Introduction: The aim of this study was to investigate the relationship between fatigue life and resonance frequency (RF) of various types of nickel-titanium (NiTi) rotary instruments. In addition, the influence of NiTi instruments with different manufacturing methods on cyclic loading was evaluated by using RF as a parameter. Methods: Twenty-eight ProFile instruments and 10 Twisted File instruments were subjected to cyclic fatigue-loading until fracture by repeated preparation with simulated root canals made of clear resin. The RF of each sample was recorded immediately after the simulated canal block was prepared. For each sample, the microscopic images on the fracture surface, change in lengthening deformation, number of canal blocks prepared, and corresponding RF changes were recorded. Results: For all the tested instruments, RF values decreased gradually before breakdown when the fatigue failure of the instruments was associated with plastic deformation. In addition, there was a linear relationship between the RF change and the corresponding deformation of the failed instruments. Conclusions: These results demonstrate that the RF analysis has potential as a tool for structural analysis in NiTi instruments.
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