Modular Mega-prosthesis — Knee

Modular mega-prostheses (also termed tumour endoprostheses or limb salvage prostheses) are custom or modular implant systems designed to replace large segments of bone and the adjacent joint following resection of primary bone tumours, aggressive benign tumours, or for reconstruction in massive bone loss from periprosthetic fractures and failed total knee arthroplasty with significant bone deficiency. They represent the cornerstone of limb-salvage surgery, replacing what was historically managed by amputation.

• Primary oncological indications: osteosarcoma (the most common primary bone tumour requiring mega-prosthesis — affects the distal femur and proximal tibia most commonly; the classic location for osteosarcoma is the metaphysis of long bones around the knee); Ewing sarcoma; chondrosarcoma; aggressive giant cell tumour (not responding to denosumab or intralesional curettage); secondary indications — massive periprosthetic fractures (Vancouver Type B3 — massive bone loss around a loose femoral stem); failed revision TKA with severe bone loss (Anderson Orthopaedic Research Institute — AORI Type III bone defects)
• Limb salvage vs amputation: the principle of oncological limb salvage is to achieve equivalent oncological control (equivalent survival) while preserving a functional limb; 5-year survival data for osteosarcoma in appropriate limb salvage candidates (resectable, no skip metastases, adequate soft tissue coverage) is equivalent to amputation; limb function and quality of life are significantly better with limb salvage; approximately 90% of osteosarcoma cases around the knee are now treated with limb salvage rather than amputation
• Neoadjuvant chemotherapy: standard of care before surgery for osteosarcoma; MAP protocol (methotrexate, adriamycin, cisplatin) or similar regimen; neoadjuvant chemotherapy allows tumour shrinkage, facilitates surgical margins, and enables assessment of tumour response (Huvos grading — histological necrosis after neoadjuvant chemotherapy is a major prognostic factor)

• Wide surgical margins: the fundamental oncological principle; the tumour must be resected en bloc with a cuff of normal tissue on all sides; the margin is classified as intralesional (through the tumour — inadequate), marginal (through the reactive zone — borderline), wide (cuff of normal tissue — the goal for most sarcomas), or radical (entire compartment — rarely used for limb salvage); achieving a wide margin reduces local recurrence; the resection level is planned pre-operatively based on MRI (which defines the extent of marrow involvement and soft tissue extension)
• Implant systems: major systems include Stanmore (UK), MUTARS (Germany/global), GMRS (Global Modular Replacement System — Stryker), Compress (Biomet) and others; all modular systems allow intraoperative adjustment of the resection length by assembling components of appropriate length; the modular design reduces the need for truly custom implants (which have longer manufacturing lead times)
• Distal femoral replacement (DFR): the most commonly performed mega-prosthesis procedure; the distal femur and the femoral component of the knee joint are replaced; the tibial component is a conventional TKA tibial tray; the patellar tendon and extensor mechanism must be carefully preserved and reconstructed to allow knee extension; medial gastrocnemius rotational flap is frequently used to provide soft tissue coverage of the prosthesis and reconstruction of the pseudocapsule
• Proximal tibial replacement: technically more challenging than distal femoral; the patellar tendon inserts into the tibial tubercle, which is resected with the proximal tibia — extensor mechanism reconstruction is mandatory; the patellar tendon must be reattached to the prosthesis (synthetic ligament — Trevira tube, LARS ligament, or Gore-Tex); the medial gastrocnemius rotational flap is essentially mandatory for proximal tibial replacements to provide vascularised soft tissue coverage and extensor mechanism reconstruction — it is wrapped around the prosthesis and the patellar tendon is secured to it
• Fixation methods: cemented (most common for primary oncological implants — allows immediate weight-bearing; suitable for shorter life expectancy); press-fit/hydroxyapatite-coated (biological fixation — for younger patients where long-term fixation is required; requires cortical bone contact of sufficient length for osseointegration); the Compress implant uses an extracortical bone-growing (ECBG) mechanism

• Rotating hinge design: mega-prostheses use a rotating hinge mechanism (unlike a fixed hinge) to allow tibial rotation in addition to flexion-extension; this reduces the torsional stress at the bone-implant interface (which would be transmitted to the stem-cement junction in a fixed hinge, accelerating loosening); the rotating hinge allows approximately 10–15° of axial tibial rotation, mimicking the natural screw-home mechanism of the knee; the hinge also provides intrinsic stability to the knee (replacing the function of the resected ligaments — the ACL, PCL, and collateral ligaments are typically removed with the tumour resection)
• The rotating hinge is constrained in flexion-extension (unlike a normal TKA which relies on ligamentous balance); this constraint is necessary because the collateral ligaments are frequently sacrificed during tumour resection; the constraint transfers stress to the implant-bone interface and is a major driver of aseptic loosening over time

• The challenge of skeletal immaturity: osteosarcoma predominantly affects adolescents; resection of the distal femoral or proximal tibial growth plates creates a limb length discrepancy that worsens as the child grows; historically this required multiple revision surgeries (conventional lengthening) or acceptance of significant leg length inequality; growing prostheses address this by allowing non-invasive or minimally invasive lengthening
• Non-invasive (extendable) prostheses: the Stanmore Non-Invasive Tumour Prosthesis (NITP) and the MUTARS Xpand system use an internal magnetic motor that is activated by an external electromagnetic coil placed over the limb; the prosthesis extends in increments of approximately 3–5 mm per session; performed as an outpatient procedure without anaesthesia; allows staged lengthening matched to the growth of the contralateral limb; reduces the need for repeated open surgery for lengthening
• Complications specific to growing prostheses: mechanical failure of the extension mechanism (motor failure); infection (as with all mega-prostheses); the prosthesis will ultimately require revision to an adult-sized system once skeletal maturity is reached; despite these challenges, growing prostheses have transformed management of paediatric bone tumours around the knee

• Huvos grading and prognostic significance: the Huvos grading system classifies the histological tumour response to neoadjuvant chemotherapy in the resected specimen; Grade I — minimal response (<50% necrosis); Grade II — partial response (50–90% necrosis); Grade III — >90% necrosis; Grade IV — complete necrosis (100%); Grades III and IV (`good responders`) are associated with significantly improved 5-year survival (~80–90%); Grades I and II (`poor responders`) have a 5-year survival of approximately 40–60%; the Huvos grade influences post-operative chemotherapy protocol adjustments
• Soft tissue coverage and the medial gastrocnemius flap: the medial gastrocnemius is the workhorse flap for distal femoral and proximal tibial tumour surgery; for proximal tibial replacements, the medial gastrocnemius is raised on its proximal blood supply (medial sural artery from the popliteal artery), rotated anteriorly, and used to wrap the implant — providing vascularised soft tissue coverage, reducing infection risk, and serving as the anchor for extensor mechanism reconstruction (the patellar tendon is sutured to the gastrocnemius); the lateral gastrocnemius can be used for lateral-sided coverage; free flaps may be required for complex wound situations
• HENDERSON classification of failures: the Henderson classification categorises failure modes of limb salvage implants — Type 1 (soft tissue failure — wound/extensor mechanism); Type 2 (aseptic loosening); Type 3 (structural failure — implant fracture); Type 4 (infection); Type 5 (tumour progression/local recurrence); this classification guides surgical management of failed mega-prostheses and is used in the oncological orthopaedic literature for reporting outcomes

• Mega-prosthesis: limb salvage after bone tumour resection; osteosarcoma — distal femur/proximal tibia; ~90% of cases now limb-salvage (not amputation); equivalent oncological survival to amputation
• Wide surgical margins: en bloc resection with cuff of normal tissue; reduces local recurrence; planned on MRI pre-operatively
• Neoadjuvant chemotherapy (MAP protocol): before surgery for osteosarcoma; enables margin assessment; Huvos Grade III/IV (`good responder` — >90% necrosis) = better prognosis (~80–90% 5-year survival)
• Rotating hinge: allows axial rotation + flexion-extension; reduces torsional stress at stem-cement junction; intrinsically constrained (replaces ligament function); necessary when collaterals resected
• Proximal tibial replacement: most technically challenging; extensor mechanism (patellar tendon) must be reconstructed; medial gastrocnemius rotational flap is MANDATORY for coverage + extensor mechanism anchor; highest infection rate of all mega-prosthesis sites
• Aseptic loosening: most common failure mode (~20–30% at 10 years); higher in young active patients; constrained hinge transfers stress to implant-bone interface
• Deep infection: ~5–10%; chemotherapy increases risk; two-stage revision or amputation for uncontrolled infection
• Growing prosthesis: non-invasive extendable prosthesis (NITP/MUTARS Xpand); magnetic motor extended by external coil; avoids repeated open surgery in growing children; used for skeletally immature osteosarcoma patients
• Henderson classification: Type 1 (soft tissue/extensor failure); Type 2 (aseptic loosening); Type 3 (structural failure); Type 4 (infection); Type 5 (tumour progression)
• Medial gastrocnemius flap: workhorse coverage flap for distal femur and proximal tibia; raised on medial sural artery; wraps the implant + anchors patellar tendon in proximal tibial reconstruction