Drive Implant Eliminates Follow-up Surgery

Researchers at the UK's University College London (UCL) have developed a prosthesis that contains a small gearbox to extend the device without surgery. The beneficiaries: young cancer patients implanted with a prosthesis to replace diseased leg bone. Device extension to keep pace with growth is simple enough to be done on an outpatient basis.

Currently, a surgeon does such extensions by accessing a telescopic mechanism in a prosthesis through a small incision and turning a "key." The procedure requires general anesthesia and a hospital stay. Such operations often need repeating three or four times a year for up to five years.

For the self-contained, extendable prosthesis, licensed to UCL's subsidiary Stanmore Implants (http://rbi.ims.ca/3851-506), the medical team designed a patented epicyclic (planetary) reduction gearbox with a 13,000:1 reduction ratio. They drive the gearbox using a rotating magnetic field in a coil around the patient's leg to spin a rare-earth magnet rotor, 12 mm in diameter by 4 mm thick. The magnet has diametrically opposed north-south poles that spin in the 3-phase field at the UK standard of 50 Hz (3,000 rpm).

Development

Originally the team used a winding without any laminations, according to Jay Meswania, research fellow at UCL and technology manager for Stanmore Implants. Without any laminations, heat build-up in the coil necessitated oil cooling and the magnetic field was not concentrated into the rotor but out the coil sides.

In the search for more efficient components, the UCL engineers looked at stator cores donated by ABB (http://rbi.ims.ca/3851-507) and various inverter controls. They ended up specifying an ABB 180-frame-size-motor lamination core wound to UCL specs (552 turns of 1.06-mm wire in a star connection) by ABB's service partner EMR Silverthorn (Wembley, UK). The final device also features inverter controls from Danfoss (http://rbi.ims.ca/3851-508).

Not efficient

Meswania points out that the motor mechanism is inherently inefficient because the stator-rotor air gap cannot be minimized as in a classic motor due to the patient's leg filling this gap. "Efficiency wasn't the goal but rather being able to capture the magnetic field at a distance," he notes. The final configuration produces a consistent magnetic field across the core. Despite the field density being only on the order of tens of milli Tesla (hundreds of Gauss), the rotor can extend the prosthesis because of the high reduction ratio of the gearbox.

The rotor and gearbox are about three-quarters of an inch in diameter and an inch long. The size of the drive and the bone prostheses it extends make them suitable for patients from about nine years and older.

Stanmore Implants has furnished the devices for 13 patients thus far. The company is in the process of studying the long-term effects and plans to market the prosthesis worldwide.

Above, Good Turns:A stator surrounding a patient's leg produces a rotating magnetic field, which turns a small magnet rotor in the implant at 3,000 rpm. The rotor drives a reduction gearbox to extend the bone-supporting prosthesis 1 mm in just over four minutes. Below: Leg Extension: Doctors limit periodic prothesis extensions that simulate growth in young patients to 2-3 mm per treatment, avoiding overstreatching tissue and inducing pain.