A Myoelectrically Controlled Prosthesis Using Remote Muscle Sites
PETER H. STERN. M D.
All members of the amputee clinic team are acutely aware of the difficulty in providing satisfactory prosthetic function for the shoulder-disarticulation or forequartor patient. These types of deficits seem to occur quite frequently in children by reason of congenital absence, trauma or disease. Hence many attempts have been made to develop externally powered devices for this group of amputees.
We have recently had quite a positive experience in fitting a myoelectrically controlled, electrically powered prosthesis to a young adult with a scapulo-thoracic disarticulation. This report is written in the belief that similar equipment can be successfully applied to juvenile amputees.
W.L., a 23-year-old male, developed a painful left upper arm with a slowly progressing wrist drop. The clinical diagnosis, supported by electrodiagnostic procedures, revealed an axon stenosis of the radial nerve. As a temporary measure, W.L. was provided with a dynamic wrist and finger-extension orthosis of our design. Neurosurgical exploration revealed the presence of an apparently malignant mass entrapping the radial nerve. Upon pathological examination, a malignant schwannoma was discovered, necessitating a scapulo-thoracic shoulder disarticulation. Initially the patient was supplied with a shoulder cap and a passive arm and hand. It was soon noticed that he had detached the functionless arm and hand, and was wearing only the shoulder cap for reasons of convenience and cosmesis. The young man was, however, in desperate need of a functional prosthesis in order to pursue his career as a commerical artist.
Patient Evaluation and Device Description
As no useful muscles remained on the amputated side, the muscles of the contralateral side were evaluated electromyographically for control, using two-channel equipment-Aided by the audiovisual display of the EMG, the patient readily learned to isolate contractions of the pectoral muscles from those of the supraspinatus. Myometric evaluation followed and the optimal locations for placement of the pick-up electrodes were determined.
The leather harness suspending the shoulder cap was used to incorporate the surface electrodes in the following manner: the electrode for closing the electric hand was placed over the pectoral muscles. The electrode for opening the hand was located over the supra-spinatus muscle and kept in place by a leather strap connecting the anterior and posterior portions of the harness. This connecting strap also contained the control circuit. Protected wire connections were made, enclosed in the harness, and attached to the rechargeable battery pack* located in the upper-arm shell, and to the terminal device. The elbow was a standard Hosmer E-400 model with a spring forearm lift assist.** This elbow was flexed and extended manually and locked and unlocked by means of a nudge control located on the anterior part of the shoulder socket. Fig. 1-A and Fig. 1-B below show anterior and posterior views of the completed prosthesis.
The patient has proportional control over grasp and release of the electric motor-driven hand, and is able to hold light or heavy objects and to carry out fine or gross manipulations. He has learned to keep the pectoral and supraspinatus muscles "silent" while using his right arm and hand, thus obviating the necessity of deactivating the electric hand by a switch located under the cosmetic glove. The unit has worked faultlessly over a period of six months, and has made a significant impact on this patient's life.
The development of myoelectrically controlled electrically powered prosthetic and orthotic equipment which is truly functional is a substantive contribution of the engineering sciences to clinical medicine. The actual and successful application of such sophisticated equipment to patients requires a collaborative effort between the physician, the bioengineer, and the prosthetist. The main focus of these efforts is, of course, the patient. He must be well motivated, have a considerable degree of learning ability, and a reasonably well-defined purpose for utilizing these products of modern technology.
At the moment, commercially available components, mostly imported, are limited in their application to selected cases, primarily for below-elbow amputees, using "physiologic" muscle sites for electrode placement. The experience gained in the described patient has encouraged us to apply similar systems to other patients where "body power" control was not feasible and physiological electrode placement not possible. The lack of a satisfactory elbow unit which is externally powered and commercially available has been a major drawback.
Summary and Conclusions
Specialists in the fields of medicine, bioengineering, and prosthetics have collaborated in the successful application of a prosthetic device to a young patient with a scapulothoracic shoulder disarticulation secondary to a rare malignancy. The device utilized remote muscle sites for myoelectric control of an electrically powered hand. The results were encouraging enough to warrant further applications of such systems.
Additional Component Specifications
Electrodes and preamplifier combination block Dimensions: 2.4cm x 1.4cm x 0.9cm
Impulse frequency modulation proportional myoelectric control circuit Dimensions: 4.3cm x 3.9cm x 0.75cm Total current drain: 0.9 m A. Weight: 20 gr.
We wish to express our thanks to Mr. Yves Lozac'h and the staff of the Department of Research, the Rehabilitation Institute of Montreal, Quebec, for their cooperation and assistance; and the Hanger Company of New York City for help given in the fabrication of the prosthesis.
1. Editorial-Is this the wave of the future? Newsletter . . . AMPUTEE CLINICS, 4 :5:7-8, October 1972.
2. Lozac'h, Yves, An improved and more versatile myoelectrical control. Inter-Clin. Inform. Bull., 11 :8:13, May 1972.