Osteoid osteomas are benign bone forming tumours that typically occur in children (particularly adolescents). They have characteristic lucent nidus <2 cm and surrounding solid periosteal reaction and classically cause night pain that is relieved by the use of salicylate analgesia, e.g. aspirin.
Osteoid osteomas are usually found in children and adolescents, between the ages of 10 and 35 years 2.
They account for ~10% of all benign bone lesions and there is a male predilection (M:F 2-4:1) 2.
Classically patients present with nocturnal pain and are relieved by salicylates (e.g. aspirin).
When in the spine they are a classic cause of painful scoliosis, concave on the side of lesion. This typical presentation is seen in over 75% of cases 2.
Soft tissue swelling may occur (see case 4) and if close to a growth plate, accelerated growth may be evident 2, presumably related to hyperaemia.
When the lesion is intracapsular, the presentation is more atypical, and more likely mimics and inflammatory arthropathy or synovitis. Joint effusion is often present 2,4.
An osteoid osteoma is composed of three concentric parts 1:
- meshwork of dilated vessels, osteoblasts, osteoid and woven bone
- may have a central region of mineralisation 2
- fibrovascular rim
- surrounding reactive sclerosis
The nidus releases prostaglandins (via Cox-1 and Cox-2) which in turn result in pain 2.
Most osteoid osteomas occur in long tubular bones of the limbs (especially proximal femur), but essentially any bone may be involved.
- long bones of the limbs: ~65-80% 1-2,7
- femur most common (especially neck of femur)
- mid tibial diaphysis common also
- phalanges: ~20%
- vertebrae: ~10%, predominantly posterior elements
- lumbar: 59% 2
- cervical: 27%
- thoracic: 12%
- sacrum: 2%
Furthermore, osteoid osteomas are usually cortical lesions but they can occur anywhere within the bone including medullary, subperiosteal (most common in talus), and intracapsular 2.
It is important to remember that the sclerosis is reactive and does not represent the lesion itself. The nidus is usually <2 cm in diameter, and is typically ovoid. It may have a central region of mineralisation 2.
May be normal or may show a solid periosteal reaction with cortical thickening. The nidus is sometimes visible as a well circumscribed lucent region, occasionally with a central sclerotic dot.
Excellent at characterising the lesion and is the modality of choice. It typically shows a focally lucent nidus within surrounding sclerotic reactive bone. A central sclerotic dot may also be seen.
Scintigraphy will show typical focal uptake and at times will show a double density sign (also known as the less catchy hotter spot within hot area sign) which if present is highly specific. The central focus showing intense uptake within a surrounding lower, but nonetheless increased uptake rim.
On ultrasound, focal cortical irregularity with adjacent hypoechoic synovitis may be present at the site of intra articular lesions. The nidus can show hypoechogenicity with posterior acoustic enhancement. Ultrasound may be able to identify the nidus as a hypervascular nidus on Doppler examination 2.
Although MRI is sensitive, it is non-specific and is often unable to identify the nidus. The hyperaemia and resultant bone marrow oedema pattern may result in the scans being misinterpreted as representing aggressive pathology 2.
The signal intensity of the nidus is variable on all sequences as is the degree of contrast enhancement 2.
Treatment and prognosis
The lesion is benign and treatment has traditionally been with surgical resection. Historically, this has, on occasion, been difficult because of the inherent inability to locate the nidus during surgery 3. However, percutaneous radiofrequency ablation is being used under CT guidance is being used with increasing frequency 5.
There is growing evidence, that osteoid osteoma finally resolves spontaneously over time and can be treated conseratively with NSAIDs in certain groups of patients 6. Average time to resolution is 33 months.
General imaging differential considerations include: