Effect of flexural stress on torsional resistance of NiTi instruments

Introduction: Previously published studies have deeply investigated the characteristics of flexural and torsional resistance of nickel-titanium rotary instruments, but none of them investigated the relationship between the 2 stresses. The objective of the present study was to evaluate the influence of flexural stresses over torsional resistance. Methods: Sixty S-One 20.04 files (Fanta Dental, Shanghai, China) were used in the present study (N = 60) and divided into 3 test groups of 20 files. A customized device made of a mobile structure with a connection that holds the handpiece and the artificial canal was used for the experiment to make the measurements repeatable. The artificial canals were created with a 90° curvature, a 60° curvature, and lastly a straight canal. Each file was rotated at 300 rpm with a maximum torque value of 5.5 Ncm with the apical 2 mm firmly secured in a vise. The torque at fracture and the time to fracture were recorded by the software integrated in the handpiece and evaluated through statistical analysis. Results: Statistical analysis found significant differences in the values of torque to fracture (TtF) between these 3 groups (P < .05). The 90° curved canal group showed the highest TtF value, and the 60° curved canal group showed a higher TtF value than the straight canal group. Conclusions: The results of the present study demonstrated a positive influence of flexural stresses over torque at fracture of rotary files. When nickel-titanium instruments were used in a 90° or 60° curvature, the values of torque at fracture increased compared with the same instruments that rotated in the straight canal.