Charge and impedance analysis of 4500/plus hours of electrical stimulation of sacral roots with a foramen electrode implant

A sacral root foramen implant has been studied for its impact an neural tissue of prolonged electrical stimulation in 3 mini pigs. Identical hardware is currently being used in human clinical trials. One foramen electrode was implanted into the sacral foramen S2 on either side, left and right. Proper placement was monitored by rectal evoked muscle potential. Only one electrode of one side was connected to an implantable self contained pulse generator. The other side was left unstimulated and served as a control whether the pure presence of the implant would cause inflammation and an adverse effect on the neural tissue. The lead-tissue impedance was constantly monitored. The stimulation consisted of an interrupted 10 Hz train of pulses, each pulse of 210 μs in duration, mean amplitude 1.7 mA, rectangular cathodal unbalanced charge, 0.5 μC/phase. After a mean of 3976 hours of this electrical stimulation with 5 sec an and 2.5 sec off (the identical stimulation pattern used in the human trials), the animals were sacrificed and the sacral roots histologically examined. The calculated charge density at the electrode-tissue interface was 4 μC/cm2. Bioelectric impedance showed an initial average value of 612 Ω and increased due to scar tissue formation to a constant mean value of 792 Ω. Nerve integrity was demonstrated functionally by evoked muscle potential which remained nearly unchanged during the entire implant period. Light microscopic evaluation of histology slides showed no demyelination within the stimulated nerve trunk or the unstimulated control. These results suggest that prolonged cycled unbalanced cathodal electrical stimulation with charge densities of 4 μC/cm² within a distance of 3 mm from the nerve trunk elicits no functionally or histologically apparent damage to the neural tissue