Osteoid Osteoma

General characteristics

Osteoid osteomas are relatively common, following only osteochondromas and fibrous cortical defects.

Epidemiology

The male-to-female ratio is 3:l. They typically occur in teenagers and young adults and are rare before 5 and after 30 years of age.

Location

These tumors occur most frequently in the femur, tibia, and humerus and involve the diaphysis and, less commonly, the metaphysis. When the spine is involved, the posterior elements are typically affected.

Clinical symptoms

The main clinical symptom is pain relieved by aspirin.

Morphology

CT is very accurate in detecting the nidus and is preferred to MRI for evaluating. osteoid ostema.

Osteoid osteomas in younger patients tend to be associated with more extensive peritumoral edema. When the osteoid osteoma is interarticular, an associated joint effusion may be detected.

Imaging

O ccasionally, an osteoid osteomi can be confused with a stress fracture on MR images because of edema in the bone marrow. On a CT scan, the typical appearance of an osteoid osteoma is an area of sclerosis surrounding a small (<1 cm) radiolucent nidus. Lesions larger than 1.5 cm are considered osteoblastomas.

The nidus may contain calcifications, the number of which can vary from none to enough to bring about almost complete calcification with only a thin peripheral rim of low density. For full evaluation of the nidus, thin-section CT with 2-mm slices is needed because the nidus is often less than 1 cm in diameter. The nidus enhances on dynamic CT scans, and depending on the degree of calcification, it has low signal intensity on TI-weighted MR images and variable intensity on T2-weighted images.

The stmounding bone marrow has low signal intensity on all pulse sequences in the presence of reactive sclerosis or high signal intensity on the T2-weighted images in the presence of edema.