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

This study was conducted to evaluate the effect of file length on the torsional resistance of nickel-titanium rotary files.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.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.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.