Orthotic characteristics of off-the-shelf ankle-foot orthoses; how size affects dimensions and stiffness

Ankle-foot orthoses (AFOs) are used to improve walking ability in people with neurological and neuromuscular disorders that cause calf muscle weakness. To maximize walking ability, the AFO characteristics, particularly stiffness around the ankle, must be individualized. In case of calf muscle weakness, relatively stiff AFOs exceeding 3.5 Nm/degree are often necessary. To facilitate an informed AFO choice, we aimed to provide an overview of the characteristics of off-the-shelf AFOs. We evaluated the characteristics of 15 off-the-shelf carbon AFOs from three international manufacturers, in all available sizes (XS - XL, 69 AFOs in total). The following characteristics were measured: AFO weight, stiffness around the ankle, and footplate stiffness around the MTP joints. Differences between AFO sizes were tested using repeated measures ANOVA's. Partial correlation analysis was used to determine the correlation between AFO weight, and ankle stiffness and footplate stiffness while correcting for AFO size. There was a large variation in ankle stiffness (0.2-4.0 Nm/degree) and footplate stiffness (0.07-0.28 Nm/degree) between the AFOs. With increasing AFO size, ankle and footplate stiffness increased significantly (p < 0.001), although the effect of size differed between AFO types. Ankle stiffness (r = 0.479) and footplate stiffness (r = 0.246) were moderately correlated with AFO weight, independently of size (p < 0.043). The ankle and footplate stiffness of off-the-shelf AFOs are highly dependent on their size, with smaller AFOs being less stiff. This in combination with the low maximum ankle stiffness (<4.0 Nm/degree) limits the applicability of off-the-shelf AFOs in patient populations were higher values are required, such as people with calf muscle weakness.