Endoprosthetic Replacement of the Distal Femur (Megaprosthesis)

Indication. A distal femoral megaprosthesis is used when a segment of distal femur must be resected en bloc — for a primary bone tumour with a clear margin, or for catastrophic bone loss that no conventional implant can reconstruct — and the limb can still be salvaged. The goal is wide local excision of the diseased segment and immediate, stable reconstruction with a modular rotating-hinge prosthesis. Oncologic indications

• Primary malignant bone tumours of the distal femur (osteosarcoma, Ewing sarcoma, chondrosarcoma) in patients with no metastatic disease and a resectable lesion with adequate margins.
• Metastatic disease with a solitary or oligometastatic lesion causing catastrophic bone loss or impending fracture when curettage and augmentation are not feasible.
• Failed limb-salvage reconstruction with a massive allograft or a previous megaprosthesis requiring revision. Non-oncologic indications
• Catastrophic periprosthetic distal femoral fracture or bone loss after total knee arthroplasty where conventional revision implants cannot achieve stable fixation.
• Non-reconstructable distal femoral bone loss from infection, osteolysis, or multiple failed revisions.
• Severe distal femoral deformity or non-union with bone loss exceeding 5 cm. Contraindications. Absolute: active deep infection at the site (must be eradicated first); an inadequate soft-tissue envelope for coverage (requires plastic surgery input); a patient unable to comply with postoperative weight-bearing or surveillance. Relative: the skeletally immature patient (consider an expandable prosthesis or rotationplasty); poor expected survival (less than 6 months), where palliative stabilisation may suffice; severe peripheral vascular disease precluding safe tourniquet use or vessel mobilisation. The one decision that matters — stem fixation. Whatever the indication, the reconstruction itself converges on the same modular rotating-hinge implant. The real choice is how the stems are fixed:

Consent specifically for: infection (the commonest early failure, greater than 10 percent at 5 years) and the possible need for staged revision or even amputation; extensor lag and some loss of knee flexion; a lifelong risk of mechanical hinge failure and reoperation; limb-length discrepancy; and, in oncologic cases, local recurrence and the need for long-term surveillance. Setup. Supine on a radiolucent table with a bump under the ipsilateral buttock; the leg is draped free to allow full flexion and extension. A tourniquet is applied high on the thigh but inflated only after the approach and vessel identification. Image intensifier is positioned for AP and lateral views of the distal femur and knee.

The goal is to expose the distal femur through the extensile medial parapatellar approach, identify and protect the popliteal vessels and sciatic nerve before any tumour mobilisation, achieve a wide margin, reconstruct with a modular rotating-hinge megaprosthesis, and restore the extensor mechanism under appropriate tension. The exposure is laid out in full below (and in depth on the medial parapatellar approach to the knee page).

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Rehabilitation | Phase | Timing | Immobilisation | Therapy | |-------|--------|----------------|---------| | 1 | 0-2 weeks | Hinged knee brace locked in extension | Touch weight-bearing; quadriceps setting and ankle pumps from day 1 | | 2 | 2-6 weeks | Brace locked in extension (4-6 weeks total to protect the extensor repair) | Passive and active-assisted flexion begins at 2 weeks; progressive weight-bearing | | 3 | 6-12 weeks | Brace unlocked | Active flexion — aim for 90 degrees by 8 weeks and 110 degrees by 12 weeks; progressive resistance | | 4 | 3-6 months | No brace | Full weight-bearing; advanced strengthening and proprioception; low-impact activities from 4-6 months | High-impact sports are discouraged permanently because of the risk of periprosthetic fracture and hinge failure. Most long-term survivors regain knee range of motion averaging 90-110 degrees; patients with an extensor lag less than 10 degrees achieve near-normal gait, and Musculoskeletal Tumour Society (MSTS) scores average 70-85 percent of normal. Oncologic surveillance: clinical and radiographic review at 6 weeks, 3 months, 6 months, then annually; MRI of the resection bed and CT chest every 3 months for the first 2 years, then 6-monthly to 5 years; CRP and ESR at every visit for the first 2 years as an infection screen. Complications

Outcomes. Wide resection with megaprosthesis reconstruction achieves local control rates of 85-95 percent at 5 years when negative margins are obtained; overall survival depends on histologic grade and response to neoadjuvant chemotherapy rather than the reconstruction itself. Limb salvage is possible in greater than 90 percent of distal femoral sarcomas with modern techniques. Cemented modular megaprostheses demonstrate 70-80 percent survival at 10 years and 50-60 percent at 20 years, with aseptic loosening and infection the dominant failure modes. Compress (force-directed) fixation with a porous collar shows improved 10-year survival (greater than 85 percent) in selected young patients with good bone stock. Surgical anatomy. The distal femur consists of the metaphysis, the medial and lateral condyles, and the trochlear groove; the flat popliteal surface lies immediately anterior to the popliteal vessels. The linea aspera gives attachment to the adductors and origin to the vastus medialis and lateralis. The epicondyles bear the collateral ligaments, which are sacrificed in a distal femoral replacement. The sciatic nerve lies in the posterior compartment and divides into the tibial and common peroneal nerves at the superior angle of the popliteal fossa; the popliteal artery lies directly on the posterior surface of the distal femur, separated only by a thin layer of fat and the joint capsule; and the genicular arteries form an anastomosis around the knee that must be controlled during the approach. The quadriceps tendon inserts on the superior pole of the patella and the patellar tendon continues to the tibial tubercle; the vastus medialis obliquus is a key dynamic stabiliser that must be preserved or repaired. Implant design. Current-generation megaprostheses are modular, allowing intraoperative length adjustment and hinge orientation; the rotating hinge reduces torsional stress at the bone-implant interface compared with fixed-hinge designs. Popular systems include the Global Modular Replacement System (Stryker), the MUTARS system (Implantcast), and the Compress device (Biomet).