Automated intraoperative EMG testing during percutaneous pedicle screw placement.

Background: EMG screw testing has been shown to be sensitive and reliable in open spinal instrumentation cases. However, there is little evidence to show its applicability to percutaneous screw placement.

Purpose: To demonstrate the utility of EMG testing in percutaneous techniques, where lack of direct visualization poses an added risk to nerve injury.

Study Design: Summary of intraoperative EMG results during percutaneous pedicle screw placement.

Methods: Percutaneous pedicle screws were placed in twenty patients (22 levels, 88 pedicles). The initial fluoroscopically-guided k-wires and the subsequent taps were insulated and stimulated via an automated EMG system. Low threshold values prompted repositioning of the pedicle trajectory.

Results: Four (5%) k-wires induced EMG thresholds less than 10mA, prompting repositioning. One was repositioned without improvement, but with improvement upon tapping. One k-wire with very low threshold (3mA) was repositioned with an improved result (13mA). In 78 pedicles (89%) the tap threshold was greater than the k-wire.

Conclusions: EMG testing helps to identify suboptimal screw trajectories, allowing for early adjustment and confirmation of improved placement. Tapping often improved thresholds, perhaps by compressing the bone and creating a denser, more insulative pedicle wall. EMG testing may improve the safety of percutaneous screw techniques, where the pedicle cannot be visually inspected.