Nonlinear stress analysis of the whole lumbar spine in torsion--mechanics of facet articulation.

The response of the whole ligamentous lumbar spine in axial torsion is studied. Attention is focused on the inter-segmental variations, role of articular facets, presence of coupled movements, intervertebral stresses and the effects of a structural alteration at a level on the response. A detailed three-dimensional finite element model (L1-S1) was used for nonlinear stress analyses under torques of up to 15 N m. In right axial torque, extra cases of larger gap limit for more effective articulation, of loss of L4-L5 disc fluid content, and of removal of L4-L5 compression facet were also studied. The intersegmental results varied from one level to the next. The facet contact occurred at the lateral posterior regions of articular surfaces. The interference gap distance between facet articular surfaces, and not the orientation of articular surfaces, was the primary factor in the relative effectiveness of facets in resisting torsion. Coupled motions of upward translation and flexion rotation were noted at all levels under torques in both left and right directions. Loss of disc fluid content at the L4-L5 level resulted in larger facet contact force and axial rotation at the same level. Removal of the L4-L5 compression facet substantially increased the axial rotation, intradiscal pressure, maximum fibre strain, and strains in capsular ligaments at the same L4-L5 level. In axial torque, the disc failure appeared to be unlikely. Relatively large facet forces were generated that further increased with a loss in disc fluid content.