Diagnostic Studies. An intense bony reaction to a small nidus is the radiographic hallmark of osteoid osteoma. Radiographs reveal an oval radiolucent nidus only 3 to 5 mm in diameter and surrounded by a disproportionately large, dense reactive zone. Although usually located in the cortex, a nidus may occur in the subperiosteal and endosteal regions. CT scans at 5-mm intervals are used to confirm a cortical nidus and to help direct the therapeutic approach. The bone scan usually shows increased radioisotope uptake.

The radiographic differential diagnosis includes osteomyelitis (chronic sclerosing osteomyelitis), Brodie’s abscess, and stress fracture.

Histologic examination reveals a nidus composed of thick, vascular bars of osteoblastic tissue surrounded by a thin zone of vascular fibrous tissue, in turn surrounded by a dense shell, or margin, of mature reactive cortical bone. The histologic differential diagnosis primarily includes osteoblastoma (see Plate 6-3). Although osteoblastoma is similar to osteoid osteoma in many respects, it is usually larger and has some subtle but distinct histologic differences. Distinguishing osteosarcoma (see Plates 6-15 and 6-16) from the small osteoblastic nidus of osteoid osteoma is rarely problematic.

Treatment/Prognosis. Although osteoid osteoma may eventually resolve spontaneously, with spontaneous ossification and the subsequent relief of pain, most patients prefer not to wait for resolution, owing to severe pain. Percutaneous radiofrequency ablation of the osteoid osteomas can be performed with up to 90% success and can be done under local anesthesia with CT scan guidance; thus, this is the current initial treatment of choice. When the nidus is located in a low-stress area such as the metaphysis, en bloc excision with a surrounding small block of reactive bone can be performed but has increased morbidity relative to percutaneous ablation. Alternatively, the overlying margin of reactive bone may be shaved until the nidus is visible as a cherry-red spot; this spot then can be removed with curettage. Although intracapsular curettage is associated with a higher recurrence rate than other types of excision, it minimizes the risk of postoperative fracture in high-stress areas such as the femoral neck. Most recurrences result from fragmentation of the lesion, partial excision, or inaccurate localization of the lesion in an inaccessible place.

After radiofrequency ablation, complete excision, or even spontaneous resolution, prognosis is excellent. No cases of malignant transformation have been reported.