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

Abstract

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.

Figures

Fig. 1

Location of the needle electrodes at the axillary midline for recording t-EMG potentials from T2 to T6 pedicle screws

Fig. 2

Mean EMG thresholds according to the post-operative position of the pedicle screws on both sides of the thoracic curve

Fig. 3

Percentage of pedicle screws in the different post-operative positions with EMG thresholds above and below 12 mA

Fig. 4

Percentage of pedicle screws in the different post-operative positions with EMG thresholds within the 6- to 12-mA range

Fig. 5

Innervation of intercostal and transversus thoracis muscles by the perforant branches of the intercostal muscles. Patient in operative position