• Rosemary E Thompson, Mark J Pearcy, and Timothy M Barker.
• School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology, Brisbane 4001, Australia. rosemary.thompson@aofoundation.org
• Clin Biomech (Bristol, Avon). 2004 Jun 1; 19 (5): 448-55.

ObjectiveTo determine the mechanical effect of individual concentric tears, radial tears and rim lesions of the intervertebral disc anulus.DesignIn vitro dynamic mechanical testing of sheep discs comparing the mechanical behaviour before and after lesion creation.BackgroundStructural 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.MethodsFifteen 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.ResultsIt 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.ConclusionsThese 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.RelevanceThese 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.Copyright 2004 Elsevier Ltd.

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