Cosmetic Covers for Modular Prostheses

WILLIAM F. SAUTER, C.P.O.


With the revival of the modular technique in prosthetics, prosthetists were once again faced with the problem of finding suitable materials and techniques for the fabrication of cosmetic covers. The general acceptance of the modular principle largely depends on whether or not it is possible to fabricate these customized covers directly and economically in the limb shop.

Ingold in Basel, Switzerland, who first used modular systems on a large scale at the turn of the century, failed in the end because the covers he used, although almost indestructible, were difficult to make, and being made from fibre acted as resonators to amplify the sounds made by loose rivets, etc.

Since then, man has created numerous new materials, many of which at first glance seem to lend themselves naturally to use for cosmetic covers.

The criteria set for cosmetic covers, however, limited acceptable materials to a very few. In short, cosmetic covers should be light, hygienic, even washable, and able to stand up for a reasonable length of time. But most important probably, the techniques used for fabrication should not require fancy equipment or highly specialized skills. Only if these requirements are fulfilled will the modular technique become part of the daily routine in the limb shops.

In recent years, several prosthetic groups have produced tangible results. From the Munster group in Germany came open-cell urethane-foam covers covered with Helanca stockinette. This method is specially suited for articulated prostheses because of the low durometer of the urethane foam. The shaping of this foam, however, is very time-consuming. The stockinette used to cover the finished leg is subject to "runs," absorbs water and body perspiration, and generally soils very easily, especially with younger patients.

Foort, Hobson, et al., in Winnipeg concentrated their efforts on producing a range of pressure-moulded urethane shin covers to complement their own modular systems. These covers are very durable but do have a wall thickness which is enough to prevent their use on patients with fleshy stumps.

The fabrication of these covers requires a whole range of expensive moulds that only a large rehabilitation centre can afford.

Another approach was taken in the U.S.A. where the Prosthetics Center of the Veterans Administration published a report on the use of a polyethylene foam which, after shaping, can either be covered with a garter hose or painted with cordo-bond skin. This method produces a nice cover. However, its drawback lies in the fact that it is impossible to feather the foam sufficiently so that it can be brought up all the way to the proximal edge of the socket. Thus typically, the portion above the patellar-tendon shelf is left exposed. This foam is also rigid, and therefore unsuitable for articulated prostheses.

As an alternative to these methods, we have in the past 18 months successfully used a vinyl foam produced and marketed by B. F. Goodrich under the designation VS-400.

It is the purpose of this paper to describe in detail the technique used in the fabrication of cosmetic covers for endoskeletal below-knee prostheses from VS-400.

At a later date the author plans to produce a complete manual which will cover the full range of endoskeletal limbs and describe the deviations from the procedures discussed in the present report.

Fabrication Procedures

The process of making a cosmetic cover is simple. This foam has a density of approximately 4 lb/ft3. Thus it has sufficient firmness to be sanded, yet is flexible enough to be used with articulated limbs. Furthermore, the cellular structure of the foam is fine and uniform, so that after sanding has been completed the covers are simply painted, thus giving the foam not only a tough but also a washable skin. After the size of the cover has been determined from the dimensions of the limb, a block of foam is cut from the sheet which can be obtained in pieces. This block, after the ends are squared off, should still be at least one full inch longer than the distance from the proximal brim of the socket to the top of the foot. The foam is compressible and this will ensure good contact with the top of the foot.

The centres of both ends of the block are then marked with a soft pencil. It is not advisable to use felt pens the ink of which tends to soak into the foam and later shows on the finished product ( Fig. 1 ).

A piece of metal tubing, sharpened to a keen edge on one side, is then used to cut a hole longitudinally through the foam block. This is easily accomplished by spinning the tubing in the chuck of a lathe. To achieve a smooth cut, it is necessary to wet the tubing inside and out. Needless to say, the tubing should be of the same outside diameter as the pylon being used ( Fig. 2 ).

Assembly

The pylon-socket assembly is placed upon the foam block and its lateral silhouette transferred to the foam with a soft pencil.

Also the outlines of the brim of the socket are drawn on the top of the foam block. If necessary, a simple paper pattern of the brim contours can be used for this purpose. Front and back should be clearly marked on the foam to prevent any "mix-up" during the "fitting-in" of the socket ( Fig. 3 ).

A hole is started in the top of the foam block with a sharp knife and is gradually enlarged using a coarse sanding cone. The hole made should ideally be of the same size and shape as the socket. In practice, however, it is better to keep the hole's circumferential dimensions somewhat smaller to ensure intimate fit between the socket and the foam. The hole should be made deep enough not only to accommodate the full length of the socket, but also to recess it at least one-quarter inch.

The maximum depth of the hole should be marked with a strip of masking tape on the shaft of the router ( Fig. 4 ).

After it has been covered with 3/4-in. masking tape along its proximal brim, the outside of the socket is given a coating of silicone spray. This procedure will prevent bonding of the foam to the socket except for a 3/4-in. strip along the proximal edge where it is desirable ( Fig. 5 ).

With a brush the hole in the foam block is then soaked with vinyl paint ( Fig. 6 ).

After the masking tape has been removed from the socket, the whole socket-pylon assembly is pushed into the foam block. If there is any gapping along the brim, the gaps should be closed by applying several turns of flexible electrician's tape. The foot should be installed to help keep the foam in place.

The vinyl paint applied inside the hole requires about half a day's drying time to form an "inner skin" and only then can shaping of the foam commence. If untrimmed sheets of foam have been purchased the skin should be sliced away to expedite the drying process.

With the foot in place, and properly toed out, the silhouettes of the leg are marked on the foam. Excessive material is then cut away on the band saw and the corners trimmed. Band-saw blades with 24 teeth per inch running at high speeds will cut the foam without tearing it ( Fig. 7 ).

Final shaping can be done on a Trautmann Carver starting with a coarse sanding cone and progressing to a 3-in. sanding drum and #120 garnet paper ( Fig. 8 ).

The sanding should never be directed against the edges of the foam but rather should start from the middle and progress toward the edges. If the first coats of paint applied inside the foam block have been allowed to form a reinforcing skin, the foam can be sanded down to this inner skin where necessary. If any doubt exists, the thickness of the foam can always be checked with a sharp awl. Any uneven spots can be sanded smooth by hand.

To finish off the prosthesis, it is simply hand painted or dipped with vinyl paint made of clear vinyl and Hosmer pigment ( Fig. 9 ). The first coat should be allowed to dry for 30 minutes before the second coat is applied. Before the second painting the foot can be taken off to allow application of paint to the distal end of the cover. When the foot is reinstalled the cover will bond itself to the top of the foot ( Fig. 10 ).

Two coats of paint will normally suffice and give the leg a beautiful lasting finish.