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Cyclic fatigue of ProTaper Gold in single and double curvature canals Algahtani, Fahda N 2018

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 CYCLIC FATIGUE OF PROTAPER GOLD IN SINGLE AND DOUBLE CURVATURE CANALS by  Fahda N Algahtani   BDS, Riyadh Colleges of Dentistry and Pharmacy, 2011  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)  August 2018  © Fahda N Algahtani, 2018  ii  Committee Page The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled:  Cyclic fatigue of ProTaper Gold in single and double curvature canals. submitted by Fahda N Algahtani in partial fulfillment of the requirement for the degree of Master of Science in Craniofacial Science.  Examining Committee:  Ya Shen Supervisor  Markus Haapasalo  Co-supervisor  Jolanta Aleksejūnienė Supervisory Committee member   Ahmed Hieawy Supervisory Committee member   Nesrine Mostafa  Additional examiner     iii  Abstract  Objectives: This study aimed to evaluate and compare the fatigue resistance of ProTaper Gold (PG) and ProTaper Universal (PU) in artificial single and double curvature canals in 5% sodium hypochlorite (NaOCl) at body temperature. Methods: PG and PU files (size F1) were subjected to fatigue tests inside two different custom-made ceramic artificial canals. The first one was a single curvature canal (group 1: 60° curvature, 5 mm radius) and the second was a double curvature canal (group 2: first [coronal] curve of 60° curvature and 5-mm radius and the second one [apical] of 30° curvature and 2 mm radius). The artificial canals were milled in an InCoris ZI zirconium oxide disc (Dentsply Sirona, Bensheim, Germany) using the inLab MC X5 Digital CAD/CAM System (Dentsply Sirona). The first 19 mm of each file tip was introduced into the artificial canal which was immersed in either distilled water or 5% NaOCl at body temperature (37oC). The total number of cycles to failure (NCF) was recorded, and the length of the detached fragments was measured. Data were analyzed using t-test and multiple linear regression analyses.   Results: The fatigue performance of PG is better than that of PU (P<0.001) in all tested groups. The NCF of ProTaper files was significantly influenced by the type of file (β=0.854; P<0.001), canal curvature (β=0.147; P=0.003), and the type of medium solutions used (β=0.100; P =0.044). The length of the broken instrument of PU was longer than PG files. Conclusions: The fatigue performance of PG is better than that of PU. The double curvature canal represents a challenge for ProTaper files and the presence of the 5% NaOCl irrigation solution adversely affects the fatigue performance.  iv  Lay Summary  The presence of a fractured instrument within the space of a root canal treated tooth is an unwelcome incident for the practicing dentist and the patient. The difficulty of removal of the broken file, together with the possible need for surgical intervention and patient inconvenience, influenced the researchers to study the effect of new file metallurgy in improving the instrument resistance to fracture. Also, the researchers were interested in studying two factors that could increase the chance of instrument failure, namely the cleaning solution and the complex canal anatomy. The findings confirm that the thermal processing of the file significantly improved the file’s resistance to fracture. Also, the findings revealed the negative impact of sodium hypochlorite (NaOCl) cleaning solution and complex curvature on the instrument’s life expectancy. The results helped clarify two causes that accelerate the occurrence of file fracture. Further research to ratify the influence of irrigation solution in mechanical properties of NiTi alloy rotary files is needed since this might propose the benefit of reducing the concentration of the NaOCl cleaning solution, especially in complex anatomical cases. Also, the findings may encourage the development of files that resist the corrosive nature of NaOCl irrigation solution that is commonly used in root canal treatment.  v  Preface  This thesis is an original work written by the candidate, Fahda Algahtani, as part of the prerequisite for a Master of Science in Craniofacial Science combined with a Diploma in Endodontics at the University of British Columbia.  Fahda Algahtani participated in 60% of the overall design of the study, the performance of the experiments, analysis, and interpretation of the results, and writing of the thesis (80%).  Dr. Ya Shen, as the main supervisor, participated in the planning of the study, constructing the test apparatus, providing guidance and tutoring during various stages of the study, contribution to study (20%), and editing of the thesis (10%). Dr. Markus Haapasalo, as a co-supervisor involved with Dr. Ya Shen, contributed to research planning and provided guidance (10%). Dr. Jolanta Aleksejūnienė, as a committee member, was involved in the design, statistical analysis, and interpretation of the data, providing guidance in statistics. Dr. Aleksejūnienė contributed to the study (5%) and participated in editing the thesis (5%). Dr. Ahmed Hieawy, as a committee member, provided consultation and support in the study. Zhe-jun Wang provided help in obtaining the SEM images needed for the study, while Xiangya Huang provided consultation for the experiment. The research team contributed up to 5% participation in the study. The Canadian Academy of endodontist provided financial support for the experiment. The SEM examination and images were attained inside the Centre for High-Throughput Phenogenomics at UBC. A Canadian dental manufacturer, Dentsply Sirona, provided the rotary files needed. The author obtained the copyrights of the figures copied from the literature.    vi  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents………………………………………………………………………………..vi List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Abbreviations .................................................................................................................. xii Acknowledgments ...................................................................................................................... xiii Dedication ................................................................................................................................... xiv  Introduction ..................................................................................................................1 1.1 Stainless steel files and the development of nickel titanium files .................................. 1 1.1.1 The basic structure and properties of NiTi alloy ........................................................ 2 1.2 The fracture mode of rotary files .................................................................................... 6 1.2.1 Methodologies used for studying cyclic fatigue failure .............................................. 7 1.2.2 The appearance of the fragments under Scanning Electronic Microscope (SEM) ... 10 1.2.3 The clinical significance of fracture ......................................................................... 11 1.3 The five generations of rotary files ............................................................................... 12 1.4 ProTaper Gold (PG): the new version of ProTaper Universal (PU) ............................. 13 1.4.1 The trajectory of PG in comparison to PU................................................................ 15 1.4.2 The effect of temperature on the performance of PG ............................................... 16 1.4.3 The effect of NaOCl irrigation on the cyclic resistance of ProTaper Gold .............. 16 vii  1.5 Rationale ....................................................................................................................... 18 1.6 Objectives ..................................................................................................................... 18 1.7 Null hypothesis ............................................................................................................. 19  Material and methods ................................................................................................20 2.1 Study design and statistical power calculations ............................................................ 20 2.2 Designing the double curvature and single curvature testing apparatus ....................... 20 2.3 The cyclic fatigue test ................................................................................................... 22 2.4 SEM analysis of broken file surfaces............................................................................ 23 2.5 Statistical Analysis ........................................................................................................ 24  Result ...........................................................................................................................25 3.1 Quantitative analyses .................................................................................................... 25 3.2 Qualitative analyses ...................................................................................................... 36  Discussion ....................................................................................................................38  Conclusion ...................................................................................................................42 References .....................................................................................................................................43  viii  List of Tables  Table 1. The performance (measurement: Number of Cycles to Failure) of the ProTaper Gold and ProTaper Universal in two curvature models and two medium solutions. ............................ 26 Table 2. Predictors of File failure ................................................................................................. 32 Table 3. Comparison of the length (mm) of fractured PG and PU files in the single and double curvature canals ............................................................................................................................ 32                  ix  List of Figures  Figure 1. The influence of temperature changes on the microstructure of NiTi alloy. A. The diagram represents the reversible transformation between the martensite and austenite phases. B. The NiTi wire diagrammatic transformation.[10] .......................................................................... 4 Figure 2. The thermal hysteresis of NiTi alloy and the four transition temperatures associated with it.[10]....................................................................................................................................... 4 Figure 3. The effect of heat treatment in NiTi file metallurgy as shown in DSC curves of TyphoonTM CM compared to Typhoon file. A. The conventional superelastic Typhoon file Af temperature is below 37oC. B. The new generation files with controlled memory technology exhibited more sophisticated phase transformation behaviour, and the Af temperature was 54oC.[15] ......................................................................................................................................... 6 Figure 4. The five methods used to study the cyclic fatigue failure. A. The metal tube.  B. Rotation against an inclined plane. C. Grooved black and rod assembly. D. Three-point bending test. E. Artificial canal. ................................................................................................................... 9 Figure 5. The typical features of torsional and cyclic fatigue failure in the fractographic analysis. (A, C) Fractographic figures of the fractured surfaces of PG and PU, respectively, in torsional failure. (B, D) The fractured surfaces of the previously mentioned files in cyclic loading.[25] .. 10 Figure 6. DSC curves of size F1 PG and PU files.[12]................................................................. 14 Figure 7. Computer-aided design and manufacturing of the single curvature model. A) Schematic drawings of the single curvature model. B) PG inside the model of the single curvature artificial canal. C) PU inside the model of the single curvature artificial canal. ......................................... 21 x  Figure 8. Computer-aided design and manufacturing of the double curvature model. A) Schematic drawings of the double curvature model. B) PG inside the model of the double curvature artificial canal. C) PU inside the model of the double curvature artificial canal. ......... 22 Figure 9. The rotary handpiece is mounted in the mobile supporting device, and the tempered glass container is held above the hot plate. The first 19 mm of the file was inserted inside the artificial ceramic canal. A) PG inside the double curvature model. B) PU inside the single curvature model. ........................................................................................................................... 23 Figure 10. Interpretation of a box-whisker plot.[58] .................................................................... 24 Figure 11. The NCF values for ProTaper Universal files in single curvature canal when tested in two different solutions of distilled water and 5% NaOCl. ............................................................ 27 Figure 12. The NCF values for ProTaper Universal files in double curvature canal when tested in two different solutions of distilled water and 5% NaOCl. ............................................................ 28 Figure 13. The NCF values for the ProTaper Gold files in single curvature canal when tested in two different solutions of distilled water and 5% NaOCl. ............................................................ 29 Figure 14. The NCF values for ProTaper Gold files in double curvature model when tested in two solutions medium of distilled water and 5% NaOCl. ............................................................ 30 Figure 15. The NCF of the ProTaper Gold (PG) and ProTaper Universal (PU) files in two curvature models and two different medium solutions of distilled water (H2O) and 5% NaOCl. SC: single curvature canal; DC: Double curvature canal. ............................................................ 31 Figure 16: The fractographic appearance of the fractured surface of the PU in distilled water and double curvature group showing the classic features of CF failure. A) One crack origin (arrow) followed by an area of steady-crack growth and rapid-crack growth. B) Higher magnification showing an area of shear lip at the peripheries of PU cross-section. C) Higher magnification to xi  show the crack origin and steady growth of the crack. D) Higher magnification of shear lip in the file peripheries. E) Higher magnification in the center of a cross-section showing the junction between the zones of steady-crack growth and the rapid-crack growth. ...................................... 34 Figure 17: The fractographic appearance of the fractured surface of PU when tested in single curvature model and 5% NaOCl. A) There are two crack origins (long arrows), steady-crack growth zone and microscopic striations representing the rapid fracture zone. The shear lip at the file peripheries is also apparent (short arrow). B-D) Higher magnification the cutting edges of PU. E) The center of the file showing the microscopic striations and dimples which are typical features of CF failure. ................................................................................................................... 35 Figure 18: The fractographic appearance of the fractured surface of the PG file after testing in double curvature model and 5% NaOCl. A) Two crack origins at the cutting edge (arrows), steady-growth zone followed by microscopic striations. B&C) The higher magnification of the crack origin in two different locations. D) The rapid zone of fracture and the characteristic feature of dimples is apparent. E) The center of the cross-section at higher magnification. ........ 36 Figure 19: The pathognomonic features of CF failure are visible across the surface of the fractured PG files when the PG files were tested in single curvature model and 5%NaOCl (A) or distilled water (C). B&D) The higher magnification at the center of the fractured surface shows the microscopic striations and dimples at the two mentioned conditions respectively. ............... 37   xii  List of Abbreviations  As                                  Austenite Start Temperature Af                                  Austenite Finish Temperature Ms                                  Martensite Start Temperature Mf                                  Martensite Finish Temperature oC                                   Degree Celsius CF                                  Cyclic Fatigue DSC                               Differential Scanning Calorimetry GCM                             Gram Centimeter NCF                               Number of Cycles to Failure  NiTi                               Nickel Titanium PG                                  Protaper Gold PU                                  Protaper Universal             RCT                               Root Canal Treatment  SEM                               Scanning Electron Microscope TF                                  Torsional Fatigue    xiii  Acknowledgments  It was a great pleasure to work on my research with Dr. Ya Shen, who is an experienced, successful researcher, and easily approachable person. Your polite manners and nice smile are only matched with your positive attitude and nature. I want to express my gratitude to Dr. Coil for accepting me in the program and for his mentorship. I was a foreigner to the system, and with his kindness, I was able to acclimatize with my surroundings quickly. You can't easily pass by Dr. Haapasalo without noticing his intelligence and inspiring modesty. Your alertness and good judgment were essential for my research. Ahmad, you showed me how to be both an excellent student and an amazing educator. You are from a similar background, and we speak the same language which made things simpler for me when needed. Thank you, Dr.Aleksejūnienė for being my source of confidence in biostatistics and for your lovely participation in my study. I want to thank God for answering my prayers and strengthening me. Special thanks to my father and my stepmother Suzan for their support. I would not be able to do this without the help of my mother Hessa who loved me unconditionally and helped me to become a mother and a resident at the same time.  Honest appreciation to my husband, life partner, and friend Abdulmaged, it is your generous delightful love that made my life lighter and happier.   xiv  Dedication  This dissertation is dedicated to my first son and wonderful love Nabeel.  Every precious minute that I spent preparing and writing my thesis was a minute that I wanted to spend with you.  My real joy will be to see you and your brother Micheal as fully-grown men who have the best future opportunities.   Introduction  The primary cause of endodontic diseases is bacteria,(1, 2) and therefore bacterial biofilm disinfection of the root canal system is essential for predictable healing and successful outcomes. The anatomy of the root canal system is complex, and bacteria find this complexity to be a desirable environment to accommodate and grow. The root canal treatment is composed of multiple steps in which the dental provider accesses the infected canals, clean the canals, then fills and seals the space to prevent reinfection. The process of cleaning the root canal involves shaping the canal with files to provide sufficient space for irrigation solution to disrupt the biofilm and kill the bacteria along the entire length of the canal. Also, these files participate in the disinfection process by removing infected dentinal debris, dead tissue, and material or foreign bodies from of the root canal space. Therefore, these files are essential instruments for achieving the objectives of a root canal treatment. Consequently, the design and metallurgy of files were frequently studied in literature and many improvements have been made to enhance the clinical performance of these instruments, the quality of care, and ultimately patient experience.(3)   1.1 Stainless steel files and the development of nickel titanium files The systemic use of endodontic files was initiated by Ingle in 1958 when he standardized the design, taper, and sizes of manual files.(4, 5) The file design referred to as a “K file” was made by twisting stainless steel tapered wires to fabricate different sizes of 0.02 taper files. Then, modifications of this design were produced with similar sizes and taper to overcome some of the limitations of the traditional K files.(6) However, none of these designs could overcome the 2  restriction that is innate to properties of stainless steel alloy - which is stiffness. The stiffness of stainless steel alloy is desirable in small sizes such as six, eight, ten, and 15 when the clinician wants to bypass the small constricted root canals to reach working length at the beginning of the therapy. Stiffness becomes an obstacle with larger sizes, especially in curved canals where many irreversible iatrogenic mishaps are expected, such as ledging, transportation, or zipping. In 1988, Walia introduced the nickel-titanium (NiTi) alloy to endodontics and published promising results indicating that the new file metallurgy resulted in the production of significantly more flexible files that were more resistance to torsional failure compared to traditional K type files.(7) This remarkable improvement in mechanical properties encouraged the market to shift the manufacturing focus toward varieties of products, including electrically driven rotary files.   1.1.1 The basic structure and properties of NiTi alloy  The nickel-titanium alloy, or Nitinol, was unintentionally made by William Buehler and Frederik Wang while working in a Naval Ordnance Laboratory to find a metal that was heat resistant and ideal for the synthesis of nose cones for navy vehicles. Buehler elaborated that for personal reasons he spent most of his time in the laboratory, therefore engaging himself in the project and shifting his interest from doing calculations of existing metals to developing new alloys.(8) After discovering the unique properties of this alloy, which are superelasticity and shape memory, there were countless applications for the use of this revolutionary material. The initiation of actually synthesizing and employing NiTi alloy in variable products was slow because of the difficulty and high expenses associated with the alloy production. Eventually improved industrial knowledge was gained over subsequent years, and there was an obvious market need. Therefore, 3  Nitinol became a well-known material for synthesizing orthodontic wires, endodontic files, many medical devices including heart stents, pipes, heat engines, eyeglasses, etc.(8)  Nitinol’s name comes from its basic composition which is nickel and titanium. While the ending –nol was added as a symbol for its discovery in the Naval Ordnance Laboratory.(8) The proportion of each component is almost equal, with a slight increase in the percentage of nickel such as Nitinol 55 or Nitinol 60. In endodontics, Nitinol 55 is more commonly used than Nitinol 60 since this nearly equiatomic composition allows for greater shape memory.(9) Shape memory is a term used to describe the ability of a material to deform at a certain temperature and to recover back to its original shape when heated. Therefore, this property is closely related to temperature changes. On the other hand, superelasticity occurs within a certain temperature range where the material can be challenged or withstand applied stresses without being permanently deformed after the stress is relieved. Shape memory and superelasticity are two properties that are almost exclusive to nickel-titanium alloy, with few exceptions, because this material expanded the initial knowledge about these qualities.(8, 10) The unique abilities of Nitinol are imparted by reversible changes between two crystal structures, which are the austenite and the martensite. In the austenite phase, also called the parent phase, the atoms form a body-centered cubic lattice, and the complete phase transformation is stable at relatively high temperatures. The transformation into the martensite phase, on the other hand, is stable at low temperatures, and the atoms form a monoclinic crystal structure (Figure 1).(10) (8)It is important to recognize the four transition temperatures associated with this kind of transformation (Figure 2). The four transition or transformation temperatures are the Austenite start (As) temperature, Austenite finish (Af) temperature, Martensite start (Ms) temperature, and Martensite finish (Mf) temperature (Figure 2).(8, 10)   4   Figure 1. The influence of temperature changes on the microstructure of NiTi alloy. A. The diagram represents the reversible transformation between the martensite and austenite phases. B. The NiTi wire diagrammatic transformation.(10)      Figure 2. The thermal hysteresis of NiTi alloy and the four transition temperatures associated with it.(10)   Multiple methods are used to measure the transformation temperature of Nitinol alloy, such as the use of Differential Scanning Calorimetry (DSC), the active Af test, constant load test, 5  etc.(11) In endodontics, there were attempts to measure the transition temperatures for multiple NiTi rotary files, such as Protaper Universal (PU), Lightspeed, and profile.(12, 13) The Af temperature for traditional NiTi rotary files such as Profile, Lightspeed, and Quantic was about 25°C, which is the same as room temperature.(13) Studying the Af temperature was particularly important since it indicates the composition, and subsequently the behavior, of the metal at room temperature or human body temperature (37°C). For example, if the Af temperature was lower than room temperature, then the material will be austenitic and will be superelastic when used in the same reference temperature. However, if the Af temperature was higher than room temperature, then the material will be partially or completely martensitic and will show controlled memory behaviour. Therefore, since traditional NiTi files are completely austenitic at room temperature, this clarifies the observable superelasticity of these files.(13)  The application of stress induces a similar transformation behaviour. For instance, if the Nitinol wire is austenitic at room temperature (Af < 25°C) and is subjected to stress that is within the elastic limits of Nitinol, the wire will reversibly change into the martensitic phase until the stress is relieved.(8, 10) The elastic flexibility of Nitinol files was reported to be two to three times higher than stainless steel files in combination with superior resistance to torsional fracture.(7) Further modifications of rotary file manufacturing techniques were implemented gradually to improve the mechanical properties of the rotary files.(14) Intially, the modifications were directed toward surface treatment by using methods such as electropolishing or ion implementation.(14) However, heat treatment of the NiTi wire before or after machining significantly enhanced the cyclic fatigue resistance of the rotary files.(14) The DSC curves of the heat-treated files revealed a higher Af temperature and the presence of two-step transformation behaviour in which an R-phase was an additional phase between austenite and martensite (Figure 6  3). The flexibility of the material after thermal processing is superior to conventional superelastic alloys.(14) Industrial companies have mentioned that the alloy, in this case, lose the shape memory and exhibit no memory or controlled memory behaviour.(14)  Figure 3. The effect of heat treatment in NiTi file metallurgy as shown in DSC curves of TyphoonTM CM compared to Typhoon file. A. The conventional superelastic Typhoon file Af temperature is below 37oC. B. The new generation files with controlled memory technology exhibited more sophisticated phase transformation behaviour, and the Af temperature was 54oC.(15)  1.2 The fracture mode of rotary files There are two main types of fracture associated with the use of rotary files.(16) The first one is known as a torsional failure and happens when the apical part of the file binds inside the canal while the coronal half continues to rotate until there is complete separation of the file into two segments.(16) Usually, the manufacturers recommend avoiding using the rotary file as the first file used to negotiate the canal and instead to establish a glide path with manual files to the working length to reduce the chance of file binding at the apex. Also, frequent irrigation, cleaning of the file flutes during use, use of files in the right order, using a crown-down technique, and a gentle picking motion were all mentioned as the best ways to ensure safe 7  practice.(16) However, the second type of fracture, known as cyclic fatigue, is predominantly material dependent.(16, 17) The instrument fails during free rotation inside a curvature due to alternating rapid cycles of tension and compression at the point of maximum curvature.(16, 17)  Multiple risk factors increase the risk of cyclic fatigue, such as acute radius and angle of curvature.(16, 17) Nevertheless, the risk remains unpredictable.(18, 19) Therefore, this type of fracture has frequently been studied since it is imperative for the clinicians to treat complicated root anatomy using instruments that are safe and durable for this task.    1.2.1 Methodologies used for studying cyclic fatigue failure  The American National Standards Institute/American Dental Association (ANSI/ADA) specification No. 28 has not identified a test for cyclic fatigue.(17, 20, 21) Rotary files need a dynamic test inside an apparatus that resembles the canal geometry.(17) Pruett et al. identified curvature and radius as essential parameters when testing the cyclic fatigue resistance of rotary files.(17)  Cheung recognized four testing methods used for studying cyclic fatigue failure.(16) The first of which is the curved metal tube (Figure 4). The metal tube can be a hypodermic needle with a single diameter of 1-2 mm, with dry testing conditions. The shortcomings are lack of visibility of the rotating instrument and a negligible disproportional diameter size in relation to the file size. Additionally, the fatigue life of controlled memory files is significantly reduced in air in comparison to liquid. Plotino and colleagues mentioned that the morphologic and geometric features of each instrument change the file rotation path inside the testing tubes, hence study results using this method might be unreliable.(21) Additionally, he acknowledged the unpredictability of replicating the same angle and radius in every metal tube, together with the 8  possibility for vibration of the file within the loosely fit tube. The second method is the grooved black and rod assembly in which the experiment runs dry within the curve. The direct visualization of the instrument is improved in comparison to the metal tube method. Additionally, some variation can be minimized by customizing the test to the size of the file being studied. The third method is the use of an inclined plane, although this has been criticized due to unstable parameters. The fourth method is the three-point bend test, which did not increase the precision of the experiment.(16)  Plotino et al. added the description of a fifth technique which involves an artificial canal that is customized to be identical to the dimensions of the chosen file system while allowing the free movement of the file within the canal (Figure 4).(21) Therefore, an additional 0.1 mm is added to the file diameter to achieve the objective of the test. The artificial canal is milled inside a metal block, and the visualization of the rotating instrument is obtained by covering the outer surface of the metal block with tempered glass.(21) The authors usually use synthetic oil for lubrication, nevertheless, the use of sodium hypochlorite to mimic the irrigation medium used in clinical situations was recommended.(21) The advantages of this method are the accurate standardization of parameters, visualization of the running experiment, and the presence of liquid inside the canal during file rotation.(21)  The importance of a liquid medium is emphasized in literature since it will reduce the temperature and the temperature is a significant factor in modifying nitinol behaviour.(16, 21, 22) There are two sources of heat during file rotation, they are the friction between the instrument and the canal walls, mixed with the internal friction between the reversible transforming phases of austenite and martensite.(16) The heat-treated alloy is more sensitive to the presence of liquid compared to the superelastic alloy.(22) One possible reason is that the 9  short life of a conventional superelastic alloy does not allow sufficient time to study the influence of various factors such as temperature.(22) Another reason that explains this finding is the increase of the austenite phase at high temperatures in heat-treated files. Therefore, this will lead to a reduction in the file flexibility and will have a noticeable effect on the number of cycles to failure (NCF). The use of an irrigation medium during file rotation was suggested since this will allow a closer clinical simulation and a chance to study the corrosion-fatigue failure in NiTi alloy.(16, 21)     Figure 4. The five methods used to study the cyclic fatigue failure. A. The metal tube.  B. Rotation against an inclined plane. C. Grooved black and rod assembly. D. Three-point bending test. E. Artificial canal.     10  1.2.2 The appearance of the fragments under Scanning Electronic Microscope (SEM) An examination of the fractured surface of nitinol under a microscope indicates the type of failure (igure 5). Pruet and colleagues identified three stages of cyclic fatigue fracture, beginning with crack initiation at the periphery, then propagation through striations, and finally a rapid zone of fracture or an ultimate ductile fracture.(17) A ductile fracture involves plastic deformation and necking before complete separation of material fragments.(23) Nitinol, gold, gray iron, and aluminum are considered ductile materials. Thus they absorb significant energy before they fracture. The surface area after a torsional failure is characterized by the presence of a dimple at the center of rotation surrounded by concentric circular abrasion marks.(24) The flutes are not distorted in a cyclic failure, while the flute’s distortion and unwinding are apparent with torsional failure.(19)  Therefore, cyclic fatigue failure is considered an unpredictable file fracture.  Figure 5. The typical features of torsional and cyclic fatigue failure in the fractographic analysis. (A, C) Fractographic figures of the fractured surfaces of PG and PU, respectively, in torsional failure. (B, D) The fractured surfaces of the previously mentioned files in cyclic loading.(25)   11    1.2.3 The clinical significance of fracture  The prevalence of rotary file fracture was estimated to be in the range of 0.9 to 23% depending on the study.(26-29) Alapati and colleagues reported the fracture of 23% of 52 discarded PU instruments in a graduate endodontic program.(29)  Case-control retrospective studies suggest that the presence of a retained instrument in the canal did not significantly affect the overall prognosis of treatment.(30, 31) However, the presence of a periapical pathology was the main factor influencing the prognosis of the treatment.(30, 31) Apparently, bacteria is the main cause of treatment failure, and the mere presence of a fractured instrument within the root canal parameter is not a cause for failure, rather than a complication of treatment.  The objectives of root canal therapy remain the same whether a fractured instrument is retained or not. Although, the retained instrument can make achieving these goals difficult to impossible, depending on the case presentation.(16) The cleaning and shaping of the root canal system are compromised if the instrument was not removed or bypassed adequately. The location of the instrument within the canal is essential for the management of this complication.(32) The presence of a fractured instrument coronal to the curvature might permit an attempt of careful removal of the obstruction, without significant removal of the tooth structure. However, if the file was broken below the curvature, the removal will be challenging, and a considerable amount of the dentin will be lost in this endeavour.(32, 33) Therefore, literature advocates leaving the broken instrument behind in these situations or surgical removal if necessary.(32, 33) Unfortunately, the location of the instrument fracture in cyclic fatigue failure is below the curvature. Additionally, the removal of NiTi rotary files is even more challenging than stainless steel files due to their superelasticity, engagement in dentinal walls after rotation, easy fracture 12  into smaller pieces when ultrasonic energy is applied, and the increased taper of these instruments.(33)     In conclusion, rotary file fracture is an inconvenient, frustrating clinical event due to the difficulties encountered during attempted removal or bypassing of the broken fragment. The patient’s perception of the instance might vary. Nevertheless, some patients might translate this incident as a negative experience and remain unhappy regardless of the treatment outcome. The need for an additional surgical procedure is considered on many occasions in cases where the patient became symptomatic, or the tooth shows radiographic signs of failure.(33)   1.3 The five generations of rotary files The process of developing new concepts in designing and producing rotary files has been continuous since the production of the first rotary system. At present, there are five recognizable generations of rotary files, as mentioned by Haapasalo and Shen.(3) - The first generation is known for the presence of non-cutting radial lands combined with a fixed taper of .04 or .06.  - The second generation marks the production of cutting edges and variable taper. Therefore, fewer files were needed to prepare the canals in comparison to the first generation. ProTaper Universal (PU), K3, EndoSequence, and BioRace belong to this category.  - The third generation is characterized by the incorporation of thermomechanical processing to increase the transitional temperature. The thermal processing increases the instrument flexibility and improves cyclic fatigue resistance. Some of the files preserved the same design as in the second generation while being heated. These include the 13  ProTaper Gold (PG), Profile Vortex, and K3XFF. Additionally, new files with this metallurgy were introduced to the market such as HyFlex and Twisted files.   - The fourth generation represents the innovation of reciprocating motion in the electrically driven system. This new motion decreased the number of files required to prepare the root canal to one file in the WaveOne system.  - The fifth generation signifies the introduction of the offset center of rotation to reduce the contact with dentinal walls during rotation. Protaper Next and One Shape are two known systems that belong to this group.       1.4 ProTaper Gold (PG): the new version of ProTaper Universal (PU)  The introduction of PU signifies the start of the 2nd generation of rotary files, in which fewer numbers of files are needed to prepare the canals, improving the efficiency and productivity of the practitioner.(3) The system is composed of seven different sizes and one orifice opener. The major distinctive features in the design are the convex triangular cross-section and the variable taper, pitch, and helical angle.(3, 34) This design allowed high cutting efficiency and preparation of canals using the crown-down concept while minimizing torsional load and stresses.(34)  Clinicians are advised to use the orifice opener, then prepare the canal for the first three sizes, before completion of the root canal treatment (RCT). However, large canals may require the use of bigger file sizes. Therefore, four additional larger sizes are included in the system. The recommended maximum number of uses range between eight canals for relatively straight canals (<10º) and two canals for considerably curved canals (>30º) or double curved canals.(35, 36) Moreover, the manufacturer warns about the risk of degradation in the hydrogen peroxide solution and advises the use of less than 5% NaOCl for disinfection.(35, 36) 14  The new generation, PG belongs to the third generation of rotary instruments, in which thermal processing was used to improve the mechanical properties.(3) The Af temperature of PG was 50oC in comparison to 21oC for PU, as was shown in the DSC analysis (Figure 6).(12) Consequently, PG files were more flexible with superior cyclic failure resistance and represented a better choice for curved canals.(12, 25) However, the torsional stress resistance and microhardness values were inferior to PU.(25)    Figure 6. DSC curves of size F1 PG and PU files.(12)   The double curvature or “S-shaped” canals are challenging, and studying CF performance of PG might illuminate the significance of heat treatment in improving the file mechanical properties. Duke et al. found no significant difference between Profile Vortex (PV) and Vortex Blue (VB) when tested in the double curvature model, although VB was more flexible than PV.(37) The following topics are three important aspects that have influenced the design of this study.  15   1.4.1 The trajectory of PG in comparison to PU  Flexibility increases the ability of the file to withstand bending stresses, and elasticity is the ability of the file to resist distortion forces and recover its original shape. The Nitinol instrument is both flexible and elastic. Therefore, the trajectory of Nitinol files within the canal may vary according to the alloy composition, manufacturer processing, the motion used (reciprocation or rotation), and instrument design - which includes the cross-section, the diameter, and the taper. The trajectory is a term used to describe the pathway that files follow during the shaping procedure since it can deviate from the canal peripheries. The centering ability of an instrument reflects its tendency to keep the outline of the original canal unaltered while shaping all the dentinal walls equally. This property allows adequate cleaning while minimizing unnecessary structure loss.   The transportation or deviation from the canal center was studied in different methodologies. Variability in testing methods led to variabilities in the results. When natural teeth were used to study transportation, there was no significant difference in the centering ability between PG and PU.(38-41) However, one study found significant differences between them when used in the mandibular mesial canals.(42) When resin blocks of identical canals were used, PG performed significantly better than PU in preserving the canal parameters.(43) In general, most of the studies mentioned that the centering ability of PG was significantly or insignificantly better than PU, which reflects the improved flexibility of the heated alloy.  Hence, these studies can further confirm previous observations of the impact of different file trajectory in cyclic fatigue test results.(16, 21) Predictable testing results can only be attained 16  when all variables are standardized. Therefore, the canal geometry in the testing apparatus should be identical to the file size while allowing unrestricted rotation.(21)   1.4.2 The effect of temperature on the performance of PG  Nitinol alloy is sensitive to temperature since temperature reversibly transforms the alloy from martensite to austenite. Further heat treatment of Nitinol made the material sensitivity even more noticeable because the martensitic component is available at room temperature. Consequently, it is expected that thermal processing will make the alloy more responsive to high thermal changes in comparison to the conventional superelastic alloy. However, the conventional superelastic alloy will remain reactive to cold temperature since the alloy is austenitic at room temperature. Because the martensite phase increases the flexibility of the file when present at room temperature, complete transformation to the austenite phase at body temperature might decrease the flexibility of the instrument and eventually decrease the cyclic fatigue resistance.(12, 25)  Studies confirm the drastic effect of changing the temperature on the cyclic fatigue resistance of heat-treated files.(44, 45) Therefore, the current recommendation is to study the properties of the file at the same temperature as that of the mouth.(44, 45)      1.4.3 The effect of NaOCl irrigation on the cyclic resistance of ProTaper Gold  The possible effect of irrigation on the properties of Nitinol remains debatable.(22, 46-50) Dentsply manufacturers recommend that all metal instruments, including NiTi files, be cleaned with an anticorrosive disinfecting agent. Also, the use of hydrogen peroxide is not advised since it will degrade the NiTi instruments. Finally, the manufacturers advise that only the active part of a NiTi file should be immersed for cleaning in NaOCl, with the concentration not exceeding 17  5%.(35, 36) Therefore, these instructions may suggest that the NiTi files are sensitive to corrosive agents, especially NaOCl at high concentrations. However, there was no significant adverse effect in the flexural fatigue with the use of irrigation solutions.(22, 47-51) Most of the studies immersed the rotary instruments in the irrigation solution for different amounts of time before performing the test.(22, 47-50) In a standard clinical scenario, the instrument is rotating inside a wet canal filled with NaOCl. The use of NaOCl during the shaping procedure is required to achieve the cleaning objective of diluting the organic components of dentin and pulp tissue.(35, 36, 52, 53) The contact time between the irrigation solution and instrument varies in clinical cases. Nonetheless, it will remain in the range of seconds to a few minutes. Previous studies have recommended testing the rotary files inside an irrigation medium to replicate a clinical environment.(16, 21) However, the majority of publications about cyclic fatigue do not use a NaOCl irrigation medium, because the testing apparatus has to be made of a non-corrosive material to prevent galvanic corrosion between dissimilar metals in the presence of a corrosive medium such as NaOCl. Most of these tests were inside artificial metallic canals, which are particularly susceptible to corrosion when they are in close contact with electrically driven rotating NiTi files.  Therefore, synthetic oil was used as a lubricating medium in these cases.(16, 21, 48, 50, 54) Cheung and colleagues used stainless steel pins and immersed the experiment device in NaOCl and found that NaOCl significantly reduced the cyclic fatigue resistance.(46) Cheung et al. were concerned about the galvanic reaction between the instrument and its handle. Therefore, the group inserted only the curved portion into the irrigation medium. In another study, an artificial canal was made from a heat-resistant glass tube, and the author found no significant differences between NaOCl, saline, and air in testing the flexural fatigue of Protaper Next files.(51)  Recently, a new artificial canal model made of a zirconium oxide disc was 18  introduced.(55, 56) The zirconium oxide was described as “ceramic steel” and has the ideal mechanical strength and chemical stability for studying the effect of irrigation during file rotation within an artificial root canal system.(55, 56) In conclusion, the use of NaOCl as an irrigation medium while running experiment was only used in a limited number of studies. There is a lack of knowledge about the effect of NaOCl in the cyclic fatigue resistance of rotary files in general and in PG in particular. 1.5 Rationale Clinicians must be able to clean a complicated root canal system, and the choice of a rotary instrument is critical. The studies of factors, and types, of instrument failure, have allowed for better clinical application and significant manufacturing improvement in file metallurgy and design. In previous studies, PG performed significantly better than PU in a single curvature model. However, this does not necessarily mean that a similar performance will be expected in more challenging curvatures. The effect of irrigation on the performance of rotary files is poorly understood and studied. Therefore, the direct comparison between PG and PU will assist in knowing the extent of the value of controlled memory technology in improving flexural fatigue resistance. This comparison may also assist in understanding the impact of the presence of NaOCl in file performance. The knowledge gained may help in modifying clinical choices, techniques, or manufacturing processes.   1.6 Objectives This study aims to investigate the flexural fatigue resistance of PG compared to PU in a double curvature model and the influence of using NaOCl as an irrigation solution.   19  1.7 Null hypothesis The cyclic fatigue performance of ProTaper Gold is similar to ProTaper Universal. The number of cycles to failure is the same in single and double curvature. The presence of 5% NaOCl has no influence in the cyclic fatigue performance of files.  20   Material and methods  2.1 Study design and statistical power calculations PU and PG files (size F1) were tested in single and double curvature models along with a third variable of interest - type of medium, which was either 5% NaOCl or distilled water. The values for cyclic fatigue resistance were calculated and expressed as the number of cycles to failure (NCF). The sample size formula for a comparison of two means in the quantitative data was used to calculate the number of files needed in each group (57): n = (Zα/2 + Zβ)2 x σ2 /d2   Zα/2 is the critical value of normal distribution at α/2, and the chosen confidence level was 95%.  Zβ is the critical value of normal distribution at β, and the chosen power was 80. The σ2 is the population variance and d is the effect size. A study by Hieawy et al. was used as the source of population variance for the PG files. Finally, the effect size was calculated. Accordingly, at least three instruments in each group were needed and the decision was made to include thirteen files in each group.  2.2 Designing the double curvature and single curvature testing apparatus  The curvature angle and radius were the major defining points in the design of the artificial canal. The intersection of the lines between the beginning and the end of the curvature is called the curvature angle. The radius is the line that is drawn from the center of the circle formed by the curvature angle. Following this same concept, the single curvature model had a 60° curvature angle and 5 mm radius. In the double curvature model, the first angle was a 60° curvature angle with a 5 mm radius and the second angle was a 30° curvature with a 2 mm radius. The position of the second angle is at a distance of 2 mm from the apex. The design incorporated the PU and 21  PG size F1 file taper with an additional 0.5 mm space to allow free rotation within the canal. The model design was accomplished using the aid of an inLab MC X5 Digital computer-aided design and manufacturing (CAD/CAM) system (Dentsply Sirona). The artificial canal was milled in an InCoris ZI zirconium oxide disc (Dentsply Sirona, Bensheim, Germany) (Figure 7&Figure 8).   Figure 7. Computer-aided design and manufacturing of the single curvature model. A) Schematic drawings of the single curvature model. B) PG inside the model of the single curvature artificial canal. C) PU inside the model of the single curvature artificial canal.      22    Figure 8. Computer-aided design and manufacturing of the double curvature model. A) Schematic drawings of the double curvature model. B) PG inside the model of the double curvature artificial canal. C) PU inside the model of the double curvature artificial canal.    2.3 The cyclic fatigue test The testing model was placed inside a tempered glass container and secured in the desired position. The container was filled with 350 mL of 5% NaOCL (The Clorox Company, Brampton, Ontario, Canada) or distilled water. The medium solutions were heated inside a circulating water bath (Thermo Fisher Scientific, Marrieta, Ohio, United States of America) to body temperature (37oC). The container was mounted on a hot plate using plastic strips, and the temperature of the medium solution was stabilized to remain at body temperature (37oC). A standard mercury thermometer was placed in the container to monitor the temperature during the procedure. The rotary handpiece was fixed in a mobile supporting device that allowed precise and reproducible insertion of the working part of the file into the artificial canal (see Figure 9). The torque control motor (AEU-20T Endodontic System) settings for the rotary handpiece were adjusted to follow the manufacturer recommendation for size F1 ProTaper files (PG: 300 rpm and 150 GCM, PU: 300 rpm and 250 GCM). The experiment was closely observed using a digital video placed a 23  fixed distance from the apparatus, and a steady video magnification power was employed for accurate detection of file breakage and registration of the fracture time. The time was recorded in seconds and multiplied by the rotation speed to calculate the NCF. Finally, the broken instruments were collected and the length of the fragments were measured in mm.   Figure 9. The rotary handpiece is mounted in the mobile supporting device, and the tempered glass container is held above the hot plate. The first 19 mm of the file was inserted inside the artificial ceramic canal. A) PG inside the double curvature model. B) PU inside the single curvature model.  2.4 SEM analysis of broken file surfaces Three samples from each group were randomly chosen for fractographic examination under SEM. The samples were prepared by cleaning the broken segments in absolute alcohol (99%) inside an ultrasonic bath for 15 minutes. The fractured surface was mounted upward to face the SEM (Helios Nano Lab 650; FEI, Eindhoven, Netherlands).  24  2.5 Statistical Analysis The IBM SPSS for Windows 25.0 software was used for statistical analyses (Chicago, IL) and the threshold for significance was set at p<0.050. The normality distribution and the assumption for the homogeneity of variance were examined using the Kolmogorov-Smirnov test and Levene's test, respectively. The independent sample t-test was used to compare the study groups. The multiple linear regression analysis was used to examine the potential predictors/explanatory factors associated with the outcome variable (cycles to failure). The group-related distributional patterns were studied in boxplots, the example of a boxplot and its interpretation is presented in Figure 10.   Figure 10. Interpretation of a box-whisker plot.(58) 25   Result  3.1 Quantitative analyses  Table 1 compares the performance of two types of files (PG and PU) in two types of curvatures. Two types of comparisons are presented in this table. Vertical comparisons compare two files separately for each of the two mediums and each of the two curvatures. The horizontal comparisons compare the performance of a specific file separately for each curvature between the two mediums. The performance of PG (vertical comparisons) was significantly better than PU in both the single curvature model and in the double curvature model and two different medium solutions, namely 5% NaOCl and distilled water (P<0.001). In the single curvature canal, the PU files were more resistant to fracture in the distilled water solution than in the 5% NaOCl solution (P<0.030) (Table 1). The Figure 11 presents the aforementioned findings in more detail, and we can see that there was a substantially higher intra-group variation in the NaOCl medium as compared to water. In the distilled water, the lowest value 230 NCF and the highest value was 420 NCF, while in NaOCl the lowest value was 110 NCF and the highest value was 440 NCF. In the double curvature model, there was no significant difference between the two mediums regarding the performance of the PU file (Table 1). Figure 12 shows that there was more intra-group variation when the PU files were tested in the NaOCl medium as compared to distilled water.  Table 1 shows that there were no significant fatigue-related differences of the PG files between the two mediums either in the single (P=0.922) or in the double curved canals (P=0.079).  Distributional patterns for the PG files in a single curvature canal are presented in Figure 13 and double curvature canals in Figure 14.  26  The comparison of the NCF values and their distribution in eight experimental groups is presented in Figure 15. Overall, there was a substantially higher resistance to failure among the PG files than among the PU files.    Table 1. The performance (measurement: Number of Cycles to Failure) of the ProTaper Gold and ProTaper Universal in two curvature models and two medium solutions.       27   Figure 11. The NCF values for ProTaper Universal files in single curvature canal when tested in two different solutions of distilled water and 5% NaOCl.   28   Figure 12. The NCF values for ProTaper Universal files in double curvature canal when tested in two different solutions of distilled water and 5% NaOCl.   29   Figure 13. The NCF values for the ProTaper Gold files in single curvature canal when tested in two different solutions of distilled water and 5% NaOCl.  30   Figure 14. The NCF values for ProTaper Gold files in double curvature model when tested in two solutions medium of distilled water and 5% NaOCl.  31   Figure 15. The NCF of the ProTaper Gold (PG) and ProTaper Universal (PU) files in two curvature models and two different medium solutions of distilled water (H2O) and 5% NaOCl. SC: single curvature canal; DC: Double curvature canal.    Tolerance values for the predictors (file type, curvature, and solution) of the multiple regression model were 1.0 indicating the assumption of absence multicollinearity was fulfilled (Table 2). The overall model was highly significant (P<0.001), and three predictors jointly explained 75.4% of the variance (Adjusted R2=0.754) in the NCF.  This analysis showed that the NCF was significantly influenced by the type of file (β=0.854; P<0.001), canal curvature (β=0.147; P=0.003), and the type of medium solution (β=0.100; P =0.044).   32  Table 2. Predictors of File failure  Table 3 compares the mean lengths of broken instruments in two type of curvature canals between the two types of files. There were significant (P<0.001) differences in file lengths between the two types of files (vertical comparisons) in both single and double curvature canals. There was a significant difference (horizontal comparisons) in the mean PU length between the two curvatures (P<0.001). However, there was no significant (P=0.999) difference (horizontal comparisons) in the mean PG length between the two curvatures.  For example, the mean length of the broken instruments in the PU group was 5.4 mm in the single curvature canal and 4.4mm in the double curvature canal. The mean length of the broken instruments was significantly longer in the PU group than in the PG group (P<0.001) (Error! Reference source not found.3).   Table 3. Comparison of the length (mm) of fractured PG and PU files in the single and double curvature canals   33  3.2 Qualitative analyses  The typical features of ductile fracture in the CF failures was apparent when examined in the randomly chosen broken instruments (Figure 16-18). The figures show that the fracture starts as one crack or more at the cross-section peripheries then the crack propagates slowly till complete separation happens in the rapid zone of fracture. The qualitative analyses were done under SEM examination.        34   Figure 16: The fractographic appearance of the fractured surface of the PU in distilled water and double curvature group showing the classic features of CF failure. A) One crack origin (arrow) followed by an area of steady-crack growth and rapid-crack growth. B) Higher magnification showing an area of shear lip at the peripheries of PU cross-section. C) Higher magnification to show the crack origin and steady growth of the crack. D) Higher magnification of shear lip in the file peripheries. E) Higher magnification in the center of a cross-section showing the junction between the zones of steady-crack growth and the rapid-crack growth.   35   Figure 17: The fractographic appearance of the fractured surface of PU when tested in single curvature model and 5% NaOCl. A) There are two crack origins (long arrows), steady-crack growth zone and microscopic striations representing the rapid fracture zone. The shear lip at the file peripheries is also apparent (short arrow). B-D) Higher magnification the cutting edges of PU. E) The center of the file showing the microscopic striations and dimples which are typical features of CF failure. 36   Figure 18: The fractographic appearance of the fractured surface of the PG file after testing in double curvature model and 5% NaOCl. A) Two crack origins at the cutting edge (arrows), steady-growth zone followed by microscopic striations. B&C) The higher magnification of the crack origin in two different locations. D) The rapid zone of fracture and the characteristic feature of dimples is apparent. E) The center of the cross-section at higher magnification.   37   Figure 19: The pathognomonic features of CF failure are visible across the surface of the fractured PG files when the PG files were tested in single curvature model and 5%NaOCl (A) or distilled water (C). B&D) The higher magnification at the center of the fractured surface shows the microscopic striations and dimples at the two mentioned conditions respectively.     A. B. C. D. 38   Discussion   The cyclic fatigue (CF) failure was regarded as the main cause of fracture of rotary files in endodontic treatment.(59) This type of fracture was described as unpredictable and the risk increased with frequent use, especially in curved canals.(59, 60) The additional heat treatment of the NiTi files improved the rotary file’s fracture resistance significantly in comparison to traditional treatment.(12, 25) Nonetheless, the impact of this influence on various testing conditions was not fully explored or comprehended. The clinical use of 5.25% NaOCl solution was recommended to achieve significantly better cleaning results in a reasonable time.(61) The use of NaOCl irrigation solution in RCT was regarded as crucial for the success of the procedure, and the disinfection results are not equally achievable using alternative solutions.(52, 53) Therefore, the present study offers a deeper understanding of the impact of the type of metallurgy, the number of curvatures, and the used of 5% NaOCl solution in affecting the CF resistance of ProTaper rotary files. The findings of this study confirm the findings of earlier studies that PG files have a higher CF resistance than PU files.(12, 25) The gold heat treatment increased the ProTaper file flexibility, therefore PG have a better ability to withstand challenging anatomical situations, such as abrupt curvature and double curved canals.(25) However, the influence of instrument size and the risk of fracture was not included in this study, and the choice of a size F1 file was intentional. This decision was made due to the fact size F1 files are the smallest finishing file, and their use can be sufficient for the completion of the procedure. The use of larger instruments in double curved canals should be carefully attempted since larger instrument sizes have lower resistance to CF fracture.(12, 60) 39  Frequent rinsing with NaOCl irrigation solution during shaping, to dilute the pulp tissues and remove dentinal debris, was suggested as a technique to reduce the risk of torsional fracture since the accumulation of dentinal debris within the file flutes can increase this risk.(62)  The real impact of NaOCl in increasing the CF failure remains unclear. The results of this study indicate that 5% NaOCl can increase the risk of CF fracture solely. However, the results have to be interpreted with caution in a clinical setting, since dentinal debris might negatively affect the strength of NaOCl and therefore lower the potentially harmful effects of NaOCl on the file.(52, 63) Nevertheless, the frequent replenishing of NaOCl from the beginning of the procedure might counter the adverse effect on the strength of the NaOCl from the dentinal debris and remnant pulp tissue in later phases of the treatment. The lubrication effect of 1% NaOCl on the mechanical properties of Profile file was comparable to deionized water in one in-vitro study.(64) Nonetheless, the lubricant ability of 5% NaOCl might be higher than distilled water which could delay the negative effect of the corrosive property. The corrosive abilities of 5% NaOCl during the flexing state of ProTaper files increased the growth rate of cracks and resulted in a decrease in the NCF. The phenomenon is known as stress corrosion cracking (SCC) in which normally ductile material, especially alloys, are expected to fail unexpectedly due to the rapid growth of the crack when the material is subjected to tensile stresses in a corrosive medium.(65) The SCC was reported to increase in elevated temperatures, which was at body temperature 37oC in this experiment.(65, 66) The findings of this study were not consistent with those previously published. (47-50)  However, those experiments did not study the influence when the file was exposed to the corrosive environment in flexing and rotating states.(47) On the other hand, findings from a previous study did not find any significance of irrigation in CF failure due to differences in the type of instruments tested.(55) The multiple linear regression 40  analysis employed in the present study allowed us to evaluate the joint effect of three predictors (file type, curvature, and irrigation solution) to the fatigue failure. All three predictors were significantly and negatively associated with the file fatigue, and the strongest predictor was the file type (PU worse than PG files), followed by curvature type (double worse than a single one) and irrigation solution (NaOCl worse than distilled water).     Pruett et al. reported that the angle and radius of canal curvature reduced the CF resistance of rotary files.(17) Wu et al., in a clinical study, stated that the incidence of PU files separation was 2.6%, with 54% of the separation happening in extremely curved canals. (60) The presence of double curvature in this study decreased the life expectancy of ProTaper files, which was similar to findings of previous in vitro studies in double curved canals. (67-69) Duke et al. found that double curved canals were more challenging than single curved canals. However, the new generation Vortex Blue did not show higher resistance to fracture in comparison to the Profile Vortex when both of these files were tested in double curved canals.(37) On the contrary, this study found that PG files were superior to PU files in all tested conditions. One explanation is that the present study compares traditional NiTi files and heat treated NiTi files, while Duke et al. compared heat treatment in Profile Vortex files and M-wire technology in Vortex Blue files.    The pathognomonic features of CF fractures were noticed in the surface cross-section of ProTaper files. The SEM examinations of the fractured surfaces confirm the type of fracture. The length of the broken segment was longer in the PU files, with the average length 1 mm shorter in the double curvature canals. On the other hand, PG had an average of 3.6 mm fractured pieces in single and double curvatures. The fractured instrument of the Profile traditional NiTi file was reported to be shorter in the double curved canal while TRUShape fractured at the same distance in single and double curvature canals.(68) Duke et al. found that the fractured segments were 41  shorter in the double curvature model.(37) However, in the present study, ProTaper files were used in customized canals that were immersed in solution at body temperature. Every aspect of this methodology has an important influence on the results, as stated in the literature.(16, 21, 54) The clinician might favor having the fracture at a shorter distance from the apex rather than at longer distance below the canal curvature since this will allow better cleaning of the remaining root canal space and easier surgical management when needed. The most important factor influencing the ProTaper file’s performance was the heat treatment, followed by the number of curvatures, then the use of 5% NaOCl irrigation medium. The limitation of this in vitro study permit careful clinical interpretation. However, the in vitro study allows precise standard comparisons of file performance in different situations and minimizes the presence of confounding factors. The results obtained guide clinicians to use the PG files since the gold treatment has improved the CF resistance of these files, especially in curved canals. Moreover, the reuse of ProTaper files needs to be cautiously considered in extremely curved canals since these canals significantly reduce the instrument life expectancy. Further research to know the extent of irrigation role in initiation and propagation of cracks during file rotation inside the curvature is recommended. A better understanding might aid in the development of improved ways to prevent or reduce the incidence of fracture.   42   Conclusion  The fatigue performance of PG is better than that of PU. 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