Ankle Rehabilitation via Compliant Mechanisms

Abstract

A flexure-based ankle rehabilitation device utilizes the linear elastic force-deflection characteristics of certain types of flexure beams to measure the forces and torques output by the human ankle joint complex (AJC). Three sets of flexures, placed so as to allow rotation about the three primary rotational axes of the AJC, allow for a measurement of all three major rotations applied by the ankle joint and coupled motions essential for walking, balancing, and running. Currently, there is no method or universally accepted device used to measure and quantify the strength, speed, and stabilizing ability of the AJC. This is especially important when it is considered that ankle injuries are perhaps the most common type of musculoskeletal injury. Furthermore, AJC integrity is recognized as critical to the maintenance of balance and prevention of falls among older adults. We have developed a device that would primarily be used by physicians or rehabilitative professionals as part of a more standardized methodology of musculoskeletal care and to determine the extent of an ankle’s recovery after injury. It has potential to help researchers better understand the recovery process and the mechanics of ankle injuries. Additionally, it could be useful to the footwear and orthopedic industries as a means of evaluating products for AJC protection and recovery. The architecture of the device could also be modified to fit other multi-DOF joints, such as the wrist, shoulder, hip, or spine. Single-DOF devices could also be developed to potentially increase efficacy of rehabilitation practices for single-DOF joints such as the knee and the elbow. Upon fulfilling its initial functional requirements, this device still has a lot of potential for further development. The current design can be tuned fairly easily by exchanging different flexure modules of varying stiffness. This allows for testing of different materials, stiffness values, and flexure module designs. Furthermore, it would be possible to expand on the design so that it can be used as a training tool, instead of just as an evaluation device. A rehabilitation program utilizing such a device would serve to isolate the ankle much more than current recovery exercises, helping to reduce the chances of re-injury. Current technology used to evaluate ankle rehabilitation focuses solely either on the foot’s range of motion or on the joint’s ability to balance the rest of the body. While the former ignores the ankle strength, the latter disregards any information regarding the ankle beyond the standing configuration. The flexure-based ankle rehabilitation device measures ankle strength and power output through a range of motion that is anatomically similar to walking, running, etc. Also, measuring power output is not feasible with currently available devices and is believed to be very important for further injury prevention. Also, a wide range of foot configurations can be achieved, suggesting that regardless of the patient’s physical abilities, the device can still be used to safely and effectively rehabilitate a person’s injured joint and return it to a healthy state. Results from the initial prototype suggest that an ankle rehabilitation device utilizing compliant mechanisms can measure the expected torque outputs from the ankle joint. Use of compliant mechanisms allows for the design of a more cost-effective, compact device that can be handled by physicians even outside of a hospital setting. The final paper will present the second design iteration and the results of measurements that will be performed on patients’ rehabilitating ankles in the interim so as to determine what other adjustments need to be made in the system to account for the coupled motions of the ankle joint. A full working system including data acquisition equipment utilized to collect patient information and the final device iteration will be presented in the final technical paper.