Potential Effect of Stress-Induced Martensite Transformation from the Reuse of Nickel-Titanium Files on Fatigue Fracture Resistance According to File Sequences

This study aimed to evaluate the changes in the number of cycles to fracture (NCF) of the Nickel-Titanium (NiTi) files after multiple uses and investigate the potential relationship with stress-induced martensite transformation. A total of 225 ProTaper Ultimate (PTUL) files were classified into three groups: Group 1 consisted of new files, Group 2 comprised files used to shape two resin simulated canals, and Group 3 consisted of files used to shape four canals. The simulated resin canals with a 16 mm length of J-shaped with 35° curvature were prepared using PTUL Slider, Shaper, F1, F2, and F3 files sequentially. After instrumentation, the cyclic fatigue resistance of each sequential file was assessed in a 35° curved canal by rotating at 400 rpm using a custom-made device. Statistical analysis was performed using one-way ANOVA with Tukey’s post-hoc test or Kruskal–Wallis with Dunn’s test with Bonferroni correction for parametric and non-parametric data, respectively. Slider and Shaper maintained consistent NCF across all groups. In contrast, F1 showed a slight increase after two uses but declined significantly after four uses. F2 demonstrated progressive NCF decline, with significant differences between new and four-use groups. F3 exhibited the most significant reduction in NCF across all conditions (p < 0.05). Repeated use of NiTi files reduces their cyclic fatigue resistance, but the degree of decline varies among files. Repeated use of a certain-sized file may enhance fracture resistance, possibly due to stress-induced martensite (SIM) transformation.