Osteoid osteoma

The tumours are painful and may be accompanied by soft tissue swelling and tenderness. Torticollis, spinal stiffness and scoliosis are among the clinical features of lesions that develop in the vertebral column in the immature skeleton. The initial clinical manifestations of intra-articular osteoid osteomas may be joint tenderness, swelling, synovitis and limitation of motion. The femur is the bone involved most frequently, followed by the tibia and then the bones in the hands and feet. Other sites of localization of osteoid osteomas are the spine (especially the lumbar vertebrae), innominate bone, skull, mandible or maxilla, clavicle, scapula, ribs and radius.

The classic radiographic appearance is that of a centrally located, oval or round radiolucent area, surrounded by a zone of uniform bone sclerosis; this combination is virtually diagnostic of osteoid osteoma.

In the long tubular bones, the osteoid osteomas that are located in the diaphysis (and some that are metaphyseal) typically occur in the cortex as a radiolucent lesion, representing the nidus, surrounded by bone sclerosis with cortical thickening due to endosteal and subperiosteal new bone formation. Osteoid osteomas that are subperiosteal in location may evoke more limited bone proliferation immediately adjacent to or at a distance from the lesion.

In the carpal or tarsal bones, an osteoid osteoma usually arises in the medullary spongiosa and appears on radiographs as a well-circumscribed, partially or completely calcified lesion.

In the small bones of the hand and foot (metacarpals, metatarsals and phalanges), cortical osteoid osteomas generally manifest a periosteal response similar to that observed in the diaphysis of the long tubular bones. Subperiosteal sites may reveal scalloping of the adjacent cortical surface; in the cancellous bone, a partially or totally calcified lesion with or without a radiolucent margin is identified. In any of these locations, soft tissue swelling may be prominent, simulating the appearance of infection or arthritis.

Intra-articular osteoid osteomas may cause pain, soft tissue swelling, a joint effusion and restriction of joint motion. A synovial inflammatory response may ensue and lead to irreversible cartilaginous and osseous destruction. Osteopenia, uniform narrowing of the interosseous space, and periarticular subperiosteal bone apposition may be encountered. Eventually, hypertrophic changes similar to those in osteoarthritis may occur.

Osteoid osteomas arising in the spine frequently cause radicular pain, especially at night and during spinal motion, and are often accompanied by scoliosis. Other manifestations include local tenderness and paraspinal muscle atrophy; neurologic abnormalities are relatively infrequent. On radiographs, the lesion is characteristically located on the concave aspect of the scoliotic curve, often near its apex. Sometimes osteosclerosis of a pedicle, lamina, articular process or, less commonly, a transverse or spinous process is observed. Identification of the nidus frequently requires CT (Fig.1).

Radionuclide bone imaging reveals these lesions to avidly accumulate bone-seeking radiopharmaceutical agents during the vascular, blood-pool and delayed phases of the examination. A distinctive pattern, the double density sign, is characterized by intense scintigraphic activity centrally in the region of the nidus and less intense accumulation of the radionuclide peripherally in the sclerotic bone (see double density sign, Fig. 1).
CT scanning has largely replaced conventional tomography in the imaging evaluation of osteoid osteomas and is most valuable in defining osteoid osteomas in the spine, osseous pelvis and femoral neck.

MR imaging is generally considered less useful than CT scanning in the detection of the nidus, and the MR imaging findings in cases of osteoid osteoma may simulate those of a malignant tumour or osteomyelitis. In cases of intra-articular osteoid osteomas, the MR imaging findings may include synovitis and joint effusion.

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