The current treatment of osteosarcoma includes neoadjuvant chemotherapy followed by wide resection of the tumor and postoperative chemotherapy. While amputation of the extremity was once the mainstay of treatment, medical and surgical developments over the last several decades have led to the success of limb-salvage surgery1,2. In the very young patient, this has led to the challenge of achieving limb-length equality. Recently, the introduction of an endoprosthesis, expandable by noninvasive means, has allowed for reconstruction of the affected limb while addressing limb-length discrepancy in skeletally immature patients3. The latest generation of the expandable prosthesis is the Repiphysis expandable limb salvage system (Wright Medical Technology, Arlington, Tennessee), originally manufactured as the Phenix prosthesis. As the prosthesis has been followed for greater than ten years, failures are being described. Aseptic loosening and prosthetic breakage have been reported, but we know of no detailed report of the mechanism of failure of this prosthesis4. We report the case of a patient who had failure of an expandable prosthesis attributable to internal prosthetic component failure that was salvaged with conversion to an adult modular prosthesis. The patient’s family was informed that data concerning the case would be submitted for publication, and they consented. A fourteen-year-old boy who had had resection of an osteosarcoma in the proximal end of the right tibia at the age of nine years and had reconstruction with use of a proximal tibial replacement, the Repiphysis expandable prosthesis, presented for a routine follow-up evaluation. The patient had undergone two prior limb-lengthenings without complication and had continued growth from the distal femoral physis documented by scanograms. At the time of the latest follow-up, the limb-length discrepancy was <1 cm. The patient complained of slowly progressive pain and a “spongy sensation” with walking. He denied any …