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

Abstract

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.

Anatomy of the L5 nerve root in the pelvis for safe sacral screw placement: a cadaveric study

“…Some reports have also focused on the complications of L5 nerve root injury caused by anterolateral misplacement of the S1 pedicle screws. [22][23][24] Sacral screw fixation is the gold standard for sacral bone purchase. The two most commonly used methods of screw insertion are into the promontory through the S1 pedicle with an anteromedial orientation and into the ala of the sacrum with an anterolateral orientation.…”

“…[19][20][21] Moreover, previous reports have highlighted the complications of L5 nerve root injury when S1 pedicle screws were inserted anterolaterally. [22][23][24] Although some cadaveric studies have demonstrated the anatomical relationship between the lumbar pedicle and adjacent dural sac and nerve roots, [25][26][27] few reports have detailed the anatomy of the L5 nerve root in the pelvis. Previous reports have measured the distance from the sacroiliac joint to the L5 nerve root, 28,29 which is not practical for use as a reference to prevent iatrogenic L5 nerve root injury when inserting the sacral screws under intraoperative fluoroscopy due to differences in patients' body sizes.…”

L5 nerve root injury caused by anterolateral malpositioning of loosened S1 pedicle screws: illustrative cases

“… 6–9 Several reports have described the risk of neurovascular injury when S1 pedicle screws are inserted into the anteromedial side of the sacrum; 10–12 however, few reports have focused on the complications of an L5 nerve root injury when the screws are inserted anterolaterally. 13 , 14 In this study, we report the cases of two patients with postoperative L5 nerve root injury caused by anterolateral malpositioning of S1 pedicle screws.…”