Blunt impact causes changes in bone and cartilage in a regularly exercised animal model

The goal of this study was to document the effect of blunt-impact trauma on the knee in a small animal model that incorporated a known level of physical exercise after impact. We hypothesized that a single blunt impact to the patellofemoral joint, of a magnitude comparable with our earlier studies, would result in degenerative changes to cartilage and to subchondral hone of the patella. Blunt impacts were delivered to rabbit patellofemoral joints without producing bone fracture, and biomechanical and histological analyses were performed on joint tissues at various times. At 12 months, the subchondral bone plate was thicker on the impacted side than on the unimpacted side and than that of the controls at comparable locations (near where surface fissures were found on the impacted side). The instantaneous modulus of cartilage was significantly less on the impacted side than that of controls at 3, 6, and 12 months after impact. The relaxed modulus of cartilage on the unimpacted side increased with time after impact and was significantly greater than that of controls at 12 months. These facts suggest that in this exercise model, the contralateral limb should not be considered a control. The retropatellar cartilage on the impacted side was significantly less thick than that of controls at 12 months, and histological analyses of the cartilage and bone indicated early-stage osteoarthrosis in the impacted joint. Thus, in this animal model a single subfracture blunt impact produced degeneration of joint tissues.