Cervical vertebral strain measurements under axial and eccentric loading

The mid to lower cervical spine is a common site for compression related injury. In the present study, we determined the patterns of localized strain distribution in the anterior aspect of the vertebral body and in the lateral masses of lower cervical three-segment units. Miniature strain gages were mounted to human cadaveric vertebrae. Each preparation was line-loaded using a knife-edge oriented in the coronal plane that was moved incrementally from anterior to posterior to induce compression-flexion or compression-extension loading. Uniform compressive loading 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 location which resulted in the least amount of deformation for a given force 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., balance point) was in this region ; it was also close to where the vertebral body strains change from compressive to tensile. Strain values from line loading in this region produced similar strains as recorded under uniform compressive loading, and this was also the region of minimum strain. The region of minimum strain was also more pronounced under higher magnitudes of loading, suggesting that as the maximum load carrying capacity is reached the stiffest axis becomes more well defined.