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Written by: Jeremy Dailami
Written on: May 4th, 2002
Tags: biomedical engineering, health & medicine
Thumbnail by: Xiong Chiamiov/Wikimedia Commons
About the Author
In Fall 2002, Jeremy Dailami was a junior at the University of Southern California pursuing a B.S. in Biomedical Engineering. As a student, he enjoys surfing, long walks on the beach, and aims to pursue a career that involves mass-producing industrial sized microwave ovens.
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Volume III Issue I > The Myoelectric Arm: It's Electrifying
With the help of scientists and engineers, individuals missing appendages can be given a chance to live a life in which their amputee status is an afterthought. The recent progress of prosthetic engineering has enabled scientists to design artificial limbs that function nearly as well as biological ones. On the forefront of this technology is a prosthetic device called the myoelectric arm. What makes this prosthesis so unique is its ability to function with the amputee's muscle movements, reacting accordingly. It enables wearers to pinch, grip, and release objects, unlike earlier prostheses. The device is compatible with daily activities and can be donned and doffed independently.

Introduction

One of the most commonly amputated or missing limbs is the arm, which is used in many vital movements. Opening doors, pulling out a chair, or even rising from bed all require the use of the upper body. The loss of an arm can be understandably overwhelming.
However, with modern technology and the emergence of rehabilitation engineering, the effects of losing a limb are not as catastrophic as they once were. The superiority of today's artificial limbs may go unnoticed, since they have become so sophisticated that many amputees are able to participate in activities that once seemed inconceivable.
Vast technological advances have been made in designing upper extremity (shoulder, arm, hand) prostheses. Progressing from peg arms and hooks, the appearance of electrically powered prostheses has revolutionized the artificial limb industry. These prostheses not only help amputees live normal lives, but also look realistic enough to alleviate self-consciousness (Fig. 1).

Prosthetic History

Afurse/Wikimedia Commons
Figure 1: Prosthetic limbs allow amputees to live normal lives while looking realistic to alleviate self-consciousness.
P​rosthetic devices have existed for centuries. In 218 B.C. the Roman general Marcus Serigus guided his troops against Carthage in the second Punic War and suffered over twenty injuries, including the loss of his right arm. An iron hand was created so he could continue to fight in the war [1].
The American Civil War prompted the establishment of the American prosthetics field. Reports show that there were at least 30,000 amputations on the Union side alone [1]. But the industry soon faded away and it was not until the United States entered World War I that a revival of prostheses production occurred. The industry was further developed by the advent of the telephone and phone directories. Doctors were able to place illustrated advertisements, attracting more customers [1].
In the 1960's, German scientists created the first functional hand, followed by American development of the first working prosthetic arm. However these prostheses were not always well received by the amputee community, due to great user discomfort.

Background

In the United States alone, there are an estimated 10,000 new upper extremity amputees annually [2]. Many of these are trauma-related, some are disease-related, and some are due to congenital deficiencies.
With the exception of children born missing a limb, individuals can begin the process of obtaining an artificial limb immediately following the loss of an appendage. Designing and fitting a prosthesis for an individual occurs in two stages. The first stage involves fitting the amputee with a preparatory prosthesis. This is perhaps the most crucial stage, for it helps the residual limb - any remaining part of the limb - to heal. During this stage, the amputee has a chance to adjust to the artificial limb, which may assist in the final design of the prosthesis.
The preparatory prosthesis is worn for two months. During this time, any sizing problems or other concerns of the wearer are addressed. The next step involves taking the preparatory prosthesis and creating a more durable and long-term prosthesis by molding it from harder materials, then giving the prosthesis a silicon or latex covering.

The Electric Arm

GeeJo/Wikimedia Commons
Figure 2: After an injury in Iraq, Lance Cpl. Brandon Mendez now has a new myoelectric arm.
The most recent additions to the prosthetic field are electrically-powered​ limbs controlled by electrical signals from the body. When first introduced in the late 1960's, electric hands and arms seemed a science fiction dream from television shows such as The Six Million Dollar Man. However the early model of the electric prosthesis was bulky, with a loud motor to control finger movement. These early models were very noticeable, and made amputees wearing them uncomfortable and embarrassed. However in the past four decades, improvements have made such prostheses more inconspicuous. The most popular and advanced arm on the market today is the myoelectric arm.

How does the myoelectric arm work?

Electric prostheses use small electric motors to move the replaced limb. These motors can be found in the terminal device (hand or hook), wrist and elbow. An electrically-powered​ prosthesis utilizes a rechargeable battery system to power the motors. Since electric motors are used to operate hand function, grip force of the hand is significantly increased in comparison to earlier functional prostheses, often in excess of 20-32 pounds [3].
There are many ways to control an electrical prosthesis, one of the more popular being myoelectric control. Whenever a muscle in the body is contracted, or flexed, a small electrical signal called an EMG in the range of 5 to 20 microvolts is created by a chemical interaction in the body [4]. A typical light bulb uses 110 to 120 volts, so the signal generated by the body is less than a millionth of the strength of a light bulb [4].
One of the key components of the myoelectric arm is the electrode attached to the surface of the skin to record the EMG signal. Once recorded, the signal is amplified, then processed by a controller that switches the motors on or off in the hand, wrist, or elbow to produce movement and function [4].
Not everyone can wear the myoelectric arm. Users must be able to produce an EMG strong enough to be recorded and sufficiently amplified. Users must also be able to separate muscle contractions. Separating contraction means that when one muscle is contracted, the opposing muscle is relaxed. If both muscles were contracted at the same time (co-contraction), the controller would receive signals to both turn the motor on and off at the same time. This would signal the hand to open and close simultaneously, resulting in no function [5].

Advantages and Disadvantages

There are several advantages to wearing an electric prosthesis like the myoelectric arm. Most people prefer this type of control because non-electric prostheses are often laborious to operate, whereas simply simply flexing a muscle can control myoelectrically powered prostheses. They eliminate the need for the tight harness amputees have to wear if they choose a non-electric prosthesis. Since electric prostheses do not have to utilize a control cable or harness, cosmetic skin made of silicon or latex can be applied to the prosthesis, greatly enhancing the cosmetic restoration [2].
Perhaps the greatest advantage of the myoelectric arm is the operational range. It can be used over the head, down by the feet, and out to the sides of the body. Such movements are nearly impossible with cumbersome, non-electric prostheses.
Unfortunately, the myoelectric hand is not perfect. One of the major inconveniences of electrically powered prostheses is the required battery system. Such a system needs a certain level of maintenance, including charging, discharging, and the eventual disposal and replacement of the battery. Electrically powered prostheses also tend to be heavier than other prosthetic options due to the weight of the motor and batteries. However, advanced suspension designs have minimized the weight greatly.
Another disadvantage is potential malfunction of the arm, resulting in costly repairs. Wearers also have to be very cautious around water. Severe damage to the motor and controller can result from water exposure.
Cosmetically there seems to be no disadvantages over traditional prostheses. Yet under extreme conditions, latex covered prostheses are prone to staining, so several coverings may be necessary throughout the device's lifetime.
There are several companies that currently produce the myoelectric arm, including Motion Control, Otto Bock Orthopedic Industry, Hosmer, and Liberty and Technology Prosthetics and Orthopedics.

Conclusion

The continuing development of artificial limbs has allowed amputees to live more normal lives while giving them a sense of hope for future improvements in prosthetic technology. Though the prosthetic industry has made enormous strides in the last century, there is still much work to be done. As prostheses expert Dudley Childress observed, improved shaping and a better understanding of soft tissue mechanics are needed to create better input data for electrically- and computer-controlled prostheses [6].
Choices of prosthetic hands are not limited to the myoelectric arm, which is in fact only one of several options. An amputee may have a prosthetic arm fastened across the shoulder and operating through a hook system. Some individuals are better suited for purely cosmetic arms, which are tremendously lifelike, but lack mechanical function. Even with all these options, approximately one half the amputated population prefers not to use any kind of prosthesis due to the often arduous method of operation.
The next century of prosthetic design will lend itself to incredible discovery and innovation. Current research findings indicate that people could actually regain perception of hot and cold through prosthetics like the myoelectric arm. Work is also being done involving designing a hand capable of moving all the fingers and joints separately. Additionally, the improvement of battery life is very important for future prosthetic design. With these technological advances on the horizon, the quality of life for amputees is bound to improve, making the loss of a limb not the loss of a normal life.

References

    • [1] Michael John. "History of Prosthetics." O&P Business News pp. 14-20, 1996.
    • [2] Advanced Arm Dynamics. Internet: http://www.advanceda​rmdynamics.com, 28 March 2002.
    • [3] "Motion Control." Internet: http://motioncontrol​.com, 28 March 2002.
    • [4] Animated Prosthetics. Internet: http://www.animatedp​rosthetics.com, 28 March 2002.
    • [5] Nissim Benjuya. "Myoelectric Hand Orthosis" Journal of Prosthetics and Orthotics, pp. 149-154, 1990.
    • [6] Dudley Childress. "The Myoelectric Arm." Journal of Prosthetics and Orthotics, pp. 30-32, 1990.