Clinical and radiological aspects of stifle bone cysts in the horse.

Thirty-three cases with subchondral bone cysts in the stifle are reported. The condition was most commonly seen in young Thoroughbreds and produced intermittent lameness of varying degree. Radiographically distinct areas of radiolucency were found in the distal femur or proximal tibia adjacent to the femorotibial joint. Lesions were usually unilateral but 5 horses had cysts in both stifles. The cases could be divided into 2 distinct groups. Horses in Group A (28 cases) had a large circular or dome-shaped cyst in the medial femoral condyle with a distinct communication with the femorotibial joint. In Group B (5 cases) the cysts were much more variable in shape, size and location. The common sites were the distal femur adjacent to the intercondyloid fossa or the proximal tibia just beneath the tibial spine. No evidence of radiological signs typical of osteochondrosis dissecans were seen in any of the cases. All the horses were treated conservatively with a 6 month rest period out at grass, followed by a gradual return to full training. Fourteen horses were re-examined 4 to 33 months after the initial examination and none of them showed disappearance of the cysts. There was a tendency to increased radiodensity of the cyst in some cases but this was only to a minor degree. The outcome of the 2 groups was different. In Group A just over half (14) made a complete recovery while a further 5 returned to full work although some persistence of mild lameness was still present. In Group B only one case was reported to have made a successful return to full work.