Prosthetic Components

Interface between the residual limb and prosthesis

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 to provide skin protection and modulate and more evenly distribute pressure. Custom molded interfaces may be necessary to accommodate irregular residual limb 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 other 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.

Integrated suspension system

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.

• 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: 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.

Structural components of a limb prosthesis

A prosthesis is composed 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. Microprocessor-controlled prosthetic knee systems for above knee, hip-disarticulation, and hemipelvectomy amputees, are computer programmable hydraulic-controlled systems that the clinician programs based on the patient’s walking. Swing- and stance-phase functions are programmed based on daily activities. This usually takes 2 to 4 weeks for initial programming, followed by several appointments over the course of 12 months to refine the programming.

In the upper limb, electrical sensors are embedded on the inner surface of the prosthesis socket (that contains the amputees residual-limb) or on the inner surface of a silicone gel sleeve that the amputee places on the residual limb. A microprocessor-controlled system is contained within the prosthetic hand. The system can recognize specific electrical signal patterns (EMG) generated when the patient intends to function the hand (eg, to point the index finger). The extensor indicis muscle is activated allowing the prosthetic hand to extend the index finger for pointing. Likewise, the EMG signal pattern of the set of muscles that are required to grasp a glass are recognized by the microprocessor, and the fingers and thumb are activated accordingly. This allows intuitive and fast-acting grip patterns.

For upper limbs, myoelectric 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.

Cover and appearance of a limb prosthesis

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 people with an amputation, especially athletes during competition, prefer to eliminate the anatomic shape and skin and leave the plastic and metal components exposed.