Osteoid osteoma

Benign osteoblastic (bone forming) tumor. It consists of a highly vascularized osteoid core. The growing core induces a peripheral zone of sclerosis and periosteal reaction.

Clinical Presentation: pain which is classically increased at night and often relieved by low doses of salicylates (prostaglandin response). Typically occurs in patients age 5-25 years old. Can also occur in the very young and very old. Male to female ratio is 3:1.

Imaging Findings: Plain film/CT description: Centrally located oval or round nidus, < 2 cm in diameter, with a uniform peripheral zone of sclerosis. Unfortunately, appearance can be highly variable because the nidus can be located in the cortex, intrameduallary space, or periosteum of a bone. Description above is classic for an intracortical lesion. Diagnosis can also be more difficult when there is intraarticular extension or occurs in the spine. Often nidus cannot be seen on plain films.

Additional pearls:

Radiolucent nidus, < 2 cm in diameter. Nidus may contain calcified bone matrix. Nidus surrounded by sclerosis. (Sclerosis sometimes is so extensive that the nidus can't be detected on plain films.) Nidus located in the metaphysis or diaphysis of long bones. (Ephysis is very uncommon.) Periosteal reaction. Monostotic or polyostotic. One or multiple niduses. Niduses may be clustered. Lesion is located on the concave side of bone in patients with painful scoliosis. Synovitis commonly occurs, especially if there are periarticular or intracapsular lesions. Limb overgrowth in children.

Distribution: Intracortical--80%, Tibia/femur--55%, Hands/feet--20%