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


METHODS
Prospective cohort.


METHODS
Level I trauma center.


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


METHODS
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).


METHODS
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.


CONCLUSIONS
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.
Ricci
William M
WM
Department of Orthopaedic Surgery, Washington University School of Medicine, One Barnes Hospital Plaza, Suite 11300, St. Louis, MO 63110, USA. ricciw@msnotes.wustl.edu
Padberg
Anne M
AM
Borrelli
Joseph
J
D000293:Adolescent; D000328:Adult; D000368:Aged; D001863:Bone Screws; D015331:Cohort Studies; D004558:Electric Stimulation; D004566:Electrodes; D004576:Electromyography; D005071:Evoked Potentials / Q000502:physiology*; D005260:Female; D005593:Fracture Fixation, Internal; D050723:Fractures, Bone / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans; D007085:Ilium / Q000503:physiopathology* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012447:Sacrum / Q000293:injuries* / Q000503:physiopathology* / Q000601:surgery