Chondroblastoma

     Chondroblastoma is a rare, benign tumor derived from chondroblasts. It is found in the epiphysis of long bones, usually of the lower extremity. The most common site is the distal femur followed by the proximal femur, proximal humerus and proximal tibia. The tumor has a preference for males over females and the mean age of presentation is approximately 20 years old. The tumor may have behavior not normally associated with benign tumors including pulmonary metastases as well as local invasion of bone and soft tissue.
Clinically, chondroblastoma presents as pain near a joint without history of trauma. A secondary synovitis can be induced by the tumor, but pathological fracture is extremely rare.
     The diagnosis of chondroblastoma can usually be made by radiograph. when the age of the patient and location of the lesion are considered. The most common site for chondroblastoma is the epiphysis. The lesion is lytic with well defined margins and can be from 1-6cm in size. Scalloping or expansion of cortical bone may be present. Fine calcifications, either punctate or in rings, may be visible. Cysts are present about 20% of the time and both MRI and CT can define the fluid levels. CT is also useful for defining the relationship of the tumor to the joint, integrity of the cortex, and intralesional calcifications. The differential diagnosis includes enchondroma, central chondrosarcoma and aneurysmal bone cyst.
     On gross examination, a chondroblastoma has a lobulated, round form and is made up of friable, soft, grayish pink tissue that may be gritty. If present, the cystic fluid is rust or straw-colored.  Chondroblastoma is made up of uniform, polygonal cells that are closely packed. These primitive cells are derived from the epiphyseal cartilage plate and have abundant cytoplasm. There is little mitotic activity. A scant chondroid matrix may be superimposed by a pericellular deposit of calcification that appears like "chicken-wire". The rapid proliferation of immature chondrocytes does not create lacunae or formal cartilage matrix. Giant cells are often present.
     Treatment of chondroblastoma is biopsy and curettage with possible use of  adjuvant liquid nitrogen or phenol, or a mechanical burr. It may be necessary to reconstruct articular surfaces due to subchondral erosion. Any joint invasion is usually secondary to previous instrumentation. All pulmonary nodules should be excised.

References
Bulloughs, Peter, Orthopaedic Pathology (third edition), Times Mirror International Publishers Limited, London, 1997.

Huvos, Andrew, Bone Tumors: Diagnosis, Treatment and Prognosis, W.B. Saunders, Co., 1991.