Brief description of various materials for CAD/CAM The fabrication process is system and material specific. The prepared tooth or teeth is scanned directly from the mouth or from a model made from a regular impression. Next the restoration or substructure is designed on the computer. The blank is attached to the milling station and the bar code scanned. The time taken for milling depends on the size and complexity of the restoration as well as the material used. For example, presintered zirconia is easier to mill than sintered zirconia. It also reduces wear of the milling tools. After milling, the structure is separated from the blank using water cooled cutting and grinding disc or burs. Subsequent processing procedures are then initiated depending on the material and system used. Feldspathic blanks (Fig. 28.33) Feldspathic restorations can be milled to full contour. The restoration is glazed after milling. Optional processing includes veneering and staining. Uses - inlays, laminates and anterior crowns. Leucite reinforced (Fig. 28.34) These blanks can be milled to full contour. The restoration is glazed after milling. Optional processing includes veneering and staining. Uses - inlays, onlays, laminates and anterior crowns. Lithium disilicate (Fig. 28.34) The ceramic is machined in an intermediate crystalline state in which the material shows its characteristic bluish shade (Fig. 28.35A). In this stage the material is easier to shape and can be tried in the mouth. This is followed by a simple, quick crystallization process (30 minutes) in a conventional ceramic oven in which it reaches its final strength and the desired esthetic properties such as tooth color, translucence and brightness (Fig. 28.35B). Optional processing includes veneering and staining. Uses - inlays, onlays, and anterior and posterior crowns. Glass infiltrated ceramics (Fig. 28.36) These are usually machined as cores or FDP substructures. Subsequent processing includes glass infiltration, veneering, and glazing. Uses In-Ceram Spinell is recommended for anterior single crowns copings. In-Ceram Alumina is indicated for anterior and posterior crowns and 3 unit anterior FDP substructures. In-Ceram Zirconia can be used for anterior and posterior crowns and 3 unit FDP substructures.

FIGURE 28.33 Feldspathic porcelain.

FIGURE 28.34 Glass ceramic blanks— leucite based (above) and lithia disilicate (bottom).

508 PART 7 Indirect Restorative and Prosthetic Materials

FIGURES 28.35A AND B (A) Milled lithia disilicate crown in the presintered state can be tried in the mouth. This is possible because there is no shrinkage during the subsequent ceramming process. Note the color change after heat treatment (B). (Courtesy: Dr. Hanan Abuasi, MOH, Kuwait).

FIGURE 28.37 Presintered zirconia blank for multiple units.

FIGURE 28.36 CAD/CAM blanks for glass infiltration method.

FIGURE 28.38 Presintered Zirconia. Blank with bar code is shown on the inset.

Presintered zirconia (Figs. 28.37 and 28.38) Fully dense zirconia is extremely difficult to machine, taking up to two hours just to fabricate a single unit. Therefore, most restorations with zirconia frameworks are fabricated by machining a porous or partially fired block of zirconia known as presintered zirconia. These are usually used as cores for crown or FDPs. In the presintered condition they are usually softer and easier to mill. They are milled to a slightly (20%) larger size, to compensate for the subsequent sintering shrinkage. Following milling they have to be sintered (called post sintering). Sintering is done in a furnace. Sintering time and temperature varies between brands. Sintering time – 6 to 7.5 hours Sintering temperature – 1350 to 1530 °C Because of the high temperatures involved special furnaces are required for zirconia sintering. All grinding and adjustments should be completed prior to sintering. Adjustments following sintering especially in the connector areas weaken the structure. Any adjustments required after postsintering should be done with water cooled, vibration free, fine diamonds. The restoration may be immersed in special coloring liquid to improve the esthetics. The restoration is then built up with compatible veneering ceramics. Uses—core construction for crowns and long span anterior and posterior FDPs.

Sintered zirconia (Figs. 28.39 and 28.40) Since these materials are already fully sintered, post sintering is not required. This material is milled in 1:1 ratio as no shrinkage is expected. Because of its extreme hardness milling takes more time and causes more wear of the milling tool. Subsequent processing includes build up with compatible veneering ceramics. Uses—

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core construction for crowns and long span anterior and posterior FDPs. (Zirconia is described in greater detail in a subsequent section).

Sintering furnaces Furnaces are an important part of CAD/CAM dentistry. A variety of furnaces are available depending on the type of blank used. For example In-Ceram alumina blanks have to be glass infiltrated in a furnace following machining. Leucite or lithia disilicate blanks have to be cerammed to induce partial crystallization. FIGURE 28.39 Sintered zirconia blanks (KAVO). The furnace for the sintering of zirconia is highly specialized as it involves very high temperatures. Zirconia sintering can involve temperatures greater than 1500 °C.

COPY MILLED (CAM) SYSTEMS Some systems use a copy milling technique to produce ceramic cores or substructures for FDPs. In copy milling a wax pattern of the restoration is scanned and a replica is milled out of the ceramic blank (see Table 28.4 for comparison of CAD/CAM and copy milling).

Commercial systems available Examples of commercially available copy-milling systems are 1. Celay (Figs. 28.41A to C) (Mikrona AG, Spreitenbach, Switzerland). 2. Cercon (Degudent, Dentsply). Cercon has both CAD/CAM and copy-milling systems. 3. Ceramill system (Fig. 28.42).

Fabrication of a copy-milled restoration substructure The Cercon system will be described (Figs. 28.43A to I).

A stone die is prepared from the impression of the preparation.

FIGURE 28.40 SEM showing microstructure of unsintered (presintered (left) and sintered (right) In-ceram Zirconia. Sintering fuses the particles to form dense ceramic.

TABLE 28.4 Comparison of CAD/CAM and copy-milling CAD/CAM

Copy-milling

Scans preparation

Scans pattern

Restoration designed virtually

Restoration designed manually

Object milled from virtual pattern

Restoration mills replica of pattern

510 PART 7 Indirect Restorative and Prosthetic Materials

FIGURES 28.41A TO C (A) Celay copy milling system. (B) Celay blanks. (C) Close-up of copying and milling process showing the wax pattern and the milled inlay.

A pattern of the restoration is created using wax. The pattern is fixed on the left side of the milling machine (Cercon Brain). A presintered zirconia blank is attached to the right side (milling section) of the machine. The machine reads the bar code on the blank which contains the enlargement information. On activation the pattern on the left side is scanned (noncontact optical scanning) while the milling tool on the right side mills out the enlarged replica (30% larger) of the pattern from the attached ceramic blank. The milled structure is removed from the machine and sectioned off from the frame. Any remaining attachment stubs are trimmed and final adjustments are made. The zirconia structure is then placed in a sintering furnace (Cercon Heat) and fired for 6 hours at 1350 °C to complete the sintering process. The restoration is completed using compatible veneering porcelains.

Ceramill system Unlike the earlier system, the Ceramill system (Fig. 28.42) is based on the pantograph type of copy milling which, according to the company, “puts the material back in the hands of the technician”. To create a zirconia coping, the user applies a light-cured resin over a traditional die, attaches the resin pattern into a plastic plate and inserts it into the milling unit, side by side with a YtZP zirconia blank. The unit has two conjoined arms that hold the

FIGURE 28.42 Ceramill.

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FIGURES 28.43A TO I Fabrication of a zirconia restoration with the Cercon system. (A) Cercon brain (milling unit). (B) Zirconia blanks. (C) Wax pattern. (D) Blank in position. (E) Milling. (F) Separating. (G) Sintering (cercon heat). (H) A completed substructure. (I) A completed prosthesis.

probe tip and the milling handpiece. The user manually traces the resin buildup with the probe tip while the other arm simultaneously mills a duplicate coping out of the zirconia block.

Networked CAD/CAM production (Procera AllCeram) The Procera system by Nobel Biocare is a unique system where restorations are produced using information sent via internet. In this system impression is sent to a local Nobel licenced laboratory. Here the impression is poured and the conventional die is scanned by a contact scanner [Procera Forte - (Fig. 28.44)]. The coping is designed (CAD) and together with the dimensions of the scanned die, the information is passed via internet to a fully automated industrial scale remote production facility which may be in another country. Here an enlarged die is milled via CAM process. The core is produced by dry pressing on to the die and is followed by sintering. The sintered copings are individually checked for quality control and shipped to the laboratory of origin where the subsequent veneering is completed. Thus in this system the laboratory needs to invest only in the scanner and the CAD software.

FIGURE 28.44 Procera contact scanner. Inset contact probe.

512 PART 7 Indirect Restorative and Prosthetic Materials YTTRIA STABILIZED ZIRCONIA CERAMICS Zirconium is one of the most abundant elements in the earth’s crust. Zirconia is the oxide of zirconium metal (ZrO2). Zirconium oxide is a white crystalline oxide ceramic with unique properties. Its most naturally occurring form is the rare mineral, baddeleyite. A form of cubic zirconia is popularly used as a diamond simulant (Figs. 28.45A to C).

Transformation toughening It has the highest strength among the dental ceramics because of its high degree of crack resistance. This is possible because of a unique property of zirconia to undergo a process known as transformation toughening (Fig. 28.46). The stable form of zirconia is the monoclinic form. When zirconia is heated, it changes to its tetragonal high-temperature phase which again reverts back to the monoclinic form on cooling. However, addition of yttrium oxide (3–5%) also known as yttria maintains the zirconia in its high temperature tetragonal form at room temperature. Thus, this form of zirconia is known as ‘yttria-stabilized zirconia polycrystal’ (Box 28.6). When a stress is applied to the zirconia as in the beginning of a crack formation, it reverts back to its monoclinic form locally with an accompanying increase in volume. The local increase in volume introduces compressive stresses around the crack and slows its growth. This is also known as ‘tension expansion’ - a phenomenon otherwise known only in the case of steel. For this reason zirconium oxide is also known as ‘ceramic steel’. The introduction of zirconia as a core material revolutionized dental ceramics. Its unique transformation toughening process, made it possible to construct relatively long span fixed partial dentures in both anterior and posterior locations. Cubic zirconia

Baddeleyite

Zirconia powder

FIGURES 28.45A TO C Various forms of zirconia.

F I G U R E 2 8 . 4 6 R e p r e s e n t a t i o n o f transformation toughening. The illustration shows the compressive forces around the crack caused by transformation and expansion of the zirconia crystals around the crack (adapted from Vita).

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BOX 28.6 Yttria stabilized zirconia The addition of minor components to the zirconia, such as yttrium, can produce a crystal that is both strong and resistant to crack generation because of the unique “transformation toughening” that occurs when zirconia goes from a tetragonal phase to a monolithic phase. It is this monolithic phase of zirconia that is resistant to breakage when used in full-coverage restorations.

Composition Composition

Wt%

Zirconium dioxide (ZrO2)

90-92

Yttrium oxide (Y2O2)

3-5

Hafnium oxide (HfO2)

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References