BME 240, Spring 2008 Leyla Esfandiari

Active Ankle Foot Orthosis

An ankle foot orthosis (AFO) is defined as a medical mechanical device to support and align the ankle and foot. This device is used to assist weak and paralyzed muscles of the ankle and foot, to prevent or correct ankle and foot deformities and to improve the functions of the ankle and foot.
Static AFO is mainly used by patients who are suffering from drop foot. These patients are not able to clear their foot during swinging motion (toe drag) or to control it during the landing (foot slap). Static AFO holds the ankle in a fixed neutral position, minimizing foot slap, and assisting with toe clearance during swing. However, because of the rigid design, it significantly limits plantar flexion during late stance.

The feature that distinguishes dynamic from static AFOs is that they allow, or have the potential to allow, sagital plane motion at the ankle. This is accomplished by incorporation of a mechanical ankle joint or, in the case of a posterior leaf spring orthosis, strategically minimized thermoplastic trimlines.

Active ankle foot orthosis (AAFO) are new generations of AFOs which usually consist of a dynamic AFO powered by an actuator. The actuator provides a controlled force to compensate for muscle deficiencies around the ankle joint. The force provided by AAFO at the ankle joint is calculated by a controller which receives both biological (EMG) and physical (GRF, joint angle ...) feedbacks.
Any actuator used in AAFO should be able to provide enough force for the movement and have the beneficial properties of muscle include its low impedance, high force-fidelity, low friction, and good bandwidth.
As a control point of view an ideal force-controllable actuator would be a perfect force source, outputting exactly the commanded force independent of load movement. In the real world, all force-controllable actuators will have limitations that result in deviations from a perfect force source. These limitations include impedance, stiction, and bandwidth. In a perfect force source, impedance is zero, stiction is zero, and bandwidth is infinite. Muscle has extremely low impedance and stiction and moderate bandwidth and is the currently best known actuation technology that approaches a perfect force source. Series Elastic Actuators and McKibben muscles share these beneficial properties with muscle and thus are well suited.
Biomechatronics lab  at MIT and
Neuromechanics Lab  at University of Michigan are the academic leaders in the filed. Series Elastic Actuators (SEA) was first designed for legged robot in MIT and is used in AAFO. Different type of pneumatic artificial muscles has also been used in University of Michigan for AAFO.

Active Ankle Foot Orthosis