Prosthetic Limbs: Conventional Technology
Up till the mid 20th century, although prosthetic limbs could be aesthetically pleasing, they still did not give the amputee much prehension (the act of grasping or seizing). In fact, the only artificial device which could be used for grasping was an invention in 1909 by W.C. Dorrance, which consisted of a hook opened by a strap across the back of the shoulder and closed by rubber bands. This way of controlling the prosthesis is still in use today, called the body-powered prosthesis.
External prosthetic limbs can also be powered by motors, and driven by the amputees themselves, usually by using switches to control the different functionalities of the prosthesis.
Myoelectric Limbs
The invention of myoelectric prosthetic limbs has been the most exciting development of the previous century. Myoelectricity signals (also known as Motor Action Potentials) are electrical impulses sent by the brain through the nervous system to produce contraction of muscle fibres in the body. A Myoelectric prosthetic limb uses electrodes placed on the skin surface to detect these signals, which then are used to move the arm, open and close the fingers etc.
Targeted Sensory Reinnervation
These technologies, however, still leave the patient with one striking handicap, the lack of a sense of touch. Amputees can only operate by visual feedback i.e. seeing that they have touched or picked up an object. They cannot feel if they have grabbed it too strongly or too lightly. In 2007, scientists in Northwestern University, in Chicago transplanted nerves from an amputated hand to the patient’s chest. Thus the patient could feel hand sensation there, and when the brain sent a signal to the arms, the muscles on the chest twitched, and these contractions were used to move a motorized elbow. This system is called targeted sensory reinnervation (TSR).
Manufacture of Artificial Limbs
Nowadays, computers are being used to design and attach prosthetic limbs. CAD/CAM (Computer Aided Design and Computer Aided Manufacture) can design a patient’s model of the limb, on which then a mould can be produced.
But how are prosthetic limbs manufactured, and what are they made of at the present time?
The most important property of the prosthetic device is the weight. It has to be lightweight, especially if the prosthesis is transhumeral or transfemoral. The device generally consists of a custom fitted socket, a pylon (the internal structure), knee cuffs and belts that attach it to the body and a ‘sock’ which acts as a cushion for the contact area. Originally the artificial limb was made out of wood and steel. Nowadays, although still used in some form, these have been mostly replaced by lighter metals like aluminium, titanium and their alloys while the emergence of strong plastics like polypropylene polyethylene, acrylics, and polyurethane have made the development of stronger, lighter and more flexible prosthetics easier. Carbon fibre is also popular.
Prosthetic Limb Design
Prosthetic limbs are not manufactured en-masse. They have to be designed and produced according to the needs of every patient. They are first prescribed by the doctor after consultation with the amputee, the prosthetist and a physical therapist.
Prosthetic Limb Manufacture
After the artificial limb is commissioned, there are three stages in the manufacturing process:
Measuring and casting: The prosthetist measures and takes a digital reading of the residual limb, after which he notes the position of the bones and tendons on the remaining part of the limb. Sometimes this is done even before the amputation, in order to minimise the wait for manufacture of the prosthesis. This done, a plaster cast is produced, from which a model of the stump is created.
Making of the socket: A thermoplastic sheet is then heated and vacuum-wrapped around the model. This becomes the test socket, on which the prosthetist checks the fitting of the limb. This test socket is then used by the patient to see that it fits correctly and comfortably. When satisfied, the permanent socket is produced.
Prosthesis formation: The plastic parts of the prosthesis are formed using vacuum-forming and injection moulding, while the metal parts are die cast. Wooden parts are sawed, planed and drilled. The various parts are then tooled together to form the whole limb. The permanent socket is then fitted to the residual limb with the whole prosthesis attached. Finally minor readjustments are made.