Work Hardening of NiTi Endodontic Rotary Instruments

Objective: We examined the nano-mechanical properties of NiTi endodontic files before and after mechanical testing to failure.

Methods: Twelve ProFile 0.04 Taper instruments from one lot were examined. The modulus of elasticity(E) and hardness(H) of each as-received file was characterized at 5mm from the tip (at the mid-point of subsequent testing deflection) by nanoindentation (Hysitron UBI-1 nanomechanical testing instrument). Each file was subjected to cyclic fatigue testing using a custom device designed to allow the instruments to rotate freely inside a stainless steel artificial canal without vertical displacement, thus concentrating stress at the desired distance from the tip. Rotation was accomplished at a constant speed of 300rpm inside the artificial canal with a 90° curvature and a 5mm radius of curvature. E and H were again measured near the fracture edge following testing. Fracture surfaces were examined with SEM to determine mode of failure. Mean values for E and H were compared using paired t-test, with alpha=0.05.

Results: All files failed within 1.5minutes of testing (<450 rotations). Both the mean hardness value, as well as the modulus of elasticity, for the files were significantly different within 20µm of the fracture edge. Further from the fracture, values for E and H were similar to the as-received instruments.

Conclusions: Nano-indentation analysis indicated that substantial work hardening in the active region of the file occurred. Although previous studies, using micro-indentation, have not found this phenomenon, this has likely been due to the large-scale sampling of those techniques. Further research to correlate structural changes in NiTi instruments with changes in mechanical properties are currently underway.