There are two approaches to building a lower extremity prosthesis: exoskeletal or endoskeletal. Exoskeletal prostheses have a hard outer plastic laminated shell or skin with a wood or urethane foam interior. In the exoskeletal prosthesis, the strength is provided by the outer lamination, and the shape or cosmesis (cosmetic cover) is an integral part of the prosthesis. This style of prosthesis is very durable, however, the choice of components is limited and the weight of the prosthesis is usually heavier than an endoskeletal design. It can be appropriate for a person who will be using the same prosthesis for many years.
Endoskeletal prostheses feature an inner support pylon composed of lighter-weight materials like aluminum, titanium, and graphite. These designs have interchangeable connectors and other components such as knees and feet. The shape or cosmesis is derived from a removable soft, foam cover that is finished with a nylon hose or other flexible material. This style of prosthesis is generally more adaptable and is well suited to people with active lifestyles. Endoskeletal prostheses offer more component options with more adjustability, but the cosmesis is less durable.
A Hanger Clinic prosthetist will help you decide which style is the best choice for you and your lifestyle.
The pylon is the portion of the prosthesis that transfers weight between the socket and the prosthetic foot. Many people prefer dynamic pylons that help reduce the shocks that are transmitted to the residual limb each time the heel strikes the ground. This is particularly important for people who participate in high-impact activities like running or other sports. The spring-like action of a dynamic pylon also helps move the person forward, compressing upon heel strike and then releasing when the toe lifts off the ground. Some pylons have torque-absorption that allow for rotation of the foot up to 45 degrees in either direction; the amount of resistance can be adjusted depending on the activity. Torque absorption can be an important feature for people who play golf or tennis, or want to go out dancing.
Like feet, there are many different models of prosthetic knees on the market today, yet all fall under two broad categories: mechanical and computerized.
Mechanical knees may be single-axis or polycentric (multi-axis). A single-axis knee operates like a basic hinge and is durable, lightweight and inexpensive. It uses sliding friction to prevent the leg from swinging forward too fast and allows for one optimal walking speed. Some single-axis knees have a manual lock which offers more stability when walking, particularly for those who are unsteady on their feet.
Polycentric knees are good for active people who would like a more stable gait without the expense of a computerized knee unit. There are also pneumatic (air) and hydraulic (fluid) knees, which allow the user to walk comfortably at varying speeds.
Computerized or microprocessor controlled knees are the latest in knee technology and offer users the most stability and the most natural gait of any knee. The on-board microprocessor samples the gait 50 times per second, automatically adjusting the response of the knee to the walking surface. Microprocessor controlled knees are more expensive but can be very beneficial to any user, particularly those with high activity levels or multiple limb amputations.
A basic hip joint operates like a hinge, allowing the joint to flex and then to extend slightly. A locking hip joint with a stride limiter offers more stability. A hip flexion bias system is a spring-loaded hip that begins to flex forward before the knee reaches its maximum forward travel, shortening the leg and allowing the foot to clear the ground. While there are numerous hip joints available, the anatomically correct placement of the hip is what ensures a person’s ability to walk.
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