Application of a Three-State Myoelectric Control System

William F. Sauter, C.P.O. (C)


Most commercially available myoelectric control systems, whether proportional or digital, are two-state systems. These systems are specifically designed for the traumatic below-elbow amputee and require two different muscles for the control of the electrically powered hands. The wrist flexor and extensor muscles of the below-elbow stump lend themselves readily for this task, for they are not associated with gross movement or elbow flexion. The prosthetist can easily identify these muscles, and the average amputee learns quickly to produce clean and strong EMG signals from them.

The congenital below-elbow amputee, however, invariably has only one good control site. Our experience has shown that there is a negligible chance of finding a good second control site in short congenital below-elbow stumps (Fig. 1 ). These patients therefore can be fitted myoelectrically only with a three-state control system in which the chosen muscle is trained to produce a strong EMG signal to open the hand and a moderate signal to close it.

The three-state system used in our facility is the U.N.B. Myo-Electric control system designed and manufactured by the Bio-Engineering Institute, University of New Brunswick, Fredericton, New Brunswick, under the Directorship of Professor R. N. Scott. Although the University of New Brunswick system is not the only three-state system in use (Prof. Schmidl in Italy uses a similar circuitry), it is the only such system available in America. It is further worthwhile to note that while this article deals exclusively with below-elbow myoelectric fittings, the University of New Brunswick three-state system has effectively been used in our centre in the fabrication of myoelectrically controlled prostheses for several fore-quarter amputees.

As early as 1961, Ontario Crippled Children's Centre and the University of New Brunswick conducted experimental fittings of the U.N.B, three-state control system to child amputees. Because of the early developmental state of the electronic components, and because of the young age of the patients (7 to 12 years), most of these fittings were short-lived. The experience gained from these fittings, however, resulted in improved circuitry and improved packaging of the control unit and powerpack.

In 1969 the Mark II system was introduced, initially as a flatpack (Fig. 2 ) and later as a double stack (Fig. 3 ). Along with this change came a removable and interchangeable battery pack (Fig. 4 ). While this unit performed reliably and well, it required the use of saline electrode paste to perform properly. The patients found it awkward to inject the paste through the tiny hole in the outside lamination and often found it impossible to gauge properly the amount of paste injected.

Seven girls, all unilateral congenital short-below-elbow amputees, were fitted with Muenster-type sockets and the Mark II U.N.B, control system. Five of these patients were left below-elbow amputees; the remaining two were right. Stump lengths varied from 5 to 9 cm. Two patients eventually abandoned the myoelectric prosthesis. One of these patients used the prosthesis for one year, the second for almost three years. In both cases, the patients stated that the prosthesis was too heavy (approximately 0.9 kg). Of the remaining five patients, three are still using their Mark II type prosthesis every day. The other two patients have subsequently (1976) been fitted with the new Mark III U.N.B. system with good results. In addition to receiving the new pasteless Mark III system, these two patients also were changed from rigid-laminate Muenster sockets to sili-cone-rubber suction sockets. These flexible sockets combine absolute suspension and total comfort through the whole range of elbow motion. Further advantages are offered by the inertness of the silicone material to patients with allergy problems. Increased elbow extension of 15 deg. to 20 deg. is obtained because of the flexibility of the socket brim (Fig. 5 ). Suction is achieved by mounting an automotive inner-tube valve transverse to the distal part of the inner socket. It is interesting to note that none of the patients wearing silicone-rubber suction sockets would ever like to return to the standard rigid polyester socket. The silicone-rubber suction socket seems to be the ideal "coupling" between a short stump and a weighty prosthesis.

The U.N.B. Mark III control unit (Fig. 6 ) has the same characteristics as its predecessors but has been considerably improved in several ways. The most important improvement is the change from Beckman electrodes requiring ECC saline paste to simple stainless-steel buttons. It is regrettable that this change was not made years ago, since it simplifies the donning of the prosthesis considerably. The second improvement is in the packaging of the unit, which is now being encased in an A.B.S. cylinder measuring 5 cm in diameter and 7 cm in length with amphenol connectors on both ends. The motor lead of the hand and the matched but noninterchangeable electrode leads are simply plugged in. This arrangement eliminates delicate solder operations for the prosthetist. The reduced overall length also means that patients with longer stumps can be fitted. The third change involves the use of the Myo-tester in the patient's initial assessment. The Myo-tester and the patient's own control unit are used together as a unit to determine the patient's own signal range. The potentiometers in the control unit are then adjusted to the proper switching levels to obtain smooth control of the electric hand. It is further possible to connect the patient's prosthetic hand to the Myo-tester to provide valuable visual feedback to the patient during the training process. The sometimes lengthy and tiring training sessions of the past are now considerably reduced (Fig. 7 ).

The girls whose fittings were described here constitute a very sensitive group from a psychological point of view. It is indeed very rewarding to hear from the patients themselves how much they appreciate the improved cosmesis of the hand, the powerful grip of the hand, and the freedom from harnesses (Fig. 8 ).

Rehabilitation Engineering Ontario Crippled Children's Centre Toronto, Ontario, Canada