by C. Michael Schuch, CPO, FISPO, FAAOP


The Amputee Coalition has long wanted to have available a comprehensive explanation of limb prosthetics written in easily understood language for amputee consumers. This material will appear first as a series of articles for InMotion magazine, and later can be assembled into a comprehensive text for further distribution.

One of the most essential needs of an amputee is knowledge about prosthetics. This knowledge needs to be scientifically accurate, up to date, and easily understood by people without training in prosthetics. The concept of the Amputee Coalition GUIDE TO LOWER LIMB PROSTHETICS is to make this type of knowledge available to all amputees.

This in no way implies that the Amputee Coalition believes that prosthetic use is the only way or the best way for any given amputee. Only the amputee can determine this: however, the position of the Amputee Coalition is that each amputee should be given accurate information upon which to base decisions about prosthetics. Through improved education, outcomes can be enhanced, and through improved outcomes, lives can be empowered and improved.

As prosthetic designs, components, materials and types of prostheses are discussed in this introductory article, please keep in mind that the discussion is an overview or introduction. More detail will follow on each of these subjects in successive articles.

Prosthetic Design: Basic Concepts

Illustration 1This endosketal hip-disarticulation prosthesis is constructed from several modular components. The finished prosthesis includes a custom made socket, a hip, joint, a thigh section, a rotator, a knee joint, a shank section, a torque absorber and an ankle/foot unit. Here if is shown without ifs cosmetic cover. (Illustration 1)

Modern prostheses consist of a variety of parts commonly referred to as components. Prosthetists no longer manufacture every part of a prosthesis as was done earlier in this century. Rather, prosthetists and their technicians use a series of manufactured components to assemble an entire prosthesis. There are still some aspects of the prosthesis that are custom fabricated or manufactured within the prosthetist's laboratory, but most of the components of a prosthesis are readily available from manufacturers, suppliers, and distributors of prosthetic supplies and components.

There are two basic types of prosthetic designs, exoskeletal and endoskeletal. Even though they provide many of the same functions, they are made very differently.

Exoskeletal prostheses are the older design with an outer plastic laminated skin or shell and with wood or urethane foam interiors. In the exoskeletal prosthesis, the strength is provided by the outer lamination and the shape or cosmesis is an integral part of the prosthesis.

Illustration 2This endoskeletal hip-disarticulation prosthesis is constructed from several modular components. The finished prosthesis includes a custom made socket, a hip, joint, a thigh section, a rotator, a knee joint, a shank section, a torque absorber and an ankle/foot unit. Here it is shown without its cosmetic cover. (Illustration 2)

Endoskeletal prostheses utilize aluminum, titanium, graphite and other tubular material to form the central supporting structure, and they usually have modular or interchangeable connectors and other components such as knees and feet. The structural strength is from the central, skeleton-like components. The shape or cosmesis is external and removable, usually a soft, foam material covered with nylon hose or flexible, skin simulating, material.

Both types of prostheses should be individually and dynamically aligned to the amputee; however, once constructed and finished, exoskeletal prostheses are permanently set, requiring significant effort and the actual cutting apart of the prosthesis to make changes. On the other hand, endoskeletal prostheses have the ability to be adjusted greatly after construction and finishing. In short, endoskeletal prostheses are generally more adaptable to a changing amputee. Endoskeletal prostheses tend to be lighter in weight, offer more component options with more adjustability, but the cosmesis is less durable and sometimes the endoskeletal prosthesis costs more.

The components that make up a prosthesis vary from one prosthesis to another, depending on the level of amputation and the needs of the individual amputee. All prostheses have certain fundamental components: however, some prostheses have extra or supplemental components. The fundamental components are required for the prosthesis to do its essential job. The supplemental components may allow some special function or provide an enhancement. For example, virtually all prostheses must have a foot component. Transfemoral (above the knee) prostheses require a knee in addition to the foot. Both are fundamental components for the level of amputation and the amputee could not adequately ambulate without them; however, the transfemoral level amputee may also benefit from a knee rotation unit, allowing him or her to rotate the knee and shin of the prosthesis and cross the prosthetic leg over their other leg or perhaps sit cross-legged on the floor. This rotation unit certainly provides a benefit, but is not necessary for basic ambulation; therefore, the knee rotator is a supplemental component. The higher the level of amputation, the more components necessary, including both fundamental components and supplemental components.

Illustration 3Illustration 6At left, the same endoskeletal hip-disarticulation prosthesis with cosmetic cover and nylon hose (Illustration 3)

To the right, This exoskeletal above-knee (trans-femoral) prosthesis consists of a custom made socket, a thigh section, a knee joint, a shank and a foot. (Illustration 6)

Fundamental Components

All lower limb prostheses generally have the following components:

Socket -- the custom made, top portion of the prosthesis that fits around the residual limb. Socket designs vary depending on the level of amputation, the needs of the amputee, and the resulting materials selected for fabrication of the socket. Most sockets are custom fabricated directly from molds or empirical data about an amputee's residual limb. Some prefabricated and volume adjustable sockets are available; these are most often used in the early fitting stages.

Foot -- the bottom or terminal portion of the prosthesis that contacts the ground. There is an abundance of prosthetic foot designs available, with a range of functional characteristics to suit the needs of most any amputee. Prosthetic feet are manufactured outside of most prosthetist's facilities and are generally ordered on an individual basis to fit the requirements of the amputee. Most feet attach solidly to the shank (shin) and do not require a moveable ankle unit; these feet simulate ankle motion in their function. Some feet require ankle units and others are functionally enhanced by ankle units.

Illustration 7On the right, a cut-away view of exoskeletal below-knee (trans-tibial) prosthesis showing socket, outer lamination, inner foam core and Solid Ankle Cushion Heel (SACH) foot attached with bolt. (Illustration 7)

Prostheses for transtibial (below the knee) amputees and higher levels also include:

Shank (shin)-- the portion connecting the foot (and ankle if used) to the upper prosthesis, usually to the socket or the knee unit. In exoskeletal prostheses, the shank is most often rigid urethane foam or wood. In endoskeletal prostheses, the shank is tubular, usually aluminum or graphite with either stainless steel or titanium connectors at the foot and socket or knee. The connectors generally have alignment capability, even after the prosthesis is fabricated and finished.

Prostheses for knee disarticulation (through the knee) amputees and higher levels include:

Knee -- the component that bends (flexes) and straightens (extends) to allow for standing, normal walking, silting, and kneeling. Exoskelelal knees are usually made of wood or rigid urethane foam. Endoskeletal knees are made of a variety of materials, such as stainless steel, titanium, and various composites of graphite, acrylic, or epoxy. There are many classes or types of knees, ranging from the very simple to extremely complicated designs. Endoskeletal knees offer a wider selection of options.

Illustration 8This exoskeletal prosthesis was sawn apart and reglued at the correct angle in order to change its alignment. Note that this above-knee prostheses has a single axis knee in a wood setup and it incorporates a hydraulic knee control unit which cannot be seen in this photo. (Illustration 8)

Prostheses for transfemoral (above the knee) amputees and higher levels must include:

Thigh-- the component between the top of the knee and bottom of the socket in transfemoral amputees, or to the hip joint in higher level amputees. The thigh section may consist of rigid urethanc foam or wood in exoskeletal prostheses and tubing, usually of aluminum or graphite with titanium or stainless steel connectors in endoskeletal prostheses.

Prostheses for hip disarticulation (at the hip) amputees and higher levels must include:

Hip joint-- the hinged component that bends (flexes) and straightens (extends) to allow for standing, walking, and sitting. For this level of amputation, exoskeletal prostheses are rarely used because of their weight. There are a variety of endoskeletal hip joint designs available out of a variety of typical endoskeletal materials.

A final fundamental for all prostheses is a means of suspension, which is essentially the method of securing the socket to the amputee's residual limb. Suspension methods vary and are related to the level of the amputation and the nature of the residual limb and activities of the amputee. Sometimes suspension is obtained by use of a component such as a belt, sleeve, or other device. Other times, suspension occurs by use of a technique such as suction within the socket or tight fit around the condyles of the residual limb. Finally, there may be times when a combination of both is necessary.

Illustration 9Several different prosthetic feet (Illustration 9)

Illustration 4Several different prosthetic knee joints. (Illustration 4)

Supplemental Components

There are a number of supplemental components available for a variety of amputation levels. There are ankle joint modules available that allow motion between the foot and the shank. For swimming prostheses, there is an adjustable ankle component that allows the foot to be adjusted into a pointed-toe position for a swimmer's kick or for use with swimmer's fins. Torque absorber units allow for rotational movements within the long axis of a prosthesis, usually occurring within the shank or thigh. The torque absorber dampens rotational forces and allows graded rotation with a Illustration 5smooth return to the normal position. They are useful for golfers, racquet sport enthusiasts, and even dancers. The knee rotation unit allows the amputee to cross the prosthetic leg over the sound leg for several purposes including dressing, changing shoes, sitting cross-legged on the floor, or gaining clearance for easier entrance into a car.

The rotation unit allows this woman with a hip disarticulation prosthesis to change shoes easily by crossing her prosthetic leg across her sound leg. (Illustration 5)

In the next issue of InMotion, we will outline the details of the materials used in prostheses.

Last updated: 01/01/2017
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