AN INVESTIGATION OF PHASE TRANSFORMATION NIECHANISMS FOR
`NICKEL-TITANIUM ROTARY ENDODONTIC INSTRUMENTS
`
`DISSERTATION
`
`Presented in Partial Fulfillment of the Requirements for
`
`the Degree Doctor of Philosophy in the
`
`Graduate School of The Ohio State University
`
`By
`
`Satish B. Alapati, BDS, MS
`
`*****
`
`The Ohio State University
`
`2006
`
`Dissertation Committee
`
`Dr. William A. Brantley, Adviser
`
`Approved by
`
`Dr. John M. Nusstein
`
`Dr. William M. Johnston
`
`
`
`Adviser
`
`College of Dentistry
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`ABSTRACT
`
`The concern of nickel—titanium instrument separation is
`
`still a challenge
`
`confronting every manufacturer and endodontist, and such separation often seems to
`
`happen without any prior signs of permanent deformation. Nickel-titanium instruments
`
`based upon the equiatomic interrnetallic compound NiTi have gained considerable
`
`popularity among endodontists because of their very low modulus of elasticity, which
`
`enables these instruments to negotiate curved root canals during conventional root canal
`
`therapy. NiTi exists in two major microstructural phases: austenite, the high—temperature
`
`and low—stress form, and martensite,
`
`the low-temperature and high—stress form. An
`
`intermediate R—phase is also sometimes observed for the transformation between
`
`austenite and martensite. The structural
`
`transformations in NiTi occur rapidly by
`
`twinning on the atomic level and are reversible for stresses below the onset of permanent
`
`deformation.
`
`The nickel—titanium rotary instruments are intentionally manufactured in the
`
`superelastic condition, which provides the capability of extensive elastic strain without
`
`fracture under clinical conditions associated with root canal therapy. The microstructural
`
`phases present in the NiTi rotary instruments were studied by Micro x-ray diffraction
`
`(Micro-XRD) and temperature—modulated differential scanning calorimetry (TMDSC).
`
`The latter analytical technique provided information about the variations in proportions
`
`of the phases with temperature.
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`The overall objective of this
`
`study was
`
`to gain new insight
`
`into the
`
`microstructural phases and their transformations that would provide the basis for
`
`improved clinical performance of NiTi rotary instruments. The phases present were
`
`identified by Micro—XRD and TMDSC, using the clinically popular ProFile GT and
`
`ProTaper nickel—titanium rotary instruments, which have two different cross-sectional
`
`designs. Instruments were analyzed in the as-received condition and after clinical use, as
`
`well as following elevated-temperature heat treatments. The first null hypothesis for this
`
`research was that microstructural phases and phase transformations do not have an impact
`
`on clinical performance and instrument failure. The second null hypothesis was that
`
`appropriate heat treatments previously used for orthodontic wires will not result
`
`in
`
`beneficial changes in microstructural phases that may significantly affect the clinical life
`
`of these instruments. Based upon this research and complimentary previous studies by
`
`this investigator, both null hypotheses were rejected. Information obtained from this
`
`research should be of importance for the future development of improved instruments
`
`with reduced likelihood of failure during clinical use.
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`DEDICATION
`
`This study is dedicated to my family, parents and friends who have provided me
`
`unwavering love, support, and motivation throughout my life.
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`ACKNOWLEDGMENTS
`
`I would like to express sincere appreciation to my advisor, Dr. William A.
`
`Brantley, for the ubiquitous role he has had in all aspects of my academic program and
`
`dissertation work.
`
`I would also like to thank Dr. John M Nusstein and Dr. William
`
`Johnston, for their unswerving willingness to actively participate in my committee and
`
`also for their encouragement and challenge throughout my academic program.
`
`I am also
`
`highly appreciative to Dr. Rudy Melfi for expert advice on dental hard tissues and pulp
`
`biology. I am also highly appreciative to Dr. Robert Seghi and to the entire faculty in the
`
`Section of Restorative and Prosthetic Dentistry for their support throughout my academic
`
`program. My sincere appreciation goes to Dr. Glenn Daehn in the Department of
`
`Materials Science and Engineering for his valuable guidance and motivation over the
`
`years. It is highly important for me to acknowledge my early mentor at the Northwestern
`
`University Dental School, Dr. James Bahcall, who constantly provided me great moral
`
`support when I needed it the most at very early stages. Lastly, sincere thanks to Dr. Scott
`
`Schricker for allowing me use of his laboratory facilities to perform the Temperature—
`
`Modulated Differential Scanning Calorimetric experiments and to Dr. Masahiro lijima at
`
`the Hokkaido Health Sciences University in Japan for performing the Micro—X—Ray
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`Diffraction experiments.
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`VITA
`
`September, 1972 ................................... Born, Andhra Pradesh, INDIA
`
`March, 1996 ......................................... Bachelor of Dental Surgery, Bapuji Dental
`
`College and Hospital, Kuvempu
`
`University, Davangere, Karnataka, INDIA
`
`1997—1998 ............................................ General Practice Residency, Truman
`
`Medical Center, University of Missouri—
`
`Kansas City.
`
`2003 ................................................... M.S., Dental Materials, Ohio State
`
`University
`
`2001-present ......................................... Graduate Teaching/Research Assistant,
`
`Section of Restorative and Prosthetic
`
`Dentistry, College of Dentistry,
`
`The Ohio State University
`
`PUBLISHED ABSTRACTS AND PUBLICATIONS
`
`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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`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).
`
`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).
`
`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).
`
`and SEM Evaluation of Fiber-reinforced Provisional
`Fracture Toughness
`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).
`
`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).
`
`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).
`
`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).
`
`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).
`
`l5. 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.003 8, 2005 (www.dentalresearch.org).
`
`l6. 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 (wwwdentalresearchorg)
`
`17. Casting Dimensions, Porosity and Mechanical Properties for Palladium—Silver
`Alloys D. Li, N. Baba, X. Hu, S.B. Alapati, W.A. Brantley, RH. 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, SB. Alapati, W.A. Brantley, RH. Heshmati, T. Das
`Gupta, P.J. McCabe, J Dent Res 2006; 85(Special Issue A): Abstract No. 0039
`(www.dentalresearch.org).
`
`PUBLICATIONS
`
`l. 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 3lst 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.
`
`reinforced With different
`5. The fracture toughness of denture base material
`concentrations of POSS. Hamza T, Wee AG, Alapati S, Schricker SR. J Macro
`Molecular Sci 2004;A4l: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.
`
`FIELDS OF STUDY
`
`Major Field: Dentistry
`
`Oral Biology — Biomaterials track
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`TABLE OF CONTENTS
`
`Page
`
`Abstract ........................................................................................................................ ii
`
`Dedication ................................................................................................................... iv
`
`Acknowledgments ........................................................................................................ V
`
`Vita ............................................................................................................................. vi
`
`List of Tables ............................................................................................................. xii
`
`List of Figures ........................................................................................................... xiii
`
`Chapters:
`
`1.
`
`Introduction ............................................................................................................ l
`
`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
`
`2. Significance and Hypotheses. ............................................................................... 13
`2.1. Significance ................................................................................................... 13
`2.2. Hypotheses .................................................................................................... 13
`
`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 ................................................................................................... l9
`
`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
`
`6. Summary and Conclusions .................................................................................... 53
`6.1. Chapter 3 ....................................................................................................... 53
`6.2. Chapter 4 ....................................................................................................... 54
`6.3. Chapter 5 ....................................................................................................... 55
`
`References Cited and Bibliography of Other Articles .................................................. 56
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`LIST OF TABLES
`
`
`Table
`
`Page
`
`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
`
`4.2 Properties determined from the TMDSC plots for the two types of rotary
`endodontic instruments that fractured during clinical use ............................................... 38
`
`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
`
`Figures
`
`Page
`
`1.1 Equilibrium diagram of the nickel and titanium binary system near the NiTi
`phase (Brantley, 2001. Adapted fiom Goldstein et al., 1987). ....................................... 8
`
`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 um. Original
`magnification x250) ..................................................................................................... 9
`
`1.3 Secondary electron image of a Liberator rotary instrument, showing straight
`flute design. (Scale bar length 800 um. Original magnification x75) ............................. 9
`
`1.4 DSC heating and cooling curves for as—received Neo Sentalloy archwire
`segments (Brantley, 2001). ......................................................................................... 10
`
`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
`
`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
`
`3.1 Conventional XRD pattern for 40°C Copper Ni-Ti with labeled austenite (A)
`and martensite (M) peaks (Brantley, 2001). ................................................................ 20
`
`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
`
`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
`
`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
`
`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
`
`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
`
`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 (AH) for transformations is illustrated. ..................... 35
`
`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
`
`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
`
`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
`
`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
`
`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
`
`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
`
`INTRODUCTION
`
`1.1 Nickel-Titanium Rotary Instruments for Endodontics
`
`Conventional endodontic treatment involves the removal of inflamed or necrotic
`
`pulp tissue from the coronal and radicular sections of the root canal, along with
`
`significant shaping of the dentin along the canal walls to facilitate better obturation. The
`
`objective is to eventually seal the apex of the root by filling the root canal with an inert
`
`material such as gutta—percha. The coronal seal is also important in the success of root
`
`canal treatment, since this is followed by placing a full coronal restoration to extend the
`
`longevity and function of the tooth in the oral cavity. The morphology of the root canal
`
`system varies from tooth to tooth and person to person (Manning, 1990a and 1990b;
`
`Sidow et al., 2000). The curvature of the root varies from 0 to 50 degrees (Cohen and
`
`Burns, 2002), and the clinician may find the negotiation of such curved canals to be both
`
`difficult and challenging.
`
`The clinical endodontic procedure involves the sequential use of several sizes of
`
`root canal files, which are typically manufactured from stainless steel or nickel—titanium
`
`alloys. In the current practice of endodontics, engine-driven rotary instruments made of
`
`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
`
`Walia et al. (1988), who investigated NiTi hand files, which are also currently marketed
`
`for clinical use. The major reason for the clinical selection of nickel-titanium instruments
`
`by endodontists is the much greater flexibility (much lower elastic modulus) of the
`
`nickel-titanium alloy compared to stainless steel (Walia et al., 1988), which offers
`
`distinct clinical advantages for shaping curved root canals.
`
`Rotary instrument designs have changed at a rapid pace for the past two decades.
`
`In recent years there have been tremendous advances in the design of these instruments to
`
`improve the quality of root canal preparation, which leads to better obturation, apical
`
`seal, and eventually to better prognosis for the treated tooth. Nonetheless, the possibility
`
`of nickel—titanium instrument fracture is still a challenge confronting every manufacturer
`
`and practitioner who performs endodontic treatment. Despite very successful clinical
`
`performance, some incidence of nickel—titanium instrument deformation and separation
`
`seems to be inescapable, even with use of new design features and modern technology.
`
`Such instrument separation, which typically occurs without any preceding signs of
`
`permanent deformation and impending failure during root canal preparation (Patino et al.,
`
`2005, Bahia et al., 2005), is a highly unpleasant experience for the patient and the
`
`clinician.
`
`There are several factors that contribute to fracture of the nickel—titanium rotary
`
`instruments (Bahia et al., 2005). They include the original alloy processing by the
`
`primary metals company, machining of the instrument by the file manufacturer, and
`
`mechanical properties of the nickel—titanium alloy. Another variables is manipulation of
`
`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
`
`these instruments can be broadly classified into two general categories: (1) failure due to
`
`alloy properties and instrument manufacture (Kuhn and Jordan, 2002) and (2) failure due
`
`to improper manipulation by the clinician in the root canal (Cheung, 1996). Scanning
`
`electron microscope (SEM) studies of rotary instruments have revealed that surface
`
`defects, such as pitting, formation of grooves, and blunting of the cutting edges increase
`
`with clinical usage (Alapati et al., 2003).
`
`The alloy composition used in the manufacture of nickel—titanium orthodontic
`
`wires
`
`and endodontic
`
`nickel—titanium instruments has
`
`an
`
`approximate overall
`
`composition of 55% nickel and 45% titanium (wt. %) (Thompson, 2000), and is based
`
`upon the intermetallic compound NiTi (Figure 1.1). For orthodontic purposes, the nickel—
`
`titanium alloys are marketed in superelastic, nonsuperelastic, and shape memory forms
`
`(Bradley et al., 1996; Brantley, 2001). It was reported that nonsuperelastic alloys (such as
`
`the original Nitinol from 3M Unitek) have a predominantly heavily cold—worked, stable
`
`martensitic structure, whereas the superelastic and shape memory alloys undergo
`
`reversible transformation by twinning on the atomic scale (Brantley, 2001) between the
`
`lower-temperature martensitic structure and the higher—temperature austenitic structure.
`
`The shape memory alloys return to a higher—temperature shape established during
`
`processing (Civjan et a1, 1975) when the temperature is raised above the austenite-finish
`
`(Af)
`
`temperature at which the transformation to austenitic NiTi
`
`is completed. The
`
`transformation process can be complex, with the formation of an intermediate R—phase
`
`between the martensitic and austenitic structures (Brantley, 2001). Low—temperature
`
`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).
`
`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)
`
`(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
`
`diffraction (Thayer et al., 1995), which only provides information about phases within
`
`less than 50 um 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
`
`in the superelastic condition. This was confirmed by DSC study of two commercial
`
`products (ProFile and LightSpeed) in the as-received condition (Brantley et al., 2002a)
`
`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
`
`stainless steel root canal files, where tapered wire blanks are twisted to form the
`
`instrument.
`
`(Thompson, 2000; Cohen and Burns, 2002). The nickel—titanium rotary
`
`instruments are milled into various designs of cutting heads, flutes, shapes, tapers and
`
`sizes. Some of the newest NiTi rotary instrument products are Sequence System (VDW,
`
`Endodontic Synergy), SafeSiders (Essential Dental Systems)
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`(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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`4
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`features. The manufacturing process for these instruments introduces a variety of defects:
`
`(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
`
`failure during clinical use or laboratory testing (Sattapan et al., 2000; Tripi et al., 2001;
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`Martins et al., 2002).
`
`Some NiTi rotary instruments, such as RaCe and Sequence, are available with an
`
`electropolished surface finish. This type of surface finish reduces the extent of milling
`
`marks and metal
`
`flash, and also minimize metal
`
`rollover
`
`(Rangel et al., 2005)
`
`(Figure 1.3). Some researchers have also proposed the use of chemical vapor deposition,
`
`boron ion implantation (Lee et al., 1996), and nitrogen ion implantation (Tripi et al.,
`
`2002), with the overall objective of making the instrument surface hard and therefore
`
`improve cutting efficiency.
`
`1.3 Nickel-Titanium (NiTi) Phases and Their Transformation Process
`
`Rotary endodontic instruments machined from nickel—titanium alloy have
`
`considerably improved mechanical behavior compared to older stainless steel instruments
`
`because of their unique superelastic capability to develop large reversible tensile strains
`
`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
`
`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
`
`temperature (.Mf), austenite—start temperature (A,) and austenite-finish temperature (Af).
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`An intermediate R—phase generally forms between martensite and austenite phases on
`
`heating and cooling for nickel—titanium orthodontic wires (Brantley et al., 2001, 20020
`
`and 2003), so that additional RS and Rf temperatures can be defined for the start and
`
`finish of the transformations involving R—phase.
`
`It was previously noted in Section 1.1 that conventional DSC (Figure 1.4),
`
`temperature—modulated DSC (Figures 1.5 and 1.6), electrical resistivity measurements,
`
`and x—ray diffraction have been utilized to study phase transformations in nickel—titanium
`
`orthodontic wires. While conventional DSC is highly convenient as it provides bulk
`
`information,
`
`the present research described in later chapters employed temperature—
`
`modulated DSC, which provides detailed information about
`
`the reversing and
`
`non-reversing heat flow character of transformations in nickel-titanium rotary endodontic
`
`instruments that is not available with conventional DSC.
`
`The thermal signatures associated with the phase transformations are exploited to
`
`determine the transformation temperatures. The calorimeter used with conventional DSC
`
`or with TMDSC monitors temperature, regulates heat flow, and records the variation of
`
`heat flow with temperature during heating and cooling. Only a very small nickel-titanium
`
`test specimen, typically 20 - 40 mg and irregular in shape, is required. A sample