by John Angelico, CP

Transfemoral prostheticTransfemoral (above knee) prosthetics refers to an artificial limb replacement where the knee joint has been removed and the individual still has part of the femur or thighbone intact.

Our goal in prosthetics is to fit the client comfortably with the greatest return of function, and to make it look as natural as possible. To achieve this goal, we must create an appropriate style socket (the part the residual limb fits into), and provide the best design knee, ankle, and foot to fit the client’s life style requirements. As with any limb replacement, the best results will be achieved when a team approach is taken. At the very least the team should consist of the amputee, a physician, a prosthetist, and a therapist.

Of all the changes and advancements in prosthetics today, none has had a greater impact on our industry as has the transfemoral (AK) socket design. We’ve gone from wooden plug sockets, to laminated quadrilateral sockets, to today’s popular design of the ischial containment socket.

Wooden Plug Socket
Wooden Plug Socket
Laminated Quadrilateral Sockets
Laminated Quadrilateral Sockets
Ischial Containment Socket
Ischial Containment Socket

Each style still has its place in our industry today. A person fit with a “plug socket” 50 years ago usually has a hard time changing to the newer design sockets. The plug socket was a design that used the soft tissue at the groin level to support the body weight. The shape of the socket was very much like the shape of the thigh, “oval.” Unfortunately, this design does not provide the femur and pelvis with adequate stability. To compensate for the lack of stability a pelvic joint and band (usually fabricated out of metal) are added to the socket and then to a waist belt . Today, versions of the “plug socket” are still used in instances where the socket can’t be fit all the way up to the ischium.

The quadrilateral socket designed in the late 1940s at the University of California, Berkeley, has been a very effective design. The socket is divided into four walls (quad) and each wall has a specific function in either positioning the ischium, controlling tissue, or stabilizing the femur. The front wall positions the ischium onto the top of the back wall. Its shape is such that it creates a space for the large rectus muscle of the thigh and still allows pressure backwards without impinging on the neurovascular bundle located on the inner top thigh. The inside wall must contain the soft tissue between the legs. Care is taken to provide sufficient relief to the strong adductor tendon at the front part of the inside wall. Failure to provide adequate relief to this tendon results in pain and burning sensations.

The side wall has the big job of controlling the side displacement of the femur. This wall should be flat and angled toward the opposite limb from just below the hip joint to just above the cut edge of the bone. At the cut edge of the bone a relief is made. When an individual with an above knee amputation walks, the femur moves out to the side because the muscles used to stabilize the hip are flexing. The position of these muscles are such that they pull the femur out to the side. The ischium sits on a shelf on the top of the back wall. This prevents the limb from dropping down into the socket, which would result in breakdown on the bottom. The disadvantage with the quad design is that the ischium sitting on the back shelf acts as a fulcrum allowing the femur to move out to the side. Because of this the individual walks with a limp or a side to side gait.

Around 1980, Ivan Long used a little of both styles to create a new design that has evolved into the “ischial containment socket”. This design has many names (Northwestern design, CAT-CAM, Sabolich socket, NSNA), but the basic idea is to stabilize the pelvis and femur by controlling the ischium and the femur.

Ischium model The basic concept behind ischial containment is that the inner (medial) wall of the socket actually cups inside ischium. This prevents the socket from shifting laterally during the stance phase of gait. Capturing the ischium provides a secure fulcrum against the pelvis so that when the amputee abducts the amputated femur, the muscle force actually moves the pelvis, not just shifting the socket from side to side. This is the primary theoretical benefit of the ischial containment socket, to control the side-to-side shift of the socket on the upper thigh. This potentially makes the thigh/prosthesis interface more stable and thus more energy efficient.

Fitting the ischial containment portion of the socket brim is very critical because having it too high makes it very uncomfortable in the perineum. Having it too low does not allow it to control pelvic shift.

Making a plaster impression All of these designs are used today to fit someone with a transfemoral amputation. Once the decision has been made as to what type of socket to use the next step is to get an impression of the residual limb. Making a plaster impression of the residual limb or scanning the limb with computer technology can do this. The first way is probably the most widely used method in the field today. A typical casting procedure will first require the prosthetist to record precise measurements of the residual limb to be used in modifying the mold. A cast separator (i.e., Vaseline) is applied to the limb to prevent the plaster from sticking and then a cotton sock is put on. The sock may be just a tube of stockinet or it could be in the form of pajamas that are held up with suspenders. Rolls of plaster of Paris or rolls of synthetic casting material are molded onto the limb. Once the plaster has hardened it is removed from the residual limb by sliding it off. It is very infrequent that a transfemoral cast will need to be cut off. The client is sent home and the prosthetist begins working on creating the prosthesis.

Plaster mold Our first step is to fill the mold we took of the residual limb with liquid plaster and then before it hardens we insert a mandrel (pipe) to allow us to put the mold in a vice to modify.

Prosthetist shaping the moldBased on the evaluation and the measurements taken, the prosthetist removes plaster in areas where weight bearing is to occur and adds plaster to bony areas or problem areas that there should be no weight bearing. The practitioners use their education of anatomy, physiology, and kinesiology, as well as biomechanics to correct the mold.

Test socketOnce the mold is corrected and smoothed, it’s ready to have the test socket fabricated. The test socket (evaluation socket or check socket) is a tool we use to evaluate the fit before actually making the prosthesis. The test socket is a clear plastic material that is easily modified with heat. If the client is experiencing too much pressure in one area, the prosthetist will be able to see it through the socket. Heat can be directed to the area, thereby softening the plastic and allowing the prosthetist to move the socket away from the excessive pressure. The material can also be heated and modified to increase pressure where appropriate. The prosthetist should make as many test sockets as are needed to create a comfortable and functional socket.

Transfemoral amputation The next step in fabricating a prosthesis is to fill the test socket with liquid plaster. Once the plaster hardens the test socket is removed. The actual socket can now begin to be fabricated. Sockets can be made of wood, polyester, acrylic, carbon, plastic or a combination of these. Socket design One of the more popular designs for a socket starts with a flexible plastic that is vacuum formed directly onto the cast. Over the flexible inner socket a rigid carbon frame is made. The carbon frame offers strength and stability and the plastic offers flexibility and comfort in appropriate areas.

Once the choice of socket has been fabricated, the next step is to add the parts that will make up the finished prosthesis. Space does not permit me to discuss all the different types of prosthetic knees or feet (look for upcoming articles to deal with these separately). The knee, shin, ankle and or foot are aligned (set-up) to the socket according to the clients needs, as well as the components chosen.

Component diagram The client then returns for the fitting of the prosthesis on all their components to achieve the best possible gait. The prosthetist can change the angles and positions of all the components, which allows them to function at appropriately and in turn provides the client the best most energy efficient gait. Some of the things the prosthetist looks for are comfort, a narrow base of gait, ease in swinging the knee, and no abnormal walking patterns.

Once the prosthetist is confident that all these things have been accomplished the next task is to finish the prosthesis and make it look as real as possible. Tracings and measurements of the opposite side are taken to finish the fabrication. Initially, a rough shaping is done and then the client comes in to match the shape as close as possible. The cover can then be finished with a nylon stocking over it or it may be covered or painted with latex to closely match the client’s skin color. Adding hair to the cover is also a possibility.

Making plaster cast Suspension of the prosthesis can be accomplished with either belts, suction, or inserts. Belts have been suspending prosthesis since the first prosthesis was ever used. Suction and inserts are more recent advances that have eliminated the need for belts on many people. It is important that when fit with suction suspension, the client’s limb size doesn’t change (i.e., poor circulation or weight change). This type of suspension holds the prosthesis on and prevents rotation the best. Inserts are made of either silicone or polyurethane and are rolled on the clients limb. On the end of the insert, is a device that attaches the insert to the prosthesis. This may be a locking pin which connects to a lock in the bottom of the socket, or a Velcro strap which is fed through a hole in the bottom of the socket and attached to the outside of the socket. The advantage with this type of suspension besides eliminating the belt is that it can be used on people with many different problems (i.e., poor circulation, anticipated weight change).

Prosthetist making cast There are two ways to make a transfemoral prosthesis, exoskeletal and endoskeletal. Exoskeletal means the outside of the prosthesis has a hard plastic finish, usually over wood or hard foam. The big advantage with this style prosthesis is its durability. The two big disadvantages are increased weight and fewer choices in componentry. Endoskeletal means the outside of the prosthesis is soft and the components are under the soft cover. Lightweight, cosmetic, with many different components to choose from are the advantages of an endoskeletal system. The majority of the components developed today are for endoskeletal prostheses. The disadvantage is the costs involved.

It is strongly recommended that every new client receive physical therapy to learn how to walk with the prosthesis as well as how to maintain the fit. It is unrealistic that someone can learn the correct way to walk with a prosthesis without a physical therapist. It is more difficult to correct a poor gait than it is to learn the proper gait with a therapist. It is also unrealistic to believe that a new prosthesis will correct a poor gait. A prosthesis that fits and functions perfectly alone will not correct a poor gait, but combined with a skilled therapist should achieve this goal.

Prosthetic legs So how do you go about receiving the best prosthesis individually suited to your needs? My feeling is if you know very little about the field of prosthetics, find a prosthetic clinic that can help you with your complete rehabilitation. The doctor, a prosthetist, and a physical therapist. The job of the team is to determine the best prosthetic prescription based on your medical history, occupation, hobbies, and goals. The team will decide when you should be fit, recommend therapy, and follow your rehabilitation and progress. Once you have worn a prosthesis, you gain some valuable knowledge. You know the things your prosthesis can and can’t do, which will help you to make possible changes. To find what changes to make, you must educate yourself with the resources available. The clinic team should always be a resource avenue. You are reading the best resource guide for prosthetics, inMotion keeps you up to date with the latest technology as well as personal experiences. Your public library and the Internet are also very valuable resources. Surfing the net will put you in touch with articles, manufactures, and chat rooms. Armed with all this information you now have the ability to make the best decisions for yourself.

About the Author

John Angelico is a Certified Prosthetist and Registered Occupational Therapist at Scheck & Siress O&P, Inc., in Oak Park, Illinois.

Last updated: 09/18/2008
Back to Top