Torsional fatigue of the lumbar intervertebral joints.

The effect of cyclic torsional loads on the behavior of intact lumbar intervertebral joints was investigated. A total of 24 specimens, divided in two groups, were tested. In the first group, a specimen was subjected to a specified constant cyclic torque, and the resulting change in angular displacement with time was recorded. The specimens in the second group were subjected to a specified constant angular displacement and the resultant torque recorded. The results of the constant torque tests showed that fatigue failure was primarily a function of the averaged initial angular displacement and secondarily of the applied torque. The critical threshold values appear to be an initial angular displacement of +/- 1.5 degrees. All specimens, displaying an averaged initial angular displacement of less than +/- 1.5 degrees, did not fail in fatigue, while the others did. However, the specimens with angular displacements of less than +/- 1.5 degrees had lower applied torques (11.3 and 22.6 Nm), while specimens with higher applied torques (33.9 and 45.2 Nm) all exceeded the +/- 1.5 degrees threshold and failed in fatigue. Failure locations occurred in such diverse regions as end-plates, facets, laminae, capsular ligaments, etc. All specimens exhibited a synovial fluid discharge from the apophyseal joint capsule sometime during testing. Post-test examinations of all the cartilage surfaces showed fibrillation, whether or not the intervertebral joint failed.