Mechanical response of a lumbar motion segment in axial torque alone and combined with compression

In the current study, a nonlinear three-dimensional finite element program has been used to analyze the response of a lumbar L2-3 motion segment subjected to axial torque alone and combined with compression. The analysis accounts both for material and geometric nonlinearities and treats the facet articulation as a general moving-contact problem. The disc nucleus has been considered as an incompressible inviscid fluid and the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The spinal ligaments have been modeled as a collection of nonlinear axial elements. Effects of loss of intradiscal pressure and removal of the facets on the joint response have been analyzed as well. Torsion is primarily resisted by the articular facets that are in contact and the disc annulus. The ligaments play an insignificant role in this respect. For the intact segment, with an increase in torque, the axis of rotation shifts posteriorly in the disc so that under maximum torque it is located posterior to the disc itself. Loss of disc pressure increases this posterior shift whereas removal of the facets decreases it. Torque, by itself, cannot cause the failure of disc fibers, but can enhance the vulnerability of those fibers located at the posterolateral and posterior locations when the torque acts in combination with other types of loading, such as flexion. The most vulnerable element of the segment in torque is the posterior bony structure.