Out on a Limb
Prosthetic foot and socket
Dynamic Response Foot 2
The Palmer Leg (click to enlarge)
The Concept
Perhaps inspired by his own participation in endurance events, Wayne Alexander decided to focus on developing prosthetics for amputees involved in sports or other active physical challenges. The sporting arena is the only real testing ground, he says. "It's much easier to reverse-engineer something that's been honed in that environment than to go the other way. It's much easier to make production bikes out of race bikes than to make race bikes out of production bikes."In recent years there have been huge advances in prosthetics, mostly in materials, progressing from wood and metal to plastics and, more recently, carbon fibre. But amputees still face the same fundamental problems: unlike living legs, artificial legs produce no power of their own and they can't receive sensory information.
An amputee, where once they relied on the complex interaction of muscles, tendons, and ligaments to provide just the right amount of thrust while maintaining balance, has to fling forward what is essentially a dead-weight. To address this problem, energy-storing feet were developed in the 1980s, featuring a spring that captures the power created at the strike of the heel and uses it to propel the leg forward.
A healthy leg continuously receives information from its nerves about positioning, terrain and speed, and sends this information to the brain. The brain, in turn, sends instructions to the leg on how it should bend and adjust. Without signals travelling in both directions, amputees find it difficult to maintain a normal gait and are at constant risk of falls and injury. A new generation of computer-controlled prosthetics promises to alleviate this problem; computer chips paired with sophisticated sensors may ultimately allow artificial limbs to replicate the complex functions of healthy limbs.
While a truly bionic leg – an artificial limb "wired" to the brain – is still the stuff of science fiction, rudimentary microprocessor-controlled limbs are available. But they come with a hefty price tag: the C-Leg, produced by German company Otto Bock, costs around US$40,000. What was required, Wayne Alexander reasoned, was a robust but light energy-storing limb that could be cheaply mass-produced.
In 1997, Wayne Alexander was introduced to Mark Inglis through the Para Fed organisation. He set about producing what he calls a "graft" – a quickly assembled prototype, brazed together from panel steel – that embodied his ideas. The aim was to establish the first principles from which a sporting limb could be produced (Conceptual statement).
From the outset it was clear that it would be a waste of time re-engineering the sockets connecting Mark's stumps to his artificial legs. An artificial leg is fitted by means of a socket that fits snugly over the stump like a thimble on a finger. The socket is custom-made of fibreglass or plastic, and held on either by suction or by a system of belts or straps. Much of the prosthetist's art lies in fitting the socket, creating a comfortable connection that minimises irritation, blistering and tissue breakdown. To complicate matters stumps swell and shrink, changing the fit. (A relatively new approach is to bypass the socket and connect the artificial limb directly via a titanium-coated stainless steel pin implanted in the cavity of the femur.)
COP Outcome development and evaluation
Mark sacrificed the sockets from an old pair of legs and mounted them on the prototypes. He describes the prototypes as looking "heavy enough to anchor a boat"; but they worked – "After two steps I wanted to run and jump".
