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 (<i>p</i> > 0.05). In contrast, F1 showed a transient increase (117.7%) after two uses but declined significantly (91.6%) after four uses (<i>p</i> < 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 (<i>p</i> < 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.