In adults, modular tumor endoprostheses, an implantable artificial metal device to replace a missing segment of bone and adjacent joint, are becoming a standard option for reconstructing the limb after resection of malignant bone tumors. However, reconstruction with the standard static prosthesis is neither a practical nor a functional option in the skeletally immature patient due to the length discrepancy [14]. In order to solve the problem in children, the concept of the expandable endoprosthesis was introduced by Scales in 1976 [48]. An expandable endoprosthesis can be lengthened as the child grows, similar to the standard endoprosthetic reconstruction. It allows immediate weight bearing with early rehabilitation and return to function. The design of expandable endoprosthesis has significantly evolved over the past 30 years (Fig. 11.7).

By then, all the lengthening mechanisms required a formal surgical procedure, which resulted in a high incidence of infection [49]. The younger the child, the more operations were needed not only for lengthening, but also for management of complications such as stiffness, infection, wear, and loosening. The latest design has a noninvasive lengthening mechanism, which relies on an electromagnet outside the body to transfer energy to the sealed implant inside the body. Today, there are several noninvasive products that have the ability to lengthen a child’s limb without having to undergo an operation [13, 36, 50, 51]. This provides a substantial advantage for both the child and the parents. Not all children are candidates for an expandable prosthesis. The size, age, and potential growth of the patients need to be evaluated carefully. Endoprosthetic reconstruction is not recommended for patients younger than 6–8 years old or for small children who have tall parents, since their potential of growth is too much to be corrected even with the use of expandable prosthesis [14, 52]. Beebe et al. [53] analyzed the functional outcome and gait of children after reconstruction with noninvasive expandable prosthesis at a mean follow-up of 31.5 months. Increased time spent in double-limb support and decreased gait velocity was observed, indicating weakness of hip abductors. However, the reconstruction produces satisfactory functional outcome and the patients maintain high levels of emotional acceptance.

Complications of expandable endoprosthesis include infection, loosening of implants, stiffness, unplanned shortening or lengthening, outgrowing the available extension, and implant and periprosthetic fracture [49]. Infection poses the largest iatrogenic risk to limb salvage. The deep infection rate is reported to be 10.9–17.6 % [54–56], most prevalent in the proximal tibia. Infection of an implant is difficult to eradicate because of the adherent colonies of bacteria in a polysaccharide matrix, collectively called a biofilm [57]. Infecting bacteria can remain on the surface of an implant for a variable length of time. Clinical symptoms can be caused by the local proliferation of bacteria shed by the biofilm, and may present themselves from several months to years after index surgery. Chronic infection usually requires a two-stage revision with a 70 % success rate, but some patients still require amputation [49]. Aseptic loosening of endoprosthesis is the most frequent complication in pediatric limb salvage, which has been reported to be 7.1–26.3 % [58, 50]. This is largely due to the long lever arm created at the bone-implant interface, and also, difficult anchoring conditions. Mechanical failure is another frequently reported complication in the literature, most frequently involving failure of the expansion mechanism. There are also reports regarding the fatigue fracture of the prosthesis [59]. The primary biological limiting factor to using an endoprosthesis in children is the diameter of the host residual shaft diameter. Under long-term cyclical loading, even strong material will fail if the stem diameter is less than 8 mm [51]. Picardo et al. reported that revision procedure was performed in 16.4 % of the cohort after the endoprosthesis was maximally extended. The revision rate due to mechanical complications was 18.2 % [54]. Despite these complications, the literature suggests the existence of a role for expandable prostheses in pediatric limb reconstruction after resection of a primary malignant bone tumor (Table 11.3).

Limb length discrepancy remains a critical late effect after limb salvage procedure in children (Fig. 11.8) [14].

Differences in length between upper extremities are not often a problem, but leg length discrepancies can cause significant functional deficits. The distal femur and proximal tibia are the most common sites for primary malignant bone tumors, and the epiphyses of the distal femur and proximal tibia contribute approximately 35 % and 30 %, respectively, to growth of the lower extremity [36]. Also, many other factors contribute to the overall limb length discrepancy, including systemic chemotherapy, slowing of the preserved growth plate in the affected joint, muscle atrophy, muscle loss and overgrowth of the contralateral limb. Several options, including conservative method and surgical intervention, such as expandable prosthesis, epiphyseal preservation, contralateral epiphysiodesis or distraction osteogenesis, have been developed to solve the problem. In order to choose the proper elongation method, it is important to estimate the expected limb length inequality resulting from both segmental resection and loss of subsequent normal growth. Several methods have been developed to estimate final height at skeletal maturity, including Paley’s “multiplier” method [62, 63], Moseley graph [64] and Green-Anderson growth remaining charts [65].

Leg length discrepancies less than 2 cm in the lower extremities is well tolerated and normally has little-to-no functional or clinical significance. These discrepancies can be easily corrected by shoe lift. Discrepancies of 2–5 cm are associated with gait abnormalities [66] and can be corrected with a shoe lift or a contralateral epiphysiodesis. Inequality larger than 5 cm often requires surgical management (Table 11.4).

Distraction osteogenesis is a surgical process used to lengthen the long bones. After corticotomy, the two bone ends are gradually moved apart during the distraction phase, allowing novo bone to form in the gap (Fig. 11.9). It is based on accelerating the functional remodeling and integration seen with fracture healing [68, 69]. Distraction is usually started 7–14 days after the osteotomy, with a rate of 1 mm/day [70], however the consolidation phase is usually twice as long as the distraction phase. Duration is estimated on the basis of 1 month/cm of increased length [71, 72]. The most common complications using this method are pin tract infection, pin break, fracture of regenerate, dislocation/subluxation of nearby joint, skin invagination, bone resorption and loss of alignment. Tsuchiya et al. [73] reported a complication rate of 52.6 %. Physeal distraction procedures have the advantages of more rapid ossification, earlier weight bearing, and fewer complications than do other lengthening techniques but should be used only in children under 12. Cãnadell et al. used distraction osteogenesis to expand the tumor-free margin and to preserve the epiphysis by physeal distraction.