Reparative cartilaginous callus in subarticular osteonecrosis of bone. A histopathological study of the femoral head.

A histopathologic study of a series of 346 resected femoral heads with gross osteonecrosis secondary to trauma, steroid therapy, idiopathic or other disease processes demonstrated the production of significant cartilaginous callus tissue at fracture clefts in 19 specimens (5.5%). Although subarticular fracture clefts were commonly present in such specimens, the formation of cartilaginous callus occurred only when one of the two opposing surfaces at the fracture cleft consisted of viable bone. Usually this was osseous tissue that originally had composed a portion of the infarct but had undergone revascularization. In another group of five cases from the same series, fragmentation of all tissue superior to the layer of cartilaginous callus had occurred by the time of surgery. The previously formed cartilaginous callus layer then became the actual weightbearing surface. Pathologically, callus must be differentiated from the cartilaginous tuft phenomenon of exposed intra-articular bone.