## 4 Discussion
These findings are consistent with those of a previous study [27], indicating that larger pecking amplitudes increase stress from the screw-in force, potentially shortening the fatigue life of NiTi instruments. Torque is produced when a rotating NiTi instrument contacts the canal wall, and instrument fracture occurs when the torsional stress exceeds the elastic limit of the NiTi alloy [37; 38]. In this study, the pecking amplitude did not exert a significant effect on torque generation, likely because the reciprocal motion cyclically and effectively alleviated torsional stress. However, further investigation using continuous rotation is needed. The canal-centering ability may exert significant influence on the prognosis of curved root canal treatments, because minimizing deviation and preserving the original shape are crucial for effective debridement and reducing procedural errors such as ledging and perforation [4]. Factors that enhance the canal-centering ability of NiTi instruments include design features such as smaller taper [38] and radial landed design [39], greater flexibility [24], heat treatment of the NiTi alloy [14], and higher rotational speed [40]. To the authors' knowledge, this is the first study to investigate how the pecking amplitude influences the canal-centering ability of NiTi instruments. Group S exhibited significantly larger outer deviations compared with other groups at levels \(\geq\)2 mm from the apex, possibly because the smaller pecking amplitude caused repeated cutting within a narrow region of the canal wall without sufficient relief provided by the upward and downward motion. Consequently, the instrument tended to cut the outer wall of curved canals in a sustained manner. The cutting of the outer wall may have been enhanced by the frequent generation of a spring-back force, which is generated by the tendency of the superelastic NiTi instrument to straighten upon withdrawal from the canal curvature [13]. Cyclic fatigue fracture is a main cause of failure in NiTi rotary instruments. It occurs due to repeated cycles of tension and compression experienced by the instrument while rotating in a curved root canal [41]. Repeated loading induces microscopic damage at stress concentration points, leading to the formation of a small crack that rapidly propagates and results in failure [41]. The present study employed a dynamic cyclic fatigue test, which is commonly used to assess the fatigue resistance of NiTi rotary instruments [9; 21; 22; 23; 42; 43]. The dynamic test better simulates clinical conditions compared to the static test by replicating clinical movements, distributing stress more evenly, and providing a more accurate assessment of instrument durability [43]. Scanning electron microscopy of the fractured surfaces (Figure 5C,D) revealed typical features of cyclic fatigue failure, such as crack initiation zones and dimples characteristic of ductile rupture [44]. The dynamic cyclic fatigue test revealed that the 3 mm pecking amplitude provided greater cyclic fatigue resistance compared with the 1 mm amplitude, which aligns with previous studies at different pecking depths [28; 29; 30]. The longer pecking amplitude may distribute stress more widely over the instrument and increase the interval between high stress concentration periods. The shorter length of the fractured segment after a longer pecking amplitude indicates a shift in stress concentration toward the instrument's tip. This may also contribute to the extension of the time to fracture, because a smaller diameter is more resistant to cyclic fatigue [45; 46]. This study used simulated resin root canal blocks to standardize conditions and eliminate variations in canal morphology [47], ensuring uniform canal wall hardness [48]. However, the physical properties of resin canal blocks differ from those of dentin, necessitating further experimentation using anatomically matched extracted teeth [49].