Aim: To assess the influence of different motions on the cyclic fatigue resistance of Reciproc instruments simulating the temperature of the clinical conditions. Methods: The sample size was determined using statistical software set with the following parameters: \u3b1=0.05, \u3b2=0.20, \u3b4=30.0, \u3c3=28.0. The experiment required 54 Reciproc files. Brand new R25 files were randomly allocated to three groups defined by the tested motion: continuous rotation at 300 rpm (n=18), \u201cRECIPROC\u201d mode (n=18), and \u201cWAVEONE\u201d mode (n=18). The same endodontic motor was used for all groups (X-Smart IQ). All files were rotated/reciprocated until fracture inside a custom-designed artificial canal with 60\ub0 angle and 5-mm radius of curvature milled in a stainless-steel block. The testing device was electrically heated to keep its internal temperature at 35\ub11\ub0C, which was constantly monitored with a thermometer. After file separation, the time to failure was registered with a digital chronometer and the length of the fractured fragment measured with a digital calliper. The fracture surface of each file was observed at the scanning electron microscope to perform a qualitative fractographic analysis. The
collected data (time to fracture and fracture length) were tested for the normality of the distribution and the equality of variances with a Shapiro-Wilk and a Levene test, respectively. The dependent variables were compared amongst groups by means of a multivariate analysis of variance and Tuckey post-hoc test (p=0.05). Results: The continuous rotation group exhibited the shortest lifespan among the considered groups (85.4\ub19.5 s to failure). Both reciprocating motions were associated with a significant improvement of fatigue resistance (p<0.001). The \u201cRECIPROC\u201d mode allowed for longer time to failure than the \u201cWAVEONE\u201d mode, with 141.6\ub119.4 s and 117.2\ub111.2 s to failure, respectively. The absence of differences among the considered groups in terms of fracture length confirmed the correct positioning of the files inside the artificial canal. The scanning electron microscopic analysis showed signs of file separation ascribable to cyclic fatigue. Conclusion: The present study preliminary demonstrated
that the native \u201cRECIPROC\u201d motion use of R25 Reciproc files should be preferred over other types of motions to prevent file separation in the clinical setting
