The role of axial rotation in the etiology of unilateral disc prolapse. An experimental and finite-element analysis.

This study was done to establish the relevance of axial rotation as a cause of disc degeneration in the lumbar spine and the role of facet asymmetry in the injury mechanism. It previously was shown that facet asymmetry does not affect the axial torque-rotation response of lumbar motion segments. This study, in both an experimental and a finite-element analysis, examined three important points previously not considered for lumbar motion segments subjected to axial torque: 1) the effect of facet asymmetry on the coupled motions; 2) the effect of combined geometric parameters on the segment response; and 3) the effect of facet asymmetry on the annular strains. Three different lumbar-coupling patterns were observed; however, they did not appear to be influenced by facet joint asymmetry. An oblique and flat compression facet may allow an increased motion-segment response, but in general, combined geometric parameters were found to have no effect on segment response. It was concluded that, without facet damage, the right or left side of the disc is not biased by a particular facet geometry to experience unusual levels of stress and strain, either as a result of increased axial rotation or any of the associated coupled motions.