Optimising rigid ankle foot orthoses design: A quantitative evaluation of trimlines on stiffness

Ankle-foot orthoses (AFOs) are of important in the management of gait deformities in most neurological conditions through stabilising and supporting the ankle and foot. Despite its importance, there is a lack of knowledge about how some design parameters, particularly trimline geometry, affect AFO stiffness. This study employs a parametric finite element (FE) model to quantify the impact of trimline design on rigidity to improve standardisation of AFO prescription manufacture, and quality control. A parametric model was developed to systematically modify trimline placement and analyse its effect on AFO stiffness. A dorsiflexion moment of 30 Nm was employed to simulate loading conditions, with experimentally determined material properties of polypropylene. The parametric model was developed and validated against experimental results. Trimline positions were manipulated systematically by 1 mm in the proximal and 10 mm in the distal direction of ankle to investigate their impact on stiffness. Thickness, loading, and constraints were controlled for in the analysis. The results of this study verify that the model accurately predicts ankle dorsiflexion, and there are small discrepancies between calculation and experiment. Having more than five transverse plates proximal to the footplate and distal to the ankle does not significantly impact stiffness. Furthermore, trimline position has significant effect in AFO rigidity, that even small changes affect stiffness. Change in trimline posterior to the ankle produced a linear decrease in stiffness, while trimline adjustments distal to the ankle had a nonlinear effect. These findings emphasise the importance of precise prescription and quality control of trimlines to optimise the AFO function.