Gédet Philippe, Thistlethwaite Paul A, Ferguson Stephen J
MEM Research Center, Institute for Surgical Technology and Biomechanics, University of Bern Stauffacherstrasse, 78 CH-3014 Bern, Switzerland.
J Biomech. 2007;40(8):1881-5. doi: 10.1016/j.jbiomech.2006.07.024. Epub 2006 Oct 25.
Apparatus-induced artifacts may invalidate standard spine testing protocols. Kinematic measurements may be compromised by the configuration of motion capture equipment. This study has determined: (1) the influence of machine design (component friction) on in vitro spinal kinetics; (2) the sensitivity of kinematic measurements to variations in the placement of motion capture markers. A spinal loading simulator has been developed to dynamically apply pure bending moments (three axes) with or without a simultaneous compressive preload. Two linear slider types with different friction coefficients, one with caged ball bearings and one with high-precision roller bearings on rails, were mounted and specimen response compared in sequential tests. Three different optoelectronic marker cluster configurations were mounted on the specimen and motion data was captured simultaneously from all clusters during testing. A polymer tube with a uniform bending stiffness approximately equivalent to a polysegmental lumbar spine specimen was selected to allow reproducible behavior over multiple tests. The selection of sliders for linear degrees of freedom had a marked influence on parasitic shear forces. Higher shear forces were recorded with the caged-bearing design than with the high-precision rollers and consequently a higher moment was required to achieve a given rotation. Kinematic accuracy varied with each marker configuration, but in general higher accuracy was achieved with larger marker spacings and situations where markers moved predominantly parallel to the camera's imaging plane. Relatively common alternatives in the mechanical components used in an apparatus for in vitro spine testing can have a significant influence on the measured kinematic and kinetics. Low-magnitude parasitic shear forces due to friction in sliders induces a linearly increasing moment along the length of the specimen, precluding the ideal of pure moment application. This effect is compounded in polysegmental specimens. Kinematic measurements are highly sensitive to marker design and placement, despite equivalent absolute precision of individual marker measurements, however marker configurations can be designed to minimize errors related to spatial distribution and system bias.
仪器诱导的伪影可能会使标准的脊柱测试方案无效。运动学测量可能会因运动捕捉设备的配置而受到影响。本研究确定了：(1) 机器设计（部件摩擦）对体外脊柱动力学的影响；(2) 运动学测量对运动捕捉标记放置变化的敏感性。已开发出一种脊柱加载模拟器，用于动态施加纯弯矩（三个轴），可同时施加或不施加压缩预载荷。安装了两种具有不同摩擦系数的线性滑块类型，一种在导轨上带有保持架滚珠轴承，另一种带有高精度滚柱轴承，并在连续测试中比较了样本响应。在样本上安装了三种不同的光电标记簇配置，并在测试期间从所有簇中同时捕获运动数据。选择了一根具有均匀弯曲刚度且大致等同于多节段腰椎标本的聚合物管，以确保在多次测试中行为具有可重复性。用于线性自由度的滑块选择对寄生剪切力有显著影响。与高精度滚柱相比，保持架轴承设计记录到的剪切力更高，因此需要更高的力矩才能实现给定的旋转。运动学精度随每种标记配置而变化，但一般来说，标记间距越大以及标记主要平行于相机成像平面移动的情况下，精度越高。体外脊柱测试设备中使用的机械部件中相对常见的替代方案可能会对测量的运动学和动力学产生重大影响。由于滑块摩擦产生的低幅值寄生剪切力会沿样本长度方向引起力矩线性增加，排除了纯力矩施加的理想情况。这种效应在多节段标本中更为复杂。尽管单个标记测量的绝对精度相同，但运动学测量对标记设计和放置高度敏感，不过可以设计标记配置以尽量减少与空间分布和系统偏差相关的误差。
