Comparison of torsional stiffness of nickel-titanium rotary files with different geometric characteristics.

INTRODUCTION

The aim of this study was to evaluate the theoretical effect from pitch and cross-sectional geometry on torsional stiffness of nickel-titanium (NiTi) instruments.

METHODS

Finite element models of NiTi rotary instruments with different cross-sectional geometries and different number of threads were made for comparison of torsional stiffness. Four cross-sectional shapes were tested: triangle, slender rectangle, rectangle, and square. Taper and external peripheral radius were the same for all models, whereas cross-sectional area and/or center core area were varied. Three pitch values (5, 10, and 15 threads) were tested for each type of cross-sectional geometry. The torsional stiffness of the 12 resulting finite element models was calculated by twisting the file shanks 20 degrees while holding the file tip at apical 4 mm.

RESULTS

The file models with larger pitch (fewer threads) had lower torsional stiffness. The models with the rectangular cross section had higher torsional stiffness than models with the triangular cross section, even when the cross-sectional areas were the same or the center core area was smaller. File models with larger cross-sectional area had higher torsional stiffness.

CONCLUSIONS

Torsional deformation and/or fracture of NiTi rotary files might be reduced by reducing the pitch (increasing the number of threads) and increasing the cross-sectional areas rather than the center core area.