Effect from Cyclic Fatigue of Nickel-Titanium Rotary Files on Torsional Resistance

IntroductionThe aim of this study was to evaluate the effect of cyclic fatigue on the torsional resistance of nickel-titanium (NiTi) rotary instruments.MethodsProFile (#25/0.06; Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper (F1, Dentsply Maillefer), both of which have the same external diameter at D5, were tested using a fatigue testing machine for the mean number of cycles of failure (mNCF). Then, new files were cyclic precycled to 4 conditions (ie, 0%, 25%, 50%, and 75% of the mNCF) before the torsional resistance test was performed on these cyclic preloaded files. A uniform clockwise rotation was applied to the file in a straight state in a torsion tester. The torsional load and distortion angle were recorded during rotation until the file succumbed to the torque. The toughness was computed. The results were analyzed statistically using one-way analysis of variance and the Duncan post hoc comparison to find any differences between groups at a significance level of 95%.ResultsIn both ProFile and ProTaper groups, the 75% preloading groups had significantly lower torsional strength than other preloaded files. In the ProFile group, the 50% and 75% preloading groups had a smaller distortion angle until fracture than the 25% and no preloading groups. The 75% preloading group showed a lower toughness value than the 25% and no preloading groups. In the ProTaper group, all preloading groups had less distortion and toughness than the no preloading group. Fractographic examinations revealed the 75% preloaded files showed less amount of reverse-wound flute than other preloading groups.ConclusionsWithin the limitations of the methodology, it could be concluded that approximate 75% cyclic fatigue may reduce the torsional resistance of NiTi rotary instruments significantly.