A limb prosthesis has 3 main parts:
The prosthesis attaches to the body by direct skin contact or by an interface made of various thin viscoelastic cushion materials worn over the residual limb.
A gel cushion interface is worn over the residual limb (stump) to provide skin protection and modulate and more evenly distribute pressure. Custom molded interfaces may be necessary to accommodate irregular stump contours (eg, deep scars, sharp bones, burns). Ideally, patients should have 2 identical interfaces for each prosthesis so they can be alternated day to day, which helps them maintain their elasticity and shape and last longer. Interfaces are typically recommended to be replaced every 6 months and, for very active patients, every 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). Prosthetic socks are used to manage volume changes during the day as a result of muscle atrophy, weather, or activities. When the patient cannot maintain a comfortable, stable fit with the use of volume-management socks, the prosthetist will make socket adjustments according to volume measurements and muscle shape changes.
An integrated suspension system, which may be part of the cushion interface, is used to help hold the prosthesis on securely. 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 provides greater fluid volume stabilization in the residual limb.
Suction: When the residual limb is put in the socket, air is forced out through a one-way expulsion valve at the bottom of the socket, which results in suction that holds the prosthesis in place.
Interface with a locking pin: A cushion interface with an integrated suspension pin at the bottom is inserted into a locking mechanism embedded in the bottom of the plastic socket. A release button disengages the pin for removing the prosthesis.
Belts and straps: Sometimes the prosthesis is attached by a belt and/or straps if keeping the prosthesis on with vacuum, suction, or pin is difficult or cannot be tolerated.
A prosthesis is comprised of
Socket (plastic receptacle in which the residual limb is contained), which is the most important component because it supports the body and transmits all associated pressure and forces developed during ambulation to the residual limb
Appendages (eg, hand, foot) and joints (eg, wrist, elbow, shoulder, ankle, knee, hip)
Connecting modules that connect appendages and joints to the socket
Advanced components are available. For lower limbs, microprocessor-controlled ankles and knees can provide greater safety, stability, reduced energy expenditure, and diminished stress to proximal joints and the spine. Myoelectric upper-limb prostheses use the natural, electrical signals of a person's muscles; sensing electrodes embedded in the socket of the prosthesis over active muscles detect muscular activity and transmit signals that operate the prosthetic hand, wrist, and/or elbow. No other body movement is required. In contrast, the body-powered upper-limb prosthesis requires a fully functioning shoulder and arm because there is a loop strap around and under the opposite axilla. The strap is connected to the prosthetic hand or hook via a wire cable. Movement of the opposing shoulder stretches the strap/cable system and thereby opens or closes the hand or hook.
Some patients choose to have their prosthesis appear anatomically natural. This is accomplished by applying and shaping a soft foam material equal in consistency to muscle and subcutaneous tissue over the plastic and metal components. This material can reduce clothing damage and be shaped to match the patient's contralateral limb.
A synthetic skin can be applied over the anatomic shape matching the patient's skin tone.
Some amputees, especially athletes during competition, prefer to eliminate the anatomic shape and skin and leave the plastic and metal components exposed.