Fourth Year Engineering

Lower Limb Prosthesis for a Racing Cyclist
(Tim Melville)

Racing cyclistThe brief for my fourth year project was to design and build a replacement lower limb prosthesis for Keith Moore, a racing cyclist from near Cambridge. He suffered a cycling accident over 10 years ago and had to have his right leg amputated below the knee.

Over the course of his rehabilitation, the NHS had provided him with a specialised cycling prosthesis. Although the fittings were bespoke, the design philosophy had been over-conservative, and there was a cumbersome arrangement of titanium tubing and heavy steel connectors. A considerable performance benefit could be achieved by making the new limb lighter and more streamlined.

Keith and his prosthetist were, however, satisfied with the geometry of his old prosthesis; the dimensions and angles between the stump socket and the pedal connection were to be preserved. What required engineering insight was the structural filling between the two ends.

A 4-ply isotropic lay-up of carbon-fibre-reinforced plastic was the material of choice, for its strength and stiffness-to-weight properties. I considered all possible loading scenarios for the structural member, to attain a design strength and stiffness. I consulted extensively with the prosthetic technicians at Addenbrooke's Hospital to guarantee the buildability of my design and that it could be funded by the NHS. I chose an aerofoil section for the main part of the member (whilst ensuring that my design would still be deemed legal for racing).

Construction took place in the Addenbrooke's workshops, with expert assistance from the technicians there. The old prosthesis was fitted into a jig which preserved the relative positions of the two ends. The socket was clad in a profiled template of stiff honeycomb foam, which provided the basis for laying up the woven carbon cloth. The epoxy resin was injected between polythene membranes either side of the carbon. After curing, the socket and foam were removed and the socket was glued back into place.

The new limb was over 600g lighter than before, and Keith felt a noticeable improvement. I thoroughly enjoyed the challenge; it touched on many fields of engineering, from structural analysis to manufacturing. It gave me an opportunity to get hands-on, producing a tangible deliverable with an obvious benefit to the end-user.