Expandable (Growing) Endoprosthesis in the Skeletally Immature

Indication. A child or adolescent with an open physis and a high-grade bone sarcoma of a periphyseal location — most often the distal femur, then the proximal tibia and proximal humerus — in whom wide en-bloc resection for cure must remove the adjacent growth plate. After induction chemotherapy has controlled systemic disease, limb salvage with a prosthesis that can grow with the child prevents the crippling limb-length discrepancy that a fixed implant would otherwise leave. The younger the child and the more growth remaining, the greater the eventual discrepancy, and the stronger the case for an expandable device. Assess the whole problem, not just the tumour. Before committing, confirm and quantify:

• Staging — local MRI for marrow extent and extraosseous spread, a CT chest and a bone scan (or whole-body MRI) for metastases, and a confirmed biopsy through a tract that can be excised en bloc with the specimen.
• Predicted limb-length discrepancy — calculate mature limb length and the projected discrepancy from the remaining growth of the contralateral physes (the multiplier method is the modern standard; the Moseley straight-line graph is the classic equivalent).
• The nearest physis and the joint — whether the tumour abuts the joint dictates an intra- versus extra-articular resection, and the soft-tissue envelope available for cover. The one decision that matters. Once limb salvage is chosen, the decision is how to deal with the growth that will be lost:

Consent specifically for a lifelong prosthesis with a high probability of one or more revisions, infection and possible implant removal, stiffness from repeated lengthenings, the possibility of mechanism failure, residual limb-length inequality, and — if chemotherapy or local control fails — the possibility of late amputation. Setup. Supine with a thigh-high tourniquet, a bump under the ipsilateral hip, and the image intensifier available for resection length and stem position. A long utilitarian incision is planned to excise the biopsy tract in continuity. Vascular and reconstructive-microsurgery backup is on standby for the proximal tibia and for large distal-femoral tumours abutting the popliteal vessels.

The goal: resect the tumour widely with the biopsy tract en bloc while protecting the popliteal neurovascular bundle, reconstruct the defect with a constrained (rotating-hinge) expandable prosthesis sized a little short so it can be lengthened through growth, and restore a stable, well-covered limb with a functional extensor mechanism. The distal femur is used below as the index operation; the principles transfer to the proximal tibia and humerus with the named dangers noted in each step.

Rehabilitation | Phase | Timing | Immobilisation & loading | Therapy focus | |-------|--------|--------------------------|---------------| | 1 | 0 to 2 weeks | Cast or locked hinged brace; non-weight-bearing | Pain control, quadriceps sets, ankle pumps | | 2 | 2 to 6 weeks | Hinged knee brace, progressive weight-bearing | Active range of motion, patellar mobilisation, gait re-education | | 3 | 6 to 12 weeks | Brace weaned; full weight-bearing | Knee flexion toward 90 degrees, closed-chain strengthening | | 4 | Through growth | None between lengthenings | Interval physiotherapy around each lengthening episode to recover flexion | Most children return to school within weeks and to low-impact activity by three to six months; contact sport is discouraged. Function is generally good on validated scores (MSTS), but these are lifelong implants: expect at least one revision over the patient's life, and lengthening continues until skeletal maturity equalises the limbs. Complications

Epidemiology. Osteosarcoma and Ewing sarcoma are the commonest primary malignant bone tumours of childhood and adolescence, with osteosarcoma showing a peak incidence in the second decade that mirrors the adolescent growth spurt and frequently presenting in patients with substantial growth remaining. The distal femur is the single most common site, followed by the proximal tibia and proximal humerus — precisely the periphyseal locations where a curative wide resection removes the growth plate that contributes most to limb length. Pathoanatomy. These sarcomas arise in the metaphysis, hard against the growth plate, and the safe oncological margin therefore demands sacrifice of the adjacent physis. The distal femoral physis contributes the greatest share of lower-limb length — roughly nine to ten millimetres of growth per year at peak — and the proximal tibial physis a little less, so loss of either produces a discrepancy that grows with the child: the younger the patient, the larger the final inequality. The biological problem the expandable implant solves is straightforward — restore the length that the lost physis would have produced — but doing so around a tumour resection, a constrained joint and a vulnerable neurovascular bundle is what makes the operation demanding.

Key evidence. Modern outcomes come from large sarcoma-centre cohorts. The Henderson failure-mode classification gave the field a shared language for why these implants fail and showed infection to be the dominant mode in children — the rationale behind non-invasive lengthening. The Stanmore experience with extendible prostheses reported good functional scores but a substantial lifetime revision burden driven by infection and loosening. The Repiphysis and later electromagnetic devices demonstrated that non-invasive lengthening is feasible in clinic, removing the repeated wound breach of minimally invasive devices and their associated infection risk. The multiplier method provides a reproducible, widely adopted way to predict mature limb length and discrepancy from a single current measurement, underpinning the planning that the whole operation depends on.