Influence of continuous rotation or reciprocation of Optimum Torque Reverse motion on cyclic fatigue resistance of nickel-titanium rotary instruments.

Aim: To evaluate the resistance to cyclic fatigue of ProTaper Next (PTN; Dentsply Sirona, Ballaigues, Switzerland), Revo-S (Micro-Mega, Besançon, France), Mtwo (Sweden & Martina, Padova, Italy), Twisted Files (TF, SybronEndo, Orange, CA, USA) and EndoWave (J Morita Corporation, Osaka, Japan) used in continuous rotation or in reciprocation of Optimum Torque Reverse motion (OTR).

Methodology: A total of 120 nickel-titanium files were tested. Twenty-four instruments for each brand were divided into two groups (n = 12) on the basis of the motion tested: continuous rotation (Group 1) or reciprocation of OTR motion (Group 2). Resistance to cyclic fatigue was determined by recording time to fracture (TtF) in a stainless steel artificial canal with a 60° angle of curvature and 5 mm radius of curvature. The TtF data were analysed by using two-way analysis of variance (anova) and Bonferroni's post hoc tests at 0.05.

Results: Mtwo and TF had significantly higher TtF when compared with all other instruments, both in continuous rotation and in reciprocation of OTR motion (P < 0.0001 and P < 0.05, respectively). No difference was observed between Mtwo and TF (P > 0.05), in both motions. PTN was associated with higher cyclic fatigue resistance than Revo-S and EndoWave, both in continuous rotation and in reciprocation of OTR motions (P < 0.0001). No difference was observed between Revo-S and EndoWave, in both motions (P > 0.05). Reciprocating OTR motion improved TtF of all instruments (P < 0.0001).

Conclusions: Reciprocation of OTR motion improved significantly cyclic fatigue resistance of all instruments tested compared with continuous rotation. Mtwo and TF had significantly higher cyclic fatigue than the other instruments, in both continuous rotation and reciprocation of OTR motion.