Department of Bioscience Research, College of Dentistry, University of Tennessee Health Science Center, Memphis, Tennessee, USA.
J Endod. 2012 Oct;38(10):1399-403. doi: 10.1016/j.joen.2012.06.008. Epub 2012 Jul 6.
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.
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.
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.
Under the same tip deflection (simulating canal curvature), flexural stiffness and stress correlated with center-core area. Increasing pitch increased flexural stiffness and stresses.
形状是镍钛旋转器械力学性能的主要决定因素。本研究评估了螺距和横截面几何形状如何影响弯曲刚度和应力。
创建了具有 4 种横截面形状（三角形、细长矩形、矩形、正方形）和 3 种螺距（5、10、15 螺纹）的旋转器械的有限元模型，具有超弹性镍钛性能。所有模型均具有相同的长度、锥度和外周半径；横截面面积和/或中心芯面积不同。夹在轴上的轴旋转，同时尖端偏转 5 毫米。计算弯曲刚度和最大 von Mises 应力。
刚度和最大应力随螺距（线程增加）的减小而减小。对于三角形或矩形横截面，将线数加倍或三倍，刚度和应力分别降低 6%和 12%；正方形横截面的影响较小（分别降低 1%和 3%）。具有相同挠度的方形横截面（较高的横截面和中心芯面积）具有较高的刚度和应力。具有相同中心芯面积的矩形和三角形模型具有相似的应力，但矩形模型的刚度高 30%-40%。细长矩形的中心芯面积最小，刚度和应力最低。两种矩形横截面均导致刚度和应力随旋转角度变化（细长矩形为 13%）；较大的螺距会导致更多的变化。
在相同的尖端挠度（模拟根管曲率）下，弯曲刚度和应力与中心芯面积相关。增加螺距会增加弯曲刚度和应力。
