Supracondylar suspension for a transradial prosthesis has been around for decades. This type of socket fabrication is commonly made from a wet lamination or thermoplastic materials.
Despite the growth of thermoplastics, Ottobock, Minneapolis, Minnesota, continues to receive questions about and orders for upper-limb, laminated, flexible-brim sockets. Let's take a closer look at two-stage laminations.
For most technicians, the greatest challenge is determining when the resin is about to gel. The chemical action heats up as the resin nears its set point. To master the two-stage lamination, you must know your resin and learn to sense temperature by touch. If your fingers can feel the resin approaching the right temperature, you can move and position the resin with precision.
To improve your success rate and boost your confidence, use the following process for your two-stage lamination.
Set up for laminating. Start with a cast that has been dried or sealed and is at room temperature. For a rush job, cover the plaster mold with a casting balloon. Secure the cast upright on the vacuum stand (Figure 1).
Apply the layup for the first stage. Pull your desired layup onto the mold, making sure the fabric extends beyond the proximal end of the cast.
Position four star washers under the layup. These will be used to attach the outer shell to the inner socket. This design allows easy access to components (Figure 2). Determine the proper location based on the contours of the mold. Avoid undercuts and other areas that will cause the outer shell to hang up on the inner socket. For patient comfort, avoid the condyles and the corners of the anterior side.
Pull a polyvinyl acetate (PVA) bag over the layup. Seal the bag around the pipe. Twist the excess bag at the center of the distal end of the cast and apply a Yates clamp. Make sure the remaining segment of the bag can hold the entire amount of resin.
We prefer to laminate horizontally, so we reposition the plaster mold, clamping it in a slight downward angle. This encourages the natural tendency of air bubbles to rise. It also allows us to simply rotate the cast to work all around it.
Outline the rigid lamination. Sketch the desired outline of the first lamination on the PVA bag. We use these guidelines to determine where the rigid lamination will end:
- Capture the entire olecranon.
- Capture the condyles.
- Cover the star washers and stop lamination just proximal to them.
Mix the resin. For the initial rigid lamination, we measure a blend that is 60 percent sealing resin and 40 percent flexible resin. The amount of promoter determines the speed of the chemical reaction. We want more working time for this step, so we stir in half the normal amount—1.5 grams of promoter per 100 grams of resin. Pour the resin mixture in the PVA bag. Twist the excess bag above the resin, and apply a second Yates clamp.
Work the resin. Apply vacuum at one to two inches of mercury (inHg). Remove the first Yates clamp and untwist the bag (Figure 3). You will have about 15 minutes of working time, depending on the temperature of the plaster mold.
Even this low amount of vacuum will continue to draw resin into the fabric until it sets. Control the resin's advance, allowing it to approach the line slowly. Note: Do not let the rigid resin extend beyond the line.
The resin must be close to the line when you sense your working time is nearing an end. When the resin starts to gel, you should be at the line (Figure 4). For most layups the resin should be about 1/8-inch thick.
Let the resin cure. Leave the vacuum on. Curing takes about a half hour. Remove the outer PVA bag once the resin cures.
Set up for the second stage. Apply additional layup directly over the previous lamination. No surface preparation is necessary due to the characteristics of acrylic resins. Pull a PVA bag over the layup and seal the bag around the pipe. Twist the excess bag at the center of the cast's distal end and apply a Yates clamp.
Mix the resin for the second stage. For the flexible lamination, we measure a blend that is 90 percent flexible resin and 10 percent sealing resin. Flexible laminations can become too flexible in warmer climates, which can affect longevity, so we use an 80/20 blend for patients living in warmer climates.
Stir in the normal amount of promoter—3 grams per 100 grams of resin. Pour the mixture in the PVA bag. Twist the excess bag above the resin, and apply a second Yates clamp.
Work the resin. With full vacuum turned on, remove the first Yates clamp and untwist the bag. Let the resin saturate the length of the mold. Work out any air bubbles. Flood the medial and lateral proximal undercuts with resin. Work out all excess resin from the proximal posterior and anterior surfaces (Figure 5).
Let the resin cure. Leave the vacuum on while the resin is curing (Figure 6). Remove the PVA bag once the resin cures.
Finish the socket. You are now ready to fabricate the outer shell. If this is a replacement socket, finish as usual (Figure 7).
Jason Kimmel is lead fabrication technician for Ottobock. He oversees all prosthetic and orthotic fabrication and is the main fabrication contact at the Ottobock technical center, Minneapolis, Minnesota.