External Power And The Amputee: An Engineer's View

Andrew L. Lippay, B. Eng.

With all due respect to the very real accomplishments of the artists and artisans of classical prosthetics, the limitations of the conventional or body-powered device must be realized, particularly as the requirements for artificial functions increase with the higher levels of upper-extremity amputation. Indiscriminate extension of basic prosthetic principles soon creates an intolerable maze of harnessing, cables, loops, and levers. In bilateral cases the prosthetist has very little to work with in his efforts to restore function, since the necessary active body segments are either absent or numerically very limited. Surgical connection of muscles to the prosthesis (cineplasty) creates cosmetic and tissue problems which outweigh the advantages of the method.

In recent years, energy sources external to the body, such as compressed gas or electricity, have found a steadily increasing field of application in orthotics and prosthetics. The current functional devices are admittedly primitive compared with their anatomical equivalents, but nevertheless they fill an essential need in partially restoring functional ability and reducing the degree of dependence. Furthermore, these initial efforts have proved beyond any doubt that such man-machine combinations are not only feasible but desirable and useful, and that routine service in the home environment can be accomplished (although not without some problems) even with child amputees.

A recent application of an electromechanical arm complex to a patient with bilateral phocomelia in Montreal1 indicated clearly that a young patient can and will adapt to the idiosyncrasies of a four-function powered device, enjoy the functional freedom afforded her, and resent the removal of the prosthesis, in spite of its rather excessive weight and nonco-ordinated control. Adult amputees equipped with the Canadian version of the myoelectric prosthesis developed in the Soviet Union report significant improvement in functionality and a high degree of integration into the body image2.

Problems of Man-Machine Complex

Disciples of the technical sciences have taken an increasing interest in the problems of powered prosthetics since World War II. It may safely be assumed that, with their assistance, most of the technical problems of hardware design, manufacturing, cosmesis, and material selection will be satisfactorily solved in the not-too-distant future. However, the interface between man and machine will continue to present the greatest single obstacle to the free passage of command f rom the brain to the device and to the flow of information feedback in the reverse direction.

The problems of man-machine communication demand a dedicated and extensive cooperation between medicine and engineering, including all related aspects of biology, neurology, psychology, body mechanics, kinematics, control systems, and information theory, in order to increase the knowledge available on the basic structure and operation of the human neuromotor and sensory nervous systems, and to develop matching systems by means of electronics and mechanisms. Both disciplines will enrich their education by learning to understand the other, and the end result will be a gratifying improvement in the services available to disabled humans.

The amputee and the prosthesis must be considered as a single system if they are to be united successfully and operate efficiently. At present, this rehabilitative man-machine complex is often created over a fairly long period of time and with little or no coordination between the various operators, such as the surgeon and the pros-thetist. Hopefully, such recent practices as the immediate postsurgical application of prostheses will eventually lead to a routine of teamwork and advanced planning involving all concerned, especially when the various "instant prosthesis" methods attain universal acceptance, thereby greatly reducing the overall time span between the incidence of disability and full rehabilitation.

Intermediate Devices and Hybrid Systems

Much remains to be done to improve the cosmesis and functional freedom provided by the appliances of today, powered or otherwise. However, our patients must not be denied any such advantages as are available in present devices or components, even if the results fall short of the concept of an ideal system or an engineering masterpiece.

At the beginning of 1967 a patient who had been a bilateral shoulder disarticulation amputee for 11 years was equipped with an experimental hybrid complex of components in Montreal. Two myoelectric control sites were established on either side of the back. The amputee was trained to associate the contraction of the rhomboid muscle with the opening of the terminal device, and the lower trapezius with its closing on the corresponding side. As he was able to move both acromium points at least 2 cm from a rest position, both vertically and sagittally, without affecting the control muscles on the back, two double-acting pneumatic control valves were installed in each of the shoulder cones of a body vest. One was to control the elbow, the other wrist rotation.

A conventional friction-type shoulder joint was attached to a Vitrathene body vest, supporting the aluminum beam of the upper arm. Internal rotation of the forearm was possible through a friction turntable located above the elbow joints. Pneumatic units marketed by the Bock Company were used to provide powered elbow flexion and terminal device supination-pronation, these functions being controlled by the valves at the shoulder. The terminal devices were the electric hands developed in the USSR, with a high-speed electric motor and gear drive, providing simple three-point prehension. The myoelectric control amplifier and battery, both designed and built in Montreal, were mounted on the upper-arm member and covered with a soft plastic shell. Surface electrodes were embedded in the plastic vest over the myoelectric control sites on the back. The pneumatic reservoir was connected through a long pressure line and carried in a convenient back pocket. Fig. 1 and Fig. 2 provide a diagrammatic outline of the chief functional components.

While operation was slow and far from smooth, the patient was able to eat his first meal in 11 years virtually unassisted and to carry out simple tasks after only one week's training. Furthermore, this installation proved that the muscles of the back, completely foreign to the function of prehension, can be used to control the terminal device through myoelectrics with convenience and quick response and with very little danger of inadvertent operation except during such abnormal conditions as a violent coughing spell. It appears that surface electrodes will maintain good contact all day, even through a light cotton shirt, with only an initial application of Aquasonic jelly to initiate operation.

It may appear to be an engineering heresy to saddle this patient with two batteries and a gas cylinder, and to force him to operate four valves and four myoelectric channels. However, similar applications on unilateral shoulder disarticulation and forequarter amputees have been completed with acceptable cosmesis and an increase in functionality. Physiological conditions or other clinical requirements may well dictate the use of an essentially conventional device, with only one function provided or assisted by external power, even after improved powered devices have become available.

Hardware Development

In the past three years at our Institute, pneumatic and electrical appliances have been prescribed and built in increasing numbers. Myoelectric control has been used in various configurations with encouraging results. A more basic form of research is presently under way, with a view to adapting single motor unit signals to the control of external power. However, in the next two years, an intensive development of hardware and modular components will be necessary to overcome the cosmetic and functional shortcomings of the present devices.

Parallel to this requirement, a control system will have to be developed to produce a smooth, coordinated movement of the arm prosthesis and an almost semiautomatic positioning of the terminal device in the desired work area. A minimum amount of concentration on the part of the patient should be incorporated, but without loss of the feeling of control and command. Multifunction artificial hands and perhaps versatile terminal devices of other configurations will need to be developed and tested in the clinical and home environment on young and child patients3.

Dynamic Cosmesis

The perennial argument between the proponents of the prosthetic hook and of the artificial hand still rages. It is difficult to say who is winning. However, with new hand mechanisms and configurations, designed around the concept of external power rather than the passive cosmetic hand image of the past, the artificial hand will steadily encroach on the functional superiority formerly held by the hook. At any rate, external power will eventually eradicate any excuse that may still exist for the use of paperweights and sleeve-fillers in prosthetics. The promise of dynamic cosmesis-i.e. cosmesis providing both functional and visual normalcy-attainable only with a hand, certainly warrants any and all efforts to bring its development to the highest possible level.

The current methods of energy storage and power transfer, namely pneumatics and electromechanical systems, also have inherent shortcomings which limit their convenient and universal application. Hydraulics, long used in industry and transportation, promise high forces and fast response, good efficiency, reliability, and economy 4 . An experimental bilateral arm prosthesis is now under construction in Ottawa, at the Northern Electric Research Laboratories, and will be applied to one of our teen-age congenital shoulder disarticulees. The energy source is a rechargeable battery, proven by our previous experience to be completely safe, reliable, and simple to use. The concept of hydraulics was chosen in an effort to combine the most advantageous features and characteristics of the pneumatic and electrical systems without incorporating their inherent drawbacks. As in the case of the terminal device, prosthetic and engineering designs must be based on the concept of external power and a complete man-machine system analysis.

Control and Function

The sophisticated prosthetic and orthotic systems expected to appear in the next few years will demand equally sophisticated control methods to present the user with high functional freedom and the sensation of positive command over the device without overloading his capabilities of concentration. The difficult task of drawing the line between voluntary control and automation, between commanded function and robotization, must be undertaken-and soon. As the complexity of arm systems increases, the proprioceptive coupling between the prosthesis and the wearer decreases due to the large "elastic" mechanism and powered joints. Information feedback (feed-in, cueback, etc.) circuits will be needed to provide bidirectional information flow across the man-machine interface. At our Institute a part of this task is presently incorporated in a research effort to define some parameters for training cues or commands given to subjects connected to myoelectric systems.

Electronics offers the possibility of constructing miniature control components capable of driving a complex arm system or other assistive devices in a coordinated fashion with a minimum of control effort or input. A quadruped model constructed in Los Angeles required only seven electronic logic gates per leg to perform a coordinated walking gait over random terrain with good stability. Computer models indicate that end-point control of prosthetic systems will be practicable5.

With careful system design and electronic control, the externally powered prosthesis becomes much more than a tool or a gadget. The patient will accept it eventually as a part of his own body, which turns his thoughts into physical action. The prosthetist, the engineer, the therapeutic staff, and medicine in general will find in it a powerful tool of rehabilitation.

Andrew L. Lippay, B.Eng. is the Engineering Consultant Department of Research Rehabilitation Institute of Montreal Montreal, Quebec

1. Nicholls, P.B., et al, "A Canadian Electric-Arm Prosthesis for Children," Canad. Med. Ass. J., 96:1135-1140, April 22, 1967. 
2. Lippay, A.L., "Clinical Experience With a Myoelectric Prosthesis," ICIB, VI:25-31, January 1967. 
3. Stevenson, David A., P. Eng., "Engineering an Artificial Arm," Proceedings, Centennial Congress of Canadian Engineers, Series "E," Paper E.9.3.1. 
4. Lambert, T.H., and Davies, R.M., "The Future of Hydraulically-Powered Prostheses," Thesis, Dept. of Mech. Eng., University College, London, November 1966. 
5. Personal communication with Professor Rajko Tomovic, University of Belgrade, Yugoslavia, 1967.