Stiffness and Stress with Nickel-Titanium Rotary File Pitch and Cross-section

Objectives: Shape is the main determinant of mechanical performance for Nickel-Titanium (NiTi) rotary instruments. This study evaluated how pitch and cross-sectional geometry affected flexural stiffness and stresses.

Methods: Finite-element models of rotary instruments with four cross-sectional geometries (triangle, slender-rectangle, rectangle, square) and three pitches (5-, 10-, 15-threads) were created, featuring NiTi shape-memory properties. All models had the same length, taper and external peripheral radius; cross-sectional area (CSA) and/or centre-core area (CCA) varied. The clamped shaft was rotated axially, while the tip was deflected 5mm. Flexural stiffness (bending force/deflection) and von Mises equivalent stresses in were calculated. Maximum stress was the mean of the top-5% values.

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 about 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 under the 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 (up to 13% for the slender-rectangle); larger pitch caused more variation.

 

	CSA (mm3)	CCA (mm3)	stiffness (N/m)			maximum stress (MPa)
threads			5	10	15	5	10	15
triangle	0.33	0.20	99±0	94±0	86±1	404±1	378±0	351±0
slender-rectangle	0.33	0.10	60±4	55±2	51±1	317±17	301±8	285±5
rectangle	0.43	0.20	133±5	124±3	115±2	401±14	384±7	372±4
square	0.50	0.39	204±0	201±0	197±0	448±1	443±1	436±1

Conclusions: Maximum stress, and thus fracture risk, in NiTi rotary files may be reduced by decreasing pitch. Reducing cross-sectional and center-core surface-area also reduces flexural stiffness and stress.