A rigid AFO should only deform a small amount to achieve its clinical goals. Material thickness and the design of reinforcing features can significantly affect AFO rigidity, but their selection has been based on anecdotal evidence, according to the authors of a recent study. With that in mind, they designed a polypropylene AFO to quantify the effect of these parameters on AFO stiffness and to set a basis for guidelines for the optimal design of rigid AFOs. The authors concluded that stiffness is maximized when reinforcements are placed at the most anterior position possible, and further stiffening is achieved by ensuring the reinforcements extend from the footplate to at least two-thirds of the AFO’s total height.
For the tests, the researchers fabricated an AFO according to standard practice in the United Kingdom, and its stiffness was experimentally measured for 30Nm of dorsiflexion. Its geometry and mechanical characteristics were used to create a finite element (FE) model of a typical rigid AFO. The model was then used to quantify the effect of material thickness and reinforcement design (i.e., reinforcement placement and length) on stiffness. A final set of AFO samples was produced to experimentally confirm key findings.
For a specific AFO geometry and loading magnitude, there is a thickness threshold below which the AFO cannot effectively resist flexion and buckles, the study said, and the stiffness of the AFO reinforced according to standard practice with lateral and medial ribbing was 4.4 ± 0.1 Nm/degree. When the orthotic technician moved the ribbings anteriorly, the stiffness increased by 22 percent.
The study, “A quantitative analysis of optimum design for rigid ankle foot orthoses: The effect of thickness and reinforcement design on stiffness,” was published in Prosthetics and Orthotics International.
