An Apposition Post Prosthesis For Transcapal Deficiencies

Carl Sumida, C.P.&O. Julie Shaperman M.A., O.T.R Yoshio Setogushi, M.D.


This work is supported by Grant No. 00489, National Institute of Child Health and Human Development, National Institutes of Health, Department of Health, Education and Welfare; and Grant No. C-199 (R 2), Children's Bureau, Department of Health, Education and Welfare.

For the past five to six years personnel at the Child Amputee Prosthetics Project have been studying the possibilities for providing a special prosthesis for children with transcarpal deficiencies. Ideally, such a prosthesis should be extremely light in weight, inoffensive in appearance, not interfere with the child's wrist movement but fully utilize the power and range of motion of the wrist.

When confronted with this type of limb deficiency in the past, it was customary to consider the length of the transcarpal or metacarpal segment as the determining factor. If it was felt that the patient had considerable functional use of his wrist, no prosthetic device was prescribed. If a very short palmar segment precluded any possibility of natural prehension, a standard long below-elbow prosthesis was the prescription of choice.

In reviewing this philosophy of prosthetic prescription, the Project staff felt that both courses of action failed to give full consideration to the child's needs and that the engineering potential which existed in a wrist that could move was being ignored. The children, with what superficially appeared to be a minor skeletal loss, often suffered as much functional loss as a child with a standard below-elbow amputation.

If no functioning digits are present, prehension is markedly restricted even when the palmar section is relatively large. Yet when the child is fitted with a below-elbow prosthesis designed to furnish prehension through the use of a conventional hook, the prosthesis is often rejected. A conventional prosthesis fitted to an arm of normal length which includes a wrist section and perhaps some nubbins has several severe disadvantages. It not only eliminates tactile sensation over a third to a half of the forearm and immobilizes the wrist, but also increases the total length of the arm. The advantages of hook prehension are insufficient to outweigh these drawbacks.

Design Criteria

It was decided to develop a new prosthetic design for these patients, adopting some orthotics principles. Although this group of patients is loosely referred to as having transcarpal deficiencies, in point of fact their deficiencies range from near-Achieria to digital loss only. Occasionally one digit will be present (Fig. 1 ). These variations require that an individual approach be taken to each patient, although the pertinent areas of concern are the amount of palmar surface present, the digital or nubbin conformation, and the strength and range of motion of the wrist segment. The design criteria established for the new device were:

  1. To provide prehension by means of a surface configuration against which the remaining carpal segment and wrist could oppose.

  2. To use the existing strength and range of motion of wrist flexion and extension as the source of power.

  3. To provide a broad holding surface comparable in shape to the remaining portion of the transcarpal segment. Also to provide sufficient frictional surface for good grasp.

  4. The device should not interfere with normal bimanual and locomotor activities.

  5. The device must not limit normal arm motions, i.e., pronation and supination of forearm, flexion and extension of the elbow.

  6. It must be easy to don and remove, fit securely and have stability when worn, so that the young child would not remove it unnecessarily.

  7. The device must be comfortable during use, and not create any undue pressures on the bony structures of the forearm.

Design and Fabrication

The apposition post is designed to provide a surface against which a functional palmar and/or carpal segment may be opposed to provide prehension. It consists of distal and proximal bracelets with one or two distal projections (Fig. 2 and Fig. 3 ). To maintain a stable grip on the object being held, leverage forces are transmitted from the forearm to the apposition post. This is done by molding a flat-surfaced continuation of the post contoured to fit along the volar aspect of the forearm; and through the bracelet design. As a continuation of the post, one bracelet starts from the ulnar side and extends around the forearm to the radial side, and the second bracelet starts from the flat surface on the radial side and curves around to the ulnar side (Fig. 4 ). To prevent slippage during supination and pronation of the forearm, the bracelets are contoured in the same general pattern as the configuration of the forearm, i.e., in a modified oval shape. When the post is being used for prehension, the forearm must not rotate medially or laterally inside the bracelet. This is the reason for locating the two bracelets on opposite sides, contouring around the forearm.

In addition it is important that the proximal bracelet on the volar surface be flared. During prehension of an object in the post the pressure transmitted to the forearm on the dorsal side by the distal bracelet and on the volar side at the proximal end of the bracelet is considerable (Fig. 5 ).

The apposition post is most satisfactory in function and appearance when constructed of a material which is easily molded and yet has a considerable amount of strength with some degree of flexibility. The Project has selected the plastic Plexidur (Plexiglas), made by the German firm of Rohm and Haas, as the material of choice at the present time. Since the plastic must be heated to 250°F. before it becomes malleable, it cannot be molded on the patient. Therefore, a permanent mold has been made simulating the tapered forearm shape of most of these infants. After the post is fitted to the mold, only minor adjustments are needed to make the post fit the patient correctly. The angle of the projections as related to the volar aspect of the bracelets is usually determined by the amount of opening desired and the patient's wrist flexion strength (Fig. 6 ).

Holding Area

The entire holding area of the post must be provided with a frictional surface; otherwise most objects will simply slide off the smooth plastic surface. Several different materials have been tried, including surgical tubing, Kemblo, and polyurethane foam of various densities. All of these surfaces have worn through quite rapidly and have required frequent replacement. A flexible polyurethane now appears to be the most satisfactory material. It is very durable and needs only to be roughened slightly in order to provide an excellent frictional surface. One post with this frictional material has now been used for over two years without need for replacement.

Fitting

In general, our philosophy is that if a child is fitted with an apposition post at approximately seven months of age, he quickly learns good grasp and release. By ten months of age the child will be transferring objects from hand to hand. Once full-time wear is accepted and the functional advantages of the post are understood, most of these children do not require much more than five pounds of prehension force to utilize the apposition post satisfactorily. The objects that require the most force are those which are unwieldy and demand fine, firm control. For example, none of these children can use a fork in the post unless they shove the fork handle between the volar surface of the forearm and the post bracelet. Lengthy longitudinal objects such as "pick-up-sticks" are also difficult to handle. When the child maintains pressure on any given object for a considerable period of time, fatigue is noticeable.

During the past six years we have found that children with transcarpal deficiencies have shown an increase in range of motion and strength after a period of active use of the apposition post. Perhaps much of this increase should be attributed to normal growth and development. However, in at least two examples, the change was quite remarkable.

Training

The child with a post must learn some of the same things that a below-elbow amputee learns. Motions for grasp and release, discriminate placement of objects in the prosthesis for secure grasp, and the necessary kinesthetic adjustments, are learned easily if the child is fitted early (Fig. 7 ). These children learn to know very quickly just what objects their post will accommodate. However, to accomplish this learning, some training is needed. Initially the therapist observes the function provided by the post and works with the prosthetist to achieve the final adjustments. Fit of the bracelets and the shape and angle of the projections must be quite precise in order to provide optimal ability to grasp objects. This simple-appearing device sometimes requires minute attention to detail if the desired level of fit and function is to be achieved.

In addition, the therapist needs to orient the family and the child for two or three days as to the post's functional potential and to its care and maintenance. Parents of young children are encouraged to place objects into the grasp of the post to stimulate use by the child. Also we have found that continued therapy once a week helps break patterns of clasping objects against the chest and builds good post-using habits. This therapy is especially important for the pre-kindergarten and school-age child. While infants usually will wear the post all their waking hours, those children not fitted until they are ready to begin school or later may use the post only for specific activities.

Conclusion

The apposition post prosthesis has been a very satisfying addition to the prosthetic armamentarium for the treatment of limb-deficient children. If it is properly made, so that the projections do not extend beyond the palmar segment, it does not interfere with crawling, grasping, or any other exploratory type of infant activity. The major limitation of the post is that of its range of opening. If the child has a carpal segment of at least an inch and a half from the radial styloid, he can achieve approximately two inches of opening. If the length of the carpal segment is shorter, his opening will be substantially less. To date 34 patients have been fitted with these posts and results have been generally favorable. Acceptance of the post by parents has been excellent. They do not object to its appearance and they are very pleased with the almost instant functional success achieved by the children.

Carl Sumida, C.P.&O., Julie Shaperman M.A., O.T.R and Yoshio Setogushi, M.D. are associated with the Child Amputee Prosthetics Project of the Univeristy Of California, Los Angeles, California