Osteoid osteomas are small, benign bone tumors that are almost always painful. Most lesions are richly innervated and surrounded by reactive sclerotic bone. The pain produced by an osteoid osteoma can be intense, but is usually relieved by aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), at least temporarily. Although the majority of patients are 10 to 20 years old, some patients are old enough to have degenerative joint disease, which may obscure the diagnosis initially.

Epidemiology

Osteoid osteoma is the third most common benign bone tumor (after osteochondroma and nonossifying fibroma), representing about 10% of all benign bone tumors. It tends to affect patients younger than 20 years old, although the age at presentation can range from 5-40 years old. Approximately 70% of patients are male.

Clinical Features

Osteoid osteoma often presents with intense pain that occurs more frequently at night. The pain is usually relieved by aspirin and NSAIDs and exacerbated by drinking alcohol. Although the lesion can “burn out” over a period of 2 to 3 years, many patients have intolerable symptoms that need treatment before that time period elapses.

The most common locations of osteoid osteoma are the proximal femur and the tibial diaphysis (representing about half of all cases), though it can be found in the spine, hand, and foot. Spinal osteoid osteoma (about 15% of cases) can cause a rigid scoliosis, thought to be caused by paraspinal muscle spasm. When osteoid osteomas cause scoliosis, they are found in the concavity of the curve. Osteoid osteoma of the hand is less common (about 10% of cases), and may be difficult to diagnose, as it is either asymptomatic or will produce symptoms that may be confused with those of DeQuervain’s tenosynovitis. When the affected bone is superficial, there may be tenderness to palpation, localized swelling and erythema.

Radiologic Features

Osteoid osteoma develops eccentrically, within the cortex of the bone. It is characterized radiographically by a small radiolucent nidus, which represents the actual neoplastic cells, surrounded by a halo of reactive sclerotic bone at the periphery (Figure 1). By definition, the nidus is less than 2cm in diameter. If the nidus is more than 2cm in diameter, the lesion is classified as an osteoblastoma.

CT scanning, the advanced imaging modality of choice, usually reveals the nidus surrounded by thick reactive bone (Figure 2). A bone scan can be used to identify the lesion by revealing intense uptake associated with reactive bone (Figure 3).

MRI imaging is useful in demonstrating extensive edema within the surrounding marrow, which is characteristic of this lesion. On MRI, the T1-weighted images show a nidus with low-intermediate signal intensity; reactive sclerosis has low signal intensity. On fluid-sensitive MRI sequences, the nidus exhibits moderate signal intensity, the reactive sclerosis is hypointense, and extensive marrow edema can be observed (Figure 4).

Pathology

As a highly vascular lesion, osteoid osteoma appears cherry red on gross specimens (Figure 5). However, the red region can be obscured if the lesion is surrounded by dense reactive sclerotic bone.

On histologic examination, osteoid, osteoclasts, vascular channels, and neoplastic bone formation can be seen within the nidus (Figure 6), which represents the true neoplastic component of the tumor. A ring of reactive bone surrounds the nidus, with no permeation of the tumor into the bone beyond the rim. Cartilage is not found in osteoid osteomas. The lesions usually have only minimal fibroblasts and collagen; capillaries, however, are abundant.

Differential Diagnosis

Although osteoid osteoma has a fairly distinct radiographic appearance, it can sometimes be confused with the following entities:

• Osteoblastoma
• Intramedullary osteomyelitis or intracortical abscess
• Stress fracture
• Arthritic subchondral cyst
• Metastasis
• Bone island
• Eosinophilic granuloma
• Osteosarcoma

Osteoid osteoma and osteoblastoma look alike, as both contain osteoblasts that produce osteoid and woven bone. Osteoblastoma, however, is larger, tends to be more aggressive, and

its characteristic pain is both less severe than that of an osteoid osteoma and less responsive to NSAIDs. The margins of malignant, metastatic, and infectious lesions are usually permeative, whereas the rim around the nidus of an osteoid osteoma is smooth.

Disease Course: Treatment and Prognosis

Osteoid osteomas are first treated with NSAIDs, which typically produce a dramatic but transient decrease in symptoms. If the response to NSAIDs is inadequate, percutaneous radiofrequency ablation under CT guidance can be tried in most cases. In this procedure, an electrode is inserted in the nidus and heated to approximately 90°C for 5 minutes to cause focal thermal necrosis (Figure 7). Although almost all patients report relief with radiofrequency ablation, there is a ~15% recurrence rate.

Radio frequency ablation is contraindicated for lesions that are subcutaneous or within a few centimeters of a critical neurovascular structure. Given the high risk of burning skin or injuring nerves and vessels under these circumstances, surgical excision is performed instead. However, finding the tumor’s nidus during open surgery can be difficult, and excisional curettage can be complicated by the dense rim of sclerotic bone surrounding the nidus. Preoperative CT-guided needle localization or intraoperative fluoros copy can be used to assist the surgery, and frozen section analysis can help confirm that the lesion has been located.

Commonly Tested on Exam

1. Appearance on X-ray and CT
2. The “classic” symptom of night pain that is very responsive to NSAIDs
3. Role of radiofrequency ablation and its contraindications