Intraoperative neurophysiological monitoring through pedicle screw stimulation

OBJECTIVES: Study the interdependence between stimulus duration and stimulus strength, to observe if by changing the stimulus duration, the current threshold levels will also change during pedicle screw stimulation. Then comparing the pedicle screw threshold data to 3-Dimensional imaging to confirm the effectiveness of pedicle screw stimulation. BACKGROUND: This study examined the stimulus parameters of triggered electromyography in the operating room during posterior spinal fusions with pedicle screw fixation, and how changing stimulus duration would affect the threshold value of a pedicle screw. Triggered Electromyography (t-EMG) or pedicle screw stimulation has been used for years as Gold Standard to test whether pedicle screws are properly placed or well insulated by bone in the pedicle of the vertebrae. In a triggered EMG test, a stimulus is sent to the pedicle screw, when the electrical stimulus activates nervous tissue, a compound muscle action potential is elicited, at this point a current threshold value is recorded. This threshold is compared to normative values, which determine if the screw is located intrapedicular or has breached the pedicle wall. A breach in the pedicle wall by a pedicle screw would cause a post-operative neurological deficit, such as nerve irritation. To examine the effects duration has on threshold values, the same screw was stimulated three different times with three different durations. The threshold values were statistically evaluated to see if there was a significance between each stimulus duration and its corresponding threshold value. Duration was examined because it is often overlooked as a parameter that may change the threshold value of a triggered EMG test. Having discrepancies due to duration can produce inaccurate results which could potentially harm the patient or change the surgical protocol. It may harm the patient by leaving a screw in place that has breached the pedicle wall of a vertebrae, which will most likely cause post-operative nerve irritation. Pedicle screw stimulation is an additional modality used to verify screw placement along with radiographic imaging in the operating theatre when posterior spinal fusions are taking place. METHODS: The current technique of pedicle screw stimulation using threshold numbers (in milliamperes) was evaluated against different stimulation parameters, and later the position of the screw was visually verified by a neurosurgeon with 3-dimensional imaging. Fundamentally, the imaging was used to validate the effectiveness that pedicle screw stimulation has on determining a well-positioned screw. Patients already scheduled to undergo spinal fusions with pedicle screw fixation were eligible for this study. A 3-Dimensional (3D) image of their spine was taken intraoperatively before insertion of spinal fixation instrumentation and one was taken after pedicle screws were inserted. These images were used to evaluate the position of the pedicle screw by a neurosurgeon. After screws were positioned, triggered EMG was utilized to check whether screws were properly placed. Screw measurements were taken from patients undergoing a posterior spinal fusion in either the thoracic, lumbar or sacral region. 213 screw measurements were taken in total from 40 patients. Factors like sex, age, height and weight were not considered for this study. The triggered EMG test threshold was then compared to the post screw insertion 3D image to verify the accuracy of the triggered EMG. In other words, the image was used to justify whether the triggered EMG test alone was an accurate indicator of a properly placed screw. In addition, the triggered EMG test itself was further evaluated, by stimulating the same screw three times, with three different stimulus durations, 300µsec, 200µsec, and 100µsec. The values produced by each stimulus duration were then compared to the corresponding stimulus threshold to see if there was a significant difference. Triggered EMG stimulus durations were assessed to see if stimulus parameter settings play a role in the threshold number. A change in stimulus duration, could change the triggered EMG threshold number, which when compared to normative data could possibly indicate a properly placed screw from a mal-positioned screw with a difference in threshold of as little as 1 milliampere. From all the stimulus parameters, a focus was put on the stimulus duration because this could directly affect the triggered EMG current threshold number, or the current value where a compound muscle action potential is elicited. Which depending on where the threshold value fell against pre-determined normative values, could directly affect whether a screw is interpreted as well-placed or not. To summarize, each pedicle screw was tested three times, at three different stimulus durations, then the corresponding threshold numbers were compared to currently established normative data thresholds and evaluated to see if duration could affect the results of a pedicle screw’s position. Remember, thresholds have been established that deem a properly placed screw, these have been used throughout the years, but no emphasis has been given to the stimulus parameters set for these thresholds. This study evaluated the importance of setting the proper stimulus parameters, mainly the stimulus duration, when using certain normative thresholds tested at specific durations to deem a properly positioned screw in the pedicle of a vertebrae. After stimulation, 3D imaging was taken intraoperatively to compare the triggered electromyography data to the actual placement of the screws. The threshold values were compared to the 3 D image of the same screws tested, to verify if indeed these established threshold values determined well-positioned screws. RESULTS: 213 screws were stimulated, out of the 213 screws, 2 screw measurements were excluded because stimulations were not obtained from all three durations. Thus 211 screws and 40 patients were included in this study. 211 screw measurements were taken in total from 40 patients with screws confirmed to be placed intrapedicular. The triggered EMG fell within normative data thresholds for 206 screws (98%), these screws were found to be intrapedicularly placed in the 3D imaging, which was confirmed by the neurosurgeon. Stimulation durations were found to be important in five of the screws, where there was a difference in the thresholds between the three stimulations, these were significant because the values were lying borderline on normative values, questioning if the screws were indeed well-placed or possibly causing a medial breach. These five screws were thoroughly examined by the neurosurgeon via 3-dimensional imaging and were found to be acceptable in placement, and not near nervous tissue. CONCLUSION: Intraoperative 3D imaging has shown that triggered electromyography is a reliable indicator of properly placed pedicle screws. Statistical data has also shown that stimulus duration can affect the interpretation of a properly placed screw, and threshold values do vary with different durations. Threshold values were gathered from 211 screws at three different stimulus durations, the first at 300µsec, the second at 200µsec and the third at 100µsec. At 300µsec stimulus duration, the mean threshold value was at 27.25mA (p=0.0078). At 200µsec stimulus duration, the mean threshold value was at 35.46mA (p=0.0028). At 100µsec stimulus duration, the mean threshold value was at 50.90mA (p=0.0676). These mean values were found to be statistically significant when run by the Kruskal-Wallis test, a non-parametric statistical significance test. Since, three groups of data were being compared, and thus were not normally distributed, a non-parametric significance test was used. In conclusion, the stimulus duration should be considered when using certain thresholds to interpret data. Different durations change the stimulus strength and thus, affect the results of the screw stimulation thresholds