Effects of Flat-Side Design on Torsional and Bending Stress of Nickel–Titanium File by Finite Element Analysis

Background: This study evaluated the effects of flattening the side of the bending resistance and torsional resistance of nickel–titanium files through finite element analysis of a novel flattened file and a standard nonflattened file. Methods: For torsion analysis, the tip of the file was fixed at 3 mm, generating a torque of 2.5 N·mm at the handle. For bending analysis of curved root canals (45° and 60°), the handle was kept fixed, a force of 1 N was applied at the tip, and the file was fixed at 3 mm. Results: The standard nonflattened file exhibited better torsional resistance. In contrast, the novel flattened file showed improved flexibility under 45° bending. Under this condition, lower maximum von Mises stress was observed in the flattened design compared with the standard file. At 60° bending, stress distribution varied with loading orientation, and higher stress concentrations were observed in the flattened file under specific bending directions, indicating reduced bending resistance under large deformation conditions. Conclusions: Since lateral flattening may reduce the cyclic resistance of files, caution should be exercised in the clinical use of such files.