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:

• Excessive translation or angulation of one vertebra relative to another
• Failure of stabilizing structures, including intervertebral discs, ligaments, facet joints, and musculature
• Symptoms may include mechanical back pain, neurological compression, or spinal deformity

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

• 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

To assess segmental instability radiographically, dynamic flexion–extension X-rays are performed. One important metric is angular motion between vertebral segments.

• L1–L5: > 10–15° of angular motion between flexion and extension
• L5–S1: > 20° of angular motion

Assume flexion and extension lateral radiographs show the following angles:

• Draw lines along the endplates of adjacent vertebral bodies (e.g., L4 inferior endplate and L5 superior endplate)
• Measure the angle formed in flexion and extension
• Subtract to get the range of angular motion

The threshold values may vary slightly by source, but generally:

• > 15° at L4–L5 or above is considered unstable
• > 20° at L5–S1 accounts for the normally greater mobility at this junction

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

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

• 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