File-Specific Cyclic Fatigue Resistance of NiTi Instruments After Repeated Use in Simulated Canals: Patterns Compatible with Potential Stress-Induced Martensite Transformation Effects

This study evaluated changes in the number of cycles to fracture (NCF) of Nickel–Titanium (NiTi) files after repeated use in simulated canals and investigated the potential relationship with stress-induced martensite (SIM) transformation effect. A total of 225 ProTaper Ultimate (PTUL) files were divided 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 steel 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 stable NCF across all groups (p > 0.05). In contrast, F1 showed a transient increase (117.7%) after two uses but declined significantly (91.6%) after four uses (p < 0.05). F2 and F3 demonstrated progressive NCF reductions (F2: 72.9%; F3: 71.5% after four uses), with F3 showing the most pronounced decline (p < 0.05). Repeated use of NiTi files reduced their cyclic fatigue resistance in a file-specific manner, with larger finishing files most affected. The distinctive F1 pattern suggests potential preload-related or SIM transformation effects that warrant further metallurgical investigation.