OSTEOCHONDROMA
Most Common Benign Bone Tumor | Cartilage-Capped Exostosis | Malignant Transformation Risk 1%
TYPES
Critical Must-Knows
- Most common benign bone tumor - represents 35% of all benign bone lesions
- Stops growing at skeletal maturity - continued growth after maturity suggests malignancy
- Cartilage cap under 2cm in adults is reassuring; greater than 2cm raises malignancy concern
- HME has autosomal dominant inheritance with 5-25% malignant transformation risk
- Surgical indications: pain, neurovascular compromise, cosmesis, or suspected malignancy
Examiner's Pearls
- "Examiners ask about continued growth after skeletal maturity - this is sarcomatous change until proven otherwise
- "Know imaging features of malignancy: cartilage cap greater than 2cm, irregular calcification, soft tissue mass
- "EXT1 and EXT2 mutations cause hereditary multiple exostoses (HME)
- "Distinguish from parosteal osteosarcoma - osteochondroma has cortical and medullary continuity
Clinical Imaging
Imaging Gallery
Critical Osteochondroma Exam Points
Red Flags for Malignancy
Growth after skeletal maturity. Any osteochondroma that continues enlarging after physeal closure must be biopsied. Also suspect if cartilage cap greater than 2cm on MRI in adults.
HME vs Solitary
Hereditary Multiple Exostoses: Autosomal dominant (EXT1/EXT2). 5-25% malignant transformation vs 1% for solitary lesions. Screen annually with clinical exam.
Imaging Diagnosis
Cortical and medullary continuity with parent bone. Cartilage cap visible on MRI. No need for biopsy unless malignancy suspected.
Surgical Indications
4 main indications: Pain, neurovascular compression, cosmetic deformity, or concern for malignant transformation. Excise with cartilage cap intact.
Quick Decision Guide
| Scenario | Cap Thickness | Management | Key Pearl |
|---|---|---|---|
| Child, asymptomatic, small lesion | Under 2cm | Observation only | Will stop growing at skeletal maturity |
| Adolescent, pain, mechanical symptoms | Under 2cm | Surgical excision | Remove entire cartilage cap to prevent recurrence |
| Adult, lesion growing, cap greater than 2cm | Greater than 2cm | MRI + biopsy + wide excision | Suspect secondary chondrosarcoma |
EXOSTOSISFeatures of Osteochondroma
Memory Hook:Think EXOSTOSIS - this IS the name for the bony outgrowth! Growth STOPS at skeletal maturity unless malignant.
PNCCIndications for Surgical Excision
Memory Hook:PNCC - Pain, Nerves/vessels, Cosmetic, Cancer worry - these are your reasons to operate.
CAPSRed Flags for Malignant Transformation
Memory Hook:Think CAPS - the cartilage CAP is the key! Thick caps, growing caps after maturity = secondary chondrosarcoma.
Overview and Epidemiology
Clinical Significance
Osteochondroma is the most common benign bone tumor, accounting for 35% of all benign osseous lesions and 8-9% of all bone tumors. It represents a developmental anomaly arising from aberrant cartilage at the growth plate periphery, not a true neoplasm. The lesion grows by enchondral ossification and ceases growth at skeletal maturity - any growth after physeal closure is highly suspicious for malignant transformation to secondary chondrosarcoma.
Demographics
- Age: 10-20 years (during active growth)
- Gender: Male predominance 2:1
- Location: Metaphysis of long bones (70%)
- Most common sites: Distal femur, proximal tibia, proximal humerus
Natural History
- Growth pattern: Enlarges during skeletal growth
- Maturity: Stops growing when physis closes
- Recurrence: 2% if incompletely excised
- Transformation: 1% solitary, 5-25% in HME
Pathophysiology and Genetics
Developmental Anomaly
Osteochondroma arises from aberrant cartilage that herniates through a defect in the perichondral ring of the growth plate. This displaced cartilage maintains growth potential and undergoes enchondral ossification, forming a cartilage-capped bony projection that is continuous with the underlying bone cortex and medullary cavity.
Not a True Neoplasm
Osteochondroma is considered a developmental error rather than a true tumor. The lesion grows by the same mechanism as normal physeal growth and stops enlarging at skeletal maturity. This distinguishes it from neoplastic processes that continue growing regardless of skeletal age.
Genetic Basis of HME
EXT1 and EXT2 Mutations
- Location: EXT1 on chromosome 8q24; EXT2 on 11p11-13
- Function: Encode glycosyltransferases for heparan sulfate synthesis
- Inheritance: Autosomal dominant with high penetrance
- Loss of heterozygosity: Required for lesion development
Clinical Implications
- Screening: First-degree relatives of HME patients
- Malignancy risk: 5-25% in HME vs 1% solitary
- Growth disturbance: Forearm and lower leg deformities
- Genetic counseling: 50% transmission risk to offspring
Malignant Transformation
Secondary chondrosarcoma develops in 1% of solitary osteochondromas and 5-25% of HME cases. Transformation typically occurs in adulthood and is heralded by renewed growth, increasing pain, or enlarging cartilage cap. The chondrosarcoma is usually low-grade (Grade 1), but wide excision is required for cure.
Classification and Morphology
Morphological Classification
| Type | Description | Location | Clinical Notes |
|---|---|---|---|
| Pedunculated | Narrow stalk with bulbous cap | Metaphysis of long bones | Easier to excise, less deformity |
| Sessile | Broad-based attachment | Flat bones (scapula, pelvis) | Higher malignancy risk due to thicker cap |
Clinical Distinction
Pedunculated lesions are more common in long bones and point away from the adjacent joint due to muscle traction during growth. Sessile lesions have broader bases and are more common in flat bones (pelvis, scapula) - these carry slightly higher malignant transformation risk due to typically thicker cartilage caps.
Clinical Presentation
History
- Painless mass: Most common presentation (found incidentally)
- Mechanical pain: Impingement on adjacent structures
- Snapping: Tendon or muscle irritation over lesion
- Fracture: Pedunculated stalk fracture (rare, usually painless)
- Growth concerns: Parents notice enlarging mass in child
Physical Examination
- Palpable mass: Firm, non-tender, fixed to bone
- Size: Variable (1-10 cm typical)
- Location: Metaphyseal region of long bones
- Neurovascular: Check for compression symptoms
- Joint motion: Assess for mechanical block
Red Flag Symptoms (Malignant Transformation)
Suspect Secondary Chondrosarcoma
Any of these findings in an adult with known osteochondroma requires urgent imaging and possible biopsy:
- New or increasing pain not explained by mechanical factors
- Growth after skeletal maturity - lesion should be quiescent after physeal closure
- Rapidly enlarging mass - suggests aggressive cartilage proliferation
- Neurovascular symptoms - new onset compression or ischemia
Complications of Osteochondroma
Local Complications
| Complication | Mechanism | Management |
|---|---|---|
| Bursa formation | Friction over bony prominence | Excision of lesion if symptomatic |
| Neurovascular compression | Mass effect on adjacent structures | Surgical decompression/excision |
| Fracture through stalk | Trauma to pedunculated lesion | Usually non-operative, excision if symptomatic |
| Limb length discrepancy | Physeal tethering in HME | Guided growth or osteotomy |
Imaging and Diagnosis
Plain Radiographs
Radiographic Features
Cortex of lesion is continuous with cortex of parent bone. The medullary cavity of the lesion also merges with the host bone medullary canal. This is the diagnostic hallmark.
Cartilage cap is not visible on plain X-ray unless it contains calcifications. Cap thickness can only be assessed with MRI.
Pedunculated lesions point away from the nearest joint due to muscle pull during growth. Sessile lesions are broad-based.
Punctate, irregular calcifications within soft tissue suggest malignant transformation. Benign caps show organized ring-and-arc calcification if any.
MRI - Gold Standard for Cap Assessment
Benign Features
- Cartilage cap under 2cm in adults (under 3cm in children acceptable)
- Smooth, regular cap with homogeneous signal
- No soft tissue mass beyond cartilage cap
- T2 high signal in cartilage (normal hyaline cartilage)
Malignant Features
- Cartilage cap greater than 2cm in adults (suspicious)
- Irregular, nodular cap with heterogeneous signal
- Soft tissue mass extending beyond cap
- Destruction of underlying bone cortex
The 2cm Rule
Cartilage cap thickness greater than 2cm in a skeletally mature patient is concerning for malignant transformation. This should prompt biopsy. In children and adolescents, caps up to 3cm may be acceptable as the cartilage is still actively growing. Serial MRI is useful to document stability.
CT Scan
Indications for CT:
- Assessment of cortical breach or bone destruction
- Preoperative planning for complex anatomy (pelvis, scapula)
- Evaluation of calcification pattern in cartilage cap
CT is less useful than MRI for measuring cartilage cap but superior for bone detail.
Differential Diagnosis
| Condition | Key Distinguishing Feature | Imaging Clue |
|---|---|---|
| Parosteal Osteosarcoma | NO cortical/medullary continuity | Lesion wraps around bone, distinct from cortex |
| Myositis Ossificans | Zonal phenomenon (mature periphery) | History of trauma, maturation pattern |
| Periosteal Chondroma | Scalloping of cortex, no continuity | Small, often in hand/foot |
Pathology
Gross Pathology
The lesion consists of a cartilage cap (hyaline cartilage) overlying a bony stalk. The stalk is composed of normal trabecular bone and marrow, continuous with the parent bone. The cartilage cap is covered by a fibrous perichondrium.
Thickness: Cap is typically 3-10 mm in adults. Pediatric caps can be up to 20-30 mm during active growth.
Histology
Benign Histology
- Hyaline cartilage cap: Chondrocytes in lacunae
- Perichondrium: Fibrous covering over cartilage
- Enchondral ossification: Active at base of cap during growth
- Normal bone: Trabecular bone and marrow in stalk
Malignant Histology
- Hypercellularity: Increased chondrocyte density
- Nuclear atypia: Enlarged, hyperchromatic nuclei
- Myxoid change: Degeneration of cartilage matrix
- Permeative growth: Invasion into adjacent soft tissue
Biopsy Pitfalls
Biopsy of the cartilage cap is challenging - active enchondral ossification in pediatric patients can show cellular atypia that mimics low-grade chondrosarcoma. Correlation with imaging (cap thickness) and clinical features (age, growth pattern) is essential. Biopsy is only indicated if malignancy is suspected.
Management Algorithm
Observation Protocol
Indications for observation:
- Asymptomatic lesion in child or adolescent
- Small size with no neurovascular compromise
- Typical imaging appearance (cortical continuity, thin cap)
- No growth after skeletal maturity
Surveillance Schedule
Clinical examination yearly during growth. X-ray if symptoms develop or size changes noted.
Once growth plates close and lesion is stable, discharge with instructions to return if new symptoms develop.
Annual clinical exam for life. Baseline MRI of any large or symptomatic lesions. Repeat MRI if growth or pain develops.
When to Stop Surveillance
For solitary osteochondroma, once skeletal maturity is reached and the lesion is asymptomatic, no further routine follow-up is needed. Educate patient to return if new growth or pain. For HME, lifelong clinical surveillance is recommended due to higher malignancy risk.
Management of Malignant Transformation
Secondary Chondrosarcoma Management
| Stage | Treatment | Margin Goal |
|---|---|---|
| Low-grade (Grade 1) | Wide excision with negative margins | Wide margin (5-10mm normal tissue) |
| High-grade (Grade 2-3) | Wide excision +/- adjuvant therapy | Wide margin, consider limb salvage vs amputation |
Prognosis: Low-grade secondary chondrosarcoma has excellent prognosis with wide excision - 90% 5-year survival. High-grade lesions have poorer outcomes and may require chemotherapy/radiation (though chondrosarcoma is relatively chemo-resistant).
Outcomes and Prognosis
Surgical Outcomes
Recurrence: 2% if cartilage cap incompletely excised. Nearly 0% if complete excision achieved. Recurrences typically occur within 2 years and may require re-excision.
Complications:
- Neurovascular injury: Rare (under 1%) if careful dissection
- Wound infection: 2-3% (standard surgical site infection rates)
- Pathologic fracture through excision site: Rare, more common with large sessile lesions
Long-term Prognosis
For solitary osteochondroma, prognosis is excellent. Most lesions are asymptomatic and require no treatment. Those requiring excision have excellent outcomes with low recurrence.
For HME, quality of life is impaired by multiple lesions, skeletal deformities (forearm, lower leg), and lifelong malignancy surveillance. Genetic counseling is important for family planning.
Complications
Surgical Complications
Intraoperative
- Neurovascular injury: Risk depends on location; common peroneal nerve at proximal fibula, axillary nerve at proximal humerus
- Incomplete excision: Failure to remove entire cartilage cap leads to recurrence (2%)
- Fracture through stalk: May occur during manipulation of pedunculated lesions
Postoperative
- Recurrence: 2% if cartilage cap incompletely excised
- Wound infection: Standard surgical site infection rates (2-3%)
- Hematoma: Rare; usually self-limiting
- Pathologic fracture: Through weakened bone at excision site (rare)
Disease-Related Complications
Complications of Osteochondroma
| Complication | Mechanism | Management | Prevention |
|---|---|---|---|
| Malignant transformation | Secondary chondrosarcoma develops in cartilage cap | Wide excision with negative margins | Surveillance for red flags; intervene if cap greater than 2cm |
| Bursa formation | Friction over bony prominence creates fluid-filled sac | Excision of lesion if symptomatic | Cannot be prevented; patient education |
| Neurovascular compression | Mass effect on adjacent vessels/nerves | Surgical decompression and excision | Early intervention for enlarging lesions |
| Limb length discrepancy | Physeal tethering in HME | Guided growth or osteotomy | Early recognition and intervention |
| Angular deformity | Forearm and lower leg most common in HME | Corrective osteotomy if functional impairment | Regular monitoring during growth |
Key Complication Pearl
Malignant transformation to secondary chondrosarcoma is the most serious complication. Risk is 1% for solitary lesions and 5-25% for HME. Key indicators: growth after skeletal maturity, new pain in adult, cartilage cap greater than 2cm on MRI. Wide excision is curative for low-grade lesions (90% 5-year survival).
Evidence Base and Key Studies
Natural History of Osteochondroma
- Imaging review of 200 osteochondromas from AFIP registry
- Cartilage cap under 1.5cm in 98% of benign lesions
- Cortical and medullary continuity diagnostic hallmark
- Malignant transformation associated with cap greater than 2cm, irregular mineralization
Hereditary Multiple Exostoses - Genotype-Phenotype
- Review of 212 HME patients with genetic testing
- EXT1 mutations cause more severe phenotype than EXT2
- Malignant transformation in 5-25% of HME patients
- Forearm and lower leg deformities most common skeletal complications
Surgical Excision Outcomes
- Retrospective series of 50 osteochondroma excisions in children
- 2% recurrence rate when entire cartilage cap removed
- Higher recurrence (10%) when cap incompletely excised
- No major neurovascular complications with careful dissection
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Incidental Finding in Adolescent
"A 14-year-old boy presents with a painless mass on his distal femur noticed by his mother. X-ray shows a pedunculated lesion with cortical and medullary continuity with the femur. What is your diagnosis and management?"
Scenario 2: Adult with Growing Lesion
"A 35-year-old woman with known osteochondroma of the proximal humerus since childhood presents with increasing pain and size of the lesion over 6 months. MRI shows cartilage cap thickness of 3.5cm. How do you proceed?"
Scenario 3: HME and Genetic Counseling
"A 10-year-old boy presents with multiple bony prominences around his knees and ankles. His father had similar lesions. What is your diagnosis and how do you manage this patient and family?"
MCQ Practice Points
Most Common Benign Bone Tumor
Q: What is the most common benign bone tumor? A: Osteochondroma - accounts for 35% of all benign bone tumors and 8-9% of all bone tumors. Peak age 10-20 years during active skeletal growth.
Diagnostic Hallmark
Q: What is the pathognomonic imaging feature of osteochondroma? A: Cortical and medullary continuity with the parent bone. The cortex of the lesion is continuous with the cortex of the underlying bone, and the medullary cavity merges with the host bone marrow.
Malignant Transformation Threshold
Q: What cartilage cap thickness raises concern for malignant transformation in an adult? A: Greater than 2cm in a skeletally mature patient. Caps under 2cm are reassuring. In children, caps up to 3cm may be acceptable during active growth.
Genetic Basis of HME
Q: What genes are mutated in Hereditary Multiple Exostoses? A: EXT1 (chromosome 8q24) and EXT2 (chromosome 11p11-13). These genes encode glycosyltransferases required for heparan sulfate synthesis. Inheritance is autosomal dominant.
When Lesion Stops Growing
Q: When does osteochondroma stop growing? A: At skeletal maturity when the growth plates close. Any growth after physeal closure is concerning for malignant transformation to secondary chondrosarcoma.
Recurrence Prevention
Q: What is essential during surgical excision to prevent recurrence? A: Complete removal of the entire cartilage cap with the base of the stalk. Recurrence rate is 2% with incomplete excision, nearly 0% with complete excision.
Australian Context
Tertiary Referral
- Peter MacCallum Cancer Centre (Victoria) for complex sarcoma cases
- Chris O'Brien Lifehouse (NSW) for limb salvage surgery
- Royal Children's Hospital (Victoria) for pediatric HME management
- Multidisciplinary sarcoma tumor boards recommended for malignant transformation
Genetic Services
- Victorian Clinical Genetics Services (VCGS) for HME genetic counseling
- NSW Health Genetics for EXT mutation testing
- Medicare rebates available for genetic testing in confirmed HME families
- Family screening recommended for first-degree relatives
Medicolegal Considerations
Consent discussion for surgical excision must include:
- Risk of incomplete excision and recurrence (2%)
- Neurovascular injury risk (site-specific)
- Risk of malignancy if cap greater than 2cm (document pre-op imaging)
- For HME patients: counsel about lifelong surveillance and genetic implications
Documentation: Record cartilage cap thickness on MRI in adults. Document rationale for observation vs surgery. For HME, document family history and genetic counseling offered.
OSTEOCHONDROMA
High-Yield Exam Summary
Key Facts
- •Most common benign bone tumor (35% of all benign bone lesions)
- •Peak age 10-20 years, male 2:1
- •Metaphysis of long bones (distal femur, proximal tibia most common)
- •Stops growing at skeletal maturity - growth after = malignancy
Diagnosis
- •Cortical and medullary continuity = pathognomonic
- •Cartilage cap under 2cm in adults (under 3cm in children)
- •MRI gold standard for cap assessment
- •No biopsy needed unless malignancy suspected
Malignant Transformation
- •Solitary: 1% risk; HME: 5-25% risk
- •Red flags: growth after maturity, cap greater than 2cm, new pain
- •Secondary chondrosarcoma (usually low-grade)
- •Wide excision curative - 90% 5-year survival for low-grade
Surgical Indications (PNCC)
- •Pain (mechanical or mass effect)
- •Neurovascular compromise
- •Cosmetic deformity
- •Concern for malignancy (cap greater than 2cm, growth)
HME
- •EXT1/EXT2 mutations (autosomal dominant)
- •Lifelong surveillance required (annual clinical exam)
- •Manage skeletal deformities (forearm, lower leg)
- •Genetic counseling - 50% transmission risk
Surgical Pearls
- •Remove ENTIRE cartilage cap to prevent recurrence
- •Recurrence 2% if incomplete, 0% if complete excision
- •Send entire specimen to pathology intact
- •If malignancy suspected, frozen section before closure