Contact combinations in epidural spinal cord stimulation. A comparison by computer modeling.

The geometrical characteristics of longitudinal fiber populations in the dorsal columns (DC), recruited by various contact combinations in epidural spinal cord stimulation (SCS), were compared in a theoretical study. A 3-dimensional computer model was used, representing the geometry and electrical conductivity of the low-thoracic spinal cord and surrounding tissues, in combination with a model representing the electrical properties of a myelinated nerve fiber. It was calculated that, among usual dorsomedial contact combinations, the ratio of mediolateral and dorsoventral extent of the recruited area in DC only varied by 2-5%. The model predicts that in bipolar stimulation the relative lateral extent was smallest at a contact separation of 3-3.5 times the dorsal cerebrospinal-fluid width, which seems to fit the highest ratio of discomfort threshold/paresthesia threshold. It is concluded that the optimal combination varies with distance between cathode and DC and that a unique 'best contact combination' does not exist. When both a decrease of primary afferent diameter in DC and collateral branching near their entrance are considered, the model predicts that recruitment of cutaneous afferents will start in lateral DC and proceed medially at increasing stimulus, which fits clinical observations on the spread of paresthesia. The model predicts that large fibers in the posterior spinocerebellar tract will also be recruited in SCS.