Instability
Jeffrey S. Ross, MD
Key Facts
Terminology
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Loss of spine motion segment stiffness, when applied force produces greater displacement than normal, with pain/deformity
Imaging
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Deformity, which increases with motion and over time
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Various parameters used to measure degenerative instability by plain films
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Dynamic slip > 3 mm in flexion/extension
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Static slip of ≥ 4.5 mm
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Angulation > 10-15° suggests need for surgical intervention
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Flexion-extension plain films best for definition of motion
Top Differential Diagnoses
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Pseudoarthrosis
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Infection
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Endplate destruction, disc T2 hyperintensity
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Tumor
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Enhancing soft tissue mass
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Postoperative
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Following multilevel laminectomy or facetectomy
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Pathology
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Degenerative instabilities
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Axial rotational
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Translational; plain films show spondylolisthesis, traction spurs, vacuum phenomenon
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Retrolisthesis; plain films show increased retrolisthesis with extension
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Degenerative scoliosis
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Post laminectomy; resection of 50% of bilateral facets alters segmental stiffness
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Post fusion; altered biomechanics
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TERMINOLOGY
Synonyms
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Spine instability (SI), segmental instability, abnormal spinal motion, degenerative instability
Definitions
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Loss of spine motion segment stiffness, when applied force produces greater displacement than normal, with pain/deformity
IMAGING
General Features
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Best diagnostic clue
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Deformity that increases with motion and over time
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Location
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Any spinal motion segment (composed of 2 adjacent vertebrae, discs, and connecting spinal ligaments)
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Size
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Displacement may vary from few mm to width of vertebral body
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Morphology
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Displacement of vertebral body with respect to adjacent body
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Stabilizing anatomic structures
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Ligaments
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Anterior longitudinal ligament
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Resists hyperextension
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Posterior longitudinal ligament
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Intertransverse ligaments
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Connect neighboring transverse processes
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Interspinous ligaments
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Resist hyperflexion
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Facet capsule
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Ligamentum flavum
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Intervertebral disc
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Main stabilizer of lumbar and thoracic spine
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Muscular attachments
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Both global (rectus and abdominal muscles) and local paraspinal muscle groups
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Radiographic Findings
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Radiography
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Various parameters used to measure degenerative instability by plain films
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Dynamic slip > 3 mm in flexion/extension
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Static slip ≥ 4.5 mm
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Angulation > 10-15° suggests need for surgical intervention
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Traction spurs
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Vacuum phenomenon
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Fluoroscopic Findings
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Increased motion with flexion/extension or translation
CT Findings
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NECT
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Nonspecific findings of degenerative disc disease ± spondylolisthesis
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MR Findings
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T1WI
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Anterolisthesis, retrolisthesis, lateral translation
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Nonspecific changes of degenerative disc disease
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Controversial as to role in defining instability: Type I degenerative endplate changes
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T2WI
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Loss of disc signal ± disc space height
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STIR
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Type I endplate changes may be more evident on this sequence
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T1WI C+
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Nonspecific enhancement of disc due to degenerative disc disease
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Enhancement of type I degenerative endplate changes
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Imaging Recommendations
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Best imaging tool
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Flexion-extension plain films
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Protocol advice
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MR findings useful as secondary tool for degeneration, endplate changes, stenosis, and herniation
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DIFFERENTIAL DIAGNOSIS
Pseudoarthrosis
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Abnormal low T1 signal extending through disc, posterior elements, and ligaments
Infection
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Endplate destruction, disc T2 hyperintensity
Tumor
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Enhancing soft tissue mass
Postoperative
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Following multilevel laminectomy or facetectomy
PATHOLOGY
General Features
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Etiology
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Multifactorial
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Associated abnormalities
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Clinical and imaging relationship of disc degeneration to instability controversial
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Many causes of spinal instability
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Fractures
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Infection (especially anterior column involvement)
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Primary bone and metastatic tumors (vertebral body destruction, neural compression, post resection)
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Isthmic spondylolisthesis (L5-S1 progressive deformity in children)
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Scoliosis
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Degenerative instabilities
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Axial rotational (recurrent pain worse with twisting)
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