Influence of the Pecking Motion Frequency on the Cyclic Fatigue Resistance of Endodontic Rotary Files

Purpose: To analyze the influence of the pecking motion frequency on the cyclic fatigue resistance of endodontic rotary files. Material and Methods: Sixty PlexV 25.06 endodontic rotary files were selected and distributed into three groups: 30 movements/min (n = 20), 60 movements/min (n = 20), and 120 movements/min (n = 20). A dynamic cyclic fatigue device was designed using Computer Aided Design/ Computer Aided Engineering (CAD/CAE) technology and manufactured by 3D impressions to simulate the pecking motion performed by an operator. Failures of the endodontic rotary files were detected by a Light-Emitting Diode (LED)/Light-Dependent Resistor (LDR) system controlled by an Arduino-Driver complex and management software. Endodontic rotary files were tested on an artificial root canal manufactured by wire electrical discharge machining (EDM), with similar dimensions to those of the instrument under examination. Endodontic rotary files were used following the manufacturer’s recommendations. The results were analyzed by ANOVA and Weibull statistics. Results: All pairwise comparisons revealed statistically significant differences in all three variables, except for the difference in the number of cycles between the groups with 60 and 120 movements/min (p = 0.298). The scale distribution parameter of Weibull statistics showed statistically significant differences in all three variables, except for the differences in the number of cycles between groups with 30 and 60 movements/min (p = 0.0722). No statistically significant differences in the three variables were observed for the shape distribution parameter. Conclusion: A low frequency of pecking motion is recommended to reduce the risk of failure of endodontic rotary files associated with cyclic fatigue.


Introduction
Endodontic rotary files have experienced continuous development since nickel-titanium (NiTi) files were introduced in the 1980s [1]. This alloy increases the flexibility and strength of rotary files compared with stainless-steel instruments [2], and it simplifies the endodontic procedure by improving the speed, accuracy, and safety of root canal shaping [3]. Despite continuous enhancements in the design and manufacture of NiTi rotary files to reduce the occurrence of failure during root canal shaping [4], failures can still occur. Many variables can contribute to file separation, but the main causes are cyclic bending fatigue and torsional overload [5][6][7][8]. Torsional overload is caused by the blockage of the endodontic files during rotational movement [9]. However, NiTi instrument failures are primarily caused by cyclic fatigue, which occurs when a NiTi endodontic instrument rotates in a curved root canal [10]. During rotation, the structure of the endodontic instrument is alternately subjected to compressive and tensile stress cycles, which produce microstructural changes that lead to the failure of the endodontic rotary file [11]. Root canal shape, instrument geometry, rotational speed, torque, instrument surface treatments, sterilization cycles, the number of clinical uses, and the chemical composition of NiTi alloys are the main factors that affect the number of cycles to failure (NCF) of NiTi rotary instruments [12,13]. However, the influence of the pecking motion (frequency of in-and-out movement) on the cyclic fatigue resistance of endodontic rotary instruments has never been tested. In 2002, the American National Standard Institute and the American Dental Association standardized a protocol for testing the torsional and flexibility resistance of stainless-steel manual files [14], which was also addressed in the 3630-1 norm by the International Organization for Standardization (ISO) [15] for endodontic instruments with a taper of 2%. However, there is no international standard for testing the cyclic fatigue behavior of NiTi endodontic rotary instruments [11], and several self-designed devices and methods have been used [11]. However, none of these custom-made devices have been capable of dynamically testing the cyclic fatigue of NiTi endodontic rotary instruments in vitro with an automatic detection system and an anatomically based artificial root canal.
The aim of this work was to analyze the influence of the pecking motion on the cyclic fatigue resistance of endodontic rotary instruments; the null hypothesis (H0) asserts that the frequency of the pecking motion does not significantly affect the time to failure, the NCF, or the cyclic fatigue resistance of endodontic rotary instruments.

Study Design
Sixty sterile NiTi CM Wire endodontic rotary instruments 25.06 (Plex V2 ® , Orodeka, Italia) were involved in this study. Before the experiment, every endodontic rotary instrument was inspected for defects or deformities under a stereomicroscope (SZR-10, Optika, Bergamo, Italy), and none were discarded. A randomized controlled experimental trial was performed at the Dental Centre of Innovation and Advanced Specialties at the Alfonso X el Sabio University (Madrid, Spain) between February and July 2019. The endodontic rotary instruments were randomized (Epidat 4.1, Galicia, Spain) and distributed into the three groups: (A) 30 pecking movements/min (n = 20), (B) 60 pecking movements/min (n = 20), and (C) 120 pecking movements/min (n = 20).

The Experimental Cyclic Fatigue Model
The cyclic fatigue tests were performed using a custom-made device (Utility Model Patent number ES1219520) that provides information about the behavior of an endodontic rotary file during a root canal treatment. The endodontic rotary instruments were neither used nor submitted to sterilization cycles before the tests. The structure was designed by 2D/3D Computer-Aided Design/Computer-Aided Engineering (CAD/CAE) (Midas FX+ ® , Brunleys, MK, UK) and manufactured by 3D impression (ProJet ® 6000. 3D Systems©, Rock Hill, SC, USA) ( Figure 1). The endodontic handpiece (X-Smart Plus, Dentsply Maillefer, Baillagues, Switzerland) was scanned (3D Geomagic Capture Wrap, 3D Systems © , Rock Hill, SC, USA) to create an accurate design for its support piece by means of inverse engineering technology (Midas FX+ ® , Brunleys) and 3D impression manufacture (ProJet ® 6000. 3D Systems © , Rock Hill, SC, USA). This support was firmly attached to the main structure and enabled the angular displacement of the endodontic handpiece by means of a spindle to test different file lengths. The support also allowed the files to be removed during the cyclic fatigue tests.
The direction and speed of the movement were produced by the brushed DC gearmotor (Ref.  Figure 2A) that was needed to design an accurate artificial root canal regarding the measurements of the endodontic rotary file tested ( Figure 2B). The artificial root canal piece was manufactured with stainless steel with a 1 mm width. The artificial root canal form was anatomically based and designed using 2D/3D CAD/CAE software (Midas FX+ ® , Brunleys), and it was manufactured by electrical discharge machining (EDM) molybdenum wire-cut technology (Cocchiola S.A., Buenos Aires, Argentina) ( Figure 2C) to simulate the tested endodontic rotary instrument's apical size, taper, and length and enable intimate contact between the endodontic rotary file and the artificial root canal, with a 60° curvature according to Schneider's measure technique [16] and a 3 mm radius of curvature ( Figure 2D). The endodontic handpiece (X-Smart Plus, Dentsply Maillefer, Baillagues, Switzerland) was scanned (3D Geomagic Capture Wrap, 3D Systems©, Rock Hill, SC, USA) to create an accurate design for its support piece by means of inverse engineering technology (Midas FX+ ® , Brunleys) and 3D impression manufacture (ProJet ® 6000. 3D Systems©, Rock Hill, SC, USA). This support was firmly attached to the main structure and enabled the angular displacement of the endodontic handpiece by means of a spindle to test different file lengths. The support also allowed the files to be removed during the cyclic fatigue tests.
The direction and speed of the movement were produced by the brushed DC gearmotor (Ref.  Figure 2A) that was needed to design an accurate artificial root canal regarding the measurements of the endodontic rotary file tested ( Figure 2B). The artificial root canal piece was manufactured with stainless steel with a 1 mm width. The artificial root canal form was anatomically based and designed using 2D/3D CAD/CAE software (Midas FX+ ® , Brunleys), and it was manufactured by electrical discharge machining (EDM) molybdenum wire-cut technology (Cocchiola S.A., Buenos Aires, Argentina) ( Figure 2C) to simulate the tested endodontic rotary instrument's apical size, taper, and length and enable intimate contact between the endodontic rotary file and the artificial root canal, with a 60 • curvature according to Schneider's measure technique [16] and a 3 mm radius of curvature ( Figure 2D).    (Figure 1).
The endodontic rotary instruments were operated by a 6:1 reduction handpiece (X-Smart Plus, Dentsply Maillefer) and a torque-controlled motor with continuous rotation at 400 rpm and 3.5 N/cm torque according to the manufacturer's instructions. The friction between the file and the artificial canal walls was reduced by applying special high-flow synthetic oil designed for the lubrication of mechanical parts (Singer All-Purpose Oil; Singer Sewing Company, Barcelona, Spain).
All endodontic rotary instruments were rotated until fracture occurred. The number of cycles to fracture (NCF) for each instrument was calculated by using the following formula: NCF = time (seconds) to failure × rotational speed (rpm)/60 seconds [17]. The time to failure, the NCF, the number of in-and-out movements, and the length of the fracture file tip were also measured and recorded. Fractographic analysis of the failure was performed under a scanning electron microscope (ZEISS Supra 35VP; Oberkochen, GmBH, Germany) to examine topographic features of the fractured endodontic rotary files.

Statistical Tests
Statistical analysis of all variables was carried out using SAS 9.4 (SAS Institute Inc., Cary, NC, USA). Descriptive statistics are expressed as means and standard deviations (SD) for quantitative variables and as absolute numbers and percentages for qualitative variables. Comparative analysis was performed by comparing the time to failure, the NCF, and the number of pecking movements (cycles of in-and-out movements) using ANOVA. In addition, Weibull characteristic strength (σ0) and Weibull modulus (m) were calculated. The statistical significance was set at p ˂ 0.05.

Results
The means and SD values for the time to failure (seconds), the NCF, and the number of cycles of in-and-out movement in the study groups are displayed in Tables 1-3, respectively. The endodontic rotary instruments were operated by a 6:1 reduction handpiece (X-Smart Plus, Dentsply Maillefer) and a torque-controlled motor with continuous rotation at 400 rpm and 3.5 N/cm torque according to the manufacturer's instructions. The friction between the file and the artificial canal walls was reduced by applying special high-flow synthetic oil designed for the lubrication of mechanical parts (Singer All-Purpose Oil; Singer Sewing Company, Barcelona, Spain).
All endodontic rotary instruments were rotated until fracture occurred. The number of cycles to fracture (NCF) for each instrument was calculated by using the following formula: NCF = time (seconds) to failure × rotational speed (rpm)/60 seconds [17]. The time to failure, the NCF, the number of in-and-out movements, and the length of the fracture file tip were also measured and recorded. Fractographic analysis of the failure was performed under a scanning electron microscope (ZEISS Supra 35VP; Oberkochen, GmBH, Germany) to examine topographic features of the fractured endodontic rotary files.

Statistical Tests
Statistical analysis of all variables was carried out using SAS 9.4 (SAS Institute Inc., Cary, NC, USA). Descriptive statistics are expressed as means and standard deviations (SD) for quantitative variables and as absolute numbers and percentages for qualitative variables. Comparative analysis was performed by comparing the time to failure, the NCF, and the number of pecking movements (cycles of in-and-out movements) using ANOVA. In addition, Weibull characteristic strength (σ0) and Weibull modulus (m) were calculated. The statistical significance was set at p < 0.05.

Results
The means and SD values for the time to failure (seconds), the NCF, and the number of cycles of in-and-out movement in the study groups are displayed in Tables 1-3, respectively.   The ANOVA revealed statistically significant differences in all three variables. The differences revealed by pairwise comparisons were all statistically significant, except for the difference in the number of cycles between the groups with 60 and 120 movements/min (p = 0.298). The scale distribution parameter (η) of Weibull statistics showed statistically significant differences in all three variables, except for the difference in the number of cycles between the groups with 30 and 60 movements/min (p = 0.0722) (Tables 4-6). However, the shape distribution parameter (β) showed no statistically significant differences in the time to failure between the groups with 30 and 60 movements/min (p = 0.0911), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8568) (Figures 4 and 5).
There were no statistically significant differences in the β value of the NCF between the groups with 30 and 60 movements/min (p = 0.0990), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8858) (Figures 6 and 7). However, the shape distribution parameter (β) showed no statistically significant differences in the time to failure between the groups with 30 and 60 movements/min (p = 0.0911), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8568) (Figures 4 and 5).
There were no statistically significant differences in the β value of the NCF between the groups with 30 and 60 movements/min (p = 0.0990), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8858) (Figures 6 and 7).   There were no statistically significant differences in the β value of the number of cycles of inand-out movement between the groups with 30 and 60 movements/min (p = 0.0990), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8858) (Figures 8 and 9). There were no statistically significant differences in the β value of the number of cycles of in-and-out movement between the groups with 30 and 60 movements/min (p = 0.0990), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8858) (Figures 8 and 9).
The lifespan of the endodontic rotary files submitted to 30 movements/min was higher (430.53 ± 77.71 s than that of instruments submitted to 60 movements/min (235.75 ± 62.55 s) and 120 movements/min (165.20 ± 42.03 s). The mean lengths of the fractured fragments were not statistically significantly different for any of the instruments tested (p > 0.05). There were no statistically significant differences in the β value of the number of cycles of inand-out movement between the groups with 30 and 60 movements/min (p = 0.0990), 30 and 120 movements/min (p = 0.1537), and 60 and 120 movements/min (p = 0.8858) (Figures 8 and 9).   The lifespan of the endodontic rotary files submitted to 30 movements/min was higher (430.53 ± 77.71 s than that of instruments submitted to 60 movements/min (235.75 ± 62.55 s) and 120 movements/min (165.20 ± 42.03 s). The mean lengths of the fractured fragments were not statistically significantly different for any of the instruments tested (p > 0.05).

Discussion
The results obtained in the present study lead to the rejection of the null hypothesis (H0), which states that the frequency of pecking motion has no statistically significant effect on the cyclic fatigue resistance of endodontic rotary instruments.
Different causes of fractures of endodontic instruments have been proposed by many authors and include operator experience [18,19], rotational speed [20,21], the number of uses [22], the number of rotations [23], pre-flaring [24], glide path [25], the angle and radius of the curvature [20,26], and the sterilization of instruments [27]. However, the influence of the frequency of pecking motion performed by the operator on the cyclic fatigue resistance of endodontic rotary instruments has not yet been analyzed.
Cyclic fatigue resistance has been experimentally analyzed by using several custom-made devices; however none of them have been capable of testing the cyclic fatigue resistance of NiTi endodontic rotary instruments using an anatomically based artificial root canal that allows intimate contact between the artificial root canal and the endodontic rotary instrument along the file, which might alter the cyclic fatigue resistance of the endodontic rotary instruments and hence the results of the experiment [11]. The dynamic motion produced by a cyclic fatigue test device can be made comparable to the pecking motion performed by the operator during a root canal treatment. The

Discussion
The results obtained in the present study lead to the rejection of the null hypothesis (H0), which states that the frequency of pecking motion has no statistically significant effect on the cyclic fatigue resistance of endodontic rotary instruments.
Different causes of fractures of endodontic instruments have been proposed by many authors and include operator experience [18,19], rotational speed [20,21], the number of uses [22], the number of rotations [23], pre-flaring [24], glide path [25], the angle and radius of the curvature [20,26], and the sterilization of instruments [27]. However, the influence of the frequency of pecking motion performed by the operator on the cyclic fatigue resistance of endodontic rotary instruments has not yet been analyzed.
Cyclic fatigue resistance has been experimentally analyzed by using several custom-made devices; however none of them have been capable of testing the cyclic fatigue resistance of NiTi endodontic rotary instruments using an anatomically based artificial root canal that allows intimate contact between the artificial root canal and the endodontic rotary instrument along the file, which might alter the cyclic fatigue resistance of the endodontic rotary instruments and hence the results of the experiment [11]. The dynamic motion produced by a cyclic fatigue test device can be made comparable to the pecking motion performed by the operator during a root canal treatment. The automatic detection system is able to objectively and accurately identify failures of endodontic rotary files. Dederich and Zakariasen (1986) were first to develop a dynamic testing device, which allowed the endodontic rotary instruments to realize displacement in a pure axial movement [28]. Ray et al. (2007) also performed dynamic cyclic fatigue studies with standardized axial movement, and they concluded that the pecking motion increased the lifespan of the endodontic rotary instruments submitted to cyclic fatigue compared with the results obtained from static cyclic fatigue studies [29]. Hülsmann et al. (2019) reported similar findings to ours and highlighted the differences between static and dynamic cyclic fatigue tests [30]. Nevertheless, only 12% of published cyclic fatigue studies have used a dynamic fatigue device [30]. Most studies that have compared static and dynamic cyclic fatigue studies have concluded that the time to fracture of endodontic rotary instruments submitted to dynamic cyclic fatigue studies was approximately 20-40% higher than that found in static cyclic fatigue studies [26,[31][32][33][34][35]. Previous dynamic cyclic fatigue devices have been unable to accurately identify the causes of endodontic rotary instrument failures because of the absence of standardization of the axial direction of the pecking motion. Lateral movements during the pecking motion of an endodontic rotary instrument can lead to a second bending point at the beginning of the artificial root canal, thus distorting the outcome of cyclic fatigue tests. In addition, Plotino et al. (2010) reported the influence of the shape of the artificial root canal on the contact between the endodontic rotary instrument and the artificial root canal. They concluded that artificial root canals must be designed with a focus on the shape of the endodontic rotary file being studied [36]. However, the above-described artificial root canals had a cylindrically shaped circular section that did not allow intimate contact with the endodontic rotary instrument. In an attempt to simulate real clinical conditions, the present study involved an artificial root canal that was designed by using the measurements of the endodontic rotary instrument being studied.
The higher β values observed for the group with 30 movements/min for all three variables reveal more predictable behavior of the failure of the endodontic rotary instruments in this study group, and the lower η values found in the groups with 60 and 120 movements/min for all three variables indicate a lower cyclic fatigue resistance of the endodontic rotary instruments in these study groups. The location of the crystal structure transformation is changed in the endodontic rotary instrument during the pecking motion, which increases the cyclic fatigue resistance [30].
The conclusion derived from this study is that a high frequency of pecking motion decreases the cyclic fatigue resistance of endodontic rotary files.
Nevertheless, further research is needed to determine the influence of the frequency of pecking motion and working time on the cyclic fatigue resistance of endodontic rotary files.

Conclusions
In conclusion, within the limitations of this study, our results show that a high frequency of pecking motion decreases the cyclic fatigue resistance of endodontic rotary instruments. A low frequency of pecking motion is recommended to reduce the risk of fractures of endodontic rotary instruments associated with cyclic fatigue.
Author Contributions: All of the authors contributed to the investigation, supervision, writing, review, and editing of the study. The study was conceptualized by R.