The effect of three rotational speed settings on torque and apical force with vortex rotary instruments in vitro

Both the number of rotations in curved canals and torque are related to fracture resistance of nickel-titanium rotaries via the respective mechanisms of brittle and flexural failure. Increased rotational speed (rotations per minute [RPM]) may lead to higher cutting ability and could overcompensate for increased fatigue. The impact of three RPM settings on peak torque (Nmm) and apically directed force (N) during root canal preparation were investigated in vitro.

S-shaped canals in plastic blocks (n = 12/group) were instrumented with Vortex rotaries (Dentsply Tulsa Dental, Tulsa, OK) sizes #15 to 30 with a .04 taper. Rotaries were used in a manufacturer-recommended sequence: #30, 25, and 20 in a crown-down approach progressively deeper into the canal, #15 to the working length, and apical enlargement with sizes 20 and 25 to WL. A total of 216 preparation procedures were performed using a custom testing platform. RPM was set at 200, 400, or 600; automated axial feed mirrored clinical handling, resulting in two in-and-out movements, each to preset insertion depths. Torque and apical force were continuously recorded and peak values statistically contrasted using analysis of variances.

No file fractures were observed in any of the three experimental groups. Peak torques and forces varied by instrument size and were highest at 200 RPM for all sizes; torque and force were reduced by 32% and 48%, respectively, at 400 RPM (P < .001). Increasing RPM to 600 did not result in further reductions. The number of discernible peaks for torque (threshold: 0.3 Nmm) and force (threshold: 0.2 N) significantly decreased from 200 RPM to 400 RPM and did not decrease further with 600 RPM.

Under the present experimental conditions, rotational speed had a significant impact on preparation with Vortex rotaries, with instruments at 400 RPM generating less torque and force compared with 200 RPM.