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 A prosthesis is a device designed to restore function to an individual who has undergone the amputation of a limb. Generally, prosthetics are categorized as either lower extremity or upper extremity. While this seems to be a simple distinction, their roles in functional restoration are quite different. |
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A transtibial or below-knee prosthesis consists of a socket, foot/ankle
assembly, suspension, and pylon. In a transfemoral or above-knee prosthesis
there is the addition of a prosthetic knee joint. The socket is the interface between the residual limb and the prosthesis. Most prosthetic sockets are contoured to provide total contact, which allows weight to be distributed throughout the entire limb. While the number of different prosthetic feet are many, they can be divided into 5 categories: |
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1. S.A.C.H. Foot: Solid Ankle Cushion Heel – The SACH foot is the
simplest foot. It is also lightweight and durable. 2. SAFE Foot: Solid Ankle Flexible Endoskeleton – This foot is more flexible than the SACH foot, thereby allowing a smooth rollover while walking. 3. Single Axis: Single axis feet contain an ankle joint which allows the foot to move up and down. The motion allowed by the ankle joint increases knee stability. 4. Multiple Axis: Similar to the single axis foot, multiple axis feet not only contain an ankle joint, allowing the foot to move up and down, but also can bend side to side. 5. Energy Storing: Also known as “dynamic response” – These state -of -the- art feet absorb energy when the foot contacts the ground and release it when the foot leaves the ground. |
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There are various means for suspending a prosthesis. Transtibial Suspension Methods: 1) Supracondylar Cuff – A supracondylar cuff, affixed to a socket, allows the prosthesis to hang from the top of the knee. 2) Sleeve suspension – An elastic or neoprene sleeve is pulled over both the prosthesis and a large area of skin, thereby suspending the prosthesis by partial suction. 3) Gel liner with shuttle-lock – One of the more advanced designs, this pin, incorporated at the end of the liner, fits into a shuttle-locking mechanism fabricated into the bottom of the socket. 4) Joint and thigh corset – This suspension method bears much of the patient’s weight on the thigh. 5) Waist belt suspension – In this design, much of the weight of the prosthesis is distributed around the waist. Transfemoral Suspension Methods: 1) Full suction – The prosthesis adheres by suction. 2) Partial suction - This method of suspension employs the use of prosthetic socks in combination with negative pressure. It is used mainly for people who can not put on a suction socket. 3) Gel liner and pin-lock - Similar to the transtibial design. 4) Silesian belt/TES belt – This is often used in a temporary prosthesis, because a new, rapidly changing limb can not be fit with suction suspension. 5) Hip joint and pelvic band – The pelvic belt is very easy for people with poor balance or hand problems to put on. It is also appropriate for people with short residuums, because it controls rotation and provides excellent side to side stability. |
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Like prosthetic feet, there are many different designs of
prosthetic knees. They are divided into 6 categories: 1. Manual Locking: This is the most stable knee available. As the patient walks, the knee does not bend at all. The knee only bends with a manual release for sitting. 2) Constant Friction: This type of knee allows for single speed walking on level surfaces. 3) Stance Control: This knee is very stable and is usually prescribed for a patient’s first prosthesis. When weight is placed on the prosthesis, the knee does not bend until weight is removed, allowing for clearance during walking. 4) Polycentric: Polycentric knees are naturally stable, are recommended for older patients and can be used with long and short residual limbs. 5) Hydraulic: This knee is prescribed for patients with the ability to walk with varying speeds. 6) C-Leg: The newest of the knee designs, this state-of- the- art knee, has an onboard computer, which provides an extremely efficient and natural gait. Click here to see a Windows Media Video of the C-Leg. (Windows Media 9 required.) |
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Upper Extremity The versatility of the arm and hand to reach, grasp, hold and release objects allows the individual to interact with his environment. If the upper-extremity amputee is to maintain independence, career and lifestyle, it is necessary to provide a prosthesis which will restore primary function. An upper-extremity prosthesis consists of a socket, suspension, power supply and terminal device. In a body-powered prosthesis, the individual harnesses body movements which, in turn, activate the terminal device and other components via a cabling system. In an externally powered or myoelectric prosthesis, electrical signals from remaining muscles are transmitted to an internal motor which controls the terminal device and other components. The terminal device is the component which replaces the lost hand. This can be in the form of a cosmetic replacement, functional hook, specialty device such as a gardening tool or a functional hand. Some upper- extremity prostheses are termed “hybrid,” that is, they are both body and myoelectrically powered and may even have the ability to “quick change” between various terminal devices. |
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