FEM analysis of NiTi rotary endodontic instruments to fatigue stress conditions: influence of geometrical parameters and design optimization

The aim of this paper was to analyse the mechanical behaviour of NiTi endodontic rotary instruments when subjected to fatigue stresses. Four different commercial endodontic files, characterized by different cross-section shapes, were tested. Two finite element models were built to identify the cross-section shape with the best performance. In the first FEM model, rotary instruments were fixed near the tip, while a transverse displacement was applied to the bottom. Von Mises equivalent stresses were evaluated to compare the bending response of the different instruments. In the second FEM model the low-cycle fatigue life of these instruments was examined, when constrained to displace in a curved root canal and contemporary rotate. The comparison of the results, in terms of cyclic fatigue, allowed to identify the commercial instrument whose particular geometric conformation ensures better mechanical strength under test conditions. Subsequently, an analysis of geometry parameters influences on the fatigue life was performed and a design optimization was carried out. In the design optimization process, the output parameter is the total deformation. The results obtained showed that it is possible to achieve an increase in fatigue life of up to 50%. Finally, the fatigue life of the optimized geometry has furthermore tested in a double curvature root canal.