Expandable Prostheses in Pediatric Oncology

Expandable endoprostheses represent a critical advancement in pediatric musculoskeletal oncology, enabling limb salvage in skeletally immature patients while accommodating ongoing skeletal growth. Without expandable technology, children with significant remaining growth would face limb length discrepancy greater than 4cm following tumour resection that includes the growth plate.

Key epidemiological context:

• Incidence: Approximately 400 new primary bone sarcomas in children annually in Australia
• Peak age: Osteosarcoma peaks at 15-19 years, Ewing sarcoma at 10-15 years
• Common sites: Distal femur (40%), proximal tibia (20%), proximal humerus (15%)
• Growth implications: Distal femoral physis contributes 70% of femoral length (approximately 10mm/year)

Indications for expandable prosthesis:

• Predicted limb length discrepancy greater than 4cm at skeletal maturity
• Malignant bone tumour requiring physeal resection
• Patient/family acceptance of multiple procedures and prolonged follow-up
• Adequate soft tissue envelope for reconstruction
• Absence of pathological fracture through tumour (relative contraindication)

Physeal Growth Contribution

Accurate prediction of limb length discrepancy is essential for surgical planning. Growth contribution varies by anatomical site:

Lower Limb:

Upper Limb:

The Multiplier Method

The Multiplier Method (Paley) provides the most accurate prediction of limb length discrepancy using skeletal age:

Calculation Steps:

1. Obtain bone age radiograph (left hand and wrist)
2. Determine multiplier value from published tables based on skeletal age and sex
3. Calculate expected remaining growth: Remaining growth = Current limb length x (Multiplier - 1)
4. Predicted LLD = Remaining growth of affected side (which will be zero if physis resected)

Example Calculation:

A 10-year-old boy (skeletal age 10) with osteosarcoma of the distal femur requiring physeal resection. Current femoral length is 35cm. The multiplier for a 10-year-old boy is 1.28.

• Expected remaining femoral growth = 35 x (1.28 - 1) = 9.8cm
• Distal femoral contribution = 70% x 9.8cm = 6.9cm predicted LLD

This patient requires an expandable prosthesis.

Timing of Expansion

Frequency:

• Typically every 3-4 months to match physiological growth
• More frequent in younger children with faster growth rates
• Aim for 5-10mm per lengthening episode

Expansion Limits:

• Individual lengthening: 10-15mm maximum (limited by neurovascular stretch)
• Total expansion per prosthesis: 80-100mm typical capacity
• May require prosthesis exchange if growth exceeds expansion capacity

Pre-operative Evaluation

Oncological Assessment:

1. Tumour staging: MRI of entire bone, CT chest, bone scan/PET
2. Biopsy: Core needle biopsy with tract planning for excision
3. Neoadjuvant chemotherapy response: Assess clinically and radiologically
4. Margins: Plan for wide surgical margins (minimum 2cm from tumour)

Growth and Skeletal Assessment:

1. Bone age radiograph: Left hand and wrist for skeletal age
2. Limb length measurement: Scanogram or CT for accurate measurement
3. Joint range of motion: Document baseline for comparison
4. Multiplier calculation: Predict LLD at skeletal maturity

Patient and Family Assessment:

Imaging

Plain Radiographs:

• Full-length views of affected limb
• Chest X-ray as baseline

MRI:

• Entire bone including skip lesions
• Joint involvement assessment
• Soft tissue extension evaluation
• Neurovascular proximity

CT:

• Chest staging for pulmonary metastases
• 3D planning for custom prosthesis

Prosthesis-Related Complications

Infection Management

Classification:

Mechanical Failure Modes

Non-Invasive Systems:

• Motor burnout (most common)
• Gear mechanism slippage
• Telescoping section jamming
• Electromagnetic coil malfunction

Modular Systems:

• Locking mechanism failure
• Taper corrosion
• Component dissociation

Management:

• Revision to alternative expansion mechanism
• Conversion to non-expandable prosthesis if near maturity
• Custom prosthesis for complex failures

Complication Imaging Gallery

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Long-term Considerations

Growth-Related Issues:

• Angular deformity from asymmetric growth
• Joint contracture limiting function
• Limb length over-correction or under-correction

Prosthesis Longevity:

• Expandable prostheses are not permanent
• Conversion to adult prosthesis at skeletal maturity
• Multiple revisions expected over lifetime

Immediate Post-operative

Days 0-3:

• Inpatient stay for wound monitoring
• DVT prophylaxis (mechanical and pharmacological)
• Drain management
• Pain control (PCA then oral)
• Non-weight bearing initially

Weeks 1-6:

• Protected weight bearing (touch or partial)
• Wound surveillance
• Range of motion exercises
• Physiotherapy referral

Rehabilitation Protocol

Conversion to Adult Prosthesis

Timing:

• At or near skeletal maturity
• When expansion capacity exhausted
• When mechanical failure precludes further expansion

Procedure:

• Elective revision to non-expandable prosthesis
• Often requires longer stem for fixation
• May need bone grafting of expansion gaps
• Joint constraint reassessment

Oncological Outcomes

Local Recurrence:

• 5-10% local recurrence with wide margins
• Higher with inadequate margins
• Surveillance MRI recommended

Overall Survival:

• Osteosarcoma: 65-70% 5-year survival
• Ewing sarcoma: 70-75% 5-year survival
• Chemotherapy response is key prognostic factor

Prosthesis Survival

Functional Outcomes

MSTS Score:

• Average 70-80% of normal
• Lower than non-expandable in adults
• Impact of multiple procedures

Limb Function:

• 85-90% limb salvage success
• Some activity restriction recommended
• Avoid high-impact sports

Quality of Life:

• Generally good with successful limb salvage
• Psychological impact of repeated procedures
• Support services important

Epidemiology

Primary bone sarcomas in children and adolescents are relatively rare in Australia, with approximately 150 new cases of osteosarcoma and Ewing sarcoma annually. The majority of cases occur in patients under 25 years, with peak incidence during adolescence coinciding with the pubertal growth spurt. The need for expandable prostheses is determined by age at presentation and tumour location, with distal femoral and proximal tibial tumours most commonly requiring growth-accommodating reconstructions.

Referral and Management

Paediatric bone sarcoma management in Australia is centralised at major tertiary paediatric oncology centres with dedicated musculoskeletal oncology units. These centres are located in state capital cities and provide multidisciplinary care including paediatric oncology, orthopaedic oncology, radiation oncology, and allied health services. Telemedicine facilitates initial consultation for patients in rural and regional areas, with definitive surgery and chemotherapy typically delivered at the tertiary centre.

Access to expandable prosthesis technology, including non-invasive magnetic expansion systems, is available at major Australian sarcoma centres. Custom prosthesis manufacturing is supported through collaboration with international implant companies. The Therapeutic Goods Administration (TGA) regulates implant availability, with most major expandable systems available under Special Access Scheme or registered for Australian use. Long-term follow-up is coordinated through the treating sarcoma unit, with transition to adult services occurring during late adolescence.