OTA 2002 Posters

Anode Location Significantly Changes Threshold Results during Stimulus-Evoked Electromyography for Iliosacral Screw Placement

William M. Ricci, MD; Anne M. Padberg, MS, CCEA; Joseph Borrelli, Jr., MD; Washington University School of Medicine, St. Louis, Missouri, USA

Purpose: Iliosacral screw fixation is commonly used for the treatment of patients with unstable fractures and dislocations of the posterior pelvic ring. Safe placement of iliosacral screws is technically demanding, with iatrogenic neurologic injury reported in up to 18% of cases. Stimulus-evoked electromyographic (SE-EMG) monitoring has been advocated to help avoid nerve injury during this procedure. The location of the anode for SE-EMG has empirically been set at the midline. The effect of anode location on current threshold during placement of iliosacral screws has not been studied. The goal of the present study was to determine the effect of anode location on current threshold during SE-EMG monitoring for iliosacral screw placement.

Methods: By using a prospective protocol, the effect of anode location on SE-EMG current thresholds during iliosacral screw placement was examined in 23 consecutive patients who had posterior pelvic ring injuries (61B or 61C). Twenty-seven iliosacral screws (7.3-mm cannulated) were inserted percutaneously into the first sacral vertebrae over 2.8-mm guide wires. Constant-current SE-EMG was performed with the cathode (guide wire) at four different stations: sacroiliac joint, first sacral neuro-foramen, final position within the body of the sacrum, and sacroiliac screw in final position over the guide wire. At each station, SE-EMG was obtained with the anode at four different locations: adjacent to the percutaneous insertion site of the guide wire (A), at the ipsilateral anterior superior iliac spine (ASIS) (B), at the midline (C), and at the contralateral ASIS (D). A current threshold of >8 mA was used to indicate unsafe placement. Therefore, when a threshold of less than 8 mA was obtained, the guide wire was redirected. Results were compared with two-tailed t-tests.

Results: Moving the anode from midline (location C) toward the guide wire entry point significantly increased current thresholds required to provoke an EMG response (up to 67.1%, P<0.05). In one case, anode locations A and B both failed to identify a dangerous implant position, whereas anode location C identified the danger, leading to guide wire redirection and safe screw insertion.

Conclusions: Physical location of the anode significantly changes the current threshold required to provoke an EMG response during SE-EMG for iliosacral screw placement. The current required to stimulate a nerve increases as the anode is moved toward the stimulating electrode. Improper anode placement (ipsilateral to the stimulating electrode) may provide a false indication of safe guidewire placement leading to potential iatrogenic nerve injury. The proper use of SE-EMG monitoring during placement of iliosacral screws requires the anode to be placed at or beyond the midline.