Traditional MRI plays an important role in guiding the management of intervertebral disc degeneration. We investigated the relation between MR lumbar degenerative changes grading systems and lumbar neural elements compromise. Up to the best of the authors’ knowledge, this research question has never been addressed by previous studies.
The mean age of the included patients was 48.2 ± 14.1 years, with nearly half the patients below the age of 45 years and 26% above 60 years. Disc degeneration is well known to correlate with the advancement of age .
The study of Brinjikji and his colleagues reported that lumbar degenerative changes imaging findings like disc bulge, disc height or signal loss are caused by normal aging not due to pathological processes. They also reported that about a half of 30 to 39 years aged asymptomatic individuals and even young adults exhibited disc bulge, disc signal or height loss. These degenerative disc changes may be seen as incidental imaging findings and not necessary to be the cause of the presenting symptoms .
The lumbar disc degenerative disease was assessed at the levels of L3–4, L4–5, and L5–S1 discs with a severer grade of degenerative changes at the level of L4–5 and L5–S1 compared to the L3–4 level. This finding is in line with previous research as the level of L4–5 was reported as the most frequent site of abnormalities, followed by L5–S1, and then L3–4 [13, 14].
The prevalence rate of the stenosis of the lumbar central spinal canal in the present study was 58%, which is higher than that reported by a recent meta-analysis (38%) in the general population-based on imaging studies . This may be due to using different criteria for defining stenosis of the spinal canal across the studies.
We found that, at all levels, Pfirrmann grades 3–5 were significantly associated with root compression as identified by Van Rijn classification (p < 0.001) and severe stenosis of the central spinal canal and lateral spinal canals (p < 0.001). These findings were supported by the moderate, significant, positive correlation observed between Pfirrmann grades and the severity of stenosis of the central and lateral spinal canals.
In the present study, the association of Pfirrmann grades 3–5 with severe grades of foraminal stenosis was significant at the levels of L4–5 and L5–S1, but not at the level of L3–4. Correlation analysis revealed a moderate, significant, positive correlation between Pfirrmann grades and the severity of foraminal stenosis. However, the strength of correlation was lower at the L3–4 level, accounting for the non-significant difference found between Pfirrmann categories 1–2 and 3–5.
The results of the current study showed that the severity of the spinal canal and foraminal stenosis tended to increase with increased bulging and protrusion as assessed by the CTF classification of NASS. However, some categories included low numbers of patients, thus we were not able to compare the severity of stenosis across all NASS categories.
The study of Li and his colleagues reported that the commonest system used for assessment of lumbar disc herniation is the CTF of NASS while other classification systems like van Rijn and Pfirrmann classification systems are used for classifying the lumbar neural element compression .
Many studies concluded that the CTF and van Rijn classifications systems had significant interobserver agreement and they considered as the most reliable systems for assessment of lumbar intervertebral disc herniation and lumbar neural element compression [16,17,18,19,20].
The studies of Zileli and his colleagues, Park and his colleagues, Kushchayev and his colleagues and Andreisek and his colleagues reported lumbar spinal canal stenosis qualitative imaging parameters. These parameters aimed to identify qualitative imaging criteria that should be used as a standard in writing radiologic reports for suspected lumbar spinal canal stenosis. These imaging parameters did not depend on any lumbar spinal canal quantitative measures as they reported that measurement of lumbar spinal canal parameters is time consuming, has moderate reliability and lacked evidence correlation between lumbar spinal canal parameters and patients symptoms [21,22,23,24].
They also reported five qualitative imaging criteria that should be radiologically reported for patient with suspected lumbar spinal canal stenosis. These criteria included central canal compromise and the relation between cauda equina and lumbar cerebrospinal fluid (CSF), compression of nerve root in both lumbar lateral recesses and both foraminal zones compromise and impingement of nerve root. According to these imaging parameters central, both lateral and both foraminal lumbar spinal canal stenosis classified into 4 grades from grade 0 (no stenosis) to grade 3 (severe stenosis) [21,22,23,24].
Many studies determined multiple quantitative parameters for assessment of lumbar spinal canal stenosis. These parameters included lumbar spinal canal antero-posterior (AP) diameter; lumbar dural sac cross-sectional area, percentage of lumbar dural sac compression, height of both lateral recess and diameter of lumbar intervertebral foramina .
In this study, we correlated the qualitative parameters of central, both lateral and both foraminal lumbar spinal canal stenosis grades with the quantitative measures of central lumbar spinal canal measures, both lumbar lateral recesses heights and both lumbar foraminal diameters at L3–4, L4–5 and L5–S1 lumbar vertebral disc levels.
The study of Zileli and his colleagues reported that the most reliable measures used for diagnosis of central lumbar spinal canal stenosis were the AP diameter and cross-sectional area of the lumbar spinal canal. They also reported that height of lateral recess is the most reliable measure used for diagnosis of lateral lumbar spinal canal stenosis and foraminal diameter is the most reliable measures used for diagnosis of foraminal stenosis .
Many studies concluded that the diagnosis of central spinal canal stenosis was established if the AP diameter of lumbar spinal canal is less than 12 mm. These results were matched with the results of this study as this study concluded that the diagnosis of grade 3 severe central spinal canal stenosis established if the measurement of AP diameter of lumbar spinal canal at L3–4, L4–5 and L5–S1 were equal or less than 13 mm, 12.5 mm and 12.6 mm, respectively [21, 25,26,27].
Many studies concluded that the diagnosis of central spinal canal stenosis was established if the lumbar dural sac cross-sectional area is less than 100 mm2. These results were matched with the results of this study as this study concluded that the diagnosis of grade 3 severe central spinal canal stenosis established if lumbar dural sac cross-sectional area measurement at L3–4, L4–5 and L5–S1 were less than or equal 106 mm2, 105 mm2 and 107 mm2, respectively [21, 25,26,27,28].
With regard to the degree of the dural sac compression at disc level (%), this study concluded that a dural sac compression ratio of 64% or less is significant for diagnosis of grade 3 central lumbar spinal canal stenosis. These results were in agreement with the results of Laurencin and his colleagues. They concluded that stenotic ratio used for diagnosis of L3–4, L4–5 and L5–S1 central spinal canal stenosis were 66%, 62% and 73%, respectively .
In this study, severe lateral spinal canal stenosis was diagnosed if the average diameter of both lateral recesses at L3–4, L4–5 and L5–S1 disc levels was equal or less than 1.4 mm, 1.5 mm, 1.45 mm, respectively. These results were in agreement with the study of Steurer and his colleagues. They reported that lateral lumbar recess height of 2 mm or less is diagnostic of lateral lumbar spinal canal stenosis . The study of Strojnik and his colleagues concluded that lateral recess height of 3.6 mm or less is considered the cut-off value used for diagnosis of lateral lumbar spinal canal stenosis .
In this study, severe lumbar foraminal stenosis was diagnosed if the average diameter of both intervertebral foramen at L3–4, L4–5 and L5–S1 disc levels was equal or less than 3.8 mm, 3.25 mm, 3.25 mm, respectively. These results were in agreement with many studies who reported that an intervertebral foramen measurement below the cut-off value of 3 mm is diagnostic of a foraminal stenosis [9, 26].
The present study attempted to fill a gap of knowledge considering the correlation between the commonly used classifications of intervertebral disc degeneration and the degree of the spinal canal and foraminal stenosis. However, the relatively small number of included patients resulted in some categories of classifications, particularly the NASS classification, lacking adequate numbers for carrying out comparisons. This limits the generalization of the current findings to other populations. In addition, due to the retrospective nature of the study, the radiological findings were not correlated with the clinical presentation of the patients and the severity of pain or disability. However, previous studies showed a poor association between the degree of spinal stenosis and clinical findings [31, 32].
In conclusion, Pfirrmann classification showed a good correlation with neural element compromise, reflecting the severity of spinal canal stenosis, foraminal stenosis, and nerve root compression at different levels of the lumbar spine.
There is significant correlation between the degrees of disc displacement in CTF classification of NASS, the qualitative grading and the quantitative measures of central lumbar spinal canal, lateral lumbar spinal canal and lumbar foraminal stenosis for assessment of severity of lumbar spinal canal stenosis.
Further studies with a larger sample size are required to reproduce these findings and assess the association of NASS classification with spinal canal and foraminal stenosis,