Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files

Abstract

Introduction: The aim of this investigation was to study cyclic fatigue resistance of various nickel-titanium (NiTi) rotary files under various root canal curvatures by correlating cyclic fatigue fracture tests with finite-element analysis (FEA).

Methods: Four NiTi rotary instruments with different cross-sectional geometries but comparable sizes were selected for this study: ProTaper (Dentsply Maillefer, Ballaigues, Switzerland), ProFile (Dentsply Maillefer), HeroShaper (Micromega, Besançon, France), and Mtwo (VDW, Munich, Germany). The ProFile and HeroShaper files were of size 30/.06 taper, the Mtwo was of size 30/.05 taper, and the ProTaper was F3. The cyclic fatigue test was conducted in a custom-made device that simulated canals with 25°, 35°, and 45° curvature. For the FEA, the file models were meshed, and 17-mm long curved canals were modeled to have same curvatures as the cyclic fatigue tests. Numerical analysis was performed to determine the stress distributions in the NiTi instruments while they rotated in the simulated curved canals.

Results: ProTaper (the stiffest instrument) showed the least cyclic fatigue resistance and highest stress concentration for all tested curvatures, whereas Mtwo showed the best cyclic fatigue resistance. A comparison between the FEA and fatigue results showed that when stresses increased, the number of instrument rotations to fracture decreased. Maximum stresses in the instruments predicted the approximate location of the fatigue fracture.

Conclusions: The stiffer instrument had the highest stress concentration in FEA and the least number of rotations until fracture in the cyclic fatigue test. Increased curvature of the root canal generated higher stresses and shortened the lifetime of NiTi files. Finite-element stress analysis reflected cyclic fatigue fracture resistance.