Into this broad category fits all devices intended primarily to control foot pathology. The traditional rigid orthotic is the pure embodiment of biomechanical control. By definition, this device does not offer any cushioning or shock absorption. The demands of active patients have lead to the popularity of less rigid devices which trade-off some control for a critical measure of shock absorption. In particular, sport devices substitute polyethylene or polypropylene for composites or acrylics, often adding a crepe or other shock absorbing post, and shock absorbing arch fill or top cover. The type of sport is a key consideration, such as in determining how much jumping, starting or stopping, or side to side motion is required. These considerations will impact the design of the orthotic.
Types of Materials
Composites: Graphite (e.g. TL-61 or TL-2100) is a typical example. Ideal for a strong, lightweight, thin rigid orthotic. Composed of two layers of carbon graphite with an acrylic core. Difficult to form a deep heel cup with.
Acrylics: The material of the classic rigid orthotic. Examples include Rohadur (no longer being manufactured) and Polydur. Polymers of methyl methylacrylate. Easily molded and modified. Highly durable.
Polyolefins: A rigid family of materials, but more flexible and shock absorbing than acrylics or composites, making them well suited for sports applications. Includes polyethylene and polypropylene. Polypropylene is the more rigid of the two. Ortholene and subortholene are types of polyethylene. The trade-off of greater flexibility is less control.
Posting Materials: Extrinsic posts are frequently made out of methyl methylacrylate for rigid orthotics. Sports devises might use EVA or crepes in durometers of 40 to 60.
Posting: Rigid orthotics are routinely posted to accommodate the varus or valgus rearfoot and forefoot dispositions. Intrinsic posts are ground out of the shell of the orthotic, while extrinsic posts are added to the shell. In general, intrinsic posts are most practical up to and including 3 degrees of correction, and extrinsic posting up to and including 6 degrees of correction. Seldom is more than 6 degrees of posting employed in an orthotic as the patient will slip off the device. If you are uncertain about how to post the orthotic you are prescribing, you can leave it to the discretion of the lab.
Orthotics must be broken in gradually. Many orthotic manufacturers include break-in instructions with their orthotics. Also, many orthotics require modification in terms of grinding down or building up part of the orthotic in order to achieve comfort or the desired function.