Motor parts from ABB help children walk again

Dec. 9, 2003
New mechanical expansion device for prosthetic implant saves repeated, invasive surgeries.

Motor stators, supplied free by ABB to a pioneering research team working at the University College London (UCL), are helping children to walk again, according to experts based in ABB's U.K. offices.

A new procedure that elminates the need for years of painful surgery helps children being treated at the Royal National Orthopaedic Hospital, Stanmore, who have lost part of their leg bones after suffering from bone cancer.

Because 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 to keep pace with the skeletal growth. Previously, this involved further operations, often three or four a year over a five-year period, each bringing pain and inconvenience for the patient and extra costs for the hospital.

The new procedure is non-invasive and involves placing a small magnetic rotor in the patient's leg. This is linked to the prosthetic implant by a gearbox and is turned by an external stator. To increase the length of the prosthesis, the patient's leg is placed inside the stator core.

When energized, the stator turns the rotor at 3,000 rpm, which drives the gearbox and extends the prosthesis by one millimeter every four minutes. A typical treatment will extend the prosthesis by four millimeters over the course of 16 minutes.

Quick and painless, the procedure can be completed in a clinic (outpatient basis), rather than as an admitted patient operation. Depending on a patient's growth rate, the implant is extended in small increments several times until the person is fully grown.

The stator cores were supplied through an ABB Motor Service Partner, EMR Silverthorn, in Wembley. Researchers from the Centre for Biomedical Engineering at UCL, working at the Royal National Orthopaedic Hospital at Stanmore, developed the device. The team had initially used six air-cored coils, configured as a 2-pole, 3-phase winding. Although this generated sufficient torque to turn the magnet, it was found to be inefficient and required oil cooling.

In a subsequent iteration of refining the device, "We were asked by the researchers at Stanmore to obtain the stator cores from ABB and rewire them according to their requirements," said EMR's Managing Director Chris Fletcher. "ABB gave the stator cores free of charge and, working with the UCL research team, we helped to develop a winding to meet the specifications," he said.

Using a stator core based on ABB's standard 180-frame-sized motor in a 2-pole stack for 3,000 rpm nominal speed, UCL specified a series-wound stator with 552 turns of 1.06 mm gauge wire, for star connection. This was the configuration that had produced the best performance during tests at Stanmore.

"Our role was to interpret the needs of the UCL team and bring our expertise in motor winding to solve the problem, using standard and readily available equipment," said Fletcher.

The first patient was treated with the new procedure in November 2002, and five have been treated in total so far. Stanmore Implants Worldwide Limited, UCL's commercial arm, has ordered five more windings from EMR Silverthorn. ABB will continue to supply laminations on a commercial basis for the additional windings.

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