Instability

Jeffrey S. Ross, MD

Key Facts

Terminology

Loss of spine motion segment stiffness, when applied force produces greater displacement than normal, with pain/deformity

Imaging

Deformity, which increases with motion and over time
Various parameters used to measure degenerative instability by plain films
- Dynamic slip > 3 mm in flexion/extension
- Static slip of ≥ 4.5 mm
- Angulation > 10-15° suggests need for surgical intervention
Flexion-extension plain films best for definition of motion

Top Differential Diagnoses

Pseudoarthrosis
Infection
- Endplate destruction, disc T2 hyperintensity
Tumor
- Enhancing soft tissue mass
Postoperative
- Following multilevel laminectomy or facetectomy

Pathology

Degenerative instabilities
- Axial rotational
- Translational; plain films show spondylolisthesis, traction spurs, vacuum phenomenon
- Retrolisthesis; plain films show increased retrolisthesis with extension
- Degenerative scoliosis
- Post laminectomy; resection of 50% of bilateral facets alters segmental stiffness
- Post fusion; altered biomechanics

(Left) Wide laminectomy destabilizing spine is shown. Flexion lateral radiograph demonstrates grade 2 anterolisthesis

at L3-4 that improves on extension (see next image). (Right) Lateral radiograph in extension shows grade 2 anterolisthesis, which is mildly reduced

compared to the flexion deformity. Note the widening of the anterior disc space with extension

(Left) Sagittal T1WI MR in an adolescent shows severe spondylolisthesis of L5 on S1, with disc degeneration and extensive type I degenerative endplate changes

. (Right) Sagittal T1WI C+ MR shows progressive deformity and retrolisthesis

at operative site following extensive posterior debridement for infection, marked thecal sac compression from the deformity and residual phlegmon

, and extensive dorsal pseudomeningocele

TERMINOLOGY

Synonyms

Spine instability (SI), segmental instability, abnormal spinal motion, degenerative instability

Definitions

Loss of spine motion segment stiffness, when applied force produces greater displacement than normal, with pain/deformity

IMAGING

General Features

Best diagnostic clue
- Deformity that increases with motion and over time
Location
- Any spinal motion segment (composed of 2 adjacent vertebrae, discs, and connecting spinal ligaments)
Size
- Displacement may vary from few mm to width of vertebral body
Morphology
- Displacement of vertebral body with respect to adjacent body
Stabilizing anatomic structures
- Ligaments
  - Anterior longitudinal ligament
    - Resists hyperextension
  - Posterior longitudinal ligament
  - Intertransverse ligaments
    - Connect neighboring transverse processes
  - Interspinous ligaments
    - Resist hyperflexion
  - Facet capsule
  - Ligamentum flavum
- Intervertebral disc
  - Main stabilizer of lumbar and thoracic spine
- Muscular attachments
  - Both global (rectus and abdominal muscles) and local paraspinal muscle groups

Radiographic Findings

Radiography
- Various parameters used to measure degenerative instability by plain films
  - Dynamic slip > 3 mm in flexion/extension
  - Static slip ≥ 4.5 mm
  - Angulation > 10-15° suggests need for surgical intervention
- Traction spurs
- Vacuum phenomenon

Fluoroscopic Findings

Increased motion with flexion/extension or translation

CT Findings

NECT
- Nonspecific findings of degenerative disc disease ± spondylolisthesis

MR Findings

T1WI
- Anterolisthesis, retrolisthesis, lateral translation
- Nonspecific changes of degenerative disc disease
- Controversial as to role in defining instability: Type I degenerative endplate changes
T2WI
- Loss of disc signal ± disc space height
STIR
- Type I endplate changes may be more evident on this sequence
T1WI C+
- Nonspecific enhancement of disc due to degenerative disc disease
- Enhancement of type I degenerative endplate changes

Imaging Recommendations

Best imaging tool
- Flexion-extension plain films
Protocol advice
- MR findings useful as secondary tool for degeneration, endplate changes, stenosis, and herniation