ATLANTOAXIAL ARTHRITIS
C1-C2 Degeneration | Occipitocervical Pain | Rotatory Loss | C1-C2 Fusion
Grisel Classification
Critical Must-Knows
- C1-C2 provides 50% of cervical rotation - arthritis causes profound functional loss
- Rheumatoid arthritis is most common inflammatory cause (30-40% prevalence)
- ADI over 3.5mm in adults = transverse ligament incompetence = instability
- SAC under 13mm = high myelopathy risk = surgical threshold
- Harms technique (C1 lateral mass + C2 pedicle screws) is gold standard fusion
Examiner's Pearls
- "Atlantoaxial joint: 50% rotation, 10% flexion-extension contribution
- "Transverse ligament is primary stabilizer - RA pannus causes erosion
- "Dynamic flexion-extension radiographs essential for diagnosis
- "Vertebral artery runs lateral to C1 lateral mass and through C2 pedicle
Clinical Imaging
Imaging Gallery
Critical Atlantoaxial Arthritis Exam Points
Functional Anatomy
C1-C2 = 50% cervical rotation. Odontoid acts as pivot. Arthritis or instability threatens cord and brainstem. Loss of rotation profoundly affects quality of life.
Rheumatoid Link
30-40% of RA patients develop C1-C2 involvement. Pannus erodes transverse ligament. Annual flexion-extension cervical radiographs mandatory for RA screening.
Instability Criteria
ADI over 3.5mm = pathologic in adults. SAC (space available for cord) under 13mm = myelopathy risk. Dynamic imaging shows progression.
Surgical Principles
C1-C2 fusion for instability or refractory pain. Harms technique (C1 lateral mass + C2 pedicle screws). 90% fusion rate. Accept 50% rotation loss.
Quick Decision Guide
| Clinical Scenario | Imaging Findings | Treatment | Key Pearl |
|---|---|---|---|
| Early degeneration, no neuro signs | Joint space narrowing, ADI under 3mm | NSAIDs, collar for flares, physiotherapy | Trial conservative 3-6 months with repeat imaging |
| Refractory pain, borderline instability | ADI 3.5-5mm, SAC over 13mm, no myelopathy | Consider C1-C2 fusion | Counsel about 50% rotation loss vs continued pain |
| RA patient with myelopathy | ADI over 5mm, SAC under 13mm | Urgent occiput-C2 fusion | Myelopathy may not fully reverse - do not delay |
TADSAtlantoaxial Instability Assessment
Memory Hook:TADS = Think Atlantoaxial Dynamic Stability - assess these 4 parameters!
VABSRheumatoid Cervical Patterns
Memory Hook:VABS = Vertebral Abnormalities in Biological Syndromes - RA cervical patterns!
PSRFHarms Technique Key Steps
Memory Hook:PSRF = Posterior Screw-Rod Fusion - the 4 pillars of Harms technique!
Overview and Epidemiology
Atlantoaxial arthritis encompasses degenerative or inflammatory changes affecting the C1-C2 articulation. The atlantoaxial joint is unique: it provides approximately 50% of total cervical rotation and 10% of flexion-extension, yet bears minimal axial load. This high-mobility, low-load environment makes primary osteoarthritis rare, with inflammatory arthritis (especially rheumatoid) being the predominant etiology.
Epidemiology:
- Inflammatory: Rheumatoid arthritis accounts for 80% of inflammatory atlantoaxial disease; 30-40% of RA patients develop C1-C2 involvement
- Degenerative: Primary osteoarthritis uncommon; secondary degeneration follows odontoid fracture nonunion or os odontoideum
- Age: Degenerative cases peak at 60-70 years; RA cases follow systemic disease onset
- Gender: Female greater than male (2:1) in RA-related cases
Clinical Impact:
- Rotation loss: Up to 50% cervical rotation deficit impairs daily activities (driving, checking blind spots)
- Pain: Chronic occipitocervical pain in 90% of symptomatic cases
- Myelopathy: 5-10% develop spinal cord compression without treatment
- Surgical need: 15-20% of RA atlantoaxial patients require fusion
Anatomy
Critical Anatomical Relationships
The atlantoaxial complex is immediately adjacent to the cervicomedullary junction. Posterior subluxation or superior odontoid migration can directly compress the brainstem, causing respiratory compromise and quadriplegia. The vertebral arteries traverse the C2 pedicles and C1 lateral masses - screw fixation requires precise trajectories.
Atlantoaxial Anatomy
Three synovial articulations:
- Median atlantoaxial joint: Odontoid process (C2 dens) and anterior arch of C1
- Lateral atlantoaxial joints (bilateral): Inferior C1 facets and superior C2 facets
- Posterior membrane: Connects posterior C1 arch to C2 lamina
Stabilizing ligaments:
- Transverse ligament: Primary stabilizer, spans C1 lateral masses posterior to dens, prevents anterior C1 translation
- Alar ligaments (bilateral): Connect dens to occipital condyles, limit rotation and lateral flexion
- Apical ligament: Dens tip to foramen magnum, minimal stabilizing role
Biomechanics
Motion Contribution
- Rotation: 50% of total cervical rotation (45 degrees each side)
- Flexion-Extension: 10 degrees total (10% contribution)
- Lateral Flexion: Minimal (coupled with rotation)
- Pivot point: Odontoid process acts as central axis for C1 rotation
Pathophysiology
- Degenerative: Lateral joint cartilage erosion from rotatory shear forces
- Inflammatory (RA): Pannus erodes transverse ligament and facet cartilage
- Instability: Transverse ligament failure allows ADI widening
- Myelopathy: Cord compression from posterior C1 translation or vertical odontoid migration
Rheumatoid atlantoaxial patterns:
- Anterior subluxation (60%): Transverse ligament erosion - most common presentation
- Vertical migration (30%): Lateral mass collapse allows odontoid to migrate into foramen magnum
- Posterior subluxation (10%): Odontoid erosion allows C1 to translate posteriorly
- Lateral subluxation: Asymmetric lateral mass erosion causes rotatory deformity
Pathophysiology
Degenerative Pathophysiology
The atlantoaxial joint is primarily a rotational joint with minimal axial loading, making primary osteoarthritis uncommon. When degenerative changes occur, they typically affect the lateral atlantoaxial facet joints:
- Cartilage degradation: Repetitive rotatory shear forces cause chondrocyte dysfunction and matrix breakdown
- Subchondral sclerosis: Loss of cartilage protection leads to bony eburnation
- Osteophyte formation: Marginal osteophytes develop at joint margins
- Joint space narrowing: Progressive cartilage loss reduces joint height
- Secondary instability: Advanced degeneration may lead to ligamentous laxity
Inflammatory Pathophysiology (Rheumatoid Arthritis)
Rheumatoid atlantoaxial disease follows a distinct pathophysiologic cascade:
- Synovitis initiation: RA pannus forms within atlantoaxial synovial joints
- Transverse ligament erosion: Pannus directly erodes the transverse ligament, the primary C1-C2 stabilizer
- Lateral mass erosion: Pannus destroys the lateral atlantoaxial facet cartilage and subchondral bone
- Instability development: Transverse ligament failure allows anterior C1 translation (increased ADI)
- Vertical migration: Lateral mass collapse allows odontoid to migrate superiorly into foramen magnum
- Neural compression: Cord compression from posterior C1 translation or odontoid migration into brainstem
Neurologic Sequelae
Atlantoaxial instability threatens the spinal cord through two mechanisms:
- Dynamic compression: Flexion increases ADI, causing intermittent cord compression
- Static compression: Vertical migration causes persistent brainstem compression
The cervicomedullary junction is particularly vulnerable - compression here affects respiratory centers, upper motor neuron pathways, and posterior column function.
Classification Systems
Grisel Classification (Inflammatory Atlantoaxial Subluxation)
| Type | Rotatory Position | ADI | Management |
|---|---|---|---|
| Type I | Rotatory fixation without anterior shift | Under 3mm | Conservative or C1-C2 fusion if chronic |
| Type II | Rotatory fixation with anterior subluxation | 3-5mm | C1-C2 fusion recommended |
| Type III | Rotatory fixation with significant anterior shift | Over 5mm | Occiput-C2 fusion |
| Type IV | Posterior atlantoaxial subluxation | Variable | Occiput-C2 fusion |
Originally described for post-inflammatory atlantoaxial rotatory subluxation in children, extended to rheumatoid and degenerative patterns.
Clinical Presentation
History
- Pain: Occipitocervical pain (90%), radiates to vertex, worse with rotation
- Stiffness: Progressive rotation loss (difficulty turning head to look sideways)
- Mechanical symptoms: Clicking or clunking with head rotation
- Neurologic: Paresthesias (C2 distribution), gait instability if myelopathy
- Red flags: Bowel/bladder dysfunction, drop attacks, dysphagia (brainstem compression)
Examination
- Look: Head fixed in rotation (cock-robin position if rotatory subluxation)
- Range of motion: Reduced rotation (under 45 degrees each direction), pain at extremes
- Palpation: Tenderness over C1-C2 (2cm below occiput)
- Neurology: Upper motor neuron signs if myelopathy (hyperreflexia, Hoffmann sign, Babinski, gait ataxia)
- Special: Sharp-Purser test (anterior C1 translation with head flexion - now rarely used)
Myelopathy Warning Signs
Progressive myelopathy from atlantoaxial instability is insidious: hand clumsiness, gait imbalance, hyperreflexia. Once established, myelopathy may not fully reverse post-fusion. Urgent surgical consultation if SAC under 13mm or neurologic signs present.
Investigations
Imaging Protocol
AP and Lateral cervical spine: Atlantoaxial alignment, ADI measurement. Flexion-Extension laterals: Dynamic instability (ADI change over 2mm indicates ligament failure). Open-mouth odontoid: Lateral mass symmetry, dens integrity.
Gold standard for bony anatomy: Odontoid morphology, lateral mass dimensions, C2 pedicle anatomy for surgical planning. Reconstructions: Sagittal/coronal views show vertical migration and basilar invagination.
Cord compression evaluation: SAC measurement, T2 signal in cord (hyperintensity = myelomalacia). Soft tissue: Pannus (enhances with gadolinium in RA), transverse ligament integrity.
Key Radiographic Measurements
Atlantoaxial Stability Parameters
| Measurement | Normal Value | Pathologic Threshold | Clinical Implication |
|---|---|---|---|
| ADI (Atlantodental Interval) | Adult under 3mm, Child under 5mm | Adult over 3.5mm, Child over 5mm | Transverse ligament incompetence |
| SAC (Space Available for Cord) | Over 13mm | Under 13mm | High myelopathy risk - surgical threshold |
| McGregor line | Dens tip below line | Dens tip over 5mm above line | Basilar invagination - requires occiput-C2 fusion |
ADI Measurement Technique
ADI measured on lateral cervical radiograph: distance between posterior cortex of anterior C1 arch and anterior cortex of odontoid. Measure in neutral, flexion, extension. Increase over 2mm between positions = dynamic instability.
Management Algorithm
Conservative Treatment
Indications:
- Mild degenerative changes, no instability (ADI under 3mm)
- Early inflammatory arthritis, intact ligaments
- Patient medically unfit for surgery
Conservative Protocol
NSAIDs: Naproxen 500mg BD or celecoxib 200mg daily. Soft collar: Temporary (under 2 weeks) for acute flares only - prolonged use causes muscle atrophy. Activity modification: Avoid extreme rotation and extension.
Physiotherapy: Gentle ROM, postural training, avoid forceful manipulation. Strengthening: Periscapular and deep neck flexor exercises. Ergonomics: Neutral cervical posture at workstation.
Clinical reassessment: Pain scores, neurologic exam. Repeat imaging: Flexion-extension radiographs if symptoms worsen. Surgical threshold: ADI over 3.5mm, new neuro signs, or refractory pain.
Surgical Technique
Patient Setup
Positioning Checklist
Prone on Jackson table or Wilson frame. Head: Mayfield 3-pin clamp, neutral alignment confirmed on lateral C-arm - avoid flexion (narrows spinal canal). Arms: Padded at sides, chest rolls under thorax.
Neuromonitoring: SSEPs and MEPs baseline before incision. Alert thresholds: 50% amplitude drop or 10% latency increase.
Exposure: Inion to C7 spinous process. Bone graft site: Posterior iliac crest if autograft planned. C-arm access: Confirm AP and lateral imaging possible.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Vertebral artery injury | 2-4% | Anomalous VA, small C2 pedicle, screw malposition | Pack with gel foam, complete contralateral screw, observe. Vascular surgery if bilateral or symptomatic |
| C2 nerve root injury | 5-10% | Excessive retraction, screw malposition | Occipital numbness, dysesthesias - usually resolves over 6 months |
| Pseudarthrosis | 5-10% | Smoking, osteoporosis, inadequate fixation | Revision fusion with bone graft, consider rhBMP-2 |
| Screw loosening/breakage | 3-5% | Osteoporosis, early collar removal | Revise if painful or progressive, otherwise observe |
Vertebral Artery Injury Management
Unilateral VA injury usually tolerated (contralateral supply). Bilateral injury can cause posterior circulation stroke. If brisk arterial bleeding during drilling: pack with gel foam, do NOT pursue bleeding or attempt repair (risk exsanguination), complete contralateral screw, consider postop angiography.
Postoperative Care and Rehabilitation
Postoperative Protocol
ICU observation: Especially if occiput-C2 fusion (respiratory risk). Mobilization: Out of bed POD 1 with rigid collar, PT/OT assessment. Drain removal: When output under 30ml per 8 hours.
Wound check: Remove staples/sutures at 10-14 days. Collar compliance: Wear rigid collar full-time (shower with assistance). Activity: No lifting over 2kg, no driving.
CT cervical spine: Assess fusion progress (early bridging bone). Collar: Continue full-time if no solid fusion. Activity: Gradually increase ADLs, still no driving.
CT cervical spine: Confirm solid fusion (bridging bone across C1-C2 facets). Collar weaning: Gradual removal if solid fusion confirmed (4-6 weeks weaning). Activity: Return to driving when off collar, no contact sports for 6 months.
Weight-Bearing and Activity Restrictions
- Rigid collar 12 weeks: Full-time except showering
- No lifting: Under 2kg for 6 weeks, under 5kg for 12 weeks
- No driving: Until off collar (vision impairment from collar, rotation loss)
- Return to work: Sedentary work at 6-8 weeks, manual labor at 3-6 months post-fusion
- Sports: Swimming at 3 months, non-contact sports at 6 months, no contact sports ever (risk hardware failure)
Outcomes and Prognosis
| Outcome Domain | Expected Result | Time to Plateau | Notes |
|---|---|---|---|
| Pain relief | 80-85% significant improvement | 6-12 months | Occipitocervical pain resolves in majority |
| Neurologic recovery | 50-70% if myelopathy present | 12-24 months | Better if myelopathy duration under 6 months |
| Fusion consolidation | 90-95% solid fusion | 12 months | CT shows bridging bone across C1-C2 facets |
Counseling on Functional Loss
Preoperative counseling essential: C1-C2 fusion eliminates 50% of cervical rotation. Patients must turn entire body to look sideways (e.g., checking blind spot while driving). Most accept this trade-off for pain relief and stability, but informed consent is critical.
Evidence Base and Key Trials
Harms Technique for C1-C2 Fusion
- Described polyaxial C1 lateral mass + C2 pedicle screw-rod fixation
- 37 patients: 100% fusion rate at 1 year
- No vertebral artery injuries, 1 C2 nerve dysesthesia
- Biomechanically superior to transarticular screws
Atlantoaxial Subluxation in Rheumatoid Arthritis
- 333 RA patients: 30% had atlantoaxial subluxation
- 7% developed myelopathy over 5 years
- ADI over 10mm and SAC under 14mm correlated with myelopathy
- Early fusion prevented neurologic progression
Outcomes of C1-C2 Fusion for Degenerative Arthritis
- 40 patients with C1-C2 fusion for degenerative arthritis
- 93% fusion rate at 12 months
- 85% significant pain improvement (VAS decrease over 4 points)
- All patients accepted 50% rotation loss
Surgical Treatment of Rheumatoid Cervical Spine Disease
- 134 RA patients with atlantoaxial instability followed long-term
- Patients with SAC under 14mm had significantly worse neurologic outcomes
- Early surgical intervention (before myelopathy) had better functional outcomes
- Posterior fusion techniques achieved 85-90% union rates
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: RA Patient with Neck Pain
"A 55-year-old woman with longstanding rheumatoid arthritis presents with 6 months of progressive occipitocervical pain and difficulty turning her head while driving. Flexion-extension lateral cervical radiographs show ADI of 4mm. How do you assess and manage?"
Scenario 2: Surgical Technique - Harms Method
"Walk me through your technique for C1-C2 fusion using the Harms method. Focus on screw trajectories and danger structures."
Scenario 3: Complication - Vertebral Artery Injury
"During C2 pedicle screw placement, you encounter brisk arterial bleeding from the drill hole. How do you manage this intraoperatively?"
MCQ Practice Points
Atlantoaxial Motion Question
Q: What percentage of total cervical rotation occurs at the atlantoaxial joint? A: 50% - C1-C2 contributes ~50% of cervical rotation (45 degrees each side) and 10% of flexion-extension. This is why C1-C2 fusion causes profound functional loss.
ADI Threshold Question
Q: What is the pathologic ADI threshold in adults? A: 3.5mm - Normal adult ADI is under 3mm. ADI over 3.5mm indicates transverse ligament incompetence. In children, threshold is 5mm.
SAC Surgical Threshold Question
Q: What SAC measurement indicates high myelopathy risk? A: Under 13mm - SAC (space available for cord) is measured from posterior odontoid to anterior C1 posterior arch. Under 13mm correlates with myelopathy and is a surgical threshold.
RA Prevalence Question
Q: What percentage of RA patients develop atlantoaxial subluxation? A: 30-40% - Atlantoaxial involvement is most common cervical manifestation of RA, due to pannus erosion of transverse ligament.
Harms Trajectory Question
Q: What screw trajectory angles for C2 pedicle in Harms technique? A: 25 degrees medial and 25 degrees cephalad - Following anatomic C2 pedicle axis. Entry point is medial-superior quadrant of C2-C3 facet.
Australian Context and Medicolegal Considerations
Australian Guidelines
- RACS Neurosurgery: C1-C2 fusion established procedure
- RA screening: Annual flexion-extension cervical XR recommended
- Informed consent: Document 50% rotation loss, fusion rate, complications
Medicolegal Pitfalls
- Delayed diagnosis: Failure to obtain flexion-extension views in RA with neck pain
- Neurologic monitoring: Intraop neuromonitoring is standard of care
- VA injury: Document anatomic variation on preop CT, navigation if indicated
- Consent: Rotation loss, pseudarthrosis, neuro injury, VA injury
Key Documentation Requirements
Preoperative counseling:
- 50% rotation loss (patient understands functional impact - driving, checking blind spots)
- Fusion rate (90-95%) and pseudarthrosis risk (5-10%)
- Vertebral artery injury risk (2-4%) and management if occurs
- Neurologic outcome (myelopathy may not fully reverse)
- Alternative treatments considered (conservative vs surgical)
Common litigation issues:
- Failure to diagnose atlantoaxial instability in RA (delayed flexion-extension imaging)
- VA injury without documented anatomic assessment or navigation consideration
- Inadequate consent regarding rotation loss
ATLANTOAXIAL ARTHRITIS
High-Yield Exam Summary
Key Anatomy
- •C1-C2 = 50% cervical rotation, 10% flexion-extension
- •Transverse ligament = primary stabilizer, prevents anterior C1 translation
- •Vertebral artery: lateral to C1 lateral mass, through C2 pedicle
- •C2 ganglion: over C2 lamina, ligate venous plexus to access C1-C2 joint
Instability Thresholds
- •ADI: Adult over 3.5mm pathologic, Child over 5mm
- •SAC: Under 13mm = high myelopathy risk, surgical threshold
- •McGregor line: Dens tip over 5mm above = basilar invagination
- •Dynamic: ADI increase over 2mm flexion to extension = instability
Surgical Indications
- •Absolute: Myelopathy, ADI over 5mm, SAC under 13mm, basilar invagination
- •Relative: Refractory pain 6 months, ADI 3.5-5mm symptomatic
- •C1-C2 fusion: Atlantoaxial instability without basilar invagination
- •Occiput-C2: Basilar invagination, severe instability
Harms Technique
- •C1 screw: 10 degrees medial, 0 cephalad, 26-30mm, entry 1mm inferior-medial to C1 arch midpoint
- •C2 screw: 25 degrees medial, 25 cephalad, 18-22mm, entry medial-superior C2-C3 facet
- •Palpate medial pedicle wall before screw insertion
- •Gentle compression, decorticate facets, bone graft, collar 12 weeks
Complications
- •VA injury 2-4%: Pack, do not pursue, complete contralateral, observe
- •C2 nerve 5-10%: Occipital numbness, resolves over 6 months
- •Pseudarthrosis 5-10%: Smoking, osteoporosis, inadequate fixation
- •Rotation loss 50%: Counsel preop, patient turns body not head