Influência da deformação cíclica em torção na resistência à fadiga flexural dos instrumentos rotatórios de NiTi Hiflex CM, Typhoon CM e Vortex Blue

Abstract

The NiTi rotary instruments undergo simultaneous flexural and torcional stresses during the root canal shaping. The mechanical properties of these instruments are influenced by many factors such as geometry, microstructure, chemical composition of the alloy and thermomechanical treatments. In this study, the influence of cyclic torsional straining on the fatigue resistance of different thermal treated instruments was assessed. Hyflex CM (HF; Coltene/ Whaladent Inc.), Typhoon CM, (TYP;Clinician´sChoice Dental Products,) e Vortex Blue (VB; Dentsply Tulsa Dental), size and taper 30/.06 were the instruments chosen, based on their geometry and specific characteristics of their manufacturing process. Ten new instruments for each system were tested in a bench test device to determine the mean value of the number of cycles to failure (Nf) of new instruments (GC, Control Group); another group (of 10instruments was submitted to 20 cycles of torsional straining between 0° and 180°. After the cyclic straining, the instruments (EG, Experimental Group) were submitted to fatigue until rupture under the same conditions of the control group. The longitudinal area and the fatigue fracture surfaces were examined by scanning electron microscopy (SEM). The results were analyzed by one-way analysis of variance ANOVA ( = .05). The new HF instruments displayed a higher resistance to fatigue (P < .05), followed by VB and TYP. During the torsional preloads, theinstruments exhibited a decrease in the torque values after the first cycle, with a tendency of stabilization in the following cycles. The lowest mean torque value was observed for the TYP instruments. After the torsional preloads, there was reduction of 20%, 39% and 45% of the mean Nf values for the instruments HF, VB and TYP, respectively. Longitudinal cracks, generated during the torsional preload were not observed in the CM instruments (HF and TYP), but were present in VB files. The instruments evaluated that involve a thermal treatment during its manufacturing, presented different mechanical behavior and possible distinct microstructure. There was a reduction in fatigue resistance after cyclic deformation in all instruments evaluated.