A finite element model of electrode placement during stimulus evoked electromyographic monitoring of iliosacral screw insertion

Abstract

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..

Fig. (1)

A CT image showing the five simulated anode locations and four drill bit positions of surgical interest along the path described as insertion motion. Translation of the drill bit from the nerve root towards the anterior cortex of the sacrum, identified as perpendicular motion, is also modeled at three distances (2 mm, 5 mm, and 10 mm).

Fig. (2)

A posterolateral view of the 5 mm model with the drill bit located at station III, and the anode placed on the ventral midline. The current density in the L5, S1 and S2 nerve roots is displayed along with the structural outline of the bone and drill bit. The peak current density occurs in the S1 nerve root immediately posterior and superior to the drill bit.

Fig. (3)

(a) Current density plotted versus axial position for three anode locations. The drill bit was 5 mm from the nerve root and located at station III. The DM location exhibited the largest current density. (b) Comparison of peak current density |Jmax| for all five anode locations. (c) Backward-cumulative histogram depicting the fraction of S1 versus current density magnitude.

Fig. (4)

(a) Current density plotted versus axial position for three drill bit insertion stations. The drill bit was 5 mm from the nerve root and the anode was located at the VM position. (b) Comparison of peak current density |Jmax| at all four insertion sites. (c) Backward-cumulative histogram of the contralateral S1 tissue fraction versus current density magnitude for the VM and CASIS anode locations.

Fig. (5)

Current density magnitude and direction is shown for anode locations IS (top) and VM (bottom) with the drill bit at station II.