TRANSVERSE TRIPOLAR STIMULATION OF PERIPHERAL-NERVE - A MODELING STUDY OF SPATIAL SELECTIVITY

Various anod-cathode configurations in a nerve cuff are modelled to pr edict their spatial selectivity characteristics for functional nerve s timulation. A 3D volume conductor model of a monofascicular nerve is u sed for the computation of stimulation-induced field potentials, where as a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of fibres. As well as the usual configur ations (monopole, bipole, longitudinal tripole, 'steering' anode), a t ransverse tripolar configuration (central cathode) is examined. It is found that the transverse tripole is the only configuration giving con vex recruitment contours and therefore maximises activation selectivit y for a small (cylindrical) bundle of fibres in the periphery of a mon ofascicular nerve trunk. As the electrode configuration is changed to achieve greater selectivity, the threshold current increases. Therefor e threshold currents for fibre excitation with a transverse tripole ar e relatively high. Inverse recruitment is less extreme than for the ot her configurations. The influences of several geometrical parameters a nd model conductivities of the transverse tripole on selectivity and t hreshold current are analysed. In chronic implantation, when electrode s are encapsulated by a layer of fibrous tissue, threshold currents ar e low, whereas the shape of the recruitment contours in transverse tri polar stimulation does not change.