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Using Triggered Electromyographic Threshold in the Intercostal Muscles to Evaluate the Accuracy of Upper Thoracic Pedicle Screw Placement (T3–T6)

Study Design. A prospective clinical study of high thoracic pedicle screws monitored with triggered electromyographic (EMG) testing. Objective. To evaluate the sensitivity of recording intercostal muscle potentials to assess upper thoracic screw placement. Summary of Background Data. Triggered EMG testing from rectus muscle recordings, which are innervated from T6 to T12, has identified medially placed thoracic pedicle screws. No clinical study has correlated an identical technique with the intercostal muscle for upper pedicle screws placed in the upper thoracic spine (T3–T6). Methods. A total of 311 high thoracic screws were placed in 50 consecutive patients. Screws were placed from T3 to T6 and were evaluated using an ascending method of stimulation until a compound muscle action potential was obtained from the intercostal muscles. Screw position was then evaluated using computed tomography and results were compared with evoked EMG threshold values. Results. Fifteen screws (5%) showed penetration on postoperative computed tomography scans. Eleven screws showed medial cortical breakthrough (3.6%), 6 had stimulation thresholds ≤6 mA, and 5 had stimulation thresholds between 6 and 10 mA. Stimulation values for all breached screws decreases 60% to 65% from their mean. Four screws (1.3%) showed lateral cortical breakthrough with stimulation thresholds >20 mA. Of the 296 screws with thresholds between 6 and 20 mA, 285 (91%) were within the vertebra. No postoperative neurologic complications were noted in any of the 50 patients. Conclusion. In this series, cortical violation is highly unlikely in patients whose stimulation threshold lies between 6 and 20 mA with values 60% to 65% decreased from the mean (98% negative predictive value). Although verification of screw placement should not depend solely on stimulation thresholds, pedicle screw stimulation provides rapid and useful intraoperative information on screw placement during procedures involving the use of thoracic pedicle screws.