`NICKEL-TITANIUM ROTARY ENDODONTIC INSTRUMENTS
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`DISSERTATION
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`Presented in Partial Fulfillment of the Requirements for
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`the Degree Doctor of Philosophy in the
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`Graduate School of The Ohio State University
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`By
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`Satish B. Alapati, BDS, MS
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`* * * * *
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`The Ohio State University
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`2006
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`Approved by
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`__________________________
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` Adviser
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` College of Dentistry
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`Dissertation Committee
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`Dr. William A. Brantley, Adviser
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`Dr. John M. Nusstein
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`Dr. William M. Johnston
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`ABSTRACT
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`The concern of nickel-titanium instrument separation is still a challenge
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`confronting every manufacturer and endodontist, and such separation often seems to
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`happen without any prior signs of permanent deformation. Nickel-titanium instruments
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`based upon the equiatomic intermetallic compound NiTi have gained considerable
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`popularity among endodontists because of their very low modulus of elasticity, which
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`enables these instruments to negotiate curved root canals during conventional root canal
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`therapy. NiTi exists in two major microstructural phases: austenite, the high-temperature
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`and low-stress form, and martensite, the low-temperature and high-stress form. An
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`intermediate R–phase is also sometimes observed for the transformation between
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`austenite and martensite. The structural transformations in NiTi occur rapidly by
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`twinning on the atomic level and are reversible for stresses below the onset of permanent
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`deformation.
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`The nickel-titanium rotary instruments are intentionally manufactured in the
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`superelastic condition, which provides the capability of extensive elastic strain without
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`fracture under clinical conditions associated with root canal therapy. The microstructural
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`phases present in the NiTi rotary instruments were studied by Micro x-ray diffraction
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`(Micro-XRD) and temperature-modulated differential scanning calorimetry (TMDSC).
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`The latter analytical technique provided information about the variations in proportions
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`of the phases with temperature.
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`The overall objective of this study was to gain new insight into the
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`microstructural phases and their transformations that would provide the basis for
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`improved clinical performance of NiTi rotary instruments. The phases present were
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`identified by Micro-XRD and TMDSC, using the clinically popular ProFile GT and
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`ProTaper nickel-titanium rotary instruments, which have two different cross-sectional
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`designs. Instruments were analyzed in the as-received condition and after clinical use, as
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`well as following elevated-temperature heat treatments. The first null hypothesis for this
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`research was that microstructural phases and phase transformations do not have an impact
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`on clinical performance and instrument failure. The second null hypothesis was that
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`appropriate heat treatments previously used for orthodontic wires will not result in
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`beneficial changes in microstructural phases that may significantly affect the clinical life
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`of these instruments. Based upon this research and complimentary previous studies by
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`this investigator, both null hypotheses were rejected. Information obtained from this
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`research should be of importance for the future development of improved instruments
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`with reduced likelihood of failure during clinical use.
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`DEDICATION
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`This study is dedicated to my family, parents and friends who have provided me
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`unwavering love, support, and motivation throughout my life.
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`ACKNOWLEDGMENTS
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`
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`I would like to express sincere appreciation to my advisor, Dr. William A.
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`Brantley, for the ubiquitous role he has had in all aspects of my academic program and
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`dissertation work. I would also like to thank Dr. John M Nusstein and Dr. William
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`Johnston, for their unswerving willingness to actively participate in my committee and
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`also for their encouragement and challenge throughout my academic program. I am also
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`highly appreciative to Dr. Rudy Melfi for expert advice on dental hard tissues and pulp
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`biology. I am also highly appreciative to Dr. Robert Seghi and to the entire faculty in the
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`Section of Restorative and Prosthetic Dentistry for their support throughout my academic
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`program. My sincere appreciation goes to Dr. Glenn Daehn in the Department of
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`Materials Science and Engineering for his valuable guidance and motivation over the
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`years. It is highly important for me to acknowledge my early mentor at the Northwestern
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`University Dental School, Dr. James Bahcall, who constantly provided me great moral
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`support when I needed it the most at very early stages. Lastly, sincere thanks to Dr. Scott
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`Schricker for allowing me use of his laboratory facilities to perform the Temperature-
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`Modulated Differential Scanning Calorimetric experiments and to Dr. Masahiro Iijima at
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`the Hokkaido Health Sciences University in Japan for performing the Micro-X-Ray
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`Diffraction experiments.
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`VITA
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`September, 1972………….………………….Born, Andhra Pradesh, INDIA
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`March, 1996…………………………........…Bachelor of Dental Surgery, Bapuji Dental
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`College and Hospital, Kuvempu
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`University, Davangere, Karnataka, INDIA
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`1997-1998……………………………………..General Practice Residency, Truman
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`Medical Center, University of Missouri-
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`Kansas City.
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`2003……………………………………………M.S., Dental Materials, Ohio State
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`University
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`2001-present…………….……………….……Graduate Teaching/Research Assistant,
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`Section of Restorative and Prosthetic
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`Dentistry, College of Dentistry,
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`The Ohio State University
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`
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`PUBLISHED ABSTRACTS AND PUBLICATIONS
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`
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`
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`1. SEM Observations of New and Used Nickel-Titanium Rotary Files. Alapati S,
`Brantley WA, Mitchell JC, Iijima M, Svec TA, Powers JM. J Dent Res 2002; 81
`(Special Issue A): Abstract No. 3859 (www.dentalresearch.org).
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`2. Fatigue Limits and SEM/TEM of Fracture Characteristics for Pd-Ag Alloys. Li D,
`Brantley WA, Guo WH, Clark WAT, Alapati S, Heshmati RH, Daehn GS. J Dent
`Res 2003; 82 (Special Issue A): Abstract No. 128 (www.dentalresearch.org).
`3. SEM Observation of Deformed and Fractured Nickel-Titanium Rotary
`Endodontic Instruments. Svec TA, Alapati S, Brantley WA, Powers JM,
`Nusstein JM. J Dent Res 2003; 82 (Special Issue A): Abstract No. 1528,
`(www.dentalresearch.org).
`4. Vickers Hardness Investigation of Work-Hardening in Used Nickel-Titanium
`Rotary Instruments. Alapati S, Brantley WA, Guo WH, Svec TA, Powers JM,
`Nusstein JM. J Dent Res 2003; 82 (Special Issue A): Abstract No. 1529
`(www.dentalresearch.org).
`5. Fracture Toughness and SEM Evaluation of Fiber-reinforced Provisional
`Restoration Resin. Hamza TA, Rosenstiel SF, Alapati SB, EL-Hosary MM,
`Ibraheem RM; J Dent Res 83 (Special Issue A): Abstract No. 409, 2004
`(www.dentalresearch.org).
`6. Contact Angles of Orthodontic Elastomeric Chains in Two Media. Webb CS,
`Alapati S, Johnston W, Brantley WA, Vig KWL, Liu R. J Dent Res 2004; 83
`(Special Issue A): Abstract No. 1387 (www.dentalresearch.org).
`7. Fatigue of Two Pd–Ag Alloys and SEM of Fracture Surfaces. Li D, Baba N,
`Alapati S, Heshmati R, Brantley WA. J Dent Res 2004; 83 (Special Issue A):
`Abstract No.2685 (www.dentalresearch.org).
`8. SEM Observations of RaCe Rotary Instruments after Simulated Clinical Use.
`Sanli Y, Alapati SB, Nusstein JM, Brantley WA, Baba N, Svec T, Powers JM.
`J Dent Res
`2004;
`83
`(Special
`Issue A): Abstract No.
`2686
`(www.dentalresearch.org).
`9. SEM Microstructural Observations and Vickers Hardness Measurements for
`Pd-Ag Alloys. Baba N, Li D, Alapati S, Heshmati R, Brantley WA. J Dent Res
`2004; 83 (Special Issue A): Abstract No.3976 (www.dentalresearch.org).
`10. Fractographic Studies of Nickel-Titanium Rotary Instruments. Svec TA, Brantley
`WA, Alapati SB, Daehn GS, Nusstein JM, Powers JM. J Endod 2004; 30: 272
`(Abstract No.69).
`11. Vickers Hardness Measurements and Microstructural Observations of Soldered
`Beta-Titanium Wires. Baba N, Alapati SB, Iijima M, Brantley WA, Kawashima I,
`Ohno H, Mizoguchi I. J Dent Res 84 (Special Issue A): Abstract No. 646, 2005
`(www.dentalresearch.org).
`12. Microstructure, Chemical Composition and Vickers Hardness of Straumann
`Implant Component. Sanli Y, Baba N, Alapati SB, Li D, Brantley WA. J Dent
`Res 2005; 84 (Special Issue A): Abstract No. 1417 (www.dentalresearch.org).
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`13. SEM and EPMA Investigation of Soldered Beta-Titanium Orthodontic Wires.
`Iijima M, Brantley WA, Kawashima I, Alapati SB, Baba N, Yuasa T,
`Mizoguchi I. J Dent Res 2005; 84 (Special Issue A): Abstract No 1475
`(www.dentalresearch.org).
`14. Micro-XRD Study of Nickel-Titanium Rotary Endodontic Instruments after
`Clinical Use. Iijima M, Brantley WA, Alapati SB, Nusstein JM. J Dent Res 2005;
`84 (Special Issue A): Abstract No. 1479 (www.dentalresearch.org).
`15. Investigation of Transformations in Used and Heat-Treated Nickel-Titanium
`Endodontic Instruments S.B. Alapati, W.A. Brantley, S.R. Schricker, J.M.
`Nusstein, U.-M. Li , and T. Svec, J Dent Res 2006; 85(Special Issue A): Abstract
`No.0038, 2005 (www.dentalresearch.org).
`16. Micro-XRD and SEM of NiTi Instruments Modified by Ion Implantation U.-M.
`Li, M. Iijima, W.A. Brantley, S.B. Alapati, C.P. Lin 4, J Dent Res 2006; 85
`(Special Issue A ): Abstract No. 0037 (www.dentalresearch.org)
`17. Casting Dimensions, Porosity and Mechanical Properties for Palladium–Silver
`Alloys D. Li, N. Baba, X. Hu, S.B. Alapati, W.A. Brantley, R.H. Heshmati, T.
`Dasgupta, P.J. McCabe, J Dent Res 2006; 85(Special Issue A): Abstract No.
`1623(www.dentalresearch.org).
`18. Mechanical Properties of Three Palladium-Silver Casting Alloys for Metal-
`Ceramic Restorations D. Li, S.B. Alapati, W.A. Brantley, W.M. Johnston, T.
`Dasgupta, P.J. McCabe, J Dent Res 2006; 85(Special Issue A): Abstract No.1932
`(www.dentalresearch.org).
`19. Effects of Porcelain-Firing and Annealing Heat Treatments on Palladium-Silver
`Alloys D. Li, X. Hu 1, S.B. Alapati, W.A. Brantley, R.H. Heshmati, T. Das
`Gupta, P.J. McCabe, J Dent Res 2006; 85(Special Issue A): Abstract No. 0039
`(www.dentalresearch.org).
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`PUBLICATIONS
`
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`1. A review of the use of conventional DSC to study phase transformations in new
`and used nickel-titanium rotary endodontic instruments. Brantley W, Svec T,
`Iijima M, Powers J, Grentzer T, Alapati S. Proceedings of the 31st NATAS
`Conference, September 2003.
`2. Scanning electron microscope observations of new and used nickel-titanium
`rotary files. Alapati SB, Brantley WA, Svec TA, Powers JM, Mitchell JC. J
`Endod 2003;29:667-9.
`3. Proposed role of embedded dentin chips for the clinical failure of nickel-titanium
`rotary instruments. Alapati SB, Brantley WA, Svec TA, Powers JM, Nusstein JM,
`Daehn GS. J Endod 2004;30:339-41.
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`4. SEM observations of nickel-titanium rotary endodontic instruments that fractured
`during clinical use. Alapati SB, Brantley WA, Svec TA, Powers JM, Nusstein JM,
`Daehn GS. J Endod 2005;31:40-3.
`5. The fracture toughness of denture base material reinforced with different
`concentrations of POSS. Hamza T, Wee AG, Alapati S, Schricker SR. J Macro
`Molecular Sci 2004;A41:1-10.
`6. Bending fatigue study of nickel-titanium Gates Glidden drills. Leubke NH,
`Brantley WA, Alapati SB, Mitchell JC, Lausten LL, Daehn GS. J Endod
`2005;31:523-5.
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`
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`FIELDS OF STUDY
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`Major Field: Dentistry
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`Oral Biology – Biomaterials track
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`TABLE OF CONTENTS
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`Abstract........................................................................................................................ii
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`Dedication...................................................................................................................iv
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`Acknowledgments........................................................................................................v
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`Vita .............................................................................................................................vi
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`List of Tables .............................................................................................................xii
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`List of Figures...........................................................................................................xiii
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`Chapters:
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`1. Introduction............................................................................................................1
`1.1. Nickel-Titanium Rotary Instruments for Endodontics ......................................1
`1.2. Processing of Nickel-Titanium Rotary Instruments..........................................4
`1.3. Nickel-Titanium Phases and Their Transformation Process..............................5
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`2. Significance and Hypotheses. ............................................................................... 13
`2.1. Significance................................................................................................... 13
`2.2. Hypotheses .................................................................................................... 13
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`3. Micro-X-Ray Diffraction Investigation of Clinically Used Nickel-Titanium Rotary
`Endodontic Instruments ........................................................................................ 15
`3.1. Introduction................................................................................................... 15
`3.2. Materials and Methods................................................................................... 16
`3.3. Results and Discussions................................................................................. 17
`3.4. Conclusions ................................................................................................... 19
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`4. Temperature-Modulated DSC Investigations of Phase Transformations in New
`and Clinically Used Nickel-Titanium Rotary Endodontic Instruments................... 24
`4.1. Introduction................................................................................................... 24
`4.2. Materials and Methods................................................................................... 26
`4.3. Results and Discussion .................................................................................. 27
`4.4. Conclusions ................................................................................................... 31
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`5. Effect of Heat-treatment on Phase Transformations in Nickel-Titanium Rotary
`Instruments........................................................................................................... 40
`5.1. Introduction .................................................................................................. 40
`5.2. Materials and Methods................................................................................... 42
`5.3. Results .......................................................................................................... 43
`5.4. Discussion ..................................................................................................... 45
`5.5. Conclusions ................................................................................................... 47
`5.6. Acknowledgments ......................................................................................... 47
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`6. Summary and Conclusions.................................................................................... 53
`6.1. Chapter 3....................................................................................................... 53
`6.2. Chapter 4....................................................................................................... 54
`6.3. Chapter 5....................................................................................................... 55
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`References Cited and Bibliography of Other Articles.................................................. 56
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`LIST OF TABLES
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`Table
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`4.1 Properties determined from the total heat flow curves on the TMDSC plots for
`the as-received ProFile GT and ProTaper rotary endodontic instruments. ......................37
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`4.2 Properties determined from the TMDSC plots for the two types of rotary
`endodontic instruments that fractured during clinical use...............................................38
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`4.3 Properties determined from the TMDSC plots for ProFile GT and ProTaper
`instruments that permanently deformed without fracturing during clinical usage. ..........39
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`LIST OF FIGURES
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`Figures
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`1.1 Equilibrium diagram of the nickel and titanium binary system near the NiTi
`phase (Brantley, 2001. Adapted from Goldstein et al., 1987). .......................................8
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`1.2 Secondary electron image of a SafeSider rotary instrument, showing flat end of
`a 180° cutting surface and machining marks. (Scale bar length 200 µm. Original
`magnification ×250) .....................................................................................................9
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`1.3 Secondary electron image of a Liberator rotary instrument, showing straight
`flute design. (Scale bar length 800 µm. Original magnification ×75).............................9
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`1.4 DSC heating and cooling curves for as-received Neo Sentalloy archwire
`segments (Brantley, 2001). ......................................................................................... 10
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`1.5 TMDSC reversing and non-reversing heat flow curves for the heating cycle of
`as-received Neo Sentalloy orthodontic archwire segments (Brantley et al., 2003) ....... 11
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`1.6 TMDSC reversing and nonreversing heat flow curves for the cooling cycle of
`as-received Neo Sentalloy orthodontic archwire segments (Brantley et al., 2003) ....... 12
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`3.1 Conventional XRD pattern for 40°C Copper Ni-Ti with labeled austenite (A)
`and martensite (M) peaks (Brantley, 2001). ................................................................ 20
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`3.2 Micro-XRD patterns at various distances from the tip for an as-received ProFile
`GT/.06 taper NiTi rotary instrument. Only austenitic NiTi peaks are present. Peak
`identifications are shown in Figure 3.1 ....................................................................... 21
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`3.3 Micro-XRD patterns for an as-received K3 NiTi rotary instrument, showing the
`variation in intensity at various distances from the tip for the main austenite peak ...... 22
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`3.4 Micro-XRD patterns for a clinically used ProTaper NiTi rotary instrument,
`showing the variation in intensity for the main austenite peak at various distances
`from the tip................................................................................................................. 23
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`4.1 TMDSC curves for the heating (lower curves) and cooling cycles (upper curves)
`of test specimens obtained from a ProTaper rotary instrument in the as-received
`condition. ................................................................................................................... 32
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`4.2 TMDSC curves (total heat flow) for test specimens consisting of segments from
`three ProFile GT instruments, comparing an instrument that was fractured during
`clinical use with an instrument that was permanently deformed during clinical use
`and an instrument in the as-received condition (AR)................................................... 33
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`4.3 TMDSC curves (total heat flow) for test specimens consisting of segments
`obtained from ProFile GT (PFGT) and ProTaper (PT) instruments that fractured
`during clinical use ...................................................................................................... 34
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`4.4 TMDSC curves for a test specimen obtained from an as-received ProFile GT
`rotary instrument. The reversing and nonreversing, as well as the total, heat flow
`curves are shown for both the heating and cooling cycles. The use of construction
`lines to obtain enthalpy changes (ΔH) for transformations is illustrated. ..................... 35
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`4.5 TMDSC curves showing total heat flow, reversing heat flow and nonreversing
`heat flow for the heating and cooling cycles of a test specimen consisting of several
`segments from an as-received ProTaper rotary instrument. ......................................... 36
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`5.1 TMDSC total heat flow, reversing heat flow, and nonreversing heat flow curves
`for a ProFile GT test specimen after heating in a nitrogen atmosphere at 400°C for
`15 minutes.................................................................................................................. 48
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`5.2 TMDSC total heat flow, reversing heat flow, and nonreversing heat flow curves
`for the heating and cooling cycles of a ProFile GT test specimen after heat
`treatment at 850°C in a nitrogen atmosphere for 15 minutes ....................................... 49
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`5.3 TMDSC total heat flow, reversing heat flow, and nonreversing heat flow curves
`for a ProTaper test specimen after heat treatment at 500°C for 15 minutes.................. 50
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`5.4 TMDSC total heat flow, reversing heat flow, and nonreversing heat flow curves
`for a ProTaper test specimen after heat treatment at 600°C for 15 minutes.................. 51
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`5.5 TMDSC plots (total heat flow) showing a comparison of the effects of heat
`treatments at 400°, 500°, and 600°C for 15 minutes on test specimens from ProFile
`GT.............................................................................................................................. 52
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`CHAPTER 1
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`INTRODUCTION
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`1.1 Nickel-Titanium Rotary Instruments for Endodontics
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`Conventional endodontic treatment involves the removal of inflamed or necrotic
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`pulp tissue from the coronal and radicular sections of the root canal, along with
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`significant shaping of the dentin along the canal walls to facilitate better obturation. The
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`objective is to eventually seal the apex of the root by filling the root canal with an inert
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`material such as gutta-percha. The coronal seal is also important in the success of root
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`canal treatment, since this is followed by placing a full coronal restoration to extend the
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`longevity and function of the tooth in the oral cavity. The morphology of the root canal
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`system varies from tooth to tooth and person to person (Manning, 1990a and 1990b;
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`Sidow et al., 2000). The curvature of the root varies from 0 to 50 degrees (Cohen and
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`Burns, 2002), and the clinician may find the negotiation of such curved canals to be both
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`difficult and challenging.
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`The clinical endodontic procedure involves the sequential use of several sizes of
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`root canal files, which are typically manufactured from stainless steel or nickel-titanium
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`alloys. In the current practice of endodontics, engine-driven rotary instruments made of
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`nickel-titanium alloy are widely used with a torque-control motor and handpiece. Nickel-
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`titanium instruments were first introduced to endodontics in the landmark research by
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`Walia et al. (1988), who investigated NiTi hand files, which are also currently marketed
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`for clinical use. The major reason for the clinical selection of nickel-titanium instruments
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`by endodontists is the much greater flexibility (much lower elastic modulus) of the
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`nickel-titanium alloy compared to stainless steel (Walia et al., 1988), which offers
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`distinct clinical advantages for shaping curved root canals.
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`Rotary instrument designs have changed at a rapid pace for the past two decades.
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`In recent years there have been tremendous advances in the design of these instruments to
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`improve the quality of root canal preparation, which leads to better obturation, apical
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`seal, and eventually to better prognosis for the treated tooth. Nonetheless, the possibility
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`of nickel-titanium instrument fracture is still a challenge confronting every manufacturer
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`and practitioner who performs endodontic treatment. Despite very successful clinical
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`performance, some incidence of nickel-titanium instrument deformation and separation
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`seems to be inescapable, even with use of new design features and modern technology.
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`Such instrument separation, which typically occurs without any preceding signs of
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`permanent deformation and impending failure during root canal preparation (Patino et al.,
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`2005, Bahia et al., 2005), is a highly unpleasant experience for the patient and the
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`clinician.
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`There are several factors that contribute to fracture of the nickel-titanium rotary
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`instruments (Bahia et al., 2005). They include the original alloy processing by the
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`primary metals company, machining of the instrument by the file manufacturer, and
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`mechanical properties of the nickel-titanium alloy. Another variables is manipulation of
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`the instrument by the clinician, who must be aware of the torque delivery limits and the
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`flexural moment produced in the root canal with a given radius of curvature. Failures of
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`these instruments can be broadly classified into two general categories: (1) failure due to
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`alloy properties and instrument manufacture (Kuhn and Jordan, 2002) and (2) failure due
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`to improper manipulation by the clinician in the root canal (Cheung, 1996). Scanning
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`electron microscope (SEM) studies of rotary instruments have revealed that surface
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`defects, such as pitting, formation of grooves, and blunting of the cutting edges increase
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`with clinical usage (Alapati et al., 2003).
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`The alloy composition used in the manufacture of nickel-titanium orthodontic
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`wires and endodontic nickel-titanium
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`instruments has an approximate overall
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`composition of 55% nickel and 45% titanium (wt. %) (Thompson, 2000), and is based
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`upon the intermetallic compound NiTi (Figure 1.1). For orthodontic purposes, the nickel-
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`titanium alloys are marketed in superelastic, nonsuperelastic, and shape memory forms
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`(Bradley et al., 1996; Brantley, 2001). It was reported that nonsuperelastic alloys (such as
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`the original Nitinol from 3M Unitek) have a predominantly heavily cold-worked, stable
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`martensitic structure, whereas the superelastic and shape memory alloys undergo
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`reversible transformation by twinning on the atomic scale (Brantley, 2001) between the
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`lower-temperature martensitic structure and the higher-temperature austenitic structure.
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`The shape memory alloys return to a higher-temperature shape established during
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`processing (Civjan et al, 1975) when the temperature is raised above the austenite-finish
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`(Af) temperature at which the transformation to austenitic NiTi is completed. The
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`transformation process can be complex, with the formation of an intermediate R–phase
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`between the martensitic and austenitic structures (Brantley, 2001). Low-temperature
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`transformations within the martensitic NiTi structure have been reported for orthodontic
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`nickel-titanium alloys, using electrical resistivity measurements (Chen et al., 1992) and
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`temperature-modulated differential scanning calorimetry (TMDSC) (Brantley et al.,
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`2002c and 2003).
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`The relationships between different phases in nickel-titanium alloys have been
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`extensively studied by means of conventional differential scanning calorimetry (DSC)
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`(Todoroki and Tamura, 1987; Yoneyama et al., 1992; Bradley et al., 1996; Brantley,
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`2001), which provides information about the bulk test specimen, in contrast to x-ray
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`diffraction (Thayer et al., 1995), which only provides information about phases within
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`less than 50 µm below the surface (Brantley, 2001). A recent review article (Thompson,
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`2000) has stated that the nickel-titanium rotary endodontic instruments are manufactured
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`in the superelastic condition. This was confirmed by DSC study of two commercial
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`products (ProFile and LightSpeed) in the as-received condition (Brantley et al., 2002a)
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`and after simulated clinical use (Brantley et al., 2002b).
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`1.2 Processing of Nickel-Titanium Rotary Endodontic Instruments
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`Nickel-titanium endodontic instruments are machined, unlike their predecessor
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`stainless steel root canal files, where tapered wire blanks are twisted to form the
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`instrument. (Thompson, 2000; Cohen and Burns, 2002). The nickel-titanium rotary
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`instruments are milled into various designs of cutting heads, flutes, shapes, tapers and
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`sizes. Some of the newest NiTi rotary instrument products are Sequence System (VDW,
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`Endodontic Synergy), SafeSiders (Essential Dental Systems) (Figure 1.2), Liberator
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`(Miltex Endodontics) (Figure 1.3) and RaCe (Brasseler), all of which have unique design
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`features. The manufacturing process for these instruments introduces a variety of defects:
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`(1) surface flaws, such as irregularities, (2) milling marks, such as grooves, and (3) metal
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`flash and rollover (deformation of cutting edge during manufacturing process) which are
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`thin strips of metal at the edges of the flutes or the flat radial lands (Alapati et al., 2003).
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`Previous investigators have considered that such defects are responsible for instrument
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`failure during clinical use or laboratory testing (Sattapan et al., 2000; Tripi et al., 2001;
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`Martins et al., 2002).
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`Some NiTi rotary instruments, such as RaCe and Sequence, are available with an
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`electropolished surface finish. This type of surface finish reduces the extent of milling
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`marks and metal flash, and also minimize metal rollover (Rangel et al., 2005)
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`(Figure 1.3). Some researchers have also proposed the use of chemical vapor deposition,
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`boron ion implantation (Lee et al., 1996), and nitrogen ion implantation (Tripi et al.,
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`2002), with the overall objective of making the instrument surface hard and therefore
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`improve cutting efficiency.
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`1.3 Nickel-Titanium (NiTi) Phases and Their Transformation Process
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`Rotary endodontic instruments machined from nickel-titanium alloy have
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`considerably improved mechanical behavior compared to older stainless steel instruments
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`because of their unique superelastic capability to develop large reversible tensile strains
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`of about 7% – 10% (Miura et al., 1986). This superelasticity arises from the diffusion-less
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`phase transformation by twinning between martensite and austenite phases (Wayman and
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`Duerig, 1990; Brantley, 2001). The major phase transformation temperatures that govern
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`the mechanical properties are termed: martensite-start temperature (Ms), martensite-finish
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`temperature (Mf), austenite-start temperature (As) and austenite-finish temperature (Af).
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`An intermediate R–phase generally forms between martensite and austenite phases on
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`heating and cooling for nickel-titanium orthodontic wires (Brantley et al., 2001, 2002c
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`and 2003), so that additional Rs and Rf temperatures can be defined for the start and
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`finish of the transformations involving R–phase.
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`It was previously noted in Section 1.1 that conventional DSC (Figure 1.4),
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`temperature-modulated DSC (Figures 1.5 and 1.6), electrical resistivity measurements,
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`and x-ray diffraction have been utilized to study phase transformations in nickel-titanium
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`orthodontic wires. While conventional DSC is highly convenient as it provides bulk
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`information, the present research described