Cyclic fatigue resistance of ground and twisted nickel-titanium rotary instruments: An in vitro study

INTRODUCTION. Nickel–titanium (NiTi) rotary instruments have been extensively employed in endodontics because of their flexibility and capability to preserve canal anatomy. However, unpredictable separation due to cyclic fatigue is still a major concern. As compared to traditional ground NiTi instruments, Twisted File (TF) instruments are produced using a thermomechanical twisting process that maintains the alloy grain structure and may improve fatigue resistance. PURPOSE. To evaluate the cyclic fatigue resistance (CFR) and fracture characteristics of conventional ground and twisted nickel-titanium rotary instruments using scanning electron microscopic (SEM) examination. MATERIALS AND METHODS. The study included thirty-two new NiTi rotary instruments, separated into four groups ( n = 8): Twisted File #25/.06, RaCe #25/.06, ProTaper Universal F1, and Helix #25/.06. Cyclic fatigue testing was conducted using an endodontic motor running at 300 rpm in a stainless-steel artificial canal with a 45° curvature and a 5 mm radius. The recorded time to fracture was used to calculate the number of cycles to failure (NCF). The lengths of fractured fragments were measured, and representative instruments were analyzed by SEM either before or after cyclic fatigue testing. A one-way analysis of variance (ANOVA) and Scheffé’s post hoc test were used to assess the data, and p < 0.05 was used to define statistical significance. RESULTS. The higher CFR was recorded for Twisted File instruments (690.00 ± 55.93 cycles), followed by RaCe (529.00 ± 45.23 cycles), ProTaper Universal (416.00 ± 29.64 cycles), and Helix (242.87 ± 21.63 cycles). A one-way ANOVA showed a statistically significant difference among groups. For every pairwise comparison, post hoc analysis demonstrates significant differences ( p < 0.001). The lengths of the broken fragments did not significantly differ among groups ( p = 0.06). SEM assessment indicated microcracks and crack propagation patterns generated by fatigue in all instrument systems. Twisted File instruments showed fewer surface flaws in comparison with the standard ground instruments, which had crack propagation linked with machining grooves. CONCLUSION. Twisted File instruments have better CFR than RaCe, ProTaper Universal, or Helix. SEM demonstrated characteristic fatigue failure patterns in all systems, but traditional ground instruments exhibited greater surface flaws. Twisted Files may have improved fatigue performance due to thermomechanical twisting.