Damage to rabbit femoral articular cartilage following direct impacts of uniform stresses: an in vitro study

Objective, To determine the acute gross and histologic damage resulting to femoral cartilage from an in vitro direct impact of uniform stress. Design. Gross and histologic evaluations were performed on rabbit femoral c ondyles impacted by a drop-tower device. Background. It is thought that impacts above a given threshold stress may i nitiate post-traumatic arthritis. The extent of damage following impacts of specific stress has not been previously studied. Methods. 12 New Zealand White rabbit medial femoral condyles were divided i nto three groups by impact type and magnitude. A drop tower was used to str ike the femoral condyle with a flat impactor, or with a custom contoured im pactor. Gross and histological grades were given depending on the depth and number of fissures and cracks in the impacted condyle. Results. The degree of damage correlated best with the type of impactor use d and with the impact force; correlation between damage and impact stress w as less significant. Contoured impacters tended to produce superficial fibr illation, while flat impacters tended to produce deep cracks. Impact forces above 500 N tended to create more severe damage than impact forces below 5 00 N. Subchondral bone remained intact in all cases and deep cartilage dama ge did not occur without disruption of more superficial layers. Poor correl ation was found between damage as graded by gross examination versus damage graded histologically. Conclusions. Acute damage corresponds best with type of impactor and impact force, and not as well with impact stress. Micro structural injuries may b e present in the absence of gross findings.