Giant cell tumors can resemble a chondroblastomas in several ways. Both arise in the ends of bone, have lytic configurations and tend to narrow the cortex and remodel bone instead of destroying and invading adjacent trabeculae. Chondroblastomas often calcify centrally whereas giant cell tumors never accumulate calcium salts in their midst. Although most chondroblastomas occur in the second decade of life, they are not exceedingly rare at age 30. However, the most telling distinction feature, as evidenced by this case, is soft tissue extension which does not occur in chondroblastoma.
Subchondral cysts form in the context of longstanding arthritis.
Large lucencies in a subarticular location occur in both osteoarthritic and rheumatoid patients. The absence of joint space narrowing and the presence of a soft tissue mass argue against this possibility. The final diagnostic choice, non-ossifying fibroma, should not be considered strongly. Non-ossifying fibromas are lytic lesions with scalloped edges and come to attention most often in adolescence. These are located predominantly about the knee.