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Effect of torsional and cyclic fatigue preloading on the fracture resistance of HyFlex EDM NiTi files Tra, Charles


Objective: Endodontic nickel-titanium (NiTi) files are submitted to a combination of cyclic fatigue and torsional stresses when used in a root canal, which could lead to their fracture. The purpose of this study was to evaluate the effect of various degrees of cyclic fatigue preloading on the torsional failure and of torsional preloading on the cyclic fatigue life of heat-treated HyFlex EDM NiTi files (Coltene/Whaledent, Altstätten, Switzerland). Methods: The mean number of cycles until failure (Nf) of HyFlex EDM and HyFlex CM NiTi (Coltene/Whaledent) files (size #40, taper 0.04 for both files) was examined in a 5mm radius and 60° single curve. Torque and distortion angle at the failure of new instruments and instruments stressed to 50% and 75% of the Nf were measured according to ISO 3630-1. Other new files were preloaded at 5%, 15%, 25% and 50% of the mean distortion angle before the fatigue test. After torsional preloading, the Nf was examined. The fracture surface of each fragment was examined with a scanning electron microscope. Results: The fatigue resistance of HyFlex EDM files was higher than that of HyFlex CM files (P < .05). The torque and distortion angle at fracture of new HyFlex EDM files were similar to those of new HyFlex CM files. For both instruments, there was a significant effect because of the torsional preloads. Over 15% torsional preloading lowered the Nf of HyFlex EDM files (P < .05), while a moderate amount of 50% torsional preloading significantly lowered the Nf of HyFlex CM files (P < .05). In the fatigue pre-stressed groups, there was no negative effect of the HyFlex EDM group even with 75% preloading on the torque and distortion angle. The fractographic patterns corresponded to the pattern defined by the last stage test. Conclusions: A low amount of torsional preloading reduced the fatigue resistance of HyFlex EDM files. The torsional resistance of HyFlex EDM files was less affected by previous load of cyclic fatigue even after extensive pre-cycling.

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