Allograft-Prosthesis Composites for the Reconstruction of the Distal Femur After the Resection of a Bone Tumor in Childhood.
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BACKGROUND: The tibial physis is responsible for about 30% of the growth in the limb1. Therefore, preservation of this growth potential in skeletally immature patients undergoing distal femoral tumor resection and reconstruction is important. Reconstruction with use of an expandable prosthesis has a high failure rate2,3. Osteoarticular allograft reconstruction of the distal femur allows for the preservation of the proximal tibial plateau; however, the small size of the child's tibial plateau relative to the femoral condyle of the allograft creates challenges in size matching2,4,5. To address this issue, resurfacing the osteoarticular allograft with the appropriate femoral component of a total knee prosthesis, referred to as allograft-prosthesis composite (APC) reconstruction, could be regarded as a viable alternative2,6. DESCRIPTION: An anterior longitudinal incision is made encompassing the biopsy tract, either medial or lateral to the midline, depending on the tumor mass. The roof of the adductor canal is opened to expose the femoral vein and artery for protection. The resection length is based on the length of medullary involvement as measured on magnetic resonance imaging (MRI), plus an additional 2-cm margin. The bone marrow from the proximal femur is sent for frozen-section analysis in order to ensure that an adequate margin has been achieved. Depending on the MRI findings, the capsule, collateral ligaments, and cruciate ligaments are incised, and the meniscus on the tibial plateau is preserved, with consideration given to achieving a negative margin. An appropriately sized osteoarticular allograft with the retained capsule and ligaments is obtained from a standard bone bank. The femoral component of the unconstrained total knee prosthesis is matched for size with the patient's tibial plateau and is fixed to the distal end of the osteoarticular allograft with use of bone cement. The resurfaced allograft is fixed to the host bone with a bridging plate. The ligaments and capsule of the allograft are sutured as closely as possible to their counterparts in the proximal tibia. Knee stability is evaluated at the end of the procedure. Two crossed pins are utilized to fix the knee in 15° flexion to prevent joint subluxation in cases of knee instability and are removed after 2 weeks. ALTERNATIVES: One alternative is reconstruction with use of an expandable prosthesis; however, high failure rates have been reported, including due to stem loosening, lengthening failure, and periprosthetic fracture3. Moreover, currently available expandable prostheses require a minimum resection length substantially longer than the actual tumor length, leading to unnecessary removal of healthy bone and loss of valuable bone stock in growing children2,3. Osteoarticular allograft presents challenges related to the small size of the tibial plateau in pediatric patients relative to the allograft femoral condyle, leading to a Charcot joint developing in all patients emerging around 8 years postoperatively4,5. RATIONALE: APC reconstruction preserves the proximal tibial growth plate and retains more bone stock for future revisions. This technique offers benefits over the use of an osteoarticular allograft, including improved knee function and a reduced risk of osteoarthritic changes, and can even be performed on patients <9 years old. EXPECTED OUTCOMES: Median knee range of motion was 100°, and the median Musculoskeletal Tumor Society score was 28. Sports activity was possible in two-thirds of cases2. IMPORTANT TIPS: Choose a medial or lateral approach on the basis of the dominant tumor location.During tumor resection, take care to preserve the neurovascular bundle.After opening the adductor canal, detach the medial gastrocnemius head from the distal femur to facilitate genicular artery ligation and the separation of the vascular bundle from the lesion.Try to use an allograft with remaining ligamentous and capsular structures, as attaching these to their host counterparts can enhance the restoration of joint stability.The compression plate should fully cover the allografts and secure them to the host bone, and compression should be maintained with at least 4 screws.The selected cruciate-retaining femoral component of the THA should be secured to the distal allograft with high-viscosity cement according to the total knee method. ACRONYMS AND ABBREVIATIONS: rAPC = resurfaced allograft-prosthesis compositesTKA = total knee arthroplastyMRI = magnetic resonance imagingDVT = deep vein thrombosisPE = pulmonary embolismMSTS = Musculoskeletal Tumor SocietyROM = range of motionDJD = degenerative joint disease.