New implantable leg prostheses that allow physicians to extend bone length without additional surgical procedures show great promise in rehabilitation of child and young adult survivors of osteosarcoma, according to an article in the February issue of Mechanical Engineering magazine.

Several new devices represent a third generation of leg prostheses for young survivors of osteosarcoma who have not finished growing yet. The first generation, developed in the late 1970s, employed screws to lengthen the affected bone. The second generation devices, which are still in use in many areas, were designed in modules that could be removed and fitted with a larger module over time. Both types required a surgical procedure for each unit of bone extension.

The newest generation requires only one surgical procedure to implant the prosthesis. Extension over time is accomplished with noninvasive means. One such device, produced by Stanmore Implants Worldwide, a British company, requires implantation of a lead screw-rotor device. During office visits, an external source of energy causes the rotor to rotate such that the lead screw advances (lengthening the bone) 0.25 mm per minute. After a painless office visit lasting approximately 15 minutes, the patient can walk away.

An American variant, The Wright Repiphysis system (which was approved for use in the USA in 2002) requires implantation of a spring-loaded extender in a pair of nested tubes, with the entire mechanism encased in a special polymer material. Use of an external electromagnetic wave source during an office visit provides the energy to make the polymer gradually soften, allowing the extender to lengthen within its nest of tubing.

Brian McDaniel, an engineer with the Wright company, said that even though the majority of American orthopedic oncology specialists still use modular prostheses, they “are giving the noninvasive forms (of prosthesis) strong consideration because they limit the surgery and rehabilitation their young patients must endure.”