A Myoelectric Prosthesis For A Forequarter Amputation

Dow S. Dorcas, M.Sc Vaughn A. Dunfield, M.Sc.E. Barbara J. O'Shea, O.T. Reg.

The research reported herein is supported in part by grants from The Department of National Health and Welfare (Public Health Research Grant No. 603-7-9), The National Research Council (Operating Grant A-1083), The Medical Research Council (Grant MA-2881), The Government of New Brunswick, The Workmen's Compensation Board of New Brunswick, The Canadian Council for The Rehabilitation of the Disabled (New Brunswick Division), and The Associated Alumni of The University of New Brunswick.

The greatest challenges in upper-extremity prosthetics are presented by the high-level amputee, especially the patient with a shoulder disarticulation or forequarter amputation. In order for a prosthesis to be accepted in these cases, the function provided must be sufficient to outweigh the problems encountered in wearing an essentially cumbersome device.

Typically, the conventional fitting for a forequarter amputee includes a passive shoulder joint, an internal locking elbow with dual control cable, and a cable-operated hand or hook. Control harnessing is difficult, often requiring thigh or waist straps. Even with these devices, the amputee frequently does not have sufficient power and/or excursion to operate the elbow and hand, with the result that the prosthesis is often rejected shortly after it has been fitted.

This report describes a prosthesis fitted to a forequarter amputee. It may be considered successful to date, in that the patient still finds it useful six months after fitting and training have been completed.

Mrs. J.A., a 25-year-old registered nurse, had a left forequarter amputation on November 28, 1966. As she was anxious to return to hospital nursing, Mrs. A. requested a hand rather than a hook, believing that a hand would be less upsetting to her patients. She also rejected the conventional cable and harnessing patterns and asked if it might be possible to provide external power. At this point she was referred to us for consultation regarding myoelectric controls for a suitable prosthesis.

After a study of the available powered components capable of providing elbow function and prehension, we concluded that the combination which offered the best possibility for satisfying her needs was an electric elbow designed at the Ontario Crippled Children's Centre1 and a Rancho Los Amigos Hospital cable puller2, operating a conventional Dorrance #3 hand. To confine the operation of such a prosthesis to the affected side, thus retaining unrestricted function of the right hand and arm, it was necessary that myoelectric control of both functions (elbow flexion and prehension) should be attempted. The project was started on June 12, 1967.

Control Sites

The first aspect of the fitting was the selection of two control sites. From the surgeon's report it was evident that for all practical purposes the lower trapezius had been completely removed. Therefore, the choice was between the pectoralis major, upper trapezius, and latissimus dorsi muscles .

A myoelectric trainer3 was used to evaluate the response from each muscle. With this instrument it was found that the strongest signals were received from the upper trapezius and the latissimus dorsi.

Controls Training

Training of these two sites was conducted, first with the myoelectric trainer and later with an electric hook. Independent use of each muscle at three levels of contraction was required-that is, complete relaxation, a small contraction, and a stronger contraction.

The two muscles were first trained separately. Control of each muscle was considered satisfactory when the patient could reach and hold all three levels of muscle tension on command without interference from other muscles. After this control had been achieved, the next step was to isolate one muscle from the other. This separation was done by using a trainer on each muscle to monitor activity. As their original functions were not similar, the patient had very littly difficulty in achieving independent control. This control could be more difficult if the two muscles were in close proximity to each other or had similar functions.

Another difficulty anticipated was that of maintaining relaxation in each muscle during head or trunk movements or activities involving the other arm. If the muscle contracted during these movements, undesired activity of the prosthetic components would result.

This possibility of inadvertent operation did not prove to be a major problem in Mrs. A.'s fitting. Some inadvertent activity occurred due to contraction of the latissimus dorsi when reaching with the right hand involved trunk rotation. With training this difficulty was partially overcome, but had not been entirely eliminated by the time the patient was discharged. However, since the patient is aware of the difficulty, she is able to take precautions against the occurrence of accidents or inconveniences which might result. If this inadvertent operation continues to be a problem, a long-term conscious effort on the part of the patient may be necessary before it can be eliminated, if indeed it can ever be overcome.

This controls training was given simultaneously with the fabrication of the prosthesis and the construction of the electronic components.

Prosthetic Fitting

The prosthesis (Fig. 1 ) was fabricated at the Bio-Engineering Institute by W. F. Sauter, an exchange prosthetist from the Ontario Crippled Children's Centre, Toronto. The socket, made of San-Splint," was formed over a positive plaster mold. It consisted of a half body jacket extending from the spinal column in the back to the sternum in front, with a cutout for the breast. A collar extended around the neck on the nonamputated side to provide some suspension and stability. A Hosmer universal shoulder joint was used. This joint permits passive movement in three planes: flexion-extension, abduction-adduction, and medial and lateral rotation. A cover of silastic-impregnated stockinette improved the cosmesis of the shoulder. A hammered aluminum half shell open on the lateral side formed the humeral segment, which contained all of the electronic components (Fig. 2 ). A cosmetic fairing of San-Splint covered the entire segment but could be easily removed for repair or adjustment of the components. The electric elbow, a forearm shell of San-Splint, a nylon constant friction wrist unit, and a Dorrance #3 hand and glove completed the prosthesis.

Electronic Control System

The unique part of this fitting was the electronic control system. Since no electric hand was available when the fitting was started, the only way in which a hand could be provided was to use an electric cable puller to operate a standard hand. The cable puller used was the one designed at Rancho Los Amigos Hospital in California. This unit is controlled by a myoelectric control unit designed and built here in the Bio-Engineering Institute of the University of New Brunswick 3,4.

Initially a three-state control was considered necessary to give active control over both opening and closing. However, on closer analysis of the functions required, it was decided that it was seldom necessary for the hand to remain in an open position. Therefore the operation was changed to a two-state control with active opening and automatic closing. This system resulted in easier operation for the patient, as well as some simplification of the electronics. The latissimus dorsi muscle was used to control this unit.

The electric elbow is controlled by a three-state University of New Brunswick myoelectric controller 3,4. The upper trapezius muscle operates the elbow, with the weak contraction providing extension and the stronger contraction providing flexion. The elbow remains in the desired position when the muscle is relaxed.

To conserve battery power, limit switches were provided in both elbow and hand. The elbow employs built-in micro-switches to turn off the motor at both extremes of the range, thus providing positioning stops as well as conserving power. Since the hand was not equipped with a micro-switch, an electronic current limit was also provided. This unit senses the increase in motor current and switches the motor off as the hand reaches the fully open or closed position.

After the controls had been fitted in the prosthesis, the gain was adjusted to suit the patient's requirements. The sensitivity or gain setting of the control unit is partially determined by the strength of contraction available for repeated usage. If the gain is too low, the muscle will fatigue quickly and the patient will be unable to operate the device as often as is necessary. On the other hand, if the gain is set too high, a very slight contraction will bring about operation of the unit. This situation is undesirable, since the patient may be unaware of very small contractions and thus be unable to control the resulting inadvertent activity. During the initial stages of training, the sensitivity of the control units was adjusted to the optimum level to ensure the best functional use of the prosthesis .

Application of Electrodes

Beckman surface electrodes5 were used with either an electrode paste or cream. In order to facilitate independence in putting on and removing the prosthesis, these electrodes were attached directly to the socket. After we experimented with several methods of attaching them, the most satisfactory means was found to be a combination of double-sided tape and sponge rubber. The electrodes were first mounted on a piece of sponge rubber approximately one-half inch thick. This material provides some accommodation for movements of the skin relative to the prosthetic jacket. The sponge is then applied to the correct position in the socket with double-sided plastic tape. The tape must be reapplied at intervals of approximately two weeks, but the patient can accomplish this re-application independently. Two active electrodes are placed over each controlling muscle, with one ground electrode placed in a convenient location. The electrodes are wired directly into the control unit, thus eliminating connectors .


Power to the control units, the electric elbow, and the cable puller is supplied by rechargeable nickel cadmium batteries. One 24-volt battery powers the two control units, one 12-volt battery supplies the two powered components (i.e. elbow and cable puller), and an auxiliary 12-volt battery powers the current limit device of the cable puller. (This additional battery was a temporary expedient to permit use of available relays in the current limit circuit.) These batteries were fastened into a flat pack covered by flesh-colored plastic, which the patient wears around her waist on the right side. The battery pack is suspended by straps from the body shell of the prosthesis in order to provide some counterbalance to the weight of the prosthesis. The total weight of the prosthesis is 5 pounds, while the batteries weigh one additional pound.


As previously mentioned, the controls training was carried out continuously during the fabrication of the prosthesis and the construction of the components. Functional prosthetic training began after the prosthesis had been completed and was carried on for a period of approximately four weeks. The methods used were similar to the established practices for any type of prosthetic training. Emphasis was placed on functional activities related to everyday living, such as dressing, eating, and household skills (Fig. 3 and 4 ); recreational and leisuretime activities; and skills related to the patient's profession as a registered nurse. Since she had not previously worn a prosthesis, the training time was longer than that normally required, as it also included organizing and preplanning activities, prepositioning of the hand, and optimum placement of objects in the terminal device.

Functional activities as opposed to drills provide a more realistic picture of the demands that will be made on the components, and will precipitate any problems the patient may encounter in day-to-day activities.

Daily care of the prosthesis and the skin is always an important aspect of amputee education. However, it assumes even greater importance than usual when myoelectric controls are used. In order to maintain proper function of the controls, the electrodes must be washed each time the prosthesis is removed. A hypodermic syringe with the needle blunted is a convenient way to force warm water through the electrodes to remove the paste. Skin care is also important to ensure against skin breakdown. If the skin and prosthesis are not cleaned each day, the skin under the electrodes may become irritated. If this occurs, the electrodes must be left off until the irritation has been cleared up.


In this paper we have attempted to describe one type of prosthetic fitting for a forequarter amputee. In no way do we mean to indicate that the fitting used is an ideal solution to the problem. It is obvious that prosthetic development has a long way to go before a high-level amputee can be fitted with a prosthesis that is indeed a satisfactory replacement for the lost functions. However, until we can reach this objective, we must use the best in equipment, skills and knowledge that we have acquired to date.


The Bio-Engineering Institute acknowledges with gratitude the assistance of Mr. W. F. Sauter, who did a first-class job under rather difficult circumstances.

San-Splint is the trade name for a thermoplastic material developed by Polymer Corporation and supplied by Smith and Nephew.

Dow S. Dorcas, M.Sc is Research Associate, of Vaughn A. Dunfield, M.Sc.E. is Research Associate and Barbara J. O'Shea, O.T. Reg. is Research Associate Bio-Engineering Institute University of New Brunswick Fredericton, New Brunswick, Canada

1. McLaurin, Colin A., "On the Use of Electricity in Upper Extremity Prostheses," Journal of Bone and Joint Surgery, British 47B:448-452, August 1965. 
2. Karchak, A., Allen, J.R., Nickel, V.L., Snelson, R., "The Electric Hand Splint," Orthopedic and Prosthetic Appliance Journal, 135-136, June 1965. 
3. Dorcas, D.S., Libbey, S.W., and Scott, R.N., "Myo-Electric Control Systems Technical Note No. 2," University of New Brunswick Bio-Engineering Institute, Research Report 66.1. 
4. Dorcas, D.S., and Scott, R.N., "A Three-State Myo-Electric Control," Medical and Biological Engineering, 4:367-371, July 1966. 
5. "Beckman Biopotential Skin Electrode," Instruction Manual, Spinco Division of Beckman Instruments Inc., Palo Alto, California, October 1965.