The effects of internal fixation on the articular cartilage of unfused canine facet joint cartilage.

Custom-made distraction instrumentation was placed in the lumbar spine of eight large dogs with care taken to preserve the integrity of two intervening apophyseal joints. Histologic staining of immobilized joint cartilage showed varying degrees of chondrolysis, cloning, invasion of the tide mark, and loss of proteoglycans as early as 2 months postoperatively. Every specimen had significant degenerative changes characteristic of osteoarthritis. The joints one segment caudal to the lower hooks also were examined and were found to have similar histologic evidence of degeneration. Three dogs had the instrumentation removed and were then sacrificed 1.5-3 months later. The degenerative changes were not reversible following instrumentation removal.