MECHANICAL CONSEQUENCES OF ANNULAR TEARS AND SUBSEQUENT INTERVERTEBRAL DISC DEGENERATION

The relationship between degeneration of the intervertebral disc and c hanges to its mechanics is unclear. The aim of this study was to exami ne, in a sheep model, the effect of creating a lesion in the outer, an terior annulus on the mechanics of the intervertebral joint complex an d the disc. Forty-one 2-year-old Merino wethers were allocated randoml y into a control group or an annular lesion group and additionally to non-survivors which were sacrificed immediately or survivors sacrifice d 6 months later. The annular lesion group had incisions made in two n on-adjacent intervertebral discs and a plate was secured across the ve rtebrae at one level. Mechanical tests were performed on specimens con sisting of the two vertebrae, the intervening disc and associated liga ments. Stiffness of the specimens was measured in flexion, extension, and in pure torsion. The tests were conducted first on the intact inte rvertebral joints and then after removal of the zygapophyseal joints a nd the interspinous and supraspinous ligaments. The results showed tha t the creation of an annular lesion caused immediate changes to the me chanics of the disc. In torsion, where no axis of rotation was imposed on the joints, there was a clear reduction in stiffness compared with controls. After 6 months the discs in the lesion groups approached th e stiffness of the controls. The plates had a marked effect on the sti ffness of the joints in flexion and extension, but after 6 months this difference was not apparent. The mechanics of the intact joints were not affected immediately by the lesion but after 6 months they were le ss stiff than the controls, There was clear evidence of a progressive degenerative response in the nucleus in all discs with a lesion. The a ddition of a plate to limit movement did not markedly affect this biol ogical response to the injury but there was some evidence that after 6 months there were fewer degenerative changes to the zygapophyseal joi nts in the plated specimens. Recovery of the mechanical integrity of t he disc was more marked in the joints that were plated, supporting the concept that limiting motion of an injured intervertebral disc facili tates a healing response in the annulus. Relevance Knowledge of the ef fect of injury to and degeneration of the intervertebral disc on the m echanics of the intervertebral joint complex is fundamental to an unde rstanding of its mechanical function and potential for healing. With t his knowledge the effect and efficacy of procedures, both surgical and physical, designed for the rehabilitation of patients with back disor ders can be predicted. This study, in particular, provides evidence th at limiting motion of an injured intervertebral disc enhances its abil ity to heal and recover its mechanical properties.