Validation of Triggered EMGs by CT Scan for Thoracic Pedicle Screw Placement

INTRODUCTION: Pedicle stimulation is an easily reproducible monitoring technique for lumbar screws. For thoracic pedicle screws, rectus abdominus triggered EMG (trEMG) thresholds are frequently used for T6-T12. However, no reliable method of obtaining triggered potentials for T1-T5 currently exists to identify medially placed screws.

PURPOSE: The purpose of this study was (1) to validate the trEMG thresholds in the thoracic spine (T6-T12), as recorded from the rectus abdominus, by postoperative CT scan and (2) to determine if a threshold effect exists for T1-T5 with this method.

METHODS: Triggered EMGs were recorded using an ascending method of stimulation for each thoracic pedicle screw in 18 consecutive spinal deformity patients (262 titanium screws) undergoing posterior spinal fusion. A postoperative fine-cut CT scan was obtained and evaluated by an independent observer to assess for possible medial pedicle violation(s). TrEMG recordings and CT findings were then analyzed.

RESULTS: The mean trEMG threshold was 13.3 mA for all thoracic screws. For T6-T12, the mean trEMG threshold was 13.6 mA (192 screws), and 12.4 mA for T1-T5 (70 screws) (p>0.05). There were 12 screws (4.5%) that were determined to have a medial violation by CT scan (T5-3, T6-2, T7-1, T8-2, T11-2, T12 –2). Of these, the average trEMG for was 11.1 mA for T6-T12 screws (10 screws)(82% of mean threshold) (p<.05), which was also <80% of the mean screw threshold for those individuals. The lowest measured trEMG was 6 mA (2 screws), both with a medial violation. However, two additional medial screws (on the curve convexity) could not be distinguished from the mean trEMG for that individual. For T1-T5 (2 medial screws on the curve convexity), the average trEMG was 12.5 mA (101% of mean threshold)(p>.05), which was > 113% of the mean screw threshold for each individual patient. These two screws (both at T5) were removed. There were no neurologic complications.

CONCLUSIONS: No absolute triggered EMG threshold could be established in this study, except for screws placed from T6-T12 with < 6 mA threshold (2 screws). Further, T1-T5 triggered EMGs are unreliable with this technique. For spinal deformity, convex screws appear to have higher threshold potentials, and therefore, this technique may be less reliable. Our findings could reflect a difference between titanium and stainless steel screws.