It was 1979, and I was working in a state-of-the-art O&P lab, laminating prosthetics with polyester resin, which was readily available and cost-effective for large-volume applications. Today, fabrication materials have advanced significantly. Acrylic and epoxy resins are the most common materials in current use. Thermoplastic technology is assisting prosthetic socket fabrications and advanced materials continue to hit our industry.
Evolution of production
Before the introduction of today’s advanced resins, thermoplastics and composites, prosthetic sockets were fabricated from materials that included leather, wood, latex and metal. For centuries, certain types of wood or leather were carved, soaked, stretched and stitched into prosthetic forms. Once dried, sealed or lacquered, they proved to be very durable. Early leather sockets were often suspended in a structural metal or wood frame.
In other early efforts, technicians would bore and carve laminated blocks of wood to produce a “plug fit” socket. Specialty pulling tools were used to finish the interior walls of the socket to match the shape of the positive model provided by the clinician. Transfer chalk would be placed on the positive model, and the inner surface of the socket would be determined by placing the socket to the model and removing any transferred chalk from the inner walls of the socket. The finished socket would then be coated in sealers and lacquers or finished with leather. Today’s CAD-CAM capabilities achieve more intimate-fitting sockets but could be applied using this technique.
If you are ever able to inspect an aluminum socket – another early model – you will be impressed by what experienced technicians were able to do using aircraft grade aluminum that was rolled, heated, contoured, shaped and peened with minimal riveted, structural seams.
Today, our fabrication work is enhanced by readily available industry-standard acrylic resins that are relatively easy to use and, once cured, can be adjusted by applying heat because of their thermoplastic features. Degassing the catalyzed resin improves the air-to-resin ratio considerably.
Industry standard epoxy resins are not as rigid but rebound more easily when stress forces are applied. Epoxy resins are commonly used with advanced laminating materials such as carbon fibers, hybrid matt and stockingette. Fiber orientation is important when fabricating advanced prosthetic sockets. The manufacturers’ resin-to-matrix ratio will ensure the resin’s optimum strength capability. Epoxy laminations are usually very light and rigid, and do not adjust easily when heat is applied.
It is worthwhile to review the fabrication process, which essentially sandwiches layers of selected materials over a cast and between two layers of polyvinyl alcohol (PVA), polyvinyl chloride (PVC) or optional advanced materials.
The selected resin is injected into the material using a vacuum assist. The cast should be smooth with no sharp edges, and dried or sealed if moist. PVA and PVC are activated by rolling the bag or sheet material in a moist towel for 5 to 7 minutes. The cast can be capped with a sheet of PVA; an electrical tape wrap will hold it in place while it dries. Apply talcum powder to the cap and trim the edge to fit the cast. Prefabricated, closed-end PVA bags are also available to simplify the capping process.
Soak the proper-sized PVA or PVC prefabricated bag for 5 to 7 minutes. Wipe off any excessive moisture with a soft, clean and dry cloth; invert the soaked bag and cloth-dry the interior surface. Apply a very light coating of talcum powder to the inner surface, but do not over-apply, and wipe off any excess. Invert the bag and wipe off any excessive moisture. Make sure there is material in between the cast and the top of the inside vacuum pipe. Gently pull the bag over the cast, but be careful not to displace the capped end. Use sharp scissors to cut a clean line in the top of the bag before it passes over the end of the cast. The excess will hold the cap in place; allow the cap to overlap as much as possible without leaving any wrinkles. Let this dry for at least 30 minutes. Contained low temperature heat can speed up the drying process. Always provide adequate ventilation.
Once the inner PVA/PVC material is applied and dried, the lay-up can be added. (We will look more closely at thermoplastic check sockets and specific lay-ups in future articles.) Apply the outer PVA/PVC bag, twist it off and allow it to dry under vacuum. Once this is dry, the selected resin is mixed, degassed and poured into the bag. Palpating the material will help to remove large air bubbles from the resin. Once the resin has degassed, the twisted bag may be opened to introduce the resin to the lay-up, being careful not to trap any air while doing so. After the resin is fully cured, the socket is ready to be removed from the cast.