Abstract: A total joint prosthesis that achieves non-invasive length modification and a method of using the same is provided. A gearbox housing within the implant contains a lengthening mechanism comprising a rotatable magnetic rod mechanically coupled to a lengthening screw through a series of parallel gear sets. The lengthening screw is coupled to an internally threaded implant neck. An external magnetic device containing at least one rotatable magnet produces a magnetic field. After total joint arthroplasty, the magnetic field of the external device forces rotation of the internal magnetic rod, causing rotation of the lengthening screw. Rotation of the lengthening screw causes the neck to either compress toward or extend away from the housing, adjusting the overall length of the implant and limb. This invention thus provides non-invasive means for post-operative limb-length modification after total joint arthroplasty and is useful in treating conditions such as limb-length discrepancy.