Effect of Different Speeds of Up-and-down Motion on Canal Centering Ability and Vertical Force and Torque Generation of Nickel-titanium Rotary Instruments

Introduction: This study was performed to evaluate how the speed of up-and-down motion affects the canal centering ability and torque/force generation of ProTaper Next rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland). Methods: Twenty-one simulated resin canal blocks with a 1-shaped canal were prepared with ProTaper Next X1, X2, and X3 instruments using an original automated root canal instrumentation and torque/force analyzing device with up-and-down speed settings of 10, 50, and 100 mm/ min (low-, medium-, and high-speed groups, respectively). Pre- and post instrumentation images were superimposed, and centering ratios were calculated at 0-3 mm from the apex. The maximum vertical force and torque were also recorded. The results were statistically analyzed using 1-way analysis of variance and the Tukey test. Results: At 0, 0.5, 1, and 2 mm from the apex, the high-speed group showed the lowest centering ratio (ie, least deviation) followed by the medium-speed and low-speed groups (P < .05). Force values (downward and upward) tended to increase as the up-and-down speed increased; with the X2 and X3 instruments, the high-speed and/or medium-speed groups generated significantly larger values than the low-speed group (P < .05). With all instruments, the high-speed and/or medium-speed groups generated significantly larger clockwise torque than the low-speed group (P < .05). One and 2 X2 instruments fractured in the low- and high-speed groups, respectively. Conclusions: The up-and-down speed affected the canal centering ability and stress generation of ProTaper Next instruments. The high-speed group showed the best centering ability but tended to generate larger vertical force and torque than the medium- and low-speed groups.