Above knee prostheses

– Knee disarticulation & transfemoral amputation

If the leg is removed above the knee it is known as a transfemoral (across the femur) amputation.  When the leg is removed through the middle of the knee without cutting the bone it is called a knee disarticulation.  The major difference between these two different levels is that it is possible to bear some weight through the end of the residual limb with a knee disarticulation but not with a transfemoral amputation.

The socket of the prosthesis must be shaped in a very specific way to allow the user to put their bodyweight into the prosthesis.  The best way to achieve this is to create a small ‘seat’ at the top of the socket for the pelvis (specifically the ischial tuberosity or ‘sit bone’) to rest on.  The socket must be designed to allow for as much movement of the hip as possible, while also providing a comfortable and supportive fit for the residual limb.

A transfemoral prosthesis is generally made from plastic, be it fibreglass, carbon fibre or polypropylene. Often a flexible polypropylene liner is used within a rigid fibreglass or carbon fibre socket to provide a more comfortable fit.  There are a number of different methods that can be used to hold the prosthesis:

• A strap around the waist. This is the simplest method.
• A suction socket uses a small valve at the bottom of the socket to allow the air inside the socket to be forced out, using a vacuum to hold the socket on
• A silicone rubber liner called an IceROSS (Icelandic Roll-On Silicone Socket) can be rolled onto the limb and features either a small pin which engages into a lock at the bottom of the socket, or a rubber seal to create suction within the socket.

A prosthetic knee joint and foot are then attached to the socket.  There are a large number of different components that can be used and their function varies depending on the need of the user.  Some knees are free to bend while others are locked straight for walking and must be released by hand for sitting.  A locked knee is very stable to walk on; a free knee requires more strength to control but gives a much more natural walking pattern.  Pneumatic, hydraulic or electronic systems can be used within the knee to control how fast it swings and to prevent it from buckling.

A prosthetic foot is then attached below the knee unit to provide a prosthesis that is the correct height and is stable for walking.  There are a range of different types manufactured from materials such as wood, rubber and carbon fibre.  Some have moveable ankle joints and others have carbon fibre springs,  some also have built-in shock absorbers for high activity uses, or adjustable ankles to allow you to wear high-heeled or flat shoes.  Your prosthetist will work with you to choose the best components to suit your needs.

For more information or if you would like to make an appointment with a member of our clinical prosthetics team, please call APOS on 091 796983.