Surlyn Sockets for Below-Elbow Myoelectric Prostheses

EDWARD SKEWES, CPO, JAMES HAAS, ANDLEON M. KRUGER, MD


Surlyn* has been widely used for upper- and lower-limb check sockets and for definitive flexible socket systems. 1-4 As a result of our affiliation with New York University, we were involved early in the ISNY aboveknee sockets, upper-limb frame sockets, and below-knee flexible socket designs.5-10 For above-knee amputees we progressed to narrow mediolateral flexible socket designs.11 Experience with these sockets suggested the use of Surlyn for the socket of myoelectric prostheses. In early 1987, we developed a self-suspending Surlyn socket. Its benefits are: 1) less fabrication time, 2) less expensive, 3) easily adjusted, 4) durable, 5) flexible proximally and 6) hygienic.

Materials and Fabrication

The patient's amputation limb is examined for tissue density, range of motion, muscle strength, and any discolorations. Myotesting is performed to insure minimum microvolt threshold, control ability, and electrode site placement. The limb is lubricated with a parting agent. Bony prominences are marked with an indelible pencil. Elastic plaster bandage is used to form a cast. For short and very short below-elbow limbs, we use a modified Otto Bock casting procedure. Suspension of the definitive socket is achieved by means of medial and lateral pressure proximal to the humeral epicondyles.

The cast is poured and modified. A 1/8 in Surlyn check socket is fabricated to evaluate fit, comfort, and range of motion. Myosites previously located are transferred to the check socket. It is poured with plaster and, after setting, two small holes are drilled in the socket to indicate exact placement of the electrode sites.

The socket is then removed from the cast and any imperfections in the cast are smoothed. Electrode lamination forms are attached to the new cast at the previously located myosites. Small steel forearm attachment plates are secured to the cast with a bonding agent and located so as not to interfere with bony prominences.

A definitive socket of Surlyn is drape molded under vacuum over the cast (Fig. 1 ), either with a posterior seam which will be used to accommodate the myoelectric unit's wires, or without a seam. The socket is trimmed and then the plaster is broken out. The electrode lamination forms are removed. A pilot hole for the larger electrode hole is drilled through the center of the electrode lamination form location. Electrode holes are drilled. Electrode mounts and dummy electrodes are located in the socket.

The socket is then poured with plaster (Fig. 2 ). After setting, a PVA bag is pulled over the socket, sealed with vacuum, and permitted to dry. The forearm shell is foamed over the socket, rough shaped, cut to the proper length, and the wrist unit is secured to the foam. The dummy battery pack is located on the medial aspect of the forearm section. With final shaping completed, a second PVA bag is pulled over the forearm socket. Four nylon stockinettes are donned over the forearm and socket and a final PVA bag is pulled over the stockinette. Polyester resin is used to laminate the forearm section. Before the lamination completely cures, the forearm section is very carefully trimmed out without cutting into the Surlyn socket.

The foam is removed from the forearm shell, and the forearm section is trimmed and smoothed. The battery receptacle is located within the forearm shell and secured with a bonding agent. The pull tube is bonded to the Surlyn socket and may be exited through the battery receptacle (Figs. 3-4 ). Because perspiration can corrode the electronics, the patient is instructed to keep the socket clean; for some children, a better approach involves tucking a tissue in the pull hole after donning in order to absorb perspiration. If one anticipates problems with perspiration, the hole can be positioned in the conventional location away from the battery receptacle. Modular components are attached to the forearm shell and socket which is then ready for final fitting (Fig. 5 ).

Total time for fabrication from molding the amputation to final delivery is 8 hours.

Discussion

Five patients have been fitted with below-elbow myoelectric prostheses using Surlyn sockets. Three previously had conventional myoelectric sockets. All report they are comfortable with their prostheses and have had no problems with skin irritation from the sockets. The sockets have not split.

Use of Surlyn for upper- and lower-limb flexible socket systems has been widely accepted. For a myoelectric fitting, the Surlyn socket can be vacuum formed over a modified cast of the patient's amputation limb and attached to a polyester laminate forearm shell. In our clinic, as the number of prescriptions for myoelectric prostheses increased, we examined the technique of fabrication and decided to use Surlyn as the material for the definitive socket. The method provides a less expensive and more practical way to fabricate a below-elbow myoelectric prosthesis. The prostheses have proven to be comfortable, functional, clean, and durable.

*Physical Support Systems, Inc., 6 Ledge Road, Windham, NH 03087

Mr. Skewes is Director, Orthotics and Prosthetics Department; Mr. Haas is a technician in the department; Dr. Kruger is Chief of Staff, Shriners Hospitals for Crippled Children, Springfield Unit, 516 Carew Street, Springfield, MA 0 1104

References:

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  2. Fomuff D: Flex-Frame Sockets for Upper Extremity Prostheses. Clinical Prosthetics and Orthotics 9:31-34, 1985.
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  9. McClellan BP, Kapp S, Stills M: The Application of lonomer Resins in Definitive Below-Knee Prostheses: A Limited Study. Clinical Prosthetics and Orthotics 8:18-21, 1984.
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  11. Snell T, Bush P, Meyer W et al: CAT-CAM Socket with Total Flexible Brim for Pediatric Amputees. Journal of the Association of Children's Prosthetic-Orthotic Clinics 21:24, 1986.