Myoelectric Single-Site Electrode Fitting for a Short Below-Elbow Amputee

CHARLES F. SCHULTZ, C.P.,* AND ALFRED E. KRITTER, M.D.**Milwaukee, Wisconsin


A boy with congenital left partial transverse terminal hemimelia is presented, with particular concern to prosthetic fitting, acceptance, and use of the prosthesis.

Since the introduction of myoelectric prostheses, problems have existed with regard to fitting the short below-elbow amputee. These issues include: 1) obtaining adequate suspension of the prosthesis, 2) maintaining good contact with the electrodes, 3) differentiating muscle contractions to operate the prosthesis successfully, 4) using the prosthesis effectively in a variety of positions and situations, and 5) locating adequate socket area to contain the electrodes. Problems are intensified when the child has a congenital limb anomaly and has a residual limb which measures less than 4.5 cm from the olecranon to the distal end (Fig. 1 ).

The patient was first seen in Juvenile Amputee Clinic at Milwaukee Children's Hospital in 1965. At that time he was approximately 2 years old. Because of the short residual limb and the age of the patient, the prescription was written to fit him as an above-elbow amputee. The elbow was fixed at 45 deg., a mitten-type terminal device was supplied, and shoulder-cap suspension was used with minimal harnessing.

When he was 4, the patient was fitted with a conventional below-elbow prosthesis using flexible hinges, a single-control cable, and a figure-of-eight harness and hook. The prosthesis was replaced three years later when it was outgrown. The same prescription was written for the new prosthesis.

At the age of 10, the boy had a new prosthesis prescribed to accommodate growth. The prescription was changed to use a below-elbow Muenster socket for suspension, figure-of-nine harnessing, and a hook.

The socket was replaced when the boy was 13, due to growth and changes in configuration of the residual limb.

From the age of 2 to 15, the patient generally demonstrated a willingness to wear and use a prosthesis, although the boy had periods of rejection.

The decision to fit a myoelectric prosthesis was made when the boy was 15. He was seen in the Amputee Clinic for routine follow-up. Because the residual limb was only 4.5 cm long, there was some doubt that a myoelectric fitting would be successful. Myoelectric testing and training were prescribed first. If the results o1 the testing and training did not indicate the probability of a successful fitting, then the prescription would be changed to fit the patient with a switch-controlled electric prosthesis.

A Fidelity*** myotester was used for testing and training. The results were marginal. Readings of 2 to 4 mA were achieved with the gain controls set at maximum. The decision to proceed with the myoelectric prosthesis was based on the assumption that use of a myoelectric prosthesis is self-reinforcing, that is, myotester results usually improve after the patient has the definitive prosthesis. Operation of the prosthesis also improves.

The patient's first myoelectric prosthesis used the Fidelity myoelectric system. The socket was a modified Muenster design (Fig. 2 ). The socket was modified to grip the residual limb proximal to the humeral condyles and the olecranon. An atmospheric suspension sleeve was added to assist suspension. The hand was the Otto Bock**** myoelectric design incorporated into the Fidelity system.

When the patient received the definitive prosthesis, hand function was sporadic but was expected to improve with training and wearing the prosthesis. The prosthesis was heavy, weighing about 2.3 kg, and it was difficult for the patient to maneuver it and to maintain its position. He cradled the prosthesis in his sound arm, even though the suspension was adequate, and he had never lost the prosthesis, nor did he complain of pain while wearing the prosthesis.

After training with the gain controls set on maximum, the patient continued to have difficulty with opening and closing the hand. At one follow-up appointment the patient said he "could only operate the hand when he pushed up against something or rested the arm on something."

At age 16 the patient was seen at a regularly scheduled Juvenile Amputee Clinic. He asked about a spare prosthesis. A prescription was written for a conventional below-elbow prosthesis with a modified Muenster socket, mechanical hand, and figure-of-nine harness. The patient did not come for casting until five months later, due to personal problems.

The patient was seen again in the Juvenile Amputee Clinic. He brought a change in prescription for his spare prosthesis. He was now to have a spare myoelectric prosthesis of modular construction. The rationale for the change was: 1) to have a myoelectric prosthesis to wear at all times, 2) to reduce the weight of the prosthesis, 3) to improve maneuverability, and 4) to improve ability to maintain position of the prosthesis.

The Clinic decided that the patient would be fitted with an Otto Bock myoelectric prosthesis. The prosthesis was to be of modular construction, have a modified Muenster socket for suspension, and use two electrode sites.

The patient was retested for myoelectric potential, using an Otto Bock testing unit. Initial results were marginal. The patient was usually able to achieve readings above 2 mV from muscle sites on the medial and lateral aspects of the residual limb. The Bock myotester registers millivoltage, rather than the milliamperage registered by the Fidelity tester. Overriding, however, occurred quite often. Generally after 30 to 60 minutes of practice, a well-differentiated signal could be achieved. The Clinic felt that, with continued training and use, the patient could be a successful user of an Otto Bock myoelectric prosthesis.

The patient was casted for a modified Muenster socket. A resin check socket was fabricated and fitted. The sites for the electrodes were marked. The check socket was sent to the Otto Bock factory for fabrication of the definitive socket and for set up with a temporary forearm. The forearm connects the socket to the hand and allows the prosthetist to set the best relationship between the socket and the hand.

The alignment set-up was fitted to the patient in 1981. Hand function was sporadic. The patient was seen in the Juvenile Amputee Clinic and referred to the occupational therapist at Milwaukee's Children's Hospital for training.

Subsequently, the patient was seen regularly in occupational therapy for training and was reviewed periodically by the prosthetist for adjustments. Most readings on the myotester were good and prosthetic function was acceptable. At times the sensitivity adjustments on the electrodes could not be set to achieve consistent results. During this time the patient began to display depression, disgust, lack of interest, and a generally negative attitude toward his prosthesis.

Larry Mott, Director of Myoelectric Central Fabrication at Otto Bock, was consulted regarding the problems in achieving good function of the patient's prosthesis. The discussion included information on a single-site electrode system which Otto Bock was developing for its myoelectric prosthesis. Consistently good readings from muscle groups were obtained, but overriding prevented optimal function. Consequently, the Clinic felt a myoelectric prosthesis with only one electrode to control hand function would solve the problems the patient was having with the two-electrode prosthesis.

The testing electrode for the single-site system was not available, so the testing electrodes for the two-site system were used. A muscle was needed which the patient could contract very quickly and intensely or very slowly with increasing intensity. By testing and training, a site on the medial aspect of the residual limb was isolated as the best available for the single electrode. A very simplified explanation of the hand function is that the single electrode used in the prosthesis employs a differential amplifier to detect and transmit impulses from the contracting muscle. If the impulse is quick and intense, the amplifier directs the motor to open the hand. If slow and increasing, the motor will reverse and close the hand.

The patient was recasted. A resin check socket was fabricated and fitted to the residual limb. The socket was marked to indicate the position of the single electrode site on the medial aspect of the residual limb. The check socket was sent to Otto Bock for fabrication of the definitive socket and set-up with a temporary forearm.

The set-up was fitted to the patient in February 1982 (Fig. 3 ). The patient was able to open and close the hand immediately. He resumed training with the prosthesis and was very successful. He wears the prosthesis at his job in a restaurant, and uses the prosthesis to repair his automobile, protecting the glove with a plastic cover. He is still wearing the original cosmetic glove. He was seen in the Juvenile Amputee Clinic for follow-up. He was doing so well that a spare prosthesis of the same type was prescribed for him (Fig. 4).

The new prostheses are light (approximately 0.8 kg), suspension is easier to maintain, and contact with the electrode is excellent. Only one muscle site is needed, so differentiation of muscle contraction at various sites is not a problem. Because of the decrease in weight, the prosthesis is more maneuverable, and the patient can maintain position of the prosthesis with less effort and for a longer period of time. With one muscle site used to open and close the hand, the opposing muscle group may possibly be used for pronation and supination of the hand. This fitting illustrates an important addition to the armamentarium of myoelectric prosthetics.

*Acme Laboratories, Inc., 10702 West Burleigh Street, Milwaukee, WI 53222

**Orthopaedic Associates of Waukesha, SC., 223 Wisconsin Avenue, Waukesha, WI 53186

*** Fidelity Electronics, Ltd., 5245 Diversey Avenue, Chicago, IL 60639

****Otto Bock Orthopedic Industry Inc., 610 Indiana Avenue North, Minneapolis, MN 55422