Aspects of the pathology of stifle bone cysts in the horse.

The gross and histological appearance of subchondral bone cysts in six Thoroughbred or partbred horses is described. The lesions were all situated in the centre of the medial femoral condyle and were not associated with any other damage to the articular surface of the femorotibial or femoropatellar joints. In three cases the cysts were unilateral but had an early non-cystic lesion present in the opposite stifle. All the cysts had a narrow channel at their distal extremity which communicated with the femorotibial joint. The more recently developed ones were fluid filled and contained fibrous strands, while those in the older horses contained dense fibrotic detritus. No signs of significant ossification of the cysts were detected. The likely pathogenesis of these lesions is discussed in the light of the pathological findings.