Experimental instability in the rabbit lumbar spine.

The authors performed mechanical, biochemical, and histologic analyses of changes in the rabbit lumbar spine occurring after instability had been induced by facet removal to find whether this intervention produced an experimental model for intervertebral disc degeneration. Sham operated animals and an unoperated control group were used for comparison. Half of the operated animals were housed under conditions to promote higher physical activity than the other animals housed individually in small cages. Acutely, the removal of facet joints increased the flexibility of intervertebral joints. Over the following year, this increase in flexibility was reduced to close to control levels in all groups of animals. Within the intervertebral discs, there was no significant change in proportions or solubility of collagen or proteoglycans after surgery, nor was there microscopic or macroscopic evidence of disc degeneration. The surgical procedure produced hypermobility of the spine, but there was a subsequent restabilization, and the intended disc degeneration was not produced. These findings indicate that some as yet unidentified soft tissue repair process, facilitated by activity, overcame the hypermobility created at surgery, so degenerative changes in the intervertebral discs did not result. We suggest that other animal models of disc degeneration may represent a failure of reparative response to acute injury.