Effect of Torsional and Fatigue Preloading on HyFlex EDM Files

The purpose of this study was to evaluate the effect of a low amount of torsional preloading on the fatigue life and different degrees of cyclic fatigue on torsional failure of HyFlex EDM (EDM; Coltene-Whaledent, Allstetten, Switzerland) and HyFlex CM (CM; Coltene-Whaledent) instruments.

EDM and CM files were used. The fatigue resistance was examined in a 5-mm radius and 60° single curve, and the mean number of cycles to failure (N_f) was recorded. The torque and rotation angles at failure of the instruments were measured according to ISO 3630-1. New files were precycled to 0%, 50%, and 75% of the N_f, and torsional tests were then performed. Other new files were preloaded at 5%, 15%, 25%, and 50% of the mean rotation angles before the fatigue test. The fracture surfaces of the fragments were examined under a scanning electron microscope.

The fatigue resistance of EDM instruments was higher than that of CM instruments (P < .05). The torque and angle of rotation at fracture of the files were similar. Torsional preloading lowered the N_f of EDM at 15% preloading (P < .05) and the N_f of CM at 50% preloading (P < .05). However, the N_f of EDM files even with 50% torsional preloading was significantly higher than unused CM files (P < .05). Fatigue prestressing even at 75% had no negative effect on the torque and rotation angle of the EDM files. Moderate precycling (50%) of EDM files increased their torsional resistance. The fractographic patterns corresponded to the pattern defined by the last stage test.

A low amount (15%) of torsional preloading reduced the fatigue resistance of EDM files, whereas even extensive (75%) precyclic fatigue was not detrimental to their torsional resistance.