Learning the Timeline of Radiographic Features of a Degenerative Spondylolisthesis

A review of literature on the progression of spondylolisthesis—a common diagnosis in older adults suffering from back pain—explores the ailment’s predictive stages and radiographic features associated with each stage in the hope of establishing a reliable clinical timeline in which each phase of the condition presents itself. 

Epidemiology 

When an anatomical displacement of the anterior or posterior vertebra disrupts the stability of the spine, the structure of the vertebral body or arch may be compromised. The condition—whether dysplastic, pathogenic, or traumatic—sets into motion a chronic degenerative process that affects the surrounding protective connective tissue. Degenerative spondylolisthesis most commonly occurs in older adults (50 years plus) and is up to six times more likely in females than in males. However, because it may also be found in six to roughly 20 percent of the general population, genetic or environmental factors may also play a role in its development.  

The lower lumbar vertebrae bear the greatest loads, making them the most vulnerable to degeneration. The cervical and (rarely) thoracic area may also develop spondylolisthesis when the costal architecture surrounding these regions attempts to stabilize. Most commonly, spondylolisthesis occurs at the single level of the L4-L5 vertebrae, with a low incidence of occurrence at L5-S1 and L3-L4 anterior displacement, often in concurrence with L4-L5. The rate of displacement is commonly graded between one-to- five, based on how far the vertebral body has slipped past the subjacent vertebrae, as noted in intervals of 25 percent. Spondylolisthesis is diagnosed at grade five, when the entire vertebral body has slipped beyond the edge of the subjacent vertebrae. 

Different Theories

Typically, high-grade anterior displacement beyond 30 percent does not occur with degenerative spondylolisthesis, and there appears to be little correlation between the progress of degeneration and the degree of pain in patients. While the method of identifying the degree of vertebral deviation is historically valid and effective, it does not take into consideration other common radiographic features, and the condition’s pathophysiology has remained a controversial subject. Some scholars attribute spondylolisthesis to spinal instability stemming from intervertebral disc changes. Others believe that age-related or other changes in the facet joints create a dynamic that eventually leads to vertebral slippage. While this idea is unrefuted, controversy over how much instability and eventual slippage are caused by facet joint degeneration persists, with some attributing the instability to arthritic changes and others emphasizing other contributing factors, such as ligament laxity due to hormonal or other changes. In any case, segmental instability occurs over time due to disc and facet degeneration, and the associated anterior/posterior ligaments and ligamentum flavum mechanical strain eventually leads to displacement. 

Common radiographic markers of lumbar segmental instability include dynamic translation of greater than 3mm on flexion-extension or sagital rotation of greater than 10 percent. The condition, however, may be difficult to detect on supine MRI images, as increased facet joint and ligament laxity allows for a dynamic in which weight-bearing vertebrae may be held in temporary suspension and revert to its proper physiologic position during supine scans. Therefore, it is advisable to compare a neutral standing scan to a supine MRI, rather than a flexion-extension radiograph, when looking for evidence of dynamic instability. 

Roughly one-third of patients may experience a further progression of their lumbar slippage, though even in those suffering from grade one or two spondylolisthesis without fusion who have undergone minimally invasive decompression surgery, the rate of slippage beyond five percent is seen in only 32 percent within two years of the procedure. The motion-limiting nature of intervertebral disc segments protects against degenerative progression, and any severe reduction of joint and disc space will lead, over time, to an auto-fusion of the vertebrae at the facets and endplates, as seen in end-stage spondylolisthesis. 

Radiographic Features Identifying Three Stages of Spondylolisthesis

The three stages of spondylolisthesis—degeneration, instability, and restabilization—are recognizable via six visible radiographic features—facet morphology/arthropathy, facet effusion, facet vacuum, synovial cyst, interspinous ligament bursitis, and vacuum disc. These features provide a reliable timeline of the disease progression. 

• In lumbar facet morphology/arthropathy, best analyzed through CT, the facets are slightly sagitally oriented and appear more coronal approaching the sacrum, and some asymmetry between same-level facets may be observed. This observation holds regardless of sex or ethnicity, though osteoarthritic remodelling due to age may or may not play a role in the noted orientation. The narrowing of joint space, subarticular bone erosions, osteophyte formation, subchondral cysts, and hypertrophy are typical radiographical indicators of facet osteoarthritis and degeneration. Similar risk factors and prevalence of facet arthropathy include increased age and female sex. 

• Facet effusion, characterized by curvilinear hyperintensities within the axial and sagital T2-weighted MR images, is prevalent within roughly 34 percent of the general population. However, a finding of facet fluid may be evidence of degeneration, as high biochemical stress caused by segmental instability affects the facet joints. This may lead to osteoarthritic changes, including synovial fluid within the joint capsule, over time. In the absence of other markers, in fact, the presence of facet fluid may be the only visible radiographic evidence of lower-level spondylolisthesis progression. Facet effusion is rarely observed in higher grades of spondylolisthesis, perhaps an indication of its transient nature and of a later spinal stability achieved as the condition progresses. 

• Facet Vacuum, a benign condition commonly seen on CT, occurs when expansion in the joint creates negative pressure and allows for an accumulation of nitrogen gas within the capsule. Viewed within a widened joint space as a radiolucent, possibly lens-shaped strip, it may be observed along with other osteoarthritic changes bilaterally at the level of spondylolisthesis and is considered a reliable marker of degenerative spondylolisthesis. Though the prevalence of facet vacuum is unknown, the feature is considered unique to degeneration and may be indicative of traction caused by vertebral displacement leading to the accumulation of gas within the joint capsule. 

• Synovial Cysts are rare within the lumbar spine and are found most frequently at L4-L5.  They may stem from the ligamentum flavum or the lining of the facet joints and present anteriorly, leading to neural compression and symptoms, posteriorly, with no neural effects, or both, with a dumbbell shape. Because of the related prevalence of symptoms, anterior cysts are more commonly detected and observed. On MR T2-weighted images—the preferred mode of cyst detection—they appear as limited structures with a hyperintense core. Though their pathogenesis is a topic of debate, visible degenerative changes are evident in almost all patients with facet cysts, and a protrusion of the synovial lining can occur when degeneration of the facet capsule creates increased stress on the associated joints. In patients, radiographic studies have shown a positive correlation between degenerative spondylolisthesis, synovial effusion, and facet cysts. Because there appears to be a lack of correlation between facet cysts and instability, it may be that they form in periods of vertebral instability and persist beyond the point at which natural or surgical restabilization occurs. 

• Interspinous ligament pathological changes are commonly associated with Baastrup’s disease—an orthopedic disorder usually found at level L4-L5 in elderly adults— when osteophytosis occurs at the posterior lumbar spinous processes. The disease is thought to be caused by friction with the adjacent spinous processes during extension. Repeated irritation eventually contributes to the formation of a bursa. Fluid and inflammation within the interspinous ligament is visible as T2 enhancement on MR imaging and may, in the absence of substantial spinous eburnation (which could indicate other pathologies), be an early indicator of segmental instability.

• Vacuum Disc, an accumulation of gas within the intervertebral disc space, is caused by the formation of clefts (due to secondary pathologies) within the disc. The empty space accumulates gas from surrounding tissues and is easily viewed on spine radiographs as radiolucent streaks within the disc. This is most easily viewed in extension, on sagital view, and may be obscured in flexed or neutral positions, making CT or gradient echo MRI the preferred methods of detection. The current literature suggests the phenomenon is a likely predictor of vertebral instability and more predictive of anterior instability when degeneration is present. However, some studies have indicated a posterior-to-anterior progression of degeneration is more common. Evidence suggests there is a positive correlation between the vacuum phenomenon and a progression from instability to restabilization. There is a positive correlation between the wider vacuum distribution, advanced disc degeneration, and disc height narrowing on MRI imaging. 

Conclusion

The process of spondylolisthesis begins with degeneration of intervertebral discs or facet joints and progresses through instability and restabilization. It may be initially marked by the presence of a facet vacuum. Fluid eventually accumulates within the facet joint space as the vertebral segment progresses through various stages of mobility. This effusion may be considered a reliable early indicator of instability, followed by (with further degeneration) facet arthropathy, degenerative disc disease, and posterior ligamentous complex pathology, including cyst(s) and bursitis. In advanced facet osteoarthritis, the effusion may be replaced by a vacuum, which may cause cleft formation and further degeneration. Eventually, an autofusion of the vertebral facets and endplates may be observed. While the review provides insight into a complex phenomenon, it emphasizes that individual patients may present the progression of degenerative spondylolisthesis uniquely and should always be treated and diagnosed with a personalized approach.