The Basics of Prosthetic Limbs and How They Work

How do prosthetics work?

Prosthetics, or artificial limbs, allow amputees to regain function resulting from the loss or absence of a limb. Prosthetic limbs are classified according to the level of amputation: above the knee, below the knee, above the elbow, and below the elbow. Preference in terms of prosthetic use and selection is often based on considerations for functionality, aesthetic, and/or health concerns. 

For leg amputees especially, prosthetics can be essential for mobility and the prevention of muscle atrophy, trunk instability and spinal curvature, depending on the level of amputation. For some arm amputees, aesthetics may be a deciding factor, due to the increased visibility and social usage of the hands. Regardless of the motivation for using a prosthetic, they are an important part of the lives of many amputees. Developments in technology have resulted in a wide range of prosthetic options to address the diverse needs of amputees. 

How do prosthetic hands and legs work?

Prosthetic limbs serve different functions according to the needs and desires of the amputee. Amputees can opt for prosthetic limbs that only serve a cosmetic purpose, that are attached by harnesses or belts, that are worn using sockets with suction systems, or even those that are surgically implanted. 

Some amputees, mostly upper extremity amputees, may choose to use a passive prosthetic, which has little function other than having a natural appearance. These are often very lightweight, which means they require less effort to wear and may be more comfortable. In some cases, passive prosthetics have joints that allow the limb to be moved into various positions to allow the user to carry or stabilize objects. Prosthetic toes and fingers can also be made for cosmetic purposes.

Another option for arm amputees is the body-powered prosthesis. This consists of cables and a harness that is controlled by the shoulders, chest, or remaining part of the arm, depending on the level of amputation. The cables and harness allow for opening, closing, and sometimes rotation of a terminal device (hook or hand) depending on the design of the prosthetic arm. These are a popular choice for people who want a durable prosthetic that allows them to engage in different types of labor. 

Technological advances have also produced myoelectric prosthetics. Myoelectric arms and hands are controlled by attaching sensors which can detect electrical impulses from existing muscles. The strength, speed, and direction of movement are controlled by the intensity of the signals from the muscles. For people without arms, or with muscle or nerve damage, the back or chest muscles can be used. Myoelectric prostheses are battery powered and therefore must be charged for use. 

These are worn using a custom socket which is pulled on using a type of sock; this creates enough suction to attach the limb. These prosthetics can be used with a variety of terminal devices. Some people prefer covers that can be matched to skin tones and resemble natural hands, others may choose terminal devices that have a simpler gripping function, while others opt to use the hands without covers. There are also hybrid, body-powered and myoelectric prosthesis that can be used for higher amputation levels or varying degrees of functionality. 

Additionally, there are function-specific prosthetic arms that have a variety of attachments based on the intended usage. These are typically used for work activities, sports, and hobbies.

For lower-extremity amputees, prosthetics can allow the option of mobility beyond crutches or a wheelchair. A prosthetic leg is fitted based on the shape of the person's residual limb. A mold of the residual limb is made in order to create a socket that serves as the point of attachment. Socket fit is very important because a good fit is necessary for functionality. Otherwise, friction, swelling, skin damage, and discomfort may prevent the limb from being used properly.  

Below-the-knee amputees have a socket that extends to a pylon and a foot. Prosthetic feet can be hydraulic, or have microprocessors, depending on the type of terrain that the person desires to walk on and the activities they want to participate in. There are also activity-based feet, such as the blades that some athletes use for running. 

Above-the-knee amputees can choose from a variety of options in terms of knee selection. Prosthetic knees can have single axis or multiple axis systems, weight-activated or muscle controlled stability, and finally, there are microprocessor knees that allow for a more controlled, natural gait that requires less effort. All amputees exert more energy when walking - oxygen consumption can increase by up to 20%for a below-the-knee amputee, up to 70% for an above-the-knee amputee, and up to 300% for a bilateral amputee. For these reasons, prosthetic selection (or even the choice to use prosthetics or not) is a carefully-considered decision that also entails gait training, numerous socket fittings, and often physical and occupational therapy. 

What are prosthetics made of?

Besides the labor-intensive adaptation process on the part of the amputee, orthotists and prosthetists must also create a wide variety of prosthetic limbs that work for the varying needs of their clients. Prosthetics should be lightweight, durable, and easily attached to the body. 

The raw materials used in creating prosthetics usually involve a variety of plastics, including polyethylene, polypropylene, acrylics, and polyurethane. Lightweight metals such as aluminum and titanium are also used. Additionally, some prosthetics are made from carbon fiber. Electronic prosthetics typically require batteries that must be charged. Some also have Bluetooth capabilities and can be synced with mobile phone apps. 

Many leg amputees wear silicone liners between their residual limb and their sockets. Sockets must be tested over time for comfort and fit, and therefore are usually made from thermoplastics that can be reheated and reshaped during the adjustment phase. When the test socket is finally up to the desired standard, plastic is usually used to create the final socket. The pylon for an artificial leg is typically made from lightweight metals or carbon fiber, which is lighter than metals. 

Prosthetic limbs are usually suspended using suction systems or by a harness, depending on the level of amputation and type of prosthetic. For many prosthetic arms and legs, the suction of the socket may also be improved or aided by a sock, or sleeve, that helps with fit and skin protection. For body-powered prosthetic arm design, harnesses are sometimes made from leather or an artificial material and attached to cables which move the arm using springs and controls the hooks which are typically made of stainless steel. 

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