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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:
- 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
Diagnosis
The diagnosis of segmental instability involves a combination of clinical assessment and imaging studies.
🧠 Clinical Features
- 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
📸 Imaging Criteria
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)
🧪 Diagnostic Criteria
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