Two-channel surface stimulation for the correction of drop foot.

Abstract

1 Functional Electrical Stimulation (FES) is used for the correction of drop foot. The clinical objective is to promote dorsiflexion to avoid tripping, and mild eversion for stability during loading. Traditional transcutaneous FES systems require the accurate positioning of surface electrodes on the skin so that the appropriate nerves are activated to give the desired muscle response. Many people find electrode positioning difficult. 2 This project examined the feasibility of using two channels of transcutaneous electrical stimulation as an adaptive system to correct drop foot. Both channels were positioned over the branches of the common peroneal nerve at the fibular head, broadly with the 'lateral' channel promoting eversion and the 'medial' channel promoting dorsiflexion. The main focus of the study was the effect on foot posture of changing the currents in each channel (the 'current balance'), and the possibility of using this in an open-loop or closed-loop control system to compensate for variation in electrode position. 3 In support of closed-loop control, a sensor consisting of switches under the heel, 1st and 5th metatarsal heads was used to assess the degree of inversion/eversion during walking. A simple controller was implemented to link the two-channel stimulation system and the foot posture sensor, with the objective of maintaining a target foot posture despite minor variation in electrode position. 4 The study found that with careful set-up the current balance could affect the inversion/eversion of the foot while also maintaining dorsiflexion. However, the range of posture control was sensitive to the electrode positions and so this approach did not significantly reduce the need to position the electrodes carefully. The signal from the in-shoe foot posture sensor was often poorly correlated with foot posture as measured by a goniometer. The control system responded appropriately to its inputs, but its overall performance was limited by the input sensor and the output range of influence. The two-channel technique may have utility as part of a leg cuff system, enabling the user to fine-tune the foot posture once the electrodes are positioned appropriately.