F170. Usefulness of t-EMG of the pedicle hole after withdrawal of lumbar pedicle screws

Abstract

t-EMG of the pedicle hole before the placement of lumbar pedicle screws, and later stimulation of the screw is an established neurophysiologic method to avoid lumbar pedicle screw malposition. Occasionally it is necessary to remove some of the instrumentation in a later surgery, which made us wonder if it is necessary to stimulate the trajectories of the screws once they have been removed, before the re-instrumentation of those levels. We have studied 12 patients with previous surgeries for the correction of vertebral deformities, in which a posterior surgery for the removal of part of the instrumentation was performed. In this second surgery, screws were stimulated before their removal, and their pedicle trajectories were performed afterwards. We have considered of risk t-EMG values below 10 mA 62 screws were removed, all lumbar. In the first surgery t-EMG values were all normal, both the trajectory and the screw. None of the patients presented neurological symptoms. A CT scan was performed before the second surgery, and 16 of the 62 screws to be removed had a certain degree of medial-caudal displacement. During the surgery for their removal, t-EMG of the screws was performed, being normal in all cases, and after the withdrawal of the screws, their trajectories were also stimulated. A 48% of the screws’ trajectories had t-EMG values under 10 mA. None of the removed screws was repositioned, regardless of their t-EMG value. In an off-line analysis of the results, 14 out of the 16 malpositioned screws (87%) had a low t-EMG value of the pedicle hole after their removal. And 16 out of the 46 well-positioned screws (34%) presented a low t-EMG value of the pedicle hole after their removal. Even though the usefulness of the t-EMG of the pedicle track before the screw placement has been proven to avoid lumbar pedicle screw malposition, it seems that the stimulation of the pedicle track once a screw has been removed does not have the same implication, at least with the current threshold values. We need more cases to be able to establish t-EMG of the pedicle trajectories of the removed screws as a useful tool to decide the replacement of a screw using the same trajectory.