Prosthetic Limb Control

Prosthetics can be designed to look like a normal limb for cosmetic purposes only or they can be made functional through various techniques.  Often an amputee will have more than one type of prosthetic device in order to use the prosthesis for different functions.
A “cosmesis” is a prosthetic device that is simply used to look like the real thing.  Good cosmetic devices are very realistic and are designed to match the wearer’s size and skin tone.   Some can be so good that you can’t really tell the difference between the prosthetic part and a real body part.  The downside of these types of prostheses is that they do not have much functionality.
Usable Prosthetics
Usable prosthetics, such as those used to grip things or carry things often do not look cosmetically pleasing but are very functional for various purposes.  There are three
main types of usable prostheses, namely, 1) body powered prosthetic devices, 2) externally powered prosthetic devices, and 3) myoelectric devices. 
Body powered prostheses are controlled by wires or cables that are attached to another body part.  The person uses their good limb to manipulate the cable so that the prosthetic device functions.  It is not unlike using the hand brakes on a bicycle to run the brakes elsewhere on the bike.  For upper prostheses, the opposite shoulder is attached to the prosthetic on the affected side; the opposite shoulder is manipulated in such a way as to make the prostheses on the other side move in certain ways.
Externally powered devices are able to move because of tiny motors manipulated by the patient.  Buttons or toggle switches are used to move the prosthetic into the proper position.  Often a prosthetic device will have more than one toggle switch so that the limb can be more functional in a complex sort of way.  The toggle switches are flipped in a certain order to obtain a prosthetic position that is useful in everyday tasks.
An advanced prosthesis uses the remaining muscles on the stump to send signals to the prosthetic device.  These are called myoelectric devices.  Muscles can generate tiny electrical signals upon contraction that can be sent to the prosthetic limb to make the prosthetic device move.  It is like a device that uses a toggle switch but instead uses movement of muscles to send the signal to the prosthesis to tell it how to move.

When there is more than one joint on a prosthesis…

When there is more than one joint involved in a prosthesis, such as an above the elbow (transhumeral) prosthesis, each joint has its own toggle switch or muscle that can send the signal to the various joints.  The movements are generally done sequentially, such as moving the elbow first with one muscle movement or cable and  then sending a signal for the prosthetic hand or claw to close around a specific object.  More advanced prosthetic devices can mimic real movement in realistic ways.
One such advanced prosthesis is an above the knee amputation prosthetic device that uses a computerized knee that will adjust its movements in order to help the wearer adjust to a normal gait and walking style.

One such device, known as the intelligent knee, consists of a microprocessor and various sensors that are able to measure the current knee angle and the forces on the knee while walking.  The microprocessor eventually “learns” how the patient’s gait goes and can adapt to the specific gait.  In one such device, there is fluid inside the artificial knee that contains metal particles.  When a magnetic field is sent through the fluid, the fluid thickens and the knee joint is able to stiffen when needed.

What makes up a modern prosthesis?
The prosthesis is made from a pylon, which is the skeleton or internal frame of the limb.  It is often made out of some type of metal, such as carbon steel or aluminum.  The pylons are often covered with something to make them more lifelike in appearance.
Each prosthetic device has a socket, which is the part of the limb that attaches or interfaces with the stump of the limb.  Sockets must fit snuggly and be able to transmit forces from the prosthetic part of the limb to the patient’s stump and must fit comfortably for longer wearability.  Sometimes socks or other types of padding are used to make sure the prosthesis sticks firmly onto the stump.
There is also the necessity of a suspension system that keeps the prosthesis attached.  The suspension system can involve a harness, belt, sleeves or straps that attach to the body in such a way that the prosthesis does not fall off.  Suction is another factor to consider.  When a prosthetic device is particularly snug fitting, suction to the stump influences the ability of the artificial limb to connect to the stump.  There is an airtight seal between the stump and the prosthesis.  Prostheses are custom made to fit exactly to the stump that is remaining.  This means that plaster casts of the stump must be made in order to have a proper fit.
The Making of a Prosthesis
The prosthetist must make particular measurements of the affected limb, even before it is amputated.  The making of the prosthesis can then begin so that it is ready for the patient when the stump has healed.  When the stump is healed, a plaster cast is made of the stump so that an airtight interface can be made without having bony prominences rubbing at the wrong places in the prosthetic device socket.  Sometimes computerized images are taken of the stump to maximize the ability of the prosthesis to fit properly.
The prosthesis is then attached, making adjustments along the way so that the stump does not have undue pressure on it.  As mentioned, sometimes several different prostheses are made that are specific to certain tasks the patient might need to do.
Because the muscles of the stump tend to atrophy over time, more padding may be necessary to keep the prosthesis on.  If this is not possible, a new prosthetic device is made that better fits the shrunken area of the stump.  Children need repeated prosthetic devices to accommodate their ongoing growth.  Even adult prostheses tend not to last longer than three years due to breakdown of the prosthesis or changing of the shape and size of the stump.
A particularly advanced form of prosthesis is called “targeted muscle reinnervation” or TMR. The nerve that was supposed to go down an arm, for example, is rerouted to a muscle on the chest wall.  When the patient wants to move the artificial arm, the nerve signal is sent to the chest wall muscle, which has an electrode attached to it that makes the amputated arm/prosthetic device move in the same way it would have if you had decided to move the intact arm outright.  The patient basically thinks of moving the amputated arm, the signals are passed on to a chest muscle that, in turn, activates the artificial limb through the use of electrode signals.
The future of prosthetic movement involves placing microelectrodes in the brain that can stimulate directly the artificial limb, just by using thoughts.  Research is now being done on primates with good success.  Eventually, humans will have microelectrodes attached to areas of the brain that can directly stimulate movement of an artificial limb without using muscle electrodes. 
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