Covering the angles

Generally, we take our bodies for granted. How its various mechanisms actually work rarely, if ever, occupies anyone's thoughts — unless they go wrong. And if you're unlucky enough to lose part of your body altogether, then replacing it suddenly shows just how complex that part was.

This is particularly true of prosthetic legs. Many different types have been designed over the centuries, from the 'peg leg' to today's advanced limb replacements. Making prosthetics mimic the function of the original limb is the most important part of the design process, and a team from the Fraunhofer Technology Development Group in Stuttgart has been making important moves towards designing the perfect prosthetic leg.

As team leader Urs Schneider points out, most people who lose a limb do so later in life. 'Most amputations today are the result of diabetes or peripheral arterial disease, and statistically speaking, the risk of contracting these increases with age,' he said.

This makes it particularly important to ensure that prosthetics copy as closely as possible the way natural limbs move. 'Older people in particular need artificial limbs which will enable them to walk without having to relearn too much,' said Schneider. Moreover, ensuring that the replacement moves like the original minimises the stresses on the rest of the body, which avoids pain, discomfort and could prevent the development of further musculoskeletal problems.

One of the biggest problems with false legs is the foot, because the ankle is a very complex joint. As well as hinging up and down, the ankle allows the foot to move from side to side and to tilt in a variety of directions. Although some prosthetics use joints which allow the foot to rotate around three axes and to adjust to irregular surfaces, Schneider said that nobody has managed to reproduce the exact sequence of movements that the foot makes during walking.

Schneider's team, however, has determined the crucial movement which keeps the body stable during walking — a tiny, barely perceptible rotation as the weight transfers from heel to toe. As the heel hits the ground, the foot first tilts inward, then rotates across its central position to the outer edge, which remains in contact with the ground and transfers the weight to the ball of the foot. While this is happening, the hip pushes forward in preparation for the next step.

Once the team had determined the necessary motion, the next move was to build a mechanism which copied it. They claim to have succeeded in this — what's more, the system is purely mechanical, and uses no electronics. The key is a Cardan or universal joint — more familiar as the component in a car which transfers torque from the gearbox to the driveshaft — which allows rotation around several axes at the same time. The team is currently awaiting a patent on the design.

Tests so far have proved encouraging, said Schneider. 'Clinical trials have shown that because of the natural-looking gait, hardly anyone notices that the person is wearing an artificial limb.'

The natural movement reduces the strain on intact knee and hip joints and the lower back, and amputees need far less time to adjust to the new leg, and learn to walk again much faster.

With prosthetics designers able to choose this type of joint, artificial legs could allow elderly users to walk naturally and without pain, and younger amputees to take part in a far wider range of activities — even, said Schneider, extreme sports.