Subaxial Subluxation | Degenerative Instability | Myelopathy Risk
- Translation over 3.5mm or angulation over 11 degrees indicates instability on flexion-extension radiographs.
- Degenerative spondylolisthesis most common at C4-C5 and C5-C6 (maximum motion segments).
- Myelopathy develops when canal diameter under 13mm or cord compression on MRI.
- ACDF (anterior cervical discectomy and fusion) is gold standard for single-level degenerative subluxation.
- Bilateral facet dislocation = complete disruption of ALL ligaments = surgical emergency.
- “White-Panjabi criteria: 3.5mm translation or 11 degrees angulation = instability
- “Degenerative subluxation: anterior (anterolisthesis) from facet incompetence
- “Traumatic dislocation: high-energy hyperflexion injury - bilateral facet fracture-dislocation
- “Post-laminectomy kyphosis: iatrogenic from facet disruption during decompression
Clinical Imaging
Imaging Atlas
White-Panjabi: Translation over 3.5mm or angle over 11 degrees. Measured on lateral flexion-extension radiographs. Indicates ligamentous failure requiring fusion.
Canal diameter under 13mm or cord signal change. Sagittal canal narrowing causes myelopathy. MRI shows T2 hyperintensity in cord = myelomalacia.
Bilateral facet dislocation = surgical emergency. Complete ligamentous disruption with high cord injury risk. Requires urgent closed reduction then fusion.
Restore alignment and achieve solid fusion. ACDF for degenerative. Posterior fusion if post-laminectomy. Combined anterior-posterior if severe deformity.
| Clinical Scenario | Imaging | Treatment | Key Pearl |
|---|---|---|---|
| Degenerative, neck pain only | C4-C5 anterolisthesis 2mm, no canal stenosis | Conservative: PT, collar for flares, NSAIDs | Under 3.5mm without cord compression - trial conservative |
| Degenerative with radiculopathy | C5-C6 subluxation 4mm, foraminal stenosis | C5-C6 ACDF | Translation over 3.5mm = instability, ACDF restores alignment |
| Myelopathy, canal stenosis | C4-C5 subluxation, canal 11mm, cord signal | Urgent ACDF or corpectomy with fusion | Canal under 13mm + myelopathy = urgent decompression |
| Bilateral facet dislocation | C6-C7 locked facets, cord compression | Closed reduction MUA, then posterior fusion | Surgical emergency - reduce within 8 hours if cord injury |
ANGELWhite-Panjabi Instability Criteria
Hook:ANGEL criteria = Angulation and Translation thresholds define ligamentous instability!
LOCKEDTraumatic Bilateral Facet Dislocation Features
Hook:Facets are LOCKED = complete ligamentous disruption requiring urgent reduction!
SMITHACDF Surgical Pearls
Hook:SMITH = Standard anterior cervical fusion technique named after Smith-Robinson!
Overview and Epidemiology
Cervical spondylolisthesis is forward (anterolisthesis) or backward (retrolisthesis) displacement of one cervical vertebra on another. Unlike lumbar spondylolisthesis (often isthmic), cervical subluxation is predominantly degenerative (facet and disc degeneration) or traumatic (bilateral facet dislocation from hyperflexion injury).
Epidemiology:
- Degenerative: Most common at C4-C5 and C5-C6 (maximum motion), age 50-70, gradual onset
- Traumatic: Bilateral facet dislocation from MVA, diving injuries (high cervical) - complete ligamentous disruption
- Iatrogenic: Post-laminectomy kyphosis from excessive facet resection (over 50%)
- Gender: Degenerative cases slight female predominance (2:1)
Clinical Impact:
- Radiculopathy: 40% have nerve root compression from foraminal stenosis
- Myelopathy: 15-20% develop cord compression if canal under 13mm
- Instability: Progressive subluxation if ligamentous failure (over 3.5mm translation)
Anatomy
The cervical spinal cord occupies 40-50% of canal diameter. Normal sagittal canal diameter is 17-18mm. Stenosis under 13mm causes cord compression. Under 10mm causes severe myelopathy. Degenerative subluxation narrows canal anteriorly (disc-osteophyte complex) and posteriorly (ligamentum flavum buckling).
Cervical Stability
Stabilizing structures (anterior to posterior):
- Anterior longitudinal ligament (ALL): Limits extension, resists anterior translation
- Intervertebral disc: Axial load distribution, limits rotation
- Posterior longitudinal ligament (PLL): Limits flexion, reinforces posterior disc
- Facet capsules: Resist flexion, rotation, translation (critical for stability)
- Ligamentum flavum: Elastic, prevents buckling into canal
- Interspinous/supraspinous ligaments: Limit flexion
Pathophysiology
Degenerative Pathophysiology
Degenerative cervical spondylolisthesis follows a predictable cascade:
- Disc degeneration initiation: Age-related proteoglycan loss leads to decreased disc height and load-sharing capacity
- Facet joint overload: Reduced disc height transfers axial load posteriorly to facet joints
- Facet arthropathy: Cartilage erosion, synovitis, and capsular laxity develop
- Osteophyte formation: Marginal osteophytes form at disc margins and uncovertebral joints
- Ligamentous failure: Chronic facet capsule incompetence allows segmental translation
- Progressive subluxation: Anterior translation progresses as facets fail to resist shear forces
Traumatic Pathophysiology
- Force vector: Forward deceleration (MVA, diving)
- Structure failure sequence: Posterior ligaments → facet capsules → disc → ALL
- Facet dislocation: Inferior facet of superior vertebra jumps anterior to superior facet of inferior vertebra
- "Perched" facets: Partial dislocation where facet tips are aligned
- "Locked" facets: Complete dislocation with facet interlocking
- Direct compression: Posterior vertebral body edge compresses cord
- Vascular injury: Anterior spinal artery compression causes ischemia
- Contusion: Cord contusion from rapid displacement
- Severity: Bilateral facet dislocation = 60-80% neurologic deficit
- ASIA Grade A: Complete injury, under 5% recovery chance
Neurologic Sequelae
The cervical spinal cord is vulnerable in spondylolisthesis through:
- Dynamic stenosis: Flexion narrows canal anteriorly, extension narrows posteriorly
- Static stenosis: Canal diameter under 13mm causes chronic cord compression
- Myelomalacia: Chronic compression causes irreversible T2 signal change on MRI
Classification and Measurements
White-Panjabi Instability Criteria
| Parameter | Normal | Unstable Threshold | Clinical Implication |
|---|---|---|---|
| Sagittal translation | Under 3.5mm | Over 3.5mm on flexion-extension | Ligamentous failure (facet capsule, PLL) |
| Sagittal angulation | Under 11 degrees | Over 11 degrees between adjacent levels | Facet and disc incompetence |
| Canal diameter | Over 13mm | Under 13mm | High myelopathy risk |
Measured on lateral flexion-extension radiographs. Change over 3.5mm or 11 degrees between flexion and extension indicates instability.
Clinical Presentation
- Neck pain: Axial pain worse with movement (degenerative)
- Radiculopathy: Arm pain, dermatomal paresthesias (foraminal stenosis)
- Myelopathy: Hand clumsiness, gait imbalance, Lhermitte sign
- Trauma history: MVA, diving, fall if traumatic dislocation
- Range of motion: Reduced flexion-extension, pain at extremes
- Spurling test: Radiculopathy if foraminal stenosis
- Myelopathy signs: Hyperreflexia, Hoffmann, Babinski, gait ataxia, inverted radial reflex
- Neurologic level: Sensory/motor deficit corresponds to compressed nerve root or cord
Progressive cervical myelopathy is insidious: hand clumsiness (difficulty with buttons, writing), gait instability (wide-based), bowel/bladder urgency. Once established, myelopathy may not fully reverse. Urgent surgical decompression if cord signal change on MRI.
Differential Diagnosis
| Condition | Key Distinguishing Feature | Imaging Clue | Pitfall |
|---|---|---|---|
| Degenerative cervical spondylolisthesis | Axial neck pain plus radiculo-/myelopathy, age over 50 | Anterolisthesis at C4-C5/C5-C6 with facet arthropathy on flexion-extension XR | Static films may look normal — dynamic instability needs flexion-extension views |
| Traumatic facet dislocation | High-energy trauma, acute deficit | Over 50% translation, naked/perched facet sign, locked facets on CT | Do not mistake unilateral (25-50%) for bilateral (over 50%) — different stability |
| Pseudosubluxation (paediatric) | Child under 8, physiological C2-C3 (or C3-C4) anterior shift | Swischuk line intact, reduces on extension, no soft-tissue swelling | Normal variant — do NOT fuse; reverts with growth |
| Rheumatoid / inflammatory subluxation | Polyarthritis, morning stiffness, often upper cervical | Atlantoaxial subluxation (ADI over 3mm), erosions, basilar invagination | Subaxial 'staircase' subluxation can coexist — image the whole cervical spine |
| Pathological (tumour / infection) | Night pain, constitutional symptoms, fevers | Vertebral body or facet destruction, marrow signal change, abscess on MRI | Translation here reflects bone loss, not ligament failure — biopsy/cultures first |
| Isolated cervical radiculopathy (no slip) | Dermatomal arm pain without translation | Foraminal disc-osteophyte but stable alignment on dynamic films | Decompression alone may suffice — do not over-fuse a stable segment |
Investigations
Imaging Protocol
Lateral C-spine: Neutral, flexion, extension. Measure: Anterior translation, sagittal angle change. White-Panjabi thresholds (3.5mm, 11 degrees). AP: Alignment, spinous process deviation.
Facet joints: Fracture, arthritis, subluxation. Canal diameter: Sagittal measurement. Trauma: Detect occult fractures missed on XR.
Cord compression: Degree and level. T2 signal change: Hyperintensity = myelomalacia (poor prognosis). Foraminal stenosis: Nerve root compression. Disc herniation: Contribution to canal stenosis.
Draw lines along posterior vertebral body cortex of superior and inferior vertebrae. Measure horizontal distance between lines. Over 3.5mm = pathological translation. Compare flexion and extension views - dynamic instability if difference over 3.5mm.
Management Algorithm
Conservative Management
Indications: Translation under 3.5mm, no myelopathy, minimal radiculopathy, patient preference.
Conservative Protocol
NSAIDs: Naproxen 500mg BD. Collar: Soft collar for comfort (under 2 weeks - avoid prolonged use). Activity modification: Avoid extreme flexion-extension.
Physiotherapy: Gentle ROM, avoid forceful manipulation. Strengthening: Deep neck flexors, scapular stabilizers. Ergonomics: Neutral cervical posture.
Repeat imaging: Flexion-extension XR if symptoms worsen. Surgical threshold: Progressive translation, new neuro signs, refractory pain.
Surgical Technique
Anterior Cervical Discectomy and Fusion
ACDF Steps
Supine, shoulder roll, neck extension. Right-sided approach (avoids recurrent laryngeal nerve injury - RLN courses in tracheoesophageal groove on left).
Transverse skin incision at disc level. Blunt dissection between sternocleidomastoid laterally and strap muscles medially. Retract carotid sheath laterally, esophagus/trachea medially. Identify disc by needle localization and fluoroscopy.
Remove disc with rongeurs, curettes. Decompress anterior spinal canal (remove posterior osteophytes, PLL if cord compression). Uncovertebral joints: Decompress bilaterally for foraminal stenosis.
Endplate preparation: Remove cartilage to bleeding bone. Cage insertion: PEEK or titanium cage packed with bone graft. Plate fixation: Anterior cervical plate with screws into vertebral bodies (prevents subsidence).
Recurrent laryngeal nerve risk 1-2%. Higher on left side (RLN loops around aortic arch). Use right-sided approach when possible. Avoid excessive medial retraction of trachea/esophagus. Neuromonitoring can detect RLN at risk.
Complications
| Complication | Incidence | Management |
|---|---|---|
| Dysphagia (ACDF) | 10-15% transient | Usually resolves 6-12 weeks, speech therapy if persistent |
| RLN injury (ACDF) | 1-2% | Hoarse voice, vocal cord medialization if permanent |
| Pseudarthrosis (ACDF) | 5% single level, 15% multilevel | Revision ACDF with bone graft |
| C5 palsy (posterior) | 5-10% | Deltoid/biceps weakness, usually recovers over 6 months |
| Subsidence (ACDF) | 10-15% | Cage sinks into endplate - plate prevents progression |
Controversies and Areas of Uncertainty
The classic teaching is to obtain MRI before reducing a facet dislocation to exclude a disc herniation that could be driven into the cord. Yet Grant (1999) and Vaccaro (1999) found awake closed reduction neurologically safe even when herniations were present. Most centres now reduce awake, cooperative patients promptly without waiting for MRI, reserving pre-reduction MRI for obtunded/unexaminable patients.
For stable Grade I anterolisthesis (under 3.5mm, no myelopathy) causing only radiculopathy, whether to decompress alone or add fusion is debated. Adding fusion treats potential instability but raises adjacent-segment disease risk (Hilibrand: ~25% at 10 years). Many surgeons reserve fusion for documented dynamic instability.
For multilevel disease the Lawrence/Brodke review found no clear winner. Choice hinges on number of levels, sagittal alignment (anterior/laminoplasty preferred in lordosis; anterior or combined for kyphosis), and where compression predominates — not on a single 'best' operation.
Methylprednisolone after acute SCI (NASCIS protocols) is no longer routinely recommended in most guidelines because of marginal benefit and complication risk. Use is now optional and protocol-dependent, not standard of care.
Guidelines, Registries & Global Practice
Global epidemiology
- Degenerative cervical myelopathy (DCM), the main morbidity of degenerative subluxation, is the commonest cause of non-traumatic spinal cord dysfunction in adults worldwide, with rising prevalence in ageing populations.
- Degenerative slips cluster at the maximum-motion segments C4-C5 and C5-C6; traumatic dislocations cluster lower, at C5-C6 and C6-C7.
- OPLL (ossification of the posterior longitudinal ligament) is a more frequent contributor to canal stenosis in East Asian populations, shifting practice toward posterior decompression/laminoplasty there.
Side-by-side guideline positions
| Body | Emphasis | Practical Recommendation |
|---|---|---|
| AOSpine / CSRS (international) | Degenerative cervical myelopathy | Surgical decompression for moderate-severe DCM; offer surgery or supervised rehab for mild disease — avoid passive observation of progression |
| AAOS / NASS (US) | Evidence-based diagnosis and surgical selection | Decompression effective for myelopathy; approach individualised to alignment and compression pattern |
| BOA / NICE-aligned (UK) | Timely referral pathways | Early specialist referral for suspected myelopathy; dynamic radiographs to confirm instability before fusion |
| AO Foundation (trauma) | Subaxial injury classification and reduction | SLIC/AO subaxial system to guide operative vs non-operative; urgent reduction of dislocations with deficit |
Registry and outcome evidence
- Large spine registries and the prospective STASCIS cohort support decompression within 24 hours for traumatic cervical SCI.
- Anterior cervical fusion registry data corroborate Hilibrand's ~25% adjacent-segment disease at 10 years, informing consent and the case for motion-preserving options (arthroplasty, laminoplasty) in selected patients.
High- vs limited-resource practice variation
- High-resource: rapid MRI, intra-operative neuromonitoring, navigation/robotics for lateral mass and pedicle screws, and arthroplasty as a motion-sparing alternative for single-level disease.
- Limited-resource: reliance on plain dynamic radiographs and CT; Gardner-Wells closed traction reduction remains a vital, low-cost, evidence-supported first step for dislocations; structural autograft and posterior wiring substitute where modern implants are scarce.
Evidence Base and Key Trials
Early Closed Reduction of Cervical Subluxation Is Safe
- 82 patients with subaxial subluxation (locked facets, burst, extension injuries) reviewed
- Early rapid closed reduction with Gardner-Wells traction successful in 97.6%, mean 2.1 hours
- Disc herniation (22%) or disruption (24%) on post-reduction MRI did NOT affect neurologic recovery
- Only 1 of 80 patients (1.3%) deteriorated, and that occurred over 6 hours after reduction
MRI Before and After Awake Closed Traction Reduction
- Prospective consecutive series of 11 patients with cervical dislocation, pre- and post-reduction MRI
- Disc herniation present in 2 of 11 before reduction; rose to 5 of 9 after successful reduction
- Awake closed traction reduction succeeded in 9 of 11 patients
- NO patient experienced neurologic worsening after awake reduction
STASCIS: Early vs Delayed Decompression in Cervical SCI
- Multicentre international prospective cohort, 313 acute cervical SCI patients
- Early (under 24h) vs late (24h or more) decompression compared
- Two-grade-or-more AIS improvement at 6 months: 19.8% early vs 8.8% late
- Adjusted odds of major AIS improvement 2.8x higher with early surgery; complication rates similar
Adjacent-Segment Disease After Anterior Cervical Arthrodesis
- 374 patients (409 anterior cervical arthrodeses) followed up to 21 years
- Symptomatic adjacent-segment disease at constant 2.9% per year; 25.6% within 10 years
- Highest risk at C5-C6 and C6-C7; single-level fusion carried HIGHER adjacent-level risk than multilevel
- Over two-thirds of new adjacent-segment disease required further surgery
Anterior vs Posterior Approach for Multilevel CSM
- Systematic review of comparative studies in multilevel cervical spondylotic myelopathy
- JOA-score improvement similar between anterior and posterior approaches
- C5 palsy rates similar; infection lower with anterior, dysphagia lower with posterior
- Strong recommendation for an individualised, pathoanatomy-driven approach
Long-Term Results of Open-Door Laminoplasty
- 204 patients, unilateral open-door laminoplasty, mean 8-year follow-up
- Overall JOA recovery rate 62.1%; muscle weakness (C5-type palsy) in 7.8%
- Better recovery with age under 60 and symptom duration under 1 year
- Recovery similar across spondylosis, OPLL and disc herniation
White-Panjabi Clinical Instability Criteria (Textbook)
- Checklist-based definition of subaxial cervical instability
- Sagittal translation over 3.5mm = ligamentous failure
- Sagittal-plane rotation over 11 degrees relative to adjacent level = instability
- Derived from sequential cadaveric ligament-sectioning experiments
AOSpine Guidelines for Degenerative Cervical Myelopathy
- GRADE-based international guideline for degenerative cervical myelopathy (DCM)
- Surgical decompression recommended for moderate and severe DCM
- Offer surgery or supervised rehabilitation for mild DCM; do not just observe progressive disease
- Standardised, multidisciplinary, evidence-to-recommendation methodology
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 60-year-old presents with 6 months of progressive right arm pain radiating to thumb and index finger. Examination shows 4/5 biceps weakness, reduced biceps reflex, and positive Spurling sign. Flexion-extension radiographs show C5-C6 anterolisthesis of 4mm. MRI shows C5-C6 disc herniation and foraminal stenosis compressing C6 nerve root. How do you manage?”
“A 25-year-old presents after diving accident. He has complete C6 motor and sensory level with absent lower limb reflexes (spinal shock). Lateral C-spine shows C6-C7 bilateral facet dislocation with 60% anterior translation. How do you manage acutely?”
“A 68-year-old reports 12 months of clumsy hands, dropping objects, and an unsteady wide-based gait. Examination shows hyperreflexia, positive Hoffmann and Babinski signs, and an inverted radial reflex. Flexion-extension radiographs show C4-C5 anterolisthesis of 3mm with degenerative changes. MRI shows multilevel canal stenosis maximal at C4-C5 with T2 cord signal change. How do you assess and manage this patient?”
MCQ Practice Points
Q: What sagittal translation threshold indicates cervical instability? A: Over 3.5mm on lateral flexion-extension radiographs. Measured between adjacent vertebral bodies. Indicates ligamentous failure (facet capsule, PLL).
Q: What angular change between adjacent cervical vertebrae indicates instability? A: Over 11 degrees on flexion-extension radiographs. Measured as sagittal plane angle change between superior and inferior endplates.
Q: What is the most common level for degenerative cervical spondylolisthesis? A: C4-C5 and C5-C6 - these are maximum motion segments in the cervical spine, subject to highest biomechanical stress.
Q: What imaging finding confirms bilateral facet dislocation? A: Naked facet sign on lateral XR - inferior facet of superior vertebra is anterior to superior facet of inferior vertebra. Translation typically over 50% vertebral body width.
Q: What degree of translation is associated with complete spinal cord injury? A: Over 50% vertebral body width (bilateral facet dislocation). Unilateral facet dislocation (25-50% translation) has incomplete injury risk. Translation under 25% rarely causes cord injury.
Q: When is MRI indicated before closed reduction of cervical dislocation? A: Controversial - traditionally if patient is obtunded/unexaminable. However, newer evidence (Vaccaro study) supports early closed reduction in awake patients regardless of MRI, as traction injury from delay may exceed disc herniation risk.
Instability Criteria
- White-Panjabi: Translation over 3.5mm or angle over 11 degrees
- Measured on lateral flexion-extension radiographs
- Canal diameter under 13mm = high myelopathy risk
- Bilateral facet dislocation = complete ligament disruption
Common Levels
- Degenerative: C4-C5 and C5-C6 (maximum motion)
- Traumatic BFD: C5-C6 and C6-C7
- Post-laminectomy: any level with over 50% facet resection
- Meyerding grade: I (0-25%), II (25-50%), III (50-75%), IV (75-100%)
Surgical Indications
- Myelopathy with cord compression (MRI T2 signal change)
- Translation over 3.5mm or angle over 11 degrees with symptoms
- Progressive neurologic deficit despite conservative management
- Bilateral facet dislocation (trauma) = surgical emergency
ACDF Technique
- Right-sided Smith-Robinson approach (avoid RLN on left)
- Complete discectomy, posterior osteophyte removal
- Decompress uncovertebral joints for foraminal stenosis
- PEEK cage + autograft + anterior plate (prevents subsidence)
Complications
- Dysphagia 10-15% (transient, resolves 6-12 weeks)
- RLN injury 1-2% (hoarse voice, higher left-sided approach)
- Pseudarthrosis 5% single level, 15% multilevel
- C5 palsy 5-10% (posterior approach, deltoid/biceps weakness)