The CAPP Terminal Device-A Preliminary Clinical Evaluation


A new type of terminal device was designed and developed by Carl Sumida, C.P.O., at the Child Amputee Prosthetics Project (CAPP) at UCLA 1 . The terminal device has undergone preliminary testing on eight patients at CAPP and is now being field-tested by the Prosthetics and Orthotics Department of New York University (NYU).

The CAPP Terminal Device incorporates a number of unique features. It has center-pull, voluntary-opening operation. Its configuration is neutral so that it can be used on either the right or left side. It is made of plastic and shaped to blend with the forearm in a continuous flowing line. It was planned to have a pleasing contour which would look quite natural during use and has some resemblance to a stylized hand in a grasping position, although it was not intended to have a recognizable form ( Figure 1 ).

The CAPP Terminal Device was designed to provide an alternative to the hand and hook. It emphasizes the functions of the nondominant extremity and, therefore, it is especially suited to the needs of the unilateral limb-deficient child. The terminal device has a wide palmar face with a frictional resilient covering which is somewhat compressible, like human soft tissue, to hold objects securely without requiring excessively strong pinch. The configuration along the cover face increases the surface area for contouring around objects, thus enhancing the quality of grasp. It was hoped that these features would allow children to hold a wide variety of object shapes securely with minimal concern as to how they were positioned in the terminal device. Since the unilateral limb-deficient child primarily needs a terminal device which holds objects securely while the sound hand performs the more precise movements, a specific design feature of the CAPP Terminal Device was the performance of this holding function.

In order to make the CAPP Terminal Device as durable and reliable as possible, considerable attention was given to the selection of materials; and quality control was stressed during manufacture. Only if the terminal device proved reliable during the testing phase would it be possible to concentrate on the evaluation of its functional features and to learn whether its design characteristics really provided the type of hold intended by the designer, and whether it was acceptable to amputees.

Description of the CAPP Terminal Device

The shell of the terminal device is made of Delrin®, a thermoplastic material produced by DuPont. The shells are made in an injection mold. The control line is of braided Dacron and full opening of the devices is obtained with 1 3/8 in. (3.5 cm) of line travel. The path of the control line inside the shell is shown in Figure 2 . The full opening dimension at the tips of the terminal device is 2 3/4 in. (7 cm), which is intermediate between the opening sizes of the Dorrance lOX and 99X hooks. The operating lever is made of aluminum, the stud is stainless steel, and the covers are made of Kraton®, a thermoplastic rubber material made by the Shell Chemical Co. The covers are injection-molded and are shaped to extend over the outer edges of the shell so as to provide friction for gross stabilizing activities.

Prehension force is supplied by a spring at the base of the operating lever and, for initial testing, four different spring strengths were made available. They provided between 1/2 lb. and 2 lb. (.2 to .9 kg) of pinch at the tips. The total weight of the terminal device is 3 oz. (85 gr) and its length is 3 in. (7 1/2 cm), without the stud.

Method of Study

Before an extensive clinical evaluation was undertaken by NYU, CAPP decided to conduct some preliminary tests on patients. This approach made it possible to develop fitting techniques, to detect any mechanical problems which could be remedied before the fitting of large numbers of patients was done, and to learn what some of the functional and learning patterns might be for children using this new terminal device. This report describes the methodology used in this preliminary evaluation and the results obtained.

The patients selected had unilateral below-elbow deficiencies since members of this group are the most active users of a terminal device. These amputees provided the best opportunity to obtain maximum information about the terminal device in a minimum of time. However, it was not intended that applications be limited to below-elbow patients beyond the period of the preliminary evaluation. The evaluation was planned to test the validity of the CAPP Terminal Device design features. No comparisons were made with a standard hook terminal device, since the two items emphasize different functions. The CAPP Terminal Device was designed to provide secure hold as needed by the assisting hand and not to grasp small objects from a surface. Furthermore, it was intended as an alternative to, not a replacement for, the hook. Therefore, it was most important to learn whether the CAPP Terminal Device provided the function, acceptance and mechanical reliability intended in its design.

The evaluation was planned to cover a six-month period with data to be recorded on mechanical, functional, and acceptance factors:

  • Function was evaluated by observation of the manner in which objects (grouped in a number of categories) were held. This method of testing permitted children to perform a wide variety of activities appropriate to a broad range of ages and interests. Also, the functional evaluation was planned to replicate the most typical use patterns of the unilateral limb-deficient child. These children tend to place objects into the terminal device with the sound hand or grasp them from a position in space rather than reach directly to grasp an object from a surface with the terminal device.
  • In the evaluation of mechanical factors notations were made as to whether the problem was one of routine maintenance or repair, or involved some other considerations.
  • Acceptance factors were rated by means of a questionnaire and from spontaneously offered reports of experiences with outsiders. Eight patients were fitted during the preliminary evaluation at CAPP ( Table 1 ). Age at fitting ranged from 10 months to 9 years. Six of the subjects were girls and two were boys. Five of the eight patients had been active users of Dorrance hooks, two others had worn hooks but had not yet learned to use them, and one was a new patient. Four of the terminal devices were applied to existing prostheses and four were applied to newly fabricated artificial limbs. The eight fittings involved four endoskeletal prostheses, two conventional below-elbow prostheses, and two specially designed limbs for children with a dislocated radial head 2 .

Installation Time

The prosthetists in the study reported that it required 30 to 60 minutes to install the CAPP Terminal Device on an existing prosthesis, and that new fabrications required only slightly longer than usual. However, it was felt that part at least of this additional time was attributable to lack of familiarity with the procedure for applying the new terminal device.

Fitting Techniques

Fitting techniques for the CAPP Terminal Device were devised by the CAPP clinical prosthetists. The first method tried was that of connecting the Dacron line from the terminal device to a steel cable inside the forearm between the end of the socket and the wrist unit. These connections tended to break, so all subsequent fittings were done using a long Dacron line which ran the entire distance from the terminal device to the hanger at the harness. A clinch knot ( Figure 3 ) was used to attach the Dacron line to the hanger. The use of the continuous long line lessened the amount of space needed inside the prosthetic forearm, and permitted the terminal device to be fitted to children with longer below-elbow stumps. However, some forearm space was still required in order to avoid a sharp bend in the line between the terminal device stud and its exit from the forearm ( Figure 4 ).


Mechanical Factors

During the six-month evaluation at CAPP, from the time that the continuous control line was used in fitting, no mechanical failures of any kind occurred. There were no broken springs, no broken lines, nor any other broken parts. The Kraton covers wore out more rapidly than had been anticipated. However, they provided excellent function and, when worn out, could be replaced easily by parents or therapists. Therefore, no need was seen to use another material. However, since the covers were made in a mold, any other promising materials that become available in the future can be tried.

Average wear time for the covers was 2½ months. However, during the preliminary evaluation, the covers were changed as soon as any sign of wear became evident, and they could have been worn longer if they had been left in place. The 2½-month average does not include data on one child-a nine-year-old girl who is an extremely vigorous user of her entire prosthesis. She chews on the covers and uses them for an eraser in school which, added to her general pattern of hard use, reduced the life of her covers to one month.

Kraton appeared to be quite stain-resistant. One child spilled ajar of paint on the terminal device which the cover soaked up like a sponge. The mother was able to remove it entirely with cleanser and a brush. Subsequently, it was found that some ink stains also tended to rub off. The covers can be thoroughly scrubbed with cleanser and water, but are attacked by chemical solvents such as alcohol, benzine, etc.

In another incident, a child dunked the prosthesis in the ocean and the terminal device was filled with sand. After it was cleaned and dried, no apparent harm had been done to the mechanism. Hence, aside from routine maintenance, and despite some hard use, the terminal device proved to be mechanically reliable.

Thus, the attention given to quality control factors during the manufacture of the terminal devices was well repaid during the evaluation period because mechanical problems were kept to a minimum and the device could be tested in terms of its other features.

Functional Results

The results of the functional evaluation are reported under five headings to correspond to the design specifications of the CAPP Terminal Device.

1. With the CAPP Terminal Device it should be possible to hold a wide variety of objects; they should be held securely and in good position for function. The preliminaryevaluation showed that the terminal device met this requirement satisfactorily. Because of the wide age span of the children in the study, a wide range of activities was explored and objects of many different shapes did fit into the terminal device in a variety of ways. In general, if the child had difficulty visualizing how the CAPP Terminal Device would hold an object, he was encouraged to try holding it with his sound hand. The CAPP Terminal Device would usually hold objects in the same way. Some children found that the handlebars of toy vehicles were difficult depending on their height and angle. However, if grasped from above they did not present a problem ( Figure 5 ). Holding a pail-an activity usually requiring a hooking action-was done easily ( Figure 6 ), but none of the children was able to hang from overhead bars using the CAPP Terminal Device.

The children themselves were quite creative in finding ways of holding objects in the CAPP Terminal Device. One child with a right prosthesis found that she could hold and turn the crank of a pencil sharpener or egg beater securely without crossing hands or reversing the beater ( Figure 7 ). Other children found specific uses for certain features of the terminal device. The lip on the tip of the wide palmar cover was used as a stop or wedge to keep small objects from pulling out, and this lip was especially useful for stabilizing long, thin objects ( Figure 8 ). Most children found that the small plastic cup-like pieces at the side of the terminal device which rests on the table were in the way for gross holding ( Figure 9 ). Moreover, long thin objects can be held securely without the cup pieces on the upper side. Since these pieces can be easily removed or applied, their usefulness is subject to further study.

During the CAPP evaluation, it was interesting to observe how much the friction cover actually contributed to the security of hold with this terminal device. It had been postulated that children would be able to obtain a firm grasp with a light pinch force because of the extra gripping function provided by the wide frictional resilient cover. Several spring tensions were available for the test period, since it was not known how much pinch would be needed when used in combination with the friction cover. In general, the children who had previously worn hooks found that they could obtain a good grip using a CAPP Terminal Device with less pinch force than they had used on the hook, but they obtained an excellent grip if they used the strongest spring they could use comfortably since the friction cover made a significant difference in holding ability. For the early learner or newly fitted young amputee, the lightest spring, which provides ½ to 34 lb. of pinch (0.3 kg) together with the friction cover, provided a grip which was secure enough for a functional hold on objects using a comfortable amount of energy expenditure right from the start. The strongest spring provided 2 lb. (0.9 kg) of pinch and was ample for even the 9-year-old child who had been accustomed to much more pinch force on her previous terminal device. Therefore, the spring tension to be used in changing from a hook to a CAPP Terminal Device was determined by equating the amount of force exerted by the amputee with that used previously, rather than by the amount of pinch measured at the tips of the terminal devices.

2. The CAPP Terminal Device should not require precise positioning of objects for secure hold. This requirement was met satisfactorily in the preliminary evaluation. The characteristic was particularly advantageous for young children just learning to operate a terminal device. These young children would push an object into the terminal device haphazardly yet found that it stayed there while they played with it ( Figure 10 ). This security of grasp meant that less assistance was required from the therapist during the early learning stages, the children were less frustrated by objects dropping out, and that, outside of therapy, they could carry over even their beginning operational skills with some satisfaction. In contrast to this situation, children who were already adept users of Dorrance hooks tended to have difficulty shifting hold patterns to the CAPP Terminal Device. They tended to place objects into the terminal device with great care, usually holding things in the tips of the terminal device or lining up objects along the operating lever. Both of these positions offer very poor grasp with the CAPP Terminal Device. In each case, the child needed to learn that placing the object further into the grasp of the terminal device and at right angles to the operating lever offered a more secure hold ( Figure 11 ).

To assist children overcome habits learned with the former terminal device, the therapist had to provide considerable support just after the change, as the children sometimes concluded that activities were not possible with the CAPP Terminal Device, since they could not be done in the same way as before.

3. The opening dimension of the CAPP Terminal Device should permit the holding of objects appropriate to the functional needs of the child wearing it. It was not known how old a child should be to be able to benefit from this size of terminal device. In the CAPP evaluation, all of the children found that the opening span at the tips was adequate, but some of the 7- to 9-year-olds found the depth of opening was inadequate.

4. Objects held in the CA PP Terminal Device should be visible to the wearer. Children were able to see what they held, although sometimes it was necessary for them to turn the terminal device in order to see well. It did not appear that an excessive amount of pre-positioning was needed and, when necessary, the prepositioning required was very obvious. Most objects were held with the terminal device positioned so that it pointed toward body midline ( Figure 12 ). A few objects such as bucket handles or trays required that the terminal device be turned up ward, and some objects-primarily vehicle handles-required the terminal device to be pointed downward. This last position was the only one in which vision of objects held was not good. However, vehicle handles are usually of sufficient size so that visibility is not a problem ( Figure 13 ). One advantage of the center-pull terminal device in regard to pre-positioning is that it can be left in the wrist unit with a minimum of friction. It needs only to be screwed in tightly enough to resist turning when holding an object down on a desk or table or to resist forces exerted by the sound hand when an object held in the terminal device is manipulated. This low-friction feature made it easy for even young children to learn to turn the CAPP Terminal Device.

5. The cover which extends to the outer surface of the shell should assist gross holding activities. This feature proved extremely useful to the children who tested the terminal device. Children through the entire age range which wore the unit found this friction useful for gross stabilization, clasping, and body-support activities. The 10-month-old relied on it for climbing, catching himself when falling, and for stabilizing objects ( Figure 14 ). Moreover, as he began to appreciate its holding function, it was interesting to see him begin to pull the operating lever of the terminal device with his sound hand to have his mother place objects into it ( Figure 15 ).

Acceptance Factors

At the conclusion of the six-month evaluation period, all but one of the eight children chose to continue wearing the CAPP Terminal Device. Those who elected to keep the new terminal device had become very spontaneous and automatic in its use. The one child who chose to return to wearing her Dorrance hook had experienced difficulty in adjusting to the change throughout the entire evaluation period. Since a number of the former hook wearers were strongly oriented to the hook and had some early learning problems, it was surprising that only one child rejected the CAPP Terminal Device. However, all families who decided to participate in the evaluation did so freely. Those families and children who showed any reluctance to try the new terminal device were not included.

Acceptance data were recorded on families at the time the device was offered to them whether or not they decided to participate in the evaluation. Those who were fitted completed a subjective reactions' questionnaire after six weeks of wear and again at the end of the six-month period. On the initial presentation of the test device, some comments from active hook users were quite negative. A typical comment was, "It's so different. I don't want it because I won't be able to do (some favorite activity)." It is probably understandable that some children would feel that no other terminal device-especially one so different-could possibly meet their functional needs. These children were not encouraged to try the CAPP Terminal Device since it is doubted that they could test it in an unbiased manner. The most positive comments on first presentation were from parents of young children-especially new patients. They were the most enthusiastic about the terminal device. A very typical response was, "It looks like it will really hold things well." They liked the idea that it has no lever sticking up which the child could fall on, and commented positively on its appearance saying, "It looks nice," or "It isn't mechanical looking."

After fitting, parents and children offered spontaneous remarks and wrote in comments on the questionnaire which described their reactions to the terminal device. One negative comment came from a child who stated that her classmates were frightened of the new terminal device at first. Others reported instances in which the new terminal device eased classroom situations, since others in the group were now quite willing to hold the terminal device during circle games. One family was approached several times in public places by other amputees asking about the CAPP Terminal Device and where they could get one. Visitors and distant relatives usually commented positively on the appearance of the CAPP Terminal Device saying that it had a natural appearance and did not attract attention.


From this preliminary investigation, an initial impression concerning the potential of the CAPP Terminal Device has been obtained. These findings-although limited to eight patients-lead CAPP personnel to be very optimistic about the CAPP Terminal Device, and to believe it has real merit. It appears to be durable and acceptable in appearance, and to function as it was designed to. Surely those who participate in the evaluation as it proceeds into the next stage will uncover many other factors about the CAPP Terminal Device, as there is still much to learn about it. Because of its many unique features, working with this terminal device offers treatment personnel an exciting opportunity to expand their thinking with respect to prosthetic function for their patients.

1. Sumida, Carl, Julie Shaperman, Wallace Sumida, and Yoshio Setoguchi, The unitized below-elbow infant prosthesis. Orth. and Pros., 22:3:11-13, September 1968.

2. Tablada, Carnian, and Susan Clarke, A fitting for the unilateral below-elbow amputee with dislocated radial head. Inter-Clin. Inform. Bull., 13 :8:1-6, May 1974.