The long-term objectives of this proposal are to characterize the changes in articular cartilage and subchondral bone in an animal model of blunt trauma to the joint; testing the hypothesis that a single sub-fracture load or repetitive increments of that load delivered across the joint surfaces simulate the early pathologic changes of degenerative joint disease. In addition, it is planned to evaluate the effects of different levels of activity on joint changes following blunt trauma to the joint by providing forced exercise for one group of animals. In view of the pathology noted in pilot studies, it is anticipated that further pathology will be demonstrated by prolonging the studies past six weeks post impaction. It is subsequently planned to evaluate surgical and medical means of altering the pathologic process at appropriate stages post impaction. Direct trauma to joint surfaces remains one of the few known etiologic mechanisms for osteoarthritic changes. The proposed animal model does not require direct invasion of the joint, but yet reproduces some of the biochemical and pathologic changes in the articular cartilage and subchondral bone similar to osteoarthritis. We propose to use this model of delivering impact across the patellofemoral joint through a subcutaneous exposure of the patella at a subfracture load on one or multiple occasions using adult canine animals for varying periods of time from three months up through twelve months post impaction and evaluating those animal joints by the use of histologic and ultrastructural methods for the articular cartilage and subchondral bone, fluorochrome labeling for the subchondral bone and zone of calcified cartilage, and biochemical studies on the articular cartilage.