Diagnostic Studies. The radiographic hallmark is an epiphyseal radiolucent lesion with fine punctate calcifications suggestive of a cartilaginous lesion. The tumor is usually bordered by a well-defined margin of reactive bone. Chondroblastomas are more epiphyseally based than the more common epimetaphyseal giant cell tumor. The radiographic differential diagnosis includes aneurysmal bone cyst (see Plate 6-11) and giant cell tumor of bone (see Plate 6-13). Inflammatory lesions such as osteoarthritic cysts also are epiphyseal but are nearly always in older patients, and chondroblastomas are generally in young patients frequently with open growth plates. Giant cell tumors are more common in young adults with closed growth plates.

Histologic examination reveals the characteristic “cobblestone” or “chicken-wire” pattern: areas of round, plump chondroblasts (the stones) are enmeshed in a sparse chondroid matrix (the mortar between the stones). Clumps of giant cells are seen in areas that contain primarily a spindle cell stroma. Unique to chondroblastoma is the fine microscopic pattern of calcifications, usually in a “chicken-wire” arrangement, in and around the islands of cartilage.

Treatment/Prognosis. Curettage is the treatment of choice of chondroblastomas. Curettage near the growth plate must be undertaken with great care in the younger patient to prevent late angular deformities, and curettage near the articular cartilage must be undertaken with great care in all patients.

The prognosis for chondroblastoma is good; most are active stage 2 tumors; however, there is a significant risk of recurrence after curettage similar to that of giant cell tumor.

CHONDROMYXOID FIBROMA

Chondromyxoid fibroma is a rare painless, benign cartilaginous lesion of bone. It occurs in adolescents, most often in the metaphyses of major long bones. This lesion most often presents as an active stage 2 lesion and is not known to undergo malignant transformation.

Diagnostic Studies. Radiographs reveal an eccentric metaphyseal radiolucent defect with no evidence of the usual calcification of a cartilaginous tumor. The radiographic differential diagnosis includes nonossifying fibroma (see Plate 6-9) and aneurysmal bone cyst (see Plate 6-11).

Histologic examination shows immature myxoid cartilage with stellate chondrocytes enmeshed in lightly staining myxomatous chondroid matrix. Intertwined throughout the lesion are strands of benign fibrous tissue and small multinucleated giant cells.

Treatment. Curettage carries a low risk of recurrence in well-encapsulated stage 2 lesions, but the size of the defect often necessitates bone grafting (see Plate 6-30).