Design, fabrication and validation of an ankle-foot orthosis simulator system to optimize mechanical characteristics of AFOs for humans with impaired locomotion

Abstract

The muscles around the ankle (calf and dorsal flexors) are essential for performing activities in daily life, like walking. Neurological and muscular pathologies, such as stroke, cerebral palsy, spinal cord injury, muscle atrophy and post-polio syndrome, affect the ability of voluntary muscle control and/or muscle strength of such muscles. This severely impairs the gait function of many people worldwide. An ankle-foot orthosis (AFO) or ankle brace, which is an assistive device that provides support to the ankle and foot, are in many cases a solution. Therefore, patients are fitted with an AFO to promote a functional gait pattern. To optimize the resulting gait pattern, the mechanical characteristics of the AFO should be matched to the specific malfunctioning muscles of a patient. This especially holds for the stiffness of the AFO. Generally, the AFO should be stiff enough to support the ankle's function, but also compliant enough to not restrict voluntary motion.

The optimal stiffness of an AFO for a patient can vary a lot, as the severity of the pathology differs and hence, the consequences, ranging from spastic to paralyzed muscles. It was found that issuing a sub-optimal AFO in the longer term may contribute to deterioration of physical function and gait. Thus, finding the optimal AFO joint stiffness for this group of patients and improving the speed of doing so, is an important clinical treatment goal.

Currently, this is achieved by a trial-and-error method consisting of fitting the patient with several orthoses. This method is time consuming and can possibly result in a sub-optimal AFO prescription. Ideally a human-in-the-loop setup is developed to find the AFO characteristics during tests using an AFO simulator. The corresponding AFO can then be fabricated and fitted to that specific patient. Hence, increasing the speed and quality of providing an AFO to a patient. However, current solutions are too expensive.

This assignment aims to create a proof of principle of a simplified, affordable, human-in-the-loop solution to vary the AFO stiffness, that enables clinicians to tune the AFO stiffness to a specific patient. This report describes the process of designing a lightweight AFO simulator with a continuously variable stiffness mechanism (VSM) and a predetermined torque-angle curve. The resulting design combines two key elements: A leaf spring of varying stiffness by changing the active length, and a cam part the serves as the transmission between the leaf spring and the AFO mockup. The design was fabricated and then validated with a dedicated AFO stiffness tester (BRUCE) based on manual deflection of the ankle-foot orthosis.

It was shown that the predicted plantarflexion stiffness range closely resembled the measured stiffness values. However, the measured dorsiflexion stiffness range was roughly two times smaller than predicted.

Concluding, the designed AFO simulator can change its stiffness, while being compact and lightweight. The potential of the design has been shown. Now it can be developed further into a fully functional AFO simulator system that can be worn by patients in a clinical gait laboratory setting.