Center for Composite Materials - University of Delaware

Research Summary

Carbon-Fiber Based Passive Dynamic Ankle Foot Orthosis for Augmenting Net Ankle Moment in Human Gait

Authors: N. S. Khattra, J. J. Tierney, S. Yarlagadda, J. Gillespie (Jr.)

INTRODUCTION

• Passive-dynamic ankle-foot orthoses (PD-AFOs) are a common form of assistive braces prescribed to improve gait of patients having impaired ankle joint function.
• Historically, PD-AFOs have been made using leather, metals and later, thermoplastics. Recently, composite based PD-AFOs are being explored as a lighter and stronger alternative.
• The composite PD-AFO can be designed to augment the lost natural ankle stiffness of a patient to match that of a healthy person.

ANKLE MOMENT vs. ANKLE ANGLE

• Human gait is characterized through the measurement of ground reaction forces.
• Using inverse dynamics, ankle moment is plotted versus the ankle angle.
• The foot-flat phase of the gait cycle (red portion of the curve) is the phase during which PD-AFO is the most effective.

DESIGN REQUIREMENTS

• Ankle moment for a patient is usually a fraction of a healthy person’s moment.
• The difference between a health person and a patient’s moment gives the required PD-AFO profile.
• Preliminary PD-AFO design is created to match linearized stiffness (Nm/deg) response.

FINITE ELEMENT MODEL

3D CAD surface model is generated using digitized anatomical points in CATIA™
Composite layup is designed using that surface as the tool surface
Layup is divided into regions to allow easier draping
• Draping analysis is done to establish correct fiber orientation on the highly curved surfaces
• FE model is crated with an additional 3D leg shank element modeled in contact with the AFO
• Moment is applied about the ankle joint center and the resulting rotation is observed
• The foot is held constrained during the loading to mimic foot-flat phase of the gait

SIMULATION RESULTS

• Simulation results from the FE model are compared with PD-AFO desired stiffness requirement.
• Layup can be modified to match requirement for different patients.
• Various designs are evaluated in order to minimize the maximum strain reached in the AFO.
• Designs with flatter sections around the heel are shown to be well within the maximum strain (1.17%) limit for the material used (VTM264 carbon prepreg).

PARAMETRIC STUDY

• Effects of ply count, ply orientation, material and geometrical design in investigated parametrically.
• The resulting PD-AFO stiffness achieved is shown in the figure along with the limit stiffness for 1st and 99th percentile population, based on their height and body weight.

ACKNOWLEDGEMENTS

This research was sponsored by the Defense Advanced Research Projects Agency and was accomplished under the Army Research Laboratory Cooperative Agreement Number W911NF-07-2-0026. The authors would like to acknowledge Dr. Stanhope’s group from the department of Kinesiology & Applied Physiology at the University of Delaware for providing human subject ankle stiffness data

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