This disclosure is for a rod-type prosthesis that, once surgically implanted, can be expanded non-invasively and repeatedly by an externally applied magnetic field. It applies to patients with a bone disease of a limb where a section of the bone must be surgically removed. If the patient is still at a growing age, current medical practice requires repeated surgical interventions to periodically implant longer and longer rods to match the normal growth of the other limbs. Similarly, in the case of adult patients a repeated lengthening of the prosthesis is often required. The present invention avoids the need for these repeated surgical interventions since the rod prosthesis can be non-invasively lengthened when the need arises.