Materials through the ages: Supporting act - the development of prosthetic limbs

Materials World magazine
,
4 Aug 2012
prosthetic legs

While prostheses have been transforming the lives of amputees for generations, only recently have advances in materials science offered disabled athletes Olympic status. Maria Felice looks at the evolution of artificial limbs.

Unless you’ve been hiding under a very large rock, you’ll be aware that the London 2012 Olympics are well underway with the Paralympic Games starting at the end of August. For the Paralympics, athletes are grouped into classes defined by their ability to perform an activity. This grading system, similar to the weight categories in sports such as boxing, ensures equality between competitors. One class of athletes for some Paralympic events is ‘amputee’, comprising competitors with partial or total loss of at least one limb.

Lower limb prostheses have improved tremendously through the ages, from the basic pegleg that simply filled the gap of missing limbs, to blade feet that enable the wearer to not only participate in sport, but do so competitively and to a high standard. Such drastic evolution owes much to advances in materials.

A typical modern lower limb prosthesis consists of a foot, a pylon and a socket. The pylon connects the foot to the socket, the latter attaching to the remaining part of the leg. The three parts are usually covered in a lightweight foam material to give a more rounded, realistic look similar in size to the sound limb, and this is often painted to match the patient’s natural skin colour. A sock is worn underneath the socket to improve functionality and ensure a comfortable fit.

Prostheses used for running races differ slightly, and consist of a blade and a socket. Although the blade is ideal for transferring the athlete’s energy to the track and enabling them to run at high speeds, walking and standing on these prostheses is not comfortable.

From wooden legs to modern pegs

Prosthetic legs have existed since ancient times, when wood and iron were the available materials. In the Dark Ages, prosthetics remained very basic and only the wealthy could a  ord ones that actually assisted them in their daily lives. Made by armourers using strong, heavy, inflexible iron, prosthetics were sophisticated in shape but not in function.

In the Renaissance developments came in the form of steel, copper, iron and wood. The first functional prosthetic was made in the 16th Century with the invention of articulated joints by Ambroise Paré, who also introduced modern amputation surgical procedures. However, the amputee was unable to flex these joints at will.

Considerable advances came in the 19th Century, many of which were initiated by (often war-wounded) amputees themselves. AA Winkley, for example, developed a slip-socket below-knee device for himself. He went on to found the Winkley Company in 1888, and marketed the legs during the National Civil War Veterans’ Reunion.

The 20th Century saw the greatest changes, mainly thanks to advances in materials. Lightweight aluminium and titanium replaced steel in the pylon, and later, carbon fibre became an option. The first aluminium prosthetic was worn by English aviator Marcel Desoutter. After losing his leg in a flying accident Desoutter was fitted with a standard wooden prosthetic, until his younger brother Charles used his knowledge of aircraft material to design a jointed leg made of the alloy duralumin – half the weight of the wooden model.

Modern plastics make prosthetic devices stronger and lighter, and require less energy exertion in use. The socket, for example, is normally made of polypropylene, a material resistant to fatigue and corrosion, and easy to form into complex shapes. New plastics, better pigments and more sophisticated procedures make the artificial skin appear more natural, although selecting materials that both perform well and look realistic remains a challenge.

The foot section of prosthetic limbs was traditionally made of wood and rubber. Although nowadays the inner part is still made of wood, the remainder is formed from lightweight urethane. The socks worn under the socket must be made of a strong yet soft material – wool has been used for hundreds of years, and now there is also the choice of cotton or various synthetic fibres.

Two materials are used in the early stages of the manufacture of a modern prosthetic limb and without them, customised and well-fitting prosthetics would be difficult to achieve. The first is a plaster mould, taken from the remaining part of the leg. The second is a thermoplastic sheet, which is formed over the mould and used to check the fit of the prosthetic.

Blade runners

Van Phillips, himself an amputee, designed the blade-shaped prostheses in the 1980s, and it remains the preferred design for about 90% of today’s Paralympic amputee athletes. The design consists of a narrow double-spring made of carbon fibre, which provides energy storage and good aerodynamics. Unlike all previous prostheses, it stores kinetic energy from the user’s steps as potential energy, allowing the user to run and jump.

Since running only uses the front part of the foot, the blades are designed without a heel, and contoured threaded insets allow for spikes. This ‘spring’ design has since been extended to heeled prostheses for use by non-athletes. By storing and releasing energy at the right time, the design enables users to develop a more natural and efficient gait.

Composite prostheses offer greater mobility and capability over any other artificial limb material. Such limbs are usually made of carbon fibres in epoxy, and for short limbs fibre glass in epoxy can be used. The addition of flexible foam over the ‘flat’ composite structure gives the prosthetic a rounded shape.

The composite structure is typically tapered in thickness, from about 80 layers at the top to 20 layers in the toe. The layers are applied over a mould one at a time, before being fused and hardened using heat and pressure. Thickness varies depending on the weight and activity level of the user, and patients can choose from different standard thicknesses or have their prosthesis custom-made.

Artificial intelligence

Nowadays, some prosthetics include electronic and computing components, such as sensors. This makes movement more natural and allows the patient to easily climb stairs and move around obstacles. Given the unfair advantage this would give some athletes in Olympic sports, competitive events require amputees to use prosthetics made only of mechanical components.

Myoelectric prosthetics detect the natural electricity produced by muscles in the remaining part of the limb, and use this to trigger the electromechanical components of the prosthesis. Such limbs are more common in upper-limb prosthetics, where fine, precise control is of great importance.

Owing much to advances in material science, sensors and microprocessors are becoming increasingly smaller and lighter, paving the way for prosthetic limbs to become even more intelligent in future. Not only will this further improve the quality of patients’ lives, it could even enable tomorrow’s Paralympians to achieve dreams of Olympic proportions.