Inflammatory involvement in rabbit knee following immobilization and resulting in osteoarthritis

The inflammatory involvement of the synovium in immobilization-induced experimental osteoarthritis in rabbit knees has been investigated. One knee of each of 33 rabbits was immobilized for up to 5 weeks in extension and then allowed to remobilize for up to 8 weeks. Histological examination revealed the presence of marked inflammatory changes in the synovium which were evident after 4 days of immobilization. These were characterized by a rapid proliferation of synovial lining cells followed by mononuclear cell infiltration and fibrosis. The synovial cell proliferation decreased during the latter part of the immobilization period but reappeared in a more severe form during remobilization, at which time gross changes could be demonstrated in the joints. Synovial fluid levels of the lysosomal enzyme N-acetyl-beta-D-glucosaminidase were elevated on day 3 and day 10 but had returned to baseline levels by day 24 of immobilization. Treatment of the animals with the nonsteroidal anti-inflammatory agent flurbiprofen (10 mg/kg/day) had neither a beneficial nor a deleterious effect on any of the parameters measured. This study has shown that immobilization-induced experimental osteoarthritis in rabbits is associated with a marked inflammatory component and that during the remobilization phase of this model the inflammation appears to be a secondary event.