Osteoid osteoma

Tumor biology and incidence

Benign osteoblastic lesion of bone
Accounts for approximately 10% of all benign lesions

Age

Typically affects persons 10-35 years of age
Has been reported as late as eighth decade

Gender

M:F = 2:1

Presentation

Focal pain that is worse at night; may awaken the patient while sleeping
Pain relieved by aspirin and low dose NSAIDs, possibly because the tumor has a diurnal production of prostaglandins
Vascular tumor; substances that cause vasodilitation such as alcohol may precipitate an acute pain crisis
May be vasomotor disturbances in the region (increased skin temperature)
May cause scoliosis secondary to muscle spasms
Intraarticular lesions can cause synovitis and joint effusions or decreased range of motion, mimicking other intraarticular pathologies
May be muscle wasting, which may lead to erroneous diagnosis of neurologic disorder
Symptoms usually plateau over time
Patients can have accelerated bone growth if nidus is near growth plate, especially in young children


Figure 1a.	Figure 1b.

Physical findings

Spinal lesions can present with scoliosis
Intracapsular lesions can cause joint effusion

Plain films

Site

Predilection for long bones (femur and tibia) as well as the vertebrae (usually posterior elements)
May occur in any bone and may be cortical, medullary, subperiosteal, or periarticular (intracapsular)

Size

Nidus usually < 1 cm
If nidus is greater than 2 cm, lesion usually considered an osteoblastoma

Tumor effect on bone and bone response to tumor

Cortical: Intense focal sclerotic reaction around the lucent nidus
Medullary: Sclerotic reaction surrounding the lucent nidus; sclerosis may be seen for a considerable distance from the nidus
Intracapsular: Radiolucent nidus; usually minimal to no reactive bony sclerosis; can cause effusion
Subperiosteal: Periosteal response is minimal, sclerosis is absent, bony scalloping/resorption.

Matrix

Lucent nidus may or may not be partially calcified centrally

Cortex

Intact, may cause marked cortical thickening

Soft tissue mass

None, except in the rare subperiosteal form (small juxtacortical soft tissue mass may be seen immediately adjacent to the bone)


Figure 2a.	Figure 2b.	Figure 2c.

Bone scan

Hot on bone scan
May be difficult to identify nidus due to uptake in surrounding periosteal reaction

CT and MRI

Thin section CT is modality of choice for localization of the nidus, often as guided by plain radiographs or a bone scan
CT useful :
- When on plain x-ray there is intense sclerosis and nidus is not visible but suspected
- To define the exact location of the lesion with respect to the cortex for pre-operative planning
- In evaluating lesions of the axial skeleton- pelvis and spine
May be necessary to obtain thin slices (1-2mm) on CT scan to visualize a small nidus
MRI useful for identification of lesions where nidus is of similar density to surrounding bone


Figure 3a.	Figure 3b.	Figure 3c.

Differential diagnosis

Osteoblastoma
Brodie's abscess
Osteoma
Osteogenic sarcoma
Stress fracture (radiolucency perpendicular to or on angle from cortex)
Subperiosteal aneurysmal bone cyst
Bone island (normal bone scan 90% of cases)


Figure 5a. Osteoblastoma	Figure 5b. Osteomyelitis	Figure 5c. Stress fracture

Figure 5d. Stress fracture MRI

Natural history

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

Pathology

Gross: Nidus is small, well circumscribed, red, and granular in appearance
Microscopic: Nidus contains osteoid at its center; matures into interconnected trabeculae of bone in a fibrovascular stroma; osteoblasts and osteoclasts rim the trabeculae
Bone appears more mature towards periphery of lesion, merges with sclerotic reactive bone


Figure 4a.	Figure 4b.	Figure 4c.

Figure 4d.

Diagnosis and treatment

Most osteoid osteomas should be diagnosed by imaging
Natural history is to undergo spontaneous resolution after a variable amount of time
If pain is unresponsive to anti-inflammatories or if patient cannot tolerate the medication, treat lesion surgically
Lesion can be resected or ablated percutaneously under radiographic control, ablated with a radiofrequency technique or toxic substance (for example, alcohol), curetted, or resected en bloc
- Curettage has higher risk of recurrence than complete resection
- En bloc resection has lower risk of relapse, but higher likelihood of requiring a reconstructive procedure folowing lesional remova.
Specimen can be x-rayed to ensure complete resection.
Defect left by removal of lesion should be bone grafted or instrumented if there is risk of fracture


Figure 6a.	Figure 6b.	Figure 6c.

Figure 6d.	Figure 6e.

Absolute contraindications to medical therapy

Allergy to medication
Uncertainty of diagnosis.
Past history of a bleeding ulcer or other bleeding disorders

Relative contraindications to medical therapy

Patient preference
Breakthrough pain at maximal therapeutic doses
Development of adverse reaction to medication

Complications

Lesion located close to growth plate in young child may lead to accelerated bone growth in area
Lesion in spine may cause scoliosis
Intracapsular lesions may produce chronic effusion
Full-thickness cortical defect produced after resection of cortical or intramedullary osteoid osteoma may put patient at risk of pathologic fracture