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 trau ma on the knee in a small animal model that incorporated a known level of physical exercise after impact. We hypothesized that a single blun t impact to the patellofemoral joint, of a magnitude comparable with o ur earlier studies, would result in degenerative changes to cartilage and to subchondral hone of the patella. Blunt impacts were delivered t o rabbit patellofemoral joints without producing bone fracture, and bi omechanical 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 fo und 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 u nimpacted side increased with time after impact and was significantly greater than that of controls at 12 months. These facts suggest that i n this exercise model, the contralateral limb should not be considered a control. The retropatellar cartilage on the impacted side was signi ficantly less thick than that of controls at 12 months, and histologic al analyses of the cartilage and bone indicated early-stage osteoarthr osis in the impacted joint. Thus, in this animal model a single subfra cture blunt impact produced degeneration of joint tissues.