Lateral mass screw stimulation thresholds in posterior cervical instrumentation surgery: a predictor of medial deviation

Abstract

<jats:sec> <jats:title>OBJECTIVE</jats:title> <jats:p>Normative data exists for stimulus-evoked pedicle screw electromyography (EMG) current thresholds in the lumbar spine, and is routinely referenced during spine surgeries to detect a screw breach, prevent injury of neural elements, and ensure the most biomechanically sound instrumentation construct. To date, similar normative data for cervical lateral mass screws is limited, thus the utility of lateral mass screw testing remains unclear. To address this disparity, in this study the authors describe cumulative lateral mass screw stimulation threshold data in patients undergoing posterior cervical instrumentation with lateral mass screws. These data are correlated with screw placement on postoperative imaging, and a novel correlation is discovered with direct clinical implications.</jats:p></jats:sec> <jats:sec> <jats:title>METHODS</jats:title> <jats:p>Using a ball-tip probe, 154 lateral mass screws in 21 patients were electrically tested intraoperatively. In each case, for each screw, the lowest (or threshold) current at which the first polyphasic stimulus-evoked EMG response was reproducibly observed by a neurophysiologist was recorded. All patients underwent postoperative CT. Screw position within the lateral mass was first measured in the axial and sagittal planes for each lateral mass screw using the CT images. Screw placement was also evaluated by 2 independent physicians, blinded to current threshold data, on a binary scale of acceptability. The predictive capacity of screw EMG threshold data was evaluated via multivariable regression analyses and receiver operating characteristic (ROC) analyses. Predictive capacity was examined with respect to screw position within the lateral mass, as well as screw acceptability.</jats:p></jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>Lateral mass screw EMG thresholds did not appear to differ significantly for screws considered “acceptable” versus “unacceptable” according to the radiographic criteria. Accordingly, ROC analysis confirmed that EMG current threshold data were of minimal utility in predicting screw radiographic acceptability. However, EMG threshold was significantly predictive of screw medial distance from the spinal canal. A screw stimulating below 7.5 mA correctly identified a screw as being within 2 mm of the spinal canal with 75% sensitivity and 92% specificity (positive predictive value 20%, negative predictive value 99.3%), independent of its distance relative to other lateral mass landmarks. EMG current threshold was not significantly predictive of screw deviation in the superior or inferior directions, and was inversely predictive of screw deviations in the lateral direction.</jats:p></jats:sec> <jats:sec> <jats:title>CONCLUSIONS</jats:title> <jats:p>In the context of uncertainty regarding the utility of cervical lateral mass EMG current threshold data, this study found that EMG current thresholds correspond significantly, and exclusively, with screw distance from the spinal canal. This association appears independent of other criteria for screw misplacement. As such, the authors recommend that EMG current thresholds be referenced in the case of a suspected medial breach as an effective means to rule out screw placement too medial to the spinal canal.</jats:p></jats:sec>

Robotic-assisted spine surgery allows for increased pedicle screw sizes while still improving safety as indicated by elevated triggered electromyographic thresholds

“…There are inconsistencies in the literature with regard to a STIM safety threshold [ 36 , 37 ]. However, it does appear that most authors hover around 8 mA as a threshold to denote what is considered safe for the patient, with anything below 8 mA being potentially harmful, especially with the lumbar pedicle screws in which the test has a higher sensitivity and specificity [ 13 , 37 , 38 ]. Screw placement was acceptable and considered safe in both cohorts based on the average STIM thresholds (34 and 30 mA).…”

Monitoreo neurofisiológico intraoperatorio en traumatismo raquimedular cervical y columna cervical compleja: Recomendaciones desde una visión dual basada en experiencia personal

“…Estimulación de tornillos a masas laterales: punto controversial y con marcada desinformación comparativa respecto a columna lumbar en la literatura. Última evidencia disponible apunta a distancia de tornillos hacia canal menor a 2 mm con umbral de corte 7,5 mA, con 75% sensibilidad y 92% especificidad 18 . Nuestros tornillos fueron todos estimulados en principio, sin embargo, comenzamos a percatar altos falsos (+), esto es, umbral percibido demostraba cercanía o contacto con estructuras neurales y TC postoperatorio además de cuadro clínico no eran compatibles.…”