The relevance of torsion to the mechanical derangement of the lumbar spine.

Cadaveric lumbar intervertebral joints were loaded simultaneously in torsion and compression, and load-deformation curves were obtained. These were repeated after each of the following structures were cut through in turn: the supra/interspinous ligaments, the apophyseal joint in compression, and the apophyseal joint in tension. From the differences in the curves, it was possible to deduce the role of each structure and of the intervertebral disc in resisting and limiting torsion. The results show that torsion of the lumbar spine is resisted primarily by the apophyseal joint that is in compression, although the intervertebral disc does play a major role. The capsular ligaments of the tension facet and the supra/interspinous ligaments are unimportant. The compression facet is the first of rotation in joints with nondegenerated intervertebral discs. Much greater angles are required to damage the intervertebral disc, so torsion seems unimportant in the etiology of disc degeneration and prolapse.