Fabricating Orthoses with a Low-Temperature Inner Layer

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By Tony Wickman, CTPO

Sometimes, advances in material sciences can provide big benefits to our patients—and be a big pain for our technicians. Over the last few years, we have seen an increase in the development and use of lower-temperature thermoplastics (e.g., DuraFlex® and PROFLEX™), which are very flexible and durable. These new materials have some great properties that allow us to make intimately fitting orthoses that are still soft enough to be very comfortable. The downside is that they can be a bit tricky to work with.

Figure 1: The inner boot is pulled, and trim lines are applied.

Figure 2: Transfer the trim lines from the real inner boot to the mold.

Figure 3: The outer frame can be pulled without a cut strip.

Figure 4 The finished boot looks great!

Low-temperature thermoplastics generally melt at a much lower temperature than olefin plastics, and since low-temperature thermoplastics are typically used as an inner layer, pulling high-temperature thermoplastics over them can present some problems. The most obvious one is that the fabricating hose can become embedded in the lower-temperature plastic because of the heat and pressure of the second pull. This can make the inner boot pretty ugly, and ugly doesn't sell.

I have seen a lot of different techniques over the years for preventing this. Most of them involve coating the low-temperature inner boot with cellulose acetate, or covering the inner boot with a PVA bag. Using cellulose acetate requires that you remove the coating from the finished boot, but it still leaves some deep stocking imprints in the finished product. Using a PVA bag works, but occasionally bubbles form in between the boot and the outer frame, leaving ugly spots in the outer frame. While these techniques work, they can be time-consuming, and, quite frankly, they still don't look very good. So we found an alternate method that takes less time and produces an attractive finished product.

The process is simple. Start by modifying the mold as usual, pulling the inner boot with the specified thickness of low-temperature material. Then apply the trim lines to the outside of the orthosis and cut it off the mold with a razor knife or a cast saw (Figure 1). Grind and finish the orthosis as you normally would. Once it is finished, reapply the orthosis to the mold and transfer precise trim lines to the mold (Figure 2). Next, remove the orthosis and pull a second, "dummy" boot with low-density polyethylene of the same thickness material that was used on the first boot. Transfer the trim lines to the dummy boot, remove it, and finish it as closely as possible to the original boot.

The dummy boot can then be reapplied to the mold, and the outer frame can be pulled over it with just a fabricating hose as an interface (Figure 3). Since the dummy boot will be thrown away, you won't need a cut strip, and only the portion that interfaces with the frame needs to be finished.

Once the frame is finished, it can be paired again with the original, lowtemperature boot, and the fit should be exact (Figure 4).

This technique is fast and simple. The polyethylene material used for the dummy boot is inexpensive and heats quickly so it is easy to pull. Plus, since the real inner boot is never formed over, the finished product will always look pristine.

Tony Wickman, CTPO, is the CEO of Freedom Fabrication, Havana, Florida. He can be reached at