Influence of rotational speed on torque/force generation and shaping ability during root canal instrumentation of extracted teeth with continuous rotation and optimum torque reverse motion.

Abstract

To evaluate how different rotational speeds affect the torque/force generation and shaping ability of rotary root canal instrumentation using JIZAI (MANI, Utsunomiya, Japan) nickel-titanium instruments in continuous rotation and optimum torque reverse (OTR) motion. Mesial root canals of extracted mandibular molars were instrumented up to size 25, 0.04 taper using JIZAI instruments, and anatomically matched canals were selected based on geometric features of the canal [canal volume (mm3 ), surface area (mm2 ), length, 15°-20° curvature and radius of curvature (4-8mm)] after micro-computed tomographic scanning. An automated root canal instrumentation and torque/force analysing device was programmed to permit a simulated pecking motion (2s downward and 1s upward at 50mmmin-1 ). The selected canals were prepared with size 25, 0.06 taper JIZAI instruments using continuous rotation or OTR motion and further subdivided according to the rotational speed (300 or 500rpm, n=10 each). Real-time clockwise/counterclockwise torque and downward/upward force were recorded using a custom-made torque/force analysing device. Then, the registered pre- and post-operative micro-computed tomographic datasets were examined to evaluate the canal volume changes and centring ratios at 1, 3, 5 and 7mm from the apical foramen. Data were analysed using two-way analysis of variance or the Kruskal-Wallis test and Mann-Whitney U test with Bonferroni correction (α=5%). Maximum upward force and clockwise torque were significantly smaller in 500rpm groups than in 300rpm groups (P<.05); however, no significant difference was found between continuous rotation and OTR motion (P>.05). OTR motion developed higher maximum counterclockwise torque than continuous rotation (P<.05). Maximum downward force, canal volume changes and centring ratios were not significantly different among all groups (P>.05). There was no file fracture in any of the groups. Under laboratory conditions using JIZAI instruments, a rotational speed of 500rpm generated significantly lower maximum screw-in forces and torque values than rotational speed of 300rpm. Continuous rotation and OTR motion performed similarly in shaping the canals.