ISNY Flexible Sockets for Upper-Limb Amputees

SIDNEY FISHMAN, PHD, NORMAN BERGER, MS, ANDJOAN E. EDELSTEIN, MA, PT


Prosthetic sockets made of flexible thermoplastics are becoming increasingly popular because the pliable material improves comfort for the wearer4,5 and, for the juvenile amputee, facilitates socket adjustment to accommodate growth3 The ISNY (Icelandic-Swedish-New York) flexible socket approach introduced in 1984 for aboveknee prostheses6 has now been extended to the full range of upper-limb sockets, including those for wrist disarticulation, below-elbow, elbow disarticulation, and above-elbow prostheses.

Unlike rigid unitary sockets traditionally fitted to individuals with upper-limb amputation, the ISNY system utilizes two separate structures, each of which serves different functions. A lightweight, translucent, flexible thermoplastic socket provides tissue containment. The socket is nested in a rigid supporting frame to which are attached other prosthetic components, such as the wrist or elbow unit, and which furnishes appropriate prosthetic length. The frame also transmits motion of the amputation limb to the terminal device, allowing the wearer to position the prosthetic hand or hook accurately in space. Socket flexibility is the most important feature enhancing comfort. The ability of the socket walls to change shape cushions the impact of moving bones and contracting muscles against these walls. Impact absorption by wall deflection translates into reduced unit pressure and thus more comfort. The thinner the socket wall, the better will be impact absorption. Walls should be approximately .05 to .07 inch thick. Heavier sockets reduce pliability and comfort, obliterating the difference between flexible and rigid sockets. An excessively thin wall, however, tends to tear too easily. Aligned low density polyethylene with its relatively parallel molecular chains provides the ideal material for uniform molding; it combines requisite thinness with durability.

The frame should be as scanty as possible to cover a minimal portion of the amputation limb. Brief coverage fosters comfort, lightness, heat dissipation, and sensory feedback, and does not interfere with socket wan deflection. In contrast, wide or superfluous struts or frame walls provide excessive coverage and cannot be considered optimal. The frame is laminated of Dacron felt, nylon, and carbon fiber. Flesh tone pigment added to the laminate increases the cosmetic appeal of the prosthesis. Struts are lined with 5 min (3/16 in) firm Pelite or similar polyethylene foam. Socket and frame are easily separable from one another to facilitate adjustment. The struts are secured to the socket by doublesided carpet tape adhered to both sides of the Pelite. Speedy rivets may be used to augment the tape.

For children, the ISNY system is ideal. As the wearer grows, either the polyethylene socket can be heated and reshaped to increase its circumference, or a new socket molded over the original plaster model modified according to measurements of the larger amputation limb. The frame is enlarged circumferentially by grinding the Pelite or removing it from one or both sides of the struts. Longitudinal growth is accommodated by repositioning the frame on the socket more distally.

Below-Elbow Designs

Children and adults with wrist disarticulation and long below-elbow amputation limbs have been fitted successfully with ISNY flexible-hinge prostheses .2 The socket and frame approach is particularly useful when the wearer attempts to lift an object held in the terminal device. The weight of the object causes the socket to rotate around the amputation limb. Rotation is arrested by forces which concentrate in the socket anterodistally and posteroproximally. Although its walls are flexible and can change shape to a small extent, the thermoplastic socket limits rotation by applying the same forces as the rigid socket. The important difference, however, is that slight wall deflection causes the forces to be applied much more comfortably by a flexible socket.

The ISNY below-elbow prosthesis features a simple frame consisting of two slender struts placed on the medial and lateral sides of the socket, with the entire anterior and posterior aspects of the residual forearm covered only by the flexible socket (Figs. I and 2 ). The socket is vacuum formed from 3/16 inch Surlyn1,7 7 or polyethylene. Polyethylene permits the prosthetist to grind and buff portions of the socket that are too thick to achieve the appropriate thinness and flexibility. Surlyn, in contrast, is more transparent, easier to mold, especially over bulbous contours; and does not shrink during the cooling process; however, it cannot be ground and buffed selectively.

The frame lay-up consists of light Dacron felt: an inner two layers of nylon stockinette; four layers of carbon fiber tape, I cm (1/2 in) wide; and an outer two layers of nylon stockinette. After lamination, the frame is trimmed to create two struts 2 cm (3/4 in) wide. The struts terminate 0.3 to 2 cm (1/8 to 3/4 in) distal to the socket trimline. The struts originate at the distal end of the frame which features a posterior curve just distal to the end of the amputation limb. The anterior curve lies immediately proximal to the end of the amputation limb to hide the site of amputation (Fig. 3 ). This design consideration was compelled by early experience with adolescents who were self-conscious about the visibility of the amputation limb through the translucent socket.

Flexible hinges are attached to the frame in the usual locations, with the cross hanger strap positioned precisely at the joint centers, directly over the humeral condyles.

Above-Elbow Designs

In terms of underlying theory, the above-elbow ISNY prosthesis presents nothing unique. The same approach, namely a thin-walled, flexible polyethylene socket nested in a laminated, carbon-fiber-reinforced frame, has worked very well (Fig. 4 ). The frame has two narrow struts placed medially and laterally. The first patient to whom we fitted this design was a strong, burly man who works for a trucking company as a loader, lifting and shoving heavy cartons of books. This is certainly unusually strenuous activity for an above-elbow prosthesis wearer. His chief complaint about his rigid plastic socket was that the anterodistal aspect of his amputation limb was continually bruised and abraded, not surprising considering his work. After three weeks of wearing his ISNY socket, he reported gleefully that all abrasions had resolved, and for the first time in years he was free of bruises and was much more comfortable. He has worn the ISNY prosthesis for well over a year and would not dream of returning to a rigid socket.

Subsequently we have fitted several children with congenital deficiencies corresponding to elbow disarticulation (Figs. 5 and 6 ). They were also much more pleased with the heat dissipating qualities of the INN Y fitting as com pared with their previous laminated sockets.

Our experience with the ISNY lower-limb and upper-limb sockets has bee most positive. Our patients have been delighted with the increase in comfort clearly validating the basic postulates that "soft is better than hard" and "flexible is better than rigid."

New York University Post-Graduate Medical School, 317 East 34th Street, New York, NY 10016

References:

  1. Banziger E: Surlyn Socket Design for the Young Child. Journal of the Association of Children's Prosthetic-Orthotic Clinics 22:21, 1987.
  2. Berger N, Fishman S, Krebs D, Webb W: The Application of ISNY Principles to the Below-Elbow Prosthesis. Orthotics and Prosthetics 39:16-20, 1986.
  3. Fishman S, Edelstein JE, Krebs DE: Icelandic-Swedish-New York Above-Knee Prosthetic Sockets: Pediatric Experience. Journal of Pediatric Orthopaedics 7:557 562, 1987.
  4. Fornuff D: Flex-Frame Sockets for Upper Extremity Prostheses. Clinical Prosthetic and Orthotics 9:31-34, 1985.
  5. Jendrzrjczk DJ: Flexible Socket Systems. Clinical Prosthetics and Orthotic 9:27-30, 1985.
  6. New York University: Fabrication Procedures for the ISNY Above-Knee and Below Knee Flexible Socket Systems. New York: Prosthetics and Orthotics, New York University Post-Graduate Medical School, 1987.
  7. Skewes E, Haas J, Kruger LM: Surlyn Sockets for Below-Elbow Myoelectric Prostheses, Journal of the Association of Children's Prosthetic-Orthotic Clinic 23:19-23, 1988.