Degenerative spinal instability

Degenerative spinal instability is frequent although somewhat controversial clinical entity with evolving theories on its clinical relevance, in particular, its role as a causative factor of low back pain (LBP). While radiographic instability may be evident, this does not always correlate with the clinical presentation. 

Degenerative spinal instability can occur anywhere in the vertebral column; however, this article focuses on lumbar degenerative spinal instability. 

Clinical presentation

Degenerative spinal instability may present with low back pain, radiculopathy and/or neurogenic claudication in some patients whereas in others it may be asymptomatic ^1-3.

Pathology

Degenerative spinal instability, although associated with low back pain, is controversial as a causal factor with improving surgical fusion rates not leading to better clinical outcomes. No strong relationship has been made between imaging-demonstrated instability and pain and might be more related to abnormal load distribution ³. 

Etiology

A prominent theory on the pathogenesis of spinal instability was proposed by Kirkaldy Willis ^1,2,4 with three phases considered:

• dysfunction phase
  • occasional LBP
  • no or minimal degenerative change
• instability phase
  • more frequent LBP becoming chronic
  • degenerative change evident on imaging including intervertebral disc spacing narrowing, disc degeneration, facet joint arthropathy
  • anterolisthesis, retrolisthesis and/or laterolisthesis
• restabilisation phase
  • reduced mobility and stiffness, often with reduced low back pain
  • compensatory remodeling is evident including marginal osteophytes, hypertrophic change of the facet joints, endplate sclerosis

Associations

• lumbar canal stenosis can be developed not only from degenerative disc disease but also from compensatory ligamentous hypertrophy secondary to abnormal loads and spinal movements as well as hypertrophic facet joints ¹
• degenerative scoliosis can occur when lumbar degeneration is asymmetrical ¹

Radiographic features

While features of instability can be seen on plain radiographs, CT and MRI, dynamic flexion/extension lumbar radiographs are still considered the gold standard ⁴. 

General features of instability include anterolisthesis or retrolisthesis with associated disc and facet joint degeneration although the relationship between imaging findings, instability and their clinical relevance is yet to be fully established ³. In instability, disc height may be preferentially preserved with increasing loss of disc height associated with less instability, which may reflect the “restabilisation phase” ⁵. 

Plain radiograph

Lumbar instability is generally accepted to be present if one of the following can be found on dynamic flexion/extension lateral radiographs:

• increased sagittal translation (thresholds between ≥3 to 5 mm ^3-6
• slip percentage of ≥8% ^4,7
• change in segmental angulation >10º ^5,6, or ≥20º on flexion or extension with ≥5º posterior opening ⁷

MRI

Features suggesting instability include ^3,6:

• Modic type 1 endplate change, especially when involving 25-50+% of the vertebral body
• bone marrow edema at the peduncle and isthmus
• severe facet joint degeneration with effusion and >1 mm articular gap
• annular fissure

These changes are not diagnostic of instability on a supine MRI but are suggestive and correlation with dynamic flexion/extension lumbar radiographs has to be considered ^3,6.