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New Leg in Two Hours
A new method of accurately fabricating artificial legs promises to get amputees back on their feet rapidly.
by James GOH

ne perennial problem faced in the production of artificial limbs is satisfactory socket fit. The patient may suffer pain arising from poor fit or an inability to use the limb effectively. This means returning to the doctor or prosthetist for refitting the limb, or revision of appliance and subsequent help with the adjustment until he or she feels comfortable with it.

Scientists at the Department of Orthopaedic Surgery and the Division of Bioengineering at the National University of Singapore have developed an accurate yet speedy method of making an artificial leg. Their hydrostatic pressure-casting technique facilitates fabrication of a direct-pressure cast socket out of braided carbon fibre in just two hours.

Lower limb prosthesis consists of a socket, a shank, and a foot. The socket, enclosing the stump (or residual limb), forms the most important component of the artificial limb because it represents the initial interface between human and machine. The prosthetic socket supports and distributes the full weight of the wearer, especially at mid-stance when walking. Even the best surgical technique cannot compensate for a poor-fitting prosthesis simply because the stump does not have the same weight-bearing capability as the sole of the normal foot. The skill and the experience of the prosthetist in designing and fitting a comfortable socket often determine successful rehabilitation of a patient who had undergone amputation.

The Direct Pressure Cast (DPCast) Prosthetic Socket for Transtibial Amputees technique devised by the NUS researchers adopts a fresh design approach. The technique is based on the hydrostatic pressure casting concept in which fluid facilitates uniform pressure around the stump. Pascal's principle of fluid mechanics assumes that fluid pressure and any additional pressure (as from a prosthetic socket) will be transmitted equally to every point in the fluid.

The most notable difference between the DPCast and a conventional patellar (knee cap)-tendon-bearing (PTB) socket is the absence of indentations at the knee tendon and the back of the kneecap area. Another enhancement is that whereas the mechanics of the PTB socket must be defined for each progression phase of the gait, the DPCast socket simply assumes that pressure at one point will be transferred to other accommodating soft tissue.

In addition, prosthetists can fabricate the DPCast socket using a quick-curing material like braided carbon fibre impregnated with resins instead of the traditional lamination method. The resin-impregnated carbon fibre is draped over the stump and then inserted into the PCast tank. The hydrostatic pressure is then introduced into the system until the patient stands with the pelvis levelled. When fully cured and hardened, the carbon-fibre socket is removed from the stump and used to form the final DPCast socket. This method does away with the positive plaster mould and reduces the usual multiple-stage process to a two-stage method.

The PCast socket has been successfully fitted onto ten subjects. Preliminary gait studies showed no significant differences between the subjects' original socket and the DPCast socket; therefore, this technique has fabricated a comfortably fitting socket. It also reduces the amount of skill required on the part of the technician fabricating the socket, resulting in the reduction of fitting error, time, and cost. Moreover, because the pressure casting technique demands little expertise, it has great potential in developing countries where prosthetic proficiency is lacking.

The project won the first prize at Singapore's first Assistive Technology Invention competition organised by the Society for the Physically Disabled's Specialised Assistive Technology Centre in collaboration with the Institute for Infocomm Research. It also received an Honourable Mention at the recent Asian Innovation Awards organised by The Asian Wall Street Journal and the Singapore Economic Development Board. The intellectual property rights to the invention remains with the inventors, who hope to interest the industry in taking up the idea to develop and market it commercially

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