OSTEOID OSTEOMA - BENIGN BONE-FORMING TUMOUR
Night Pain | Aspirin Relief | Nidus Under 2cm | RFA vs Excision
LOCATION CATEGORIES
Critical Must-Knows
- Night pain relieved by NSAIDs/aspirin - classic presentation
- Nidus under 2cm - larger is osteoblastoma (same histology)
- CT is gold standard - shows nidus within reactive sclerosis
- RFA first-line treatment - 90%+ success, day case procedure
- Surgical excision if RFA fails or for specific locations
Examiner's Pearls
- "Nidus under 2cm differentiates from osteoblastoma
- "Bone scan hot with double-density sign
- "Pain mechanism: prostaglandins from nidus
- "May cause scoliosis in spine (painful type)
Critical Osteoid Osteoma Exam Points
Size Matters
Under 2cm equals osteoid osteoma while over 2cm equals osteoblastoma. Same histology, different behaviour. Osteoblastoma is more aggressive and has higher recurrence. This is the key exam distinction.
Night Pain Pattern
Classic history is nocturnal pain that wakes patient, relieved by aspirin or NSAIDs within 20-30 minutes. Due to prostaglandin E2 production by the nidus.
Imaging Sequence
X-ray may be normal or show reactive sclerosis. CT is diagnostic and shows the nidus. MRI can be misleading due to extensive oedema. Bone scan shows characteristic uptake.
Treatment Options
RFA is first-line in most locations. Surgical excision reserved for RFA failure, spinal canal locations, or surgeon preference. Both achieve over 90% success.
Quick Decision Guide - Treatment Selection
| Location | First Choice | Alternative | Special Considerations |
|---|---|---|---|
| Long bone cortex | CT-guided RFA | Surgical excision (en bloc) | Most common location, excellent RFA access |
| Spine (posterior elements) | RFA with nerve monitoring | Surgical excision | Risk of thermal injury to neural structures |
| Intra-articular (hip) | Arthroscopic excision | RFA with chondroprotection | Cartilage damage risk with RFA |
| Hand/foot phalanges | Surgical excision | RFA | Risk of fracture, limited bone stock |
| Near growth plate | Surgical excision | Cryoablation | Thermal damage to physis with RFA |
NIDUS - Osteoid Osteoma Features
Memory Hook:The NIDUS is what you're looking for on CT - under 2cm with reactive sclerosis
SCLEROSIS - Imaging Features
Memory Hook:Remember the SCLEROSIS you see is reactive - focus on finding the small nidus
ABLATE - Treatment Principles
Memory Hook:ABLATE the source of prostaglandins to cure the night pain
RFA - Radiofrequency Ablation Steps
Memory Hook:RFA destroys the nidus that produces the prostaglandins causing pain
Overview and Epidemiology
Definition
Osteoid osteoma is a benign osteoblastic bone tumour characterised by a small nidus (under 2cm) of osteoid tissue surrounded by reactive bone sclerosis. It is histologically identical to osteoblastoma but distinguished by size and clinical behaviour.
Epidemiology
Incidence:
- Accounts for 10-12% of benign bone tumours
- Annual incidence approximately 3 per million population
- More common than osteoblastoma (3:1 ratio)
Demographics:
- Peak age 10-25 years (80% under 30)
- Male predominance 2:1 to 3:1
- All ethnic groups equally affected
Location Prevalence:
- Lower extremity 50-60% (femur and tibia most common)
- Upper extremity 20%
- Spine 10-15%
- Hand and foot 10%
Natural History
Without treatment, osteoid osteomas typically follow a predictable course with persistent symptoms for 3-5 years before spontaneous resolution. Approximately 5-10% resolve within 2 years while most cases take 3-6 years for complete spontaneous regression. Some cases persist beyond 6 years without resolution.
The regression occurs due to maturation of the nidus into inactive sclerotic bone, with gradual cessation of prostaglandin production.
Risk Factors
No clear risk factors have been identified. The tumour is not associated with trauma, infection, or inherited conditions. It represents a true neoplasm rather than a reactive process.
Anatomy and Pathophysiology
Lesion Characteristics
Osteoid osteoma is a benign bone-forming tumour characterised by a small central nidus (under 2cm) surrounded by reactive sclerotic bone. The nidus consists of osteoid tissue in various stages of maturation, surrounded by highly vascularised connective tissue stroma.
Nidus Components:
- Central zone of osteoid and woven bone
- Highly vascularised connective tissue stroma
- Osteoblasts rimming the osteoid trabeculae
- Variable mineralisation (central calcification common)
Pain Pathophysiology
The nidus produces high levels of prostaglandin E2 (PGE2), which causes local vasodilation and oedema, sensitisation of nerve fibres, and the characteristic nocturnal pain pattern.
Why Night Pain?
- Circadian rhythm of prostaglandin production
- Decreased cortisol levels at night (anti-inflammatory)
- Venous stasis during recumbency increases pressure
Why NSAID Relief?
- COX inhibitors block prostaglandin synthesis
- Aspirin particularly effective (irreversible COX inhibition)
- Relief typically within 20-30 minutes
Location Distribution
| Location | Percentage | Clinical Features |
|---|---|---|
| Femur | 30-35% | Proximal femur/femoral neck common |
| Tibia | 20-25% | Diaphysis most frequent |
| Spine | 10-15% | Posterior elements, painful scoliosis |
| Hand/foot | 10% | Phalanges, atypical features |
| Humerus | 5-10% | Similar to femur/tibia |
| Other | 10-20% | Any bone possible |
Anatomical Subtypes:
- Cortical (75%): Dense reactive sclerosis, classic presentation
- Cancellous (15%): Less sclerosis, more difficult to identify
- Subperiosteal (10%): Joint pain, synovitis, minimal sclerosis
The cortical type produces the classic radiographic appearance with extensive reactive sclerosis around a small nidus.
Classification Systems
Anatomical Classification
Anatomical Classification
| Type | Location | Percentage | Clinical Features |
|---|---|---|---|
| Cortical | Diaphyseal cortex | 75% | Classic presentation, dense sclerosis, night pain |
| Cancellous | Metaphysis/epiphysis | 15% | Less sclerosis, harder to diagnose |
| Subperiosteal | Under periosteum | 10% | Joint effusion, synovitis, minimal sclerosis |
| Intra-articular | Within joint capsule | Subset of above | Hip most common, mimics synovitis |
The anatomical classification has important treatment implications. Cortical lesions are ideal for RFA while intra-articular and subperiosteal lesions may require arthroscopic or open excision to protect articular cartilage.
Clinical Assessment
History
Classic Presentation:
- Night pain that wakes the patient from sleep
- Pain relieved by aspirin or NSAIDs within 20-30 minutes
- Dull, aching pain gradually increasing over months
- Well-localised pain (can point with one finger)
History Taking Pearls
Ask specifically about: time of pain (worse at night), NSAID response (dramatic relief), duration of symptoms (usually under 2 years), and any prior imaging studies.
Atypical Presentations:
- Intra-articular: Joint effusion, stiffness, mimics synovitis
- Spinal: Painful scoliosis in adolescents (concavity towards lesion)
- Near growth plate: Growth disturbance possible
- Hand/foot: Swelling, fusiform soft tissue enlargement
Physical Examination
Standard Findings:
- Point tenderness over the lesion
- Possible soft tissue swelling or warmth
- May have muscle wasting from chronic pain
- Usually normal range of motion (unless intra-articular)
Specific Location Findings:
| Location | Examination Findings |
|---|---|
| Femoral neck | Hip pain, limited internal rotation, antalgic gait |
| Tibial diaphysis | Localised tenderness, may feel cortical thickening |
| Spine | Paravertebral muscle spasm, painful scoliosis |
| Intra-articular hip | Effusion, limited ROM, synovitis signs |
Differential Diagnosis
Differential Diagnosis
| Diagnosis | Key Differentiating Features | Investigation Findings |
|---|---|---|
| Osteoblastoma | Larger nidus over 2cm, less pain relief with NSAIDs, can be aggressive | CT: nidus over 2cm, may have expansion |
| Brodie abscess | History of infection, systemic symptoms possible, different pain pattern | MRI: rim enhancement, sequestrum possible |
| Stress fracture | Activity-related pain, improves with rest, recent increase in activity | MRI: linear signal, cortical involvement |
| Ewing sarcoma | Age 5-25, systemic symptoms, permeative pattern, soft tissue mass | MRI: large lesion, aggressive features |
| Osteoid osteoma (confirmed) | Night pain, NSAID relief, nidus under 2cm on CT | CT: small nidus with reactive sclerosis |
Investigations
Imaging Algorithm
Step 1: Plain Radiographs
- May be normal in early cases or cancellous locations
- Cortical thickening with central lucency (nidus)
- Variable sclerosis depending on location and duration
Step 2: CT Scan (Gold Standard)
- Defines nidus size, location, and calcification
- Essential for treatment planning (RFA trajectory)
- Thin-slice acquisition (1-2mm) through affected area
Step 3: MRI (Supplementary)
- Shows extensive marrow and soft tissue oedema
- May overestimate lesion extent (appears aggressive)
- Useful for intra-articular and spinal lesions
- Nidus may be obscured by reactive changes
Step 4: Bone Scan (If diagnosis uncertain)
- Very sensitive (nearly 100%)
- Double-density sign: focal uptake within larger area of uptake
- Useful for screening multiple sites or occult lesion
Imaging Characteristics
Imaging Modality Comparison
| Modality | Sensitivity | Key Findings | Limitations |
|---|---|---|---|
| Plain radiograph | 75% | Cortical thickening, central lucency | May miss early/cancellous lesions |
| CT scan | Over 95% | Nidus with variable calcification, reactive sclerosis | Radiation exposure |
| MRI | 65-85% | Extensive oedema, nidus low signal | May obscure nidus, appears aggressive |
| Bone scan | Over 95% | Double-density sign, intense focal uptake | Non-specific, poor anatomical detail |
CT is Diagnostic
Always request CT if osteoid osteoma suspected. MRI alone may be misleading due to extensive oedema that can mimic infection or malignancy. The nidus may be obscured on MRI.
Laboratory Studies
Laboratory investigations are typically normal. FBC, ESR, CRP, and alkaline phosphatase are usually within normal limits. Laboratory tests mainly serve to exclude infection (elevated WCC, CRP, ESR in Brodie abscess) rather than to diagnose osteoid osteoma.
Management Algorithm
Treatment Decision Tree
Step 1: Confirm Diagnosis
- Clinical history (night pain, NSAID relief)
- CT scan showing nidus under 2cm
Step 2: Assess Location
- Standard location: Proceed to RFA
- Special location (spine, intra-articular, physis): Consider alternatives
Step 3: Patient Factors
- Preference for non-surgical: Trial of NSAIDs (lesion may resolve in 2-5 years)
- Requires definitive treatment: RFA or surgical excision
Treatment Options Summary
| Option | Success Rate | Recovery | Best For |
|---|---|---|---|
| RFA | Over 90% | 1-2 weeks | Most cortical lesions |
| Surgical excision | 90-95% | 4-8 weeks | RFA failure, physis, articular |
| NSAIDs | Variable | N/A | Patient preference, observation |
The algorithm guides treatment selection based on lesion characteristics, location, and patient factors.
Surgical Technique
CT-Guided Radiofrequency Ablation
Pre-operative Planning:
- Review CT for optimal trajectory
- Identify safe corridor avoiding neurovascular structures
- Position patient for access to lesion
Anaesthesia:
- General anaesthesia preferred (patient comfort)
- Local anaesthesia possible in cooperative patients
- Periosteal local anaesthetic injection essential if LA
Procedure Steps:
Step 1: Patient Positioning Position patient to allow CT scanner access and comfortable probe trajectory. Mark skin entry point based on planning CT.
Step 2: Sterile Preparation Standard sterile technique. Drape to allow CT imaging during procedure.
Step 3: Access Needle Placement Under intermittent CT guidance, advance coaxial needle to the cortex overlying the nidus. Confirm position on CT.
Step 4: Cortical Penetration Use hand drill or powered drill to penetrate cortex. Advance to nidus but not through it. Confirm position on CT.
Step 5: Biopsy (Optional) Core biopsy through needle for histological confirmation if diagnosis uncertain.
Step 6: RFA Probe Placement Insert RFA probe through coaxial system. Position tip at centre of nidus. Confirm on CT imaging.
Step 7: Ablation Heat to 90 degrees Celsius for 4-6 minutes. Monitor temperature and impedance.
Step 8: Probe Removal Allow probe to cool before removal. Remove coaxial system. Apply simple dressing.
This technique achieves complete nidus destruction while preserving surrounding bone.
Post-operative Protocol
After RFA:
- Weight bearing as tolerated immediately
- Simple analgesia for 24-48 hours
- Return to normal activity within 1 week
After Surgical Excision:
- Weight bearing dependent on bone removed
- May require protected weight bearing if significant cortex removed
- Activity restriction 4-6 weeks for bone healing
Complications
RFA Complications
RFA Complications
| Complication | Incidence | Prevention/Management |
|---|---|---|
| Skin burn | Under 5% | Maintain adequate distance from skin, use cooling |
| Nerve injury | Under 2% | Maintain over 1cm from nerves, use monitoring |
| Fracture | Under 1% | Avoid excessive cortical destruction, protect post-procedure |
| Recurrence | 5-10% | Ensure complete nidus ablation, repeat RFA if needed |
| Cartilage damage | Under 2% | Avoid RFA near articular surface, use chondroprotection |
| Infection | Under 1% | Sterile technique, prophylactic antibiotics |
Surgical Complications
| Complication | Incidence | Risk Factors |
|---|---|---|
| Fracture | 5-10% | Large cortical window, weight-bearing bone |
| Recurrence | 5-15% | Incomplete excision |
| Infection | Under 2% | Standard surgical risk |
| Nerve injury | Under 2% | Deep location, poor visualisation |
| Stiffness | Variable | Intra-articular location |
Managing Recurrence
Recurrence typically occurs within 2 years of treatment. Assessment includes return of typical night pain pattern and repeat CT to confirm residual or recurrent nidus.
Treatment Options:
- Repeat RFA (successful in most cases)
- Surgical excision if RFA fails twice
- En bloc excision for definitive cure
Persistent or recurrent pain after treatment requires repeat imaging. Do not assume treatment failure without confirming residual nidus on CT.
Postoperative Care
After RFA
Day 0-1:
- Discharge same day or next morning
- Weight bear as tolerated
- Simple analgesia (paracetamol, NSAIDs)
Week 1-2:
- Wound check at 1 week
- Return to normal activities
- No activity restrictions
Long-term:
- Clinical follow-up at 6 weeks
- CT only if symptoms persist/recur
- Discharge if asymptomatic at 6 months
After Surgical Excision
Immediate:
- Weight bearing status depends on extent of resection
- Minor cortical window: Weight bear as tolerated
- Significant resection: Protected weight bearing 4-6 weeks
Week 1-6:
- Wound management standard
- Physiotherapy if needed
- Monitor for fracture if significant bone removed
Long-term:
- Clinical review at 6 weeks, 3 months, 6 months
- Imaging only if symptoms recur
- Bone grafting site assessment if applicable
Return to Activity
| Treatment | Return to Work | Return to Sport |
|---|---|---|
| RFA | 1-3 days | 1-2 weeks |
| Surgical (small) | 1-2 weeks | 4-6 weeks |
| Surgical (large) | 2-4 weeks | 8-12 weeks |
RFA Recovery
One of the major advantages of RFA is rapid recovery. Most patients are back to full activity within 1-2 weeks compared to 6-12 weeks for surgical excision.
Outcomes and Prognosis
Treatment Outcomes
RFA Outcomes:
- Primary success rate: 90-95%
- Recurrence rate: 5-10%
- Recurrence usually within 2 years
- Repeat RFA success: 85-90%
Surgical Excision Outcomes:
- Success rate: 88-100%
- Recurrence: 5-15%
- Higher morbidity but definitive
Prognostic Factors
Favourable Factors:
- Cortical location (best RFA access)
- Complete nidus ablation/excision
- Small nidus size (under 1cm)
Less Favourable:
- Cancellous or intra-articular location
- Large nidus (approaching 2cm)
- Spinal location (technical challenges)
Long-term Prognosis
The prognosis is excellent. Osteoid osteoma is a benign lesion with no risk of malignant transformation. After successful treatment, long-term cure is expected in over 95% of cases. Untreated lesions will eventually spontaneously resolve in 3-6 years.
Functional Outcomes
After RFA:
- Full return to pre-morbid function
- No long-term restrictions
- Excellent patient satisfaction
After Surgical Excision:
- Generally full recovery
- May have residual bone defect on imaging
- Rarely causes functional limitation
Evidence and Guidelines
RFA vs Surgical Excision
- RFA success rate 89-95% in pooled analysis
- Surgical excision success 88-100%
- RFA has lower morbidity and faster recovery
- No significant difference in recurrence rates
RFA Safety Near Nerves
- Safe within 1cm of nerves with monitoring
- Thermal injury rare with appropriate technique
- Neuromonitoring recommended for spinal lesions
- Recovery expected even if transient deficit
Natural History
- Most osteoid osteomas resolve spontaneously
- Average time to resolution 3-5 years
- Some cases persist beyond 6 years
- Regression due to nidus maturation
MRI vs CT Accuracy
- CT sensitivity over 95% for nidus detection
- MRI sensitivity 65-85%
- MRI may obscure nidus due to oedema
- MRI useful for intra-articular lesions
Long-term Outcomes After RFA
- 94% success rate at mean 5-year follow-up
- Recurrence usually within 2 years
- Repeat RFA effective for recurrence
- Excellent patient satisfaction scores
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Classic Presentation
"A 16-year-old male presents with 6 months of left thigh pain. Pain is worse at night and wakes him from sleep. He finds that ibuprofen helps significantly. Examination shows point tenderness over the mid-femur. X-rays show cortical thickening with a central lucency."
Atypical Presentation - Hip
"A 22-year-old female presents with 9 months of right hip pain and stiffness. She has been treated for hip synovitis with NSAIDs which provided significant relief. MRI shows extensive bone marrow oedema in the femoral neck with joint effusion. No clear lesion is identified."
Spinal Osteoid Osteoma
"A 14-year-old female presents with progressive painful scoliosis over 18 months. Pain is worse at night. Examination shows a 20-degree left thoracic curve with paravertebral muscle spasm. There is no neurological deficit."
MCQ Practice Points
Size Distinction
Q: What is the key size criterion distinguishing osteoid osteoma from osteoblastoma?
A: Osteoid osteoma nidus is under 2cm while osteoblastoma is over 2cm. Both lesions are histologically identical (osteoid and woven bone surrounded by vascular connective tissue), so size is the primary distinguishing feature. Osteoblastoma has higher recurrence rate and potential for malignant transformation.
Imaging Modality
Q: What is the gold standard imaging for osteoid osteoma diagnosis?
A: CT scan is the gold standard - it shows the nidus with surrounding reactive sclerosis clearly. MRI can obscure the nidus due to extensive bone marrow oedema. Bone scan shows "double density" sign but is non-specific. Always order CT if clinical suspicion is high, even with negative MRI.
Treatment
Q: What is the first-line treatment for osteoid osteoma?
A: CT-guided radiofrequency ablation (RFA) is first-line treatment with over 90% success rate. It is minimally invasive, performed as day case, and allows immediate weight bearing. Recurrence rate is 5-10%. Surgical excision is reserved for RFA failure, intra-articular lesions near cartilage, or spinal lesions near neural structures.
Painful Scoliosis
Q: An adolescent presents with painful scoliosis that is worse at night. What diagnosis should be excluded?
A: Osteoid osteoma in the posterior spinal elements. Painful scoliosis in adolescence is osteoid osteoma until proven otherwise (idiopathic scoliosis is painless). The lesion is typically on the concave side of the curve. Early treatment allows curve resolution; curves present over 18 months may become structural.
Classic Presentation
Q: What is the classic clinical presentation of osteoid osteoma?
A: The classic triad is: (1) Night pain that wakes the patient from sleep, (2) NSAID relief - pain significantly improves with aspirin or ibuprofen (due to prostaglandin inhibition), (3) Young male aged 10-25 years. The mechanism is prostaglandin production by the nidus causing pain and local vasodilation.
Australian Context
Epidemiology
Osteoid osteoma occurs in Australian populations with similar demographics to international data. The condition predominantly affects young males aged 10-25 years with a 2:1 male predominance. Lower extremity involvement (femur and tibia) accounts for most cases.
Management Considerations
Australian orthopaedic practice follows international guidelines with RFA as first-line treatment for most osteoid osteomas. RFA is available in major metropolitan centres through interventional radiology services. Rural and remote patients may require transfer to tertiary centres for treatment. Surgical excision remains an option when RFA is not available or has failed.
Referral Pathways
Suspected osteoid osteoma should be referred to an orthopaedic surgeon or musculoskeletal oncology unit for definitive diagnosis and treatment planning. Multidisciplinary care involving interventional radiology is standard for RFA procedures. Spinal lesions typically involve neurosurgical or spinal surgery consultation.
OSTEOID OSTEOMA
High-Yield Exam Summary
Key Diagnosis
- •Nidus under 2cm differentiates from osteoblastoma
- •Night pain relieved by NSAIDs (prostaglandin mechanism)
- •CT is gold standard - MRI may obscure nidus
- •Painful scoliosis = search for nidus on concave side
Treatment Algorithm
- •RFA first-line treatment - over 90% success rate
- •Spontaneous resolution possible in 3-5 years
- •Surgical excision if RFA fails or near critical structures
RFA Surgical Steps
- •CT guidance for probe trajectory planning
- •Coaxial needle to cortex, drill through
- •Position RFA probe at nidus centre
- •Ablate at 90 degrees for 4-6 minutes
- •Confirm position on post-procedure CT
Common Mistakes
- •Relying on MRI alone - always get CT
- •Confusing with osteoblastoma (size over 2cm)
- •Performing RFA near cartilage without protection
- •Missing spinal lesion in painful scoliosis
Exam Triggers
- •Night pain in young patient = think osteoid osteoma
- •NSAID-responsive bone pain = prostaglandin source
- •Painful scoliosis = search for nidus on concave side
- •CT showing nidus under 2cm = osteoid osteoma (not osteoblastoma)