Osteoid Osteoma

Osteoid Osteoma

 Most commonly involves the diaphysis of long bones especially femur and tibia, and the proximal femur is the most common site.
 50% of tumours involve the lower extremity
 Osteoid osteoma may have a unique ‘pathogenic’ nerve supply
 Three types have been described: Intracortical (80%), cancellous and subperiosteal
 The pain may be referred to an adjacent joint and when the lesion is intracapsular it may simulate arthritis with effusions ,spasms and contractures
 Occasionally pain precedes the appearance of radiographic changes, and leads to multiple incorrect diagnoses including neurosis
 In the spine, posterior elements of the lumbar spine is most commonly involved (next common thoracic spine). An associated scoliosis is often present
 If the nidus is in proximity to a nerve root, root irritation can develop.
 In the lumbar spine, this pain can present as sciatica and suggest the diagnosis of a herniated intervertebral disc
 Torticollis may be seen if the cervical spine is involved.
 Aspirin or nonsteroidal anti-inflammatory agents relieves pain secondary to a high concentration of prostaglandins in the nidus

Histology:
• There is a distinct demarcation between the nidus and the reactive bone
• The nidus consists of an interlacing network of osteoid trabeculae with variable mineralisation.
• The trabecular organization is haphazard and the greatest degree of mineralisation is in the centre of the lesion

Investigation:
• X rays: Central lytic nidus with extensive reactive sclerosis. The nidus is always less than 1.5 cm although the area of the reactive bone sclerosis may be larger.
• The radiolucent nidus may be obscured by dense sclerotic bone
• When the lesion is intramedullary there is less sclerotic bone
• CT scan is the investigation of choice
• Double density sign on bone scan (Focal areas of increased uptake with a second smaller area of increased uptake)
• MRI scans will show extensive edema, which may be confused with a marrow-replacing neoplasm and is therefore not recommended if osteoid osteoma is the suspected lesion

DD:
Bone island (enostoses):
‣ Mimic osteoid osteoma on X-rays but MRI changes are different from an Osteoid osteoma.

Treatment: If surgery is undertaken, it is important to eradicate the entire symptomatic nidus.
 Removal of a large amount of the surrounding sclerotic bone should be avoided because it can severely weaken the bone and may result in a pathologic fracture
 Intralesional resection by simple curettage of the nidus followed by high-speed burring is done often (Burr down technique).
 Intraoperative localization of the lesion may be done by technetium labeled methylene diphosphonate and detection by a Geiger counter
 If block excision is performed, intraoperative roentgenograms of the specimen are advised to document complete removal of the nidus
 CT-guided percutaneous resection for small tumours is becoming popular.
 Percutaneous Radiofrequency ablation is being tried. Under CT Guidance a radiofrequency probe is placed into the lesion and the nidus is heated upto 80degree C. This induces ionic agitation and frictional heat to cause tumor necrosis
 The patient may be also treated nonoperatively using NSAIDS. About 50% of the patients treated with NSAIDs will have their lesions burnt out with no further medical or surgical treatment necessary

Ref:
1. Cantwell CP et al: Current trends in treatment of osteoid osteoma with an emphasis on radiofrequency ablation. Eur Radiol 2004; 14(4):607.