Leg Prostheses

There have been many advances in leg prostheses over the years and advances are always being made, even now.  There are hundreds of different prostheses available for the leg, some of which make use of straps, while others primarily use the vacuum pressure inside the prosthesis in order to hold the prosthetic device in place.
In order to maximize the ability of the patient to walk, the prosthesis must be so snug so as to avoid the phenomenon of pistoning, which is the natural tendency of the prosthesis to slip slightly out of place during the swinging phase of walking, only to be stuffed back in the proper place when the prosthetic foot hits the floor.  Pistoning can cause chafing of the skin on the stump and can unnaturally lengthen the prosthetic leg while the leg is in mid-air, making it difficult to walk without a limp.
 
Suction Sockets
 
These have been available since their invention around the time of World War II.  Because they provide for a snug fit, they are the attachment method of choice for amputations of the leg.  There is usually a valve inside the prosthesis to allow for the air to escape as the prosthesis is being attached to the leg.  When all of the skin of the stump is in contact with the prosthesis, the valve is closed so that the prosthesis is held on by suction alone.  Some prostheses of the leg involve both suction and strap methods in order to keep the prosthetic device in the proper position.

There are some problems with this type of device, however.  The suction may not be able to be held the whole time so that the prosthesis slips off.  It is also difficult to take these prostheses off and put them back on quickly.  The skin of the stump must be in complete contact with the rigid plastic of the prosthesis so that the skin can become irritated.

Absorbable lotion is often used to help the stump effectively fit in the way it needs to with respect to the prosthesis.  If any air pockets remain in the stump-prosthesis interface, this skin can become more irritated because of the vacuum pressure being introduced on this skin.  It can lead to breakage of capillaries in the stump, serous fluid drainage from the stump and thickening of the skin in the affected area.
A newer method was described in the 1980s that seems to work better and is a variation of the suction socket.  A silicone liner is peeled onto the stump, much like the application of a condom.   The skin is then able to adhere to the liner and a cable or attachment hook Is used to attach the prosthesis.  There can be stockings between the liner and the artificial limb which can account for changes in swelling of the stump and which adds to comfort.  It can be applied in a sitting position, which is more comfortable for the amputee.  More recently a “shuttle lock” mechanism has become popular in which the prostheses is locked onto the distal part of the liner that has been rolled onto the stump.
 
The thicker the liner, the more padding can there be for the stump and the more comfortable can the stump be.   Another method of attaching the prosthesis is even more comfortable.  Suction is used to attach the roll on liner and a second suction mechanism using a one-way valve is used to attach the prosthesis to the liner, also using suction as a means to hold on the prosthesis.

Another method of attaching an artificial limb involves the use of textile socks that have a silicone band at the top, which uses friction and suction to hold the prosthesis on while being able to absorb sweat from the stump with the textile sock.
For things like foot amputations, the prosthetist creates a prosthetic device that makes use of the bony prominences to hold onto the prosthesis.  For example, the ankle bones can be used to hold onto a prosthesis by “locking” the ankle bone prominence into a window in the prosthetic device.
 
For knee disarticulation or below the knee amputations, the prosthesis can be attached to the supracondylar area of the distal end of the femur.  This allows for the prosthesis to become wedged in the supracondylar area so that it doesn’t fall off.
 
Strap Suspensions

Straps can be used to hold the leg prosthesis in place.  In one such type of prosthesis, the strap is attached above the knee with the artificial leg wedged in where the patella (knee cap) is at.  To make sure the leg is properly attached, some patients use a waist strap to keep the prosthesis in place.  Waist belts and straps are especially helpful when it comes to making sure that the prosthesis doesn’t rotate out of position but maintains a forward position at all times.

Another way to attach the prosthesis is to attach the below the knee prosthetic device via laces that go up the thigh and uses stabilizers on both sides of the thigh in order to keep the prosthesis from rotating.  This type of attachment has the
advantage of taking the weight of the body and spreading it to involve the entire thigh rather than just on the end of the stump.

While straps can be successful in holding on the prosthesis and preventing rotation, they are not considered better than using suction to hold the prosthesis onto the stump of the lower extremity.  Some prosthetics, depending on the situation, use both suction and strap suspension in order to have the best chance of keeping the prosthetic functional and relatively painless.

Foot Prostheses

If the person has had an amputation just involving the foot, there are several choices.   Initially, there existed foot prostheses that had only a single axis.  That means that they flexed and extended but were not able to rotate at the level of the ankle.  There are others that have a single axis but that involve cushioning of the heel when the artificial heel hits the ground.  This is the cheapest and lightest-weight alternative to having a normal foot.

Now, many artificial feet are multi-axial, which means they can flex, extend, invert and evert to a limited degree.  They tend to wear out faster than single axis feet but, replacing the parts can easily be done and the feet tend to work much better than other types of artificial feet. 

Dynamic Response feet are even more accepted since being invented in the 1980s.   They involve a spring-like mechanism in the heel that allows for potential energy to be stored in the heel so the wearer can have a “spring in their step” upon pushing off with the artificial heel.  These are very popular designs in use today for foot amputations at the level of the ankle.

Knee Mechanisms
 
There are five different artificial knees available.  Some simply work by bending in single axis, such as is found in flexion and extension of the knee.  There are knee devices that lock in place and that don’t bend without manual help—used when ultimate stability is necessary but which offer poor gait control and is awkward when sitting.  The most advanced artificial knee has a microprocessor built into the knee so that the knee can be used at different speeds.  It uses a fluid-controlled hydraulic mechanism that allows for a more normal gait along with standing stability.  It is more costly than simpler designs and needs more maintenance but it is the most natural of all artificial knees. 
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