The purpose of our multicenter survey was to compare the reliabilities of surgical decisions (regarding the surgical level and modality) set based on cMR and dMR and to compare the diagnostic accuracies of cMR and dMR by evaluating the improvement in clinical outcomes after surgical treatments indicated using preoperative plans set using each of the modalities. Our study showed that dMR resulted in more consistent inter- and intraobserver reliabilities, and more accurate diagnostic accuracy than cMR.
The pathophysiology of CSM includes both static and dynamic factor. Traditionally, the static factors have been the main issues considered in the development of CSM; these include congenital canal stenosis, osteophyte formation, facet joint hypertrophy, ossification of the posterior longitudinal ligament, ossification of the ligamentum flavum, and other minor static compressive lesions around the cervical spinal cord. For the static factors, some radiologic parameters have been developed, including the Pavlov ratio and sagittal canal diameter [22, 23]. However, to date, more focus is laid on dynamic factors which are now believed to also play a role in the development and aggravation of CSM. Dynamic compression of the cervical spine is highly related with buckling of the ligamentum flavum and segmental translational instability during neck motion, especially during neck extension. CSM from a dynamic compression can occur with dynamic cervical spine instability. It is defined as a translation of > 3.5 mm and an angulation of > 11° in flexion–extension views of a dynamic radiograph, that resulting in the narrowing of the spinal canal [3, 12, 14, 24]. Moreover, increased strain or shear forces of the spinal cord can induce axonal damage within the cord, increase membrane permeability, and cause conduction losses in myelinated axons. With repetitive compression or injury from the dynamic factors, irreversible loss of neuronal cells and ischemia could occur, thus contributing to the aggravation of CSM. Therefore, dynamic factors play crucial role in the induction and development of CSM; however, dynamic cord compression has rarely been defined clearly, except when there is confirmed instability of the cord segment on cervical spine radiographs.
Recently, dMR has been frequently used to evaluate static and dynamic spinal cord compression; though it is not yet a routinely requested work-up [14, 15, 25]. Clinical decision-making processes using edMR to evaluate the number of compression levels was introduced, and clinical decisions made based on edMR findings were reported to yield better results . Some studies have reported the clinical applications of edMR in the determination of surgical levels, such as in cervical selective laminoplasty . Regarding reliability, both intra- and interobserver reliabilities showed higher agreements concerning surgical segment choice and modality decision with dMR (especially edMR) than with cMR. From the outcomes of the intraobserver reliability, neutral sagittal MR showed moderate reliability, and extension sagittal MR showed moderate reliability. The two imaging modalities had statistically significant difference.; therefore, the authors suggested extension sagittal MRI could produce greater intraobserver reliability when compared to neutral sagittal MRI. Similarly, the results of intraobserver reliability also showed that extension sagittal MRI had a greater reliability than neutral sagittal MRI, and there was a statistically significant difference between the two imaging modalities (p = 0.001). Therefore, edMRI is expected to provide consistency in surgical planning for CSM and consequently, to reduce the variation of surgical outcomes among operators and institutions. In a previous study, Kim et al. confirmed that the diagnostic reliability of dMR was significantly higher than that of cMR ; however, their study only confirmed interobserver reliability and not intraobserver reliability. In addition, since the surgeons participating in this survey had completed a fellowship in spine surgery at the same institution, the rationale behind the surgical decision-making process could be the same. Because our survey was conducted on surgeons who had completed their education and fellowship in spine surgery at different institutions, the bias from uniform surgical decisions could also be minimized.
If CSM is not treated properly and in a timely manner, the patient’s symptoms gradually worsen, leading to negative consequences. Therefore, an accurate investigation of the structure and the level of the spinal cord compression should be established before the surgery. However, cMR does not reflect the movement of the cervical spine, such as segmental motion and instability, and can miss out some hidden levels of cord compression. The confirmation of diagnostic accuracy through the evaluation of the improvement of clinical outcome after surgery also showed that edMR has a significantly higher accuracy when compared to cMR. Specifically, neural sagittal MR provided “moderate” accuracy and extension sagittal MR provided “high” accuracy. More importantly, there was a significant difference between the two modalities (p = 0.02). This means that some segments (or patients) classified as being non-pathological on cMR developed dynamic cord compression, contributing to myelopathy symptoms, as being depicted at Figs. 1 and 2. In real situation, some patients showed typical symptoms and/or signs of CSM, but they had no typical CSM findings suggestive of CSM (such as significant cervical cord compression and snake-eye appearance of signal change). For such patients, establishing a proper surgical treatment plan in terms of surgical approach and surgical level is challenging. Thus, a surgical plan based on cMR is incomplete to treat spinal cord decompression. Consequently, there is a possibility that the patient’s postoperative outcome based on cMR be poorer than expected. In addition to increasing medical expenses, this can ultimately lead to socioeconomic problems such as delays in returning to previous jobs or unemployment due to reduced self-reliance. If dMR can detect “hidden cord compression lesions,” this will ensure high diagnostic accuracy, improve surgical outcomes, and reduce the socioeconomic burden. We confirmed that dMR has a positive effect on postsurgical outcome and prognosis. It also improves the diagnostic accuracy and helps in the selection of appropriate surgical treatment by revealing hidden cord compressions. Therefore, we believe that dMR, which reflects the daily movement of the cervical spine, should be performed prior to surgical planning.
This study has some limitations. First, the sample size was relatively small, making it prone to type 1 errors. Second, there was no clear flexion-extension position protocol for dMR. Third, because of the fundamental fact that MRI can only be done in the supine position, it was not possible to observe the actual cord compression in the erect position (under the effect of the weight of the head and gravity). Fourth, we did not include computed tomography (CT) scans of the cervical spine in the survey images of our study. Calcification of soft tissue structures around the spinal cord is an important factor in the decision on surgical method, and this is a clear limitation of our study. However, in order to include CT in our study design, the patient must be exposed to radiation twice before and after position change. As it is well known, it is very difficult to determine the degree of spinal cord compression from a CT image, so CT was not used because the benefit compared to the patient’s risk from radiation hazard was insignificant. Fifth, while conducting this study, we did not prepare for the fact that the patient’s CSM symptoms could worsen when the extension posture was applied. However, since edMR only acquires sagittal images for a short time after cMR imaging, no patients experienced worsening of symptoms during that time. Lastly, researchers should discuss the results and interpret them from the perspective of previous studies and working hypotheses. These findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.