A PROSPECTIVE ANALYSIS OF INTRAOPERATIVE ELECTROMYOGRAPHIC MONITORINGOF PEDICLE SCREW PLACEMENT WITH COMPUTED TOMOGRAPHIC SCAN CONFIRMATION

Study Design. In a prospective study of 90 patients undergoing lumbar pedicle screw instrumentation, 512 screws were tested intraoperatively using electrical stimulation. The accuracy of this technique was veri fied after surgery by computed tomograph. Objectives. Computed tomogra phic scans taken after surgery were used to evaluate the efficacy of i ntraoperative screw stimulation and electromyographic monitoring of pe dicle screw placement. Summary of Background Data. Previous cadaveric and clinical studies showed the risk of pedicle screw malposition and the inadequate reliability of intraoperative radiographs to identify m isplaced screws. Methods. Screws (total, 512) in 90 patients were stim ulated intraoperatively, and stimulation threshold was recorded. Compu ted tomographic scans were taken after surgery to document pedicle scr ew position. Electromyographic thresholds and computed tomographic dat a were evaluated independently and compared to assess the accuracy of the electromyographic screw stimulation technique. Results. Intraopera tive screw stimulation was extremely accurate in confirming the adequa cy of screw position. A stimulation threshold greater than 15 mA provi ded a 98% confidence that the screw was within the pedicle. In eight o f 90 patients (9%), electromyographic monitoring detected a screw malp osition that was not identified on lateral radiograph. Conclusions. Sc rew stimulation monitoring is a valuable and efficacious adjunct to lu mbar pedicle screw instrumentation. A stimulation threshold greater th an 15 mA reliably indicates adequate screw position. A stimulation thr eshold between 10 and 15 mA was generally associated with adequate scr ew position, although exploration of the pedicle is recommended. A sti mulation threshold less than 10 mA was associated with a significant c ortical perforation in most instances.