Allograft-Prosthetic Composite (APC) Reconstruction

Indication. A segmental bone defect that spans from the diaphysis into the adjacent joint after a wide resection of a bone sarcoma (most often osteosarcoma or chondrosarcoma) or, less often, an aggressive benign tumour or a solitary metastasis with massive bone loss. APC is chosen when the resection leaves too little metaphysis and joint surface to support a standard prosthesis, and the site demands a reconstruction that tendons can attach to. The classic sites are the proximal femur (abductors), the proximal tibia (extensor mechanism) and the proximal humerus (rotator cuff); the principles transfer to the distal femur and humerus. The one decision that matters — APC, megaprosthesis or osteoarticular allograft. After any limb-salvage resection you are choosing how to bridge the defect. The three options are not interchangeable, and the trade-off is biological reattachment versus mechanical durability versus articular biology:

Pre-operative assessment. Confirm the diagnosis on a tractable biopsy and complete staging — MRI of the whole bone for local extent, skip lesions and joint involvement, and CT chest (plus the usual sarcoma staging) for distant disease. Templating is decisive: measure the expected resection length from the MRI and order a size-matched fresh-frozen allograft (matched medullary diameter and length, within roughly 1 to 2 cm). Plan the soft-tissue envelope — a proximal tibial APC usually needs a medial gastrocnemius flap, and the proximal humerus a deltoid/rotator-cuff plan. Consent specifically for junction nonunion and a possible second procedure to bone-graft or plate it, late allograft fracture, infection (which often means removal of the entire construct), leg-length or rotational discrepancy, and the rare but feared transmissible-disease and rejection risks of allograft. Setup. Lateral (proximal femur, humerus) or supine (proximal/distal tibia) on a radiolucent table; prophylactic antibiotics (with re-dosing for a long case); an image intensifier for leg length, rotation and junction alignment; and often two scrub teams working in parallel to save tourniquet and ischaemia time. The allograft is thawed in warm saline on the back table only once the resection length is confirmed.

The goal: after a wide tumour resection, bridge the segmental defect with a structural allograft that carries a cemented long-stem prosthesis, fix the host-allograft junction rigidly enough to unite, and reattach the tendons to the allograft so the patient keeps active abduction, extension or rotation. The sequence below is written for the proximal femur (the archetype); site-specific differences are flagged at each step.

Rehabilitation | Phase | Timing | Protection | Therapy focus | |-------|--------|------------|---------------| | 1 | 0 to 6 weeks | Abduction brace / orthosis; non-weight-bearing or toe-touch | Isometrics only; control swelling, protect the tendon repair | | 2 | 6 to 12 weeks | Brace weaned; partial weight-bearing if the junction is uniting | Gentle active motion; protect abductor/patellar-tendon reattachment | | 3 | 3 to 6 months | Radiographs at 6 weeks, 3 months and 6 months for junction union | Progressive weight-bearing as the junction unites; closed-chain strengthening | | 4 | 6 to 12 months | None once united | Full function; impact sport generally avoided | Weight-bearing advances on junction union, not on a fixed calendar — a proximal-femur APC may need protected weight-bearing for 3 to 6 months, and a nonunion at 6 months prompts bone-grafting and replating rather than simply waiting longer. Functional results are good in the majority (MSTS scores typically in the 70 to 85 percent range), with the best function at sites where tendon reattachment succeeds. Complications

Epidemiology. APC reconstruction is used in limb-salvage surgery for primary bone sarcomas, which together affect roughly 1 to 2 people per 100,000 per year (osteosarcoma has a bimodal distribution in adolescence and the elderly; chondrosarcoma peaks in mid-adult life). As chemotherapy and imaging have improved, limb-salvage rates for extremity osteosarcoma now exceed 90 percent at specialist centres, and the APC is one of the reconstructive tools that makes salvage possible around joints where tendon reattachment is essential. Pathoanatomy. The tumour dictates the resection, and the resection dictates the defect. A sarcoma of the proximal femur or tibia typically takes the metaphysis, the joint surface and the tendon insertions with the specimen, leaving a segmental defect that no standard prosthesis can fill without a means of reattaching the abductors or extensor mechanism. The allograft is avascular and incorporates only at its ends by creeping substitution, which is why the host-allograft junction — the one place host vessels can reach — is both the key to incorporation and the site of nonunion. The prosthesis supplies immediate mechanical stability while that slow incorporation proceeds. Why the construct is built the way it is. Cementing the stem into the allograft fixes implant to graft immediately (the graft has no blood supply to be harmed by cement); a long stem crossing into host bone shares load across the junction; and a step-cut with compression plating holds the junction rigidly until it unites. The tendons are reattached to allograft bone rather than metal precisely because only bone-to-bone and tendon-to-bone healing is durable.

Key evidence. The landmark comparison comes from Zehr and colleagues (1996), who set the terms of the APC-versus-megaprosthesis debate at the proximal femur by showing better abductor function and stability with the composite, at the cost of graft-specific complications. The Rizzoli series (Donati, 2002) reported durable proximal-femur APC results with functional scores in a good range, confirming the technique in routine practice. The Massachusetts General Hospital allograft series (Mankin, 1996) defined the biology — fracture, nonunion and infection as the dominant modes of allograft failure, with outcomes best for intercalary grafts and worst for osteoarticular grafts — and remains the reference for counselling patients on what an allograft can and cannot do. The Enneking staging system (1980) is the framework that determines who is a limb-salvage candidate in the first place.