A limb prosthesis has 4 main parts:
The prosthesis attaches to the body by direct skin contact or by an interface made of various, thin cushion materials worn over the residual limb (stump).
A gel cushion interface, worn over the residual limb, protects the skin and helps even out pressures. Custom molded interfaces may be necessary for irregular stump contours (because of, for example, deep scars, sharp bones, or burns). Ideally, people should have 2 identical interfaces so they can be alternated day to day. Alternating the interface helps it maintain its elasticity and shape and last longer. Interfaces are typically recommended to be replaced every 6 months and, for very active patients, every 3 to 4 months.
A prosthetic sock may be worn instead of or with a gel interface. Socks are made of wool, nylon, or synthetic fabrics, sometimes with gel sandwiched between the layers of fabric. Socks are available in different thicknesses (plies). The residual limb changes size normally throughout the day as a result of activities, weather, and other factors. Prosthetic socks and special pads are used to manage those changes. By putting on one or more socks of different thicknesses or by taking socks off, a person can adjust the fit of the socket to make it more comfortable as the residual limb changes size during the day. When a comfortable, stable fit cannot be maintained with the use of prosthetic socks or special pads, the prosthetist (an expert who designs, fits, fabricates, and adjusts prostheses) can adjust the socket.
Suspension refers to how the prosthesis is held to the residual-limb. Certain gel interface materials are better suited for specific suspension applications (such as suction, pin, or vacuum).
The following suspension systems are commonly used:
Vacuum: An electric or mechanical vacuum pump removes air from the socket. This is the most effective method for holding a prosthesis to the residual limb and also helps stabilize the amount of fluid in the residual limb. Urethane gel interface materials are preferred for this type of suspension.
Passive suction: When the residual limb is put in the socket, air is forced out. A seal above prevents air from reentering, creating suction. A one-way valve may be incorporated in the bottom of the socket to allow air out.
Interface with a locking pin: A cushion interface that has a removable, adjustable stainless steel suspension pin at the bottom is inserted into a locking mechanism in the bottom of the plastic socket. To remove the prosthesis, the person presses a release button to disengage the pin.
Anatomical: Bumps at the ends of bones, such as at the knee, ankle, or elbow, can be used to help hold the socket to the body.
Belts and straps: A belt and/or straps may be used to hold the prosthesis on if the person cannot tolerate or finds the vacuum, suction, or pin systems too difficult.
The basic components of a prosthesis include the following:
The socket is the most important component because it supports the body and transmits the pressure and forces that come with movement to the residual limb.
For lower limbs, microprocessor-controlled ankles and knees can provide greater safety, stability, reduced energy expenditure, and diminished stress to nearby joints and the spine.
For upper limbs, body-powered prostheses require a fully functioning shoulder and arm to operate the loop strap that controls the prosthetic hand or hook. Myoelectric upper-limb prostheses do not require a fully functioning shoulder and arm and instead use the natural, electrical signals of a person's muscles. Electrodes in the socket detect muscle activity and transmit signals that operate the prosthetic hand, wrist, and/or elbow. No other body movement is required.
Some people choose to have their prosthesis appear anatomically natural. Technicians apply and shape a soft foam material equal in consistency to muscle and tissue over the plastic and metal components. This material can reduce clothing damage and be shaped to match the person's natural limb.
A synthetic skin can be applied over the anatomic shape using a color that matches the person's skin tone.
Some people, especially athletes during competition, prefer to eliminate the anatomic shape and skin and leave the plastic and metal components exposed. This reduces weight and allows more adjustments that can improve performance.