Anatomic Pathology: Bone and Joint Pathology

• Osteoblastoma is a rare benign bone-forming lesion that produces woven bone spicules bordered by prominent osteoblasts. It is seen in young individuals 10 to 40 years old; however, the peak incidence is in the second and third decades. Men are affected more often than women.

• Pain is a common symptom; it may be dull or more significant and is relieved by antiinflammatory medication. However, it lacks the typical pattern of pain relief with aspirin that is characteristic of osteoid osteoma.

• Osteoblastoma has a predilection for the medullary (cancellous) compartment of the bone, whereas osteoid osteoma has an intracortical location. The skeletal distribution also differs from osteoid osteoma. Osteoblastoma has a predilection for the spine (40% of cases), affecting less frequently the long bones, whereas osteoid osteoma predominantly involves the long bones of the extremities, particularly the neck of the femur (22% of cases); osteoid osteoma affects the spine in only 5% of cases.

• Osteoblastoma is larger (>2.0 cm) than osteoid osteoma, and size is an important feature in distinguishing between the two tumors. Osteoblastoma has a greater potential for unlimited growth, and it may grow to 10.0 cm or more.

• Osteoblastoma comprises a nidus containing numerous small trabeculae of bone in a cellular, vascular stroma populated by osteoclasts and osteoblasts. Large osteoblasts proliferate in clusters in the stroma of osteoblastoma; this is not found in osteoid osteoma. Osteoid matrix may be deposited as very small, immature trabeculae. However, the bone is always covered by active osteoblasts and is not deposited in “lacy” patterns or “streamers” characteristic of osteosarcoma. In contrast to osteosarcoma, cellular atypia and mitoses are not present. Osteoblastoma never shows infiltration of preexisting trabecular bone.