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Segmental Instability

Segmental instability refers to the loss of the normal pattern of motion between two adjacent vertebrae, leading to abnormal movement under physiological loads. It typically involves:

It is often evaluated using dynamic flexion-extension radiographs, and is a key concept in conditions like:

  • Degenerative spondylolisthesis
  • Isthmic spondylolysis
  • Post-laminectomy instability

The diagnosis of segmental instability involves a combination of clinical assessment and imaging studies.

  • Mechanical back or neck pain exacerbated by movement or prolonged posture
  • Possible radiculopathy or neurogenic claudication
  • Sensation of “giving way” or spinal locking/unlocking
  • Instability catch or painful arc on motion

1. Dynamic Radiographs (Flexion–Extension X-rays)

  • Lumbar spine:
    • > 4 mm of translation
    • > 10–15° of angular motion (L1–L5), > 20° at L5–S1
  • Cervical spine:
    • > 3.5 mm of translation
    • > 11° of angular motion between adjacent vertebrae

2. MRI

  • Disc degeneration or high-intensity zone (HIZ)
  • Facet joint effusion (correlates with instability)
  • Ligamentous disruption (e.g., interspinous ligament)

3. CT Scan

  • Pars defects (in spondylolysis)
  • Osteophytes or vacuum phenomena (suggest segmental hypermobility)

4. Functional Tests

  • Standing vs supine MRI
  • Upright dynamic MRI (where available)

Instability is diagnosed when there is:

  • Abnormal segmental motion beyond physiological limits
  • Correlation with clinical symptoms
  • Structural or dynamic evidence of failure of spinal stabilizers
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