How to Make a Transtibial Silicone Socket Interface

Start with an existing liner or create one of your own. An example is 5mm pelite with an extra 3mm added over the tibial crest and the fibula head. Invert the liner and add strips of wet plaster bandage on the outside to reinforce the liner. The plaster bandage should extend 6 inches beyond the trim line. The plaster strips that extend beyond the trim lines should be pressed together to form the smoothest transition possible. Suspend the pipe so it does not touch the bottom of the liner, leaving a space of at least one half of an inch. Use talcum powder as a parting agent before filling the liner. Fill the liner with the material of your choice ( plaster, hyderostone or hydracal.) Remove the plaster bandage, being very careful not to disturb the liner. Smooth the top of the exposed plaster to eliminate the rough edges. To help align the outer mold halves to the inner model, make registration dimples in the cast with a large drill bit. Place two depressions on the anterior and two on the posterior sides of the model. Draw the seam line on the model to visualize easy removal of the two plastic mold halves later on. You need to cut two pieces of plastic large enough to allow you to “sandwich” the liner and mold you just created. Leave the protective cover on both sides of the plastic, place them in oven and allow the plastic to soften to the correct forming temperature. The two pieces of plastic should form a tight seal when pressed together- this will help to form crisp edges and reduce the amount of the flash that needs to be removed from the cured liner. Do not use any stocking for wicking suction as it makes removal of the protective layer very difficult. Two people should each hold a piece of plastic so that it can be brought together on either side of the liner mold There should be an equal amount of plastic on all sides of the mold (there always seems to be enough time to do this). Try and follow the target seam line as close as possible. Keep the two pieces of plastic as smooth and flat as possible as the plastic cools. Once the plastic has cooled enough to hold its shape, the seam should look flat and straight.

To insure an accurate reproduction of the liner, it is necessary to drill a small hole through the outer mold , the liner, and into the cast. If the distal moves, the resulting liner will be thicker on one side and thinner than planned on the other. Drill a series of holes through the seam on both sides of the newly formed mold. These holes will be used later to bolt the mold back together. Try and drill the holes as close to the model as possible because this will help prevent the silicone from being forced between the two mold halves. Once you have made sure the distal pin and all bolt holes are drilled and the final trim of the plastic is done, the two halves can be separated. The protective covering that was left in now serves as the parting agent. It will be necessary to separate the two halves with a screwdriver the first time. If the plastic is at the right temperature and the vacuum strong enough during forming, the seam as viewed from the inside, will be minimal. Remove the liner. Inspect the inner cast for any defect.

The smallest bubble in the casting will result in a corresponding bump on the inside of the finished product, because liquid silicone has an unbelievable ability to fill the smallest cavity. Fill any major defects with plaster slurry. Use a tool ( a hollow piece of tubing works well ) to create a small groove from the highest point of the liner to the top of the plaster cast. This serves as a vent to allow air to escape and form a reservoir for any excess silicone. Choose a low point on the outer mold half as a starting point for injecting silicone. Drill a hole just large enough to accept the static mixing nozzle without allowing it to enter too far into the mold. Fit the injection hole with a threaded tapered plug. Either wax or petroleum jelly can be worked into the cast to fill even the smallest of defects because any marks left will be faithfully reproduced in the finished liner. Place the mold half with the alignment pin back on to the mold first as this will make it easier to find the corresponding hole in the cast without damaging the cast . Place the second half of the mold on the model and bolt the halves together. The silicone material is packaged in 400 ml side- by- side tubes and automatically mixed in the correct ratio by the static mixing nozzle. The silicone can be dispensed by hand or pneumatically using injection guns designed to hold the side- by- side configuration. The silicone should completely fill the groove created earlier. The extra bit of silicone becomes necessary if there is any leakage when the static mixing nozzle is removed and the threaded plug inserted. Check the cure time of the silicone being used before opening the mold. If necessary, all the platinum cure silicones can have their cure time accelerated by the application of heat , a large heating pad works well for this purpose. As soon as the silicone has cured, it is safe to remove the mold halves. Do not forget to remove the distal alignment pin. You will probably need to trim any silicone that was forced between the mold halves (flashing) and then proceed to the final trim. The liner can now be used as is or converted into a multi durometer liner.

There are a few things to consider about the multi durometer liner, most of which depend on decisions made while creating the original liner or pattern. The ideal liner is thicker and softer in any bony or sensitive area. Remember, in our example, we started with a 5mm pelite liner and added a second layer of 3mm pelite down the crest of the tibia and the fibula head. In this example, the primary injection is done with a 20 durometer silicone. The silicone to be removed from the crest of the tibia measures 8mm thick. Cut through the silicone with a very sharp blade and remove the section you want to replace with a softer silicone. The silicone that was removed from the crest of the tibia and measures 8mm thick. Keep the edges clean and begin the second injection as soon as possible. Replace the mold halves and bolt them together. It may be necessary to add another injection hole in the plastic at the distal point of the void created by removal of the 8mm silicone from the crest of the tibia. It is also necessary to drill a few small vent holes in the plastic at the exact top of this void. Create a small reservoir made of clay so that any silicone that flows from these holes can be trapped.

The excess silicone will flow back into the mold if any silicone is lost when the static mixing nozzle is removed and the threaded plug inserted. Inject the softer silicone into the void and proceed as previously described The finished multi durometer liner is ready for the final trim. For the heavy user, it may be necessary to laminate a finish stockinette over the silicone for added strength.