During the course of spinal instrumentation performed with pedicle screw and interbody fusion device insertion, the nerve roots, spinal cord and cauda equina may be injured. To minimize this possibility, we developed a neurophysiology monitoring technique that provides immediate evaluation of bone integrity in patients undergoing instrumented lumbar fusion by using electrified surgical instruments throughout the pedicle screw instrumentation procedure. This technique also provides continuous monitoring during the placement of interbody fusion devices. Initially, this was performed as a prospective study. Our experience has been used to gain a long-term evaluation of the reliability and efficacy of this neurophysiological monitoring technique.

Intraoperative evoked electromyographic (EMG) stimulation was performed using a pedicle probe and feeler as monopolar stimulators during the insertion of 396 pedicle bone screws in 89 patients undergoing pedicle screw instrumentation placement alone. An additional group of 42 patients underwent placement of either anterior lumbar interbody fusion devices or posterior lumbar interbody fusion devices with pedicle screw augmentation.

Neurophysiologic monitoring consisted of suprathreshold stimulation of the intact pedicles for EMG response prior to pedicle screw insertion followed by assessment of EMG activity to subthreshold stimulation intensities during pedicle probing, tapping and screw insertion. EMG activity during pedicle preparation indicated cortical bone compromise. All cases of cortical bone compromise were confirmed visually or manually. Interbody fusion cases were monitored with continuous somatosensory evoked potentials (SSEPs) and spontaneous EMG activity during insertion of posteriorly placed interbody fusion devices.

Our results indicated that evoked EMG activity in appropriate muscle groups identified pedicle cortical bone compromise in 8 patients (true positives) using stimulated pedicle probes in patients undergoing pedicle screw instrumentation alone. This correlated well to pedicle fracture. One false positive and one false negative evoked EMG was noted. Pedicle screws were re-directed or removed in each of the true positive and in the single false positive case. During interbody fusion device insertion, six patients had abnormal spontaneous EMG activity and one patient had reversible SSEP changes with cauda equina retraction. One patient with abnormal spontaneous EMG activity awoke with L5 root deficits.

This preliminary study demonstrated that intraoperative evoked EMG monitoring alerted the surgeon that re-direction of the pedicle probe or screw was necessary to avoid nerve root irritation or injury. The determination of spontaneous EMG activity and continuous monitoring of SSEP activity provided important and useful information during interbody fusion device insertion. These neurophysiologic determinations are sensitive, specific, reliable and efficacious and provided important neurophysiologic information to the surgical team.

Based on this and other similar studies, the surgeons at UPMC Presbyterian utilize ongoing neurophysiologic monitoring data to reduce the possibility of nerve, spinal cord or cauda equina injury during surgical positioning, decompression, fusion and instrumentation insertion.