Inferences from referred clinical cases of osteochondritis dissecans

Summary This report relates to cases of osteochondritis dissecans from my surgical case records. Two categories of OCD were recognised: (1) those showing clinical and radiographic signs, and (2) those showing clinical without radiographic (but arthroscopic) signs. Case material examined included OCD in 318 tarsocrural joints in 225 horses, 261 femoropatellar joints in 161 horses, 170 fetlock joints in 71 horses, and 49 shoulder joints in 43 horses. In 64 tarsocrural joints, other lesions accompanied the OCD lesion including articular cartilage fibrillation (31 joints) and degenerate or eroded articular cartilage (24 joints). In 13 joints, lesions of OCD were present at arthroscopy without being identified by radiographic examination. There was discrepancy in the radiographic demonstration of lateral trochlear ridge lesions and their arthroscopic manifestations. Lesions in femoropatellar joints were divided into three grades and it was common for the arthroscopic lesion to be of a higher grade than the radiographic lesion. Lesions on the medial trochlear ridge with no radiographic manifestations and where the separated articular cartilage was the same thickness as normal articular cartilage were recognised commonly. In the shoulder, the humeral head alone was involved in 12 joints, the glenoid alone in 11 and both humeral head and glenoid were involved in 26 joints. Osteochondritis dissecans of the dorsal aspect of the distal metacarpus and metatarsus was divided into type I, II and III lesions. Type I lesions usually resolved with conservative treatment but type II lesions did not. Based on the arthroscopic findings of these cases, it is noted that instances do occur where there is no thickening of the cartilage, which is inconsistent with previous statements regarding pathogenesis. Breed predispositions were apparent (Standardbred in the hocks, Thoroughbred in the femoropatellar, and metacarpophalangeal/ metatarsophalangeal joints, Quarter Horse and Thoroughbred in the shoulder). The consistent location of lesions within the joint (very close to the limits of articulation) suggests that physical factors may be involved in the pathogenesis. The frequent bilateral nature of lesions in the femoropatellar and tarsocrural joints (and quadrilateral nature in the fetlock joint) while infrequently involving different joints is consistent with a ‘window of vulnerability’ in the endochondral ossification of that specific joint when an environmental insult occurs.