Osteochondritis dissecans in ancient British skeletal material.

OSTEOCHONDRITIS DISSECANS (which is not to be confused with osteochondritis juvenilis) is a common disease but it is also a somewhat mysterious one. Its essential pathology consists of an avascular necrosis occurring in the sub-chondral bone of a joint followed by degenerative changes in the overlying cartilage. As the disease progresses, a zone of demarcation forms around the avascular area and the necrotic fragment separates from the rest of the bone, forming a loose body within the cavity of the joint. These lesions are seldom much more than 20 mm. across and about 10 mm. deep. As a rarity the condition seems able to develop in most joints but at least 80 per cent of cases are found in the knee, 10 or more per cent in the elbow, five per cent in the ankle, and probably less than one per cent elsewhere. It usually first presents itself clinically between the ages of 12 and 25 years, the great majority of cases occurring from 15 to 18. The cause of osteochondritis dissecans is uncertain. It is often said to be due to trauma. In the knee joint it most commonly occurs on the medial femoral condyle close to the intercondyloid fossa. In this position it is attributed to damage caused by the patella impacting against the condyle, to hair-line fractures, rotational strains and other injuries. Some workers believe that it is due to embolus or thrombosis occurring in one of the end arteries immediately below the cartilage, though no satisfactory explanation for such blocking of the vessel is available. Perhaps an inborn constitutional susceptibility is involved because it may be present in several joints in one patient and sometimes appears to be familial. Osteochondritis of the elbow joint most commonly attacks the capitulum, which is the lateral part of the distal articular surface of the humerus, or, sometimes, that part of the head of the radius which is in contact with the ulna when the forearm is supinated. In the ankle joint, it is usually on the summit of the trochlea (or upper) surface of the talus. Untreated lesions eventually tend to heal spontaneously but deformity of the joints may result, with osteoarthritis as a usual aftermath. The osteochondritic pit sometimes fills with regenerated tissue which calcifies, is often irregular and may rise rather above the level of the normal condylar surface. In archaic bones, in Great Britain at least, osteochondritis, or lesions which are indistinguishable from it, presents a somewhat different picture. The commonest place for it is also the medial femoral condyle (Fig. 1) but not so predominantly as in