Novel Electronic Device to Quantify the Cyclic Fatigue Resistance of Endodontic Reciprocating Files after Using and Sterilization

: Background: The aim of this study was to analyze the e ﬀ ects of the time of use (TU) and sterilization cycles (SC) of endodontic reciprocating ﬁles on cyclic fatigue resistance. Methods: One-hundred-and-twenty (120) Procodile NiTi endodontic reciprocating instruments were selected at random and distributed into the following study groups: A: 0 sterilization cycles / 0s time of use ( n = 10); B: 0 / 60 ( n = 10); C: 0 / 120 ( n = 10); D: 1 / 0 ( n = 10); E: 1 / 60 ( n = 10); F: 1 / 120 ( n = 10); G: 5 / 0 ( n = 10); H: 5 / 60 ( n = 10); I: 5 / 120 ( n = 10); J: 10 / 0 ( n = 10); K: 10 / 60 ( n = 10); and L: 10 / 120 ( n = 10). A dynamic cyclic fatigue device was designed using computer-aided design / computer-aided engineering (CAD / CAE) technology and created with a 3D printer to simulate the pecking motion performed by the clinician. Failure of the endodontic rotary instrument was detected by a light-emitting diode-light-dependent resistor (LED-LDR) system controlled by an Arduino driver complex and management software. The results were analyzed using the ANOVA test. Results: All pairwise comparisons presented statistically signiﬁcant di ﬀ erences between the time to failure, number of cycles to failure and number of cycles of in-and-out movement for the time of use study groups ( p < 0.001), but not in the number of sterilization cycles ( p > 0.05). Conclusions: The time of use of NiTi endodontic reciprocating ﬁles negatively a ﬀ ects dynamic cyclic fatigue resistance. Dynamic cyclic resistance is not a ﬀ ected by the number of sterilization cycles. movements for the time of use and ( B ) sterilization cycles study groups.


Introduction
Nickel-titanium (NiTi) endodontic rotary instruments and the subsequent development of mechanical preparation has improved the prognosis of root canal treatment, as their increased taper and automated motion enable more effective cleaning and shaping of the root canal system [1]. Many advances have been made in the last few years, including innovations in instrument design, new NiTi alloys, thermal treating of NiTi alloys, and the addition of new movements to instrumentation

Study Design
One-hundred-and-twenty (120) sterile 250 µm apical diameter and 6% taper (25.06) austenite NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo, Germany) with a variable tapered core, one-file system, 25 mm in length, counter-clockwise (CCW) reciprocating motion, and double-S cross-section were utilized in this in vitro study. All NiTi endodontic reciprocating files were first inspected under magnification (SZR-10, Optika, Bergamo, Italy), and all samples were included. A randomized controlled experimental trial was performed at the Department of Endodontics of the Faculty of Health Sciences at Alfonso X El Sabio University (Madrid, Spain), between September and November 2019. The NiTi endodontic reciprocating files were randomized (Epidat 4.1, Galicia, Spain) and categorized into the following study groups: A: 0 sterilization cycles and 0 seconds of dynamic cyclic fatigue (n = 10); B: 0 sterilization cycles and 60 s of dynamic cyclic fatigue (n = 10); C: 0 sterilization cycles and 120 s of dynamic cyclic fatigue (n = 10); D: 1 sterilization cycles and 0 s of dynamic cyclic fatigue (n = 10); E: 1 sterilization cycles and 60 s of dynamic cyclic fatigue (n = 10); F: 1 sterilization cycles and 120 s of dynamic cyclic fatigue (n = 10); G: 5 sterilization cycles and 0 s of dynamic cyclic fatigue (n = 10); H: 5 sterilization cycles and 60 s of dynamic cyclic fatigue (n = 10); I: 5 sterilization cycles and 120 s of dynamic cyclic fatigue (n = 10); J: 10 sterilization cycles and 0 s of dynamic cyclic fatigue (n = 10); K: 10 sterilization cycles and 60 s of dynamic cyclic fatigue (n = 10); and L: 10 sterilization cycles and 120 s of dynamic cyclic fatigue (n = 10).

Experimental Cycling Fatigue Procedure
Dynamic fatigue procedures were performed regarding the previously described cyclic fatigue device (utility model patent number ES1219520) [17]. The structure of the dynamic cyclic fatigue test device was planned by computer aided design/computer aided engineering (CAD/CAE) 2D/3D software (Midas FX+ ® , Brunleys, Milton Keynes, UK) and created using 3D printing (ProJet ® 6000 3D Systems©, Rock Hill, SC, USA) ( Figure 1A-F).
The Procodile 25.06 NiTi endodontic reciprocating file (Komet Medical, Lemgo) was assessed using a microcomputed tomography scan (Skyscan 1176, Bruker-MicroCT, Kontich, Belgium) to design an accurate standard tessellation language (STL) digital file. The STL file was used to generate a replica of an artificial root canal of 60 • angle and 3 mm radius of curvature using the CAD/CAE 2D/3D software for inverse engineering technology ( Figure 2A) [18]. The artificial root canal was created by means of a highly accurate subtractive technique from a stainless steel piece (Cocchiola S.A., Buenos Aires, Argentina). This process ensured intimate contact between the NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) and the artificial root canal walls ( Figure 1H). The artificial root canal was positioned on its own support ( Figure 2B) and failure of the endodontic reciprocating files was detected by analyzing the amount of light using emitted from a Light-Emitting Diode (LED) (20000 mcd) (Ref.: 12.675/5/b/c/20k, Batuled, Coslada, Spain) ( Figure 1G) by a Light-Dependent Resistor (LDR) sensor (Ref.: C000025, Arduino LLC ® , Ivrea, Italy) located opposite at 3 mm of the working length, with a frequency of 50 ms to accurately detect the time of failure. The axial pecking movement of the NiTi endodontic reciprocating files affects the measurement of the light signal received by the LDR sensor, which were shown in real time on a Liquid Crystal Display (LCD) (Ref.: LCD-09568, Spark Fun Electronics, Niwot, CO, USA). Therefore, the absence of variation in light values during three cycles of light analysis was interpreted as the NiTi endodontic reciprocating file's failure. The sensor data was conditioned by a processor (Arduino UNO Rev. 3, Arduino LLC ® , Ivrea, Italy), and the hardware was managed by software that receives input signals from the Arduino board. Dynamic fatigue procedures were performed regarding the previously described cyclic fatigue device (utility model patent number ES1219520) [17]. The structure of the dynamic cyclic fatigue test device was planned by computer aided design/computer aided engineering (CAD/CAE) 2D/3D software (Midas FX+ ® , Brunleys, Milton Keynes, UK) and created using 3D printing (ProJet ® 6000 3D Systems© , Rock Hill, SC, USA) ( Figure 1A-F).   was assessed using a microcomputed tomography scan (Skyscan 1176, Bruker-MicroCT, Kontich, Belgium) to design an accurate standard tessellation language (STL) digital file. The STL file was used to generate a replica of an artificial root canal of 60° angle and 3 mm radius of curvature using the CAD/CAE 2D/3D software for inverse engineering technology (Figure 2A) [18]. The artificial root canal was created by means of a highly accurate subtractive technique from a stainless steel piece (Cocchiola S.A., Buenos Aires, Argentina). This process ensured intimate contact between the NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lem go) and the artificial root canal walls ( Figure 1H). The artificial root canal was positioned on its own support ( Figure 2B   reciprocating motion, according to the manufacturer's instructions [19]. The endodontic handpiece (EndoPilot, Komet Medical, Lemgo) was digitalized (3D Geomagic Capture Wrap, 3D Systems©, Rock Hill, SC, USA) to enable accurate adjustment to the endodontic handpiece supports during the dynamic cyclic fatigue tests. Initially, the NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) were randomly (Epidat 4.1, Galicia, Spain) unsterilized or pre-sterilized 1, 5 or 10 times using the following parameters: 3 bar, 134 • C and 45 min. Next, the NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) were randomly (Epidat 4.1, Galicia, Spain) subjected to no fatigue, fatigued for 60 s or fatigued for 120 s in the dynamic cycle fatigue device. The NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) were used until fracture occurred in order to analyze the effect of sterilization cycles and the time of use on the resistance of NiTi endodontic reciprocating files to cyclic fatigue.
All NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) were used in the dynamic cyclic fatigue device at a frequency of 60 pecking movements/min according to a previous study [17]. To reduce the friction between the reciprocating files and the artificial canal walls, a special high-flow synthetic oil designed for the lubrication of mechanical parts (Singer All-Purpose Oil; Singer Corp., Barcelona, Spain) was applied.
All NiTi endodontic reciprocating files (Procodile ® , Komet Medical, Lemgo) were used until fracture occurred. The time to failure, the number of cycles to failure, the number of cycles of in and out movements, and the length of the fractured file tip were measured and recorded.

Statistical Tests
Statistical analysis was performed by means of SAS 9.4 (SAS Institute Inc., Cary, NC, USA). Descriptive analysis is described as mean and standard deviation (SD) for quantitative data. Comparative statistics was carried out by comparing the time to failure (in seconds), the number of cycles to failure, the number of pecking movements (cycles of in-and-out movements), and the length of the fractured file tip (mm) using the ANOVA test. Furthermore, Weibull statistical analysis was also calculated. Statistical significance level was established at p < 0.05.

Results
The mean and SD values for time to failure (in seconds) and the mean length of the fractured file tip (mm) for each of the study groups are displayed in Table 1 and Figure 3. The ANOVA analysis showed statistically significant differences between time to failure and the time of use of NiTi endodontic reciprocating files (p < 0.001) ( Figure 3). However, no statistically significant differences were observed between time to failure and the number of sterilization cycles The horizontal line in each box represents the respective median value.
The ANOVA analysis showed statistically significant differences between time to failure and the time of use of NiTi endodontic reciprocating files (p < 0.001) ( Figure 3). However, no statistically significant differences were observed between time to failure and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p = 0.848) (Figure 3). The m ean length of the fractured file tip did not show sta tistically significant differences between tim e to failure of the time of use (p > 0.05) and the sterilization cycles (p > 0.05) of the NiTi endodontic reciprocating files ( Figure 3).
The scale distribution parameter (η) of Weibull statistics found statistically significant differences between time to failure and the time of use of NiTi en dodon tic reciprocating files (p > 0.001) (Table 2, Figure 4A); however, th ere were no statistically significant differences in time to failure between the number of sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) (Table 2, Figure 4B). Th e shape distribution parameter (β) of Weibull statistics found no statistically significant differences between time to failure in relation to time of use (p > 0.05) (Table 2, Figure 4A), and the sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) (Table 2, Figure 4B).  The scale distribution parameter (η) of Weibull statistics found statistically significant differences between time to failure and the time of use of NiTi endodontic reciprocating files (p > 0.001) (Table 2, Figure 4A); however, there were no statistically significant differences in time to failure between the number of sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) ( Table 2, Figure 4B). The shape distribution parameter (β) of Weibull statistics found no statistically significant differences between time to failure in relation to time of use (p > 0.05) (Table 2, Figure 4A), and the sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) (Table 2, Figure 4B).  The mean and SD values for number of cycles to failure and the len gth of th e fractured file tip (mm) of the study groups are displayed in Table 3 and Figure 5.   The mean and SD values for number of cycles to failure and the length of the fractured file tip (mm) of the study groups are displayed in Table 3 and Figure 5.  The ANOVA test revealed statistically significant differences between the number of cycles to failure and the time of use of NiTi endodontic reciprocating files (p < 0.001) ( Figure 5). However, no statistically significant differences were observed between the number of cycles to failure and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p = 0.848) ( Figure 5 ). The ANOVA test revealed statistically significant differences between the number of cycles to failure and the time of use of NiTi endodontic reciprocating files (p < 0.001) ( Figure 5). However, no statistically significant differences were observed between the number of cycles to failure and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p = 0.848) ( Figure 5). The mean length of the fractured file tip did not show statistically significant differences between the number of cycles to failure and the time of use (p > 0.05) and number of sterilization cycles (p > 0.05) applied to the NiTi endodontic reciprocating files ( Figure 5).
The scale distribution parameter (η) of Weibull statistics showed statistically significant differences between the number of cycles to failure and the time of use of NiTi endodontic reciprocating files (p < 0.001) (Table 4, Figure 6A); however, there were no statistically significant differences between the number of cycles to failure and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) ( Table 4, Figure 6B). The shape distribution parameter (β) of Weibull statistics did not show statistically significant differences between the number of cycles to failure and the time of use (p > 0.05) ( Table 4, Figure 6A), nor the number of sterilization cycles applied to NiTi endodontic reciprocating files (p > 0.05) (Table 4, Figure 6B).  sterilization cycles study groups.
The mean and SD values for the number of cycles of in-and-out movement and the length of th e fractured file tip (mm) of the study groups are displayed in Table 5 and Figure 7. Table 5. Descriptive statistics of the number of cycles of in-and-out movement for the time of use and sterilization cycles study groups.  The mean and SD values for the number of cycles of in-and-out movement and the length of the fractured file tip (mm) of the study groups are displayed in Table 5 and Figure 7.  The ANOVA test revealed statistically significant differences between the number of cycles of in and out movement and the time of use of NiTi endodon tic reciprocating files (p < 0.001) (Figure 7 ). However, no statistically significant differences were observed between the number of cycles of in and out movement and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p = 0.848) (Figure 7). Th e mean length of the fractured file tip did not show stati stically significant differences between the number of cycles of in -and-out movement and the time of use (p > 0.05) or the number of sterilization cycles (p > 0.05) applied to the NiTi endodontic reciprocating files (Figure 7).

Time of Use
The scale distribution parameter (η) of Weibull statistics showed statistically significant differences between the number of cycles of in-and-out movement and the time of use of NiTi endodontic reciprocating files (p < 0.001) (Table 6, Figure 8A); however, no statistically significa nt differences were found between the number of cycles of in-and-out movement and the number of sterilization cycles applied to the NiTi endodontic reciprocating files (p > 0.05) (Table 6, Figure 8B). The shape distribution parameter (β) of Weibull statistics did not show statistically significant differences between the number of cycles of in-and-out movem ent and the time of use (p > 0.05) (Table 6, Figure 8A) or th e number of sterilization cycles applied to the NiTi endodontic reciprocating files (p > 0.05) (Table 6, Figure 8B). The ANOVA test revealed statistically significant differences between the number of cycles of in and out movement and the time of use of NiTi endodontic reciprocating files (p < 0.001) (Figure 7). However, no statistically significant differences were observed between the number of cycles of in and out movement and the number of sterilization cycles applied to NiTi endodontic reciprocating files (p = 0.848) (Figure 7). The mean length of the fractured file tip did not show statistically significant differences between the number of cycles of in-and-out movement and the time of use (p > 0.05) or the number of sterilization cycles (p > 0.05) applied to the NiTi endodontic reciprocating files (Figure 7).
The scale distribution parameter (η) of Weibull statistics showed statistically significant differences between the number of cycles of in-and-out movement and the time of use of NiTi endodontic reciprocating files (p < 0.001) (Table 6, Figure 8A); however, no statistically significant differences were found between the number of cycles of in-and-out movement and the number of sterilization cycles applied to the NiTi endodontic reciprocating files (p > 0.05) ( Table 6, Figure 8B). The shape distribution parameter (β) of Weibull statistics did not show statistically significant differences between the number of cycles of in-and-out movement and the time of use (p > 0.05) (Table 6, Figure 8A) or the number of sterilization cycles applied to the NiTi endodontic reciprocating files (p > 0.05) (Table 6, Figure 8B). Table 6. Weibull statistics of the number of cycles of in-and-out movements for the time of use and sterilization cycles study groups.

Study Group
Weibull

Discussion
The results obtained in the present study reject the null hypothesis (H0) that states that time of use would have no effect on the dynamic cyclic fatigue resistance of NiTi endodontic reciprocating files. However, it also accepts th e null h ypothesis (H0) that s tates that th e number of sterilization cycles applied to NiTi endodon tic reciprocating files would have no effect on their resistance to dynamic cyclic fatigue.
The cyclic fatigue resistance of NiTi endodontic rotary and reciprocating instruments has been widely compared with regard to the NiTi alloy used, cross -section design, curva ture angle, radius, pecking motion frequency, etc., with both static and dynamic custom -made cyclic fatigue devices having been used [20,21]. However, dynamic custom-made cyclic fatigue devices are th e only ones that faithfully simulate the pecking movement performed by th e clinician, more accurately representin g the amount of time during which the NiTi endodontic rotary instrument remains in the artificial root canal. In addition, th e artificial root canal should be custom-designed for th e specific NiTi endodontic rotary instrument being tested in order to ensure immediate contact with the artificial root canal (as happens in a clinical setting). This also ensures that compression and tensile stress cycles are localized at the maximum flexure point [22], distributes the shear resistance homogeneously along the NiTi endodontic rotary instrument surface, and leads to the formation of microcracks on the instruments' microstructure [23] without any signs of plastic deformation in the static model; the area of these stresses spreads through the instrument shaft in th e dynamic model [21]. Most of th e static and dynamic custom-made cyclic fatigue devices provided a non-instrument-based artificial root canal [24,25], so the results extracted from these studies should be carefully considered and not applied to clinical practice. Di Nardo et al. reported statistically significant differences (p < 0.05) between the time to failure of Wave On e Gold NiTi endodontic reciprocating files (50.75 ± 20.06 s) and Reziflow (30.13 ± 9.40) conventional NiTi endodontic rotary

Discussion
The results obtained in the present study reject the null hypothesis (H0) that states that time of use would have no effect on the dynamic cyclic fatigue resistance of NiTi endodontic reciprocating files. However, it also accepts the null hypothesis (H0) that states that the number of sterilization cycles applied to NiTi endodontic reciprocating files would have no effect on their resistance to dynamic cyclic fatigue.
The cyclic fatigue resistance of NiTi endodontic rotary and reciprocating instruments has been widely compared with regard to the NiTi alloy used, cross-section design, curvature angle, radius, pecking motion frequency, etc., with both static and dynamic custom-made cyclic fatigue devices having been used [20,21]. However, dynamic custom-made cyclic fatigue devices are the only ones that faithfully simulate the pecking movement performed by the clinician, more accurately representing the amount of time during which the NiTi endodontic rotary instrument remains in the artificial root canal. In addition, the artificial root canal should be custom-designed for the specific NiTi endodontic rotary instrument being tested in order to ensure immediate contact with the artificial root canal (as happens in a clinical setting). This also ensures that compression and tensile stress cycles are localized at the maximum flexure point [22], distributes the shear resistance homogeneously along the NiTi endodontic rotary instrument surface, and leads to the formation of microcracks on the instruments' microstructure [23] without any signs of plastic deformation in the static model; the area of these stresses spreads through the instrument shaft in the dynamic model [21]. Most of the static and dynamic custom-made cyclic fatigue devices provided a non-instrument-based artificial root canal [24,25], so the results extracted from these studies should be carefully considered and not applied to clinical practice. Di Nardo et al. reported statistically significant differences (p < 0.05) between the time to failure of Wave One Gold NiTi endodontic reciprocating files (50.75 ± 20.06 s) and Reziflow (30.13 ± 9.40) conventional NiTi endodontic rotary instruments [26]. Although this was a static cyclic fatigue test with an artificial root canal of parallel walls and 90 • and 5 mm curvature radius, Reziflow has a similar design and manufacturing process as Procodile. In this study, a curvature angle of 60 • was designed, because Topçuoglu et al. reported that artificial root canals with a 45 • angle of curvature did not show statistically significant differences (p > 0.05) between the time to failure of WaveOne Gold (412.4 ± 55.2 s) and R-Pilot (394.5 ± 45.3 s) glider files; however, artificial root canals with a curvature angle of 60 • showed statistically significant differences (p < 0.05) between the time to failure of WaveOne Gold (368.3 ± 44.1 s) and R-Pilot (247.2 ± 36.2 s) endodontic instruments [27].
Kim et al. state that the endodontic reciprocating instruments must be disposed of after the first use because the mechanical properties, metallurgical features and risk of failure of the endodontic reciprocating instruments result from repeated usage [28]. Furthermore, Generali et al. reported that the continued usage of Reciproc and Reciproc Blue endodontic reciprocating instruments reduced resistance to cyclic fatigue, also reducing the martensite and R-phase in Reciproc Blue endodontic reciprocating instruments and causing microcracks near the tip of both endodontic reciprocating instruments after usage. In addition, there were statistically significant differences (p < 0.05) between the cyclic fatigue resistance of new and used Reciproc Blue endodontic reciprocating instruments, in keeping with the findings of the present study. However, no statistically significant differences (p > 0.05) were observed between the cyclic fatigue resistance of new and used Reciproc endodontic reciprocating instruments. Regardless of the NiTi alloy of the endodontic reciprocating system, the double-S cross-section design of Reciproc and Reciproc Blue endodontic reciprocating instruments and the reciprocating movement are similar to the Procodile NiTi endodontic reciprocating instruments; even the DSC curves confirm that the Reciproc Blue endodontic reciprocating instruments are made up of a mix of austenite and R-phase, as observed in XRD patterns [29]. However, this was a static cyclic fatigue test with an artificial root canal machined on a quenched martensitic carbon steel plate. The endodontic instruments are designed to cut dentin, but not materials with different cutting resistance values, so the results of the present study can hardly be accurately extrapolated to a clinical setting. However, in the present study, the NiTi endodontic reciprocating instruments were selected in the austenite phase due to their physical and metallurgical properties in terms of hardness and cutting capability in order to ensure the advance of the NiTi endodontic reciprocating instrument inside the stainless steel artificial root canal. In addition, the automatic, objective detection of the NiTi endodontic reciprocating instruments failure process using a LED-LDR system provided an accurate and standardizable measurement procedure, regardless of the subjective measurement protocol used in most studies to detect the precise moment of failure of the endodontic instrument tip. No statistically significant difference was found between the mean lengths of fractured file tips, whose maximum area of stress corresponded to the mid-point arc of the apical curvature, in keeping with the findings of Pruett et al. [22]. This result confirms the correct and repeated positioning of the tested NiTi endodontic reciprocating instruments within the artificial root canal.
Some researchers have postulated that the gamma-ray and autoclave sterilization processes used for fully sterile packaged Hyflex EDM and TRUShape endodontic rotary instruments might impact on the life span of NiTi martensitic endodontic rotary instruments in terms of their resistance to fatigue [30]. Furthermore, the heat sterilization method can also cause changes in the physical and mechanical properties of NiTi endodontic rotary instruments and even impact the torsional properties of NiTi endodontic rotary instruments [31]. Yang et al. reported that the surface roughness of K3XF R-phase NiTi endodontic rotary instruments increased after autoclaving, but this did not affect the cyclic fatigue resistance after 30 sterilization cycles [32]. Pedullà et al. found that repeated sterilization cycles did not impact resistance of NiTi endodontic rotary instruments to fatigue except for Twisted Files R-phase NiTi endodontic rotary instruments, which showed a significant decrease in flexural resistance after three sterilization cycles (p < 0.05) [33]. Champa et al. showed that multiple sterilization cycles significantly increased the cyclic fatigue resistance of Reciproc M-Wire NiTi endodontic reciprocating instruments, but decreased the cyclic fatigue resistance of Wave One M Wire NiTi endodontic reciprocating instruments in an artificial root canal (not anatomically modeled) with 60 • of curvature in a static cyclic fatigue device [34]. Özyürek et al. also reported that the sterilization cycles significantly increased the cyclic fatigue resistance of Protaper Next M-Wire NiTi endodontic rotary instruments (p < 0.05) and Protaper Gold alloy NiTi endodontic rotary instruments (p < 0.05) [35]. Zhao et al. showed that the cyclic fatigue resistance of pre-sterilized HyFlex CM CM Wire NiTi endodontic rotary instruments and K3XF R-phase NiTi endodontic rotary instruments increased significantly (p < 0.05) after 10 sterilization cycles in an artificial root canal (not anatomically modeled) with a curvature of 60 • and a radius of 3 mm in a static cyclic fatigue device [36]. Most of the martensitic endodontic rotary instruments (M-Wire alloys) have demonstrated an increase in cyclic fatigue resistance after heat sterilization procedures. However, in the present study, the cyclic fatigue resistance of austenite NiTi endodontic reciprocating files was not affected after 10 autoclave sterilization cycles (p > 0.05). The thermal treating of M-Wire endodontic rotary instruments makes the martensitic phase of NiTi alloy less subject to breakage during clinical use and stabilizes the NiTi alloy. The thermal heating induced by heat sterilization procedures make it even more stable, increasing cyclic fatigue resistance [31].

Conclusions
The conclusion derived from the present study is that the time of use of austenite NiTi endodontic reciprocating files negatively affects their dynamic cyclic fatigue resistance; however, resistance to dynamic cyclic fatigue is not affected by the number of sterilization cycles.