Can triggered electromyography thresholds assure accurate pedicle screw placements? A systematic review and meta-analysis of diagnostic test accuracy

doi:10.1016/j.clinph.2014.11.026

Clinical Neurophysiology

Volume 126, Issue 10, October 2015, Pages 2019-2025

https://doi.org/10.1016/j.clinph.2014.11.026 Get rights and content

Highlights

•
Triggered electromyography (t-EMG) by direct pedicle screw stimulation was introduced two decades ago and its efficacy remains disputed.
•
The diagnostic accuracy of t-EMG was weak in the condition of overall thresholds.
•
Using titanium-alloy pedicle screws, a stimulation threshold of ⩽8 mA in the lumbar spine has high accuracy (sensitivity, 0.82; specificity, 0.97; diagnostic odds ratio (DOR), 147.95) as an indication of possible pedicle screw malpositioning.

Abstract

Objective

Triggered electromyography (t-EMG) for pedicle screw placement was introduced to prevent the misplacement of screws; however, its diagnostic value is still debated. This study aimed to clarify the diagnostic value of t-EMG and to compare thresholds.

Methods

We searched MEDLINE, EMBASE, and the Cochrane Library, and 179 studies were identified. Among them, 11 studies were finally enrolled. The pooled sensitivity, specificity, diagnostic odds ratio (DOR), and summary receiver operating characteristics (SROC) plots were analyzed.

Results

The enrolled studies included 13,948 lumbar and 2070 thoracic screws. The overall summary sensitivity/specificity/DOR values of t-EMG were 0.55/0.97/42.16 in the lumbar spine and 0.41/0.95/14.52 in the thoracic spine, respectively, indicating a weak diagnostic value. However, subgroup analysis by each threshold value showed that the cutoff value of 8 mA in the lumbar spine indicated high sensitivity (0.82), specificity (0.97), and DOR (147.95), thereby showing high diagnostic accuracy of identifying misplaced screws.

Conclusion

The most useful application of t-EMG may be as a warning tool for lumbar pedicle screw malpositioning in the presence of positive stimulation at a threshold of ⩽8 mA.

Significance

t-EMG by screw stimulation may be valuable in the lumbar region at a threshold of ⩽8 mA.

Introduction

Pedicle screws are commonly used in thoracic and lumbar spine fixations. The biomechanical superiority of pedicle screws over other spinal fixation methods, along with the increasing comfort level of surgeons with the pedicle screw techniques, has driven the popularity of this technique (Wang et al., 2010). However, spine pedicle screw applications carry potential complications involving the great vessels, the spinal cord, and spinal nerve roots. Clinically, pedicle cortex screw violations have been reported at a rate of 8% (Shi et al., 2003).

The method of using triggered electromyography (t-EMG) after pedicle screw electrical stimulation for placement evaluation was developed by Calancie et al. 20 years ago (Calancie et al., 1994a). This method evaluates EMG activity from the lower extremities while electrically stimulating screws below the threshold level. Each pedicle screw is electrically stimulated with an increasing intensity from 5 to 30 mA (duration, 0.2 ms; frequency, 0.8 Hz) (Calancie et al., 1994a, Gavaret et al., 2013). Some prior investigators insisted that pedicle screw testing was the most appropriate available technique and provided rapid and useful intraoperative information regarding screw placements during procedures (Lenke et al., 1995, Shi et al., 2003, Gavaret et al., 2013). By contrast, other studies revealed that the t-EMG technique had low sensitivity in predicting screw malpositioning and asserted that imaging-based modalities remain more appropriate for assessing percutaneous pedicle screw trajectory until more robust and sensitive intraoperative neurophysiologic monitoring methods are devised (Reidy et al., 2001, Wang et al., 2010, de Blas et al., 2012).

Debate has remained not only regarding the efficacy of t-EMG but also regarding its threshold value. One prior study suggested that the threshold stimulus intensity in the lumbar spine is >8 mA if the screw is entirely in the pedicle; 4.0–8.0 mA represents the potential for a pedicle wall defect, and <4.0 mA represents a strong likelihood for a pedicle wall defect with potential for nerve root and dura contact (Lenke et al., 1995). Other investigators suggested appropriate threshold values of 8 mA (Raynor et al., 2007, Alemo and Sayadipour, 2010, Parker et al., 2011), 10 mA (Rodriguez-Olaverri et al., 2008, Gavaret et al., 2013), 10–11 mA (Shi et al., 2003), and 14 mA (Nair, 2013). The lower the stimulation threshold required to evoke a response, the higher the probability of a breach. However, there is little consensus regarding threshold stimulus intensities because these studies are limited by a low incidence of radiographic breaches.

The aim of this study was to estimate the sensitivity and specificity of t-EMG in assuring accurate pedicle screw placement and to compare threshold values with a systematic review and meta-analysis.

Section snippets

Literature search

We conducted a comprehensive literature search to identify studies that dealt with t-EMG by screw stimulation. The searched databases included MEDLINE, EMBASE, and the Cochrane Library from their inception to April 2014. The keywords and medical subject headings related to the condition and potential treatments were identified prior to initiating the search. The following search strings were used: intraoperative [All Fields] AND (“bone screws” [MeSH Terms] OR “screws” [All Fields] OR “bone

Search results

A flowchart shows the literature search and the study selection process (Fig. 1). A total of 179 studies were obtained. After the duplicate studies, case reports, and technical notes were excluded, 115 papers were left to screen. Based on the title and abstract, 71 reports were excluded because the topic of the article was not relevant to the objective of the review. Cadaver and animal model studies, as well as studies that lacked threshold descriptions, were excluded after full text reviews.

Discussion

In this systematic review, we analyzed and summarized data from diagnostic accuracy studies on the use of t-EMG to identify pedicle screw malpositioning. This test usually detected medial and inferior breaches because its detection ability is related to the distance between a nerve and a screw. This meta-analysis shows that t-EMG is a useful method for predicting pedicle screw malpositioning in limited situations. The strengths of this study are that we conducted an extensive systematic search

Conclusion

t-EMG by direct screw stimulation may be valuable only in limited situations because of its poor sensitivity. The application of t-EMG appears to be useful as a warning tool for likely cases of lumbar pedicle screw malpositioning in the presence of positive stimulation at threshold values of ⩽8 mA.

Conflict of interest statement

None.

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Cited by (27)

The effect of hydroxyapatite on titanium pedicle screw resistance: an electrical model
2022, Spine Journal
Citation Excerpt :
Second, this study provided clinically relevant data: no evidence of tEMG and HAC incompatibility. Our data suggests the use of tEMG, a powerful modality to detect malpositioned screws, and HAC screws are compatible [8,19,52,53]. These findings are particularly relevant for surgeons considering the increased awareness of misplaced screws [16,17].
Intraoperative detection of a pedicle wall breach implicitly reduces surgical risk, but the reliability of intraoperative neuromonitoring has been contested. Hydroxyapatite (HA) has been promulgated to increase pedicle screw resistance and negatively influence the accuracy of electromyography.
The primary purpose of this experiment is to evaluate the effect of HA on pedicle screw electrical resistance using a controlled laboratory model.
Controlled laboratory study.
Stimulation of pedicle screws was performed in normal saline (0.9% NaCl). The experimental group included 8 HA coated (HAC) pedicle screws and matched manufacturer control pedicle screws without HAC (Ti6Al4V). All screws were stimulated at 5, 10-, 15-, 20-, and 25-mm submersion depths. Circuit current return was recorded, and pedicle screw electrical resistance was calculated according to Ohm's Law. Data were assessed for normality and variance. Mann-Whitney U and Kruskal-Wallis tests compared groups with Bonferroni correction for multiple testing. Effect size is reported with 95% confidence intervals (95CI). p values <.05 were considered significant.
Current return was detected for all screws (N=24) following subclinical 8.5 µA stimulation at 5, 10-, 15-, 20-, and 25-mm submersion depths (N=144). The effect estimate of HA on pedicle screw electrical resistance is -0.07 (-0.17 to 0.01 95CI). The estimated effect of HA on pedicle screw electrical resistance did not differ across manufacturers. Electrical resistance values were inversely related to submersion depth. Electrical resistance values were lower in the experimental group at 10 mm (p=.04), 15 mm (p=.04), and 25 mm (p=.02) submersion depths. The HA effect ranged from -0.03 to -0.08 as submersion depth varied.
We found no evidence that HA increased pedicle screw electrical resistance in a matched manufacturer control laboratory model. Electrical stimulation of pedicle screws may be reliable for pedicle breach detection in the presence of HA. Future research should investigate if laboratory findings translate to clinical practice and confirm that electrical stimulation of pedicle screws is a reliable method to detect pedicle breach in the presence of HA.
Does Neurologic Electrophysiologic Monitoring Affect Outcome?
2022, Evidence-Based Practice of Anesthesiology
Neurologic injury from surgery results in substantial increased morbidity, mortality, and cost. Most importantly, it is devastating to patients and their families. Thus techniques to lessen, reverse, and even avoid neurologic injury are very valuable. Neurologic intraoperative electrophysiologic monitoring (NIOM) can identify impending or ongoing intraoperative injury, allowing for interventions. Techniques such as somatosensory evoked potentials, motor evoked potentials, visual evoked potentials, brainstem evoked potentials, electroencephalography, electromyography, and brain mapping are used individually or in combination depending on the neurologic structures at risk during procedures to identify injury in the anesthetized patient. Changes to a patient’s neurologic electrophysiologic baseline during the procedure alert the operative team that a potential injury may be occurring. The goal of NIOM is to detect dysfunction caused by ischemia, mass effect, stretch, heat, or direct injury in real time before it causes permanent neurologic injury. Monitoring may also be useful for identifying and preserving neurologic structures during a procedure where they are at risk (mapping). Establishing efficacy has been difficult in NIOM because of a lack of controlled trials, general acceptance of its usefulness, ethical and medical-legal concerns, and the challenge of determining whether an intervention that reverses a NIOM change was, in fact, real in an anesthetized patient. This chapter reviews the current evidence supporting NIOM techniques and its use in specific procedures, ongoing areas of uncertainty, and current NIOM guidelines published by national organizations and stakeholders. It then presents the author’s recommendations on the use of NIOM.
Optimal “Low” Pedicle Screw Stimulation Threshold to Predict New Postoperative Lower-Extremity Neurologic Deficits During Lumbar Spinal Fusions
2021, World Neurosurgery
Citation Excerpt :
Previous studies have shown that stimulation thresholds of ≤6 mA can indicate medial pedicle wall breaches and penetration of dura and adjacent nerve root.11-13 A recent meta-analysis by Lee et al8,14 of 13,948 lumbar pedicle screws showed a high diagnostic accuracy of pedicle screw testing at thresholds ≤8 mA in identifying malpositioned screws. However, not all screw misplacements result in postoperative LE neurologic deficits or symptoms.13
Previous studies have shown that pedicle screw stimulation thresholds ≤6−8 mA yield a high diagnostic accuracy of detecting misplaced screws. Our objective was to determine the optimal “low” stimulation threshold to predict new postoperative neurologic deficits and identify additional risk factors associated with deficits.
We included patients with complete pedicle screw stimulation testing who underwent posterior lumbar spinal fusion surgeries from 2010−2012. We calculated the diagnostic accuracy of pedicle screw responses of ≤4 mA, ≤6 mA, ≤8 mA, ≤10 mA, ≤12 mA, and ≤20 mA to predict new postoperative lower-extremity (LE) neurologic deficits. We used multivariate modeling to determine the best logistic regression model to predict LE deficits and identify additional risk factors. Statistics software packages used were Python3.8.5, NumPy 1.19.1, Pandas 1.1.1, and SPSS26.
We studied 1179 patients who underwent 8584 pedicle screw stimulations with somatosensory evoked potential and free-run electromyographic monitoring for posterior lumbar spinal fusion. Twenty-five (2.1%) patients had new LE neurologic deficits. A stimulation threshold of ≤8 mA had a sensitivity/specificity of 32%/90% and a diagnostic odds ratio/area under the curve of 4.34 [95% confidence interval: 1.83, 10.27]/0.61 [0.49, 0.74] in predicting postoperative deficit. Multivariate analysis showed that patients who had pedicle screws with stimulation thresholds ≤8 mA are 3.15 [1.26, 7.83]× more likely to have postoperative LE deficits while patients who have undergone a revision lumbar spinal fusion surgery are 3.64 [1.38, 9.61]× more likely.
Our results show that low thresholds are indicative of not only screw proximity to the nerve but also an increased likelihood of postoperative neurologic deficit. Thresholds ≤8 mA prove to be the optimal “low” threshold to help guide a correctly positioned pedicle screw placement and detect postoperative deficits.
Cortical screw fixation using CT-navigation coupled with real-time electrophysiological monitoring of individual screw placement for unstable degenerative lumbar spondylolisthesis
2021, Interdisciplinary Neurosurgery: Advanced Techniques and Case Management
Citation Excerpt :
A defect in the bony wall, by a malpositioned screw provides a conduit for the electrical current to freely flow leading to subsequently diminished stimulus threshold [21,38–40]. Screw stimulation at thresholds <8 mA have been shown to have 97% specificity in detecting a malpositioned screw, however, with a lower <90% sensitivity [21,37]. As such, it is commonly only one of the tools used in a multimodal approach to ensure safety.
Cortical screws offer a less invasive alternative compared to traditional pedicle screws. These screws are inserted in an inferomedial to superolateral trajectory achieving greater cortical bone purchase. Similar fusion rates, pain relief, and decreased surgical morbidity at 12-month follow-up have been documented when compared to traditional pedicle screws. Using intra-operative imaging, neuronavigation, and individual neurophysiological monitoring of each screw, we showed that this is a safe surgical approach.
Institutional review board (IRB) approved retrospective review of medical records for 173 patients to determine eligibility. Cases had to be elective one-or two-level fusion with surgical indication. Functional improvement was measured with Oswestry Disability Index (ODI) at pre and post-operative visits. Surgical morbidity data, individual screw thresholds, and radiographic evidence of stability was collected from electronic medical records (EMR).
A total of 153 patients met criteria with mean age 66.60 ± 9.30 (range: 34–84) and mean BMI of 28.76 ± 4.47 kg/m². Of 558 screws inserted, no screws recorded <8 mA upon stimulation, thus no screw required repositioning. There were no pedicle or central canal breach related to screw insertion. ODI decreased from 44.83 ± 18.02 to 19.46 ± 19.52 at 3-months post-operative (p < 0.0001). A subset of 30 patients had 12-month post ODI, which showed a change from 41.22 ± 18.42 to 22.63 ± 22.01 at 12-months (p < 0.001).
Within, we discuss our approach for inserting cortical screws in posterior lumbar fusion for patients with unstable degenerative lumbar spondylolisthesis. Our study shows that the implementation of intraoperative imaging, neuronavigation, and neurophysiological monitoring of individual screws can provide a safe environment for cortical screw insertion. This approach allows for a less invasive approach and greater quality bone purchase while mitigating the associated risks.
Accuracy of t-EMG stimulation of the middle pedicle track to prevent radiculopathies as a result of misplaced lumbar screws
2020, Clinical Neurology and Neurosurgery
To describe the accuracy of middle pedicle track stimulation for the detection of pedicle breaches causing misplaced lumbar screws and subsequent neurological symptoms.
In a comparative observational study with two cohorts, 1440 lumbar pedicle screws were implanted using the freehand technique in 242 patients undergoing surgery for spinal deformities. In the first two-year period (2011−2012), the accuracy of screw placement (802 screws) was assessed by conventional intraoperative palpation of the pedicle track, t-EMG screw stimulation, and fluoroscopic monitoring. In the second period (2012–2013), the middle aspect of the lumbar pedicle tracks was systematically stimulated with a probe (638 screws). When thresholds in the middle track showed <9 mA, potential neurological risk was considered, and therefore, new pedicle tracks were performed.
Six patients (4.4 %) in the first period presented postoperative radicular pain and a normal intraoperative screw t-EMG threshold. CT scans showed seven screws (0.9 %) with >2-mm medial-caudal invasion of the foramen. Before screw removal, t-EMG thresholds of these screws were again normal (≥10 mA). After removal of the screws. t-EMG of the middle part of the pedicle track showed thresholds below 9 mA (mean 5.2 mA). In the second period, the pedicle tracks were systematically stimulated. Low t-EMG thresholds (<9 mA) were found in 11 tracks (1.7 %) and were therefore reworked before screw placement. CT scans in these 10 patients showed that all of the 11 screws were correctly repositioned.
This study shows that caudal or medial pedicle cortical breaches can be detected effectively by stimulating the middle part of the pedicle track. This technique is strongly recommended to prevent postoperative lumbar radiculopathies due to screw malposition.
Electromyographic monitoring for pedicle screw placements
2020, Neurophysiology in Neurosurgery: A Modern Approach
The use of pedicle screws for spinal stabilization has become commonplace during different spinal surgical procedures. However despite the usage of surgical inspection and imaging techniques, the placement of these screws is largely performed blindly. The incidence of misplaced pedicle screws resulting in neurological impairment has been reported to be quite high even with experienced surgeons. New imaging techniques may help to reduce the incidence of misplaced hardware but they are still not free from error. As a result surgeons and clinical neurophysiologists have utilized electrophysiological monitoring techniques for assessing nerve root function and pedicle screw placement during surgery. Based on the published results of numerous investigators, the combined use of spontaneous and triggered myogenic activity for intraoperative monitoring during pedicle screw placement has been shown to be practical, cost effective, and improves surgical outcomes.

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Can triggered electromyography thresholds assure accurate pedicle screw placements? A systematic review and meta-analysis of diagnostic test accuracy

Highlights

Abstract

Objective

Methods

Results

Conclusion

Significance

Introduction

Section snippets

Literature search

Search results

Discussion

Conclusion

Conflict of interest statement

World Neurosurg

Orthop Traumatol Surg Res

Pedicle screws with high electrical resistance: a potential source of error with stimulus-evoked EMG

Spine (Phila Pa 1976)

Stimulus-evoked EMG monitoring during transpedicular lumbosacral spine instrumentation. Initial clinical results

Spine (Phila Pa 1976)

Stimulus-evoked EMG monitoring during transpedicular lumbosacral spine instrumentation: initial clinical results

Spine (Phila Pa 1976)

Safe pedicle screw placement in thoracic scoliotic curves using t-EMG: stimulation threshold variability at concavity and convexity in apex segments

Spine (Phila Pa 1976)

Intraoperative neuromonitoring: can the results of direct stimulation of titanium-alloy pedicle screws in the thoracic spine be trusted?

J Clin Neurophysiol

A prospective analysis of intraoperative electromyographic monitoring of pedicle screw placement with computed tomographic scan confirmation

Spine (Phila Pa 1976)

Users’ guides to the medical literature. III. How to use an article about a diagnostic test. A. Are the results of the study valid? Evidence-based medicine working group

JAMA

Users’ guides to the medical literature. III. How to use an article about a diagnostic test. B. What are the results and will they help me in caring for my patients? The Evidence-based medicine working group

JAMA

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

Eur Spine J.

Triggered electromyographic threshold for accuracy of pedicle screw placement. An animal model and clinical correlation

Spine (Phila Pa 1976)