Flexural Stiffness and Stresses in Nickel-Titanium Rotary Files for Various Pitch and Cross-sectional Geometries

Introduction: Shape is the main determinant of mechanical performance for nickel-titanium rotary instruments. This study evaluated how pitch and cross-sectional geometry affected flexural stiffness and stresses. Methods: Finite element models of rotary instruments with 4 cross-sectional geometries (triangle, slender-rectangle, rectangle, square) and 3 pitches (5-, 10-, 15-threads) were created, featuring superelastic nickel-titanium properties. All models had the same length, taper, and external peripheral radius; cross-sectional area and/or center-core area varied. The clamped shaft was rotated axially, while the tip was deflected 5 mm. Flexural stiffness and maximum von Mises stresses were calculated. Results: Stiffness and maximum stress decreased with decreasing pitch (increasing threads). Doubling or tripling the threads for the triangular or rectangular cross sections decreased the stiffness and stress 6% and 12%, respectively; square cross sections were less affected (1% and 3% decrease, respectively). Square cross sections (higher cross-sectional and center-core areas) had higher stiffness and stresses than other models with same deflection. Rectangular and triangular models with the same center-core areas had similar stresses, but the rectangular model was 30%-40% stiffer. The slender-rectangle had the smallest center-core area and the lowest stiffness and stresses. Both rectangular cross sections caused stiffness and stress variations with rotation angle (13% for slender-rectangle); larger pitch caused more variation, Conclusions: Under the same tip deflection (simulating canal curvature), flexural stiffness and stress correlated with center-core area. Increasing pitch increased flexural stiffness and stresses. (J Endod 2012;38:1399-1403)