Abstract: The disclosure is for a prosthesis for children and young adults that, once surgically implanted, can be expanded non-invasively later and periodically, as dictated by the growth of the unaffected limbs. It applies particularly to patients at the growing age who suffer from bone cancer or other diseases of the bones of the limbs. Whereas current medical practice requires repeated surgical interventions to periodically implant longer and longer prostheses to keep up with normal growth, this invention requires only a single surgical intervention. Subsequent lengthening of the prosthesis is accomplished by means of a magnetic device that is external to the affected limb and is non-invasive.