Making a Custom Gel Insert

You may have seen commercials advertising gel or "custom" insoles on television. Those inserts are a good solution for many people, but they are not for everyone. Despite how they are marketed, those off-the-shelf (OTS) inserts are not actually custom but rather are generic and tailored to fit a few different shapes of feet. When the anatomic structure of a person's foot is so pathological it requires a custom insert, an OTS device will not do the job. With the current reimbursement climate and levels, getting reimbursed for arch supports is difficult enough, so adding extra fabrication time and material cost is not ideal. But a person with extreme sensitivity, a graft, or scar tissue on the plantar surface of his or her foot may have difficulty getting a comfortable fit with a traditional multidensity insert. Being able to offer a custom-molded gel insert could be a great alternative to help some patients, and I find, in my opinion, that the balance it offers between the lack of profit and the desire to achieve the best results for the patient may be worth it. Gel inserts are not new inventions, but the solution I offer provides a different approach by completely encapsulating the gel in a foam shell. I used the following fabrication method, which could take three or more hours to complete, with inserts for one of our patients who has a large skin flap over the heel, a lot of scar tissue, and sensitivity on the plantar surface. He had tried many different devices to no avail. Using this device allowed him to walk comfortably, and he was very happy with it. Selecting a gel for the orthotic is the first priority. Choose a material that has low durometer, high durability, and easy molding properties; I used Plastisol. This particular orthotic was used in conjunction with a double upright AFO, so the boot to which the stirrup was attached was used to make the mold. Begin by making a mold to capture the anatomic structure of the patient's foot; this can be done using BIO-FOAM® Impression Foam. An existing AFO cast of our patient's foot was used to make the BIO-FOAM impression for the positive mold. The final positive mold may need modifications after being poured. It is recommended to wall up the BIO-FOAM impression to make it easily usable for the mold (Figure 1). To make a mold of the inside of the shoe, first remove the existing insole and laces. Then place a trash bag inside the shoe and wrap it around the shoe onto itself using a nylon stockinette as a wick. Place a vacuum line inside the trash bag and wick. Then seal around the vacuum line with playdough and electrical tape (Figure 2). Apply vacuum, after which you can manipulate the trash bag inside the shoe to achieve a good mold shape; the fewer wrinkles in the bag the better. Pour plaster into the shoe and insert a pipe. After the plaster cast has set, carefully remove it from the shoe; this is the positive mold of the shoe. To make the bottom section of the mold, use an appropriate-size cardboard box to pour plaster into and fit around the positive mold; a shoebox will work. Clean and smooth the positive mold of the shoe; it will need to have a slight outward taper so the mold will disconnect easily. Wrap a new plastic bag around the positive mold of the shoe, and apply vacuum using a mobile vacuum line (Figure 3). Then place the positive mold into the wet plaster in the shoebox; leave enough of the surface exposed so that it can be removed. Before the plaster completely sets, wiggle and loosen the positive mold so no chiseling is required for removal. Now, turn your attention back to the BIO-FOAM impression. After pouring plaster into the impression and adding a pipe, shave the plaster until the sides are not quite horizontal but have a slightly outward taper. This will help loosen the impression from the lower mold (Figure 4). Cut the cast to the metatarsals so it will fit into the bottom mold (Figure 5). The sides of the upper cast may need to be modified, which allows it to fit properly into the lower mold. Do not make major changes, however, as doing so may change the anatomical structure. Once the molds mate together well, determine the thickness of the orthotic and mark that depth by placing a nail in the heel of the bottom mold (Figure 6). Place the upper cast back into the lower mold, and use an indelible pencil to trace around the edge where the casts meet. Insert a few nails horizontally around the edge of that line on the upper cast. Once this is done, the nail in the heel of the lower mold can be removed, and the upper cast can be inserted into the lower mold. Make alignment marks around where the nails set (Figure 7). Use petroleum jelly to grease the cut end of the upper section so that plaster can be used to shore up the hollow toe section. This prevents the gel from flowing away from where it is needed. The toe plate will eventually be added and made from multidensity foams. Once a desired mold fit is achieved, place the Plastisol in a saucepan and heat it over a hot plate (Figure 7). Water can be poured into the mold to estimate how much gel to use. The mold only needs to be filled one-half to three-quarters full. Pour that amount of water into a cup, and then mark the fill line on the cup. Put that amount of gel into the saucepan and heat it at medium heat. Plastisol starts out a milky white color; when it gets close to working temperature it begins to gel, then it turns clear and has the consistency of syrup. Because you will be working with hot liquid, it is imperative that you wear gloves and goggles. Practice inserting the upper section in one movement to match the alignment marks a few times because accuracy will be important. Quickly insert the upper section into the mold, but be sure to only seat it once. Any retraction will cause bubbles in the inserts. The insertion may cause the gel to overflow, so be cautious (Figure 8). Let the gel set until it cools, which could take up to an hour. When the gel is cool, it can be removed from the mold with ease. Cut the gel to the desired trim lines. Don goggles, and use a rough sanding cone to grind the edges of the gel to achieve smooth transitions. While the orthotic could be used in this form, it would likely break down quickly. Containing the gel in foam will extend the lifespan of the orthotic. Using the positive mold of the shoe, heat and form one-eighth-inch Pelite or similar material over the mold to make the bottom layer of the insert. The top layer can be made from a variety of materials such as P-Cell™, Spenco, or leather, but find a balance between a soft durometer and durability. P-Cell is soft and conforming but may tear easily. Spenco is durable and soft but may be difficult to get a nice-looking transition. A soft garment leather will take the shape well and be durable but will also be difficult to get a smooth transition around the edges. In order to glue the outer layers to the gel, the gel must be prepared first using superglue and Zip Kicker. Do not use all-purpose contact cement since it will peel off and create an odd goo when in contact with the gel. While wearing gloves, smear a thin layer of superglue over the surface of the gel, doing small patches at a time, and set it off with the Zip Kicker. After the entire surface of the gel is covered with superglue, regular glue may be applied to the gel. Glue the bottom layer over the gel insert (Figure 9). Then glue a piece of Poron® over the toe plate, and grind a smooth transition into the metatarsal area; be careful not to grind off the superglue. Next glue on the top cover of the chosen material. If using leather, first glue a section of P-Cell over the toe plate for extra padding, then glue on the leather. Leave excess top material overlapping the edge (Figure 10). This will be glued and then folded over the edge to seal the insert. After the sides are glued down, one-quarter to one-half inch of a border is sufficient to seal the insert. Trim off the excess, and smooth the sides. Since this was molded from the inside of the shoe, it should fit perfectly into the shoe. The finished orthotic should look something like Figure 11. Travis Petersen, CPOA, works as an assistant and technician at Alpha-Omega Orthotics & Prosthetics, Springfield, Missouri. He has also worked as a technician for other companies in Missouri. He can be contacted at travispetersen@hotmail.com .

Making a Custom Gel Insert

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