OSTEOCHONDRITIS DISSECANS OF THE DISTAL METACARPUS IN THE ADULT RACING THOROUGHBRED HORSE

The distal metacarpi from 11 sound Thoroughbred horses in active race training at the time of death were examined grossly, with thin section radiography, and histologically. Results were compared to those found in the distal metacarpi of five normal nonracing horses and two racing horses which had developed lameness and typical large osteochondral defects in distal palmar MC III seen often in the racing Thoroughbred. Lesions which had a similar appearance, consisting of an area of nonviable subchondral bone covered by normal appearing articular cartilage and surrounded by reactive bone, were found in the distal metacarpi of five of the 11 sound racing horses. These lesions were located on the palmar surface of distal MC III in a location similar to the large osteochondral defects in the two lame racing horses. The authors postulate that subchondral bone fracture is the etiology of this subchondral necrosis and that unless the bone fragment loosens or the overlying articular cartilage is disrupted, the animals remain sound and the lesions are capable of healing. The histologic appearance of these lesions is similar to many pathologic descriptions of osteochondritis dissecans (OCD) in man with normal articular cartilage covering necrotic subchondral bone. The lesion found in these horses differs from descriptions of OCD in other animals in that the lesions are primarily osseous rather than cartilaginous.