Prosthetic Devices

(using cybernetic devices to enhance human mobility)

Advanced-technological prostheses to provide greater physical achievements

Prosthetics have been defined broadly as device by which humans not only regain abilities thy have lost, but also to add new "unnatural" abilities. In this sense telescopes, bull horns, and airplanes are prosthetic devices. The term is customarily applied to the applications of artificial limbs.

The thought of amputation is repugnant and understandably so. It is instinctive to fear the loss of a limb; this is part of nature's survival mechanism. Mention of an artificial limb in the past made people think of a replacement with a frightening metal hook at its end. Captain Hook, who wielded his sharp prosthetic "hand" as a horrible weapon. Another vicious character in fiction, Long John Silver, got about on a peg leg.

It is unfortunate that the connotation of evil has become associated with the artificial arm and leg. Added to our natural squeamishness about such things, it has made acceptance of amputees a difficult thing, and particularly for the handicapped themselves.

Electronically powered devices making advances in prostheses

With the sophistication of electronic equipment and devices, it is possible to identify and amplify individual tiny voltages produced in nerves by signals from the brain. Electorencephalographers have done this for some time in charting the electrical activity of the brain and of individual portions of it. Now, the idea is being applied to those nerves leading to the patient's extremities with electromyographic signals.

The myoelectric hand prosthesis is an alternative to conventional hook prostheses for patients with traumatic or congenital absence of forearm(s) and hand(s). These prostheses have a stronger pinch force, better grip, and are more flexible and easier to use than conventional hooks.

Myoelectric control is used to operate electric motor-driven hands, wrist, and elbows. Surface electrodes embedded in the prosthesis socket make contact with the skin and detect and amplify muscle action potentials from voluntarily contracting muscle in the residual limb.

The amplified electrical signal turns on an electric motor to provide a function (e.g., terminal device operation, wrist rotation, elbow flexion). The newest electronic control systems perform multiple functions, and allow for sequential operation of elbow motion, wrist rotation and hand motions.

Myoelectric prostheses increase ranges of bodily functionality

Myoelectric prostheses provide improved function and range of functional position as compared to “hook” prostheses. Myoelectrical prostheses can be used for patients with congenital limb deficiencies and for patients with amputations sustained as a result of trauma or surgery.

The device is appropriate for both above-the-elbow as well as below-the-elbow amputees, and for both unilateral and bilateral amputees. Patients must possess a minimum microvolt threshold (i.e., minimum strength of microvolt signals emitting from the remaining musculature of the arm) and pass a control test to be considered a candidate.

Myoelectrical prostheses are indicated for patients at least one year of age or older. Children with congenital absence of the forearm(s) and hand(s) are usually fitted with a conventional passive prosthesis until approximately age 12 to 16 months, at which time they may be fitted with a myoelectrical prosthesis.

It almost goes without saying that the success of any prosthesis depends a great deal upon the patient's own motivation. Motivation is difficult to assess, and many clinics feel that merely participating in the fitting steps, such as muscle training, demonstrates adequate motivation.

Others require consistent use of a conventional prosthesis before a myoelectric limb can be provided. That strategy, indeed, resolves many questions about the patient's motivation (and provides a backup prosthesis as well), but risks denying a myoelectric limb to some who could use it, but cannot or prefer not to use a conventional prosthesis.

Advantges and disadvantages of some of the advanced myoelectric devices

There have been many articles in the news about the latest developments in prosthetic devices that allow amputees to feel heat and cold and even to feel differentiation in pressure. They are high-tech, expensive prostheses that are wonderful to see and hear about.

  • The advantage that makes these devices attractive is also their main problem.
  • They are complicated mechanical devices that are not yet ready to stand up to everyday use and abuse.
  • They are prohibitively expensive for the majority of people.
  • The hope that such devices bring to the world is inspiring.
  • At the same time they can also raise the expectations of people to such a degree that amputees can become dissatisfied with their current prosthesis or prosthetist.
  • Nonamputees also have false expectations of the abilities of normal amputees.
  • They see videotapes of amputees performing in athletic events and think that this option is available for everyday amputees.
  • These athletes are special people who have wonderful talents.
  • They also have very expensive prostheses that are custom fabricated for a specific purpose.
  • These devices are economically unavailable to the general public.
  • Such highly specialized prosthetic devices are great tools for researchers, just as automobile manufacturers use the experience gained at the racetrack to better the everyday automobile.
  • The general amputee population has seen a remarkable gain in function and cosmesis with this research.
  • While the research is valuable, how does it aid the amputee population’s everyday use of prostheses?
  • Do the promises of this research aid the common amputee who has lost a limb due to vascular insufficiencies?
  • The research into different materials and electronic advancements has offered the below elbow (BE) amputee an increased ability to mimic—prosthetically—the movements and look of a normal human hand.
  • The availability of myoelectric prostheses has opened new doors for the “transradial” BE amputee. No longer do prostheses have to look like mechanical devices.
  • Myoelectric terminal devices are patterned after the human hand. The “thumb” and “forefingers” actually move via the electric power provided by the battery.
  • A powered terminal device adds an intriguing option for the transradial amputee. A battery is positioned in the proximal forearm of the prosthesis, which supplies electric power to open or close a terminal device.
  • The distance that the device opens or closes is dependent upon the user being able to control electrical impulses from remaining musculature.
  • Adept users have been able to control the motion of the terminal device to the extent that even an egg could be carefully picked up.
  • There are also devices similar to the human wrist. They enable the terminal device to be rotated to provide a more natural movement or placement.
  • Transhumeral above elbow (AE) amputees can also take advantage of this new technology with the addition of an electrically controlled elbow.
  • This is a major advantage for a patient who has little or no available muscle power to operate a mechanical elbow.
—Excerpts from Cyborg: Evolution of the Superman by D.S. Halacy, Jr.;
Harper & Row, Publishers, New York, 1965.

and from "Great Expectations" by Paul T. Webber,
Orthopedic Technology Review, January/February, 2001.

Pointing to a page about prostheses. The prostheses unit of words.