To examine the effects of transarticular loading on articular cartilage and subchondral bone, we used a canine model that we had developed previously, in which a standardized load of approximately 2000 newtons is delivered across the patellofemoral joint. The purpose of the study was to define and describe the initial changes, as seen on histopathological and magnetic resonance-imaging studies, that occur in the early stages after injury to the joint by transarticular loading. Scanning electron microscopy was used to define the extent and characteristics of the fractures produced in the subchondral bone of four patellae that were examined on the day of loading. We found multiple, extensive fractures through the zone of calcified cartilage and the subchondral bone, frequently with step-off displacement, and with little or no change in the gross appearance of the articular cartilage. Specimens from four patellae were examined histologically two weeks after loading, and the observed changes were correlated with those that had been demonstrated by scanning electron microscopy. Fractures through the zone of calcified cartilage and the subchondral bone, with step-off displacement, were prominent. Clefts were present in the surface of the articular cartilage and, in some areas, there was a focal loss of proteoglycan from the extracellular matrix, as indicated by the complete absence of staining with safranin O. Six dogs were examined one year after loading. There was healing of the subchondral fractures and restoration of proteoglycan in the extracellular matrix. However, superficial clefts and fissures were still present in the articular cartilage. Sequential magnetic resonance-imaging studies were also carried out on these six dogs, at two, eight, sixteen, thirty-six, and fifty-two weeks after loading. Two weeks after loading, all knees had soft-tissue swelling, effusion, and a decreased marrow signal in the medullary cavity of the patella. The decreased marrow signal and effusion were still present eight weeks after the impact, and then the findings gradually returned to normal. One year after loading, it was found that the histopathological changes had not been progressive; in fact, they had been ameliorated and, to some extent, reversed by repair processes. The early, severe magnetic resonance-imaging changes had also been reversed, so that this study demonstrated normal findings by one year after loading. CLINICAL RELEVANCE: There have recently been reports of magnetic resonance images demonstrating changes in the subchondral bone after trauma to the joint when a fracture was not identifiable on conventional radiographs. The load across the surfaces of the joint that is necessary to produce a severe ligamentous disruption has been correlated with similar findings in bone on magnetic resonance images, in the presence of intact articular surfaces, as seen on arthroscopy. Forces applied across the articular surfaces that result in changes in subchondral bone, as seen on magnetic resonance imaging, in the presence of grossly intact articular cartilage, may produce substantial changes, similar to those described in this canine model, in subchondral bone and in the zone of calcified cartilage. Our observations suggest that, under certain conditions, there is the potential for repair of these histopathological abnormalities and for restoration of some semblance of normalcy to the articular cartilage and subchondral bone.