Effects of injury on the spinal motion segment mechanics in the axial compression mode.

The changes in the biomechanical characteristics of a ligamentous motion segment as a result of two clinically relevant injuries are investigated using the finite element technique. A three-dimensional, finite element model of a one-half intact motion segment was developed from 1 mm thick transverse CT cross-sections of a lumbar spine specimen. A total denucleation with other structures left intact, and bilateral total discectomy injuries induced in the intact model, led to an approximately two-fold decrease in the disc height and a similar increase in flexion-rotation, in comparison to the intact model. The total removal of the nucleus simulated in both of the injury models increased the force per unit area of contact across the facets. The stresses in the region adjacent to the facets also increased as a consequence of this. Due to the presence of facets and an increase in load transmitted across them, the change in stresses in the vertebral body for the two injury models in comparison to the intact model was not severe. The disc bulge in the posterior region decreased following the two injuries.