Children who have lost part of their leg bones to cancer are benefiting from a new procedure that eliminates years of painful surgery.

When a large amount of bone has been lost, a prosthesis is implanted in the patient’s leg to support the remaining bone. As a child grows, the implant must be extended, a task previously accomplished through a series of painful operations — often three or four over a five-year period.

Stator cores from ABB Inc., New Berlin, Wis., are helping to change things. A new procedure involves placing a small magnetic rotor, which links to the prosthetic implant by a gearbox, in a patient’s leg. To increase the length of the prosthesis, the patient’s leg is placed inside a stator core. When energized, the stator turns the rotor at 3,000 rpm, driving the gearbox and extending the prosthesis 1 mm every 4 min. A typical treatment will extend the prosthesis by 4 mm over the course of 16 min. Quick and painless, the procedure can be completed on an outpatient basis. Depending on a patient’s growth rate, the implant is extended in small increments several times until the person is fully grown.

Researchers from the Center for Biomedical Engineering at UCL, working at the Royal National Orthopedic Hospital in Stanmore, England, developed the device. The team had initially used six air-cored coils, configured as a two-pole, three-phase winding. Although this generated sufficient torque to turn the magnet, it was inefficient and required oil cooling.