Intraoperative Triggered And Free Running EMG For Percutaneous Pedicle Screw Placement: An Unreliable Adjunct To Fluoroscopy

Presented at SMISS Annual Forum 2014
By Jacob Kushkuley BS
With Christopher Martin BS, CNIM, Frederik Pennings MD, PhD,

Disclosures: Jacob Kushkuley BS None Christopher Martin BS, CNIM E; Safe Passage Neuromonitoring, Frederik Pennings MD, PhD None,

Introduction:
Fluoroscopy is considered the golden standard for placing percutaneous pedicle screws (PPS) during minimally invasive spinal surgery. Intraoperative monitoring (IOM) is advocated as a method to improve safe placement of pedicle screws.

Aims/Objectives:
The goal of this study is to investigate the predictive value of IOM in detecting a malpositioned screw during percutaneous pedicle screw placement.

Methods:
A retrospective analysis was performed in patients undergoing PPS as a posterior fixation technique for fracture stabilization or to support a direct lateral lumbar interbody fusion. Intraoperative fluoroscopy and intraoperative monitoring were used during PPS. Intraoperative monitoring consisted of triggered EMG obtained from an electrified Jamshidi needle (normal values> 10mA), and free-running EMG. Fluoroscopy superseded IOM in the decision making process for final screw placement, therefore abnormal EMG triggered threshold of <8 mA would be ignored if intraoperative imaging showed correct placement, which is defined as the tip of the pedicle screw remaining lateral to the medial wall of the pedicle before it enters the vertebral body. Post-operative neurological exam and radiological imaging was used to assess adequate screw placement.

Results:
In 24 patients a total of 129 percutaneous pedicle screws were placed. The radiological malposition rate was 4/129 (3.1%), 2 of which produced neurological impairment (1.6%). 8/24 patients had normal trigger EMG and free running EMG values. One patient of this group displayed medial breech of left L5 and a drop foot. In 16/24 patients abnormal trigger EMG was found for at least one pedicle screw, of which 15 showed normal radiological and clinical outcomes. In one patient trigger EMG showed abnormal values for 3 out of 6 screws without abnormalities of free running EMG and acceptable intraoperative imaging. Post-operatively a medial breech of L3 pedicle with right thigh weakness was seen on CT and clinical exam. These data correspond with a sensitivity and specificity of 25.0% and 57.3% respectively per pedicle screw placed.

Conclusions:
Fluoroscopy remains the gold standard for percutaneous pedicle screw placement. IOM using an electrified Jamshidi needle for trigger EMG and free running EMG is an unreliable technique to detect abnormal pedicle screw placement. The application IOM results in unnecessary intraoperative adjustments of screws and potentially an increase in morbidity. Further technical improvements are required before intraoperative monitoring can be recommended as a standard of care in the placement of pedicle screws.