The significance of anode location for stimulus-evoked electromyography during iliosacral screw placement.

OBJECTIVES: To determine the effect of anode location on the current threshold required to provoke an electromyograph response during stimulus-evoked electromyography for iliosacral screw placement.

DESIGN: Prospective cohort.

SETTING: Level I trauma center.

PATIENTS: Nineteen consecutive patients with 23 unstable posterior pelvic ring injuries treated with iliosacral screws.

INTERVENTION: Iliosacral screws were inserted percutaneously over guidewires. Twenty-seven screws were inserted, all into the first sacral vertebrae. The guidewire was used as the cathode for constant-current, stimulus-evoked electromyography for all data collection. Stimulus-evoked electromyographs were obtained with the guidewire at four different stations: at the sacroiliac joint (station I), at the first sacral neuroforamen (station II), in the body of the sacrum (station III), and when the iliosacral screw was in final position over the guidewire (station IV).

MAIN OUTCOME MEASURE: Stimulus-evoked electromyographs were obtained with the anode at four different locations for each of the implant stations. Location A had the anode adjacent to the percutaneous insertion site of the guidewire, location B at the ipsilateral anterior superior iliac spine, location C at the midline, and location D at the contralateral anterior superior iliac spine.

RESULTS: Moving the anode from midline (location C) toward the entry point of the guidewire increased the current threshold required to provoke an EMG response as much as 67.1% (p < 0.05). Moving the anode from midline to the contralateral anterior superior iliac spine decreased thresholds as much as 3.4% (p > 0.05). In one case, anode placement close to the guidewire insertion site (locations A and B) failed to identify a potentially dangerous implant because current thresholds were >8 mA. With the anode at the midline, current thresholds were <8 mA, indicating unsafe guidewire position leading to redirection of the guidewire.

CONCLUSION: The physical location of the anode during stimulus-evoked electromyography monitoring for iliosacral screw placement significantly changes the current thresholds required to provoke an electromyograph response. Current thresholds required to stimulate nerves increase as the anode is moved toward the stimulating electrode. Anode placement ipsilateral to the stimulating electrode may provide a false indication of safe guidewire placement. We recommend anode location at or beyond the midline for stimulus-evoked electromyography monitoring during iliosacral screw placement.