Recording triggered EMG thresholds from axillary chest wall electrodes: a new refined technique for accurate upper thoracic (T2-T6) pedicle screw placement.

This study was aimed at evaluating the sensitivity and safety of a new technique to record triggered EMG thresholds from axillary chest wall electrodes when inserting pedicle screws in the upper thoracic spine (T2-T6). A total of 248 (36.6%) of a total of 677 thoracic screws were placed at the T2-T6 levels in 92 patients with adolescent idiopathic scoliosis. A single electrode placed at the axillary midline was able to record potentials during surgery from all T2-T6 myotomes at each side. Eleven screws were removed during surgery because of malposition according to intraoperative fluoroscopic views. Screw position was evaluated after surgery in the remaining 237 screws using a CT scan. Malposition was detected in 35 pedicle screws (14.7%). Pedicle medial cortex was breached in 24 (10.1%). Six screws (2.5%) were located inside the spinal canal. Mean EMG threshold was 24.44 ± 11.30 mA in well-positioned screws, 17.98 ± 8.24 mA (p < 0.01) in screws violating the pedicle medial cortex, and 10.38 ± 3.33 mA (p < 0.005) in screws located inside the spinal canal. Below a threshold of 12 mA, 33.4% of the screws (10/30) were malpositioned. Furthermore, 36% of the pedicle screws with t-EMG stimulation thresholds within the range 6-12 mA were malpositioned. In conclusion, assessment of upper thoracic pedicle screw placement by recording tEMG at a single axillary electrode was highly reliable. Thresholds below 12 mA should alert surgeons to suspect screw malposition. This technique simplifies tEMG potential recording to facilitate safe placement of pedicle screws at upper thoracic levels.

Recording diffusion responses from contralateral intercostal muscles after stimulus-triggered electromyography: refining a tool for the assessment of thoracic pedicle screw placement in an experimental porcine model.

STUDY DESIGN: A new stimulus-triggered electromyography (EMG) test for detecting stimulus diffusion to contralateral intercostal muscles during thoracic pedicle screw placement was assessed in a porcine model. OBJECTIVE: To determine if electromyographic thresholds in the intercostal muscles of both sides of the thorax could discriminate thoracic pedicle screw malpositions with and without neural contact at different aspects of the spinal cord and nerve roots. SUMMARY OF BACKGROUND DATA: There is controversy about the value of triggered EMG stimulation for aiding precise insertion of thoracic pedicle screws. A universally validated threshold that confirms screw malposition has not been established. Diffusion of EMG responses to the contralateral intercostal muscles has not previously been investigated. METHODS: Nine domestic pigs weighing 60 to 75 kg had 108 pedicle screws placed bilaterally in the thoracic spine from T8-T13. Before spine instrumentation, neural structures were stimulated in 4 animals under direct vision at different anatomic locations from T9-T12. Recording electrodes were placed over the right and left intercostal muscles. Increasing stimulus intensity was applied until muscle response was detected at the contralateral side (EMG diffusion phenomenon). After this first experiment, the thoracic spine was instrumented in all 9 animals. Screws were placed in the pedicle in different positions, the anatomic intrapedicular location and within the spinal canal, with and without contact with the neural elements. RESULTS: Response thresholds to direct nerve root stimulation were significantly lower than those obtained by stimulation of the dorsal aspect of the spinal cord (0.44 +/- 0.22 mA vs. 1.38 +/- 0.71 mA, P < 0.01). However, a 14-fold stimulation intensity (6.50 +/- 0.29 mA) was necessary to obtain diffusion of the EMG response to the opposite (left) side if the right nerve root was stimulated. A 2-fold increment (3.17 +/- 0.93 mA) elicited diffusion of the EMG responses to the contralateral side when stimulation was applied to the dorsal aspect of the spinal cord. EMG recordings of the 108 stimulated screws showed a significant decrease in the EMG response when the screw was in contact with the spinal cord (2.72 +/- 1.48 mA; P < 0.01) compared with that found when the pedicle track was intact (mean: 5.01 +/- 1.89 mA). Screws violating the medial wall of the pedicle, but not touching neural tissues, responded to slightly lower intensities than well-positioned screws, but this was not statistically significant (3.91 +/- 1.39 mA vs. 4.89 +/- 1.30 mA, P > 0.05). CONCLUSION: Stimulus-triggered EMG can identify screws that violate the medial pedicle wall if they are in contact with neural tissues. EMG thresholds could not discriminate screws that violated the medial pedicle wall without neural contact from screws with accurate intraosseous placement. However, recording EMG potentials at the contralateral intercostal muscles (stimulus diffusion phenomenon) proved to be a reliable method for identifying the neural structures at risk.