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This study aimed to evaluate the mechanical properties of contemporary heat-treated nickel-titanium (NiTi) rotary instruments used in the single-length technique [ProTaper Next (PTN), HyFlex EDM (EDM), and JIZAI (JZ)]. Bending loads, cyclic fatigue resistance, torque/force values and canal-centering ratios were evaluated for the three instruments and a non-heat-treated experimental NiTi instrument with the same geometry as JZ (nJZ). EDM and JZ exhibited significantly lower bending load and more cycles to failure compared with nJZ and PTN (p<0.05). PTN and JZ exhibited significantly better centering ability than nJZ and EDM (p<0.05). JZ and nJZ generated significantly smaller upward force and maximum torque than PTN and EDM (p<0.05). Under the present experimental condition, JZ exhibited flexibility and cyclic fatigue resistance comparable to EDM, better maintained the canal curvature than the other instruments, and generated smaller torque and screw-in force than PTN and EDM.

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This study aimed to evaluate the influence of a post-machining thermal treatment on canal-centering ability and torque/force generation of reciprocating nickel-titanium instruments. Simulated J-shaped resin canals were prepared with reciprocating instruments sharing identical geometric architecture and with/without post-machining thermal treatment (Reciproc Blue/Reciproc, VDW, Munich, Germany). Using an original automated root canal instrumentation and torque/force analyzing device, files were operated in a combination of reciprocation and up-and-down motion, and torque/force values were monitored. Canal-centering ratios were measured after superimposition of pre-and post-instrumentation images. Compared with Reciproc, Reciproc Blue showed a significantly lower canal-centering ratio (i.e., less deviation; p<0.05) at 0-1 mm from the apex and generated a significantly smaller upward maximum vertical force (p<0.05). Under standardized conditions using the automated device, Reciproc Blue showed better canal-centering ability and reduced screw-in forces than Reciproc, indicating that the post-machining thermal treatment confers superior performance to reciprocating nickel-titanium instruments.

Aim 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. Methodology 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 (4–8 mm)] after micro‐computed tomographic scanning. An automated root canal instrumentation and torque/force analysing device was programmed to permit a simulated pecking motion (2 s downward and 1 s upward at 50 mm min−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 500 rpm, 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 7 mm 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%). Results Maximum upward force and clockwise torque were significantly smaller in 500 rpm groups than in 300 rpm 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. Conclusions Under laboratory conditions using JIZAI instruments, a rotational speed of 500 rpm generated significantly lower maximum screw‐in forces and torque values than rotational speed of 300 rpm. Continuous rotation and OTR motion performed similarly in shaping the canals.