Cyclic Fatigue Evaluation of Nickel-Titanium Rotary Instruments Using an Electrical Resistance-based Method

Introduction

This in vitro study aimed to establish the correlation between electrical resistance and cyclic fatigue of endodontic nickel-titanium (NiTi) rotary instruments as a quantifiable cyclic fatigue indicator.

Methods

Two kinds of geometrically identical NiTi rotary instruments ProTaper Universal F2 and ProTaper Gold F2 were selected and allocated into 2 experimental groups: a cyclic fatigue test group and a sterilization test group. In the cyclic fatigue test group, instruments were rotated in an artificial root canal until fracture, with the number of cycles (rotations) to fracture (NCF) recorded. In the sterilization test group, instruments were rotated to 20%, 40%, 60%, and 80% of the NCF, followed by autoclaving sterilization. Electrical resistance was measured after each 10 rotations, as well as pre- and post-sterilization. Scanning electron microscopy was applied to analyze the instrument surfaces, focusing on the expanding of microcracks over rotations and the effect of autoclaving. The collected data were then statistically analyzed.

Results

The results indicated that the electrical resistance of tested instruments increased quadratically with the cyclic fatigue degrees along with the expanding of microcracks. At the 80% NCF, the electrical resistance increased by 2.38% and 2.63% for PTU F2 and PTG F2 respectively. Autoclaving had no significant effect on electrical resistance.

Conclusion

Electrical resistance could be used as a quantifiable indicator to evaluate and predict the cyclic fatigue degree of NiTi rotary instruments and seems unaffected by autoclaving.