Effect of Shaft Length on the Torsional Resistance of Rotary Nickel–titanium Instruments

Introduction: This study was conducted to evaluate the effect of file length on the torsional resistance of nickel-titanium rotary files. Methods: Forty-five new files with a #25 tip size and variable taper for ProTaper Next (PTN; Dentsply Sirona, York PA), ProTaper Gold (PTG, Dentsply Sirona), and HyFlex CM (HCM; Coltene-Whaledent, Allstetten, Switzerland) were divided into 3 subgroups (n = 15) according to the file length (21, 25, and 31 mm). Torsional tests were performed using a custom-made device (AEndoS; DMJ System, Busan, Korea). The maximum torsional load and distortion angle were measured until file fracture occurred. The toughness was calculated using these data. All data were statistically analyzed using 2-way analysis of variance at a confidence level of 95%. One-way analysis of variance and Duncan post hoc comparison were also conducted to assess the difference among the independent variables, shaft length, and file system. Results: A statistically significant interaction was observed between the file system and instrument length with respect to the maximum torsional load (P <. 05). The 31-mm shaft length of HCM showed higher maximum torsional load than that of the 25- and 21-mm shaft length in the HCM groups (P <. 05). Among the instrument systems, PTG showed the significantly highest maximum torsional load followed by PTN and HCM (P <. 05). The 31-mm shaft length of PTG showed significantly higher toughness than that of the 21- and 25-mm shaft lengths (P <. 05), whereas PTN and HCM did not show any difference in relation to the shaft length. Conclusions: Within the limitations of this study, the instrument with a longer shaft may have higher maximum torsional load or toughness than that with a shorter shaft.