Anterolateral S1 screw malposition detected with intraoperative neurophysiological monitoring during posterior lumbosacral fusion

BACKGROUND: The standard of care is to utilize intraoperative neurophysiological monitoring (IOM) of triggered electromyography (tEMG) during posterior lumbosacral instrumented-fusion surgery. IOM should theoretically signal misplacement of S1 screws into the neural L5–S1 foramen or spinal canal, utilizing screw stimulation, and recording of the lower limb muscles and the anal sphincter. Here, we evaluated when and whether anterolateral S1 screw malposition could be detected by IOM/tEMG during open posterior lumbosacral instrumented fusion surgery. METHODS: tEMG, somatosensory-evoked potential (SSEP), and transcranial electrical motor-evoked potential (TcMEP) data were retrospectively reviewed from 2015 to 2017 during open posterior lumbosacral instrumented fusions. We utilized screw stimulation alert thresholds of <14 mA (tEMG) and recorded from the lower extremity muscles and anal sphincter. Furthermore, all patients underwent routine postoperative computed tomography (CT) scans to confirm the screw location. RESULTS: There were 106 S1 screws placed in 54 patients: 52 bilateral and 2 unilateral. In 6 patients (11.1%), 7 screws (6.6%) registered at low tEMG thresholds. In 1 patient, the postoperative CT scan documented external malposition of the screw despite no intraoperative IOM/tEMG alert. When S1 misplaced screws were stimulated, the most sensitive muscle was the tibialis anterior; the sensitivity of the IOM/tEMG was 87.5%, the specificity was 97.9%, the positive predictive value was 77.8%, and the negative predictive value was 98.9%. TcMEP and SSEP did not change during any of the operations. Notably, no patient developed a new neurological deficit. CONCLUSION: Anterolateral S1 screw malposition can be detected accurately utilizing IOM/tEMG stimulation of screws. When alerts occur, they can largely be corrected by partially backing out the screw (e.g., a few turns) and/ or changing the screw trajectory.