Multiple Hereditary Exostoses
Osteochondromas Throughout the Skeleton
Key Issues
Critical Must-Knows
- EXT1/EXT2: Tumor suppressor genes.
- Malignant Transformation: 1-5% to chondrosarcoma.
- Warning Signs: New growth, pain, size greater than 5cm in adults.
- Forearm Deformity: Common (ulnar shortening).
- Excision: For symptomatic lesions.
Examiner's Pearls
- "EXT1/EXT2 mutations
- "1-5% malignant transformation
- "Pain/growth in adults = concerning
- "Forearm deformity common
Malignant Transformation
Warning signs of malignant transformation to chondrosarcoma:
- New pain in a previously painless lesion.
- Growth after skeletal maturity.
- Size greater than 5cm cartilage cap on imaging.
- Irregular margins, scattered calcifications.
- Obtain MRI. If concerning, biopsy or excise with wide margins.
MHE Features
| Issue | Details | Management |
|---|---|---|
| 1-5% to chondrosarcoma | Wide excision if suspected | |
| Ulnar shortening, radial bowing | Ulnar lengthening, osteotomy | |
| Pain, nerve compression | Excision |
MHE Features
Memory Hook:EMM - EXT, Multiple, Malignant risk.
Malignant Warning Signs
Memory Hook:PGS - Pain, Growth, Size.
Forearm Deformity in MHE
Memory Hook:URD - Ulnar short, Radial bow, Dislocation risk.
Overview/Epidemiology
Multiple Hereditary Exostoses (MHE) is characterized by multiple osteochondromas.
- Genetics: Autosomal dominant. EXT1 or EXT2 mutations (tumor suppressor genes).
- Incidence: 1 in 50,000.
- Pathophysiology: Loss of EXT function leads to abnormal cartilage growth at physes → osteochondromas.
- EXT1 vs EXT2: EXT1 mutations (chromosome 8) typically cause more severe phenotype than EXT2 (chromosome 11).
- Natural History: Lesions grow until skeletal maturity, then stop. Malignancy risk persists lifelong.
Pathophysiology and Mechanisms
Osteochondroma Structure
- Cartilage-capped bony outgrowth arising from the metaphysis.
- Continuous with host bone cortex and medulla.
- Grows away from the adjacent joint (important for diagnosis).
- Cartilage cap: Normally less than 1cm in adults, greater than 2cm concerning for malignancy.
Why Deformity Occurs
- Osteochondromas at the distal ulna → ulnar shortening → radial bowing.
- Tethering effect on growth plate.
- Similar mechanism at ankle (fibular lesions) → ankle valgus.
Common Sites
- Distal femur (most common).
- Proximal tibia.
- Proximal humerus.
- Distal ulna/radius (causes forearm deformity).
Classification Systems
Masada Classification (Forearm Deformity)
- Type I: Distal ulna osteochondroma → ulnar shortening.
- Type IIA: As above + distal radial osteochondroma → radial articular surface tilt.
- Type IIB: As above + subluxation/dislocation of radial head.
- Type III: Proximal radioulnar involvement.
Clinical Assessment
History:
- Age of first lesion.
- Symptomatic lesions (pain, cosmesis, nerve compression).
- Family history.
- Any new pain or growth in adults (malignancy concern).
Physical Exam:
- Palpable Masses: Typically at metaphyses.
- Deformity: Forearm (short ulna), ankle (valgus).
- ROM: Limited by impingement.
- Neurovascular: Peroneal nerve at knee, etc.
Investigations
Imaging:
- X-ray: Multiple osteochondromas, broad-based or pedunculated.
- MRI: If malignancy suspected (cartilage cap thickness greater than 2cm or irregular).
Genetic:
- EXT1/EXT2 testing if diagnosis uncertain.
Management Algorithm
Observation
- Asymptomatic lesions: Watch and wait.
- Regular clinical follow-up.
- Educate on warning signs for malignancy.
Surgical Techniques
Osteochondroma Excision
Indications: Symptomatic lesions (pain, nerve compression, impingement).
Technique:
- Marginal excision at the base.
- Include entire cartilage cap to prevent recurrence.
- Careful of adjacent neurovascular structures.
Pearl: Incomplete excision of cartilage cap leads to recurrence.
Complications
| Complication | Context | Management |
|---|---|---|
| Malignant Transformation | 1-5%, lifelong risk | Surveillance, wide excision |
| Recurrence | Incomplete excision | Complete cartilage cap removal |
| Nerve Injury | Peroneal at knee | Careful dissection |
| Deformity Progression | Childhood growth | Early intervention |
| Vascular Injury | Popliteal region | Pre-op imaging |
Postoperative Care
- Simple Excision: Early mobilization, wound care.
- Forearm Reconstruction: Splinting, protected ROM.
- Lengthening: Fixator care, daily adjustments.
- All Patients: Continue surveillance for other lesions.
Outcomes/Prognosis
- Most lesions remain benign.
- Malignant transformation: 1-5%.
- Significant deformity may limit function.
Evidence Base
- EXT genes and pathogenesis
- Malignant transformation risk
- Surveillance guidelines
- MHE management overview
- Forearm deformity treatment
- Excision for symptoms
- Classification of forearm deformity
- Types I, IIA, IIB, III defined
- Treatment recommendations by type
- Natural history of MHE
- 1-5% malignant transformation rate
- Long-term outcomes
- MHE clinical burden
- Quality of life impact
- Functional outcomes
Viva Scenarios
Practice these scenarios to excel in your viva examination
New Pain in an Osteochondroma
"30-year-old with known MHE. One of his knee lesions has become painful and appears to have grown over the past year."
This is **concerning for malignant transformation**. In an adult with known MHE, new pain and growth are red flags. I would order an **MRI** to assess the cartilage cap thickness (greater than 2cm is concerning) and look for irregular margins. If suspicious, I would perform **wide excision** rather than marginal. Histology is essential. If chondrosarcoma is confirmed, further oncological management is needed.
Forearm Deformity
"10-year-old with MHE has progressive forearm deformity with limited pronation/supination."
Forearm deformity is common in MHE due to osteochondromas affecting the distal ulna, causing **ulnar shortening and radial bowing**. Management: **Excise symptomatic osteochondromas**. If significant deformity, consider **ulnar lengthening** and **radial osteotomy** to restore alignment and forearm rotation. Early intervention may prevent radial head dislocation.
Nerve Compression
"12-year-old with MHE presents with foot drop. X-ray shows large osteochondroma at proximal fibula. How do you manage?"
This is a **peroneal nerve compression** from an osteochondroma at the fibular head. This is a common site for nerve compression in MHE. Management: I would obtain an **MRI** to assess the lesion and relationship to the nerve. Given the neurological deficit, this is an indication for **surgical excision**. I would excise the osteochondroma with careful dissection around the peroneal nerve. Post-op, I would monitor for nerve recovery and consider AFO for foot drop.
MCQ Practice Points
Genetics MCQ
Q: What genes are mutated in MHE? A: EXT1 and EXT2.
Malignancy MCQ
Q: What is the malignant transformation rate? A: 1-5% to chondrosarcoma.
Warning MCQ
Q: What are warning signs for malignancy? A: Pain, growth after skeletal maturity, cartilage cap greater than 2cm.
Forearm Pearl
Q: What causes forearm deformity in MHE? A: Distal ulnar osteochondroma causes ulnar shortening, leading to radial bowing and potential radial head dislocation.
Classification Pearl
Q: What is the Masada classification used for? A: Forearm deformity in MHE. Types I-III based on ulnar shortening, radial deformity, and radial head status.
Nerve Pearl
Q: Which nerve is commonly compressed at the knee in MHE? A: Common peroneal nerve (fibular head osteochondroma).
Australian Context
- Genetic Testing: EXT1/EXT2 available through clinical genetics.
- Multidisciplinary Care: Orthopaedics, genetics, oncology.
- Support Groups: MHE Australia (patient support).
- Surveillance: Regular clinical review, educate on warning signs.
- PBS: No specific medications; surgical management covered.
MULTIPLE HEREDITARY EXOSTOSES
High-Yield Exam Summary
GENETICS
- •EXT1/EXT2
- •Autosomal Dominant
- •Tumor suppressors
- •1 in 50,000
CLINICAL
- •Multiple osteochondromas
- •Metaphyses
- •Forearm deformity
- •Knee most common
MALIGNANCY
- •1-5% transformation
- •Pain in adults
- •Growth after maturity
- •Cap greater than 2cm
TREATMENT
- •Observe if asymptomatic
- •Excise if symptomatic
- •Wide excision if malignant
- •Correct deformity
FOREARM
- •Ulnar shortening
- •Radial bowing
- •Masada classification
- •Early intervention prevents RH dislocation
NERVES
- •Peroneal at fibular head
- •Sciatic at hip
- •Foot drop = excision
- •Pre-op imaging
Self-Assessment Quiz
Differential Diagnosis
Multiple Cartilaginous Lesions:
| Condition | Key Features | Differentiator |
|---|---|---|
| MHE | Multiple osteochondromas | Continuous cortex, metaphyses, EXT mutation |
| Ollier Disease | Multiple enchondromas | Medullary lesions, not cortical |
| Maffucci Syndrome | Enchondromas + hemangiomas | Soft tissue vascular lesions |
| Metachondromatosis | Osteochondromas + enchondromas | Both lesion types present |
| Dysplasia Epiphysealis Hemimelica | Epiphyseal osteochondroma | Single limb, epiphysis involved |
Key Distinguishing Points:
- MHE: Metaphyseal, cortex continuous with host bone.
- Enchondromas (Ollier): Medullary, radiolucent with stippled calcifications.
- DEH (Trevor Disease): Single limb, epiphyseal involvement.
- Malignancy risk: MHE 1-5%, Ollier 25-30%, Maffucci 25-30%.