Mechanical Properties of Various Heat-treated Nickel-titanium Rotary Instruments

Aim: The purpose of this study was to compare the bending stiffness, cyclic fatigue, and torsional fracture resistances of heat-treated and conventional nickel titanium rotary instruments. Methods: V-Taper 2 (VT2; #25/.08), V-Taper 2H (V2H; #251.06), Hyflex CM (HCM; #25/.06), HyFlex EDM (HDM; #25/variable taper), and ProTaper Next X2 (PTN; #25/variable taper) were tested. The bending stiffness was measured with the customized device (AEndoS), and the files (n = 15) were fixed at 3 mm from the tip and bent at 45 with respect to their long axis. Cyclic fatigue resistance was tested by pecking and rotating instruments (n = 15) in artificial canal with a 7.8-mm radius and 35 angle of curvature until fracture. The ultimate torsional strength and toughness were estimated by using AEndoS. The file tip of 5 mm was fixed between resin blocks and driven clockwise at 20 rpm until fracture. The results were analyzed by using one-way analysis of variance and Duncan post hoc comparison. The fracture surfaces and longitudinal aspect of each group were examined under the scanning electron microscope. Results: CM-wire instruments had lower bending stiffness than others. HDM showed the highest cyclic fatigue resistance, followed by VTH and HCM (P<.05). VT2 showed the highest ultimate strength, followed by HDM, VTH-PTN, and HCM. HDM and VT2 showed significantly higher toughness than VTH, HCM, and PTN (P<.05). Scanning electron microscope analysis showed typical fractographic features of cyclic fatigue and torsional fractures. Conclusions: CM-wire instruments showed higher flexibility and cyclic fatigue resistance than M-wire and conventional nickel-titanium instruments. Large cross-sectional area and conventional nickel titanium showed high torsional resistance.