Early pathogenesis of hemophilic arthropathy. Evolution of the subchondral cyst.

Observations of the knee joint made at surgical synovectomy in young children with the chronic synovitis stage of hemophilic arthropathy permitted description of the evolution of the subchondral cyst in this disease. Beginning as a subchondral hematoma beneath the load-bearing surfaces of the joint, the subchondral cyst expands, destroys bone and cartilage locally to become an osteochondral cyst, and, finally, presents as an osteochondral defect with discrete vertical margins and a depressed, sclerotic bony base. Since the thickness of articular cartilage around these osteochondral defects is well maintained, however, even weight-bearing radiographs did not reflect the extensive destruction observed at surgery. Radiographic staging of hemophilic arthropathy, while clinically useful, consistently underestimated the extent of damage to the joint surface. The pathogenesis of subchondral cysts demonstrated in the present study correlated with and explained the known radiographically observed changes of condylar flattening and intercondylar notch widening in hemophilic arthropathy.