The Air-Cushion Principle Applied to Upper-Extremity Prostheses

B. Ray Buddin, C.P.O. Gael R. Frank, M.D.


In 1961 the air-cushion socket for below-knee amputees was developed at the University of California at Berkeley1,3. This socket had a conventional patellar-tendon-bearing configuration proximally. Distally the stump was suspended in a sling or inner socket of stockinet impregnated with RTV (room-temperature vulcanizing) Silastic (Registered trademark Dow Corning Corp.). Air space between this socket and the outer shell of the prosthesis provided a "cushion" for the stump.

Excellent results with the air-cushion socket have been reported by a number of institutions2,4 and the technique has been incorporated in courses of "Advanced Below-Knee Fitting Techniques" by the three prosthetics schools.

This report describes the application of the air-cushion socket fitting principle to an upper-extremity amputee with severe scarring-the first time to our knowledge that this type of fitting has been done.

The Patient

The patient was a 12-year-old white male who sustained bilateral upper-extremity amputations in a farm accident on December 14, 1969. Both upper extremities were considered to be in satisfactory condition for replantation and bilateral upper-extremity replantations were attempted. Two days later it became necessary to amputate the left limb because of inadequate circulation. Approximately three weeks following the replantation the right upper extremity also had to be amputated just distal to the elbow because of compromised circulation plus infection which had developed in the forearm. We were able to salvage the elbow joint, but because of the extreme shortness of the remaining radius and ulna (approximately one inch distal to the elbow joint) the elbow was nonfunctional. Skin closure was a problem but the skin was finally closed over the end of the stump using a pedicle graft from the abdomen (Fig. 1 ). Further skin grafts may be necessary to obtain satisfactory coverage over the anterior aspect of the stump but we felt it advisable to proceed with an upper-extremity prosthesis. Our plan is to do the final plastic surgery once growth has been completed. This can be accomplished between prosthetic changes two to three years from now.

Fabrication of the Prosthesis

Examination of the patient prior to fitting revealed severe scarring and distorted contours over the entire distal two-thirds of the stump. The skin grafts and scar tissue were too thin and sensitive to tolerate the friction and pressure of a regular socket. It was then that we decided to try the air-cushion principle, fabricating the socket in the same manner as for the below-knee PTB air-cushion prosthesis. However, the technique was modified by waxing around the entire RTV Silastic portion of the socket to form a cylindrical outer shell. The air space between the inner (Silastic) and outer (polyester) socket walls would protect the scarred portions of the stump from any direct pressure. With this type of fabrication the stump is first inserted into the Silastic inner socket using a light stump sock. The outer socket and the rest of the prosthesis is then donned with a minimum of friction (Fig. 2 and Figs. 3 and 4 ).

In preparing the cast for lamination, the deep depressions in the male mold which represented the scarred areas at the line of replantation were filled with plaster (Fig. 5 ) to eliminate socket pressures in these areas.

To fabricate the inner socket (Fig. 6 ), one layer of dacron felt was placed over the proximal portion of the model to a point just above the scarred area. Two layers of nylon stockinet tailored at the end were then applied over the entire cast. To complete the lay-up of the proximal portion of the socket for lamination with polyester resin (90 per cent rigid-10 per cent flexible), one additional nylon sleeve corresponding in length to the layer of dacron was inserted between the two full-length nylon layers. A polyvinyl alcohol (PVA) sleeve was then pulled over the lay-up and banded at the distal border of dacron to provide an RTV stop line. The two layers of nylon were then impregnated with RTV Silastic. The PVA sleeve was removed (together with the band) and a new PVA sleeve applied with a small PVA tube held in place to feed polyester resin proximally to complete the inner socket.

A wax buildup (Fig. 7 ) was fashioned around the entire RTV portion of the inner socket to relieve this section of the stump from pressure and to form a cylindrical outer socket which would allow the inner socket to be slipped in and out. The elbow turntable was set in place on the wax buildup. A PVA sleeve was applied over the rigid proximal socket to serve as a parting agent and taped off at the proximal border of the waxed RTV sleeve.

The outer socket was then laminated in the usual manner for an above-elbow prosthesis. The lamination was put in an oven until the wax reached its melting point. To release the inner socket the proximal outer flare over the acromion was trimmed down to a point midway between the acromion and the medial or axilla level. The socket was then pulled and the wax removed.

A figure-8 ring-type harness with a standard elbow-lock-control strap was applied (Fig. 8 ). However, locking and unlocking the elbow proved difficult because of lack of shoulder mobility. An across-the-chest elbow-lock control strap was then substituted with excellent results.

The amputee was able to use the prosthesis satisfactorily and stated that he experienced no discomfort in the RTV sleeve. He has experienced only minor skin problems, and is gradually increasing the amount of time he wears the prosthesis each day.

Summary

The application of the air-cushion socket principle in a problem upper-extremity fitting is reported. Since the outcome was relatively successful, other clinics may wish to try this technique in similar problem cases.

Amputee Clinic-Children's Hospital The University of Oklahoma Medical Center Oklahoma City, Oklahoma

References:
1. Foort, James, The patellar-tendon-bearing below-knee prosthesis for below-knee amputees, A review of technique and criteria, Artif. Limbs. 9:1:4-13, Spring 1965. 
2. National Academy of Sciences, Below-knee prosthetics, A Report of a Symposium Sponsored by the Committee on Prosthetics Research and Development of the Division of Engineering, December 1968. 
3. Radcliffe, C. W., and J. Foort, The patellar-tendon-bearing below-knee prosthesis, Biomechanics Laboratory, University of California, Berkeley and San Francisco, 1961. 
4. Wilson, L. A., E. Lyquist, and C. W. Radcliffe, Air-cushion socket for patellar-tendon-bearing below-knee prosthesis, Department of Medicine and Surgery, Veterans Administration, Tech. Rep. 55, May 1968.