Osteoid osteoma
Overview
- Benign osteoblastic tumor characterized by a small nidus (<1.5 cm) of osteoid and woven bone
- Classic triad: nocturnal pain, NSAID-responsive, radiolucent nidus
- Self-limiting over time; can resolve spontaneously
Epidemiology
- Age: Mostly adolescents and young adults (10–30 years)
- Male > Female (~2–3:1)
- ~10–12% of all benign bone tumors
Clinical Features
- Nocturnal pain, dramatically relieved by NSAIDs
- Localized tenderness
- In intra-articular lesions:
- Pain may mimic arthritis
- Less sclerosis, more synovitis
- In spinal lesions:
- May cause painful scoliosis (convexity pointed away from the lesion: pain side = growth side)
- Lumbar > cervical > thoracic
Common Locations
| Type | Sites |
|---|---|
| Cortical (classic) | Diaphysis of long bones (femur > tibia) |
| Medullary | Small bones (hands/feet) |
| Subperiosteal / Intra-articular | Femoral neck, spine (posterior elements) |
Histology
- Central nidus of osteoid and woven bone
- Surrounded by vascular stroma
- Peripheral reactive bone formation
Imaging Features
X-ray
- Small radiolucent nidus (<1.5 cm) with variable central mineralization
- Surrounded by dense reactive sclerosis (especially in cortical lesions)
- May be occult on plain films, particularly in intra-articular or spine locations
CT
- Modality of choice
- Clearly shows:
- Central nidus
- Possible central calcification
- Reactive cortical thickening
MRI
- Variable appearance
- T1: Low signal nidus
- T2: Hyperintense nidus, surrounding marrow and soft tissue edema
- May be misinterpreted as infection or stress fracture
Bone Scan
- Double-density sign: intense focal uptake in nidus with surrounding less intense zone
- Useful for localizing clinically occult lesions
Differential Diagnosis
| Diagnosis | Key Differences |
|---|---|
| Osteoblastoma | Nidus >1.5–2 cm, less sclerotic reaction, not NSAID-responsive |
| Brodie abscess | Serpiginous border, clinical infection signs, sequestrum |
| Stress fracture | Linear lucency, periosteal reaction, history of activity |
| Ewing sarcoma | Aggressive, soft tissue mass, systemic symptoms |
| Intra-articular pathology | Synovitis or arthritis on MRI may obscure nidus |
Management
- NSAIDs: Symptomatic relief, often sufficient; lesions may burn out over years
- CT-guided radiofrequency ablation (RFA): Minimally invasive, curative in most cases
- Surgical excision: Rarely needed now unless RFA fails or not feasible
Prognosis
- Excellent
- Rare recurrence after RFA
- Symptoms resolve rapidly post-treatment
FRCR / Thai Board Exam Tips
- Buzzwords:
- “Young male + night pain + relieved by NSAIDs”
- “Lucent nidus with surrounding sclerosis”
- Intra-articular and spinal variants may lack classic features → high clinical suspicion needed
- Nidus <1.5 cm = osteoid osteoma
Nidus >2 cm = think osteoblastoma
Real-Life Reporting Tips
- Look for tiny nidus on CT if pain and MRI suggest inflammation but no mass
- Describe:
- Location (cortical, medullary, subperiosteal)
- Nidus size
- Central mineralization
- Surrounding sclerosis or edema
- Suggest RFA referral if diagnosis is confident and lesion is symptomatic
