Expandable prostheses for the leg in children.

T he most common locations for bone sarcomas in children are the distal femur and proximal tibia. 1 The epiphyses of the distal femur and proximal tibia contribute approximately 35% and 30%, respectively, to growth of the lower extremity. 1 Therefore, resection of the physis will result in leg-length discrepancy at skeletal maturity, functional deficit, gait disturbances , and cosmetic issues. Until the 1970s, amputation was the main treatment for bone sarcomas in children. Thereafter, advances in medical treatments , imaging, surgical techniques, and biomedical engineering have revolutionized the management and survival rates of children with bone sarcomas. Currently, 85% of children undergo limb-salvage surgery using allografts, vascularized bone transfer, bone transport, and megaprostheses. 2-5 Megaprosthetic reconstructions for tumor surgery were introduced in the 1970s. Adult-type megaprostheses can be used for children who are near skeletal maturity. If the resection of the physis is expected to result in a limb-length discrepancy of ,30 mm, the limb can be lengthened by 10 to 20 mm during the reconstruction, leaving an acceptable leg-length discrepancy at the completion of growth. If the anticipated leg-length discrepancy is .30 mm, alternative reconstructions should be used. Currently, several manufacturers offer expandable implants, which are desirable for younger, skeletally immature patients with long expected growth. 6-9 First-generation expandable megaprostheses appeared in the late 1970s. They were invasive, and open surgery was necessary to perform the elongation. 7 In 1976, the Centre for Biomedical Engineering manufactured the first expandable implant that used a simple worm drive mechanism to extend the prosthesis. 10 In 1983, the Lewis Expandable Adjustable Prosthesis (Dow Corning Wright Corporation, Arlington, Tennessee) was introduced. It used a fixed stem with a screw extension mechanism that expanded the prosthesis. 7 Second-generation expandable megaprostheses were minimally invasive; lengthening was achieved with an elongating screw or a telescopic mechanism. Although they also required an open procedure, the need for soft tissue dissection was dramatically reduced. 11 In 1987, the minimally invasive Growing Kotz prosthesis was introduced; it included a growth module that matched the Kotz Modular Femur and Tibia Reconstruction system (Stryker Howmedica Osteonics, Rutherford, New Jersey) and its successors, the fixed-hinge Howmedica Modular Resection System and the rotating-hinge Global Modular Replacement System (Stryker Howmedica Osteonics). 12 The growth module had an encapsulated elongation mechanism containing a threaded spindle driven by a bevel gear pair that moved a titanium sleeve by a threaded bush. When growth had ceased, …