Nano‐indentation testing of new and fractured nickel‐titanium endodontic instruments

Abstract

To investigate the effect of cyclic fatigue on nickel-titanium (NiTi) endodontic instruments using a nano-indentation test. Eight ProFile NiTi rotary instruments (size 30, taper 0.06; Dentsply Maillefer, Ballaigues, Switzerland) were tested using a cyclic fatigue set-up until fracture. The fractured instruments and eight new NiTi instruments of the same size and taper were used for a nano-indentation test on the internal surfaces of a NiTi instruments in the region just adjacent to their fractured edge (group I) and in the same region of the new group (group II), and the cutting part beside the shaft for both instruments [group III (fractured) and group IV (new)]. Data were statistically analyzed using one-way analysis of variance and Games-Howell post hoc test. The alpha-type error was set at 0.05. Significant differences in terms of hardness and elastic modulus for each group (P < 0.05) were found, with group I having the lowest mean values followed by group III. Additionally, standard deviations increased remarkably after failure, as represented by groups I and III. The nano-indentation technique can be applied to determine the performance and the failure mechanism of NiTi instruments. The fatigue process revealed a significant decrease in the hardness and elastic modulus of the NiTi instrument. As indicated by the low hardness, the fatigue process did not result in work hardening but rather work softening.