Intraoperative evoked EMG monitoring in an animal model. A new technique for evaluating pedicle screw placement.

An electric drill was used to introduce holes in the L4 through L7 pedicles in pigs. Constant-voltage stimulating pulses (5.5 V, stimulus rate = 3/sec) were delivered through a ball-tipped probe used to palpate the walls of each pedicle, and observation was made of electromyogram (EMG) evoked from hind limb muscles. Screws were placed in each pedicle hole, and evaluated for absolute voltage necessary to evoke EMG (threshold). At the conclusion of each experiment, screw positions were ascertained by removal and dissection of the lumbosacral spine. Approximately 50% of screw placements resulted in defects of the pedicle. In each of these cases, 5.5 V stimuli delivered through the probe evoked EMG from muscles innervated by adjacent motor axons. Conversely, for those cases where the pedicle was intact, significantly higher voltages were needed to evoke EMG. The authors believe that this is a promising intraoperative technique to simply and reliably identify mispositioned screws, thereby minimizing neurologic complications.