Evoked and spontaneous electromyography to evaluate lumbosacral pedicle screw placement

Study Design. A prospective study was performed to evaluate the effectiveness of evoked and spontaneous electromyography in predicting pedicle wall breakthrough and subsequent lumbar rediculopathy occurring after placement of pedicle screw instrumentation of the lumbar spine. Objectives. To correlate cortical breakthrough of the pedicle wall with an electrically evoked electromyography threshold of stimulation, to assess the sensitivity of mechanically evoked electromyography for nerve root irritation, and to correlate postoperative nerve root irritation with intraoperative findings. Summery of Background Date. Pedicle wall breakthrough has been evaluated by radiographic means and found to be difficult to evaluate. Methods to perform both electrically evoked and mechanically evoked electromyography have been developed more sensitive tests for breakthrough. Methods. Twenty-five patients receiving 112 pedicle screws were evaluated. Results. Cortical breakthrough was associated with electrically evoked electromyography threshold of less than 11 milliAmps. Not all screws that had broken through the pedicle wall caused a postoperative radiculopathy. Electromyographic activity was sensitive to nerve root stimulation. Conclusions. Measuring the electrically evoked electromyography threshold of stimulation helps to assess pedicle screw placement. Mechanically evoked electromyography indicates intraoperative nerve root displacement. Postoperative rediculopathy correlated with pedicle wall breakthrough, but did not occur in every case.