A Finite Element Model of Electrode Placement During Stimulus Evoked Electromyographic Monitoring of Iliosacral Screw Insertion

M.A Kopec1, B.R Moed2, D.W Barnett*, 1
1 Department of Biomedical Engineering, Saint Louis University, St. Louis, MO, USA
2 Department of Orthopaedic Surgery, Saint Louis University School of Medicine, St. Louis, MO, USA

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2008 Bentham Science Publishers Ltd.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestrictive use, distribution, and reproduction in any medium, provided the original work is properly cited.

* Address correspondence to this author at the Department of Biomedical Engineering, Saint Louis University, St. Louis, MO, USA; E-mail:


Pelvic ring fractures that occur as a result of substantial orthopedic trauma are frequently repaired using iliosacral screws to stabilize the fracture. Stimulus evoked electromyography, using pulsed current stimuli provided through the drill bit cathode, has been advocated to prevent nerve root injury during iliosacral screw insertion. Our objective was to examine the effects of anode location, drill bit position, and anatomical structure on the nerve monitoring technique. A three-dimensional finite element model was constructed from computed tomography data to evaluate the effectiveness of five anode locations at four stations of drill bit insertion. Results indicate that the anode location should be at the midline or on the side contralateral to drill bit insertion. Locating the anode at other positions, such that the nerve root is outside of the primary electromagnetic field, leads to an attenuated electromyographic response that will ultimately lead to the failure of the monitoring technique.

Keywords: Finite element modeling, spinal nerve roots, stimulus-evoked electromyography.