Effects of Additional Vertical Movements on the Torque, Force, and Root Canal Shaping Performance of Nickel-Titanium Rotary Instruments in Optimal Torque Reverse Motion.

Introduction: Optimal manipulation techniques for nickel-titanium rotary instrumentation using optimum torque reverse (OTR) motion, a torque-sensitive reciprocating motion, remain underexplored. This study evaluated the effects of applying "forced in-and-out motion" (i.e., downward followed by immediate upward movement upon OTR activation) on the performance of ProTaper NEXT instrumentation.

Methods: An automated instrumentation with torque/force sensing was conducted in 35 J-shaped resin canals with ProTaper Next X1 to X3, employing periodic up-and-down movements. The applied motions were; continuous rotation (group CR); OTR motion without forced in-and-out motion (group OTR-N); and OTR motion with forced in-and-out motion, featuring downward movements lasting 0.25 s, 1.0 s, and 2.0 s (groups OTR-0.25s, OTR-1s, and OTR-2s). Maximum torque/force values, instrumentation time, and canal-centering ratios were evaluated. Statistical analysis was conducted adopting two-way analysis of variance followed by a simple main effect test or Tukey test (α = 0.05).

Results: Groups OTR-0.25s and OTR-1s typically demonstrated significantly smaller clockwise torque, and groups CR, OTR-0.25s, and OTR-1s generally exhibited significantly smaller downward force, relative to the other groups (P < .05). Groups CR and OTR-0.25s demonstrated significantly higher canal-centering ratios (more deviations) at apical 0 mm and 0.5 mm than the other groups (P < .05). Group OTR-0.25s had the longest instrumentation time (P < .05).

Conclusions: When adopting OTR motion in ProTaper Next rotary instrumentation, incorporating forced in-and-out motion with 1.0 s of downward movement reduced force/torque generation and canal-centering ratios without extending instrumentation time.