Pruett J P, Clement D J, Carnes D L
Department of Endodontics/Dental School, University of Texas Health Science Center, San Antonio 78284-7898, USA.
J Endod. 1997 Feb;23(2):77-85. doi: 10.1016/S0099-2399(97)80250-6.
Cyclic fatigue of nickel-titanium, engine-driven instruments was studied by determining the effect of canal curvature and operating speed on the breakage of Lightspeed instruments. A new method of canal curvature evaluation that addressed both angle and abruptness of curvature was introduced. Canal curvature was simulated by constructing six curved stainless-steel guide tubes with angles of curvature of 30, 45, or 60 degrees, and radii of curvature of 2 or 5 mm. Size #30 and #40 Light-speed instruments were placed through the guide tubes and the heads secured in the collet of a Mangtrol Dynamometer. A simulated operating load of 10 g-cm was applied. Instruments were able to rotate freely in the test apparatus at speeds of 750, 1300, or 2000 rpm until separation occurred. Cycles to failure were determined. Cycles to failure were not affected by rpm. Instruments did not separate at the head, but rather at the point of maximum flexure of the shaft, corresponding to the midpoint of curvature within the guide tube. The instruments with larger diameter shafts, #40, failed after significantly fewer cycles than did #30 instruments under identical test conditions. Multivariable analysis of variance indicated that cycles to failure significantly decreased as the radius of curvature decreased from 5 mm to 2 mm and as the angle of curvature increased greater than 30 degrees (p < 0.05, power = 0.9). Scanning electron microscopic evaluation revealed ductile fracture as the fatigue failure mode. These results indicate that, for nickel-titanium, engine-driven rotary instruments, the radius of curvature, angle of curvature, and instrument size are more important than operating speed for predicting separation. This study supports engineering concepts of cyclic fatigue failure and suggests that standardized fatigue tests of nickel-titanium rotary instruments should include dynamic operation in a flexed state. The results also suggest that the effect of the radius of curvature as an independent variable should be considered when evaluating studies of root canal instrumentation.
通过确定根管弯曲度和操作速度对LightSpeed器械折断的影响，研究了镍钛机动器械的循环疲劳。引入了一种新的根管弯曲度评估方法，该方法兼顾了弯曲角度和弯曲突变情况。通过构建六个弯曲的不锈钢导管来模拟根管弯曲度，其弯曲角度为30度、45度或60度，弯曲半径为2毫米或5毫米。将30号和40号LightSpeed器械穿过导管，并将头部固定在Mangtrol测力计的夹头中。施加10克厘米的模拟操作负荷。器械能够在测试装置中以750、1300或2000转/分钟的速度自由旋转，直至发生分离。确定至折断的循环次数。至折断的循环次数不受转速影响。器械并非在头部分离，而是在轴的最大弯曲点分离，该点对应于导管内的弯曲中点。在相同测试条件下，直径较大的40号器械在明显较少的循环次数后折断，而30号器械则不然。多变量方差分析表明，随着弯曲半径从5毫米减小到2毫米以及弯曲角度增加超过30度，至折断的循环次数显著减少（p<0.05，检验效能=0.9）。扫描电子显微镜评估显示延性断裂为疲劳失效模式。这些结果表明，对于镍钛机动旋转器械，在预测分离方面，弯曲半径、弯曲角度和器械尺寸比操作速度更为重要。本研究支持循环疲劳失效的工程概念，并表明镍钛旋转器械的标准化疲劳测试应包括在弯曲状态下的动态操作。结果还表明，在评估根管治疗器械的研究时，应考虑弯曲半径作为自变量的影响。
