Chronic lower back pain is a common cause of disability and missed work. Over the past three decades, epidemiological studies have shown that it is the leading cause of years lived in the US with disability, and it is a tremendous economic burden on society. It affects up to 84% of the adult population.11 Lower back pain can stem from different etiologies including muscular, ligamentous, or neurologic pain, as well as the more difficult to diagnose discogenic or facetogenic pains. Unless there are red flags such as fevers, chills, increased age, a cancer diagnosis, or focal neurologic symptoms, no imaging is recommended for lower back pain for at least 6 weeks.
When conservative treatment has failed, or imaging is recommended, MRI is the imaging modality of choice.12 Neurologic symptoms may stem from causes such as disc herniations or spinal stenosis from bony or ligamentous hypertrophy. Once a clear radiographic finding correlating with symptoms is established, and all conservative measures including pain medications, physical therapy, and steroid injections have been exhausted, the patient may decide to pursue surgical options, such as a discectomy or spinal decompression surgery. At this point, surgeons often use CT imaging to better understand the bony anatomy to help them prepare for surgery.13 Traditionally, lateral flexion-extension radiographs are also utilized to determine if any anterior-posterior dynamic subluxation of adjacent vertebrae is present.14 If so, this may indicate vertebral instability and potentially necessitates the need for a spinal fusion with rods and screws or interbody devices in addition to spinal decompression (i.e. laminectomy).
With lower back pain causing such a societal burden, it is important to continue to discover the nuances of spinal radiology and surgery in order to refine the diagnosis and treatment of lumbar spine disorders. Recently, there has been much interest in finding validated radiographic indexes or direct measures to better measure vertebral instability pre-operatively. Examples of this include using facet joint effusion on MRI to determine translational or rotational instability. Another potential method involves using the Quantitative Stability Index based on MRI facet- fluid and flexion-extension radiographs. Still, even more potential methods exist, such as defining the Neutral Zone, which is the magnitude of residual displacement after loading and unloading a vertebral segment in a certain direction and waiting 30 seconds.15–17 In this same vein, we have looked at the presence of VP in the lumbar spine as a potential indicator of vertebral instability.
VP was first reported within IVD’s by Magnusson in 1937.18 In 1942, Knuttson stated that VP was due to disc degeneration.19 The gaseous make-up within the discs was analyzed by Ford et al. in 1977 and determined to be approximately 92% nitrogen, along with oxygen, carbon dioxide, and other trace gases.7,9 VP resurfaced in literature in the 1980’s and was thought to be caused by multiple etiologies including trauma, infection, malignancy, and degenerative changes,4 but multiple studies have demonstrated that its presence is largely due to end stage degenerative changes.20 Its presence in literature has been increasing in recent years, and it may be more than simply an incidental and anecdotal finding.
Past studies have looked for a correlation between IVD VP presence and vertebral instability. Lin et al. focused on surgical outcomes in patients with same segment spondylolisthesis and IVD VP presence.8 They compared two different types of spondylolisthesis, isthmic and degenerative (IS) and (DS). While there was a larger presence of VP in IS patients compared to DS patients, the fusion rate post-operatively was also higher in IS patients. Therefore, this study concluded that IVD VP presence was not directly correlated with instability. In another study, Liao et al. also studied surgical outcomes in same segment spondylolisthesis and VP.21 In their study, they treated patients surgically with posterolateral fusion either with or without the addition of an interbody device. Less translation in the patients with interbody devices led them to conclude that VP presence at the same segment of spondylolisthesis, does indicate vertebral instability. These conflicting results led us to research if certain variables, such as size and shape, of VP may play a role in instability.
A 2018 classification system designed by Murata et al. demonstrated that morphologic characteristics on sagittal CT and location on axial CT may be more closely associated with degenerative changes or disc height loss on MRI. This was the first time VP was classified on CT and may help radiologists determine the nuances of VP to gain a better understanding of when to mention VP in their readings. Morphology on CT images was separated into three categories: spot (less than 2 mm), island (greater than 2 mm), and linear. Larger island VP’s greater than 2 mm were significant indicators of decreased disc height. Both island and linear VP’s were significant indicators of high-grade degenerative disc changes on MRI.3 Due to the fact that most patients undergoing spine surgery have received CT imaging and lateral flexion-extension x-rays for pre-operative surgical guidance, we decided to retrospectively look for the presence of VP on sagittal CT.22 and use the Murata et al. morphologic component of the classification system of island (less than 2 mm), spot (greater than 2 mm), and linear morphologies to determine which morphology is correlated with increased dynamic subluxation or loss of disc height on lateral flexion-extension x-rays. Furthermore, we performed these analyses between patients who have and who have not had previous lumbar surgery.
We hypothesized that the larger sized island and linear VP’s would have larger amounts of subluxation and decreased ADH and PDH compared to the smaller spot VP’s. After performing a statistical analysis, we in fact, did not find a significant difference in any of these parameters during a 3- way comparison of the morphologies. There may be multiple reasons for this finding. First, by strictly adhering to the categorical classification system, we did not separate within island or linear VP based on their size. For example, a 3 mm island VP was considered equal to a 20 mm island VP. A tiered classification distinguishing between smaller, medium, and larger sizes of island and linear VP may have provided more clarity. Second, while attempting to elucidate if one or two VP morphologies were more likely to cause vertebral instability, we compared the three VP groups to themselves instead of comparing each individual VP group against instability itself. In other words, studies have defined a subluxation of 2-4.5 mm as vertebral instability.23,25 If the subluxation measurements of each group would have been compared to these parameters, they very well could have indicated instability. Our overall goal was to determine if certain morphologies were better at determining instability, though, we failed to prove this. Third, one may find a multitude of spine literature questioning the validity of dynamic flexion-extension x-rays in determining vertebral instability.17,24−26 We decided to use this radiographic option as it is still traditionally used by spine surgeons in determining instability pre-operatively, and these x-rays are usually available in the radiographic records of most patients with a history of a spine CT. Lastly, our study may have lacked sufficient power due to sample size. A larger retrospective study may have allowed the statistical analysis to reach significance when comparing between the three groups.
An interesting finding from this study is that there was a significant difference between the VP size of spot and linear morphology between patients who had never had lumbar fusion surgery and those who had a lumbar fusion surgery. This is further evidence that VP is associated with degenerative changes because decompressive surgery decreased the size of VP. Furthermore, fusion and the improvement in segmental stability leading to a decreased VP size may indicate a correlation between VP size and instability. Island VP size was not significantly decreased following fusion, and this may, again, be due to the wide spectrum of sizes assigned to the island classification under this CT classification system. Further dissection between small, medium, and large sized circular-shaped VP may provide a more helpful classification system when determining vertebral instability.
Another interesting finding from this study was that there was a significant difference between ADH and PDH on flexion and extension island VP x-rays, and extension linear x-rays, while there was no difference between ADH and PDH on any spot x-rays. This may suggest that increased VP size may lead to less uniform discs, again making the case for VP and disc degeneration and potential instability. Further studies on this subject with the addition of MRI grading of disc degeneration is recommended.
Limitations to our study include the lack of a validated gold-standard metric for determining vertebral instability. This may limit the generalizability of our study, but it does not limit the within-study comparative studies. Furthermore, measurements were taken by individuals of different levels of training (2 medical students, 1 orthopedic surgery resident, and 1 neuroradiologist) and intra- and inter-observer reliability were not statistically analyzed. Although, intra-and inter- observer reliability were found to have sufficient kappa values in the Murata et al. study with two orthopedic surgeons. Our study only analyzed 1 part of the 2-part Murata classification system by looking strictly at VP size and morphology and neglecting the location within the IVD on axial CT images. This limited our CT readings to a 2-dimensional analysis, whereas the Murata et al. classification system was 3-dimensional. This was done intentionally as we hoped to focus on one aspect of the classification system in determining its utility, and to potentially see how it may be improved upon. Another consideration that would allow future studies to expand on our research is to factor in the surgical outcomes as the Lin et al. and Liao et al. studies mentioned earlier did. This may factor in possible confounding variables that may be present in our study. For instance, the Lin et al. study determined that not only did IS patients have more VP, but that they had less dynamic subluxation and higher fusion rates than DS patients. This may be due to IS and DS resulting from different mechanisms. Our study also did not factor in the age, sex, or comorbid conditions of the patients. Lastly, as mentioned previously, our statistics may have been affected by a lack of power due to insufficient sample size. Still, this retrospective study provided a follow-up to the Murata et al. CT classification paper, and our data highlights potential improvements that may be made to the morphologic portion of the system.