Optimal timing of research after anterior cruciate ligament resection in rabbits.

We sought to determine whether the post-traumatic pathology in the rabbit knee induced by anterior cruciate ligament (ACL) transection is a model of cartilage injury and repair (i.e., irreversible osteoarthritis) or one in which phases of degeneration and regeneration occur. After knee instability was induced in 15 New Zealand White rabbits by ACL transection, the animals were separated into two groups according to the time of euthanasia (i.e., 8 weeks [n = 9] or 16 weeks [n = 6] after ACL transection). The macroscopic parameters evaluated were location, type, and extent of articular changes. The microscopic parameters evaluated were morphology of the articular cartilage and subchondral bone (degeneration indices) and articular cartilage thickness, chondrocyte arrangement, and chondrocyte metabolic activity (regeneration indices). A total score of all histological sections from each animal was based on a modified Mankin's grading system characterizing the progression of osteoarthritic damage. The only statistically significant differences (P < 0.05) between groups were those regarding chondral thickness, chondrocyte arrangement, and metabolic activity, indicating repairing of the articular cartilage. ACL transection led to rapid osteoarthritic changes during the first 8 weeks. However, during the later post-traumatic period, these changes progressed slowly because of a concurrent repair effort. Therefore, this experimental animal model offers investigators the choice of which phase-degenerative (early) or regenerative (late)-is most appropriate for their research purposes.