ABSTRACT This project is the development of a non-invasively extending distal femur replacement (DFR) for children and adolescents subsequent to bone tumor resection. Each year in the US there are 200-250 malignant bone tumor diagnoses in the distal femurs of patients 15 years and younger. With advances in chemotherapy, limb salvage surgery utilizing a megaprosthesis has increasingly become the treatment of choice. For growing patients these megaprostheses are required to extend so they can “grow” as the contralateral limb grows. Only one non-invasively lengthened DFR is FDA approved to treat these patients. The implant is actuated with a large electromagnetic appliance in a clinical setting. It is exceedingly complex and the average revision rate has been reported at 48%, including revisions for: insufficient lengthening capacity, lengthening mechanism failure, implant fracture, aseptic loosening and infection. MRI is contraindicated and the implant is extremely expensive. Our device is non-invasively, manually extended by pressing on the patient's skin. It's designed for a significantly reduced complication rate, the mechanism is simple and robust, it's MRI compatible and can be lengthened at home for a more gradual growth and constant leg length equality. It is capable of up to 240% of the extension that its sole US competitor offers and is easier and less expensive to manufacture so it can be offered at a reduced price to a greater population. The specific aims of the project are to (1) complete the implant design, (2) design specialized surgical instruments for implantation and (3) conduct non- clinical bench testing in preparation for FDA submission.