Mar 16 2001
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.
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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
For more information on Materials World please visit The Institute of
Materials