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