Overview

Osteoid osteoma is a benign tumor usually less than 1.5 cm in size. Although it is small, it can produce significant pain and discomfort for prolonged periods. There is virtually no risk of malignant transformation. Its radiological features can mimic other conditions, and surgical treatment is not always successful.

Epidemiology

It is the 3rd most common benign tumor comprising 12% of benign tumors and 3% of all tumors. The most common sites are metaphysis or diaphysis of long bones (75%) but can affect posterior element of spine (10%) hands or feet (12%). It is more common in males with a male to female ratio of 2:1. Three-quartes of patients will be between 10-30 years of age. It is more uncommon in blacks.

Clinical presentation

Patients will usually present with focal pain that is worse at night. In 30% of patients, it will awaken them while they are sleeping. There is dramatic relief with low dose NSAID's. Because osteoid osteoma is a vascular tumor, substances which cause vasodilitation such as alcohol may precipitate an acute pain crisis. In the spine, it can cause scoliosis secondary to muscle spasms. Intraarticular lesions can cause joint effusions or decreased range of motion mimicking other intraarticular pathologies.

Radiographic Findings

Classic radiographic features are an area of cortical thickening and sclerosis containing a lucent nidus of less than 1 cm. A central nidus is seen in 75% of patients. This defect is usually smaller than 1.5 cm in diameter and is associated with a variable degree of cortical and endosteal sclerosis. Cortical tumors tend to provoke more sclerosis than the medullary form. CT scans wil better visualize the nidus and can show varying degrees of mineralization. CT remains the best diagnostic tool, but MRI can be helpful in the diagnosis of cancellous osteoid osteoma. Bone scans will show significant uptake at the site and may be helpful when there is doubt based on radiographs.

Classification

• Based on Location
  • Cortical (most common)
  • Cancellous
  • Subperiosteal

Histology

The pathogenesis of osteoid osteoma remains controversial. Some believe it a neoplasm, some feel it arises on an inflammatory basis, and some suggest it to be an unusual healing and reparative process. The nidus is composed of thick vascular bars of osteoblastic tissue surrounded by vascular fibrous tissue. This is surrounded by mature reactive cortical bone, creating a zonal pattern to its growth. These aspects make it unlike any other true neoplasm.

Natural History

Spontaneous regression of clinically and radiologically diagnosed osteoid osteomas can occur during an average of 4-6 years. The nidus gradually calcifies, ossifies, and finally, blends into the sclerotic surrounding bone. Many patients are able to tolerate the pain with or without the prolonged use of NSAID's. Those who cannot are candidates for surgical intervention.

Differential Diagnosis

• Osteoblastoma
• Brodie abscess
• Osteoma
• Osteogenic sarcoma
• Stress fracture
• Subperiosteal aneurysmal bone cyst
  

Treatment

In most cases, it is worth a trial of NSAIDs before embarking on more invasive therapeutic procedures. This is especially so if the site of the lesion is less accessible to surgical treatment. If surgical treament is indicated, nidus removal is essential for pain relief and to prevent deformity in the case of spinal lesions. Accurate intraoperative localization of the nidus is crucial for predictable success of surgical intervention. Methods for localization include landmarks determined by preoperative imaging, after oral tetracycline loading with identification of the golden yellow fluorescent nidus under ultraviolet light, or preoperative CT-guided needle or Kirschner wire marks made on the bone surface near the nidus. The nidus can be removed en bloc or using the burr-down technique. The latter does not weaken the bone as much, but risks not removing the entire nidus.

Percutaneous ablations of the lesion by CT-guided core-drill excision and destruction of the nidus by thermocoagulation, radiofrequency, or laser have recently been reported as alternative treatments. This method seems to be particularly suited to management of lesions in deep sites such as in the neck of femur and in the pelvis. They are not indicated in most lesions of the spine with close anatomical relationship to dural and neural structures, in small bones, or in cases of recurrent lesions.  This technique should also not be used in lesions close to the joint or epiphysis as the heat can lead to permanant growth plate or cartilage damage.  Success rates of 77% to 100% have been reported with these methods.