Introduction: Structural changes to the intervertebral disc (IVD) in the form of anular lesions are a feature of IVD degeneration. Degeneration has been related to changes in the mechanical function of the IVD. This study determined the mechanical effect of individual concentric tears, radial tears and rim lesions of the anulus in an in vitro experiment.

Methods: The lumbar spines from five sheep were taken post mortem and divided into three motion segments. The disc body units were tested on a robotic testing facility, using position control, in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated after the introduction of each lesion. The mechanical response after the lesion creation was compared to the undamaged response to assess the mechanical effect of each lesion.

Results: It was found that an anterior rim lesion reduced the peak moment resisted by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak moment resisted, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of IVD lesions.

Discussion: 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. 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 IVD.