A Universal Terminal Device: Preliminary Report



The functional superiority of a prosthetic hook over a prosthetic hand has been well established over the years through clinical observation. The hook is not intended to resemble a hand, but rather serves as a built-in tool which offers the amputee many advantages. Because of the hook's slender configuration, the patient is capable of fine manipulation of objects, and its relative light weight and ruggedness provide a rather efficient instrument or tool. Prosthetic hands, while resembling the shape and size of a normal hand, are clumsy, heavy, and are prone to mechanical and cosmetic maintenance problems, i.e., wear and tear on the cosmetic glove.

Based on clinical experience, we know that children, with rare exceptions, become very proficient in the use of the prosthetic hook and do not have any difficulty in accepting it as a terminal device while they are young (generally under the age of 5 or 6). However, when they enter kindergarten or public school they are frequently subjected to derogatory remarks relating to their appearance and the use of a hook. This factor, combined with a generally increased concern about their appearance as they grow older, frequently results in disuse of the prosthesis or produces a request for a normal-looking terminal device, i.e., a prosthetic hand. Clinical observation has shown that switching such a patient from a prosthetic hook to a prosthetic hand is most often disastrous, since the patient is accustomed to the light weight and functional capability of the hook, which a hand does not offer. The result, again, may be total disuse of the prosthesis. This dilemma was one of the major reasons for exploring the design of a terminal device which would combine all the advantages of a prosthetic hook with the cosmesis and appearance of a hand.


Ideally, a universal terminal device (UTD) should combine the best features of a prosthetic hook with the looks of a hand, while eliminating the problems associated with prosthetic hands. Thus, the design criteria were rather clear. The device should have:


  • Approximately the same weight as a prosthetic hook
  • Function and mechanical simplicity similar to a prosthetic hook
  • The shape and feel of a natural hand
  • A color-fast, tear-resistant skin.


Although the last criterion has so far not been achieved, all the other criteria have been incorporated to a marked extent in the present design.

A great number of preliminary designs have been explored. However, all but the last design were unsatisfactory because they did not follow the criteria in all respects as established above. For illustrative purposes only, one of the earlier designs ( Figure 1 ) consisted of a prosthetic hand which produced a hook-type prehension pattern between the index and middle fingers. This action was accomplished through sequential retraction of digits IV and V. abduction-extension of the thumb, and finally abduction of the index finger from the middle finger ( Figure 2 ). Depending on the excursion of the cable, one, two, or all three movements could be accomplished. By retracting (flexing) digits IV and V, visual feedback was enhanced; and by abducting-extending the thumb, larger objects could be placed between the thumb and index finger ( Figure 3 ). For finer manipulation, the hook type of prehension between index and middle fingers could be used as the excursion of the cable was increased ( Figure 4 ). The complexity, weight, and maintenance of this design resulted in exploration of other alternatives.

The criteria were fulfilled better with the following presently used design. Recalling that hook-type function is generally considered excellent, a prosthetic hook, slightly modified, was indeed used as the "skeleton" of the hand prosthesis ( Figure 5 ). To enhance visual feedback, the thumb, ring and little fingers were made proportionally smaller than the index and middle fingers, which contained the fingers of the hook ( Figure 6 ). The nonfunctioning fingers were made flexible to further approximate the hook by reducing rigid bulk and allowing prehension in cramped places through displacement of the soft, flexible material. Although the resulting shape is not strictly anthropometric, a great deal of care was taken while sculpturing the original to make the prosthesis appear natural ( Figure 7 ).

For the "thumb" of the hook to fit within the hand shape, it must be placed on the ulnar aspect. Thus, when the cable is pulled, the middle finger abducts with respect to the index finger. Another model was built with the "thumb" of the hook placed on the radial aspect, as in conventional hook action. But when this is done, the hook "thumb" protrudes out of the hand shape, and it therefore must be cut approximately in half. Since the force necessary to open the hook is thus doubled, a force-amplifying pulley is added inside the hand ([popup8a] and [popup8b]), so that the opening force and cable excursion are the same as for a conventional hook.

Until a better prosthetic skin covering is found, a stock prosthetic glove is placed over the foam-rubber core. To get the proper fit, a glove size is selected such that the thumb, ring and little fingers match those of the prosthesis. When this is done, it is found that the index and middle fingers of the glove are too short. After this undersized glove is forced on, the prosthesis is placed in boiling water for a few minutes so that the glove can be made to conform to the new hand shape and reduce glove resistance to opening.

Rubber bands are applied in the manner used with conventional prosthetic hooks, after removal of the foam covering. The weight of the complete unit is approximately half that of a conventional Dorrance prosthetic hand of comparable size.

Prototypes were made in child sizes, left and right, and adult left (It is anticipated that the unit will be made available through one of the major prosthetic industries in a full range of sizes).

Patient Evaluation

Patients were asked to try the UTD in their daily activities away from the clinic. Since direct investigator observation was not possible, a self-report evaluation instrument was employed. The questionnaire gathers biographical and historical data, including the length of wear of the patient's prosthesis and of the UTD. Patients are also asked to compare the UTD with a conventional hook and/or a functional hand. General function, functional tasks, and cosmesis are then covered. Additional questions attempt to ascertain whether or not the nonfunctioning parts of the UTD obstruct vision or manipulative ability during task performance. The form concludes with questions regarding the patient's desire to continue involvement in the project. If the patient does not wish to continue, he is asked to cite the major reason for dissatisfaction. The form also asks whether the UTD concept should be continued, and with further improvement, whether the patient wouldagree to future participation.

Because of the subjective nature of this preliminary investigation, personal interviews were conducted whenever possible to substantiate responses, to enlist the help of parents in pediatric cases, and generally to furnish greater detail of individual experiences.


To date, 12 upper-limb amputees have been fitted with the UTD. These patients used the device for a period of at least one month. Pertinent characteristics of the patient group are itemized in Table 1. Age at time of fitting ranged from 5 to 26 years, with an average age of 13 years. Amputation levels of the patients included 10 (83.3 per cent) below-elbow amputations and two (16.7 per cent) wrist disarticulations. All patients had unilateral upper-limb amputations.

To gather information for this preliminary report, questionnaires were mailed to 10 of the 12 individuals in the patient group. Of the remaining two, one could not be reached, and the other had just received the UTD at the time of this writing. There were seven respondents as indicated in Table 1 . The patients gave varied responses concerning the UTD in the areas of operation, function, and cosmesis. All of the respondents, however, agreed that the idea of the UTD was worthwhile and should be continued.

In comparing the amount of effort needed to operate the UTD as opposed to the conventional hook and hand devices, four of the seven respondents indicated that the UTD required somewhat more effort, which is probably a result of the added resistance imposed by the cosmetic glove. Lately, this problem has been partially alleviated by subjecting the UTD to a heating process, as described above. In addition, after using the UTD on a regular basis for a few weeks, two patients (P.C. and C.Z.) reported that the amount of effort required to operate the terminal device decreased significantly, equaling the force needed to open a conventional hook. Presumably, this effect was caused by repeated stretching of the cosmetic glove with use.

Four of the six respondents who had previously been fitted with conventional hooks noted that the UTD could perform certain functional tasks as well as the hook. Negative feedback centered on the lack of sufficient dexterity resulting in part from the obstruction posed by the nonfunctioning thumb and fingers of the UTD. R.D. noted that the UTD exerted too much force to handle small or fragile objects. In contrast, P.C., who received the UTD in July 1977 and continues to use it regularly, noted that the UTD could perform most tasks as well as the hook. He also stated that he was better able to manipulate brittle objects with the UTD because of the cushioning effect of the foam covering at the prehensile tips.

The three respondents who had previously been fitted with conventional hands varied in their opinions concerning function of the UTD. T.C. noted that the UTD could perform most tasks as well as the hand. P.C., however, reported that the UTD was more functional than the .hand because of its increased grasping power and surface. As a result of these factors, he claimed to be able to use the UTD for a variety of tasks, including activities of daily living (eating, grooming), sports activities, driving, and carpentry work. In addition, P.C. stated that the UTD was, good for lifting and carrying because of the hook shape."

All but one of the respondents indicated that the UTD looked better than the conventional hook, but somewhat worse than the hand. R.D. was displeased with the appearance of the UTD because he felt that the fingers were very disproportionate. P.C., however, liked the appearance of the UTD better than that of the mechanical or myoelectrically controlled hands that he had previously received because he felt that the position of the hand looked more natural. C.Z., who had previously been fitted with a hook which she was reluctant to wear because of its appearance, was very pleased with the appearance of the UTD. As a result of this factor, she now uses the UTD exclusively.

Although the results of this preliminary study are rather subjective, important evaluation information was gained regarding patient acceptance of the UTD. Clinical feedback at this state of development will help guide further modification of the device.


For her enthusiastic support and encouragement of this study, the authors express their appreciation to Selene Jaramillo, M.D., Chief of Clinical Services, Children's Division, Institute of Rehabilitation Medicine. We are also very grateful to the Occupational Therapy Service, Children's Division, for their substantial technical assistance and help in developing guidelines for testing procedures for this project.