Recent advances in intraoperative neuromonitoring of spinal cord function: Pedicle screw stimulation techniques

The evolution of internal spinal instrumentation is grounded in the therapeutic constructs and spinal biomechanics associated with spinal deformities. Newer instrumentation methods for spinal fusion, which involve placing screws into the vertebral pedicles to hold the rods, have evolved for the treatment of degenerative spinal disease and traumatic insult to the lumbosacral spine. Two significant advantages of spinal instrumentation using segmental, transpedicular, screw fixation are enhanced rigidity and stability of the spine, and greater flexibility for the surgeon to accommodate a patient's individual anatomy. Despite these advances in instrumentation design, neurological deficits from improper screw placement may occur. The determination of screw placement using traditional methods like anatomical identification of bony landmarks and radiographic techniques have significant limitations. An alternative method for monitoring pedicle screw placement involves an electrophysiological approach called pedicle screw stimulation technique. Evoked electromyographic activity is recorded from muscle groups innervated by the spinal nerve roots at risk. Either constant current or voltage stimulation applied through the hardware used to form the pedicle holes, inside the pedicle holes, and through the implanted screws is performed to determine the stimulation threshold needed to produce a compound muscle action potential. These threshold values for evoked EMG are then used for evaluating the success or failure of screw placement. A relatively high EMG threshold suggests proper pedicle screw placement with an intact pedicle wall surrounding the screw; a relatively low threshold indicates a malposed screw resulting in a possible cracked or breached pedicle.