Correlation Between Obliquity of Exiting Nerve Root on Lateral Sagittal MRI Images and Degenerative Spondylolisthesis
Keywords: Degenerative spondylolisthesis, exiting nerve root, low backache, lumbosacral radiculopathy, obliquity of the angle
Low backache, with or without pain in the lower limbs, is a common complaint nowadays. Magnetic resonance imaging (MRI) of the lumbosacral spine has become the investigation of choice for these patients.It has been observed that listhesis may get reduced in the supine position and hence may be missed on MRI and CT scans as both these studies are done with patients in the supine position., Weight-bearing, standing lateral flexion-extension X-rays are the gold standard to confirm or rule out spinal instability.,,
Several investigators have tried to find the correlation between various degenerative changes seen on MRI and spinal instability.However, the sensitivity and specificity of these signs are not very high.,,,,
Far lateral T2 sagittal MRI images show the exiting nerve roots coming out of the intervertebral foramen in an inverted teardrop shape. Usually, they come out and descend straight downwards, but in some cases, they descend obliquely. We carried out a pilot study and described the angle between the exiting nerve root and the lower end plate of the vertebra.
We observed that in patients without spondylolisthesis, this angle was almost a right angle. However, in patients with spondylolisthesis obvious on MRI, this was always an obtuse angle. In some patients, we found this angle was obtuse, but there was no obvious spondylolisthesis on MRI. When we did standing lateral flexion-extension radiographs of these patients, they revealed spondylolisthesis.
The aim of this study was to determine the incidence of the obliquity of the exiting nerve root visible on far lateral Sagittal T2WI MRI and to assess its correlation with lumbar instability present on standing lateral flexion-extension X-rays.
This is a retrospective, observational study.
We retrospectively studied patients who underwent discectomy or laminectomy and stabilization for degenerative lumbar disease at our institute between January 2017 and February 2020.
The criteria of inclusion were patients who were operated for lumbar discectomy or lumbar laminectomy and stabilization for degenerative lumbar disease and whose MRI studies and standing lumbosacral spine lateral flexion-extension radiographs were available for review.
The exclusion criteria were patients whose MRI and standing lateral flexion-extension X-rays were not available for review.
We considered degenerative spondylolisthesis to be present when a vertebral slip greater than 3 mm was seen on lateral standing flexion-extension radiographs.
For objectively measuring the obliquity of the exiting nerve root, we described an angle which the exiting nerve root made with the body of vertebra.We selected far lateral sagittal T2-weighted image just lateral to the facet joints or the image that clearly showed exiting nerve roots coming out of intervertebral foramen in an inverted teardrop manner.We drew a line parallel to the lower endplate of the vertebra and another line along the exiting nerve root and measured the angle between these two lines. We considered this the “angle of obliquity of the exiting nerve root”[Figure 1].
We studied the correlation of this obliquity of exiting nerve root on lateral sagittal MRI images with spondylolisthesis on standing lumbosacral spine lateral flexion-extension X-rays.
Statistically, we determined the (1) sensitivity of our test, (2) specificity of our test, (3) predictive value of a positive test, and (4) predictive value of a negative test. We calculated Yule's coefficient of association, Q. This showed that the angle of obliquity of exiting nerve root of more than 105° is “very highly associated”with spondylolisthesis.
The demographic data of our study are shown in [Table 1].
There were 129 (52%) males and 119 (48%) females, with an average age of 47.21 years (range: 26–76 years).
In our study, 108 (43.54%) patients had spondylolisthesis, and it was more common in females [Table 2].
Out of 108 patients with spondylolisthesis, in 67 (62.03%) patients, it was also reported on MRI.However, in 41 (37.96%) patients out of 108 patients, spondylolisthesis was not reported on MRI. This shows that in 37.96% of patients, the spondylolisthesis was missed on MRI [Table 3].
[Table 4] shows a very high correlation between spondylolisthesis and the angle of obliquity of the exiting nerve root of >105°. Moreover, there is very high correlation between the angle of obliquity of the exiting nerve root of <105° and theabsence of spondylolisthesis.
Statistically, we calculated the parameters for our test of “angle of obliquity of exiting nerve root in spondylolisthesis” as compared with standing lateral flexion extension X rays, the current gold standard. The sensitivity of the test was found to be 98.14%. The specificity of the test was found to be 97.85%.The predictive value of a positive test was 97.25%.The predictive value of a negative test was found to be 98.56%. Yule's coefficient of association Q for our study was 0.99.
All the above parameters show that the angle of obliquity of the exiting nerve root >105° is very highly associated with spondylolisthesis.
Lumbar spondylolisthesis is a comparatively common condition causing low back pain with or without radiculopathy.In patients with lumbosacral radiculopathy, the presence of spinal instability will dictate the need for stabilization in addition to decompression, whereas in the absence of spinal instability, only decompressive surgery is needed.
To diagnose spondylolisthesis, standing lumbosacral lateral flexion-extension X-rays are the gold standard. As standing X-rays are loading ones, they tend to pronounce the slip of the vertebra. As MRI and CT scans are done in the supine position, they may allow the realignment of slipped vertebra in some patients and the listhesis may be missed in them. Because MRI does not show the degree of slip uniformly, fluid effusion distending facets seen as T2 hyperintensity is used to predict listhesis. Ben-Galim et al. reported that facet fluid effusion bigger than 1.5 mm highly correlates with degenerative spondylolisthesis.,,,
Many authors have studied the association between lumbar facet degeneration and other findings of degeneration on MRI with spondylolisthesis, but the outcomes of these studies are equivocal.,,,,,
As many investigators have detected linear correlation in facet joint effusion and lumbar instability,,,, the facet fluid sign, despite being imperfect, has become more acceptable among all the tests to the extent of being called thegold standard.,
Thus, the quest for a test with high sensitivity and specificity that can reliably predict spondylolisthesis on MRI continues.
We feel that the far lateral sagittal images of the T2-weighted MRI sequence are not usually looked at very carefully. This is the slice that is just lateral to the facet joint. This slice shows exiting nerve roots coming out from the intervertebral foramen.They look like a vertical row of inverted teardrops. If we look carefully, they travel parallel to the posterior border of the vertebra or at right angles to the lower end plate.
Arslan et al., who studied the anatomy of the nerve root, observed that immediately after coming out of intervertebral foramen, the exiting nerve root descends vertically for a few millimeters, crosses the intervertebral disc, and then turns anterio-inferiorly.,
Thus, we concentrated on this descending part of the exiting nerve root coming out of the intervertebral foramen.We observed that these roots usually come out of the foramen and descend straight [Figure 2]; however, in some patients, they come out of the foramen and descend obliquely, directing posteriorly. When we studied this further, we realized that at the level of spondylolisthesis, this root descended obliquely [Figure 3]. In some patients, surprisingly, we saw the oblique root without obvious spondylolisthesis at that level on MRI. When we did standing lateral flexion-extension radiographs of these patients, they showed spondylolisthesis [Figure 4]a, [Figure 4]b, [Figure 4]c. This obliquity of the exiting nerve root was quite obvious to a discerning eye, but to have an objective measurement, we described the angle of obliquity exiting nerve root[Figure 1].
During our study, we found that whenever the exiting nerve root looks straight descending down, the angle is less than 105°, and if the exiting nerve root looks oblique, the angle is usually more than 105°[Figure 2] and [Figure 3].
We did a pilot study of 30 patients for studying the obliquity of the exiting nerve root. Encouraged by the outcome, we decided to do a retrospective study of patients who had undergone discectomy or lumbar laminectomy and stabilization at our institute to evaluate the correlation between the obliquity of the exiting nerve root on MRI and spondylolisthesis.
Whenever there is instability, body and the nature try to create stability by adding more tissue. In lumbar spondylolisthesis, it happens by osteophyte formation, additional bone formation, and fibrosis, which try to hold the slipping vertebrae together.
During this process, whenever the added tissue occupies a lot of space and starts compressing the nerve roots, patients get the signs and symptoms of nerve root compression and canal stenosis. We feel that whenever this type of additional tissue formation is there between the two vertebrae, it engulfs the exiting root and makes it oblique because of the slip [Figure 5]a.
In the supine position, the slip may get neutralized in some patients and vertebrae may get aligned. However, as the exiting nerve root is engulfed in fibrosis, it does not come back to its natural position when the patient becomes supine and it continues to be in the oblique direction [Figure 5]b.
We measured the angle of obliquity for the exiting nerve root in patients with and without spondylolisthesis. When we correlated this data, we came to a conclusion that obliquity of the exiting nerve root on MRI has a very high correlation with spondylolisthesis.
All our statistical parameters show that the obliquity of exiting the root on MRI is an excellent indicator of spondylolisthesis with very high sensitivity and specificity.
Schellinger et al. found out that only 50% of the patients with facet fluid had spondylolisthesis.
Ben-Galim et al. provided the explanations why the face fluid sign may be an imperfect indicator of instability.Hipp et al. mentioned that a perfect relationship between spondylolisthesis and the facet fluid sign is not to be expected.
Rihn et al. described the method for estimating the facet fluid wherein multiple axial T2 MRI images must be selected through the L4–L5 facets and the ratio of the sum of width of facet fluid to the sum of width of thefacets must be calculated.
As compared to this, to see the oblique root, we simply have to study the far lateral sagittal image of T2-weighted MRI where the roots are coming out of the foramina and see the obliquity of the exiting nerve root. In the majority of the cases, we can judge the obliquity by looking at the image. To further endorse the observation, we have specified the angle between the body of the vertebra and the exiting nerve root.
Apart from being highly predictive, we found our method of predicting the listhesis on MRI with obliquity of the exiting nerve root to be more user-friendly, less cumbersome, and easy to apply for all lumbosacral levels. This test, obliquity of the exiting nerve root on MRI, has a very high sensitivity (98.14%) and very high specificity (97.85%) for diagnosing spondylolisthesis on supine MRI.
Limitations of our study: As our study is retrospective and observational, the sensitivity and specificity observed by us need to be further validated by prospective studies.
In conclusion, this test of obliquity of the exiting nerve root on MRI has very high sensitivity (98.14%) and very high specificity (97.85%) for predicting spondylolisthesis on MRI.
The angle of obliquity of the exiting nerve root >105° is highly associated with spondylolisthesis.
The author thanks Dr. Tanmay Mahale for sketching [Figure 5]a and [Figure 5]b.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4]