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Finite Element Analysis of Ventral Ankle-Foot Orthosis Under Cuff and Ground Reaction Force Loading

Ankle Foot Orthosis (Photo: AliMed)

Tuberculosis (TB) has become a global problem, especially in developing countries. More than 10 million people in the world suffered due to TB in 2019, with India as the highest contributor in this number (26%), followed by Indonesia in second place at 8.5% and China in third place at 8.4%. Spinal TB accounts for about 50% of cases of bone TB and is a hazardous type of TB because spinal infections caused by TB bacteria can cause neurological disorders with a prevalence of 10% to 43%. The formation of kyphosis due to a decrease in the mechanical strength of the bone can lead to a sagittal imbalance of the body when walking so that the body will implement a compensatory mechanism characterized by knee flexion and ankle extension inducing pelvic translation. However, the existence of this compensatory mechanism makes spinal TB sufferers more prone to falling in the late stance phase.

Most spinal TB patients undergo medication to eradicate the bacteria, usually for six months. During this period, the patient still has to suffer from the deformation in the spine, which affects their gait. Some treatments for extrapulmonary TB sites, such as the use of orthopedic hardware, have been recommended by some experts. The spinal deformation in spinal TB cases could lead to gait disorders. It could be treated by using orthosis, which can give more stability to the user’s stature when standing and support during walking.

Ankle Foot Orthosis (AFO) is a medical instrumentation that can assist the rehabilitation process of paraplegia in spinal TB patients so that the patient’s gait can approach normal conditions. The type of AFO suitable for spinal TB cases is ventral AFO, which has an anterior resistance called a cuff to keep the legs balanced and sturdy enough to support body weight. To ensure the convenience of using AFO, several parameters must be considered in AFO design: rigidity, geometric shape, and the type of material used. However, even if these parameters were as well defined as possible, for AFO whose parameters were explicitly adjusted to the situation of one patient, the probability of AFO to fracture was twice as high as for AFO whose parameter values were fixed. Model design and simulation based on the Finite Element Method (FEM) are carried out with the help of computers to reduce the duration of trial and error when looking for the most optimal AFO model parameters.

Our previous study carried out the design and simulation of ventral AFO for cases of spinal tuberculosis. The differences in safety factors from variations in the thickness of the AFO were found. The AFO model’s thickness, such as the AFO’s stiffness, will affect the AFO’s behavior when the load is applied. Furthermore, the material choice is also essential since the AFO should maintain its mechanical integrity when walking. The Ventral AFO has a design that resembles a solid-ankle AFO that covers the posterior and inferior surfaces of the calf and foot. Still, the Ventral AFO is also equipped with a stiff anterior surface to resist excessive tibial displacement in the stance phase, increase knee stability, and provide knee torque. Extensor externally during the stance phase. This AFO is also known as ventral AFO due to the addition of the ventral part.

Generally, Ventral AFO treats weakness of the plantar flexor and dorsiflexor muscles and excessive tibial shift. In this study, the solid design of the ventral AFO model was adapted to the condition of spinal TB patients who experience spinal deformities in the form of kyphosis. The parameters varied in the AFO model, namely in terms of the material and thickness of the AFO. The simulation of the FEM numerical method was carried out to determine how the response of the variation in the AFO model to the loading simulates the gait stage during the stance phase and also loading in the cuff section. The analysis was carried out on the deformation, stress, and safety factors. The final objective of this study is to obtain a suitable and safe variation of the AFO model in rehabilitating the gait of spinal TB patients so that the gait condition can approach a normal state.

The varied dimensions and types of AFO materials affect the results of deformation and stress, which lead to different safety factor values for each variation of the AFO model. In this simulation, the variation of the AFO model made from Polypropylene (PP) with a thickness of 7 mm has a higher safety factor than the AFO model made from Carbon Fiber (CF) with a thickness of 5 mm. The high-value safety factor indicates that AFO has sufficient ability to accept stress from gait cycle loading. The AFO considered suitable for rehabilitating spinal TB patients is the AFO variation two models made from CF 395 GPa with a thickness of 7 mm. By having this result, a more dedicated AFO for spinal TB patients could be optimized.

It can be an optional rehabilitation strategy for spinal TB cases that undergo a medication procedure of tuberculosis drugs. Future studies could apply loading to the AFO model using dynamic loading based on the ground reaction force data of spinal TB patients and test the effect of the AFO model on the gait of spinal TB patients directly. In addition, the high safety factor value of the AFO carbon fiber model can be overcome by reducing the value in dimensional variations, which can make the fabrication process of the AFO model made from carbon fiber easier. The design should be fabricated and validated using an experimental set-up indicating the condition of spinal TB patients for further implementation.

This study has been published in Mathematical Modelling of Engineering Problems.


Putra, A.P., Syahananta, L.H.D., Rahmatillah, A., Pujiyanto, Rahma, O.N., Pawana, I.P.A., Qulub, F., Andarini, E. (2024). Finite element analysis of ventral Ankle-Foot Orthosis under cuff and ground reaction force loading. Mathematical Modelling of Engineering Problems, Vol. 11, No. 3, pp. 673-679. https://doi.org/10.18280/mmep.110311 

Author: Alfian Pramudita Putra