A paradigm shift for torsional stiffness of nickel-titanium rotary instruments: a finite element analysis

Introduction: The aim of this study was to investigate the influence of mass and the polar moment of inertia on the torsional behavior of nickel-titanium rotary instruments to understand which parameter of cross-sectional design had a key role in terms of torsional resistance. Methods: Four different instrument models were designed and meshed using computer-aided engineering software (SolidWorks; Dassault Systems, Waltham, MA). Instrument models shared the same characteristics, except for cross-sectional design; triangle, rectangle, square, and hollow square geometry was selected. Finite element analysis was performed simulating a static torsional test using the FEEPlus internal solver (Solid Works). Von Mises stress and torsional load at fracture were calculated by the software. Linear regression analysis was performed to investigate the relationship of the polar moment of inertia, cross-sectional area, inner core radius, and mass per volume on the torsional resistance of nickel-titanium rotary instruments. Results: The polar moment of inertia positively affected the maximum torsional load with the highest level of correlation (R2 = 0.917). It could be stated that the higher the polar moment of inertia is, the more maximum torsional load at fracture is present. Mass and cross-sectional area had a lower level of correlation compared with the polar moment of inertia (R2 = 0.5533). According to this, 2 instruments with the same mass/mm and/or cross-sectional area could have different torsional resistance. Conclusions: The polar moment of inertia can be considered as the most important cross-sectional factor in determining the torsional resistance of rotary instruments over metal mass and cross-sectional area.