CERVICAL VERTEBRAL STRAIN-MEASUREMENTS UNDER AXIAL AND ECCENTRIC LOADING

The mid to lower cervical spine is a common site for compression relat ed injury. In the present study, We determined the patterns of localiz ed strain distribution in the anterior aspect of the vertebral body an d in the lateral masses of lower cervical three-segment units. Miniatu re strain gages were mounted to human cadaveric vertebrae. Each prepar ation was line-loaded using a knife-edge oriented in the coronal plane that was moved incrementally from anterior to posterior to induce com pression-flexion or compression-extension loading. Uniform compressive lending and failure runs were also conducted. Failure tests indicated strain shifting to ''restabilize'' the preparation after failure of a component. Under these various compressive loading vectors, the locat ion which resulted in the least amount of deformation for a given forc e application (i.e., stiffest axis) was quantified to be in the region between 0.5 - 1.0 cm anterior to the posterior longitudinal ligament. The location in which line-loading produced no rotation (i.e., balanc e point) was in this region; it was also close to where the vertebral body strains change from compressive to tensile. Strain values from li ne loading in this region produced similar strains as recorded under u niform compressive lending, and this was also the region of minimum st rain. The region. of minimum strain was also more pronounced under hig her magnitudes of loading, suggesting that as the maximum load carryin g capacity is leached the stiffest axis becomes more well defined.