Above Knee Prosthesis
Above knee amputees are unable to bear weight on the bottom of the femur, thus body weight is supported by loading the ischial tuberosity and the soft tissues of the limb [primarily the gluteal muscles]. The socket design is termed an ischial containment socket. In an ischial containment socket the amputee is essentially ‘sitting’ while walking because they are bearing the majority of their body weight through their ischial tuberosity.
Goals of fitting the Ischial containment design
The goals of the above knee prosthesis are to provide a comfortable prosthesis, that enables the amputee to regain stability during walking and any other balance related activities, and a prosthesis that is cosmetically appealing.
Comfort is attained by having a socket that has a hard external frame and soft inner shell. The hard external frame is a lamination comprised of fiber glass, carbon fiber, and epoxy acrylic resin. The soft inner shell is made a plastic composite. This external framing allows for musculature to fire different phases of walking without causing discomfort within the prosthesis.
Biomechanical Functional Goals
Frontal plane (left to right) stability goals
Provide ML stability of pelvis at midstance of gait. By minimizing lateral Center of mass movement. This is attained by creating a narrow medial-lateral dimension of the prosthetic socket. Weight bearing on the ischium is attained by having a ‘bony lock’ at the midstance phase of walking. A ‘bony lock’ is maintained by having a high lateral wall on the prosthesis that is contoured to the long axis of the femur and cups over the hip joint. This high lateral wall prevents the ischium from moving laterally. This is significant as it enables the amputee to bear their weight through their ischium. If the ischium is not seated properly, the patient will feel pressure in their ramus (pubic area) which is very uncomfortable.
Sagittal plane (front to back) stability goals
When a person has an above knee amputation, they tend to have weakness in their hip extensor musculature due to the gross change in anatomical properties of the amputation. This coupled with them being forced to either sit or remain in bed while the amputation site heals, causes hip flexion contractures. A hip flexion contracture is when the person’s hip remains in a flexed position (pointing up) when they are standing rather than straight down towards the ground. This can cause complications in function and cosmesis of the prosthesis when they are severe (20 degrees or more).
In order to build a prosthesis that is safe to walk on, the center of the prosthetic knee must be behind the weigh line to provide the most stability during walking. If the amputee has good muscular control of their residuum, then the weight line can fall through the prosthetic knee center. If the knee center is ever anterior of the weight line, the prosthetic knee will buckle and cause very unstable gait. In patients that have a very severe hip flexion contracture the, prosthetic knee center tends to be in anterior of the weight line in order to provide a cosmetically pleasing prosthesis. This is unsafe and the patient is typically given a manual locking knee that is locked during walking and standing, but free to bend during sitting.
Choosing the proper prosthetic knee
Microprocessor Prosthetic Knees
The microprocessor knee is designed to provide stability in most walking situations for the user. The microprocessor is designed control the swing phase of gait (when the prosthesis is in the air) and the stance phase of gait (when the prosthesis is supporting the user’s body weight as they walk).
During swing phase, the microprocessor knee controls the speed and ease at which the prosthetic knee swings to match the user’s sound side walking pattern. This results in an energy efficient and cosmetic gait pattern.
The primary benefit of the microprocessor knee occurs in stance phase. When all of the user’s body weight is on their prosthesis as they are walking, a Microprocessor knee provides resistance to the knee bending. This results in an energy efficient walking pattern, shock absorption, more normal walking pattern, and reduced falls during walking.
The microprocessor knee is most beneficial to any above knee amputee that is able or was able to be independent in their home and community. They should be able to go shopping, care for their home, walk outside on uneven ground, and walk with variable speeds.
The primary advantage of a microprocessor knee over a mechanical knee is seen when the amputee is walking up and down stairs, ramps, or on uneven ground. The stance phase resistance that is provided when the knee is flexing reduces the overall muscle exertion the amputee must use during these difficult tasks.
At OPS we are trained to work with all microprocessor knees available and would be happy discuss the benefits of them with you and your family.
Mechanical Prosthetic Knees
Mechanical prosthetic knees are the predecessor for microprocessor knees. They are typically more heavy duty, require less maintenance and care, and are available for an amputee of every activity level.
The low activity mechanical knees are typically best suited for those who walk at a single speed and primarily on flat surfaces. There are some mechanical knees that better suited for highly active individuals that enable biking, running, and other high impact activities.
Hundreds of non-microprocessor knees are available worldwide. They all use a mechanical hinge; the speed and ease of the hinge’s swing is controlled by one of the following mechanisms: Free swing, Manual lock, Constant friction, Weight-activated friction, Geometrically locking, and Hydraulics.
The hinge swings, then locks manually when pressure is placed on the leg during stance phase. Mechanical-knee users must exert muscular and mechanical control to alter speed and step length and provide stability in the weight-bearing phase of gait.
At OPS we are trained to work with all prosthetic knees available and would be happy discuss the benefits of them with you and your family.