Effect of Cyclic Fatigue on Static Fracture Loads in ProTaper Nickel-Titanium Rotary Instruments

The aim of the present study was to evaluate static fracture loads of ProTaper Nickel-Titanium instruments that had been subjected to various degrees of cyclic fatigue. Torque and angle at failure of new instruments and instruments that had been stressed to 30, 60, or 90% of their cyclic fatigue rotations in a simulated canal (90 degrees and 5 mm radius) were tested according to ISO 3630-1. With unused ProTaper instruments, resistance to cyclic fatigue decreased with diameter increase and ranged from 158 to 450 rotations. Torque at failure ranged from 0.5 to 2.1 Ncm and showed a strong linear relationship to instrument diameter (r = 0.9) while angle at failure was weakly related to diameter (r = 0.46). Cyclic prestressing significantly reduced torsional resistance in finishing files, while shaping files were largely unaffected. In conclusion, build-up of tension within NiTi rotary instruments depends on instrument diameter. Clinically, larger instruments that have been subjected to some cyclic fatigue should be used with great care or discarded.