A New Approach to Sealing Thermoplastic Edges

A technical challenge solved by identifying a thermal window. Sealing thermoplastic edges simply and smoothly in a way that adds to patient comfort has always been problematic. Although the nature of low-temperature thermoplastic makes it ideal for molding around an extremity or torso to quickly custom fabricate devices such as a humeral fracture brace, cuffs for a knee brace, a hand/wrist orthosis, LSO, or TLSO, the edges it leaves are not smooth, which is not cosmetically appealing or comfortable for the wearer. Traditional Thermoplastic Seals Top: Silicone edging strips. Bottom: Small silicone edge applied to an orthosis. Images courtesy of Allard USA. Traditionally, the edges of thermoplastic products are sealed as the final step before delivery. There are many appropriate edging materials on the market such as pressure-sensitive foam and moleskin, and these materials are often self-adhering with just a small degree of elasticity. If applied correctly, such traditional edgings work quite well, but they need to be peeled off and replaced if the product is reheated and reshaped. In addition, with exposure to excessive moisture or temperature changes, the pressure-sensitive materials tend to peel loose from the device. We wanted to find an alternative solution to sealing the edges of various thicknesses of a low-temperature thermoplastic that was pre-cut into appropriate anatomical shapes, versus the standard flat sheets, to expedite the custom fabrication of orthotic devices. The solution had to be such that it could be applied to the pre-cut thermoplastic during the manufacturing process. Orthoses that are fabricated from thermoplastic often require further reshaping and/or heat modification to meet changing patient needs, such as weight loss or edema. Because of this, the ideal solution must also address how to re-seal the edges of the pre-cuts if such modification is required. And finally, because these types of orthoses come in contact with the patient's skin, the surface must be soft and smooth. In short, an alternative solution to thermoplastic edging material must meet the following four criteria: Must be able to be applied following modifications and withstand additional heat modification. Must be able to accommodate the varying degrees of curves of the fabricated orthotic device. Must offer a smooth surface for comfortable skin contact. Must not peel loose when exposed to excessive moisture, temperature variances, or extended wear. Finally, we came up with a solution. On the surface, it appears to be a simple solution—a silicone extrusion—but the core of the solution can be found in the identification of the thermal window—the interval between two temperature levels (see chart below). The Silicone Extrusion Through a thorough study of various materials' physical parameters, we developed a silicone extrusion that offers a number of useful features to address the edging material criteria described above. The supplier of the silicone extruded it into two widths—5mm and 7mm—to seal the edges of the corresponding thicknesses of the pre-cut anatomical shapes of the thermoplastic. The Thermal Window The "thermal window" describes the gap between different materials' molding temperatures. For example, if one material can be molded at T1 and another at T2, the gap between T1 and T2 is the thermal window. There is a wide variety of thermoplastic materials available, and their features vary. The common denominator is that they become moldable when heated, and when they cool, their original properties are regained. This procedure can be repeated. Low-temperature thermoplastics are not heated beyond 175 degrees Fahrenheit. Using silicone in combination with low-temperature plastics enables a positive thermal window (see graph). The characteristics of the silicone selected for the extrusion and adhesive do not change until 300 degrees Fahrenheit. In this case, the window is 300°F>T>160°F, which makes the process possible. Because of the positive thermal window, the silicone edging can be added before heating the thermoplastic material. The silicone is flexible and can be draped into almost any geometry, and it will not come off during heating. We chose Dow Corning 732 Multi-Purpose Sealant to adhere the silicone extrusion to the edges of the low-temperature thermoplastic. The silicone adhesive cures in approximately 15 minutes. Applying the silicone extrusion to the edges of the pre-cut low-temperature thermoplastic is the last step in the manufacturing process. The silicone extrusion is also available separately should the orthotist need to replace any of the silicone edging that may have to be removed as a result of trimming or other modifications that may be necessary to accommodate specific patient needs. This process was developed specifically for a line of pre-cut spinal orthoses fabricated with the low-temperature thermoplastic, carrying the brand name 3D-Lite. However, it can be used for many other applications as a method of adding a smooth, soft finish to otherwise rough edges of many other low-temperature thermoplastics. Christian Hick is the operations manager for Runlite SA Allard Int, Belgium. His background is in the production and development of thermoplastic orthopedic products. Gunilla Ström, CPO, works in product development for Camp Scandinavia AB in Helsingborg. Ström has a clinical background in treating patients with neurological deficits and spinal problems.

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