Comparative evaluation of the shaping ability and mechanical properties of a heat-treated nickel-titanium rotary instrument featuring a flat-sided design

A flat-sided design has been introduced into nickel-titanium (NiTi) rotary endodontic instruments with the intention of improving clinical performance. Our aim was to compare the canal-centering ability and mechanical properties of a flat-sided NiTi instrument with those of three different NiTi rotary instruments. Four heat-treated NiTi continuous rotary instruments (#25/.06) were tested: Platinum V.EU (PL: flat-sided), VDW.ROTATE (RT), Vortex Blue (VB), and JIZAI (JZ). Shaping tests were conducted on J-shaped resin canals with an automated instrumentation device that recorded the torque and vertical force. Canal-centering ratios were evaluated using pre- and post-instrumentation images. Cyclic fatigue resistance was assessed using a dynamic test at 37 ± 1 °C in a stainless-steel canal (60° curvature and 3-mm radius) at 2-mm axial amplitude, and the number of cycles to failure (NCF) was determined. Bending resistance was measured at 37 ± 1 °C using a cantilever bending device at deflections of 0.5 and 2.0 mm. Differential scanning calorimetry was used to examine the phase transformation. The Kruskal–Wallis test with Bonferroni correction was used to analyze the data ( α = 0.05). The canal-centering ratios of PL at 0–2 mm from the apex were smaller than or similar to those of the other instruments. PL and VB generated larger downward vertical forces than RT ( p < 0.05). VB produced higher torque than PL and RT ( p < 0.05). RT and JZ exhibited significantly higher NCF than PL ( p < 0.05). PL exhibited a bending load lower than those of RT and VB at 0.5-mm deflection, but a higher load than JZ at 2.0-mm deflection ( p < 0.05). PL had the highest austenite finish temperature of 45.83 °C. The flat-sided PL instrument demonstrated apical (0–2 mm) canal-centering ratios, torque, and bending loads within the elastic range, as well as NCF that were comparable to or lower than those of the non-flat-sided instruments tested. In addition, it exhibited a higher vertical force. DSC indicated that PL contained a greater proportion of martensite than the other instruments at body temperature. These results indicate that PL combines enhanced flexibility and canal-centering ability with lower cyclic fatigue resistance compared with the other instruments tested.