A Finite Element Study of Electrode Location during Stimulus Evoked Electromyographic Monitoring of Iliosacral Screw Insertion

INTRODUCTION: Pelvic ring fractures are a relatively common form of orthopaedic trauma. While mechanical stability may be restored by the insertion of iliosacral screws, the proximity of the ideal screw trajectory to the fifth lumbar and first sacral nerve roots (S1), as well as the spinal canal make nervous tissue injury a potential hazard. Fluoroscopy has typically been used to ensure proper screw placement by inserting guide wires followed by the screw. However, this approach may produce less then satisfactory results, as nerve root damage has been reported in as many as 18% of cases. To avoid this surgical complication, Moed et al. [1] introduced a novel technique for determination of the proximity to a neural structure based on the electromyographic (EMG) response evoked by passing a small current between the tip of the drill-bit and a reference electrode. In order to evoke an EMG response small current pulses (<50 mA, 0.2 ms) delivered at 3 Hz are passed between the tip of the dill-bit and the reference electrode (anode) to stimulate action potential propagation from the nervous tissue towards the muscles they innervate. The threshold current required to induce a 20 mV EMG response serves as a measure of the distance between the tip of the drill bit and the neural structure intraoperatively. The location of the anode relative to the drill bit determines the path that current flows through the body and the potential at any point on the nervous structure. The efficacy of the nerve monitoring technique is clearly dependent on the anode location. However, due to the subtle anatomical intricacies of the neural structure and the complex effects of a three dimensional anatomical model of varying conductivity, a computational study of anode sites is necessary to determine optimal electrode location. A three dimensional finite element model has the potential to provide this information through a comprehensive investigation of the current density observed in the first sacral nerve root using different anode configurations. In order to evaluate the importance of reference (anode) location in this procedure, a three dimensional finite element model was constructed from computed tomography data to evaluate the effectiveness of five anode locations.