LAUSR

Introduction: The purpose of this study was to evaluate differences in torsional resistance using up‐regulated speed of rotational spindle. Methods: Three NiTi rotary instrument systems were selected in this study: K3XF (SybronEndo, Glendora, CA), BLX (B&L Biotech, Ansan, Korea), and OneShape (MicroMega, Besançon, France). The tip size and taper for all files were #25 and 0.06. Experimental groups (n = 10/group) were assigned to 2‐, 60‐, 350‐, and 600‐rpm groups by the rotational speed of spindle. Forty new files were used for each test. The file tip of 5‐mm length was secured between brass plates. While keeping the file straight, it was rotated clockwise at a constant rotational speed until fracture occurred. The parameters of torsional resistance, torsional load (Ncm), and distortion angle (°) were measured using an AEndoS‐k (DMJ System, Busan, Korea), and the toughness until fracture was computed from these data. The 1‐way analysis of variance test was used to analyze the torsional resistance at a significance level of 95%. All fractured fragments were observed under a scanning electron microscope to evaluate the topographic features of the fractured surfaces. Results: No significant difference in torsional resistance was found among groups when they were compared for ultimate strength at the maximum torque, fracture angle, and toughness. Scanning electron microscopic examination of the fractured cross‐sectional surfaces revealed typical features of torsional fractures, concentric abrasion marks, and fibrous dimples from the torsional center. Conclusions: Under the conditions of the study, the torsional resistances of the rotary instruments were not affected by the rotational speed. HighlightsThe difference of torsional resistance was evaluated according to the up‐regulated speed of rotational spindle.Four conditions of rotational speed (2, 60, 350, and 600 rpm) were applied.The conventional nickel‐titanium alloy (BLX and OneShape) and the R‐phase heat‐treated alloy (K3XF) were tested.The different speeds of rotation did not change the torsional resistance of the rotary files.