Periosteal chondroma masquerading as osteochondroma

The authors describe two cases of a lesion that closely resembled osteochondroma occurring at the posterior aspect of the distal femur in the popliteal region. Histopathologic examination after open biopsy revealed that the lesions were periosteal chondromas, and they were resected. Osteochondroma, particularly that undergoing malignant transformation, should perhaps be considered in the differential diagnosis of periosteal chondroma; the key to distinguishing between these two lesions is the separation of periosteal chondroma from the medullary portion of the bone by intervening cortex.