The Michigan Electric Hook: A Preliminary Report on a New Electrically Powered Hook for Children



Children with severe bilateral upper-limb deficiencies who are fitted with conventional very-short-above-elbow or shoulder-disarticulation-type prostheses obtain only limited functional advantage from them. Unfortunately, these children frequently lack the necessary force and excursion to fully open their terminal devices with the elbow flexed as little as 90 degrees. Even if full opening can be achieved, it is usually possible only with a minimal rubber-band loading and hence minimum pinch force. Opening the terminal device at the mouth is even more difficult. Yet for many of these children use of their prostheses is essential for independence in such activities as feeding, writing and carrying objects while walking.

When externally powered prostheses became available for children, a marked increase in functional capabilities was made possible with minimal energy expenditure by the child. The first electrically powered shoulder-disarticulation-type prosthesis for children, the Michigan feeder arm, was fitted to children as early as 1963. It provided electrically powered elbow flexion and extension with optional wrist rotation. The terminal device was body-powered. However, the powered elbow motion made it possible to utilize a single cable housing, thereby reducing the amount of excursion required to open the terminal device when the elbow was flexed. Its size and functional capacities limited its application to children from three to seven years of age.

Later, the Michigan Mark IV electrically powered shoulder-disarticulation-type prosthesis was developed at the Area Child Amputee Center to meet the needs of older children with severe bilateral upper-limb deficiencies. The Mark IV, like the early Michigan feeder arm, was designed to provide independent feeding skills with powered elbow motion and a wrist-rotation unit that could be locked in or out. Larger than the feeder arm, the Mark IV also had an electrically powered hammerhead-shaped terminal device; a universal shoulder joint replaced the Wilson wedges of the feeder arm.

The Ontario Crippled Children's Centre designed the first total-system, electrically powered shoulder-disarticulation-type prosthesis. The coordinated arm provided electrically powered elbow and shoulder motion as well as terminal-device opening. It was not until the development of the OCCC electric elbow, however, that it became possible to fit externally powered components to children's prostheses other than the shoulder-disarticulation type.

Recently developed and currently being feild-tested by the Area Child Amputee Center is the new Michigan electric hook. This is a Dorarnce Size 10 hook (both 10X and 10P modles) modified for electric control ( Fig. 1 ).

To date, four children have been filled with the Michigan electric hook. The patients were selected so as to cover a wide age range and a variety of limb-deficiency types (Table 1 ). One of the patients had had extensive experience with external power, and two had been wearing conventional prostheses. The fourth child was fitted with a Michigan feeder arm on a trial basis, but this fitting was discontinued because the child lacked sufficient trunk motion to open the terminal device. She recently has been fitted with a new shoulder-disarticulation-type prosthesis, simultaneously field-testing the OCCC electric elbow (through New York University) and the Michigan electric hook ( Fig. 2 ). The Michigan electric hook was incorporated into the most recent prostheses of the other three candidates (Table 2 ).


Prior to fitting the Michigan electric hook, objective measurements of the system were made. These are recorded in Table 3 . In addition, the hook was cycled for 1500 revolutions (estimated to be the maximum number of times a patient would open the hook during a day) without noticeable battery depletion.

Following this initial testing, each patient was to participate in a one-week functional evaluation. A complete evaluation of prosthetic skills prior to and after incorporation of the Michigan electric hook was to be made. Training was to be provided as needed. The patients then were to be sent home with the new hooks, with clinical reevaluation after four to six weeks. If, at the end of a three-months' evaluation period, the Michigan electric hook was found to be mechanically satisfactory and preferred by the patient, the patient would retain the device.


To date, two of the patients have completed the three-months' evaluation, one will complete it in four weeks, and the fourth has just completed training with her new prosthesis which incorporates the OCCC electric elbow and Michigan electric hook. All of the patients have demonstrated improved function with the experimental Michigan item and prefer it to their previous terminal devices.

The initial functional evaluations of the powered hooks revealed the following;

B.G. Although the Michigan electric hook and the Mark IV hammerhead hook were essentially similar with regard to pinch force and terminal-device opening, the design of the hook lingers and the relatively slower rate of opening/ closing of the Michigan electric hook made it significantly easier for this patient 1) to grasp flat objects directly from a table surface; 2) to hook heavy objects for carrying; 3) to place the hook accurately for grasp; 4) to grasp more securely small round objects which had a tendency to "snap out" of the hammerhead; and 5) to control the amount of hook opening in order to pick up objects within a confined space. As the wrist-rotation unit was retained, feeding and writing skills were not affected by incorporation of the Michigan electric hook ( Fig. 3 and Fig. 4 ).

M.S. and M.V. Both of these patients previously had been fitted with body-powered prostheses. Forearm lift and terminal-device operation were controlled by posterior perineal straps to their opposite legs. In order to keep active forearm flexion, it was necessary to retain the posterior perineal straps ( Fig. 5 ). However, this disadvantage did have a secondary benefit. It became possible to provide these two patients with interchangeable hook systems for use in case the Michigan electric hook should fail to operate ( Fig. 6 ). The patients' families had only to unplug and unscrew the Michigan electric hook from the wrist unit, insert a body-powered hook with a triple swivel, and then tie the dacron cable to the swivel rather than to the keeper on the forearm where it had been attached when the electric hook was used.

Because the hook designs are identical, the change to an electric hook has had little effect upon the types of objects that can be grasped. Most noticeable was the fact that these patients no longer displayed the trunk flexion and rotation previously necessary to open their terminal devices. This enabled both of them to place their terminal devices more easily and accurately, and whether they were sitting or standing no longer affected ease of terminal-device opening. Similarly, it became equally as easy for them to open their terminal devices at the mouth as when the elbow is fully extended at the side.

P.G. As this patient had not worn a prosthesis recently, it was not possible to actually perform a comparative evaluation. It was apparent, however, that fitting her with both an electric elbow and the Michigan electric hook was of functional advantage. She previously had demonstrated inability to open a body-powered terminal device; she now had full opening with the elbow in any position. Further, by using the OCCC electric elbow, it was possible to utilize a six-way shoulder joint rather than Wilson wedges. This change enabled her to preposition the prosthesis in any amount of shoulder flexion-extension, abduction-adduction, or rotation.

Fabrication and Maintenance

Incorporation of the Michigan electric hook into the patient's most recent prosthesis required approximately two hours' labor. The major part of that time involved wiring modifications (i.e., wiring two electrical components to a single battery, modifying or changing switch types, etc.) and tying the wiring down for adequate protection.

The patients' families were instructed in routine maintenance, including battery charging and changing the cable from the terminal device to the motor. The latter was as easy as changing the cable on a conventional prosthesis.

There have been two incidences of breakdown, and these were due to defective gears in the gearheads.

During this evaluation, other minor problems with wiring and maintenance were encountered, and recommendations to eliminate these problems in future hooks have been made. These modifications should make fabrication easier and the terminal device more versatile as far as the types of prostheses and levels of amputation to which it can be applied are concerned.

Technical Aspects

The design of the Michigan electric hook is one of compromise. Pinch force, efficiency, and cosmesis are sacrificed for simplicity of construction, repair and control. The unit is constructed around the model 10X and 10P hooks produced by Hosmer Corporation. A motor-housing is attached to the stud of the hook as shown in Fig. 7 . The motor (Faulhaber, 250/1055), gearhead (spur gear, 141:1), and pulley simply slide into this housing and are covered by two plastic caps. The pulley cord then is attached to the "thumb" of the hook. The entire power train is made from commercially available components and may be completely replaced for less than $20.00.

Control is accomplished by a single, momentary contact switch. This single-site operation makes it applicable for a wide range of prosthetic needs. When the switch makes contact, the motor is activated and opens the hook against the rubber band. When the switch is released, the energy stored in the band closes the hook. Dynamic braking of the motor is employed to oppose the closing movement so opening and closing velocities of the hook can be adjusted so as to be approximately the same. The electrical circuit is shown in Fig. 8 .

The electric hook described approaches the maximum possible simplicity for a powered prehension device. This simplicity suggests potentially low manufacturing costs, ruggedness, and ease of repair.

Descriptors: electric hook; upper-limb deficiencies; Dorrance # 10 hook; external power.

Rehabilitation Therapist, Area Child Amputee Program (Michigan Dept. of Public Health), Grand Rapids, Mich.

Director, Prosthetic Research Laboratory, Northwestern University Medical School, Chicago, IL