Upper Limb Prostheses
A prosthesis is an artificial device that substitutes for a missing part of the body. Upper limb prostheses can be applied anywhere from the shoulder joint through the fingers, including the fingers, the hand, the wrist, the forearm, the elbow, the upper arm, and the shoulder.
Most patients require prostheses as the result of amputation. The affected body part must be removed due to severe damage or disease that threatens the patient's survival or is too damaged to be repaired. Amputations of upper limbs are usually due to accidents, particularly in industrial settings. Victims tend to be younger and in good health otherwise, and often have a normal life expectancy. It is particularly important for them to regain
substantial upper-limb function to maintain independence. Upper limb prostheses are also important for those who are missing upper limbs due to congenital conditions. This group includes children, who may use prostheses from very early in life and require regular refitting and revision of their prostheses as they grow.
Patients use upper limb prostheses for two general purposes: to improve their appearance and to increase their ability to perform tasks. Unfortunately, these two purposes often conflict with one another. Prostheses that look like normal hands are often limited in their functionality, while highly functional devices may look unattractive. Many patients use two different prostheses: one for situations in which appearance is most important, and another for situations in which adequate function is desired.
The most important goal for function-oriented upper limb prostheses is reproducing actions performed by the hands. The human hand is capable of many distinct and complex actions, which are often crucial for independent functioning. The patient must be able to grasp and manipulate objects of varying sizes and shapes in order to carry out basic activities such as dressing, grooming, and eating, as well as work-related activities. Most prosthetic devices can perform only one or two distinct actions, and so a large number of specialized prostheses have come into being, each designed for a particular purpose. These include devices designed for particular work functions, such as using tools, and also devices intended for leisure activities, such as holding a golf club or throwing a bowling ball. New developments in the field of prosthetics are raising hopes for a "bionic hand" that is capable of multiple actions, but devices of this type are still experimental.
Materials and construction
Upper limb prostheses can be constructed of a variety of materials, depending on the purpose of the prosthesis. Prostheses used for cosmetic purposes are usually constructed of lightweight plastics, and are designed to match the color and shape of the patient's intact hand. Prostheses used to perform work usually need to be much more durable. These devices usually include components made out of different materials, such as soft plastic or silicon for the socket that fits the device to the patient's
Prostheses can be classified as endoskeletal or exoskeletal. Endoskeletal prostheses consist of a hard inner core covered by a soft outer material. These devices tend to be lightweight, but they are usually less durable than exoskeletal prostheses. Cosmetic prostheses are often constructed with an endoskeletal design. Exoskeletal prostheses have a hard outer shell, which can usually withstand considerable force. Exoskeletal designs are usually preferred for prostheses designed to perform work.
Amputations are usually classified according to the point at which the limb is removed. In general, amputations below the elbow require simpler devices than those that occur above the elbow, because above-the-elbow prostheses require some sort of substitute for the elbow joint. Amputations at or above the shoulder joint add yet another level of complexity to the prosthesis.
TERMINAL DEVICES. Virtually all upper limb prostheses involve some sort of terminal device, which is the most distal part (farthest from the patient's trunk). The simplest terminal devices include a hook, a cosmetic hand, or some other element that has no moving parts, and are referred to as passive terminal devices.
Active terminal devices, which involve moving parts, are much more common. These devices can be shaped like a hook, a hand, or any specialized tool. They often involve one stationary part and one moving part. The patient controls the moving part using a body control device or a myoelectric control, allowing the patient to grasp things between the stationary part and the moveable part. Some devices allow the patient to have voluntary control over closing the device, while others allow voluntary control over opening the device. Patients are able to perform a variety of work-related and self-care activities using these devices.
CONTROLS. The most common control system is the body-powered or mechanical system. With this system, the user operates the terminal device by flexing a muscle near the stump of the amputated limb. The energy from the user's movement is transferred to the prosthesis by means of a stainless steel cable. Body-powered prostheses are popular and are used by about 90 percent of amputees who use a prosthesis. These devices are simple, durable, and easy to use. These systems are also preferred because they provide some feedback to the user, who can detect the action of the terminal device through the cable.
An alternative control system involves myoelectric control of the terminal device. Myoelectric devices detect the electrical potential of contracting muscles and use the potential to control an electric motor that operates the terminal device. Myoelectric devices allow a stronger grip than body-powered devices, and they also provide the ability to regulate the amount of force in the grip. Despite these advantages, myoelectric systems are less popular than body-powered systems. They are expensive, they break down more easily, and they force the user to rely on battery power. These systems also provide less feedback to the user, who must rely on vision to regulate his or her activity. Technological improvements are making these devices more reliable, and they may become more popular in the future. Some devices combine myoelectric controls for the terminal device with body-powered controls for the elbow joint.
SOCKETS AND HARNESSES. The fit between the prosthesis and the body is an important element in assuring that the prosthesis is comfortable and functional. Most upper limb prostheses have a pliable socket that is custom molded over the stump to assure an exact fit. Many above-the-elbow systems depend on a harness to hold the prosthesis in place and provide an attachment point for control devices. Harnesses are usually made of Dacron straps that fit over the shoulders or around the upper arm. Some upper limb prostheses can be attached without a harness.
Prompt fitting is very important. Research has shown that patients are more likely to reject an upper limb prosthesis if it is fitted more than 30 days after amputation surgery. Early fitting also helps control swelling and pain in the stump. If the patient's surgeon feels it is appropriate, the mold for a temporary prosthesis can be made immediately after surgery. The patient can begin adjusting to the prosthesis and can begin to make decisions about the features he or she wants in a permanent device. The patient may wear the temporary prosthesis for several weeks while the size and shape of the stump are stabilizing.
Patients require considerable time and training in order to accept and use upper limb prostheses. They must learn to detect and control fairly subtle movements in or near the stump in order to control the prosthesis. They must also learn how to care for the stump and how to prevent pressure sores. Finally, they must adjust to the changes in their lives brought about by the loss of an upper limb. Training usually takes place over the course of several weeks, during which the patient increases wearing time and begins to use the prosthesis for functional
activities. The training and adjustment period can be lengthy, and it is important for the patient to have a continuing relationship with supportive therapists.
Patients can use upper limb prostheses to perform a variety of functions, but motivation is often a limiting factor. The biggest obstacle to using the prosthesis may be the patient's reliance on the intact hand. If the patient becomes used to doing things with one hand, it will be much more difficult to adjust to using a prosthesis. Patients who refuse an upper limb prosthesis may eventually suffer from overuse injuries in the intact limb. Patients with bilateral amputations are forced to rely on prostheses in order to perform the functions of daily living, and tend to accept the prostheses readily, since they are a means of restoring function.
The maintenance needs for upper limb prostheses vary with the complexity of the device. Devices that incorporate electric motors require regular battery changes and tend to need more maintenance than body powered or passive devices. Cosmetic gloves that are used to make the prosthesis look more like a hand also require maintenance. The gloves can become stained or torn and may need frequent cleaning and replacement. Patients also need periodic adjustments to their prostheses to ensure that they work and fit properly.
Health care team roles
The process of creating and employing upper limb prostheses involves several health care professionals. Physicians who specialize in physical medicine and rehabilitation usually prescribe the prostheses, and patients learn to use the devices in a rehabilitation setting. Prostheses are fitted and custom-built by prosthetists, who are specially trained technicians in this field. Occupational therapists help patients learn to perform adaptive tasks using the prostheses. Patients tend to respond best when the professionals involved work as a team and provide the patients with ongoing support.
Prosthetics is a specialized field with a complex body of knowledge. Certification as a prosthetist requires a baccalaureate degree in the field of orthotics and prosthetics, or a degree in another field, followed by a six-month to one-year certificate training program. Prosthetists must also pass a certification exam, and may require additional certification in order to fit certain specialized devices.
Body-powered prosthesis—A prosthesis that is controlled by mechanical action. The user flexes a muscle and the energy from this action is transferred to the prosthesis by means of a cable.
Endoskeletal prosthesis—A prosthesis that is constructed with a hard core and a soft outer covering.
Exoskeletal prosthesis—A prosthesis that is constructed with a hard outer layer.
Myoelectric prosthesis—A prosthesis that is controlled by electrical impulses. The patient flexes a muscle and the electrical potential of the activity is used to control an electric motor that operates the prosthesis.
Prosthetist—A technician who specializes in the fitting and building of prosthetic devices.
Terminal device—A device attached to the distal end of a prosthesis. The patient uses the terminal device to perform functional activities.
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Denise L. Schmutte, Ph.D.