A new day is dawning in the fabrication of lower-extremity braces – ankle-foot orthoses (AFOs) and knee-ankle-foot orthoses (KAFOs). When the full potential of this new technology is realized, the outcome will be stronger, thinner, and lighter devices, some of which will deliver dynamic response to facilitate ambulation. For good measure, the fabrication process will be shorter and easier on the patient and potentially more cost-effective.
For the past 30 or so years, AFOs have been fabricated primarily of polypropylene and other thermoplastics; KAFOs have been enhanced with metal struts.
Thermoplastics are durable and allow orthotists to build in required flexibility by varying thickness and geometry. However, they also present certain drawbacks, including a thick profile that can limit footwear choices, heat discomfort (thermoplastic is an insulator), and a sometimes lengthy fabrication process.
With KAFOs the weight of the orthosis can be a deterrent to patient tolerance and effective ambulation, as well.
Just as the introduction of plastics to orthotic fabrication several decades ago was a quantum improvement over the then-prevalent leather, wood and metal, so now is the emerging application of advanced composite materials—layers of carbon or other fabric bound with a resin—promising to at least partially supplant plastic construction.
Composites have been applied in prosthetics for some time in socket fabrication and advanced ankle-foot construction. Now, a recent variation, pre-impregnating the composite with a thermosetting resin, gives the fabrication team some exciting new applications for orthotic components as well.
“Pre-pregs” as these new materials are coming to be known, provide high durability at low weight and can be designed to flex, providing dynamic response.
Compared to similar orthoses made from plastics, pre-preg AFOs and KAFOs offer a high strength-to-weight ratio, thin profile and enhanced patient comfort. While early applications appeared primarily in prefabricated devices, fully custom pre preg braces are beginning to enter the mainstream.
Energy storing and release has become a mainstay characteristic of many prosthetic feet models but heretofore has not found major application in orthotics. Now, the dynamic properties of prepreg composites make possible the same sort of gait assistance from an AFO.
A dynamic-response AFO can be designed to allow motion and a gradient resistance throughout the stance phase, capitalizing on the patient’s strengths while supporting weaknesses. Customized dynamic forces throughout the gait cycle enable the patient to ambulate longer and farther before exhaustion.
An exciting pre-preg option still in development is interrupted cure processing, in which a partially cured pre-impregnated composite pre-shaped into the general form of a particular AFO design can be custom-molded, trimmed and adjusted for the needs of a particular patient, sometimes in a single office visit. This technique could conceivably eliminate the casting, vacuum forming and extensive sanding commonly required in the fabrication of plastic orthoses. Pre-preg thermoformable composites may accept several fine adjustments before the material completely cures.
Applications and Contraindications
Pre-preg lower-extremity orthoses can be prescribed to provide improved function and enhanced comfort for patients with drop foot secondary to stroke, multiple sclerosis, polio, spinal cord injury, and other neurological conditions, as well as various ankle instabilities. On the other hand, patients with spasticity or mild-to-severe clonus can be stimulated by a dynamic-response device and are thus not considered good candidates for a pre-preg brace.
Still in their infancy, pre-preg composite orthoses are just beginning to stir excitement and gain acceptance in the rehabilitation community. We can look forward to further news and advances about this technology in the near future.
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