Mechanical response of nickel-titanium instruments with different cross-sectional designs during shaping of simulated curved canals

To evaluate how different cross-sectional designs affect stress distribution in nickel-titanium (NiTi) instruments during bending, torsion and simulated shaping of a curved canal. Four NiTi rotary instruments with different cross-sectional geometries were selected: ProFile and HeroShaper systems with a common triangle-based cross section, Mtwo with an S-shaped rectangle-based design and NRT with a modified rectangle-based design. The geometries of the selected files were scanned in a micro-CT and three-dimensional finite-element models were created for each system. Stiffness characteristics for each file system were determined in a series of bending and torsional conditions. Canal shaping was simulated by inserting models of the rotating file into a 45 degrees curved canal model. Stress distribution in the instruments was recorded during simulated shaping. After the instruments were retracted from the canal, residual stresses and permanent bending of their tips due to plastic deformation were determined. The greatest bending and torsional stiffness occurred in the NRT file. During simulated shaping, the instruments with triangle-based cross-sectional geometry had more even stress distributions along their length and had lower stress concentrations than the instruments with rectangle-based cross sections. Higher residual stresses and plastic deformations were found in the Mtwo and NRT with rectangle-based cross-sectional geometries. Nickel-titanium instruments with rectangle-based cross-sectional designs created higher stress differentials during simulated canal shaping and may encounter higher residual stress and plastic deformation than instruments with triangle-based cross sections.