Osteoid osteoma
The plain radiograph of the femur (see Fig. 1, page 52) demonstrates eccentric sclerosis and cortical thickening or periosteal reaction of the upper femoral shaft (arrows). The radioisotope bone scan (see Fig. 2, page 52) reveals an intense focus of increased uptake of the radioisotope (arrow), superimposed on a region of mildly increased activity (arrowhead) along the medial aspect of the femur. Note the linear bands of uptake at the wrists and both ends of the femora. This corresponds to normal osteoblastic activity at the epiphyses. This finding on a radioisotope bone scan is known as the double-density sign and strongly suggests the diagnosis of osteoid osteoma. 1
Computed tomography confirms the diagnosis by demonstrating a nidus. Axial computed tomography in another patient with similar clinical and radiologic features as in this patient demonstrates an area of sclerotic cortical thickening (Fig. 3, arrowhead) surrounding a well-defined round radiolucent nidus (arrow), characteristic of an osteoid osteoma.2 Calcifications may be identified within the nidus.
Benign tumours of bone, osteoid osteomas are usually found in patients between the ages of 7 and 25 years and are more prevelant in males than females (3:1). Pain is the hallmark of this condition, and it is generally worse at night and relieved with nonsteroidal anti-inflammatory medication. Initially, symptoms may be inconstant and can be present for a year or more before a diagnosis is made.3 The tumour most commonly occurs in the diaphysis of long tubular bones (especially the femur and tibia) but may arise elsewhere, including the posterior elements of the spine, where it is associated with scoliosis.
Histologically, these tumours consist of a central core of vascular osteoid tissue surrounded by a peripheral zone of reactive sclerotic bone.3 The nidus is usually cortical in location but may arise in a subperiosteal or intramedullary location.4
Plain radiographs, as in this case, may not demonstrate the diagnostic radiolucent nidus within the thickened cortex. In the absence of this finding, the differential diagnosis includes the following: trauma or stress fracture, Brodie’s abscess or osteomyelitis, syphilis and, particularly for subperiosteal osteoid osteomas, Ewing’s sarcoma or osteogenic sarcoma. In the past en-bloc excision was the operation of choice. In such cases, computed tomography was often used to provide preoperative localization of the tumour nidus. A needle was advanced into the cortex, methylene blue dye was injected and sometimes a self-retaining localization wire was left in place. More recently, percutaneous excision under computed tomographic guidance (via trochar or arthroscope) or ablation (radiofrequency or laser photocoagulation) has given comparable results with lower morbidity.5 Medical management has been advocated by some.6
Fig. 4 demonstrates radiofrequency ablation of the lesion seen in Fig. 3. The tip of the radiofrequency electrode is centred on the nidus (arrow), which is ablated by the heat generated. Although complete destruction or excision of the nidus is essential for cure, the reactive sclerotic bone does not require treatment and may resolve with time.