The effect of injury on rotational coupling at the lumbosacral joint. A biomechanical investigation.

The lumbosacral joint is frequently indicated as a source of low-back pain, a cause of which may be abnormal patterns of vertebral motions. The goal of this study was to describe the influence of injury on the coupled motions of the L5-S1 joint in a human cadaveric model. Nine whole lumbosacral spine specimens were studied under the application of flexion, extension, left/right axial torque and right/left lateral bending pure moments. Injuries to the posterior ligaments, intervertebral disc, and articular facets at L5-S1 were produced, and the motion at L5-S1 was determined after each sequential injury. No significant coupled rotations were observed under flexion or extension moments. Under axial torque, lateral rotation at L5-S1 occurred to the same side as the applied torque and increased significantly only after injury to the intervertebral disc. Also coupled to axial torque was flexion rotation in the intact specimen, which became extension rotation after facetectomy. Under lateral bending moments, coupled axial rotation was to the opposite side of the applied moment and increased significantly only after removal of the facets of L5. Based on these results, it was concluded that intervertebral disc most resisted the coupled motion of lateral rotation under the application of axial torque, whereas the articular facets most resisted the coupled axial rotation under the application of lateral bending at the lumbosacral joint. Also, the facets were the structures that produced the flexion rotation of L5 on S1 under axial torque loading.