Cyclic Fatigue Resistance of Two Different Nickel-Titanium Rotary Instruments in Interrupted Rotation

Objective: To investigate the influence of interrupted rotation on cyclic fatigue of two different nickel-titanium rotary instruments. Methods: Three hundred new ProTaper Next size X1; X2 and Mtwo size #10/0.04; #15/0.05; #20/0.06 and #25/0.06 were tested for cyclic fatigue resistance using rotation continuous or stopped. Fifty files of the same brand and size were randomly assigned to five different groups (n=10). Files of group 1 were tested in continuous rotation; group 2 and 3 in interrupted rotation for one second every 10 or 20 seconds, respectively; group 4 and group 5 interrupted rotation for five seconds every 10 or 20 seconds, respectively. Cyclic fatigue was determined by counting the numbers of cycles to failure (NCF) obtained from the real seconds of continuous rotation in an artificial canal with 60° angle and a 5 mm radius of curve. A two-way analysis of variance were used to evaluate the data. Results: Cyclic fatigue resistance of groups 2 and 4 of ProTaper Next X2 and Mtwo #25/0.06 was lower than group 1 of the same instruments (p<0.01). ProTaper Next X2 showed a reduced cyclic fatigue also in groups 3 and 5 (p<0.05). No differences were found by interrupting the rotation for one or five seconds in all instruments (p>0.05). Fatigue of all other instruments were not affected by interrupted rotation (p>0.05). Conclusions: The interrupted rotation reduced cyclic fatigue resistance of instruments with a greater size especially when a major number of interruptions were performed.