Finite-element evaluation of contact loads on facets of an L2-L3 lumbar segment in complex loads.

In this study, the contact loads and regions on facets of a lumbar motion segment are quantitatively determined. The analysis accounts for both material and geometric nonlinearities and treats the facet articulation as a frictionless, nonlinear, moving contact problem. The loadings consist of axial torque and lateral bending acting alone or in combination with axial compression force. Complex loadings simulating symmetric and nonsymmetric liftings are also considered. Under identical magnitude of rotation, axial torque generates the largest contact forces, followed by extension, flexion, and lateral bending. Contact forces under loads simulating heavy symmetric lifting are as, or even larger than, those under large extension and axial torque loadings. The presence of axial and lateral rotations during forward flexion significantly increases the loads transmitted through the compression facet. The tension facet, in this case, became nearly unloaded. The large loads on the facets, predicted especially under nonsymmetric, combined flexion loadings, are likely to play a role in the observed disorders of the facet joints. Under various loadings, the transfer of forces from one facet to the adjacent one occurs through three distinct areas, namely, extension-type, flexion-type, and torsion-type contact areas. The relative magnitude of different components of the transmitted forces are also predicted to vary considerably, depending on these regions of contact.