Accuracy of t-EMG stimulation of the middle pedicle track to prevent radiculopathies as a result of misplaced lumbar screws

Objective: To describe the accuracy of middle pedicle track stimulation for the detection of pedicle breaches causing misplaced lumbar screws and subsequent neurological symptoms. Patients and methods: In a comparative observational study with two cohorts, 1440 lumbar pedicle screws were implanted using the freehand technique in 242 patients undergoing surgery for spinal deformities. In the first two-year period (2011 - 2012), the accuracy of screw placement (802 screws) was assessed by conventional intraoperative palpation of the pedicle track, t-EMG screw stimulation, and fluoroscopic monitoring. In the second period (2012-2013), the middle aspect of the lumbar pedicle tracks was systematically stimulated with a probe (638 screws). When thresholds in the middle track showed 9 mA, potential neurological risk was considered, and therefore, new pedicle tracks were performed. Results: Six patients (4.4 %) in the first period presented postoperative radicular pain and a normal intraoperative screw t-EMG threshold. CT scans showed seven screws (0.9 %) with 2-mm medial-caudal invasion of the foramen. Before screw removal, t-EMG thresholds of these screws were again normal (>= 10 mA). After removal of the screws. t-EMG of the middle part of the pedicle track showed thresholds below 9 mA (mean 5.2 mA). In the second period, the pedicle tracks were systematically stimulated. Low t-EMG thresholds (< 9 mA) were found in 11 tracks (1.7 %) and were therefore reworked before screw placement. CT scans in these 10 patients showed that all of the 11 screws were correctly repositioned. Conclusions: This study shows that caudal or medial pedicle cortical breaches can be detected effectively by stimulating the middle part of the pedicle track. This technique is strongly recommended to prevent postoperative lumbar radiculopathies due to screw malposition.