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Bionic bone

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Last week you had a story from ABB about a new surgical technique for children suffering from osteosarcoma, a form of bone cancer. Many of you asked for more technical detail.

Surgeons replace a section of diseased bone with an adjustable insert that allows the bone to be lengthened, keeping pace with the healthy limb. In the past, this involved frequent surgery to access the implant and operate a screw jack to lengthen it.

Engineers at the University College London, based at the Royal National Orthopaedic Hospital at Stanmore, have produced an implant that can be activated by electromagnetic induction.

The ?bionic bone? has a miniaturised high-ratio gearbox driven by a rare-earth permanent magnet synchronous rotor no larger than a one pence piece and 4mm thick. The gearbox is only 21.5mm diameter and 18.5mm long and drives a power screw which lengthens the ?bone?, developing the 200 Newton force required for every 1mm of extension. The complete rotor / gearbox assembly is embedded within the implant and activated by applying a rotating magnetic field .

The patient?s leg is inserted into a cast-resin three-phase stator, powered by a Danfoss VLT 2800 variable frequency inverter. The variable torque control of the drive enables the rotor to produce sufficient torque to drive the gearbox at as low as 40 to 60 volts at 50Hz, and provides accurate flux control to protect the tiny gearbox from torque overload. To ensure smooth rotation at this low stator voltage, the drive output waveform is filtered before being applied to the stator.

A single-phase to 3-phase VSD is essential so that the stator can be powered from a domestic 240v single-phase supply.

With gearing of 13,061 : 1 and the rotor spinning at 3,000 revs, the gearbox develops 4Nm torque and extension of 0.25mm per minute is achieved and timed rather than measured. Despite the asymmetrical air-gap, and the tissue of the leg, between the stator and the Niobium Iron Boron (NdFeB) rotor, there is excellent magnetic stator / rotor coupling, regardless of the fit of the patient?s limb within the stator and so there is no need for immobilisation or discomfort for the patient during a typical 15 minute treatment.

The cost of the implant is ?12,000 against ?4,000 for initial surgery but savings in subsequent surgery and aftercare offsets the cost.

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