Electrode array for reversing the recruitment order of peripheral nerve stimulation: a simulation study

Electrical stimulation of peripheral nerve activates large-diameter fibers before small ones. Previous studies using computer simulations and animal experiments showed that selective activation of small fibers could be achieved using an array of four cathodes and five anodes to reshape the extracellular voltage along the nerve and that the technique was independent of stimulating pulse width. In this simulation study, electrode arrays of 5, 7, 9, and 11 contacts were tested using finite element model of ventral sacral root. Contact separation of the array was 0.75 mm. The 5-contact array activated small axons having internodal distance smaller than contact separation before larger axons (< 7.5 μm in this case). Arrays of 7, 9 and 11 contacts suppressed the excitability of axons having internodal distance close to the intercathodic distance (∼15μm in this case). The recruitment orders were identical for 50 μs and 200 μs-pulse stimulations. Simulations suggested that electrode arrays of 5 and 7 contacts could be used to achieve selective activation of small axons independently of stimulating pulse width. Arrays of 5 and 7 contacts also decreased the recruitment curve slope to 43% and 72% of the tripolar electrode.