Foot Drop Stimulator

Abstract

A foot drop stimulator is a neuroprosthesis which directly or reflexively activates motor systems in the leg in order to restore foot dorsiflexion, thereby facilitating the gait in humans with compromised ankle joint control. The activation is achieved by bursts of low-intensity electrical charge pulses of peripheral nerves via surface electrodes or implantable electrodes. The bursts of stimulation pulses need to be timed to generate dorsiflexion during the swing phase of the leg. With more complexity, the stimulation can be added to control dorsiflexion during the early stance phase and plantar extension during push-off phase of the gait cycle. The main difficulty in application of the surface electrode-based foot drop systems is insufficient stimulation selectivity that results with nondesired ankle rotations and the habituation (no response to stimulation). The implantable systems typically provide good selectivity, and so far habituation was not reported. The differences in the gait pattern when surface and implantable systems are applied are not significant if the surface electrodes are correctly positioned. Foot drop stimulators in some patients have therapeutic effects, especially if they were applied early after the impairment occurred. The therapeutic (carry-over) effect follows the changes at the upper motor neuron level (cortical plasticity) due to electrical stimulation and intensive gait exercise. Many clinical and home applications of the foot drop systems indicated that there are no side effects nor other problems. We present here the anatomy and physiology responsible for ankle movement, describe the operation principle of a foot drop system, and review current systems with surface electrodes and implantable cuff electrodes.