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: Primary bone sarcomas are rare (approximately 8-9 per million children/adolescents per year worldwide); osteosarcoma and Ewing sarcoma account for the majority requiring growing reconstruction
• 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

When a skeletally immature child needs a growing or growth-equivalent reconstruction, the expandable endoprosthesis is one of several alternatives. Examiners expect you to compare them and justify your choice.

Unresolved questions for the literature:

• No randomised data compare expandable endoprosthesis with rotationplasty or biological reconstruction; all comparisons are Level IV.
• Patient-reported and long-term quality-of-life outcomes are inconsistently reported, limiting true device comparison.
• Optimal antibiotic prophylaxis duration for these high-risk implants is still being defined (informed by broader tumour-endoprosthesis infection trials).

Global Epidemiology

Primary bone sarcomas of childhood and adolescence are rare worldwide (approximately 8-9 per million children/adolescents annually). Osteosarcoma and Ewing sarcoma dominate, with peak incidence during the pubertal growth spurt. The proportion needing a growing reconstruction is driven by age at presentation and tumour site - distal femoral and proximal tibial lesions most often require growth-accommodating implants because they straddle the most active lower-limb physes.

Side-by-Side Guidance

Across all major bodies the principles converge: referral to a specialist sarcoma centre, MDT decision-making, R0 resection before reconstruction, and shared decision-making with family. Genuine differences are organisational (network structure) rather than oncological.

Registry and Outcome Notes

There is no dedicated international registry for paediatric expandable prostheses; evidence is dominated by single-centre and multi-institutional case series (Level IV). Adult tumour-endoprosthesis registries and the Henderson failure-mode classification (soft tissue, aseptic loosening, structural, infection, tumour progression) are widely applied to standardise reporting. Collaborative groups (e.g. PARITY trial network for infection prophylaxis) are improving the evidence base.

High- vs Limited-Resource Practice Variation

• Well-resourced centres: Non-invasive magnetic/electromagnetic systems (REPIPHYSIS, JTS, Stanmore non-invasive) with outpatient lengthening; 3D-printed custom implants; routine MDT and long-term surveillance.
• Limited-resource settings: Higher reliance on modular/minimally invasive expansion or non-expandable reconstruction; rotationplasty and amputation remain important, durable, cost-effective options where growing implants or repeated lengthening logistics are not feasible.
• Universal principle: Implant choice should match the local capacity for repeated follow-up, expansion, and revision - a sophisticated implant without reliable surveillance can underperform a simpler, durable reconstruction.