A New Fabrication Technique for Partial-Foot Prostheses

CARMAN TABLADA


At the Child Amputee Prosthetics Project, UCLA, our experiences in fitting young partial-foot patients have shown that the major problems are those of normal wear, outgrowing the prosthesis, mechanical breakdown, or- more commonly a combination of all these conditions. Children spend almost all their waking hours in strenuous activity, subjecting the partial-foot prosthesis to the most severe kinds of stress. It is not uncommon for these patients to require a new prosthesis in less than a year after a fitting, and this need for frequent replacement places a considerable financial burden on the parents or funding agency.

At CAPP a design study was undertaken to develop a partial-foot fabrication technique which would reduce time and materials costs and yet provide function and cosmesis equal or superior to that of existing prostheses. The design criteria were:

  1. Fabrication should be by existing methods, utilizing materials normally available in prosthetics shops.
  2. Fabrication time should be reduced by simplification of procedures.
  3. Function should be satisfactory, i.e., the patient should be able to utilize any potential ankle motion and achieve a near-normal gait.
  4. Cosmesis should be improved as compared to hard-shelled or metal-frame-and-leather types.

Illustrative Case

The history of one of our partial-foot patients which illustrates the problems of wear, outgrowing, and breakdown which we have encountered with these prostheses is summarized in the table on page 8. This child has a congenital transtarsal left apodia ( Prosthesis History of One CAPP Partial-Foot Patient ) She was first fitted with a modified Canadian Syme's-type prosthesis with a Velcro tab for flap closure at the age of 1 year ( Fig. 1 ). Function was very satisfactory, but she outgrew the prosthesis rapidly and it was necessary to fit her with a new prosthesis after only five months.

Each of this patient's subsequent prostheses was experimental in the sense that design modifications were made and new materials tested in an effort to improve function, wear characteristics, and cosmesis. The fourth prosthesis, for example ( Fig. 2 ) consisted of a short plastic socket with a posterior opening closed by a Velcro strap, and a forefoot covered with soft leather.

Her ninth prosthesis, fitted at the age of 7 years, 9 months, was of a special design and used a vinyl-acrylic-styrene blend with urethane filler and a reinforced sole ( Fig. 3 ). It was of ankle length with a zipper on the medial side. Fit and function were most satisfactory, but a period of especially rapid growth plus unusually severe wear made replacement necessary after six months. On the tenth prosthesis the zipper had to be replaced several times; also, frequent repairs in the instep area were necessary. With the eleventh prosthesis (made at CAPP) the only repairs necessary were the result of zipper breakdown; the prosthesis itself required no attention except for some grinding out to accommodate for normal growth.

The frequency of and causes for replacement are not unique with this patient, but are fairly representative of children with partial-foot amputations, and therefore it is not surprising that the cost to parents or an assisting agency is a matter of real concern.

The fabrication technique described below was developed in an attempt to reduce costs in both materials and fabrication time, as well as to maintain or improve function, cosmesis, and durability.

Fabrication Procedures

The cast is made in the conventional manner and the positive mold is coated with plastic parting lacquer. Three layers of nylon stockinette are pulled over the cast, a PVA bag is applied, and a slight vacuum* is applied. Catalyzed 384 RTV is used to saturate the stockinette. 384 RTV (as well as the 385 RTV and 386 RTV mentioned below) is a Dow Corning Silastic available from the Hos-mer Corporation. If these materials are allowed to warm to room temperature for an hour before use, they will be of the proper viscosity to apply without thinning.

The sole piece is made of 3/16-in. belting material, cut on a band saw to fit inside the shoe, and is bevelled on the plantar side. A ¾-in.-wide strip of spring steel is attached lengthwise to the belting with copper rivets. Proper alignment in the shoe is determined, and the sole piece is positioned in a cardboard box which has ½ in. clearance on each side.

The cast is placed on the sole piece, positioned according to toe-out and other necessary orientations, and secured by masking tape. A mixture of catalyzed 386 RTV and 385 RTV silicone is then poured around the cast-socket and sole piece and is allowed to foam and set tack-free. This material seals the sole piece to the socket and also forms the forefoot build-up. The shaping of the solidified foam is done with a combination of scissors, bandsaw, and drum/ router sanding.

Anterior reinforcement is accomplished with overlapping l½-in. dacron webbing. The lacing area is reinforced with 1-in. dacron webbing. All of the dacron reinforcement is held in place with nylon thread.

The final lamination is done with three layers of nylon stockinette, saturated with 384 RTV. The socket is then trimmed to form a smooth brim, and a ¾-in. wide vertical cut is made to provide the lacing closure. Two small triangular pieces are cut from the distal end of this aperture to prevent pinching and binding. The flaps are trimmed with horsehide, and the eyelets inserted. A tongue made of tanned elkhide is sewn to the anterior flap. Overcast stitching both strengthens and improves the appearance of this lacing closure ( Fig. 4 ).

If a zipper closure is desired, it is necessary only to make a slit on the medial side posterior to the ankle ( Fig. 5 ).

It will be noted that the prosthesis with the lacing closure, illustrated in Figure 5, extends further up the leg than the zippered prosthesis in Fig. 5 . The exact height of the prosthesis must be a matter of judgment on the prosthetist's part in providing the patient with the best possible function and cosmesis. For example, a female patient will usually prefer, for cosmetic reasons, a prosthesis which does not extend above the ankle. A boy, on the other hand, will probably want the increased function and security provided by the higher ankle support.

Summary

This fabrication technique has proved to be so successful at CAPP that all of our partial-foot patients are now being fitted with this new design. It has provided excellent function for a variety of partial-foot patients at this Project, including congenital transtarasal; congenital left partial foot; right Lisfranc tarsal-transtarsal, acquired; congenital bilateral Chopart-type apodia; congenital bilateral partial apodia. Both function and cosmesis appear to be considerably improved over conventional designs, and patient and parent acceptance is unanimously positive. A more normal-appearing gait has also been evident in the patients fitted. The single girl patient has a prosthesis with a zipper closure, which she prefers for cosmetic reasons. The boys (ranging in age from 4 to 18 years) were all provided with a lacing closure, and this has proved to be not only more functional but also much more durable than the zipper.

It has also been our experience that the cost of fabrication can be appreciably reduced by the technique described, and this of course is of major importance: both growth and intensive wear are inevitable, and it can be expected that the young partial-foot patient will require a new prosthesis at 12- to 18-month intervals.

CP Child Amputee Prosthetics Project University of California, Los Angeles, California

References:
1. Gazeley, William E., and James E. Holmblad, Congenital foot deformities requiring surgery and prosthesis in Lisfranc's amputations. Inter-Clin. Information Bull.,
6:5:14-20, Feb. 1967.
2. Gazeley, William E., and William Sampson, A prosthetic replacement for congenital forefoot meromelia. Inter-Clin. Information Bull., 7:10:12 13, July 1968.
3. Levy, Sherwin E., Total contact restoration prosthesis for partial foot amputations. J. Amer. Podiatry Assoc., 50:11, Nov. 1960