Mechanical consequences of annular tears and subsequent intervertebral disc degeneration

The relationship between degeneration of the intervertebral disc and changes to its mechanics is unclear. The aim of this study was to examine, in a sheep model, the effect of creating a lesion in the outer, anterior annulus on the mechanics of the intervertebral joint complex and the disc. Forty-one 2-year-old Merino wethers were allocated randomly into a control group or an annular lesion group and additionally to non-survivors which were sacrificed immediately or survivors sacrificed 6 months later. The annular lesion group had incisions made in two non-adjacent intervertebral discs and a plate was secured across the vertebrae at one level. Mechanical tests were performed on specimens consisting of the two vertebrae, the intervening disc and associated ligaments. Stiffness of the specimens was measured in flexion, extension, and in pure torsion. The tests were conducted first on the intact intervertebral joints and then after removal of the zygapophyseal joints and the interspinous and supraspinous ligaments. The results showed that the creation of an annular lesion caused immediate changes to the mechanics 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 the stiffness of the controls. The plates had a marked effect on the stiffness 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 less stiff than the controls. There was clear evidence of a progressive degenerative response in the nucleus in all discs with a lesion. The addition of a plate to limit movement did not markedly affect this biological response to the injury but there was some evidence that after 6 months there were fewer degenerative changes to the zygapophyseal joints in the plated specimens. Recovery of the mechanical integrity of the disc was more marked in the joints that were plated, supporting the concept that limiting motion of an injured intervertebral disc facilitates a healing response in the annulus.