(PO05) Effect of Downward Loads on Torque and Force Generation and Shaping Ability of Differently Heat-Treated Nickel-Titanium Rotary Instruments

Abstract: Limited data exist regarding the impact of downward loads on the degree of torque/force generation and apical transportation during nickel-titanium rotary instrumentation. This study aimed to evaluate how different downward loads influence the torque/force generation and canal-centering ability of heat-treated and non-heat-treated nickel-titanium rotary instruments. ProTaper Gold (PTG; F1, F2, and F3) and ProTaper Universal (PTU; F1, F2, and F3) were used to prepare J-shaped resin canals employing a load-controlled automated instrumentation and torque/force measuring device. Depending on the torque values, the handpiece was programmed to move as follows: up and down; downward at a preset load of 1 N, 2 N, or 3 N (Group 1N, 2N, and 3N, respectively; each n=10); or upward. The torque/force values were recorded, and the canal-centering ratio was calculated. The results were analyzed using a two-way or one-way analysis of variance and Tukey’s test (α = .05). In PTG, the downward load did not significantly affect the centering ratio and upward force (p > .05). In group PTU/1N, ledge formation occurred in all samples. At the apical level, group PTU/2N exhibited larger canal deviation than group PTU/3N (p < .05). In group PTU/3N, the upward force, representing the screw-in force, was larger than that in Group PTU/2N (p < .05). The impact of downward loads was detected only in PTU, where larger downward loads produced larger screw-in forces, and smaller downward loads caused larger canal deviations.