Moment Measurements in Spine Segment Dynamic Tolerance Testing using Eccentric Compression are Susceptible to Artifacts Based on Loading Configuration Van Toen, C.; Carter, J. W.; Oxland, Thomas R.; Cripton, Peter Alec, 1965-
The tolerance of the spine to bending moments, used for evaluation of injury prevention devices, is often determined through eccentric axial compression experiments using segments of the cadaver spine. Preliminary experiments in our laboratory demonstrated that eccentric axial compression resulted in ‘unexpected’ (artifact) moments. The aim of this study was to evaluate the static and dynamic effects of test configuration on bending moments during eccentric axial compression typical in injurious cadaver spine segment testing. Specific objectives were to create dynamic equilibrium equations for the loads measured inferior to the specimen, experimentally verify these equations, and compare moments from various test configurations using synthetic (rubber) and human cadaver specimens. Dynamic equilibrium equations were developed based on a generic spine testing apparatus. The equations were verified by performing quasistatic and dynamic experiments on a rubber specimen and comparing calculated shear forces and bending moments to those measured using a six-axis load cell. Additional quasistatic and dynamic experiments with various test configurations were performed on rubber and human cadaver cervical spine specimens (consisting of three vertebrae and the interconnecting ligaments and intervertebral discs). Calculated shear force and bending moment curves had similar shapes to those measured and the values in the first local minima differed from those measured by 3% and 15%, respectively, in the dynamic test, and these occurred within 1.5 ms of those measured. In the rubber specimen experiments, for the hinge joint (translation constrained), quasistatic and dynamic posterior eccentric compression resulted in flexion (‘unexpected’) moments. For the slider and hinge joints and the roller joints (translation unconstrained), extension (‘expected’) moments were measured quasistatically and initial flexion (‘unexpected’) moments were measured dynamically. In the human cadaver experiments with roller joints, anterior and posterior eccentric compression resulted in extension moments, which were ‘unexpected’ and ‘expected’, for those configurations respectively. The ‘unexpected’ moments were due to the inertia of the superior mounting structures. This study has shown that eccentric axial compression produces ‘unexpected’ moments due to translation constraints at all loading rates and due to the inertia of the superior mounting structures in dynamic experiments. It may be incorrect to assume that bending moments are equal to the product of compression force and eccentricity, particularly where the test configuration involves translational constraints and where the experiments are dynamic. In order to reduce inertial moment artifacts, the mass, and moment of inertia, of any loading jig structures that rotate with the specimen should be minimized to the extent possible. Also, the distance between these structures and the load cell should be reduced.
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