Stimulus-evoked EMG monitoring during transpedicular lumbosacral spine instrumentation. Initial clinical results.

OBJECTIVES: The authors developed and evaluated an electrophysiologic method for minimizing the risk of nerve root trauma associated with the placement of pedicle screws during transpedicular lumbosacral fixation in humans. SUMMARY OF BACKGROUND DATA: Various methods have been evaluated to reduce the high complication rates associated with lumbosacral transpedicular fixation, but none are without significant limitations or drawbacks. Using a pig model, we previously developed a technique for assessing, by electrophysiologic means, the potential risk associated with placement of a screw at a given site. In this report, the authors describe their experience with this technique in patients.
METHODS: Electromyogram (EMG) was monitored from eight lower extremity muscles bilaterally. Square-wave electrical shocks (200 microseconds, 7 mA) were delivered through the instruments used to form and evaluate each pedicle hole, and through the screw itself if placement was deemed safe. Provided that the instruments used (e.g., awl, tap, probe) do not exit from bone in any direction below the entry point, the resistance of bone to the 7 mA stimulus intensity is high enough such that no nerve roots are stimulated, and the EMG traces remain flat. Conversely if EMG is evoked, it warns of a potential perforation in a pedicle wall or in the anterior body.
RESULTS: The authors evaluated this technique in 18 patients in whom a total of 102 screws were placed. Based on results of electrophysiologic and palpatory evaluation, 68% of the screws were placed in a satisfactory manner. Electrophysiologic evidence of a perforation, which could not be confirmed by palpation or visualization, was seen in another 13% of this total. The remaining 19% of screw placements involved sites where a defect was missed originally by palpation alone, but was located based on electrophysiological testing combined with palpation and visualization (11%), and where a perforation was initially palpated (8%). There was no postoperative morbidity associated with malpositioned screws.
CONCLUSIONS: Our results indicate that the technique is sensitive and, based on early clinical results, reliable in the detection of perforations in pedicle screw placement. Moreover, the method is inexpensive, rapid, and easily implemented into a standard intraoperative monitoring protocol.