Evaluation of pedicle screw insertion monitored by intraoperative evoked electromyography

The insertion of pedicle screws monitored by evoked electromyography (EMG) was prospectively evaluated in the 132 consecutive patients. The technique involved constant-voltage stimulation and was statistically evaluated at both the arbitrary 20- and 40-V settings. The patients were postoperatively evaluated clinically and radiographically. Computed tomography (CT) scanning was performed for new neurologic deficits. Results were divided into three groups: type 1, a negative EMG response; type 2, a positive EMG response, but no corrective action taken; and type 3, a positive EMG response and corrective action undertaken. Nonparametric statistics were used to evaluate the results at both the 20- and 40-V settings. In the type 3 group, in which corrective action was undertaken, there were no neurologic injuries or screw removals, a statistically significant result. Looking at the two intensity levels, at 20- and 40-V settings, there were no statistically significant differences in the three classifications at either intensity level. We concluded the evoked EMG for monitoring pedicle screw insertion is an efficacious adjunct. A positive response at < 20 V with the constant-voltage technique warrants corrective action.