Improved control of ankle movement using an array of mini-electrodes.

The usual approach to correct foot-drop is to use one electrode to stimulate both the deep and superficial branches of the common peroneal nerve. Electrode position is critical for correct foot response: a 5mm electrode movement may produce a 20˚ change in foot position1. Difficulty in locating the correct electrode site is the principal reason (after improvement in mobility), for discontinuing FES treatment2. The use of arrays of electrodes with a random search strategy has been suggested as a means of automatically finding the correct stimulation location3. A shortcoming of this approach is that it does not use any a-priori knowledge: we do know approximately where the site of stimulation should be and in general terms how the response of the foot changes with electrode movement. Patients are usually able to locate the correct site to within a couple of centimetres. Our approach is to use an array of 'mini-electrodes' with an overall size just large enough to cover the stimulation site, allowing for error in placement. Groups of these electrodes may be activated simultaneously to produce one or more 'virtual electrodes' - see figure. Our simple search strategy is to find one 'virtual electrode' that maximises dorsiflexion (presumed to correspond to the deep peroneal branch) and another that maximises eversion (presumed to correspond to the superficial peroneal branch). The stimulation amplitudes of these virtual electrodes are then independently adjusted to achieve the desired foot movement.