MECHANICAL RESPONSES OF THE RABBIT PATELLOFEMORAL JOINT TO BLUNT IMPACT

Various studies suggest impact trauma may initially soften cartilage, damage subchondral bone, or a combination thereof The initial damages are commonly thought due to excessive contact pressures generated on c artilage and the underlying bone. The objective of this research was t o develop a small animal model for studying post-traumatic OA and to c orrelate contact pressures with tissue damage. Blunt insult was graded by dropping a rigid mass onto the hyperflexed hind limb of rabbits. C ontact pressure in the patello-femoral joint was measured with pressur e sensitive film. One, 3, 6, and 14 days later the animals were euthan ized. Damage to cartilage and the underlying bone was assessed visuall y and in microscopic sections. Indentation experiments were performed on the patellar cartilage with a rigid, flat probe. Contact pressures were nonuniform over the articular surfaces and a high frequency of su rface fissures were generated on the lateral facet in severe insults. The appearance of surface fissures correlated better with the magnitud e of contact pressure gradients in the damage zone than the magnitude of contact pressures on the facet, per se. Blunt trauma causing surfac e fissures resulted in a measurable degree of softening in the patella r cartilage, especially close to the defects. Surgical intervention of the joint to insert pressure sensitive film, however, also resulted i n significant softening of the cartilage.