Orthodontic Materials E-Book

Wiliam A. Brantley

• Herausgeber: Thieme
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 685

Veröffentlichungsjahr: 2000

Beschreibung

In this important new textbook all scientific and clinical aspects of orthodontic materials are described. Recent developments in science and technology have led to the introduction of a plethora of new orthodontic products. This work serves as an excellent source of information for a field that requires knowledge of basic elements of materials science, engineering, chemistry, and physics, as well as clinical orthodontics. The subject has been part of graduate orthodontic education for almost three decades. Besides servicing the orthodontic training programs, the book also: investigates the interactions of orthodontic materials with other dental materials as well as hard tissues in the oral cavity, gives a background to allow for proper material selection for efficient orthodontic mechanics, and treats the issues of biocompatibility, cytotoxicity and mutagenicity of materials.

Leseprobe

Library of Congress Cataloging-in-Publication Data

Orthodontic materials: scientific and clinical aspects/[edited by ] William A. Brantley, Theodore Eliades; foreword by T. M. Graber.

p.; cm.

Includes bibliographical references and

index.

ISBN 3131252812 (GTV)-

ISBN 0-86577-929-5 (TNY)

1. Orthodontic appliances. 2. Dental bonding.

3. Orthodontics. I. Brantley, William A.

II. Eliades, Theodore

[DNLM: 1. Orthodontic Appliances. 2. Dental

Bonding. 3. Dental

Materials-chemistry. WU 426 O77 2000]

RK527.O78 2000

617.6’43-dc21

00-05519

Any reference to or mention of manufacturers or specific brand names should not be interpreted as an endorsement or advertisement for any company or product.

Some of the product names, patents, and registered designs referred to in this book are in fact registered trademarks or proprietary names even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain.

This book, including all parts thereof, is legally protected by copyright. Any use, exploitation, or commercialization outside the narrow limits set by copyright legislation, without the publisher's consent, is illegal and liable to prosecution. This applies in particular to photo-stat reproduction, copying, mimeographing or duplication of any kind, translating, preparation of microfilms, and electronic data processing and storage.

Important Note: Medicine is an everchanging science undergoing continual development. Research and clinical experience are continually expanding our knowledge, in particular our knowledge of proper treatment and drug therapy. Insofar as this book mentions any dosage or application, readers may rest assured that the authors, editors, and publishers have made every effort to ensure that such references are in accordance with the state of knowledge at the time of production of the book.

Nevertheless, this does not involve, imply, or express any guarantee or responsibility on the part of the publishers in respect to any dosage instructions and forms of application stated in the book. Every user is requested to examine carefully the manufacturer's leaflets accompanying each drug and to check, if necessary in consultation with a physician or specialist, whether the dosage schedules mentioned therein or the contraindications stated by the manufacturers differ from the statements made in the present book. Such examination is particularly important with drugs that are either rarely used or have been newly released on the market. Every dosage schedule or every form of application used is entirely at the user's own risk and responsibility. The authors and publishers request every user to report to the publishers any discrepancies or inaccuracies noticed.

© 2001 Georg Thieme Verlag,Rüdigerstrasse 14,D-70469 Stuttgart, GermanyThieme New York, 333 Seventh Avenue,New York, NY 10001, USA

Typesetting by Mitterweger & PartnerKommunikationsgesellschaft mbH, Plankstadt

Printed in Germany by Aprinta, Wemding

ISBN 3-13-125281-2 (GTV)ISBN 0-86577-929-5 (TNY)     1 2 3 4 5 6

To Drs. M. Max Sharpe and the late Dale B. Wade:they showed the way, laid out the route, shared the thrill (T.E.)

To Vivian and my parents. (W.A.B.)

Contributors’ Addresses

George Eliades, D.D.S., Dr. Dent.Associate ProfessorDepartment of Dental MaterialsSchool of DentistryUniversity of AthensAthens, Greece

Margrét Rósa Grímsdóttir, D.D.S., M.S.,Dr. Odont.Private Practice in Orthodontics and PediatricDentistryDomus MedicaReykjavik, Iceland

Arne Hensten-Pettersen, D.D.S., M.S.,Dr. Odont., Dr. Odont. H.C. (Lund)Director, NIOM - Scandinavian Instituteof Dental MaterialsHaslum, Norway

Nils Jacobsen, D.D.S., M.S., Dr. Odont.ProfessorInstitute of Clinical DentistryFaculty of DentistryUniversity of OsloOslo, Norway

William M. Johnston, Ph.D.ProfessorSection of Restorative Dentistry,Prosthodontics and EndodonticsCollege of DentistryThe Ohio State UniversityColumbus, OH, USA

Aphrodite Kakaboura, D.D.S., Dr. Dent.Assistant ProfessorDepartment of Operative DentistrySchool of DentistryUniversity of Athens Athens,Greece

Alan S. Litsky, M.D., Sc.D.Associate ProfessorBiomedical Engineering Center andDepartment of OrthopaedicsCollege of Engineering and College ofMedicine and Public HealthThe Ohio State UniversityColumbus, OH, USA

Marion L. Messersmith, D.D.S., M.S.D.Associate Professor, Chair and Program DirectorDepartment of OrthodonticsUniversity of Texas - Houston Health ScienceCenter, Dental BranchHouston, TX, USA

Bjørn Øgaard, D.D.S., Dr. Odont.ProfessorDepartment of OrthodonticsInstitute of Clinical DentistryFaculty of DentistryUniversity of OsloOslo, Norway

Efstratios Papazoglou, D.D.S., M.S., Ph.D.Formerly Assistant ProfessorSection of Restorative Dentistry,Prosthodontics and EndodonticsCollege of DentistryThe Ohio State UniversityColumbus, OH, USA

John M. Powers, Ph.D.Professor and Vice ChairDepartment of Restorative Dentistryand BiomaterialsDirector, Houston Biomaterials Research CenterUniversity of Texas - Houston Health ScienceCenter, Dental BranchHouston, TX, USA

George Vougiouklakis, D.D.S., M.S., Dr. Dent.Professor and HeadDepartment of Operative DentistrySchool of DentistryUniversity of AthensAthens, Greece

John C. Wataha, D.M.D., Ph.D.ProfessorDepartment of Oral RehabilitationSchool of DentistryMedical College of GeorgiaAugusta, GA, USA

David C. Watts, Ph.D.University Reader in Biomaterials ScienceHead, Biomaterials Science UnitTurner Dental School, University of ManchesterManchester, UK

Foreword

In this fast-changing world of orthodontic materia technica, the life cycle of many materials can be amazingly short. Too often we hear, “Oh yes, we used to use that!“The surge in improved bonding materials and the techniques, particularly, has been due to concentrated research, as have the new generation of archwires, brackets, and elastomeric ligatures. Even cements and impression materials have been subjected to searching analysis to improve their most desirable qualities.

This current compendium on the scientific and clinical aspects of orthodontic materials is edited by William Brantley and Theodore Eliades, with a supporting cast of international chapter authors.

Chapter 1 on structures and properties of orthodontic materials by William Brantley not only reports his ongoing research, but definitive work of other material scientists. Atomic and interatomic arrangements in metallic, ceramic, and polymeric materials are presented in a concise and understandable fashion.

William Brantley teams up with co-editor Theodore Eliades and Alan Litsky on Mechanics and Mechanical Testing of Orthodontic Materials for Chapter 2. Bending and torsional deformation, so important to orthodontic manipulation, fatigue properties, and bond strength tests are part of a potpourri of materia technica covered. This is again quite readable for the average clinician and has direct clinical practice application.

Chapter 3, by George Eliades and William Brantley, addresses instrumental techniques for the study of orthodontic materials. This is a “first“ in any orthodontic text. Scientific principles and experimental procedures are presented for a variety of laboratory analyses.

Chapter 4 on orthodontic wires and their alloys is a major assignment, and William Brantley accepts the daunting challenge.

Enamel etching and bond strength are of course vital to successful, non-iatrogenic bonding of attachments and are authored by John Powers and Marion Messersmith in Chapter 5.

Chapter 6 by Bjørn Øgaard covers the role of oral microbiota, the oral microbiological changes, long-term enamel alterations due to decalcification and caries prophylactic aspects. State of the art diagnostic methods are presented.

Orthodontic brackets coverage is a major challenge, because of the multiplicity of bracket designs and brackets, and the frequent advances being made for easier, safer bonding. This challenge is accepted in Chapter 7 by a threesome-Theodore and George Eliades and William Brantley.

Chapter 8 on elastomeric ligatures is again multi-authored. Recent evidence of structural alterations during use is discussed by Theodore and George Eliades, David Watts and William Brantley.

Chapter 9 on orthodontic adhesive resins and composites and principles of adhesion is authored by David Watts.

Closely related to Chapter 9 is Chapter 10, which presents chemically cured, light-cured, and dual-cured adhesive systems, together with recently introduced moisture-active adhesives. Again it is authored by the Eliades team.

Chapter 11 is closely allied to the two previous chapters, and covers traditional cements used in bonding, as well as compomers and glass-ionomer cements. It is authored by Aphrodite Kakaboura and George Vougiouklakis.

In Chapter 12 impression materials are covered from an orthodontic perspective by William Johnston. The emphasis is on alginates.

Chapter 13 discusses bonding to non-conventional surfaces (e.g., amalgam, gold, porcelain). It is authored by Efstratios Papazoglou.

Vital to all oral materia technica are biocompatibility and cytotoxicity of the materials used. Chapter 14 by John Wataha addresses this major challenge.

The final chapter in the book, Chapter 15, is closely related to Chapter 14 and is badly needed in this litigious world. This again is presented by an international team, Arne Hensten-Pettersen, Nils Jacobsen, and Margrét Rósa Grímsdóttir on a vital subject-allergic reactions and safety concerns.

A convenient appendix on the names and addresses of orthodontic materials’ manufacturers makes it easy for the reader to make immediate use of the research studies reported.

It is a pleasure to write a preface for another Thieme book. Thieme means quality personified – and is a role model for orthodontic publishers. Rakosi, Jonas, and Graber's Atlas on Orthodontic Diagnosis, published by Thieme, is available in six languages. I anticipate similar success for this opus.

T. M. Graber, DMD, MSD, PhD,Odont. Dr. DSc. ScD, MD, FRCSProfessorUniversity of IllinoisCollege of Dentistry

Preface

Seminars or courses on clinically relevant materials have been a component of graduate orthodontic education for several decades. Originally, students received instruction in the scientific principles and proper manipulation for the relatively small number of dental materials used in clinical practice. Major subject areas were archwires and stainless steel brackets, elastics and elastomerics, alginate impression material, zinc phosphate cement, and acid-etching of enamel and adhesive bonding of brackets. Technological developments over the past two decades have led to the introduction of a plethora of new orthodontic products at a dramatically increasing rate, necessitating more comprehensive training in the field of dental materials science. The further progression of materials development has resulted in additional new products that are significant advances from their predecessors (e.g., Copper Ni-Ti archwires and compomer adhesives). Accordingly, the teaching format for this discipline has had to expand to encompass a much wider variety of materials.

While the educational format in the United States varies substantially among teaching philosophies, the graduate orthodontic curriculum typically contains one to two credit hours devoted exclusively to orthodontic materials over a single quarter or semester. This level of educational experience is mandated by the Accreditation Standards that have been developed by the American Dental Association for Advanced Specialty Education Programs in Orthodontics and Dentofacial Orthopedics. The seminar or course in orthodontic materials is generally complemented by courses in the biomechanics of tooth movement, and is taken by graduate students during their first or second year of specialty trainig.

A modern book on orthodontic materials should include the following topics: an introduction to the concepts of materials science; study of the composition, structure, and properties of specific orthodontic materials used in clinical practice; interactions of orthodontic materials with other dental materials and dental hard tissues in the oral cavity; and the issues of biocompatibility, cytotoxicity, and allergic reactions for both patients and operators. This background should permit the orthodontist to make rational material selections for efficient treatment mechanics and to appreciate the complex effects of the oral environment on orthodontic materials. Moreover, it is essential for a textbook to integrate the dental materials science aspects with the clinical manipulation and use of these materials.

In the absence of a textbook, courses or seminars in orthodontic materials have often been taught on a literature review basis, using relevant published articles that might be supplemented by lecture notes. From an academic perspective, this teaching approach is inverted, since literature review sessions should follow development by students of the required background, which is traditionally achieved through formal textbook-driven education. This approach appears to have led frequently to the establishment of empirical performance criteria, which were usually derived from the clinical experience of individuals who served as consultants for the orthodontic materials industry.

Additionally, the lack of a focused textbook source of information has hindered the research efforts of graduate students in orthodontics and dental materials who undertake MS thesis and PhD dissertation projects in the field of orthodontic materials. As previously noted, the field of orthodontic materials has greatly expanded, with the introduction of new materials for archwires, auxiliaries, adhesives, elastomerics, cements, and other applications. Within each group of these materials, there is frequently a bewildering array of products with different properties. A noteworthy example is the wide spectrum of orthodontic adhesives that have quite different setting mechanisms and properties. Concurrently, the study of orthodontic materials has become much more comples owing to the availability of new, relatively sophisticated instrumentation.

The previous lack of an orthodontic materials textbook derives from the limited numer of types of orthodontic materials available in the past, along with the traditional emphasis in the graduate curriculum on various treatment modalities and the related orthodontic mechanics. With the latest advances in technology, the former is no longer the case, while the latter point must be reconsidered since modern orthodontic mechanics can be greatly affected by the choice of materials used. For example, while often mistakenly overemphasized, selection of orthodontic archwire alloys possessing specific mechanical properties may alter the duration of therapy.

The enormous changes in orthodontic treatment mechanics, treatment duration, and appliance efficiency due to improvement in materials are perhaps the most prominent differences that distinguish contemporary orthodontic practice from the way Dr. Angle was practicing at the dawn of the development of the specialty. Although our knowledge about the sequence of biological events leading to tooth movement and craniofacial adaptation has been improved substantially, the basic concepts of treatment mechanics have not essentially changed. However, materials are now available that generally allow the application of forces of a known magnitude, direction, and duration, in a predictable manner and with minimum substrate alterations induced. No other contribution appears to have made such a change in the way the profession is being practiced as has the development of modern orthodontic materials.

The development of an array of distinctively different materials has necessarily modified the teaching methodology for the orthodontic materials course in the majority of the graduate orthodontic programs. Teaching methods have evolved from a “manual concept, “which was limited to the description of proper materials manipulation on a subject-driven basis, to the formulation of a principle-based approach involving materials science. The latter has given rise to the requirement for a more organized teaching method, of which an essential adjunct would be the availability of a suitable textbook.

We hope that the scope of this book provides the background needed for the comprehensive study of orthodontic materials, and that students and faculty will find this book useful in efficiently organizing their research and teaching activities, respectively. Portions of this book are based upon a course in orthodontic materials that one of us (W.A.B.) has taught for over two decades, first at the Marquette University Schhol of Dentistry and for the past decade at the College of Dentistry of The Ohio State University. Numerous graduate students at both institutions will find citations to their thesis or dissertation research in several chapters. During these two decades, many individuals contributed greatly to the tremendous advancements that have occurred in the field of orthodontic materials. Individuals (other than contributors to this book) who should be cited for their substantial body of significant work include George Andreasen, Samir Bishara, Charles Burstone, Jon Goldberg, Robert Kusy, Fujio Miura, Ram Nanda, and Robert Nikolai. There is certainly also a lengthy list of other researchers who have published highly important articles in the orthodontic materials literature over the past two decades.

We are most grateful to our contributors from the United States and Europe for their collaboration in preparing this book, and to Clifford Bergman, Gabriele Kuhn, and Gert Krüger (Thieme, Stuttgart) for undertaking the publication of such a demanding project.

September 2000

William A. Brantley

Theodore Eliades

Credits for illustrations borrowed from other sources are provided in an abbreviated form in the figure legends. For full source details, please see page 302.

Table of Contents

1 Structures and Properties of Orthodontic Materials

William A. Brantley

2 Mechanics and Mechanical Testing of Orthodontic Materials

William A. BrantleyTheodore EliadesAlan S. Litsky

3 Instrumental Techniques for Study of Orthodontic Materials

George EliadesWilliam A. Brantley

4 Orthodontic Wires

William A. Brantley

5 Enamel Etching and Bond Strength

John M. PowersMarion L. Messersmith

6 Oral Microbiological Changes, Long-Term Enamel Alterations Due to Decalcification, and Caries Prophylactic Aspects

Bjørn Øgaard

7 Orthodontic Brackets

Theodore EliadesGeorge EliadesWilliam A. Brantley

8 Elastomeric Ligatures and Chains

Theodore EliadesGeorge EliadesDavid C. WattsWilliam A. Brantley

9 Orthodontic Adhesive Resins and Composites: Principles of Adhesion

David C. Watts

10 Orthodontic Adhesive Resins

Theodore EliadesGeorge Eliades

11 Cements in Orthodontics

Aphrodite KakabouraGeorge Vougiouklakis

12 Impression Materials

William M. Johnston

13 Bonding to Non-Conventional Surfaces

Efstratios Papazoglou

14 Principles of Biocompatibility

John C. Wataha

15 Allergic Reactions and Safety Concerns

Arne Hensten-PettersenNils JacobsenMargrét Rósa Grímsdóttir

Appendix

Figure and Table Credits

Index

1 Structures and Properties of Orthodontic Materials

William A. Brantley

Schematic illustration of an edge dislocation in a crystalline material

Introduction

Interatomic Bonding and Atomic Arrangement

Modes of Interatomic Bonding

Atomic Arrangements for Metallic Materials

Atomic Arrangements for Ceramic Materials

Atomic Arrangements for Polymeric Materials

Properties of Orthodontic Materials

Property Areas of Importance for Orthodontic Materials

Mechanical Property Concepts for Orthodontic Materials

Surface Property Concepts for Orthodontic Materials

Structures of Orthodontic Materials

Metallic Materials

Ceramic Materials

Polymeric Materials

References

Introduction

Knowledge of fundamental principles governing the relationships between compositions, structures and properties is central to an understanding of orthodontic materials. Because wide arrays of metallic, ceramic, and polymeric materials are used in the profession, and new materials are continuously being introduced, it is essential that the scientific basis for the selection and proper use of materials for clinical practice be thoroughly understood.

The general plan of this chapter is to develop these concepts following the standard basic approach that has historically been used in engineering materials science. First, the different modes of interatomic bonding will be described, followed by a description of how these modes of bonding lead to the atomic arrangements found in the metallic, ceramic, and polymeric materials. Second, terminology will be presented to describe the clinically relevant properties of these orthodontic materials. After these foundations have been acquired, the linkages between the structures and properties of these materials can readily be understood.

It is expected that graduate students in orthodontic programs, as well as practicing orthodontists, will already have been exposed to many of these concepts in an introductory biomaterials course or a series of lectures that are found in the predoctoral dental school curriculum. Because of the very wide scope of the subject matter involved, it is not possible in a single chapter to fully develop all of the topics germane to the structure-property relationships of orthodontic materials. The reader is referred to the excellent textbooks on dental materials science and engineering materials science listed at the end of this chapter for further information.

Interatomic Bonding and Atomic Arrangement

Modes of Interatomic Bonding

Traditionally, textbooks on dental materials science have subdivided the modes of interatomic bonding into two major categories: chemical bonding and physical bonding. Chemical bonding involves the valence electrons of atoms that participate in chemical reactions and includes the three modes of covalent, ionic, and metallic bonding. Physical bonding, also known as ver der Waals bonding, occurs between atoms or molecules with closed electronic shells.

Covalent bonding involves the sharing of valence electrons, such as that which occurs for carbon-carbon bonding in polymer chains. Another example is the sharing of valence electrons between silicon and oxygen to form SiO2. In both of these examples, after the sharing of valence electrons the atoms have the closed shell or noble gas electron configurations. The strongest covalent bonding occurs with the carbon atoms in diamond, where each of the four valence electrons (two 2s and two 2p) for a given carbon atom is shared with a valence electron of an adjacent carbon atom in a hybridized sp3 arrangement. Covalent bonding is highly directional, and materials dominated by covalent bonding typically have relatively open structures and low densities, and also lack the ability to undergo permanent deformation except at high temperatures.

Ionic bonding involves a transfer of valence electrons, such as that which occurs between sodium and chlorine atoms to form the NaCl molecule. The sodium atom has a single 3s electron in the M shell and the chlorine atom has a single 3p vacancy in the M shell (Table 1.1). Transfer of the valence electron from the sodium to the chlorine atom allows both atom species to have the closed shell noble gas configuration, and the bonding arises from the strong electrostatic attraction between the resulting positive and negative ions. Because of the symmetric electron charge distributions around the ions, ionic bonding is not directional. Materials dominated by ionic bonding tend to have higher densities than materials dominated by covalent bonding.

Generally, materials that are dominated by covalent or ionic bonding should be considered to have a mixture of both modes of interatomic bonding. Pauling developed an empirical concept, in which the fractional ionic bonding character for simple materials consisting of two different atomic species is related to the difference in electronegativity of the two atoms. The electronegativity for a given atom is approximately proportional to the sum of the ionization potential and the electron affinity. For example, from a plot of the electro-negativities for the different atoms, it can be estimated that the fractional ionic character is 0.52 and 0.63 for SiO2 and Al2O3, respectively.

Table 1.

1

Electronic configurations for some atoms

    Element

Atomic Number

Electronic Configuration

    C

6

1s

2

2s

2

2p

2

    O

8

1s

2

2s

2

2p

4

    Na

11

1s

2

2s

2

2p

6

3s

1

    Al

13

1s

2

2s

2

2p

6

3s

2

3p

1

    Si

14

1s

2

2s

2

2p

6

3s

2

3p

2

    Cl

17

1s

2

2s

2

2p

6

3s

2

3p

5

    Ti

22

1s

2

2s

2

2p

6

3s

2

3p

6

3d

2

4s

2

    Cr

24

1s

2

2s

2

2p

6

3s

2

3p

6

3d

5

4s

1

    Fe

26

1s

2

2s

2

2p

6

3s

2

3p

6

3d

6

4s

2

    Co

27

1s

2

2s

2

2p

6

3s

2

3p

6

3d

7

4s

2

    Ni

28

1s

2

2s

2

2p

6

3s

2

3p

6

3d

8

4s

2

    Cu

29

1s

2

2s

2

2p

6

3s

2

3p

6

3d

10

4s

1

    Ag

47

1s

2

2s

2

2p

6

3s

2

3p

6

3d

10

4s

2

4p

6

4d

10

5s

1

    Au

79

1s22s22p63s23p63d104s24p64d104f145s25p65d106s1

In metals the valence electrons are loosely bound and can be considered to form a gas that permeates the atomic arrangement of the resulting ionic cores, in contrast to the localization of valence electrons around parent atoms for covalent and ionic bonding. Metallic bonding is nondirectional on the atomic scale, and the metallic atoms form arrangements leading to materials of high density. Table 1.1 shows the electronic configurations for the atoms of some metals that are found in orthodontic and restorative alloys. The valence electrons that are lost in forming the cations from these metals are obvious. Many of the metals listed in Table 1.1 are transition elements, in which the outermost subshells (4s, 5s, 6s or 7s) are filled before the interior subshells. Transition metals frequently have multiple valences and are important for orthodontic and restorative alloys.

Physical interatomic bonding occurs with symmetric atoms (noble gas atoms) or molecules, such as methane (CH4), where weak dispersive forces arise from fluctuations in the electronic charge distribution. This bonding mode can have significance for the adhesion between different materials where a polymeric material is involved. The other type of physical bonding occurs with asymmetric molecules having permanent dipole moments, such as water (H2O), where strong electrostatic forces exist between the positive and negative centers of charge.

Atomic Arrangements for Metallic Materials

In general, materials can be subdivided into two categories according to their atomic arrangements. In crystalline materials there is a three-dimensional periodic pattern of the atoms, whereas no such long-range periodicity is present in noncrystalline materials, which possess only short-range atomic order.

It is appropriate to first consider the pure metals, which have the simplest compositions (a single element) and atomic arrangements. Except for extraordinary conditions of preparation (solidification from the liquid state or condensation from the vapor state under extremely rapid conditions) that are not applicable to dental alloys, metals always have crystalline structures. There are seven crystal systems (cubic, tetragonal, orthorhombic, rhombohedral [trigonal], hexagonal, monoclinic, and triclinic), with lattice parameters as summarized in Table 1.2. The corresponding fourteen space lattices (Bravais lattices) are listed in Table 1.3. Inherently, a space lattice is a geometric construct wherin each point has identical surroundings. Crystal structures of real materials are based upon space lattices, where there is a single atom or a group of atoms at each space lattice point.

The axial lengths (a, b and c) are the atomic repeat distances or lattice parameters along the three crystallographic axes, except for the hexagonal system where the axial lengths (a1,a2 and a3) in the basal plane are used.

For crystal systems other than hexagonal, the axial angle α is the angle between the b and c axes, and the β and γ angles have analogous definitions. In some textbooks the rhombohedral system is referred to as the trigonal system.

The three identical axial angles in the basal plane of the hexagonal system are designated α1, α2, and α3. γ is the angle between each of the three axes in the basal plane and the axis in the c-direction

Fig. 1.1 Unit cells for the simple cubic (A), body-centered cubic (B), face-centered cubic (C), and hexagonal close-packed (D) structures. (Adapted from Anusavice, 1996)

It is most convenient to visualize the crystal structures of metals in terms of their unit cells, where a unit cell is the smallest portion that can be repeated in three dimensions to produce the crystal structure. Figure 1.1 shows the unit cells of the important body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) structures for metals in dentistry. For comparison, the unit cell for the simple cubic structure is also shown. The hcp structure can be considered as formed from two interpenetrating simple hexagonal structures. Important pure metals found in orthodontic alloys for archwires (Chapter 4) and brackets (Chapter 7) that have these crystal structures are as follows:

    fcc

Fe (above ~ 910 °C), Ni

    bcc

Fe (below ~ 910 °C and above ~ 1400 °C), Cr, Ti (above ~ 880 °C)

    hcp

Co, Ti (below ~ 880 °C).

It can be seen that, while nickel and chromium have the fcc and bcc structures, respectively, at all temperatures below their melting points, iron and titanium have crystal structures that depend upon temperature. For example, titanium has the hcp α structure from room temperature up to ~ 880 °C, where there is a transformation to the bcc β structure. At room temperature iron has the bcc α structure and transforms to the fcc γ structure at ~ 910 °C. The different crystal structures of a given metal (or other crystalline material) are termed polymorphic or allotropic forms. Cobalt ordinarily has the hcp α structure but can also exist in an fcc β polymorphic form.

Table 1.

3

The fourteen space (Bravais) lattices

    Crystal System

Space Lattice

    Cubic

Simple cubic

Body-centered cubic

Face-centered cubic

    Tetragonal

Simple tetragonal

Body-centered tetragonal

    Orthorhombic

Simple orthorhombic

Body-centered orthorhombic

Face-centered orthorhombic

Base-centered orthorhombic

    Rhombohedral (Trigonal)

Simple rhombohedral

    Hexagonal

Simple hexagonal

    Monoclinic

Simple monoclinic

Base-centered monoclinic

    Triclinic

Simple triclinic

In textbooks on crystallography, the term simple is by the term primitive

Atomic Arrangements for Ceramic Materials

Ceramics, which consist of more than one atomic species, can have crystalline or noncrystalline structures, depending upon the material and sometimes the mode of preparation. Important ceramics for orthodontic applications are aluminum oxide (alumina) and zirconium oxide (zirconia), which are used as bracket materials (Chapter 7). Other ceramics are found in the powder portions of cements (Chapter 11). Silicon dioxide (silica) is an important filler in composite restorative resins.

The crystal structure of aluminum oxide is illustrative of the principles involved with ceramics having substantial ionic bonding character. The crystal structure consists of a nearly hcp arrangement of the larger oxygen anions (O2−), with the smaller aluminum cations (Al3+) located in two-thirds of the octahedral interstitial sites in the hcp structure (Fig. 1.2). The layers in the structure provide the maximum separation of the Al3+ ions. These octahedral sites have six-fold coordination, i.e., each aluminum ion is surrounded by six oxygen ions. The crystal structure is determined by the ratio of the radii of the aluminum and oxygen ions and the requirement of an electrically neutral unit cell. The crystal structure of zirconia at room temperature consists of a distorted simple cubic (monoclinic) arrangement of the oxygen ions, with the zirconium cations (Zr4+) located in half of the available sites (eightfold coordination), similar to Figure 1.3.

Fig. 1.2 Structure of alumina.(From Kingery et al, 1976)

Fig. 1.3 Structure of calcium fluorite (CaF2), which is similar to the structure of zirconia. (From Kingery et al, 1976)

Fig. 1.4 Structure of cristobalite. (From Greener et al, 1972)

Silica is principally found in dental materials (investments, composite resins, and elastomeric impression materials) in the crystalline quartz or cristobalite polymorphic forms or as the glassy form of vitreous silica. Moreover, both cristobalite and quartz have high-temperature (the more stable form) and low-temperature polymorphic forms. The basic unit in the structure of SiO2 is the SiO4 tetrahedron, where the small Si4+ cation is bonded to four much larger O2− anions in a tetrahedral arrangement. The high-temperature form of cristobalite (Fig. 1.4) has a diamond cubic (the structure for the diamond form of carbon) arrangement of the SiO4 tetrahedra. The high-temperature form of quartz has a complex structure consisting of connected chains of SiO4 tetrahedra. A schematic illustration of the structure of crystalline SiO2 in two dimensions in provided in Figure 1.5.

The structure of fused or vitreous silica has the short-range order of the silicon and oxygen ions shown in Figure 1.5 but lacks the long-range periodicity found in crystalline materials. For feldspathic dental porcelain, where glass-modifying or fluxing oxides such as Na2O, K2O, and CaO are added to SiO2, the Na+, K+, and Ca2+ ions disrupt the three-dimensional silicate framework structure. Nonbridging oxygen ions that do not connect Si4+ ions are created by these large cations, which are situated in open spaces in the structure (Fig. 1.6).

Fig. 1.5 Two-dimensional illustration of the structure of quartz. (From Kingery et al, 1976)

Fig. 1.6 Structure of feldspathic dental porcelain, showing typical locations of glass-modifying cations and nonbridging oxygen ions. (From Kingery et al, 1976)

Fig. 1.7 General structure of a urethane group (OCONH) formed by the reaction of a diol and a diisocyanate. (From Anusavice, 1996)

Atomic Arrangements for Polymeric Materials

A variety of polymeric materials are used in orthodontics, such as elastomeric impression materials (Chapter 12) and polyurethane modules for tooth movement (Chapter 8), adhesive cements for bonding brackets to enamel (Chapter 10), and polycarbonate brackets (Chapter 7). All of these polymeric materials are based upon macromolecules with varying compositions, molecular weights, and degrees of cross-linking. The general structures of a polyurethane elastomer for tooth movement and an alginate impression material are shown in Figures 1.7 and 1.8, respectively. The polymers have predominantly noncrystalline structures without long-range periodicity. Illustrations of important structural components for adhesive resins will be shown in Chapter 10. Information on the structure of polymeric materials is presented in references listed at the end of this chapter.

The elastomeric materials are lightly cross-linked and undergo a glass transition below room temperature. Below the glass transition temperature, the elastomers are relatively rigid, whereas above the glass transition temperature these materials become highly flexible with thermal activation of the polymeric backbones and side chains. The elastomers also typically contain a second phase of polymer crystallites, which undergo melting at temperatures above the glass transition temperature. In contrast, the polycarbonate material used for brackets is highly cross-linked and relatively rigid, although these brackets are subject to problematic deformation under the clinical loading imposed by archwires.

Mechanisms for permanent deformation of polymeric materials include chain stretching, slippage between adjacent chains, and chain scission. There is no mechanism on the molecular level in the noncrystalline orthodontic polymers that is analogous to the dislocation motion or twinning that occur in crystalline metals. Consequently, the cross-linked adhesive resins and the polycarbonate brackets fail in a brittle manner, although slow, time-dependent creep deformation under static loads may be possible. Under normal loading conditions, these brittle polymers will fail by propagation of cracks from microstructural flaws, analogously to the situation for ceramics.

Fig. 1.8 Schematic polymer structure of alginate impression material in which sodium alginate molecules have cross-linked to form calcium alginate (Chapter 12). (From Anusavice, 1996)

Properties of Orthodontic Materials

Property Areas of Importance for Orthodontic Materials

The clinically important property areas for orthodontic materials include mechanical and surface properties for a variety of materials, and the corrosion properties for archwire alloys. The purpose of this section is to introduce property terminology that will be germane to the subject matter presented in subsequent sections of this chapter and that will provide a background for understanding the properties of the different types of orthodontic materials discussed in Chapters 4 to 15.

While thermal and optical properties are highly important for restorative dental materials, these areas have been omitted from this section. Little work has been reported on the roles of thermal properties for orthodontic materials, and the importance of optical properties for the clinical performance of ceramic orthodontic brackets will be discussed in Chapter 7.

Mechanical Property Concepts for Orthodontic Materials

A material specimen can be subjected to three basic modes of force or load application, as shown in Figure 1.9. (A force is considered to be a vector quantity with both magnitude and direction, whereas a load is generally considered to have some magnitude only without any direction being specified.) A tensile force causes an elongation in the direction of load application, whereas a compressive force causes a contraction in the direction of load application. A shear force causes either a sliding displacement of one side of a specimen with respect to the opposite side or a twisting about the specimen axis (termed torsion).

Fig. 1.9 Three basic modes of load application. (Adapted from Greener et al, 1972)

For relatively small loads, the stress is below the elastic limit of the material, and reversible elastic strain occurs that disappears completely when the specimen is unloaded. When the stress reaches a sufficiently high value, a ductile material begins to undergo irreversible plastic or permanent deformation, whereas a brittle material will fracture without any significant permanent deformation. The permanent deformation of a ductile material will continue with increasing stress until fracture takes place.

This behavior can be portrayed by the familiar engineering stress-strain plot, which is illustrated for the tensile loading of a ductile metal specimen in Figure 1.10. This plot contains an initial linear region that corresponds to elastic deformation, followed by a curved region corresponding to permanent deformation and work hardening that terminates when the metal fractures.

For elastic shear loading, the stress and strain are related by the shear modulus (G). With homogeneous (properties independent of position) and isotropic (properties independent of direction) materials, there are only two independent elastic constants. For such materials, the values of Young's modulus, shear modulus, and Poisson's ratio are related by the expression

While most metallic and ceramic dental materials can be considered homogeneous and isotropic, an archwire will have different mechanical properties in directions parallel and perpendicular to the axis because of the wire drawing.

An important mechanical property for orthodontic alloys is the modulus of resilience, or resilience, which is the area of the stress-strain plot up to the PL. The modulus of resilience represents the energy per unit volume required to load a specimen of the alloy to the end of the elastic range and is equivalent to the bio-mechanical spring energy of the alloy. Since this area on the stress-strain plot is a right triangle, it follows that the modulus of resilience is given by (PL)2/2E.

The proportional limit is determined by placing a straightedge on the stress-strain plot, and the elastic limit is determined with the aid of precise strain measurement apparatus in the laboratory. While these two properties thus do not have the same value for an alloy, many investigators and textbooks treat the two terms as synonymous. Because there is some degree of subjectivity in determining the value of the PL, the yield strength (YS) is generally used for designating the onset of permanent deformation. Conventionally, the 0.1% YS is reported, which represents the value of stress corresponding to 0.1% permanent strain. The 0.1% YS is determined by the intersection of the curved portion of the stress-strain plot with a construction line that begins at the position of 0.1% strain on the horizontal axis and is parallel to the linear portion of the stress-strain plot (Fig. 1.10). Some investigators report the yield strength for 0.2% or 0.5% permanent strain (which will be higher than the 0.1% offset yield strength), so it is essential that the very small amount of permanent deformation used for the determination of YS be specified. Values of the 0.1% offset yield strength for archwire alloys are provided in Chapter 4.

Fig. 1.10 Tensile stress-strain curve for a ductile nickel-chromium dental alloy exhibiting linear elasticity. (Illustration prepared by Dr. William M. Johnston)

The maximum point on the stress-strain plot for tensile loading is called the ultimate tensile strength (UTS), or simply the tensile strength or strength. With compressive loading, the corresponding maximum stress is called the compressive strength. For relatively brittle materials, the tensile strength or compressive strength corresponds to the stress at fracture. However, the nominal stress will decrease after the UTS for materials with substantial ductility, as the test specimen undergoes substantial necking down (decreasing cross-sectional area) within the gauge section.

Figure 1.10 shows the unloading behavior that would also be observed for the tension test of an archwire alloy (such as stainless steel, cobalt-chromium-nickel and β-titanium) displaying standard linear elasticity (Chapter 4), where the path of unloading is parallel to the initial linear portion of the stress-strain plot. (The unloading path is designated on Fig. 1.10 as the line for ductility.) The loading and unloading behavior for the nickel-titanium archwire alloys that display nonlinear elasticity will be discussed in Chapter 4. In principle, the relevant stress-strain behavior for an archwire alloy would always be the unloading plot rather than the loading plot.

A highly important clinical property for archwire alloys is the springback after the maximal elastic deflection, which would be given by PL/E in Figure 1.10. However, since clinicians may activate archwires slightly into the permanent deformation range, a more practical expression of YS/E is given for springback in the orthodontic literature. In a similar manner, some investigators use the expression (YS)2/2E to determine a more practical value for the modulus of resilience.

In Figure 1.10, the intersection of the line for unloading with the horizontal axis corresponds to the amount of permanent deformation that had taken place just before the unloading occurred. This permanent deformation is termed the percentage elongation, given by the expression [lf − l0)/l0 × 100% where lf is the final specimen length before unloading. However, permanent deformation (termed ductility for tensile loading) has little significance for orthodontic alloys, where the focus is on elastic deformation and force delivery. For prosthodontic and implant alloy test specimens, which typically have diameters of 3 mm or similar dimensions, the ductility is determined by fitting together the fractured specimen segments and carefully measuring the permanent increase in the original gauge length.

For brittle orthodontic materials, such as cements, the diametral compression test may be performed to obtain the tensile strength. This test is particularly convenient because small disk specimens can be used. The loading mode is shown in Figure 1.11, where the vertical compression causes tensile strain in the lateral direction. The specimen undergoes fracture along the indicated midline; in the case of triple-cleft fracture, each of the fractured specimen halves further fractures into two pieces. This test is discussed further in Chapter 3.

Only a few hardness measurements for archwire alloys have been reported in the orthodontic materials literature (Chapter 4), where the Knoop and Vickers indentation techniques have been employed. Both of these microhardness tests use a diamond indenter to apply a known load to the surface of a metallurgically polished specimen. After the dimensions of the indentation are measured microscopically (Fig. 1.12), the Knoop or Vickers hardness number can readily be found from published tables. The Vickers technique uses a pyramidal indenter with a square base, and the lengths of the two diagonals of the surface indentation are averaged for each measurement. The Knoop indenter is specially cut so that the recovery of elastic deformation upon unloading takes place only along the short diagonal of the diamond-shaped indentation, and the length of the long diagonal is measured.

Fig. 1.11 Loading mode for the diametral compression test. (From Craig, 1997)

Viscoelastic properties are highly important for both impression materials and elastomeric modules. For these materials, the rate and duration of loading can have significant influences on mechanical properties, in contrast to the lack of dependence typically found for metals and ceramics with the usual rates of loading in the laboratory. A schematic plot is shown in Figure 1.13 for a viscoelastic material that is subjected to a constant load in tension for a period of time and then unloaded. Initially, the material undergoes instantaneous elastic deformation, in a similar manner to that for an archwire alloy. However, as the load is maintained, there is a retarded elastic deformation (or anelastic deformation), followed by a viscous or creep deformation that is irreversible if the applied load is sufficiently great. Upon unloading, the instantaneous elastic strain is recovered immediately, followed by a decay of the retarded elastic strain until only the permanent strain remains. This behavior is frequently described in terms of the creep compliance, which is the quotient of the total strain in the viscoelastic material and the stress. The creep compliance thus contains the three contributions from the instantaneous elastic deformation, the retarded elastic deformation, and the permanent deformation.

Fig. 1.12 Geometry of diamond indenters and schematic appearance of surface indentations for the Vickers and Knoop microhardness tests. The lengths of the diagonals designated by M are measured microscopically. (Adapted from Anusavice, 1996)

Many investigators have employed models using springs and dashpots to describe the mechanical behavior of viscoelastic materials. These mechanical models employ various combinations of Maxwell and Voigt-Kelvin elements, consisting of a spring and a dashpot in series and parallel, respectively. The spring is the basic linear elastic element, for which the force and displacement are linearly related through the spring constant by Hooke's law. This is analogous to the linear relationship between stress and strain through the modulus of elasticity below the elastic limit. The dashpot is the basic linear viscous element, for which the force is linearly related to the rate of extension of the dashpot piston by the viscosity. The analogy is a linear relationship between stress and strain rate with the coefficient of viscosity in shear as the constant of proportionality.

Fig. 1.13 Loading and unloading behavior for a viscoelastic material. The initial instantaneous elastic strain on loading corresponds to 1, and the region between 1 and 2 is a combination of the retarded elastic (anelastic) and the permanent (creep) strains. The first portion of the unloading curve between 2 and 3 is the recovery of the instantaneous elastic strain. The remaining permanent strain is designated by 4. (Adapted from Dowling, 1993)

Fig. 1.14 Four-element model used to simulate the viscoelastic behavior of an elastomeric impression material. (Adapted from Craig RG, editor. Restorative dental materials, 9th ed. St. Louis: Mosby, 1993)

Fig. 1.15 Maxwell-Weichert model that has been used to simulate the force degradation behavior of orthodontic elastomeric modules. (From Stevenson and Kusy, 1994)

Figure 1.14 shows a four-element model that has been used to simulate the viscoelastic behavior of an elastomeric impression material subjected to the loading regime in Figure 1.13. Figure 1.15 shows a Maxwell-Weichert model that has been found to fit the force degradation behavior of orthodontic elastomeric modules, as discussed in Chapter 8. While a detailed discussion of the mathematical equations that describe the mechanical models in Figures 1.14 and 1.15 are beyond the scope of this textbook, the reader can readily visualize how the behavior of the elements in each model qualitatively leads to the observed viscoelastic behavior.

Surface Property Concepts for Orthodontic Materials

Fig. 1.16 Generalized concept of the three interfacial energies and the surface tension for a liquid droplet on a solid surface. (From Guy and Hren, 1974)

Figure 1.16 illustrates a liquid droplet on a solid surface, where the three interfacial energies and the contact angle (θ) are designated. The interfacial energies exist between the solid and liquid (γSL), the liquid and vapor (generally air) (γLV), and the solid and vapor (γSV). Considering an equivalent force balance in the horizontal direction in Figure 1.16 for unit area and distance, the interfacial energies and contact angle are related by the expression

When bonding failures are observed, either visually or microscopically, in order to determine the failure modes, these failures are characterized as primarily adhesive or cohesive. Frequently, mixed-mode failure occurs, where there are both cohesive and adhesive failure sites. Cohesive failure refers to a failure through only a single material, where cohesive forces between the same atomic species are involved. Adhesive failure refers to failure at the interface between two different materials, where adhesive forces between two different atomic species are involved. The locations of cohesive and adhesive failure for two different materials that have been bonded to each other are illustrated in Figure 1.17. When assessing the failure modes in bonding systems, high-magnification observations with the scanning electron microscope are recommended, since examination with the optical microscope can be misleading because of the limit of less than × 500 useful magnification and the limited depth of focus.

Recent research suggests that the surface properties of orthodontic bracket materials can have significant influence on early pellicle formation. A standard series of six liquids and measurements of contact angles were used to determine the critical surface tension, the total work of adhesion, and its components due to polar and nonpolar forces for raw materials utilized to fabricate metallic, ceramic, and polymeric brackets (Chapter 7). It was necessary to employ the raw materials because of differences in the geometries, surface compositions (including the presence of a hydrophobic silane coating on ceramic brackets) and surface roughness of commercial brackets. The highest critical surface tension was found for stainless steel (40.8 dyn/cm), followed by polycarbonate (32.8 dyn/cm) and alumina (29.0 dyn/cm), suggesting greater plaque-retaining potential with stainless steel brackets. A similar ranking among the three bracket materials was found for the total work of adhesion and its polar and nonpolar components. For the three materials, the nonpolar component of the work of adhesion was higher than the polar component, suggesting the likelihood of an increased proportion of microorganisms having dispersive van der Waals forces as the predominant attachment mechanism.

Fig. 1.17 Locations of simple cohesive (C) and adhesive (A) failure sites for two different materials that have been bonded to each other

Variations observed by Fourier transform infrared spectroscopy (Chapter 3) in the adsorbed biofilms for two human subjects, following 30 and 60 minutes of intraoral exposure, suggested that there was an influence of the surface properties of these substrates on the structure of the pellicle formed in vivo. However, a subsequent in vitro study of the adherence of Streptococcus mutans to metal, ceramic, and polymeric orthodontic brackets found that the initial affinity of this pathogen for metal brackets was significantly lower than that for ceramic and polymeric brackets. Moreover, a saliva coating caused decreased adherence to all three bracket materials. Additional complementary in vitro and in vivo studies will be needed to resolve this uncertaintly about the microbial attachment to orthodontic brackets and its clinical relevance.

Structures of Orthodontic Materials

Metallic Materials

The alloys for archwires (Chapter 4) and brackets (Chapter 7) are used in the wrought form, where the shape is determined by the manufacturer using thermomechanical processing (mechanical deformation at elevated temperatures). The starting point is the formation of an ingot by melting the component metals together and allowing the alloy to solidify in a mold. After solidification, the alloys have a polycrystalline structure, consisting of microscopic single crystals (termed grains). These cast alloys are subjected to a series of mechanical deformation operations at elevated temperatures that result in a decrease in the cross section and severe deformation of the initial grain morphology for the cast microstructure. For example, the microstructure of a wire consists of highly elongated grains with a microscopic appearance analogous to that of parallel strands of spaghetti. An analogous distinctive appearance is found for the microstructure of a metallic orthodontic bracket. In contrast, the microstructure of a dental gold alloy casting consists of equiaxed grains that have very similar dimensions. The alloy microstructures are determined by polishing the surfaces with a series of abrasives, ending in a submicrometer slurry (typically 0.05 μm) that does not leave visible scratches, followed by use of an appropriate etchant (either an immersion or electrolytic etching technique).

The alloys for orthodontic wires and brackets have relatively complex compositions and can consist of more than one structure (phase). While X-ray diffraction can be used to investigate the phase relationships in an orthodontic alloy (Chapter 3), there is difficulty detecting secondary phases present in small proportions with this technique, and examination of the polished and etched microstructures with the optical microscope and scanning electron microscope is recommended.

Phase diagrams are used to portray the phases present in alloys as a function of composition and temperature. A phase is a region having a specific composition and structure, and separated from other phases by a boundary. Phases can be in the solid or liquid state. The vapor phase, which is always present, is generally ignored in most materials science applications because of its very low concentration and minimal influence. Since these diagrams are obtained in the laboratory under conditions that approach equilibrium as much as possible, they are frequently referred to as equilibrium diagrams. Only those phase diagrams for simple binary alloys consisting of two component elements can be fully portrayed in two dimensions. The horizontal axis represents composition, and the vertical axis represents temperature.

Fig. 1.18 Phase diagram for the Ti-V binary alloy system. (From Brick et al, 1977)

Figure 1.18 presents the phase diagram for the binary Ti-V system. At the left edge of this figure, it can be seen that pure titanium has the α (hcp) structure until 882 °C, where it transforms to the β (bcc) structure; the melting temperature is 1720 °C. At temperatures below about 1620 °C, titanium and vanadium form a substitutional solid solution, in which vanadium atoms are located on the sites normally occupied by titanium atoms in its crystal structure. In the top portion of this diagram, the upper line, termed the liquidus, and the lower line, termed the solidus, define a two-phase region corresponding to coexisting solid and liquid phases as the alloy undergoes solidification. Above the liquidus curve, all alloy compositions are in the liquid state; below the solidus curve, all alloy compositions have completed solidification. In the lower left corner of the figure, the region of stability for single-phase α-structure alloys is indicated. The region between the two curves corresponds to a two-phase region of coexisting solid α and β phases. It can be seen that the presence of vanadium causes the β phase to exist at a lower temperature than would ordinarily occur for pure titanium. Hence, vanadium is a stabilizing element for the β phase of titanium.

Fig. 1.19 Quasibinary phase diagrams for Fe-18Cr-4Ni and Fe-18Cr-8Ni (wt%) alloys as a function of carbon content. Up to at least 0.5% C, the carbide is (CrFe)4C. (From Brick et al, 1977.) These two diagrams show that increasing the percentage of nickel increases the temperature range for stability of the austenite (γ) phase.

Figure 1.19 presents a quasibinary phase diagram for Fe-18Cr-8Ni (wt%) alloys containing varying amounts of carbon up to 1.0%. This phase diagram is relevant to the discussion of austenitic stainless steel orthodontic wire alloys in Chapter 4. With four component elements in this case, a different strategy is needed for indicating alloy compositions. The origin at the left corner of the right phase diagram corresponds to the baseline composition of 74% Fe, 18% Cr, and 8% Ni. On the horizontal axis, the percentage of iron in the quaternary alloy decreases as the carbon content increases. Under equilibrium conditions the alloy structure is three-phase (α + γ + carbide) or two-phase (γ + carbide) at room temperature, depending upon the carbon content, where the carbide phase has the composition (CrFe)4C. Alloy compositions having less than about 0.65% C contain both the α (ferritic) and γ (austenitic) phases, rather than the single-phase austenitic structure (along with the carbide phase).

Among the three classes of orthodontic materials, only metals have the capability of undergoing extensive permanent deformation at room temperature. This characteristic is often termed ductility although, strictly speaking, ductility in dental materials science traditionally refers to the ability of a metal to undergo extensive permanent tensile deformation before fracturing. This plastic deformation is accomplished by the generation and movement of vast numbers of defects termed dislocations in the atomic structure when the stress in the metal reaches the PL or YS.

A schematic illustration of an edge dislocation in a metal with a cubic structure is shown in Figure 1.20. The dislocation lies at the edge of the missing half-plane of atoms and moves along a slip plane in response to a shear stress (or the shear stress component of a tensile or compressive stress). Movement of the dislocation causes the portion of the atomic structure on one side of the slip plane to be displaced (to undergo slip) with respect to the structure on the other side of the slip plane. The combination of this slip direction and the slip plane is referred to as a slip system. Since the dislocation movement in Figure 1.20 creates only one unit of slip deformation at the atomic level, it can be appreciated that practical levels of permanent deformation in metals require the generation and movement of vast numbers of dislocations. While the edge dislocation is the easiest configuration to visualize, other dislocation geometries also exist in actual metals. The study of dislocations requires the use of transmission electron microscopy, for which specimen preparation and interpretation of the observations are advanced subjects in materials science. Nevertheless, the reader can understand many important aspects of the mechanical properties of metals by considering the model suggested by Figure 1.20.

Fig. 1.20 Illustration of an edge dislocation in a metal with a cubic structure and its movement along a slip plane in response to a shear stress. (Adapted from Guy and Hren, 1974)

A second important mode of permanent deformation at the atomic level for the nickel-titanium orthodontic alloys is twinning. Figure 1.21 provides a schematic illustration of this process, where it can be seen that the atoms on either side of the twinning plane have a mirror orientation relationship with each other.

The fundamental mechanisms for strengthening of a ductile metal are generally associated with the different means by which dislocation motion is impeded, and increases in the strength of a metal are accompanied by increases in hardness and decreases in ductility. The major strengthening mechanisms for orthodontic alloys arise from the addition of alloying elements to the principal atomic species in the composition and the effect of extensive cold working (work hardening) during fabrication of the archwire or bracket.