| Description |
Spinal motion segments consisting of two vertebral bodies and the disc in between were loaded in compression, and the deformation of the annulus fibrosus and vertebral endplate were measured to determine the relative contributions of each of these structures to the strain of the motion segment. Volume changes were calculated from the results using a geometric model, and the tensile stress developed in the endplate was estimated by applying the theory of plates from solid mechanics. Further testing examined the indentation stiffness of the outer annulus to complement the findings on annular bulge. Deformation of the endplate increased from the periphery to the center, indicating curvature and disproving claims that displacement transducers reflect only vertebral strain artifact remote from the interface with the disc. The shape of the deformed endplate deviated from axisymmetric with its highest point located in the posterior aspect. Characterization of the load-displacement behavior determined that endplate deformation occurs early in the loading event, indeed displaying its highest compliance in the first 500 N of load. Thus, its contribution to shock absorption is immediate rather than successive to disc deformation. Annular bulge was highest in the posterolateral disc, consistent with the site of most herniations. Statistically derived bulge contours showed a local minimum in the region of the posterior longitudinal ligament, corroborating its perceived role in preventing disc prolapse directly posteriorly. Indentation stiffness of the annulus was not affected by load, suggesting that little of the intranuclear pressure developed under compression is transmitted to the periphery of the disc. Only half the disc volume corresponding to the loss of height of the loaded motion segment was redistributed into annular bulge and endplate deformation. The remaining half is believed to represent volume strain of the disc, measuring about 8% of the initial volume. The solid-mechanics model of endplate stresses concluded that curvature is caused by less than 5% of the applied load being converted effectively to bending load through inhomogeneities in the reactive stress of both the vertebra and disc. These results improve the understanding of how compressive load is reacted through the spine and why the disc and endplate fail. They represent the behavior of the spine under the fundamental component of all loading conditions associated with injury. Secondary modes of loading, especially bending, likewise are of critical importance and must be added to the protocols developed in this thesis to complete the characterization of the mechanical response of the spinal motion segment. Future work may use these methods to determine the effects of age and degeneration on the integrity of the spine and to recommend limits on activity for those at risk. |
