The impact of composition, core metal mass and phase transformation behaviour on the dynamic cyclic fatigue of Ni-Ti files at different temperatures

Aim: To assess impact of elemental composition, core metal mass and phase transformation behaviour on the dynamic cyclic fatigue resistance of three Ni-Ti rotary files at room and body temperatures. Methods: Twenty instruments of each system were tested for dynamic cyclic fatigue resistance in a simulated root canal with a 90 degrees angle of curvature and a 5-mm radius of curvature at room and body temperature. The core metal mass at the fractured surface of each instrument was calculated by Image J software analysis of SEM images. The energy dispersive X-ray analysis was used to assess file composition. Scanning calorimetry was used to assess the structural phase state and the transformation temperature. One-way analysis of variance (ANOVA) was performed to determine any statistical difference amongst groups. For inter-group comparison, the unpaired t-test was used. Results: HEDM showed significantly higher TtF and NCF values than AFBS and ZB-F6 instruments, at both temperatures tested. The mean core metal mass was smallest in HEDM followed by AFBS with no statistical difference between them, while ZB-F6 had the significantly largest metal core. EDX analysis showed that all the instruments were mainly composed by nickel and titanium. DSC analysis revealed that HEDM and AFBS exhibited a martensitic phase at body (37 degrees C) and room temperature (25 degrees C), whereas ZB-F6 revealed an austenitic phase at body temperature. Conclusions: Dynamic cyclic fatigue resistance increased when the instruments had less cross-sectional metal mass, less Ni (wt%), a thermally treated surface, and a martensite phase at body temperature.

Automated summary

This study assessed the impact of elemental composition, core metal mass, and phase transformation behavior on the dynamic cyclic fatigue resistance of Ni-Ti rotary files. The results showed that the resistance increased when the files had less metal mass, lower Ni content, a thermally treated surface, and a martensite phase at body temperature.