Cyclic fatigue resistance of two nickel-titanium rotary instruments in interrupted rotation

Aim To investigate the influence of interrupted rotation on cyclic fatigue of two nickel-titanium rotary instruments. Methodology Cyclic fatigue of 300 new ProTaper Next size X1; X2 and Mtwo size 10,.04 taper; size 15,.05 taper; size 20,.06 taper and size 25,.06 taper instruments was tested in continuous or interrupted rotation. Fifty files of the same brand and size were randomly assigned to five groups (n = 10). Group 1 instruments were tested in continuous rotation; groups 2 and 3 in paused rotation for 1 s every 10 or 20 s, respectively; groups 4 and 5 in interrupted rotation for 5 s every 10 or 20 s, respectively. Cyclic fatigue was expressed in time to fracture (TtF) in an artificial canal with 60 degrees angle and 5 mm radius of curvature. The fracture surface was examined with a scanning electron microscope (SEM). Data were evaluated by two-way analysis of variance. Results Cyclic fatigue of groups 2 and 4 of ProTaper Next X2 and Mtwo size 25,.06 taper was significantly lower than that of group 1 of the same instruments (P < 0.01). ProTaper Next X2 had significantly reduced cyclic fatigue in groups 3 and 5 (P < 0.05). No differences were found by interrupting the rotation for 1 or 5 s in all instruments (P > 0.05). Fatigue of other instruments was not affected by interrupted rotation (P > 0.05). Conclusions Interrupted rotation reduced cyclic fatigue resistance of ProTaper Next X2 and Mtwo size 25,.06 taper, especially when a higher number of interruptions was performed.