Effect of Cross-sectional Designs on Torsional Resistance of Endodontic Nickel-Titanium Files: A Finite Element Study.

Objectives:  This study aimed to assess the influence of two key design parameters on the torsional resistance of endodontic rotary files: the ratio of the equivalent radius (re ) to the polar moment of inertia (J), or re /J ratio, and the percentage of the inner core area. Understanding these factors can guide the development of files with improved performance during root canal procedures.

Materials and methods:  Finite element analysis was employed to simulate the behavior of rotary files under torsional loading conditions. This method allowed for the investigation of maximum shear stress across various cross-sections (D₄-D₁₆) of the files. The relationship between the re /J ratio and the maximum shear stress was also evaluated. To assess the impact of cross-sectional design modifications on stress distribution, the study analyzed files with progressively changing configurations.

Results:  Regions situated outside the inner core circle experienced lower shear stress compared with a circular shaft. Furthermore, a strong linear correlation was observed between the maximum shear stress experienced by the file, the applied torque during operation, and the re /J ratio. Significantly, the study established a connection between the percentage of the inner core area and the torsional resistance of the file. Files with a larger inner core area exhibited a lower coefficient (C) within a newly derived torsional formula. This lower C value directly translated to a reduction in the maximum shear stress experienced by the file. In essence, files with a higher percentage of inner core area demonstrated enhanced torsional resistance, allowing them to withstand higher torsional loads encountered during root canal procedures.

Conclusion:  This study identified the re /J ratio and the percentage of inner core area as the most critical design factors influencing the torsional resistance of rotary files. Files with a lower re /J ratio and a larger inner core area experienced lower shear stress, resulting in enhanced torsional resistance and potentially reducing the risk of torsional fracture during use. These findings offer valuable insights for both clinicians selecting rotary files and manufacturers designing future iterations, ultimately contributing to improved safety and efficacy during root canal treatments.