Study of mechanical characteristics of ankle-foot orthoses using finite element analysis method
Ankle-foot orthoses (AFOs) are external devices applied to the lower leg segment in patients with ankle joint impairment due to neuromusculoskeletal disorders resulting in abnormal gait. In the last three decades, plastic AFOs have superseded conventional metal AFOs. Prescriptions of AFOs by orthotists are usually based on trial and error and if gone wrong, cost time, money and increases the patients' rehabilitation treatment time. This is due to lack of understanding of the mechanical characteristics (i.e. stiffness) and stress distribution of AFOs. There is evidence indicating an optimal match exists between AFOs stiffness and successful rehabilitation. To date, most investigations on AFOs to determine mechanical characteristics are mechanical based. In this investigation an attempt to validate the use of finite element analyses (FEA) in order to predict stiffness and stress distribution of an AFO. Abaqus was used to conduct FEA. The AFO model was subjected to range of loads from 100N to 700N to dorsiflex and plantarflex the AFO. The resulting analysed change in ankle angle, change in length of foot and shank segments, stress distributions were analysed. A linear relationship was found to exist between ankle angle and moment of up to 24.9 Nm and 3.4° during dorsiflexion and moment of 23.9 Nm and 2.7° during plantarflexion and above which, non-linear behaviour is seen. FEA showed asymmetry in buckling displacements at the malleoli in medial and lateral surfaces of the AFO, where the lateral surface was found to be stiffer compared to the medial. However, this result may be inaccurate due to imprecise geometry of the AFO model. High stress concentrations were mainly around the Achilles tendon and malleoli region of the AFO during tension (plantarflexion) and compression (dorsiflexion). It is thought that FEA may be reliable method of studying mechanical characteristics and stress distribution on AFOs but further work is needed to validate the model at its and its accuracy under various conditions of use.
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