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Influence of Previous Angular Deformation on Cyclic Fatigue Resistance of K3XF Instruments Abdullah, Riyahi 2014

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INFLUENCE OF PREVIOUS ANGULAR DEFORMATION ON CYCLIC FATIGUE RESISTANCE OF K3XF INSTRUMENTS   by Abdullah Riyahi  BDS, King Saud University 2008  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE  in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES  (Craniofacial Science)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  July 2014  © Abdullah Riyahi, 2014    ii  Abstract  Objective To evaluate the effect of preloading various degrees of the maximum distortion angle on the cyclic fatigue resistance of post-machining heat-treated nickel-titanium (NiTi) instruments. Methodology New K3XF and K3 NiTi instruments (size 25/.04 taper)( n = 15) were tested to obtain the mean number of cycles to failure (Nf) using a 3-point bending apparatus. Torque and distortion angles at failure were determined according to the ISO 3630-1 standard. New files were then pre-cycled to four conditions (0%, 25%, 50%, and 75% of the angular deflection) and fatigue resistance tests were performed. After torsional preloading, the total number of revolutions to failure (Nf) was measured for each file. The fracture surface of each fragment was examined with a scanning electron microscope. The crack-initiation sites and the percentage of dimple area of the whole fracture cross-sectional area were recorded.  Results The fatigue resistance of K3XF instruments was two times higher than that of K3 instruments (P < 0.05). The angles of rotation at fracture of unused K3XF instruments were similar to those of K3 instruments. With both the K3 and K3XF instruments, the 25%, 50% and 75% torsional preloading groups had significantly lower Nf than the no preloading group (P < 0.05). The K3XF instruments had significantly higher Nf than the K3 in all corresponding preloading groups (P < 0.05). The fracture surface of the K3XF and K3 instruments in which the fatigue test was applied after an incomplete torsional test was characterized as the fatigue fracture pattern.   iii  Conclusions The fatigue resistance of K3XF instruments was higher than that of K3 instruments. The K3XF instruments displayed a similar torque resistance to the K3 instruments. A small amount of torsional preloading reduced the fatigue resistance of both the K3 and K3XF instruments.  iv  Preface  This dissertation, ―Influence of Previous Angular Deformation on Cyclic Fatigue Resistance of K3XF Instrument‖ is an original, independent and unpublished work by Abdullah Riyahi.   The project was performed under the supervision and guidance of Dr. Markus Haapasalo and Dr. Ya Shen.  Abdullah Riyahi performed all aspects of the research project, including the fatigue testing, data collection and scanning electron microscopy imaging.  The relative contributions to this research project were as follows:  60% Dr. Abdullah Riyahi, 30% Dr. Ya Shen and 10% Dr. Markus Haapasalo.  Dr. Markus Haapasalo and Dr. Ya Shen reviewed the manuscript.  This research was generously supported by The Canadian Academy of Endodontics. SybronEndo donated the files used in this study.     v  Table of Contents  Abstract…………………………………………………………………………………………..ii Preface…………………………………………………………………………………………....iv Table of Contents………………………………………………………………………………...v List of Tables…………………………………………………………………………………….vi List of Figures…………………………………………………………………………………..vii List of Abbreviations……………………………………………………………………………iv Acknowledgements………………………………………………………………………………x Dedication……………………………………………………………………………………….xii Chapter  1: Introduction………………………………………………………………………...1 1.1 Nickel-titanium (NiTi) rotary instruments…..………………………………………….1 1.2 NiTi instrument fracture……...…………………………..………………………………2 1.3. Heat-treated NiTi files………..…………………………....………………………….….4     1.4 Metallurgical properties………….……………………………………………………….8 1.5 Study rationale…………………………………………………………………………...11 Chapter  2: Methods……………………………………………………………………………18 Chapter  3: Results……………………………………………………………………………...23 Chapter  4: Discussion………………………………………………………………………….37 Chapter  5: Conclusions………………………………………………………………………..43       5.1. Future directions……………………………………………………………………….43 References ………………………………………………………………………………………44  vi  List of Tables  Table 1. Mechanical properties of superelastic K3 instruments…………………………………12 Table 2. Mechanical properties of K3XF instruments…………………………………………..15 Table 3. Baseline scores for K3 and K3XF instruments…………………………………….…..22 Table 4. The number of revolutions until fracture of K3 and K3XF after preloading with different levels of the maximum distortion angle…………………………………………………………25      vii  List of Figures  Figure 1. K3XF NiTi instruments…………………………………………………………………7 Figure 2. (A) Differential scanning calorimetry curves of K3XF and K3 instruments……….…10                 (B) X-ray diffraction patterns for K3XF and K3 at 25 oC…………………………….10 Figure 3. Three-point bending apparatus for fatigue test………………………………..……….20 Figure 4. The apparatus for torsional test………………………………………………………..21 Figure 5. The number of revolutions (Nf ) until fracture after preloading with different levels of the maximum distortion angle of K3 and K3XF instruments………………………………........26 Figure 6. Lateral-view scanning electron micrograph of K3XF files with 25% preloading of the maximum distortion angle……………………………………………………………………….27 Figure 7. Lateral-view scanning electron micrograph of K3 files with 25% preloading of the maximum distortion angle……………………………………………………………………… 28 Figure 8. Lateral-view scanning electron micrograph of K3XF files with 50% preloading of the maximum distortion angle……………………………………………………………………….29 Figure 9. Lateral-view scanning electron micrograph of K3 files with 50% preloading of the maximum distortion angle……………………………………………………………………….30 Figure 10. Lateral-view scanning electron micrograph of K3XF files with 75% preloading of the maximum distortion angle…………………………………………………………………….…31 Figure 11. Lateral-view scanning electron micrograph of K3 files with 75% preloading of the maximum distortion angle……………………………………………………………………….32 Figure 12. Fracture surfaces of K3 and K3XF instruments after separation due to fatigue…......33 viii  Figure 13. Fracture surfaces of K3 and K3XF instruments after instrument separation due to  torque.............................................................................................................................................34 Figure 14. Fracture surface of K3 and K3XF instruments after fatigue failure with 25% preloading of the maximum distortion angle…..…………………………………………….......35 Figure 15. Fracture surfaces of of K3 and K3XF after fatigue failure with 75% preloading of the maximum distortion angle …………………………………………………………………........36 ix  List of Abbreviations  NiTi: Nickel-Titanium  RT: Room temperature  DSC: Differential scanning calorimetry  Af: Austenite finish temperature  Nf : Total number of revolutions to failure ISO: International Organization for Standardization  x  Acknowledgements  I want to express my deepest appreciation for Dr. Ya Shen.  She answered every one of my questions with broad knowledge and understanding. Though she had many responsibilities, she always took the time to discuss scientific matters with me in detail. I have gained so much from her knowledge and massive experience in the field of research and, on a personal level, I learned even more from her kindness. In addition, I would also like to thank Dr. Ya Shen for her help in the statistical analysis. Without Dr. Shen, this project would not have been possible.  I want to express my deepest appreciation to Dr. Markus Haapasalo who provided me with the chance to work under his supervision. Dr. Haapasalo enlightened my vision and broadened my understanding of research. His knowledge and guidance inspired me throughout my education and will have a great influence in my future career.   I am grateful to my committee member, Dr. Fernanda Almeida, for her valuable scientific comments. I am grateful for the times she listened to me and provided honest advice. Dr. Almeida's input was important for my research and she helped me improve my presentation skills.        xi  I was blessed to have Dr. Jeffrey Coil as my program director and mentor. He offered  endless support and guidance in the theoretical and clinical parts of my education. He is a great teacher who is always ready to help students in every aspect starting from general patient care to details such as setting and obtaining high quality clinical photographs. Dr. Coil's presence and influence in this program created a consistently positive atmosphere which made us eager and excited to learn more. I am grateful to him for being supportive at both the educational and personal level.  My appreciation goes to the UBC Faculty of Dentistry, my clinical supervisors, the laboratory staff, the clinical staff and all of my classmates for their encouragement and support through our program. My thanks go to Dr. Les Campell who performed the torque pretesting. A special thank you to Huimin Zhou, Zhejan Wang, Tiangfend Du and Wei Qian.  Finally, I am greatly thankful to King Saud University and the Saudi Cultural Bureau for their full financial support that has made my education dream become a reality.          xii  Dedication  I dedicate this work to my family who have supported me unwaveringly. I am grateful to my father who has given me such motivation and support through my whole education. I will be always grateful for your guidance.   I also dedicate this work to my brother Mohammad who travelled all the way here to Vancouver when I needed him.  I am really thankful to my mother who always has enlightened my visions throughout my life. I will always appreciate everything that she has done for me.  Lastly, I want to dedicate this work to my wife Hanan. I thank her for being positive and supportive.  Without you, my family, I wouldn‘t be here today.     1  Chapter 1: Introduction  1.1 Nickel-titanium (NiTi) rotary instruments       The aim of root canal instrumentation is to produce a tapered canal of adequate shape to allow effective irrigation and root filling. Realizing this objective in fine, curved root canals is difficult with the use of traditional stainless steel instruments as they are stiff and tend to create aberrations such as ledges, zips and perforations (Peters et al. 2003). Over the years, modified instrumentation techniques and new, flexible instruments have been introduced to prevent or minimize these errors. Nowadays, nickel-titanium (NiTi) instruments play an important role in root canal preparation. NiTi instruments are highly flexible and elastic (Thompson 2000). Experimental and clinical evidence suggests that the use of NiTi instruments with rotary movement results in improved quality of root canal preparation (Peters et al. 2003, Cheung & Liu 2009). In particular, the incidence of gross preparation errors is greatly reduced (Bryant et al. 1998; Schäfer & Florek 2003; Bürklein et al. 2013). However, despite considerable improvements in instrument materials and design, instrument fracture during canal preparation can still occur (Shen et al. 2006, 2009).        2  1.2 NiTi instrument fracture       While root canal treatment has benefitted from the introduction of NiTi rotary instruments, the possibility of instrument separation during use is a concern to clinicians. Instrument fracture is a potentially serious mishap. This does not by itself predispose a case to direct failure or post-treatment disease; rather, a retained instrument fragment limits access of disinfecting irrigants to the root canal system, possibly impeding elimination of sufficient amounts of microorganisms (Haapasalo et al. 2004). A recent survey of endodontic specialists and general dental practitioners in the United Kingdom (Madarati et al. 2008) indicated that the great majority (93%) of endodontists used NiTi rotary systems. Most of the users (94% of endodontists and 85% of general practitioners) had experienced at least one instrument fracture during their clinical work.        Fracture of a NiTi instrument may occur in either one or in a combination of two ways: torsional and flexural (fatigue) (Sattapan et al. 2000; Cheung et al. 2005; Wei et al. 2007). Torsional fracture occurs when the torque ("force of rotation") resulting from the contact between the instrument and canal wall exceeds the torsional strength of the instrument, or when the instrument tip is locked in a canal whilst the rest of the instrument continues to rotate. This may also occur when instrument rotation is sufficiently slowed in relation to the cross-sectional diameter. Fracture due to flexural fatigue occurs when a rotary endodontic instrument that has already been weakened by metal fatigue continues to be or is again placed under similar stress, during instrumentation of a curved root canal. Thus, a rotary instrument needs to be resistant to cyclic fatigue and also possess a sufficient resistance to torsional failure in order to reduce the incidence of instrument separation. Parashos et al. (2004) found cyclic fatigue to be the more 3  common mechanism of instrument fracture in clinical practice: 70% of the fractures were attributable to ‗flexural fatigue‘. Conversely, Sattapan et al. (2000) found that torsional stress was slightly more prevalent as the cause of fracture. However, these studies were based on structural analysis, which according to present understanding could not reliably differentiate between fracture by fatigue and fracture by torque. Cheung et al. 2005and Shen et al. 2006 later demonstrated that only microscopic examination of the fractured surfaces of the instrument can reliably identify the cause of the fracture, either fatigue or torque or both at the same time. These studies reported that while both fatigue and torsion are important mechanisms leading to instrument fracture, fatigue is the more common cause of fracture in rotary instruments whereas torque is more commonly the cause of fracture in manually used files.        The type of instrument breakage has been categorized macroscopically, based on the presence or absence of plastic deformation (in the form of unscrewing or overscrewing of the flutes) adjacent to the fracture site, into ‗torsional‘ or ‗flexural‘, respectively (Sattapan et al. 2000). However, while such examination in lateral view allows the detection of plastic deformation in the separated instrument, it fails to indicate the actual mechanism involved in the fracture process. The handbook of the American Society for Metals International (ASM International 1987) points out that fractographic examination aims to identify features on the fracture surface that would indicate the origin and propagation of the crack(s) leading to material failure. This method was introduced into endodontics to properly identify the cause of instrument fracture by Cheung et al. in 2005.  In fatigue failure there are characteristic fatigue striations on the fracture surface. In cases with shear failure, such striations are not found on the fracture surface. In this study, a total of 122 NiTi instruments discarded after distortion of the helix or instrument 4  separation were collected from dental offices. Twenty-seven of the instruments were fractured. The broken files were first examined for their lateral view following the method of Sattapan et al. (2000). The specimens were then re-mounted and the detailed characteristics of the fractured surfaces were observed by scanning electron microscopy (SEM). Under a lateral view, two demonstrated torsional failure while the other appeared to be caused by fatigue. At high magnification of the fracture surface, the presence of fatigue striations in 18 specimens indicated that fatigue failure had occurred. Nine instruments exhibited the characteristics of shear fracture. Seven of them were originally classified as either flexural or fatigue failure as seen by the lateral view. This study indicated that examination of the fracture surface at high magnification is essential to reveal the type of the fracture.  1.3. Heat-treated NiTi files       K3 second generation NiTi instruments (SybronEndo, Orange, CA), designed and developed by Dr. McSpadden, were designed with positive rake angles, with the idea that this would give them greater cutting efficiency compared to the first generation files, which had negative rake angles. The special features of the K3‘s unique cross-sectional design thus included a slightly positive rake angle for greater cutting efficiency, wide radial lands, and a peripheral blade relief for reduced friction. In addition, the K3 file features a third radial land to help prevent threading-in of the file in dentin which might cause fracture from torque. In the lateral view of K3, a variable pitch and core diameter can be identified, which according to the manufacturer make the file stronger close to its apical tip. The mechanical properties of K3 files have been widely studied (Table 1). 5        Reducing the likelihood of instrument separation has been one of the main goals of manufacturers in the development of new NiTi rotary instruments, aiming at improved safety through innovative design and manufacturing processes (Gambarini et al. 2008 & 2012; De-Deus et al. 2010; Gutmann & Gao 2012; Gao et al. 2012; Shen et al. 2012a & b, 2013a & b). Heat treatment, also known as thermal processing, is one of the most fundamental approaches toward adjusting the NiTi alloy‘s phase transition temperatures, which have a major impact on the fatigue resistance of these files (Frick et al. 2005, Gutmann & Gao 2012, Shen et al. 2013a, Haapasalo & Shen 2013).       In 2008, SybronEndo (Orange, CA, USA) presented the first fluted NiTi file, Twisted Files, which were manufactured by plastic deformation, a process similar to the twisting process that is used to produce the majority of stainless-steel hand files such as K-files and reamers. According to the manufacturer, a thermal process allows twisting during phase transformation into the so-called R-phase of NiTi (Hou et al. 2011). Theoretically, the main advantage gained by a specific heat treatment is not only to improve the flexibility and strength of the file, but at the same time, by modifying the crystalline structure of the alloy, to accommodate some of the internal stresses caused by the grinding process (Zhou et al. 2012).        The design of K3 instruments was recently updated by SybronEndo, and a new version of the file was introduced in 2011,  under the name "K3XF" (Fig. 1). K3 and K3XF instruments are identical in shape and differ only in that K3XF instruments undergo post-machining heat treatment (http://www.sybronendo.com/cms-filesystem-action?file=/sybronendo-pdf/k3xf-ss.pdf). 6  This new treatment aims to eliminate many of the drawbacks of the grinding process and produce instruments with superior mechanical properties. The manufacturer claims that K3XF (SybronEndo, Orange, CA, USA) has the basic features of the original K3, plus an "extraordinary new level" of flexibility and resistance to cyclic fatigue with the proprietary R-Phase technology (Hou et al. 2011, Shen et al. 2013c). A couple of studies (Gambarini et al. 2011, Plotino et al. 2012, Ha et al. 2013, Shen et al. 2014) showed that K3XF instruments have much higher fatigue resistance than K3 NiTi instruments (Table 2). On the other hand, K3XF instruments maintain the same torsional properties as conventional superelastic K3 files (Ha et al. 2013; Shen et al. 2013c) (Table 2).     7  Figure 1. K3XF NiTi instruments (Courtesy of SybronEndo,Coppell, TX, USA)             8  1.4 Metallurgical properties       NiTi alloy can exist in various crystallographic forms. Three distinct microstructural phases are possible: austenite, martensite, and R-phase. The transformation temperatures – that is, the temperature at which one phase begins to transform into another – determine the relative proportions of the various phases within the material, and hence the behavior of the final material at different temperatures (e.g. room temperature [RT] and body temperature [+37oC]) (Brantley 2001). The transformation temperatures can be altered by small changes in composition, impurities, and heat treatment during the manufacturing process. Thermal treatments have been shown to have an impact on some of the mechanical and superelastic properties and transformation characteristics of NiTi shape-memory alloy depending on their thermomechanical history (Frick et al. 2005). When heat treatment is coupled with instrument configuration, the mechanical properties of the metal may be significantly altered, resulting in either favorable or unfavorable outcomes, although specific thermal manipulations have resulted in enhanced alloy flexibility. Characterization of these new instruments can provide useful information for the understanding of their performance. Apart from simple mechanical testing, various metallurgical techniques (Shen et al. 2011b, 2013a & c) have been used to investigate the microstructure and phase transformation of different makes of NiTi instruments.        Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. In this analytical method, the difference in thermal energy supplied to a test specimen and an inert control, which are heated at the same rate, is 9  precisely measured and compared. Phase transformations, e.g. of endodontic instrument metals, are revealed as endothermic peaks on the heating curves and as exothermic peaks on the cooling curves for the test material. The difference between the material‘s transformation temperatures and the ambient/working temperature determines the stability of the various NiTi phases. The DSC results (Shen et al. 2013) showed that the K3XF and K3 instruments had an Af temperature below 37oC (Fig. 2). Therefore, both types of instruments have an austenite structure at body temperature. These instruments would therefore be expected to exhibit a superelastic property during clinical application. However, the phase transformation points of K3XF instruments were significantly higher than K3 instruments. Two overlapping endothermic peaks were observed on the heating plot of K3XF instruments, indicating that reverse transformation of the alloy passes through the intermediate R-phase, which reflects the complex phase transformation behavior tracking back to the manufacturing process. Hence, it was perhaps not surprising that K3XF instruments were more flexible and resistant to cyclic fatigue than the K3 instruments (Gambarini et al. 2011, Ha et al. 2013).         10  Figure 2. (A) Differential scanning calorimetry curves of K3XF and K3 instruments. Heating (upper) and cooling (lower) curves are shown. (B) X-ray diffraction patterns for K3XF and K3 at 25 oC, which contain 3 major peaks for the (110), (200), and (211) atomic planes in the austenite phase. (Shen et al. 2013c)     11  1.5 Study rationale       Although both failure modes (fatigue and torque) may occur simultaneously in a clinical situation, most fracture simulation studies of NiTi files have investigated cyclic fatigue or torsional failure separately (Sattapan et al. 2000; Cheung et al. 2005; Gambarini et al. 2008; Larsen et al. 2009; Kim et al. 2010; Bhagabati et al. 2012; Ha et al. 2013, Shen et al. 2014). While the determination of fatigue resistance and ultimate torque of unused instruments can be helpful for comparison or screening purposes, an understanding of the impact of preloading by either type of stresses on the behavior of NiTi instruments is important for those dentists who reuse their instruments. There are very few studies that have correlated these two factors of fracture, especially for heat-treated NiTi instruments. Clinically, NiTi rotary instruments are subjected to a combination of torsional load and cyclic fatigue, but little information is available on such combined effect on the K3XF instruments. Furthermore, it is not known how a previous torsional loading may affect the properties of post-machining heat-treated K3XF instruments. Therefore, the purpose of this study was to evaluate the effect of various degrees of torsional preloading on cyclic fatigue failure of K3XF instruments.  Hypotheses: Torsional preloading may affect the fatigue resistance of K3XF and K3 instruments.   12  Table 1 Mechanical properties of superelastic K3 instruments   Files Bending Torsional property Fatigue property Ninan & Berzins 2013 HyFlex CM, Phoenix Flex,  GT Series X and ProFile Vortex,  ProFile ISO and K3 The shape memory files were more flexible The shape memory files showed a high angle of rotation before fracture but were not statistically different from some of the other files.  Wealleans et al. 2011 K3, Twisted Files and EndoSequence   K3, Twisted Files better than EndoSequence. Hou et al. 2011 K3 and Twisted Files The bending load values were significantly lower for TF than for K3   Gambarini et al. 2008 K3, GTX and  Twisted Files   Twisted Files had significantly more resistance to fatigue than K3. No significant difference between K3 and GTX. Bahia et al. 2008    K3  Cyclic torsional loading caused no significant differences in maximum torque or in maximum angular deflection of the instruments analysed. Significant difference on fatigue resistance between new and previously cyclic torsional loading K3 instruments. 13  Barbosa et al. 2008a K3  No significant difference between torsion only and torsional test after incomplete fatigue test. As the previous angular deformation increases, the number of cycles to fracture in the flexural fatigue test decreases. Barbosa et al. 2008b K3  Electrochemical polishing has no influence on torsional resistance to fracture of K3 Electrochemical polishing has no influence on fatigue resistance to fracture of K3 Melo et al. 2008 K3 Bending moment of the tested files increased significantly with diameter and cross-sectional area at 3 mm from the instrument tip.  Torque increased significantly with diameter. The fatigue resistance decreased as the diameter of the instruments increased. Miyai et al. 2006       EndoWave, HERO 642, K3, ProFile, and ProTaper The bending load values of HERO and K3 were significantly higher than those of EndoWave, ProFile, ProTaper and K-file. The maximum torsional torque values of HERO, K3 and ProTaper were significantly higher than those of EndoWave, ProFile and K-file.  14      Yared et al. 2003a K3  The torque at fracture values of new K3 instruments increased significantly with the diameter  Yared et al. 2003b K3  The torque and angle of rotation at fracture were significantly affected by the repeated use of.06 K3 instruments in resin blocks  15  Table 2 Mechanical properties of K3XF instruments  Files Bending Torsional property Fatigue property Shen et al. 2014 K3, K3XF   The K3XF instruments had a fatigue resistance superior to K3 instruments under dry and aqueous environments. The fatigue life of K3 instruments was similar under both conditions, whereas the Nf of K3XF was greater under water than in air, especially at the size 40/.04 taper. Gambarini et al. 2013          K3XF, ProFile Vortex   No significant difference in resistance to cyclic fatigue when rotary nickel titanium instruments are used in clockwise or counterclockwise continuous rotation. In both directions of rotation, size 04-25 K3XF showed a significant increase in the mean number of cycles to failure when compared with size 04-25 ProFile Vortex. 16  Pérez-Higueras et al. 2013 K3, K3XF and Twist Files   The probability of a longer mean life was greater under reciprocating motion for all of the files. Under continuous rotation, K3XF was more resistant than K3 and TF. TF lasted significantly longer than K3. TF was more resistant to CF when rotated at 300 rpm instead of 500 rpm. Under reciprocating motion, there were no significant differences between K3XF and TF mean lives, but both were significantly longer than the K3 mean life Shen et al. 2013c K3, K3XF The bending load values were significantly lower for K3XF than for K3 in the superelastic ranges. There was no statistically significant difference between K3 and K3XF in the maximum torque or maximum angular deflection before failure. The torque at fracture values of K3 and K3XF increased significantly with the diameter  Ha et al. 2013 K3, K3XF  No significant difference in torsional resistance in terms of ultimate strength, fracture angle, and toughness between K3XF and K3. K3XF showed superior cyclic fatigue resistance to K3. 17  Gambarini et al. 2013 K3XF   Movement kinematics (reciprocating movements in various angles) had a significant influence on the cyclic fatigue life of the tested K3XF instruments. Plotino et al. 2012 K3, K3XF, Mtwo, Vortex   Repeated cycles of autoclave sterilization do not seem to influence the mechanical properties of NiTi instruments except for the K3XF instruments that demonstrated a significant increase in cyclic fatigue resistance.      18  Chapter 2: Methods       For the determination of resistance to cyclic fatigue, unused size 25/.04 taper K3XF and K3 instruments (n = 15 in each group) were placed in a 3-point bending apparatus with a 7 mm radius and 45° curve in deionized water (Shen et al. 2011a & 2012a) (Fig. 3). Briefly, each NiTi instrument was constrained to a curve by 3 rigid, stainless-steel pins; a calibrated digital photograph of the curvature was taken. A 16-mm length from the tip of the instrument was immersed in deionized water at the temperature of 23° ± 2°C. The instruments were then rotated at 500 rpm until fracture to determine baseline scores (Table 3). The fatigue life, or the total number of revolutions to failure, Nf, was recorded.        The torsion tests were performed according to International Organization for Standardization standard ISO 3630-1 (International Organization for Standardization, 2008) using a torsion machine; 3 mm of the instrument tip was secured firmly in a specifically designed soft brass holder. The apparatus was composed of a torque sensor (Futek Model TFF 400, Futek, CA, USA) and a low-speed rotating motor (Fig. 4). The instrument shank was then rotated at 2 rpm until fractures occurred. Before testing, each instrument handle was removed at the point where the handle is attached to the metal shaft. The end of the shaft was clamped into a chuck connected to a reversible geared motor. The torsional load and maximum distortion angle were recorded until the instrument broke (Ullmann & Peters 2005; Gao et al. 2012; Lopes et al. 2013, Campbell et al. 2014).  19       To evaluate the effect that torsional preloading may have on cyclic fatigue, torsional preloading was done on the files under three conditions. Fifteen unused instruments in each group were exposed to either 25%, 50%, or 75% of their respective mean distortion angle at torsional fracture (Table 3). After preloading to various extents, fatigue tests were performed to measure the number of cycles to failure (Nf) and to compare these to instruments that were not preloaded.       The fracture surfaces of the fragments from all instruments were examined under a scanning electron microscope (SEM; Stereoscan 260; Cambridge Instruments, Cambridge, UK). In the instruments that failed due to fatigue only, the number of crack origin(s) for each specimen was recorded. The region in which the dimple area could be found was outlined on the photomicrograph for fatigue failure groups, and measured with ImageJ 1.4 software (National Institutes of Health, Bethesda, MD) on each photomicrograph (Shen et al. 2011a & 2012a). The results were analyzed with the t-test, two-way ANOVA and post hoc analysis using statistical software (SPSS for Windows 11.0, SPSS, Chicago, IL) at a significance level of P < 0.05.   20  Figure 3. Three-point bending apparatus for the fatigue test.    21  Figure 4. Apparatus for the torsional test.      22  Table 3. Baseline scores for K3 and K3XF instruments: fatigue lifespan [the number of revolutions (Nf)], and the maximum angular deflection (Ɵ) at torsional fracture as well as 25%, 50% and 75% pre-loading of Ɵ   Different superscript letters indicate statistically differences between groups (P < .05).    Nf Ɵ (degree) 25% Ɵ 50% Ɵ 75% Ɵ K3 647 ± 139a 797 ± 147  199  399  598  K3XF 1507 ± 52b 810 ± 130  202  405  607  23  Chapter 3: Results       The Nf of K3XF was two times higher than that of K3 instruments (Table 4, Fig. 5)(P < 0.05). The angle of rotation at fracture of K3XF was similar to that of K3 instruments (Table 3). In the torsional preloading groups, a small amount of preloading (25% of the mean of the distortion angles) significantly reduced the Nf of the K3 and K3XF instruments (Fig. 5) (P < 0.05). There was no difference in Nf between the 25%, 50% and 75% preloaded groups of both K3 and K3XF files (Table 4, Fig. 5) (P > 0.05). K3XF instruments had a significantly higher Nf than the corresponding K3 instruments (P < 0.05). There was little difference in the longitudinal or lateral view appearance between new and 25% torsionally preloaded K3 and K3XF files (Fig. 6 & 7); the lateral aspect of the preloaded file did not show any specific topographic features. However, after 50% and 75% torsional preloading, all files had plastic deformation about 3 mm away from the tip (Fig. 6-11) under the lateral view. After 50% & 75% of torsional preloading, the instruments showed numerous microcracks in the area of plastic deformation (Fig. 8,10). Of particular interest, the microcracks did not seem to follow the machining grooves on the instrument surface, but rather ran irregularly (Fig. 8). K3XF instruments had unique surface characteristics with numerous irregular micropores (Fig. 10). The length of the fractured piece ranged from 2.7 - 3.2 mm.        Fractographically, in instruments failed by fatigue only or fatigue after torsional loading, the crack origins, and areas showing microscopic fatigue-striations and dimple rupture, could be identified on all fracture surfaces (Fig. 12-15). Most K3XF and K3 instruments had a single crack origin (Fig. 12). The areas occupied by the dimple region of the total surface area of the 24  fractured cross-sections in these files were slightly larger in K3 (70.1 ± 11.9%) instruments than in K3XF (67.6 ± 10.3%) (Fig. 12), although this difference was not statistically significant (P > 0.05). The fracture pattern of K3 and K3XF with and without preloading was similar. The fractography corresponding to the torsional failure showed the torsional fracture pattern with circular abrasion marks and skewed dimples near the center of rotation (Fig. 13).     25  Table 4. The number of revolutions until fracture of K3 and K3XF after preloading with different levels of the maximum distortion angle (Ɵ)       Different superscript letters indicate statistically differences between groups (P < .05).    0 25% Ɵ 50% Ɵ 75% Ɵ K3 647 ± 139a 462 ± 87b 442 ± 148b 401 ± 121b K3XF 1507 ± 52c 889 ± 213d 849 ± 133d 856 ± 132d 26  Figure 5. The number of revolutions (Nf ) until fracture after preloading with different levels of the maximum distortion angle of K3 and K3XF instruments                    27  Figure 6. Lateral-view scanning electron micrograph of K3XF files with 25% preloading of the maximum distortion angle (A & B); (C) high magnification view of the top one in (B).    28  Figure 7. Lateral-view scanning electron micrograph of K3 files with 25% preloading of the maximum distortion angle (A &B); (C) high magnification view of the top one in (B).         29  Figure 8. Lateral-view scanning electron micrograph of K3XF files with 50% preloading of the maximum distortion angle (A); (B) high magnification view of the top one in (A) with plastic deformation; (C) high magnification view in (B) with micropores and a microcrack.             30  Figure 9. Lateral-view scanning electron micrograph of K3 files with 50% preloading of the maximum distortion angle (A & B); (C) high magnification view of (B) (arrow), an area with plastic deformation; (C) high magnification view of (B) (blank arrow), and (D) high magnification view of (B) (arrow).        31  Figure 10. Lateral-view scanning electron micrograph of K3XF files with 75% preloading of the maximum distortion angle (A); (B) high magnification view of  (A) (arrow), an area with plastic deformation; (C & D) high magnification view of (B) showing micropores and a microcrack.      32  Figure 11. Lateral-view scanning electron micrograph of K3 files with 75% preloading of the maximum distortion angle (A); (B) high magnification view of the bottom two files in (A); (C) high magnification view of (B) (arrow), an area with plastic deformation.         33  Figure 12. Fracture surfaces of K3 (A) and K3XF (B) instruments after separation due to fatigue. (A) K3 instrument with the region of fatigue crack propagation and dimple area outlined (dotted line) with crack origin (arrow); (B) overall view of fatigue crack propagation and dimple area outlined (dotted line) with crack origin (arrow) of K3 XF instrument.      34    Figure 13. Fracture surface after instrument separation due to torque of K3 (A) and K3XF (B). Corresponding surfaces after fracture by fatigue of K3 (C) and K3XF (D).        35  Figure 14. Fracture surface of a file after fatigue failure with 25% preloading of the maximum distortion angle in K3 (A) and K3XF (B).   36  Figure 15. Fracture surfaces of a file after fatigue failure with 75% preloading of the maximum distortion angle in K3 (A) and K3XF (B).     37  Chapter 4: Discussion      Instrument fracture occurs if the forces applied on the instrument exceed its ultimate strength. Flexural fatigue (Pruett et al. 1997, Haikel et al. 1999, Yared et al. 1999, 2000) or torsional failure (Sattapan et al. 2000) of NiTi rotary instruments has been extensively evaluated as separate phenomena. The determination of fatigue and torque at fracture of unused instruments can be helpful for further comparative studies of the complex mechanical properties of these instruments. The impact of torsional fracture, metal fatigue, or fracture of NiTi instruments caused by a combination of torsional stress and accumulation of fatigue is still being debated. When analyzing clinical factors involved in instrument fracture, one must consider both torsional load and cyclic fatigue (Sattapan et al. 2000). However, these are not separate entities, especially in curved canals (Booth et al. 2003). Increasing numbers of NiTi rotary instruments of various designs are now available. Knowledge of simple and complicated fracture modes of thermomechanically treated NiTi instruments is important for the clinician to understand their special mechanical characteristics as well as to provide a rational basis for the selection of new generation instruments.        Only one instrument size of both brands (size 25) was tested because this is a commonly used size for root canal instrumentation. It is difficult if not impossible to quantitatively evaluate the effect of a single variable on mechanical behavior when the instruments have different material properties, designs, dimensions and rotary speeds. K3 and K3XF instruments are identical in structural design and differ only in that K3XF instruments undergo post-machining heat treatment. In the current study, the rotational speed of K3XF and K3 instruments was standardized to 500 rpm to permit direct study of the differences to K3 in the mechanical 38  properties of the K3XF caused by the thermomechanical treatment. The main aim of this study was to provide deep insight into the effect of torsional preloading on cyclic fatigue failure of K3 and K3XF instruments.       The mechanical behavior of NiTi alloy is determined by the relative proportions and characteristics of the microstructural phases. Heat treatment (thermal processing) is one of the most fundamental approaches toward adjusting the transition temperatures of NiTi alloys (Liu & McCormick 1994; Hou et al. 2011; Shen et al. 2011b & 2013c) and affecting the fatigue resistance of NiTi endodontic files. The Twisted File (TF; SybronEndo) is a NiTi rotary instrument with R-phase alloy created through a special thermal process and twisting of the triangular wire into helical form. TF has been reported to have a higher fatigue fracture resistance than ground files (Gambarini et al. 2008; Larsen et al. 2009; Kim et al. 2010; Bhagabati et al. 2012). The R-phase shows good superelasticity and shape memory effects; its Young modulus is typically lower than that of austenite. Thus, an instrument made from the R-phase wire would be more flexible and fatigue resistant. Recently, one study (Shen et al. 2013c) showed that both K3XF and K3 instruments have an austenite structure at body temperature. However, the phase transformation temperature (austenitic transformation finishing temperature) of the K3XF instruments (25 ºC) was significantly higher than it was for the K3 instruments (17 ºC). Differential scanning calorimetric analyses found two overlapping endothermic peaks in the heat plot of K3XF instruments, indicating that reverse transformation of the alloy passes through the intermediated R-phase, and thus reflecting the complex phase transformation behavior relating back to the manufacturing process (Shen et al. 2013c). Hence, it was perhaps not surprising that K3XF instruments were more flexible and resistant to cyclic fatigue than the K3 39  instruments (Gambarini et al. 2011, Ha et al. 2013). The higher resistance to cyclic fatigue was also confirmed by the present study.        It is expected that thermomechanically treated NiTi instruments maintain the same torsional properties as conventional superelastic NiTi instruments. An instrument made from the R-phase alloy would be more flexible, allowing a greater amount of deformation at a similar torque than austenitic NiTi. Wycoff and Berzins (2012) found that the Twisted Files displayed the least amount of torsional stress resistance and the highest distortion angle angle of rotation, compared with traditionally manufactured NiTi instruments of a similar cross-sectional design. In the present study, the torque and angle of rotation at fracture of K3XF were similar to those of K3 instruments. Similar findings have been reported in previous studies (Ha et al. 2013).         The influence of previous torsional angular deformation on the flexural fatigue life on conventional NiTi files has also been studied (Galvão Barbosa et al. 2007, Bahia et al. 2008). Unused K3 instruments were submitted to a pre-defined rotation (90°, 180°, or 420° angular deformation) before undergoing the flexural fatigue test. The results indicated that as the prior angular deformation increases, the number of cycles attained under the flexural fatigue condition decreases (Galvão Barbosa et al. 2007). A reduction in the fatigue resistance was registered even with prior torsional loads below the elastic limit of the material (as low as a 90° angular deformation). However, Cheung et al. (2013) found that the torsional preloads within the superelastic limit of the material may improve the cyclic fatigue resistance of conventional NiTi instruments. The 50% and 75% torsionally preloaded ProFile and all ProTaper preloading groups had a higher number of cycles to failure than the group(s) without preloading. Comparisons 40  between studies, however, cannot be made because different instruments, materials, sample sizes and methodologies were used. In the present study, the slight torsional pre-loading of 25% reduced the flexural fatigue resistance of K3 and K3XF files. This reduction is probably associated with the generation of surface defects during the torsional pre-loading, which can act as crack nucleation sites for flexural fatigue. Interestingly, the fatigue life was affected by even 25% torsional pre-loading, although there was no visible plastic deformation on the files. One explanation may be that any residual stress after the torsional preloads may interact with the propagating fatigue crack and manifest as branching of cracks from the fracture site. It seems that torsional overloads would act in tandem with flexural fatigue to reduce the resistance of NiTi files to failure in clinical situations. The file is likely to be exposed to greater torsional stresses from contacts with the canal, more than with cyclic fatigue during the early stage of canal enlargement (Blum et al. 2003). Therefore, the clinician should be careful when conventional superelastic and heat-treated NiTi files are used for instrumentation of curved canals with elevated friction between the file and root canal wall dentin.        Deformed or fractured instruments after clinical use have been examined with scanning electron microscopy to determine the mode of failure (Peng et al. 2005; Cheung et al. 2005; Cheung & Darvell 2007; Shen et al. 2009). Two distinct fracture mechanisms have been identified on detailed examination of the fracture surfaces: (1) fatigue failure, characterized by the presence of fatigue-striation marks, and (2) torsional failure, characterized by circular abrasion marks on the fracture surface. Fractographic examination, in conjunction with longitudinal examination, is necessary to reveal features that indicate the crack origin and the mode of material failure. The fractographic characteristics of the K3 and K3XF instruments were 41  very similar. In most K3 and K3XF instruments, the crack origins were usually found at the cutting edge region. This is to be expected because when a circular beam is bent, its outermost areas are subjected to the greatest stress and strain. Numerous micropores with various diameters were seen on the surface of the instrument flute on K3XF instruments. According to the SEM images, these small pores did not seem to contribute to the failure. A similar finding was reported by Ha et al. (2013). The vast majority of K3 and K3XF instruments had one crack origin. In related studies (Shen et al. 2011a & 2012a), controlled memory (CM) wire instruments (thermomechanically treated NiTi instruments) had a higher number of multiple crack origins than conventional NiTi wire instruments. Despite the significantly greater number of crack origins, the values of the fraction area occupied by the dimple region seemed to correlate with fatigue life (Shen et al. 2011a & 2012a). This is in accordance with the present study. The fatigue life of unused K3XF was two times higher than that of K3 instruments. Therefore, the values of the fracture area occupied by the dimple region (indicating the final, momentaneous separation) in K3XF were slightly larger than they were in K3 instruments. However, the difference between K3 and K3XF dimple area sizes was much smaller than previously reported for CM files and conventional superelastic NiTi files with identical design and dimension as the CM files (Shen et al. 2011a). This together with the present study and several other studies is one of many pieces of evidence suggesting that there are a variety of different outcomes in instrument metal characteristics resulting from various types of heat treatments (Gutmann & Gao 2012, Shen et al. 2013a). The patterns established in this study for the fractured files supply a consistent base for the interpretation of fracture occurring under complex loading. The results showed that in the case of both heat-treated NiTi instruments and conventional superelastic NiTi instruments in which the fatigue test was applied after an incomplete torsional loading the 42  fractography corresponded to the fatigue fracture pattern. These data are in accordance with previous studies with conventional NiTi files (Barbosa et al. 2008; Kim et al. 2012). It should be emphasized though that the complexity of the clinical situation cannot be fully reproduced in vitro. However, the test method in the present study may be one step closer to a more comprehensive representation of clinical conditions.      43  Chapter 5: Conclusion        The fatigue resistance of K3XF instruments was twice as high as that of K3 instruments. The angles of rotation at fracture of K3XF instruments were similar to those of K3 instruments. After 50% and 75% torsional preloading, all files had plastic deformation about 3 mm away from the tip in lateral view.  Even a small amount of torsional preloading significantly reduced the fatigue resistance of both K3 and K3XF instruments. In post-machining heat-treated K3XF instruments and conventional superelastic NiTi K3 instruments in which the fatigue test was applied after submaximal torsional loading, the fractography corresponded to the fatigue fracture pattern. Further study is needed to evaluate the effect of various degrees of cyclic fatigue on torsional failure of K3XF and K3 instruments.  5.1. Future directions The limitations of earlier studies, including the present study, are that the testing is not done in /on dentin and that there is no accepted way to simultaneously expose the instruments to both fatigue and torque. Therefore, to improve the validity of the results of instrument research, new models need to be developed which would allow simultaneous exposure of the instruments to different challenges in a natural environment.    44  References  ASM International (1987) ASM Handbook, Vol. 12: Fractography. Metals Park, OH, USA: The Materials Information Society, pp. 12-165.  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