Hydraulic ankle foot orthosis emulator for children with cerebral palsy

William Durfee, Saeed Hashemi, Andrew Ries

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Objective: Ankle foot orthoses (AFOs) are used by nearly 50% of children with cerebral palsy (CP) to ameliorate gait impairments. The methods used to prescribe and tune the mechanical properties of an AFO, including its angular stiffness about the ankle, are based on the intuition and experience of the practitioner. The long-term goal of this research is to develop and deploy a technology-based solution to prescribing passive AFOs that uses an AFO emulator to be used in the clinic that can, under computer control, vary its stiffness in real-time to determine the best stiffness for walking. The objective of this project was to design and bench-test a first-generation wearable hydraulic ankle exoskeleton, and to conduct a small clinical trial to determine whether walking in a conventional plastic AFO was the same as walking in the hydraulic exoskeleton whose stiffness was programmed to match that of the conventional AFO. Methods: The hydraulic ankle exoskeleton was comprised of a wearable ankle exoskeleton tethered by small-diameter hydraulic hose to a push-behind cart that contained the hydraulic power supply and control components. The ankle component contained a novel double-ended cylinder with a cable anchored to the piston. The system was controlled to emulate a rotary spring. Bench top tests were performed to validate the performance of the system. In addition, an early feasibility clinical trial was conducted with five children with cerebral palsy who walked in three conventional AFOs (flexible, medium and stiff) and the hydraulic AFO controlled to match each stiffness. Kinematics and dynamics of gait were measured with a 12-camera motion capture system and a force plate. Results: The weight of the wearable exoskeleton plus shoe was 1.5 kg, 60% over the design goal. The system, running at a rail pressure of 141 bar (2,050 psi), could produce 62 Nm of torque and could emulate springs from 1 to 4.6 Nm/deg, the stiffness range of most conventional AFOs. Once calibrated, the torque-displacement properties were similar to the matched conventional AFO. Walking metrics were the same for hydraulic and conventional AFOs. Interpretation: Small-scale hydraulics are effective for a wearable exoskeleton that is designed to mimic a passive AFO and hydraulics can be used to emulate a rotary stiffness. While heavier than the design target, the added weight of the hydraulic system did not seem to impact walking in a significant way. The metrics used to evaluate walking were not sensitive enough to detect any subtle differences between walking with the hydraulic system and walking in a normal AFO.

Original languageEnglish (US)
Title of host publicationBATH/ASME 2020 Symposium on Fluid Power and Motion Control, FPMC 2020
PublisherAmerican Society of Mechanical Engineers
ISBN (Electronic)9780791883754
DOIs
StatePublished - 2020
EventBATH/ASME 2020 Symposium on Fluid Power and Motion Control, FPMC 2020 - Virtual, Online
Duration: Sep 9 2020Sep 11 2020

Publication series

NameBATH/ASME 2020 Symposium on Fluid Power and Motion Control, FPMC 2020

Conference

ConferenceBATH/ASME 2020 Symposium on Fluid Power and Motion Control, FPMC 2020
CityVirtual, Online
Period9/9/209/11/20

Bibliographical note

Funding Information:
Thanks go to Rachel Anderson for developing the clinical trial protocol, Elizabeth Duffy for coordinating the clinical trial and Allison Schmitz for participating in the clinical trial data collection. This project was supported by the National Institutes of Health, grant number R21 EB 019390-02, and by the Center for Compact and Efficient Fluid Power, a National Science Foundation Engineering Research Center, EEC-0540834.

Funding Information:
Thanks go to Rachel Anderson for developing the clinical trial protocol, Elizabeth Duffy for coordinating the clinical trial and Allison Schmitz for participating in the clinical trial data col- lection. This project was supported by the National Institutes of Health, grant number R21 EB 019390-02, and by the Center for Compact and Efficient Fluid Power, a National Science Foundation Engineering Research Center, EEC-0540834.

Publisher Copyright:
Copyright © 2020 ASME.

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