Spinal cord direct current stimulation: finite element analysis of the electric field and current density

Applied low-intensity direct current (DC) stimulates and directs axonal growth in models of spinal cord injury (SCI) and may have therapeutic value in humans. Using higher electric strengths will probably increase the beneficial effects, but this faces the risk of tissue damage by electricity or toxic reactions at the electrode–tissue interface. To inform the optimisation of DC-based therapeutics, we developed a finite element model (FEM) of the human cervical spine and calculated the electric fields (EFs) and current densities produced by electrodes of different size, geometry and location. The presence of SCI was also considered. Three disc electrodes placed outside the spine produced low-intensity, uneven EFs, whereas the EFs generated by the same electrodes located epidurally were about three times more intense. Changes in electrical conductivity after SCI had little effect on the EF magnitudes. Uniformly distributed EFs were obtained with five disc electrodes placed around the dura mater, but not with a paddle-type electrode placed in the dorsal epidural space. Replacing the five disc electrodes by a single, large band electrode yielded EFs > 5 mV/mm with relatively low current density (2.5 μA/mm²) applied. With further optimisation, epidural, single-band electrodes might enhance the effectiveness of spinal cord DC stimulation.