The mechanical effects of intervertebral disc lesions

Objective. To determine the mechanical effect of individual concentric tears, radial tears and rim lesions of the intervertebral disc anulus. Design. In vitro dynamic mechanical testing of sheep discs comparing the mechanical behaviour before and after lesion creation. Background. Structural changes to the disc in the form of anular lesions are a feature of disc degeneration and degeneration has been related to changes in the mechanical function of the disc. However, the effect of individual lesions is unknown. Methods. Fifteen ovine, lumbar disc body units were tested in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated. The mechanical response after lesion creation was compared to the undamaged response. Results. It was found that an anterior rim lesion reduced the peak resistive moment produced by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak resistive moment, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of disc lesions. Conclusions. These results show that rim lesions reduce the disc's ability to resist motion. Radial tears change the hysteresis of response indicating an altered stress distribution in the disc. Relevance These changes may lead to overloading of the spinal ligaments, muscles and zygapophysial joints, possibly damaging these structures. This suggests a mechanism for a cycle of degeneration that is instigated by small changes in the mechanical integrity of the intervertebral disc.