Osteochondrosis in the horse. I. A clinical and radiologic investigation of osteochondritis dissecans of the knee and hock joint.

The clinical and radiologic features of osteochondritis dissecans in the knee and hock joint of horses are described. The material includes 91 horses, of which 43 had the lesion in one or both knee joints, and 48 in one or both hock joints. It was found that osteochondritis dissecans of the knee joint was more common than the one in the hock joint in thoroughbreds and halfbreds, while the lesion in the hock joint was most common in standardbred trotters. In the knee the lesion was most often located to the lateral trochlear ridge. In the hock joint the predilection site was the intermediate ridge of the tibia. The lesions were of obvious clinical importance and the knee joint was usually more severely affected than the hock joint. Prognosis was also more guarded in horses with knee joint involvement, regardless whether or not surgical treatment was instituted. Arthrotomy and removal of fragments in the osteochondral defects or of loose bodies was the standard surgical procedure. It was felt that this treatment improved prognosis somewhat, particularly in the horses with hock joint lesions. It was found that there are many similarities between osteochondritis dissecans in horses and in pigs and dogs. However, further investigation is needed to shed more light on etiology and pathogenesis and on the nature of osteochondritis dissecans in the horse.

Osteochondrosis in the horse. II. Pathology.

An investigation was made of the pathology of osteochondritis dissecans of young foals and horses with clinical signs of the lesion. A randomly selected material of fetuses and young foals without clinical signs was also examined. It was demonstrated that osteochondritis dissecans is primarily a cartilaginous disease, as previously described in pigs and dogs. Thickening, disturbance of endochondral ossification, degeneration and necrosis of the cartilage were the four main features of osteochondritis dissecans. Cracks and fissures occurred in the degenerated and necrotic parts of the cartilage. This led to formation of cartilage flaps and eventually to loose bodies. It was shown that small pieces of subchondral bone could be ripped off when a cartilage flap was formed. This was one explanation as to why many flaps and loose bodies contained bone in contrast to the findings in pigs and dogs. Endochondral ossification could also take place in the thickened joint cartilage in some cases. Even some loose bodies could undergo endochondral ossification if they were well nourished. Osteochondritis dissecans was often found bilaterally in the knee and hock joint and this was interpreted as an indication that osteochondritis dissecans is a manifestation of a generalized condition called osteochondrosis. Simultaneous occurrence of lesions in joints other than the knee and hock and in several metaphyseal growth plates was another indication of the generalized nature.