Advanced Materials for Athletes Prostheses

Topics Covered

Background
Design of Prosthetic Limbs
The Future

Background

Competition amongst paralympians is no less fierce than that experienced by their able bodied compatriots, with competitors producing athletic performances that are truly inspiring. The current world record for the 100 metre sprint by an amputee athlete is 11.03 seconds, only about a second slower than the fastest Olympic sprinters. What makes this possible?

Major factors are, of course, the strength, technique and determination of the competitors, but an important part of the equation is the materials technology found within the prosthetic limbs they use. The days of the wooden leg are long gone and, as they have for many other sports, advances in materials technology have revolutionised performance levels in disabled sport.

Design of Prosthetic Limbs

The process of designing a prosthetic limb is a complex one. Consider the case of an athlete with a below knee amputation. The remaining stump is often very tender, and is composed of a variety of tissue types, some of which are pressure sensitive and some of which are pressure tolerant. The prosthetic practitioner fitting the athlete with the limb begins by designing a hard socket that supports the limb under the stump's pressure tolerant areas. These hard sockets are made from polypropylene or woven carbon fibre composite materials. To provide protection for the pressure sensitive tissue a soft silicone rubber liner is worn over the stump, and together the hard and soft socket combination provides comfortable support for the athlete.

To replace the tibia and fibula of the lower leg a hollow circular bar is attached to the hard socket via a metallic nut and bolt assembly. The bar is made from several different carbon fibre materials, with woven and unidirectional fibres being used in combination with filament wound fibres. Attached to the base of the circular bar is a curved foot section, also made from carbon fibre composites. Its purpose is to act as a spring to aid forward motion.

To understand how this is possible the human walking pattern, or gait cycle, must be considered. Walking is an activity that we seldom think about, but the process can be separated into four distinct stages - heel strike (HS), foot flat (FF), heel off (HO) and toe off (TO), (see figure 1). The point at which the heel contacts the ground is known as the heel strike. At the midpoint of the stride the foot is flat on the ground, and as the stride progresses the heel leaves the ground followed by the toe, the cycle then repeats. At the point of heel off, the foot section begins to bend under the load of the athlete, and in doing so the section stores elastic strain energy. The load on the foot, and the energy stored, reaches a maximum midway between the heel off and toe off stages, beyond this point the stored energy is returned, providing a forward impulse. This spring-like action helps the athlete achieve a more natural gait, which contributes to the faster times for the 100 metre sprint.

Figure 1. The human gait involves four distinct stages, heel strike (HS), flat foot (FF), heel off (HO) and toe off (TO).

Carbon fibre composites are used to manufacture the foot sections partly because the resulting structure exhibits high strength and stiffness together with relatively low mass. These materials are also used because they enable such a high degree of design flexibility. Varying the degree of fibre orientation in the foot varies the bending stiffness, which is fundamentally important given the variation in mass of the competing athletes. The bending stiffness of a given foot can now be tailored to ensure that the loading is elastic and that energy storage is maximised.

The Future

The future of prosthetic limb technology is exciting, and may lie particularly in the area of osteointegration, in which the prosthetic device is fixed directly to the bone via a titanium implant. This technology is not without difficulties. The fact that the prosthetic limb is connected to the bone means that the skin does not form a continuous surface over the area of the limb and bone fixing, increasing the potential threat of infection and subsequent illness. However, if the problem of infection is solved then we could see Paralympic athletes competing alongside their Olympic counterparts.

Primary author: Dr. Mike Jenkins

Source: Materials World, vol. 8, p. 11, September 2000

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