EFFECTS OF ELECTRODE GEOMETRY AND COMBINATION ON NERVE-FIBER SELECTIVITY IN SPINAL-CORD STIMULATION

The differential effects of the geometry of a rostrocaudal array of el ectrode contacts on dorsal column fibre and dorsal root fibre activati on in spinal cord stimulation are analysed theoretically. 3-D models o f the mid-cervical and mid-thoracic vertebral areas are used for the c omputation of stimulation induced field potentials, whereas a cable mo del of myelinated nerve fibre is used for the calculation of the excit ation thresholds of large dorsal column and dorsal root fibres. The si ze and spacing of 2-D rectangular electrode contacts are varied while mono-, bi- and tripolar stimulation are applied. The model predicts th at the highest preferential stimulation of dorsal root fibres is obtai ned in monopolar stimulation with a large cathode, whereas dorsal colu mn fibre preference is highest in tripolar stimulation with small cont acts and small contact spacings. Fibre type preference is most sensiti ve to variations of rostrocaudal contact size and least sensitive to v ariations of lateral contact size. Dorsal root fibre preference is inc reased and sensitivity to lead geometry is reduced as the distance fro m contacts to spinal cord is increased.