1. KENNETH J.ANUSAVICE, PhD, DMD Associate Dean for Research Chair, Department of Dental Biomaterials Director, Center for Dental Biomaterials College of Dentistry University of Florida Gainesville, Florida ELEVENTH EDITION with 572 illustrations Selected artwork by: Jos6dos Santos, Jr. Graphical illustrations by: Chiayi Shen I ELSEVIER I

2. SAUNDERS ELSEVIER 11830 Westline Industrial Drive St. Louis, Missouri 63146 PHILLIPS' SCIENCE OF DENTAL MATERIALS Copyright O 2003, Elsevier.All rights reserved. This edition publishedunder licence by Elsevier Ltd for distributionin the Middle East and Africa O2007 No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permissions may be sought directly from Elsevier's Health Sciences Rights Department in Philadelphia, PA, USA: phone: (+I) 215 239 3804, fax: (+I) 215 239 3805, e-mail: healthpermissions@elsevier.com. You may also complete your request on-line via the ElsevierScience homepage (http://www.elsevier.com), by selecting 'Customer Support' and then 'Obtaining Permissions'. NOTICE Dentistry is an ever-changing field. Standard safety precautions must be followed, but as new research and clinical experience broaden our knowledge, changes in treatment and drug therapy may become necessaryor appropriate. Readers are advised to check the most current product information provided by the manufacturer of each drug to be administered to verify the recommended dose, the method and duration of administration, and contraindications. It is the responsibility of the licensed prescriber, relying on experience and knowledge of the patient, to determine dosages and the best treatment for each individual patient. Neither the publisher nor the editor assumes any liability for any injury and/or damage to persons or property arising from this publication. Previous editions copyrighted 1936, 1991, 1982, 1973, 1967, 1360, 1354, 1346, 1940, 1936 by W.B.Saunders Company Library of Congress Cataloging-in-Publication Data Phillips' science of dental materials / [edited by] Kenneth J.Anusavice; selected artwork by Josedos Santos Jr.- 11thed. p. ; cm. Includes bibliographical references and index. ISBN-13: 978-0-7020-2903-5 ISBN-10: 0-7020-2903-3 1. Dental materials. I. Title:Science of dental materials. 11.Anusavice, Kenneth J. 111. Phillips, Ralph W. [DNLM: 1. Dental Materials. WU 190 P5625 20031 RK652.5.P4352003 617.6'95-dc21 2003045747 Publishing Director: Linda L. Duncan Executive Editor: Penny Rudolph Senior Developmental Editor: Kimberly Alvis Publishing Services Manager: PatriciaTannian Project Manager: Sharon Core11 Designer: Gail Morey Hudson Cover Design: Julia Dummitt Printed in China Last digit is the print number: 9 8 7 6 5 4 3 2 1 3. Contributors Sibel A. Antonson, DDS, PhD Assistant Professor and Director of Dental Biomaterials Department of Restorative Dentistry NOVA Southeastern University College of Dental Medicine Fort Lauderdale,Florida William A. Brantley, PhD Professorand Director,Graduate Program in Dental Materials Science Section of Restorative Dentistry,and Prosthetic Dentistry College of Dentistry The Ohio State University Columbus, Ohio Paul Cascone, PhD Senior Vice President, Technology The Argen Corporation San Diego, California JosephineF. Esquivel-Upshaw, DMD, MS Assistant Professor Department of General Dentistry University of Texas Health Science Center at San Antonio San Antonio, Texas Crayson W. Marshall, Jr., DDS, MPH, PhD Professor and Chair, Division of Biomaterials and Bioengineering Department of Preventive and Restorative Dental Sciences University of California, San Francisco San Francisco, California Barry K. Norling, BSChE, MSChE, PhD Associate Professor Department of Restorative Dentistry University of Texas Health Science Center at San Antonio San Antonio, Texas Rodney D. Phoenix, DDS, MS Associate Professor and Head, Removable Partial Denture Division Department of Prosthodontics The University of Texas Health Science Center at San Antonio San Antonio, Texas H. Ralph Rawls, PhD Professor and Head, Division of Biomaterials Department of Restorative Dentistry University of Texas Health Science Center at San Antonio San Antonio, Texas Chiayi Shen, PhD Associate Professor Department of Dental Biomaterials University of Florida Gainesville,Florida JohnC. Wataha, DMD, PhD Professor Department of Oral Rehabilitation Medical College of Georgia Augusta, Georgia Sally J.Marshall, PhD Professor and Vice Chair for Research Department of Preventive and Restorative Dental Sciences University of California, San Francisco San Francisco, California 4. The eleventh edition of Phillips' Science of Dental Materials is dedicated to the memory of Dr. Harold Stanley who passed away in 2001. Stan's remarkable contributions to the field of dental materials will benefit the dental profession and our dental patient population well into the future. He has served dentistry with the highest level of moral and ethical standards, and he has set the standard for excellence in scholarship for all future dental materials scientists. 5. Preface This book represents a comprehensiveoverview of the composition, biocompatibility, physical properties, mechanical properties, manipula- tive variables, and performance of direct and indirect restorative materials and auxiliary materials used in dentistry. The book is intended as a textbookfor dental students,den- tal hygiene students and practicing hygienists, laboratory technicians, and dental materials scientists. It is also designed as an authoritative reference book for dentists, dental assistants, and corporate marketing staff. Although the sci- entific concepts presented in some chapters are somewhat advanced, the text information in most chapters can be readily understood by individuals with a general college education. The eleventh edition of Phillips' Science of Dental Materials is divided into four sections to reflect the focus of the chapters contained in each part. Part 1,General Classes and Properties of Dental Materials, consists of eight chapters that cover the structure, physical properties, mechanical properties, and biocompatibility of restorative and auxiliary materials used in dentistry. Part 11: Auxiliary Dental Materials, contains five chapters on impression materials, gypsum products, dental waxes, casting invest- ments and procedures, and finishing and pol- ishing abrasives and procedures. Part 111: Direct Restorative Materials, is focused on five areas, bonding, restorative resins, dental cements, dental amalgams, and direct-filling gold. Part 1V Indirect Restorative Materials, consists of five chapters including dental casting and solder- ing alloys, wrought metals, dental ceramics, denture base resins, and dental implants. Direct and indirect materials are used to restore function and/or aesthetics in mouths containing damaged, decayed, or missing teeth by producing the restoration directly within the prepared tooth or a prosthesis indirectly in a dental laboratory before placement in the oral cavity, respectively. The previous 30 chapters have been con- densed into the 23 chapters of the eleventh edi- tion by combining Chapters 3 and 16 into the new Chapter 3, Physical Properties of Dental Materials; Chapters 6, 7, and 8 into the new Chapter 9, Impression Materials; Chapters 22 and 23 into the new Chapter 12, Casting Investments and Procedures; Chapters 24 and 25 into the new Chapter 16, Dental Cements; Chapters 17 and 18 into the new Chapter 17, Dental Amalgams; and Chapters 20 and 27 into the new Chapter 19, Dental Casting and Soldering Alloys. This condensed format places similar topics into one chapter, making it easier to find information on any given topic. Each of the chapters contains an introduc- tory terminology section that is designed to familiarize the reader with key words and defi- nitions and a number of critical thinking ques- tions, which are intended to stimulate thinking and to emphasize important concepts. The answers to these questions are generally found in the section or sections immediately after each question. Although the terminology is associated with generally accepted scientific and dental definitions, it is not intended to be a comprehensivedictionary of all terms used in dental biomaterials science. Several of the chapters represent totally new approaches to the specific subject. Chapter 1 has been revised to provide an introductory overview of the use of dental materials, the his- torical evolution of biomaterials,and standards for safety and quality assurance. Chapters 5, 6, 19, and 20 have been restructured to reflect an updated review of casting and wrought metals. Chapter 7 reflects a new approach on the 6. x Preface science of dental polymers. Chapter 8 is a totally new summary of the basic principles and clinical implications of biocompatibility evaluation. Chapter 9 represents an integration of the three previous chapters on impression materials. Chapter 14 is a new overview of the systems and principles for use of dental adhe- sives. Chapter 15 reflects a more applied review of restorative resins. Chapter 16 on dental cements is an expanded description of cement composition, manipulative characteristics, and clinical performance based on the integration of the previous Chapters 24 and 25. Chapter 21 represents an updated summary of ceramics used for metal-ceramic and all-ceramic prosthe- ses. Finally, Chapter 23 is a new overview of dental implants with an emphasis on implant material and design considerations relative to clinical performance. Aims and Need for This Book The aims of this textbook are: (1) to introduce dental materials science to students with little or no dental background and facilitate their study of physical and chemical properties that are related to selection of these products by the dentist, (2) to describe the basic properties of dental materials that are related either to clini- cal manipulation by dentists and/or dental laboratory technicians, (3) to characterize the durability and aesthetics of dental restorations and prostheses made from the restorative mate- rials, and (4) to identify characteristics of materials that affect their biological safety. It is assumed that the reader possesses an introduc- tory knowledge of physics as well as inorganic and/or organic chemistry. The information in this book is intended to bridge the gap between the knowledge obtained in basic courses in materials science, chemistry,physics,and the dental clinic. As pre- viously noted, a dental technique does not need to be an empirical process, but rather it can be based on sound scientific principles as more information is available from further research. In any basic science,principlesshould be emphasized. The chapters that follow focus more on why the materials react as they do and how the manipulation variables affect their performance in a dental laboratory or dental clinic. One of the differences between a profes- sional and a tradesman is that the former pos- sesses basic knowledge with which he or she can establish conditions for a situation such that a prediction of eventual success of a proj- ect is reasonably ensured. A riveter must be responsible for the joined beams in a bridge, but the engineer is responsiblefor the design of the bridge, especially where the rivets and every truss and beam are to be placed and joined, and for the selection of the materials with which the structure is constructed. If the engi- neer knew nothing about the physical and chemical properties of the steels and other met- als with which the bridge is made, the structure would be more likely to fail. The dentist and the engineer have much in common. Dentists must estimate the stresses present in a dental prosthesis that they will build and be guided by such analyses in the design of the structure. They should possess a sufficient knowledge of the physical properties of the different types of materials that they use so that they can exercise the best judgment pos- sible in their selection. For example,they must know whether the clinical situation requires the use of an amalgam, a resin-based composite, a cement, a casting alloy, a ceramic, or a metal- ceramic. Only if they know the physical and chemical properties of each of these materials are they in a position to make such a judgment. In addition to the mechanical requirements of the materials, there are also certain aesthetic and physiologic requirements that often com- plicate the situation beyond the difficulties usu- ally experienced by the engineer. Once the dentist has selected the type of material to be used, a commercial product must be chosen. It is the intention of major dental manufacturers to cooperate with den- tists in supplying them with materials of the highest quality. The competition is keen, how- ever, and the dentist should be able to evaluate the claims of the respective manufacturersfrom an informed, intelligent perspective.It is unfor- tunate that there are a few unprincipled dental manufacturers who make preposterous claims and who exploit the dentist for their own profit. For the dentists' protection and for the protection of their patients, they must be able to recognize spurious practices of this sort. Courses or lectures in dental materials attempt 7. Preface xi to provide dentists with certain criteria of selection so as to enable them to discriminate between fact and unproven claims. Furthermore, it is hoped that students of dental materials are given an appreciation of the broad scientific scope of their chosen pro- fession. Because a great deal of the daily prac- tice of dentistry involves the selection and use of dental materialsfor patient treatment proce- dures, it is obvious that the science of dental materials is critically important. The advances being made in dental materials science suggest that intriguing changes will continue to occur in the practice of dentistry. Based on your knowledge of materials science principles, you should be prepared to analyze the benefits and limitations of these dental materials to make rational decisions on their selection and use in a clinical practice. Not all the materials used in dentistry are included in this book. For example,anesthetics, medicaments, and therapeutic agents such as fluoride varnish, xylitol, and chlorhexidine are not within the scope of this book. The science of dental materials generally encompasses some of the properties of natural oral tissues (enamel, dentin, cementum, pulp tissue, peri- odontal ligament, and bone) and the synthetic materials that are used for prevention and arrest of dental caries, for periodontal therapy, and for reconstruction of missing,damaged, or unaesthetic oral structures. These categories include materials employed in dental disci- plines such as preventive dentistry, public health dentistry, operative dentistry, oral and maxillofacial surgery, orthodontics, periodon- tology, pediatric dentistry, and prosthodontics. Organization The engineeringcurriculum of most major uni- versities includes the discipline of materialssci- ence.This is concerned with the microstructural features of materials and with the dependence of properties on these internal structures. The sequence of instruction generally progresses from atomic to macroscopic structures, from the simple to the more complex. Knowledge in this field is derived from various disciplines, such as physical chemistry, solid-state physics, polymer science,ceramics,engineering mechan- ics, and metallurgy. Because fundamental prin- ciples of the physical sciences and engineering and microstructure govern the properties of all materials, it is logical to study the microstruc- tural characteristics before proceeding to the macrostructuralfeatures. Following the overview of dental materials (Chapter 1), Part 1focuses on the structure and properties of materials. This importance of relating properties of a material to its atomic or crystalline structure is emphasized in Chapter 2, which deals with the structure of matter and certain principles of materials science that are not usually included in a college physics course. These principles are in turn related to the properties of dental materials, as discussed in Chapters 3 and 4. The requirements placed on dental structures and materials are demand- ing and unique. To design prostheses appro- priately, the dentist must be aware of the limitations of restorative materials and the demanding conditions that exist in the oral cavity. These factors are also discussed in Chapters 3 and 4. One should be increasingly aware of the difficulties involved in selecting a material that is technique insensitive, biocom- patible, and durable. These characteristics are emphasized in the discussions that follow on specific materials. Following the chapters on the structure of matter and the physical and mechanical prop- erties of dental materials are overview chapters dealing with metals and alloys, polymers, and ceramics, and the biocompatibility of dental materials. The basic science of physical metallurgy is concerned with the properties of metals and alloys, whereas the study of metallography involvesthe microstructureof metals that result from their solidification (Chapter 5). The con- stitution of alloys represents the equilibrium phases that result in an alloy system as a func- tion of temperature and composition (Chapter 6). Chapter 7 focuses on dental polymers. It is obvious from the earlier discussion of the regulatoryagencies in dentistry,such as the ADA Council on Scientific Affairs, the FDA, the IS0 and the FDI that the precursor to the marketing or selection of a dental material is its biocompatibility with oral tissues. These biological considerations are covered in Chapter 8 and are noted thereafter throughout the book. 8. xii Preface Chapters 9 through 13 in Part II describe auxiliary materialsand techniques that are used to fabricate and finish the surfaces of dental restorations and prostheses. These materials include impression materials (Chapter 9), gyp- sum products (Chapter lo), dental waxes (Chapter 11), casting investments and proce- dures (Chapter 12), and finishing and polish- ing materials (Chapter 13). The chapters in Part III for direct restorative materials include bonding (Chapter 14), restorative resins (Chapter 15),dental cements (Chapter 16), dental amalgams (Chapter 17), and direct filling gold (Chapter 18). Chapters in Part IV on indirect restorative materials include dental casting and soldering ACKNOWLEDGMENTS The eleventh edition of Phillips' Science of Dental Materials, previously named Skinner's Science of Dental Materials in the ninth and ear- lier editions, has undergone significant changes that are consistent with the rapidly changing trends in the field of dental materials science and the practice of dentistry. Increased empha- sis has been placed on biocompatibility, adhesion, dentin bonding principles, fluoride- releasing materials, resin-based composites, ceramic-based prostheses,dental polymers,and dental implants. Many individualsshould be recognized both for their contributions to the fields of dental materials science and to the revision of this textbook. Foremost is Chiayi Shen of our Department of Dental Biomaterials at the University of Florida. Dr. Shen has made signif- icant recommendations for modifying the for- mat of the eleventh edition by consolidating the 23 chapters into four main sections. He is also one of the main contributors to Chapters 9 and 16. William Brantley also made significant contributions to the revision of Chapters 3, 5, 6, 19, and 20. New chapters were written by Ralph Rawls, John C. Wataha, Barry Norling, and Josephine Esquivel-Upshaw. Much of the new artwork was created by Jose dos Santos, Jr. Other individuals who provided significant input include Michael Bagby, Wulf Bramer, Paul Cascone, Ivar Mjor, and Sam Sarma. alloys (Chapter 19), wrought metals (Chapter 20),dental ceramics (Chapter 21),denture base resins (Chapter 22), and dental implants (Chapter 23). Many branches of science are incorporated in the information presented and various spe- cialized branches of chemistry are applied. Practically all of the engineering applied sci- ences have contributed to the subject. There is also an increasing awareness by the dentist that the biological properties of dental materials cannot be divorced from their mechanical and physical properties. Thus, interwoven through- out the book are discussions of the pertinent biological characteristics to be considered in the selection and use of dental materials. Kenneth J.Anusavice, PhD, D M D I express my appreciation to those who con- tributed to the tenth edition of this textbook, but who were not contributors to the eleventh edition. Several of the revised chapters may contain portions of the sections they created in the last edition. They include Charles F. DeFreest,Jack Ferracane, J. Rodway Mackert, Jr., Miroslav Marek, Victoria A. Marker, Robert Neiman, Karl-JohanSoderholm, and Harold R. Stanley. These individuals provided significant input to the tenth edition in which several sig- nificant changes had been introduced to enhance readability and the clinical perspec- tives of dental biomaterials. In their quest to promote evidence-based dentistry, they blended basic science and applied research findings with manipulative variables to provide improved balance between science and clinical practice. Proofreading assistance was provided by my wife, Sandi, who has supported my academic pursuits in many ways. Her patienceand under- standing during the preparation of the eleventh edition were critically important to its timely completion. I also express my gratitude to those who helped to shape my professional career. These individuals include Robert T. DeHoff, Professor of Materials Science and Engineering at the University of Florida, who guided my PhD training and enhanced my technical writing skills, Robert Kinzer, a former Chairman of 9. ...Preface XIII Restorative Dentistry at the Medical College of Georgia, who encouraged me to pursue dental school training and who supported the devel- opment of my didactic and clinical teaching skills, and Carl W. Fairhurst, former Professor of Restorative Dentistry at the Medical College of Georgia, who provided opportunities to advance my research skills. My research career has advanced more rapidly because of their guidance and support. In addition, financial support during my dental career has been pro- vided by the National Institute of Dental aqd Craniofacial Research of the National Institutes of Health. This support is greatly appreciated. Finally, I would like to thank the staff at Elsevier Inc. for their assistance in organizing and expediting the activities related to publish- ing the eleventh edition. These individuals include Penny Rudolph, Kimberly Alvis, and Courtney Sprehe. Kenneth J.Anusavice, PhD, DMD 10. Contents PART l GENERAL CLASSES AND PROPERTIES OF DENTAL MATERIALS 1 Overview of Materials for Dental Applications, 3 Kenneth I.Anusavice What Are Dental Materialr? 4 Historical Use of Restorative Materials, 6 Standards for Dentdl Materials, 9 ADA Acceptance Program, 9 The ADA Seal of Acceptance 10 Classlflcation of Products Evaluated by the ADA 3 Coun~llon Scientific Affa~rs, 10 General Provislorls for ADA Acceptance 11 Corripo~t~or~N,iturc nntl Furict~on I1 IZeq~~~redIrirorliidllori 11 Infortnatlon IBqulr etl lor liellewal or A L ~cptdntc I L 115 root1 drld Drug Adrrllri~strat~on liegrllnllc~ll~ 13 International 5tantla1ds 14 IS0 Stantlards, Suljcornrnlttees, ailif Work~ng Group' 15 150 Tc.tlililta1C ornrn~ttce106 15 How Are IS0 Sta~ldartlsDevelopcd~ Ib Otkle~Derlt'il Stdllddrtl~Orga~i~/at~ot~s16 IIow Safe Are Uenrnl liesrorarlve Mater 1dl7 18 2 Structure of Matter and Principles of Adhesion, 21 Kenneth J. Anusavice Change of State, 22 Interatomic Primary Bonds, 23 Ionic Bonds, 23 Covalent Bonds, 23 Metallic Bonds, 24 Irltcratoinic Secondary Bonds, 25 Hydrogcri Bonding, 25 Van der Waals Forces, 26 4 Interatorriic Bond Distance and Bonding Energy, 26 Bo~ltlDistance, 26 Boriding Ellergy, 28 Tllerrr~;llErlergy, 28 CryslallilreStructure, 30 Noncryslalline Solids and 'Their Structures, 32 Diffusion, 33 Adhcsioil and Bonding, 34 Mecl-ianicalBonding, 34 Surfare Energy, 35 Wetting, 36 Contact Angle of Wetting, 37 Adhesion to Tooth Strurlure, 39 Physical Properties of Dental Materials, 41 Kenneth /.Anusavice and William A. Brantley What Are Pllysical l'ropcrties 42 Ahrasiorl arltl Ahr;~sio~rResistalrre, 43 Viscosily, 43 Struriural anil Stress Relaxation, 45 Crccp and Flow, 46 Color.and C:olor Perceljtio~i, 46 T1-ireeIlillieri~ionsor Color, 48 , , Iher~riol)l-iysicalProperties, 52 'I'hrrmal Contluctivity, 52 Therinal Diffusivity, 53 Cocfficicilt of Tl~cr~r~alExp;lrlior~, 54 Irilroduction Io Ttrrriisl-1a11t1Corrosioil, 56 Causes or Tarnis11a i d Corrosion, 57 Classification of Corrosior~, 57 Electrocheniical Corrosion, 58 Dissimilar Metals, 61 Helerogeneous Surface Composition, 62 Stress Corrosion, 63 Concentratioil Cell Corrosion, 63 Protection Against Corrosioil, 65 Corrosion of Derital Restorations, 65 Evaluation of Tarnish and Corrosion Resistance, 66 Clirrical Signiricance of Galvarrir Currents, 69 Mechanical Properties of Dental Materials, 73 Kenneth /. Anmavice What Are Mrchanlt a1 Propcrtlch 74 Stre(, dncl Strdms. 75 11. xvi Contents Tcnsilr Stres3, 77 Conlpressive Stress, 77 Shear Stress, 78 Flexural (Bentiitlg) Stress, 78 Mecharlical I'roperties Based on Elastic Dcfor~nation. 79 Elastic Modulus (Youi~g'sModulus or Modulus of Elasticity), 80 Dynamic Young's Modulus, 8 3 Flexibility, 8 3 Kesiliencc, 84 Poisson's Ratio, 8 5 Strerlgth Pro~~erlies,85 Proportional Lirnit, 86 Elastic Limit, 87 Yield Strength (Proof Stress), 87 Pcrinanent (Plastic) Deformation, 8 8 Cold Working (Strain Harderlirlg or Work Hardening), 8 8 Diarrletral Tensile Strength, 8 8 Flexure Strength, 89 Fatigue Strength, 90 Impact Strength, 91 Mechanical Properties of Tooth Structure, 92 Mastication Forces and Stresses, 93 Other Mechanical Properties, 94 Toughness, 94 Fracture Toughness, 94 Brittleness, 94 Ductility ailrl Mallealjility, 95 Measurcrncnt of Dl~ctility, 96 Iiard~lesx, 96 Stress Concerlli.ation Factors, 99 Criteria for Selectiorl of Restorative Malerialx, 100 5 Solidification and Microstructure of Metals, 103 WiIIiam A. Brantley Metals, 104 Mctallic Bontls, 107 Alloys, 107 Solidification of Metals, 108 Nucleus Formation, 110 Solidification Modes and Effects on Properties, 112 Grain Refinement and Grain Size, 116 6 Equilibrium Phases in Cast Alloys, 1 19 William A. Brantley Classification of Alloys, 120 Solid Solutions, 121 Solutes and Solvents, 121 Conditiorls for Solid Solubility, 122 Physical Properties of Solid Solutions, 124 Constitution or Equilit,riui~lPhase Diagrams, 124 Interpretation of the Phase Diagram, 126 Coring, 128 Hoillogerlizatior~, 129 Dcntlrite Pornlation irr Allo>ys, 130 Et~tecticAlloys, 130 Silver-Copper System, 130 Pl~ysicalPropcrties, 134 Perilectic Alloys. 134 Solid State IZeactions, 135 Goltl-Coppcr Systcrn, 135 Silver-Copper Systern, 139 Other Binary Syslerns, 139 Gold Alloys, 139 Pallatliurrt Alloys, 140 Ternary and 1lighcr-Order Alloy Systems, 140 7 Dental Polymers, 143 H. Ralph Rawls Applications of Resins in Dentistry, 144 Classification, 144 Requisites for Dental Resins, 145 Biological Compatibility, 145 Physical Properties, 145 Manipulation, 145 Aesthetic Properties, 146 Economic Considerations, 146 Chenlical Stability, 146 Funtl;~rrrcrltalNaturc of Polyrnrrs, 146 Chain 1.engtI1alltl Molecular Weight, 146 Chain B~.ancllitrgatltl C:ross-I.i~~king,147 Molecular Organi~ation, 1/19 Plrysical Properties of Ir'olyiners, 150 DeTorrr~ationand Rt.covcry, 150 1Zheorne~r.icProperties. 150 --Solvation Properlies, 151 Thcrrnal Properties, 152 Che~r~istryof l'olymcrization. 154 Atltlitiorl Polymcrizatiorr. 155 Stages ill Atlditiori Polyrner.izatiotl, 155 Inhihilio~rof Atltiitiorr Polynlel.i~alion, 160 Step-Growth Polymerization, 161 Copolymerization, 162 - Acrylic Dental Resins, 163 .-Acrylic Resins, 163 Methyl Methacrylate, 165 Poly(methyl Methacrylate), 166 Multifunctional Methacrylate and Acrylatc Resins, 166 8 Biocompatibility of Dental Materials, 171 John C. Wataha Biocompatibility: f-Iistorical Background, 172 Atlverse Effects from Dental Materials, 174 Toxicity, Inflainrnaliot~,Allergy, and Mutagenicity, 174 12. Contents xvii Local and Systerilic Effects of Materials, 177 Key Principles That Deterlllille Adverse Effect> frorn Materials, 178 Biological RCsponse111 the Dcntal Fr~vnol~lilerlt.181 Oral Andtorny T1-1'1t Irlfluencec rhe Biologlcnl R~~IOIISC.181 Special Biological Intcrtacec with Dental Matclials, 185 Osseoir~tegration, 187 The Oral Irnlrluric Systern, 187 Measuring the Uiororrlpatibility of Materials, 188 Dcf'inirig the Use of a Material, 188 Typcs ol'l'ests: Advantages and Disadvalltages, 189 How Tests Are Used Together to Measure Biocompatibility. 190 Standards: Advantages and Disadvarltages, 192 ANSIIAUA Document 41, 193 IS0 Standard 10993, 193 Current Biocompatibility Issues in Dentistry, 194 Latex, 194 Nickel, 196 Beryllium, 197 Mercury and Amalgam, 197 Estrogenicity, 199 Oil-ler Riological Effects of liesins, 200 Clinical C;uiclclines for Selecting Uioco~npatible Materials, 200 Ilefine the Use of the Maret.ial, 200 PART lil AUXILIARY DENTAL MATERIALS 9 Impression Materials, 205 Chiayi Shen Irnpression Materials: Purpose and Requirements, 207 Materials Used for Making Impressions, 208 Sctting Mechanism, 208 Mechanical Properties, 208 Uses of Impression Mater~als, 208 Elastomeric Impression Materials, 210 Characteristics. 210 Viscoelastic Properties, 211 Elastonieric Impression Materials. Chemistry and Composition, 212 Polysulfide, 212 Condensation Silicone, 213 Add~tionSiliconr, 214 E'olyethcr, 216 Elastonleric Materials: Making an Prc~~arationof I~llpressiollMaterials, 216 Impression Trays, 219 Steps liequired to Make an Tlriprcssion, 219 Relrloval of the Irilpressioll, 222 Preparation of Stone Casts and Dies, 222 E1;rstomcric Irnpression Malerials: Prol~erties, 223 Working and Sctting Tiriles, 223 Diliiensional Stability, 224 liel.~oductionof Oral Detail, 225 Disinfection, 225 Rllcological Properties, 226 El . " asilclty, 226 Tear Strerlgtl~, 227 Biocompatibility, 228 Shelf Life, 229 Effect of Mishandling, 230 Hydrocolloids, 231 Sol-Gel Transformation, 234 Gel Strength, 234 Dimensional ETfects, 234 Agar (Reversible) Hydrocolloids, 234 Composition, 235 Manipulation, 235 Preparation and Conditioning of the Agar Material, 236 Tempering of the Material, 237 Makillg the Agar Irrrpression, 237 Acctrr;~cy, 238 Visrosily of thtt Sol, 238 Distctrtiorr Dr~rir~gGclation, 239 Algir~ate(Irreversil~le)Hytlrocolloicls, 239 Co~np(~sition,2/10 Gelatior1 Process, 241 Conlrollir~gSetting Time, 242 Manipulatiorr, 243 Making the Irrlpressiorl, 243 Strerlgth, 244 Acctrracy, 244 Other Applicatiorls and 1 Iandlir~gof Hydrocolloids, 245 Laminate Technique (Alginate-Agar Method), 245 Duplicatirig Materials, 245 Modified Alginates, 245 Biocompatibility, 245 Disinfection, 246 Dirnensional Stability, 246 Corrlpatibility with Gypsurn, 247 Shelf Life, 248 Effects of Mishandling, 248 Iriclastic Inipression Materials, 248 In11xession Compound, 250 Composilion, 250 Manipulation, 250 Dirnensional Stability 251 Disinfection, 251 13. xviii Contents Zinc Oxicte-Eugenol (ZOE) Impression Pasles, 251 Coml~ositio~l,251 Marlipi~laiion, 252 Dirne~lsionalStability, 253 Disinfection, 253 Norleugrnol Pastes, 253 Surgical Pastes, 253 Bite Registratiorl Pastes, 253 10 Gypsum Products, 255 Kenneth J. Anusavice Uses of GYI)SIIIIIill Dentistry, 255 Dental Plaster and Stone. 257 Produciiorl of Calciurrl Sulfate Hemihydrate, 257 Commercial Gypsum Products, 258 Setting of Gypsum Products, 258 Setting Reactions, 259 WIP Ratio, 261 Tests for Working, Setting, and Final Setting Times, 261 Mixing Tirne (MT), 261 Working Time (WT), 261 Setting Tirne (ST), 261 Loss of Gloss 'rest for Initial Set, 261 Initial Gillmore Test for Initial Set, 262 Vic;ri Tesf for Seiting 'I'irnc, 262 Gillmore 'l'esi for Final Scttir~gTirrle, 263 Iieaily-for-Usc Criterior~, 263 Corrtrol of t l ~ cSetting Tirne, 264 T ~ r i ~ ~ ~ ~ i t i c s .261 Firierress. 264 WIP IZatio, 264 Mixing, 264 Tcrnpcraturc, 265 Rctarclc1.s ant1 Acceleraiors, 265 Seiiirlg Expansion, 2615 Corrirol of Setting Expansion, 267 Arcelerators ancl Rctarclcrs: Practice arid Theory, 268 Accelerators, 268 Retarders, 269 EIygroscopic Setting Expansion, 270 Strength, 271 Types of Gypsurn Produris, 273 Inipression Plaster (Type I), 273 Model Plaster (Type II), 274 Dental Stone (Type 111), 274 Dental Stone, High Strength (Type IV), 275 Derrtal Stone, High Strength, High Expalision (Type V), 276 Synthetic Gypsum, 276 Proportio~iing,Mixing, a ~ i dCaring for Gypst1111 Products, 276 Prop(xtion~ng, 276 M ~ x ~ n g ,277 C,~r~ngfor the Cast, 278 Specid Gypsum Products, 278 Caring lor Gypsurrl Ploclucts, 279 I~rle~tror~Control, 280 11 Dental Waxes, 283 Kennelh J. Anusavice Type of lrllay Wax, 284 Composition, 285 Des~rablcP ~ o p e ~ t ~ e s ,286 Flow, 287 ?herrnal PI opcrt~cx, 288 Wmshes,and rnoulh rinses Classification of Products Evaluated by the ADA Council on Scientific Affairs I'roducts that meet the standards of acceptance with respect to safety, efficacy, com- position and labeling, package inserts, advertising, and other promotional material are accepted. Once accepted, the products are listed and may be described in suitable reports and advertisements in TheJournal of the American Dental Associution. The man- ufacturer may then use the Council's Seal of Acceptance and may be required to use an authorized statement if the ADA Seal is used in the advertisement. Products are usually accepted for a period of up to 5 years. Acceptance is renewable and may he reconsidered at any time. If there is a change in the manufacturer or distributor of a product, the period of acceptance expires automatically. PT-ovisionallyaccepted prod- ucts consist of those that lack sufficient evidence to justify classificatiol~as ~~ccept~(l, but for which there is reasonable evidence of safety and usef~~lness,including clirlical feasibility. 'l'hese products meet the other qualifications established by the Council. 30. CHAPTER 1 Overview of Materials for Dental Applications 11 I he Council may authorize the use of a suitable statement to define specifically the area of i~sefulnessof pr-oductsclassified as provisionally accepted. Classification in this category is reviewed each yedr and is not ordinarily continued for mom than 3 years. Products that are obsolete, markedly inferior, ineffective,or dangerous to the health of the user ale declared unat-~eplcd.When it is in the best irlterest of the public or the profession, the Council may submit reports on unaccepted products to the editor for publication in The Iournal of the Ame~zcunDcntal Associalion. Decisions of the Council are based on available scientific evidence and are subject to reconsideration at any time that a sigtiifica~ltamount of new evidence becomes available. CRITICAL QUESTION GENERAL PROVISIONS FOR ADA ACCEPTANCE Composition, Nature, and Function A quantitative statement of composition and adequate information on the propei- ties of all ingredients must be provided to the Council. For instruments and equip- ment, a description of the materials used in the construction and the method of operation must be provided. Any change in the composition, nature, or function of an accepted product must be submitted to the Council for review and approval before a modified product is marketed. I'he company that seeks ADA acceptance should piovide evidence that manufac- turing and laboratory contiol facilities are under the supervision of qualified per- sonnel, that these facilities ale adequate to ensule purity and unifolmity of pioducts, and that p~oductsale ploduced in cornpliallce with the Good Manufactu~ingPractice Code. The company 111ust per-nlit Iepresentatlvcs of the C:ouncil to visit Iabordtories and fevice [lirective that became effective on Janua~y1, 1995 All medical devices marketed in the Luropean Union countries must have the CB mark of con- folmity For certain products, some couiltlies may enforce their own standards when othel countries or the international con~munityhave not developed mutually acceptable requi~ementsFor example, Sweden restricts the use of niclzel in cast den- tal alloys because of biocompatibility concerns, whereas no such restriction applies to those alloys in the United States Iceland, Liechtenstein, and Nolway ale also sig- natories of the European Economic Area Agreement and require the CE ~nalliiilg and NIOM's Notified Body registration numbel on medical device paclzdging An increasing number of universities in the llnited States and abroad have estab- lished lahorato~iesfor research in dental materials. In the past few yeas, this source of bdbi~i11fo11llatio11011 the subjec~has exceeded 111atof all other sources combined. Llntil recently, dental research activities in universities were centered solely in den- tal schools, with most of the investigations being conducted by the dental faculty. Now, however, research in dental materials is also being conducted in some univer- sities that do not have dental schools. This dental-oriented research in areas such as metallurgy, polymer science, materials science, engineering, and ceramics is being conducted in basic science departments. lhese expanding fields of research in den- tal materials illustrate the interdisciplinary aspects of the science. Since the final cri- terion for the success of any material or technique is its service in the mouth of the patient, countless contributions to this field have been made by dental clinicians. The observant clinician contributes invaluable information by his or her keen obser- vations and analyses of failures and successes Accurate record keeping and well- cont~olledpractice procedules fo111ian excellent basis fol valunble clinical research I he impo~tanceof clinical documentation f o ~claims made relative to the in vivo performance of dental malcri,lls is now readily appalent l+ol example, the Acceptance Program of the Councll on Scientific Affairs requires clinical data, when- ever appropriate, to support the lahoratoly tests for-physical properties Lluring the past two decades there has been an escalation in the number of clinical investigations designed to conelate specific properties with clinical perf01rnance criteria. These studies ale designed to establish the p~ecisebehavior of a given material or system. In the chapte~sthat follow, frequent ~eferenceis made to such investigations Another soulce of info~mationis derived from man~~factu~ers'research laborato- lies. The fal-sighted manufactu~errecogniws the value of a resenlch laboratory rel- ative to the development anti p~oduction concl-ol of p~oducts,and unbiased information from such groups is particularly valuable Duling the writing of this textbook, as with the previous edition, the counsel of scientists from dental and noildental industries was called upon. In this way the product formulations described in the succeeding chapters reflect with greater accuracy the commercial materials used by the dentist. This diversity of research activity is resulting in an accelerating growth in the body of knowledge related to dental materials. 1:orexample, in 1978 approximately 10% of all U.S support for dental research was focused on restorative dental materials. The percentage would no doubt be considerably higher if the money spent by indus- try for the development of new materials, instruments, and appliances were included. This growing investigative effort is resulting in a marked increase in the number of new materials, instruments, and techniques being introduced to the pro- fcssion. Fol these and othel leasons, an intimace knowledge of the p~operciesand behavior of dental mnterials is imperative if the modem dental practice is to remain abreast of changing developments. 37. 18 PART I General Classes and Properties of Dental Materials CRITICAL QUESTION HOW15 11 possible for dc~nfalmaterials th'lt have not been acc q3ied by the American Dental Assot ration to Oe sold to cknlists and ~onsurncrsc HOW SAFE ARE DENTAL RESTORATIVE MATERIALS? Specifications and starldards have been developed to aid producers, users, and con- sulnels in the evaluation of the safetyand effectiveness of dental products. TIowever, the decis~onof produce~sto test their materials according to national and interna- tional standards is purely volunta~yThe existence of mate~ialsevaluation standards does not preclude anyone from manufacturing, marketing, buying, ol using dental o~medical devices that do not meet these standards However, producers 01 mar- keters of products and devices are expected to meet the safety standards established for those products in the countries in which they are sold. Thus it is possible for a producer to be given premarket approval by the 1'DAto sell a dental device such as a dental restorative material without the device being approved by the ADA in accor- dance with the specification or Acceptance Program requirements Nevertheless, these agencies are becoming increasingly dependent on one another to ensure that all products marketed wol-ld-wideare safe and effective. No dental device (including restorative materials) is absolutely safe Safety is rela- tive, and the selection and use of dental devices or materials are based on the assump- tion that the benefits of such use far outweigh the known biological rislzs. However, there is always uncertainty over the probability that a patient will experience adverse effects f~orndental t~eatment.I he two main biological effects ale alle~gicancl toxic ~eactionsPa~acelsus(1493-1 5411, a Swiss physician and alchemist, fo~mulatedrevo- lutiona~yp~illciplesthat have lemained nn integral p a t ofthe ctunent field of toxic01 ogy. 1lestated thal "all s ~ ~ b s t ~ ~ n c e sale poisons, thele is none which is not a poison The light dose ciifferentiatesa poison from zr ~emt,cly."(Chllo and Doull, 1991 ) The ninjo~ioutes by which toxic agents ente~the body a1c tll~oughthe gastioin- teslinal tiact (ingestion), lungs (inhalation), skin (topical, pelcutaneous, or delnial) and parenteral routes (Klaassm nnd I:aton, 1911).l+,xposul-eto toxic agents can be sub- divided into acute (less than 24 h ~ ) ,subacute (repeated, 1 month or less), subch~onic (1 lo 3 months), and chronic (longerthan 3 months) ldor Inally toxicagents, the effects o la single exposure are diffe~enlfrom those associated with repeated exposures Like toxicity, chernical allergy may also he dose-dependent, hut it often results from low doses of chemical agents once sensitization has occurred. For a dental restorative material to produce an allergic reaction, most chemical agents or their metabolic prod- ucts function immunologically as haptens and combine with endogenous proteins to form an antigen The synthesis of sufficient numbers of antibodies takes 1 to 2 weeks. A later exposure to the chemical agent can induce an antigen-antibody reaction and clinical signs and symptoms of an allergy. Munksgaard (1992) concluded that occupa- tional risks in dentistry are low and that patient risk for side effects of dental treatment is extremely low. Adverse reactions to dental materials have been reported to occui in only 0.14% of a general patient population (IZallus and Mjor, 1931) and in 0.33% of a prosthetic patient population (Hensten-Pettersen and lacobsen, 1331). Acknowledgment The author expresses appreciation to I)I Wayne Wozniak and Dr. Sharon Stanford of the American llental Association for their helpful suggestions. 38. CHAPTER 1 ill Overview of Materials for Dental Applications 19 SELECTED READINGS Arncric,in Ile~ltalAssociation Sfill I'rogr.im, ALIA website: I lc11ste11-PettcrsenA, ,ind lacohsen N: Perceived side effcc-ts h~~p://~o~w~~~u~lia.oi;y/publir/topil:s/sei~l.l~tml of hioni,iterials in prosthclic dentistry. J I'rosilict Llcrit Coleman RI.: Pliysic,ll I'roperties of Dcn(,il Matcrinls. 65:138, 1991. National K~crcauof Standai-ds Research Paper No. 32. Inter~i,itionalOrganizatiorl for St~~nd.irdiz.ition(ISO) web- W,isliingtoii, DC, (IS (;ovcrnmcnt I'rinling Clflicc, 1328. silc: h~~p://rowuriso.o~;~ '17ris pziblirotion is thefirst rniljoieffort lo ielntr p~rj~sicalpmp- ~rti(?siofdrnlal rr~i~trrialslo rhe cliniccrl si~nulion.l h r Ameririri~ Dc.nl/tl Associi~tiorrspecific-/ition pn{yiom rocrs c~sl/rblishc~rlbrrseil on this hiscorizal ro?vii~urof thc~philosoplry unrZ the crontcJntooj thr fircilily rreiatell at the N(ationa1Bui-en11of Strrnd~rrtls. t'cdcrdl Register: Medical 1)cviccs; tlcnl,il 1)cvicc Classific,ition; Final Rulc and Withdr,iwal of Proposed Kulcs. Auyst 12, 1987, p 30082. A listing of the ~lerrtalnratvi-ials anrl /le~~icilsi-l/rssifiijd in (illegory 111 by 111~.Fooil ar~iiL)i-r~gA~Zn~inislnrtionas 01lhirl dale. Intcrnatio~ial O~-g~~niz,ilionfor Standardization (ISO) 'I'(: 106-l)cn tistry website: htt/>://w~u~i~.iso.oi;y/iso/en/ stilsrle~~elopmc~r~~/~i~c~i~prog/worlrpr-og/'~e~-~r~~ira~P~-o I>c?~aill'irgc~.'IkchrricalPi~~~r~~~rrrn~~'ICII~~Iiril!C;OMMIIl=291 6 Irgc v~ri~ilionsalong molecules inrlucc polar for( es that ,Ittr,lc t olhcr molecules. (Courtesy oi' I inter,lc[ion between molecules. (Coul-tesyof K-J. Siiclerholm.) 44. 26 PART I General Classes and Properties of Dental Materials efficiently by the electrons, and the proton s ~ d eof the watel n~oleculeis positively charged On the opposite side of the water molecule, the elect1ons that fill the outer orbit of the oxygen dtoin piovide '1 negative charge 111~1sa permanent dipole exists that repiesents an asyrnrnetiic ~noleculeThe hydrogen bond, which is associated with the positive charge of hydrogen caused by poldii~ation,is an important exam- ple of this type of secondary bonding When a watel molecule intermingles with other watel molecules, the hydrogen (positive) portion of one molt~culeis attiacted to the oxygen (negative) portion of its neighbo~~ngmolecule and hydrogen b~idgesare foiined Polarity of t h ~ snature is Inll.>oitautin accountirlg lo1 the ~nterrnoleculnlreactions in many organic com- pounds, such as the sorption of water by synthetic dental iesins Van der Waals Forces Van der Waals forces form the basis of a dipole attraction (Fig. 2-5).For example, in a symmetric molecule, such as an inert gas, the electron field constantly fluctu- ates. Normally, the electrons of the atoms are distributed equally around the nucleus and produce an electrostatic field around the atom. However, this field may fluctuate so that its charge becomes momentarily positive and negative, as shown in Figure 2-5. A fluctuating dipole is thus created that will attract other similar dipoles Such interatomic forces are quite weak. INTERATOMIC BOND DISTANCE AND BONDlNG ENERGY Bond Distance Regardless of the type of matter, thetc 1s a limiting fact01t h ~ tpievents the atoms 01 molecules from approachnig each other too closely This factoi 1s the d ~ s t ~ ~ n c e between the center of '111 atom anci lbal ol nelghbol, which 1s I~mittxlby the d~nmeteiof the atoms rnvolved Although the ,toin is lledled as n disc~etepa~ticle w~thbounda~lesand volume, its bounda~~esale estahl~shedby the elect~ostat~c f~eldsof the electrons If the atoms approach too closely, [hey ale lepelled fiom each otlle~by the11elect~oncharges 0 1 1 the other hnnd, fo~cesolall~act~ontend to dlaw the dtoms togethe1 I lie p o s ~ t ~ o ~ iat wh~chthese fo~cesof ~cpul~ondlld dttiaction heconle equdl 111 inagn~tude(hut opposlte In d ~ ~ e c t ~ o n )1s the eclurl~b~iunlposltlon oi the ntoms shown In I Igure 2-6 In thr pos~tion,the ~epell~ngfolccs are equal rn ~nagnitudeto the attlactlng fo~cesAtom 5 c,tn he d~splacedto p o~ t ~ o nB' by a dls tu~brngn~echan~cal,theimal, ol electi~calfo~ceA foice may also cause the atoms to move more closely together (pos~t~onB" In Ilg 2-6) As the forces of attraction Fig. 2-5 Flu~tuat~ngd~polethat b1nd5Inert gas rnolecules together The arrows show how [hc iieltls may flu( t~~,~tt>so [tiat the cllargcs hccomc momentarily positivc ant1 negative. (Courtesy of I1,liLic.c types of tlrnhl interr5t. A, Rhoml~oheclral.B, Orthorlloml~ic. C, Monoc linic. D, Iriclillic.. E, T(>II-agondl.F, Sirnplc hexagon.~l.G, ('loicl-patkc.d Ii(~x~lgon~I. H, IKIholrll~ic.fio~tl-te5yof /te~-4), with viscosity ttie analog of the elas- tic lnodul~ls(elas~icstress divideti by elastic strain). Viscosity is me,isured in units of MPa per second, or centipoise (cl'). Pure water at 20" C has a viscosity of 1.0 cP, whereas the viscosity of ~nol;lsscsis appi-oximately300,000 cl? 'I'his value is similar to that of tempered agar llyclrocolloid impression material (281,000 cl' '11 45" (:). Of the el as tome^-ic impression materials, light-body polysulfide has viscosity of 109,000 cl' compared with a value of 1,360,000 cl' fol- heavy-body polysulfide at 36" (;. Many dental materials exhibit pscudoplasfic behavior, as i1luslr;lted by (he change in slope of thc plot in I:ipire 3-2. 'l'heir viscosity decrt-.aseswith increcrsing strain rate ~lntilit reaches a neal-ly constant value. Licluids that show the opposite tendency are describecl as rlilntanr. 'Jhese liquitis become more rigid '1s the rate of deformation (sheclrstrain rate) increases. Finally, some classes of materials behave like a rigid body until some minirnum value of shear stress is reached. This is represented by the offset along the shear stress axis.These fluids, which exhibit rigid behavior initially and then attain constant vis- cosity, are referred to as plastic. Ketchup is a familiar example-a sharp blow to the bottle is usually required to produce an initial flow. 'l'he viscosity of most liquids decreases rapidly with increasing temperature. Viscosity may also depend on previous deformation of the liquid. A liquid of this type that becomes less viscous and more fluid under repeated applications of pres- sure is referred to as thixotropic. Dental prophylaxis pastes, plaster of I'aris, resin cements, and some impression materials are thixotropic. 'l'he thixotropic nature of impression materials is beneficial because the material does not flow out of a mandibular impression tray until placed over dental tissues, '111d 'I p~-opIiylaxispC1ste does not flow oul of a rubber cup until it is rotated against the teeth to bt. cleaned. If these materials are stirred rapiclly and the viscosity is measured, a value is obtained that is lower tlian tllc value for a sample that has been left undisturbed. The viscosity of a dental materi'll may deterrninc its suitability for '1 given appli- cation. Likewise, the nature of zlle sheal-stress versus shear stl-ainrate curve can he iluporta~tin dcterrnining the best way to manipulnte a material. As explained in liiore delail latel; the viscosity as a f~~nctionof time can also be used to measure the wol-kingtime of a material that undergoes a liquid-to-solid transformation. STRUCTURAL AND STRESS RELAXATION Aftel a sctbstance has been permanently defo~med(plastic defolmation), here are tlapped internal stresses. Tor example, in a crystalline substance such as a metal, the atoms in the crystal structure are displaced, and the system is not in equilibrium. Similarly, in amorphous structures, some molecules are too close together and others too far apart when the substance is permanently deformed. It is understandable that such situations are unstable. lhe displaced atoms are not in equilibrium positions I'hrough a solici-state diff~~sionprocess driven by ther- mal energy, the atoms can move back slowly to their equilibrium positions 'l'he result is a change in the shape or contour of the solid as the atoms or molecules change positions. The material wales or dzstorts. 'This slress reluxutzon leads to distor- tion of elastorneric impressions. I-he rate of relaxation increases wilh an increase in temperature. kor example, if a wire is bent, it may tend to straighten out if it is heated to a high temperatule. At room temperature, m y suclr relaxation caused by rearlangenlent of metal atoms may be negligible On the other hand, there are many nonciystalline dental n1'1te~i- als (such as waxes, resins, and gels) that, when manipu1,lted and cooled, can then 64. 46 PART I Fd General Classes and Propertiesof Dental Materials undergo relaxation (distortion) at an elevated temperature. Considerable attention is given to this phenomenon in succeeding chapters, because such dimensional changes by relaxation may result in an inaccurate fit of dental appliances CREEP AND FLOW If a metal is held at a temperature near its melting point and is subjected to a con- stant applied stress, the ~esultingstrain will increase ovel time. Creep is defined as the time-dependent plastic strain of a material under a static load ol constant stless. The related phenomenon of sag occurs in the permanent deformation of long-span metal bridge structures at porcelain-firing temperatules under the influence of the mass of the prosthesis. Tor a given thiclzness, a greater bridge rnass is related to greatel flexulal stress and, thus, greater flexurul ~reep.Metal cleep usually occurs as the temperature increases to within a few hundred degrees of the melting range. Metals used in dentistry for cast restorations or substrates for porcelain veneers have melting points that are much higher than mouth temperatures, and they are not sus- ceptible to creep deformation intraorally. IIowever, some alloys used for metal- ceramic prostheses can creep at porcelain veneering temperatures. This phenomenon will be discussed further in Chapter 21 Dental amalgams contain from 42 to 52 wtOhHg and begin melting at tempera- tures only slightly above room temperature. ('Fhe melting range of an alloy is dis- cussed in Chapter 6.) Because of its low melting range, dental amalgam can slowly creep from a restored tooth site under periodic sustained stress, such as would be imposed by patients who clench their teeth. Because creep produces continuing plastic deformation, the process can be destructive to a dental prosthesis 'I'he rela- tionship of this ploperty to the behavior of the amalgam restoration is discussed in Chapter 17 A creep test is required in American National Standalds Tnstitute/Ameiican Dental Association Specification No 1 and Addentturn l a for dental dmalgarn ploducts The telm flow, rathe1 than creep, has generally been used in dentistry to describe the rheology of amo~phousmate~ialssuch as waxes The flow of wax is a measule of its potential to deform unde~a small static load, even that associated with its own mass Although creep or flow may be measured unde~any type of stress, compres- sion is usually employed in the testing of dental mate~ials.A cylindel of prescribed dimensions is subjected to a given compressive stress for a specified lime and tern- peratu~e'lhe creep or flow is measured as the percentage decrease in length that occurs under these testing conditions. Creep may cause unacceptable defo~mation of dental restorations (such as low-copper dental amalgam) made from a material that is used clinically at a temperature near its melting point for an extended period. Creep may also lead to an unacceptable fit of fixed partial denture frameworks when a cast alloy with poor creep (sag) resistance is veneered with porcelain at relatively high temperatures (N1000" C). COLOR AND COLOR PERCEPTION The preceding sections have focused on those properties that are necessary to per- mit a material to restore the function of damaged or missing natural tissues. Another important goal of dentistry is to restore the color and appearance of natural dentition. Aesthetic considerations in restorative and prosthetic dentistry have received greater emphasis over the past several decades. The search for an ideal, gen- eral pulpose, technique-insensitive, direct-filling, tooth-colored restorative material is one of the continuing challenges of current dental materials research. 65. CHAPTER 3 Physical Properties of Dental Materials 47 Since aesthetic dentistry imposes severe dernd~ldson the artistic abilities of the dentist nnd technician, Iznowledge of the ~~nderlyingscientific principles of color is esseritial 'I'his is especially true for the increasingly popular resto~dtionsthat involve ceramic materials (see Chapter 21). A more colnprehensive treatment of this subject call be found in otllei texts (see the Selected Readings list at the end of this chapter). Light is elec~romdgnetic~ddiationthat can be detected by the llumn~leye. file eye is sensitive to wavelengths fiom approximately 400 11111(violet) 10 700 nm (dark red), as shown in the color version of I'igu~e3-3 (see also color plates). I he reflected light intensity and the combined intensities of the wavelengths present in incident and reflected light deternl~nethe appearance p~operties(hue, value, and chrorna). Pol an object to be visible, it Inlist ieflect or transmit light irlciclent on 11 fiom an external source. 'Jhe incident light is usually polychromalic, that is, a rnixtu~eof the va~iouswavelengths. Incident light is selectively absorbed o~scattered (01 both) at certain wavelengths. I h e spectral distribution of the transmitted or reflected light resembles that of the incident light, although certain wavelengths are reduced in magnitude. l h e phenomenon of vision, and certain related terminology, can be illustrated by considering the response of the human eye to light reflected from an object Light from an object that is incident on the eye is focused in the retina and is converted into nerve impulses that are transmitted to the brain Cone-shaped cells in the retina are responsible for color vision. 'l'hese cells have a threshold intensity required for color vision and also exhibit a response curve related to the wavelength of the inci- dent light. I igure 3-4 illustrates such curves for individuals with normal color vision and for individuals with color-deficient vision The normal observer curve shown in kigure 3-4 indicates the human visual iesponsiveness to light reflected or emitted from a particular source o~object. 1his figtue indicates that the eye is most sensitive Fig. 3-3 Spectrum oi'visiblc light ranging in w,~vclcngthfrom 400 nm (violct) to 700 nm (rctl). Thc most visually pcrccplil~leregion of the rqu,ll rnergy spc,c.lrLlm under ticjylighlcontiition5 is bc,twern w,~velengths of 540 and 570 nm, with a ~n,rximumv,~lueof visu,ll prrc:eptibilily at 555 nm (set: Fig. 3-41. See ~ l s o colol- platr. 66. 48 PART I BE4 General Classes and Properties of Dental Materials Wavelength (nm) Fig. 3-4 Relative visual response of humans to wavelength of light for a normal observer and one with protanopia (reti-green) color blindness. Protanopia is experienced by 1'%of tlie malc population and 0.02% oi the (ernale population. to light in the green-yellow region (wavelength of 550 nm) and least sensitive at the red or blue regions of the color spectrum. Because a neural response is ~nvolvedin color vision, constant stimc~lationby a single color rnay result in color fatigue and a decrease in the eye's response The sig- nals Siorn the retina ale piocessed by the blain lo produce the psychophysiological perception of coloi Defects 111 ce~tdi~lportions of the color sensing receptors result in the diSSerent types of colo~blindness, , ~ n dthus, human observe~svary greatly in their ability to distinguish colols. 111 a scientific sense, one might lilten the normal human eye to an exceptionally sensitive differential colo~imeter,'I scientific ins1111- ment that measures the intensity and wavelength of light. Although the colorimete~ is more p~ecisethan the human eye in measuring slight diffeiences in colored objects, it can he extremely irlaccurate when used on rough or cu~vedsulfaces The eye is able to differentiate between two colors seen side by side on smooth o~irreg- ula~surfaces, whether curved or flat. CRlTlCAL QUESTION Why do some tooth-colored restorations appear to be missing when viewed under "disco" Three Dimensions of Color Verbal descriptions of color are not precise enough to describe the appearance of teeth. For example, to describe a brownish-purple color called puce, Webster's 'l'hild New International Dictionary defines the word as "a dark red that is yellower and less strong than cranberry, paler and slightly yellower than average garnet, bluer, less strong, and slightly lightel than pomegranate, and bluer and paler than average wine." This definition is Sar too complex and imprecise to describe a desired coloi of a dental crown to a laboratory technician. Such a written description does not 67. CHAPTER 3 H Physical Properties of Dental Materials 49 clearly and unambiguously allow one to perceive the color. Three variables must be measured to accurately describe our perception of light reflected from a tooth or restoration surface: hue, value, and chroma. Hue describes the dominant color of an object, for example, red, green, or blue. This refers to the dominant wavelengths present in the spectral distribution. Tlle continuum of these hues creates the color solid shown in I:igure 3-5 (see also color plates). Value increases toward the top (whiter) and decreases toward the bottom (darker or more black). 'lteth and othel- objects can be separated into lighter shades (higher value) and darker shades (lower value). For example, the yellow of a lemon is lighter than is the red of a cherry. I:or a light-diff~isingand light-reflecting object such as a tooth or dental crown, vt~lz~eiden- tifies the lightness or darkness of '3 color, which can be measured independently of the hue. Figure 3-6 (see also color plates) repr-esentsa horizontal plane though the color solid in Figure 3-5. This color chart is based on the CIB L*a*b*color space in which L* represents the value of an object, a* is the measurement along the red-green axis, and b* is the measurement along the yellow-blue axis. The color of a red apple is shown by the letter A in the upper and lower charts. Its color appearance can be Fig. 3-5 Color soli(l t h ~ tis 11sc.d to dcscrihc tti(x three tliniensioris of c:olor. V,llue increases irom hl~clc'11 Ihc bollorn ccnlt'r lo whilc , ~ tthe top center. (:lirom,~ in( rr,lsri fl-orn tlie center ouiwa~.tl,anti hue ch;tngcs occur in ,i c-ircumfc,~-cnlial tlircc lion. See ,rlso ~ o l o rplate. (Co~rrtesy of Minolta Corporc~lion,Inslrumcnl Syslcrns Division, RCrmsey,NI.) (Yellow) I b* fin (Yellow) +b* Chroma C' 6 0 (Blue) Fig. 3-6 I.*a*b* color chart showing thc color of a red apple at point A (top and h o ~ ~ o m ) .For this chart, the appearance is expr-esscdby L* (v,iluc) = 42.83; a" (red-green axis) = 45.04; dntl I-," (yellow-blue ,xis) = 9.52. In contrast, the color ol' shade A2 porc:rl,~inc ,n he described by 1.j = 72.99; a X= 1.00; and 13%= 14.41. Sec also colol pl,ile. (Courtesy of Minolta Corporation, lristrurnent Syslems Division, Ramscy, NI.) 68. 50 PART I General Classes and Properties of Dental Materials expressed by I,* = 42.83, a* = 45.04, and b* = 3.52. In comparison, a dental body (gingival) porcelain of shade A2 can be described by a higher (lighter) I,* of 72.99, a lower a* ol1.00, and a higher h* of 14.41. The yellow color of a lemon is more "vivid" than that of' a banana, which is a "dull" yellow. 'l'his is a difference in the color intensity. Chroma represents this degree o lsaturation of a particular hue. Justas value varies vertically, chroma varies radially (see Fig. 3-6, bouom). Colors in the center are dull (gray). In other- words, the higher the chroma, the more intense the color. Chroma is not considered sepa- rately in dentistry. It is always associated with hue anct value of dental tissues, restorations, and prostheses. In a similar manner-, the adjustments on a color televi- sion set make use of these hue, value, and chroma principles. In the dental oper-atoiyor laboratoiy, color matching is usually performed by the use of a shade guide such as the ceramic shade glide shown in Figure 3-7 (see also color plates) to select the color of ceramic veneers, inlays, or crowns to be made by a laboratory technician. The neck region of these shade tabs has been removed because its shade is darker and its presence would complicate the matching of the correct shade. Llnfortunately, although a reasonable match can be achieved between a tooth (or restoration) and one of the shade guide tabs, it is difficult to describe this information to a laboratory technician who may not have a chance to see the patient. Furthermore, the thickness of the shade tab may be quite different from that of the prosthesis to be made, and the shade of one porcelain crown may look dif- ferent from that of another crown made from the same batch of porcelain powder. Also, the porcelain of a given shade made by the same manufacturer may valy from batch to batch. Thus the challenges are formidable for the dentist and technician who work as a team to restore the proper appeararlce o i teeth that ar-e damaged, decayed, or defective. The sl~adeguide shown in Iiigure 3-7 was specially prepared by grinding away the necks of the porcelain labs, because the cor-rectshade is determined from the gingi- val half of the lab and not irom the neck. 'l'hese tabs are usccl in much the same way as paint chips are used to match the color o l house paint. Using these shade tahs, one can specify the colol-characteristics (hue, value, chroma) and tr-anslucency LO the technician who will produce the proper appearance in the laboratory. The tooth- shaped tabs in Figure 3-7 have been arranged in decreasing order of value (lightesl Fig. 3-7 Dental sh,ltlc gulcle tab5 of IheV~taI umln type arr,~iigcxclIn tlccressing ortltv of value (lighter lo tlarker) Thc nccks ol the loot11 slidpr,d tabs have bccn gro~~litiaway to f ~ c ~ l ~ ~ a t etlie selccl~ono i loolh sli,idcs See po1t1o11