A CT-based study investigating the relationship between pedicle screw placement and stimulation threshold of compound muscle action potentials measured by intraoperative neurophysiological monitoring

Abstract

Purpose Neurophysiological monitoring aims to improve the safety of pedicle screw placement, but few quantitative studies assess specificity and sensitivity. In this study, screw placement within the pedicle is measured (post-op CT scan, horizontal and vertical distance from the screw edge to the surface of the pedicle) and correlated with intraoperative neurophysiological stimulation thresholds. Methods A single surgeon placed 68 thoracic and 136 lumbar screws in 30 consecutive patients during instrumented fusion under EMG control. The female to male ratio was 1.6 and the average age was 61.3 years (SD 17.7). Radiological measurements, blinded to stimulation threshold, were done on reformatted CT reconstructions using OsiriX software. A standard deviation of the screw position of 2.8 mm was determined from pilot measurements, and a 1 mm of screw-pedicle edge distance was considered as a difference of interest (standardised difference of 0.35) leading to a power of the study of 75 % (significance level 0.05). Results Correct placement and stimulation thresholds above 10 mA were found in 71 % of screws. Twenty-two percent of screws caused cortical breach, 80 % of these had stimulation thresholds above 10 mA (sensitivity 20 %, specificity 90 %). True prediction of correct position of the screw was more frequent for lumbar than for thoracic screws. Conclusion A screw stimulation threshold of [10 mA does not indicate correct pedicle screw placement. A hypothesised gradual decrease of screw stimulation thresholds was not observed as screw placement approaches the nerve root. Aside from a robust threshold of 2 mA indicating direct contact with nervous tissue, a secondary threshold appears to depend on patients' pathology and surgical conditions.

Intraoperative electromyographic monitoring to optimize safe lumbar pedicle screw placement – a retrospective analysis

“…Our analysis showed that a positive response to stimulation thresholds below 10 mA was highly suggestive of pedicle breach; but, a positive response for stimulation thresholds above 10 mA was frequently false positive [ 18 ]. Besides that, a negative response only suggests that the screw is not in proximity or abutting the nerve [ 10 ].…”

Transcranial motor evoked potential waveform changes in corrective fusion for adolescent idiopathic scoliosis

“…6 Tc-MEPs are increasingly used to assess the functional integrity of motor pathways and are particularly effective for corticospinal tract monitoring because they can be easily monitored and have high sensitivity and specificity. 11,17,28,29 Spinal cord monitoring is widely used in surgery for spinal deformity, 23,25,27,30,31,35 but only a few studies have included patients with adolescent idiopathic scoliosis (AIS). 7,18 Studies of intraoperative factors such as operative time, estimated blood loss (EBL), body temperature, and changes in blood pressure are particularly rare in patients who are younger than 20 years and have AIS.…”

Intraoperative Neurophysiological Monitoring for Spinal Fusion

“…From the 31 articles, by applying the article type filtering, there were 9 case reports, 22-30 5 reviews, 28,[31][32][33][34] and 2 clinical trials. 11,35 Twenty five articles (80.64%) were published in the period between 2013 and 2017. On the LILACS database, with the use of the narrow term intraoperative neurophysiological monitoring, we found 13 articles, but only 9 were related to spinal surgery.…”

“…6 Over the last decade, the use of intraoperative neurophysiological monitoring has become essential in neurosurgical procedures, because it brings safety to the patients, [7][8][9][10] as it can detect impending neurologic compromise, modifying the actions of the operating team to avoid injuries, and it might be considered as a minimal acceptable standard of care for all spinal procedures. 11,12 The decision of using intraoperative neurophysiological monitoring is still made mostly by the surgeon. 13 The effectiveness of intraoperative neurophysiological monitoring has been challenged by some studies based on the assumption of a low risk of postoperative deficits and additional costs with small differences in clinical outcomes.…”