Stiffness and Deflection of Custom-Fit, 3D-Printed Ankle-Foot Orthoses During Walking, and the Influence of Anthropometric Variability

Additive manufacturing enables the rapid production and customization of ankle-foot orthoses (AFOs), offering substantial advantages over traditional fabrication methods. Understanding the mechanical properties of these devices, particularly stiffness and deflection during ambulation, is essential for their effective deployment as it may inform future benchtop performance tests, such as fatigue life analysis. However, previous studies seemingly disregard the combined effects of the passive and active ankle joint contributions to stiffness during ambulation, limiting the predictive accuracy of the mechanical performance tests. Accordingly, this study investigates how the AFO-ankle complex quasi-stiffness (the combined stiffness of the AFO and the ankle joint) and participant-specific anthropometric measures (height and body mass) influence AFO deflection throughout the gait cycle. Nine healthy, unimpaired participants were fitted unilaterally with custom-fit AFOs fabricated using XO-Armor® 3D-scanning and 3D-printing technology. Gait analyses were conducted under AFO and no-AFO conditions, focusing on three subphases of stance: controlled plantarflexion (CPF), controlled dorsiflexion (CDF), and powered plantarflexion (PPF). The results revealed significant positive correlations between anthropometric measures and both AFO-ankle complex quasi-stiffness throughout stance phase and peak AFO deflection in dorsiflexion ( $\theta _{\textit {DF}, \textit {max}}$ ). Additionally, AFO-ankle complex quasi-stiffness during CDF ( ${k}_{\textit {AFO}, \textit {CDF}}\text {)}$ exhibited an inverse relationship with $\theta _{\textit {DF}, \textit {max}}$ . No significant relationships were identified between maximum deflection in plantarflexion ( $\theta _{\textit {PF}, \textit {max}}\text {)}$ and either anthropometric measures or AFO-ankle complex quasi-stiffness during CPF ( ${k}_{\textit {AFO}, \textit {CPF}}\text {)}$ . The findings of this study suggest that height, weight, and ${k}_{\textit {AFO}, \textit {CDF}}$ should be considered when developing standardized protocols for custom-fit AFOs to enhance the predictive accuracy of benchtop testing, particularly when estimating $\theta _{\textit {DF}, \textit {max}}$ .