Natural history of femoropatellar osteochondrosis in three crops of Thoroughbreds

Summary This paper reports the incidence of radiographic lesions of osteochondritis dissecans (OCD) of the femoropatellar joint relative to the development of clinical symptoms in the 1989 foal crop, and the clinical symptoms, radiographic features and outcome of the 1990 and 1991 foal crops affected with OCD at the same Thoroughbred farm. In the 1989 foal crop, 23 of 96 yearlings were identified with clinical and/or radiographic signs of OCD. They had an average age of 12.6 months at identification. Eighteen cases had clinical and radiographic signs of osteochondrosis, 3 had clinical signs in the stifles without radiographic signs and 2 had radiographic signs of osteochondrosis but no clinical signs (one eventually became symptomatic). Of the cases with clinical signs but no radiographic signs, one subsequently developed radiographic signs and the other 2 had bilateral clinical signs and unilateral radiographic signs. Seven horses had surgery at the time of clinical and radiographic diagnosis; 5 subsequently raced, 1 is in training and 1 has persistent problems. Of 11 cases not operated initially, 6 raced, 3 had no follow‐up, 1 became a jumper and 1 was operated later. In the 1990 foal crop, 13 of 120 foals were diagnosed with femoropatellar OCD. The average age at diagnosis was 9 months. Nine of the foals had radiographic lesions, 3 had equivocal radiographic lesions (1 progressed to an obvious lesion) and 1 had no radiographic lesions of the femoropatellar joint (but did have them in the fetlocks). All but one horse were treated conservatively. Of the horses with radiographic lesions initially, 2 have raced, 2 are still unraced, 3 have lameness and 2 were used for non‐racing careers. One horse had equivocal lesions initially but these progressed. In the 1991 foal crop, there were 16 cases of femoropatellar OCD diagnosed out of 125 foals. The average age at clinical recognition was 6 months. Ten horses had radiographic lesions at the time of initial diagnosis, 5 had no radiographic lesions initially but subsequently developed them, and 1 never had a radiographic lesion. Four horses were sold as yearlings reportedly without synovial effusion or lameness, 1 died of unrelated causes, 10 are currently racing or training and 1 subsequently developed OCD of the metatarsophalangeal joints.