Use of Copolymer for Interfaces in All Levels of Prosthetic Applications

Richard J. Rosenberg, C.P.O.
Robert Terry, Prosthetic Technician

Thermoplastic

For the past nine years or so, we have been using thermoplastic technology to fabricate socket interfaces for all levels of prosthetics. We have found that copolymer suits our needs extremely well as it demonstrates more fracture resistance than polypropylene. In all of the applications, we drape mold the material rather than use the bubble molding technique. We find that drape molding is satisfactory within our system of fabricating in that a more uniform wall thickness is easier to achieve and the posterior placed seam is usually not visible. Also with drape molding, the problem of having to evacuate a large amount of air during the initial draw is eliminated. The thickest material we use is 1/4 inch, which keeps the heating time to a minimum.

The Oven

The oven we use is an infrared type which consists of two banks of four elements each. Infrared waves heat the material directly rather than heating the air which must then heat the material. Therefore, an infrared oven is not insulated as a convection or conductive oven must be; also, we do not lose heat when the oven door is opened. The outer shell of the infrared oven consists of a single layer of sheet metal which is designed to reduce drafts through the oven. Different materials heat better at different wave lengths in reference to infrared waves.

Fabrication of a Below-Knee Thermoplastic Interface

A nylon hose is applied over the cast before the Pe-Lite insert is fabricated. We have found that two layers of Pe-Lite are necessary for the distal end cap; otherwise, the distal end becomes extremely thin after the copolymer is formed over the insert. If the below-knee (BK) is of a modular design, either temporary or definitive, a plaster buildup is fabricated (Figure 1 and Figure 2 ). The buildup has a flat surface on the distal end and is approximately 2 inches square to accommodate Otto Bock componentry. The buildup is shaped to allow the prosthesis to be assembled in neutral static alignment (Figure 3) . Over the insert and plaster buildup, we apply a heavy cast sock and then a copolymer plate shaped to the distal end of the buildup. Two layers of nylon are added under a PVA bag which is capped distally. Since we are drape molding and will end up with a posterior seam, the cast is placed into a horizontal vacuum manifold and a small piece of stockinette is applied, on top of the PVA bag, from the proximal area of the cast above the trim line of the socket, and tucked into the manifold. The stockinette is a wick to make sure vacuum is not cut off proximally. Small holes are made through the PVA bag in the area of the patella tendon and the popliteal. The PVA bag provides a smooth inner surface to the socket. The copolymer is heated in the infrared oven until it turns to a clear glasslike state. We normally use 3/16-inch thick material, but use 1/4-inch for extremely active patients.

The thermoplastic is draped over the horizontal cast and sealed posteriorly and around the pipe. The excess material is trimmed before the plastic has a chance to cool. We normally use one vacuum pump but two pumps may be necessary if the cast is very large. We have not found that a reservoir for the vacuum system is necessary. We pull as many pounds of vacuum as possible. Once the plastic has cooled, we trim the proximal end and either drive the interface off the cast, or if this is not possible, the cast is broken out.

What we have designed is a total contact insert with a void between the soft socket insert and the rigid outer socket. Depending on the shape of the socket, the insert might tend to migrate into the socket during ambulation. To rectify this situation, a pad fabricated from material such as Ensolite or Plastizote can be fitted between the insert and the socket. It takes a small amount of contact to keep the insert in position.

If the prosthesis is to be of exoskeletal design, the fabrication technique is the same as stated previously with the exclusion of the plaster buildup. The interface is treated like a laminated socket in that it is sanded on the exterior surface and foamed onto an adjustable pylon.

Fabrication of an Above-Knee Thermoplastic Interface

The distal end of the modified cast is sized, coated with polyester resin, and a polyurethane foam buildup is fabricated onto it. Holding the cast in the correct amount of adduction and flexion, a plumb line is dropped to indicate the center of the buildup in the A-P and M-L planes. A mark is drawn 1/2-inch posterior to the lateral alignment line. The buildup is then shaped to a flat distal end and decreased in circumference proximally as dictated by the size and shape of the cast (Figure 4) . The surfaces of the buildup are kept as flat as possible to obtain the most strength from the copolymer material.

A nylon is pulled over the prepared cast, which has vacuum holes drilled in the undercut areas and distal plate fabricated from 1/4inch copolymer. A valve dummy is attached if indicated. A PVA bag is pulled over all and holes are punched in the bag in the undercut areas as well as the area around the valve. After the copolymer is heated to its clear state, it is pulled over the cast while it is in the horizontal vacuum manifold, and as much vacuum as possible is used to draw the plastic to the cast. We use two vacuum pumps when doing this procedure. After the plastic has cooled, the socket is removed from the cast. The urethane distal buildup is retained and used to provide total contact. A layer of 1/8-inch or thicker is attached with Plastizote to the top of the urethane buildup.

In the fabrication of an exoskeletal prosthesis with a thermoplastic socket, the same procedures are used with the elimination of the foam buildup. The thermoplastic socket is treated as if it were a laminated socket, just as described for the BK laminated socket.

We have been using the copolymer material for socket retainers for flexible wall AK prostheses from the time flexible wall sockets were first introduced. The process is similar to the above instructions with a few minor changes. We use plaster as the distal buildup on the flexible wall interface (Figure 5) . To attach the plaster, we first apply some moleskin to the distal end where the plaster must adhere. After the retainer is fabricated, the plaster is discarded. We have been able to provide an anterior and posterior window in the retainer, as well as allowing for a flexible proximal area.

At a Symes level, we have fabricated sockets with copolymer, once again treating the socket as if it were a laminated one.

For a hip disarticulation prosthesis using an Otto Bock modular hip joint, the attachment plate is shaped and trimmed. The proximal surface of the attachment plate is covered with 3/16-inch Pe-Lite and the Pe-Lite is sanded thin until the two threaded inserts are flush with the surface of the Pe-Lite. The PeLite is tapered back, or undercut, from the edge of the aluminum plate and notches are ground into the edge of the plate to allow the thermoplastic to grip it.

Conclusion

We have been using copolymer for virtually all of our interfaces at all levels for endo as well as exoskeletal designs. In an AK situation, we were able to verify that there was a 2-pound weight savings by eliminating the wooden socket attachment block and the outer lamination. The Otto Bock endoskeletal AK prosthesis was reduced in weight from 8 pounds to 6 pounds. Also, by using thermoplastic in lieu of thermoset technology, we have reduced the volume of dust we generate in the lab. Thermoplastic techniques also make the lab less environmentally hazardous as well as reducing the fabrication time.

After using thermoplastic technology in virtually all of our interfaces, we have come to the conclusion that it produces a lighter and stronger prosthesis and also takes less time to fabricate compared to thermoset-plastic techniques which are in wide use today.

The author is with R.J. Rosenberg Orthopedic Lab., Cincinnati, Ohio.