A Mapping of Applied Electric Fields in the Spinal Cord via Finite Element Analysis

Approximately 17,000 new cases of spinal cord injuries (SCI) occur every year in the United States, resulting in temporary or permanent paralysis in patients. Presently, there are no known FDA approved treatments that restore functional loss after SCI. Previous in vitro studies have shown that axonal outgrowth and pathfinding can be enhanced through the application of weak DC electric fields. For the past several decades, our laboratory has exploited this phenomenon of electric field-induced axon regeneration by developing an implantable device that delivers weak DC current across the spinal lesion. Although the device has shown positive improvements in functional and behavioral recovery in previous animal studies and in FDA Phase 1 trials, few studies have been done to map these electric fields in vivo. Thus, the aim of this research is to characterize and optimize applied DC electric fields in the spinal cord. To achieve this, a finite element analysis model of the human spinal cord and its surrounding tissues was first constructed using human MRI data. Then, electrodes from the device were defined in the model and the resulting electric fields emitted by the device were characterized. Parameters such as electrode to cord distance and electrode shape were varied to optimize the electrode placement for optimum therapeutic effect. Characterization of the electric fields showed that the fields were lower than the fields measured in guinea pig trials. However, optimization of the electrodes and their placements resulted in electric fields reaching the threshold necessary to induce axonal growth, but clearly below the threshold necessary to induce damage. This computational modeling suggests important technical refinements to maximize functional recovery in future clinical trials