The Effects of Torsion on the Lumbar Intervertebral Joints

Sixty-six necropsy specimens of human lumbar spines were studied. After preliminary screening by discograms ninety intervertebral joints were subjected to torsional loading; the torsional strengths of the intact lumbar intervertebral joint and of its component parts were determined. The intact intervertebral joint with a normal discogram failed at an average torque of 881.4 x 106 dyne-centimeters. The disc of this joint supplied 35 per cent of the resistance to the torque, while the remaining 65 per cent of the resistance could be attributed to the posterior assembly comprising the articular processes, their capsules, and the interspinous ligaments. Whole intervertebral joints with degenerated discs were found to be weaker than those with normal discs. There was no gross damage to the vertebrae associated with experimental torsional failure of the intervertebral joint. The torsional strength of the disc was found to depend on the shape and area of the disc, on the integrity of its annulus, and on the loading rate. It was found that injury to the joint could be produced by slowly applied rotation in amounts within the range of normal lumbar movement. Annular fibers do not deteriorate purely as the result of age. The deterioration seems principally due to damage and scarring of annular fibers. Disc rupture induced experimentally by torsion produced changes similar to those seen in naturally occurring disc degeneration suggesting that both changes were the result of the same causative mechanism. It is postulated that in vivo disc degeneration is due to imposed torsional strains rather than to compressive loads. Since the joints between the articular processes stabilize the intervertebral joint against torsion, it is suggested that any impairment of the function of the joints between the articular processes may result in a higher risk of disc degeneration.