Fabricating a Polytol Socket for a Transfemoral Prosthesis
Wearer comfort with a prosthesis depends on the fit and design of the socket as well as the material used. New materials for residual-limb bedding have done much to improve users' comfort and expand technicians' skills. Every innovation in materials for prosthetic fitting tends to require a new fabrication technique, as is the case with Ottobock's Polytol® polyurethane (PU) lamination resin system.
At the Ottobock technical center, we went through a learning curve with Polytol. Compared to silicon, epoxy, and acrylic resins, Polytol acts differently, and we needed experience with its distinct properties. Now, we understand how Polytol works and have refined our process to manage the material.
Polytol can be used in a full spectrum of prosthetic and orthotic fittings. It can be fabricated as a flexible inner socket with an external frame or as a one-piece socket with the frame sandwiched between two layers of Polytol. Polytol is suitable as the inner socket of a transfemoral prosthesis for a short residual limb (Figure 1), the subject of this column.
Preparation for laminating. Start with the plaster mold inverted and secured in a vacuum-equipped lamination stand. You will be pulling six layers onto the plaster mold.
- PVA bag. Wrap the appropriate-size PVA bag in a moistened towel; let it soak for a maximum of ten minutes. Thoroughly clean the bag inside and outside, using cellulose paper to remove solubilized particles (Figure 2). Apply Ottobock Special Lubricant Powder, which is formulated for Polytol, to the inside of the bag. Note: Do not use talcum when working with Polytol; it is too coarse. Pull the PVA bag over the plaster mold (Figure 3). Tie off the bag around the vacuum pipe, and turn on the vacuum. Alert: The plaster mold must be completely dry. Polytol is more sensitive to moisture than conventional lamination resins. Moisture in the mold can cause the PVA bag to adhere permanently to Polytol.
- Dacron. Cut two pieces of Dacron felt slightly longer than the plaster mold. With a sewing machine set to the largest possible length and width of a zigzag stitch for maximum strength, sew a butt seam in each layer. Apply the first layer of Dacron and position the seam laterally, where the least flexing occurs.
- Elastic stockinette. Cut off two sections 1.5 times as long as the plaster model, and sew each end into a semi-circular shape. Apply the first layer.
- Dacron. Apply the second layer, and position the seam laterally.
- Elastic stockinette. Apply the second layer.
- PVA bag. Soak the appropriate-size PVA bag for a maximum of ten minutes, and then thoroughly clean it. Tie off the bag around the vacuum pipe. At the center of the distal end, use string to tie off the bag in a bow for quick release (Figure 4). Make sure the remaining segment of the bag can hold the entire amount of Polytol.
Mix the Polytol. The Ottobock Polytol PU Resin Kit must be processed without interruption, so start mixing only after all preparation work is completed. Because this is a fast, precise sequence of events, you should use a stopwatch at every step.
When processing Polytol, use appropriate personal protective equipment (gloves, goggles, and breathing protection) and ensure there is adequate ventilation and exhaust in workspaces.
The kit consists of three pre-measured components, so use all contents of each container. Do not shake the containers, which can create bubbles. Mix at room temperature and normal humidity.
- Open the PU resin (component A) and stir slowly with a wooden spatula for at least two minutes but no longer than ten minutes to blend the sediment. Mix thoroughly, scraping material off the sides and bottom of the container while stirring.
- Open the catalyst (component C) and, to get every drop, pour it into the resin container for one minute (Figure 5). Slowly stir the two components for an additional minute.
- Add pigment, if requested. Weigh the recommended amount of Ottobock resin color paste, the only pigment designed for Polytol, and add to the resin container. Thoroughly mix the pigment, stirring slowly for one minute.
- Open the isocyanate (component B) and empty it into the resin container for one minute. Now slowly stir it for two minutes.
Laminate with Polytol. The processing time of Polytol is 12 minutes, and it starts the second you finish mixing.
- Quickly pour the Polytol into the PVA bag (Figure 6). Tie off the bag distally so it is airtight.
- Reposition the plaster mold. Some prefer to laminate vertically, with the resin above the mold. We laminate horizontally, with the mold clamped in a slight downward angle.
- Just as you would with conventional lamination resin, work out the air bubbles. Note: Failing to remove air bubbles will result in pockmarks on the socket's outer surface.
- Use the standard lamination technique. With the vacuum on to pull out air and pull resin into the fabric, massage the resin and fabric (Figure 7). Polytol is thicker than conventional lamination resins, but it can evenly saturate the fabric. The wall thickness should be between 4mm and 5mm to ensure proper saturation. When you are done, tie off the bag at the center of the distal end.
Work quickly because you have only 12 minutes to work out air and get layers evenly saturated before Polytol sets. For larger sockets, such as a hip, the 12-minute processing time still applies, so make it a three-person job.
- With the vacuum on, let the socket cure for at least four hours.
Remove the socket from the mold. After the mold is removed, there are a few finishing touches.
- Cut and remove the wrinkled section of the PVA bag covering the socket at the distal end (Figure 8). Leave the remaining PVA bag on the socket surface to protect it against sanding dust.
- Smooth excess resin at the distal end. Carefully cut off Polytol wrinkles with a knife. While using breathing-protection and dust-collection equipment, rough-grind the area with a sanding drum or 100-grain sanding belt, then smooth with a 180-grain polishing sanding drum (Figure 9).
- Remove the socket from the plaster mold. Use a knife to cut the laminate free exactly 1cm proximal to the socket brim. Lift the inner socket off the plaster model with the aid of compressed air. Remove the inner PVA bag.
- Finish the brim. Roughly cut off excess laminate at the socket brim using scissors (Figure 10). Finish-grind the socket brim. Remove the remaining outer PVA bag. To reduce static friction, rub a thin layer of Ottobock Derma Prevent on the inside and outside of the socket but not where double-sided adhesive tape or self-adhesive hook or loop strap will be attached.
Don't get discouraged if your first try at a Polytol inner socket results in a redo. With repetition, you'll quickly get comfortable with this material.
Jason Kimmel is lead fabrication technician for Ottobock in the United States. He oversees all prosthetic and orthotic fabrication and is the main fabrication contact at the Ottobock technical center in Minneapolis, Minnesota.