C1-C2 Degeneration | Occipitocervical Pain | Rotatory Loss | C1-C2 Fusion
- 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
- “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 Atlas
C1-C2 = 50% cervical rotation. Odontoid acts as pivot. Arthritis or instability threatens cord and brainstem. Loss of rotation profoundly affects quality of life.
30-40% of RA patients develop C1-C2 involvement. Pannus erodes transverse ligament. Annual flexion-extension cervical radiographs mandatory for RA screening.
ADI over 3.5mm = pathologic in adults. SAC (space available for cord) under 13mm = myelopathy risk. Dynamic imaging shows progression.
C1-C2 fusion for instability or refractory pain. Harms technique (C1 lateral mass + C2 pedicle screws). 90% fusion rate. Accept 50% rotation loss.
| 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
Hook:TADS = Think Atlantoaxial Dynamic Stability - assess these 4 parameters!
VABSRheumatoid Cervical Patterns
Hook:VABS = Vertebral Abnormalities in Biological Syndromes - RA cervical patterns!
PSRFHarms Technique Key Steps
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
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
- 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
- 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
- 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)
- 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)
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.
Differential Diagnosis
| Condition | Discriminating Features | Key Investigation |
|---|---|---|
| Atlantoaxial arthritis / instability | Occipitocervical pain, rotation loss, RA history; UMN signs if myelopathic | Dynamic lateral radiographs (ADI), CT, MRI (SAC, pannus) |
| Atlantoaxial rotatory fixation (Grisel after infection) | Fixed cock-robin torticollis, often paediatric or post-pharyngitis/ENT surgery | Dynamic CT in rotation (fixed C1 on C2) |
| Os odontoideum / odontoid nonunion | Painless or post-traumatic instability, mobile ossicle separate from C2 body | CT (dens morphology), flexion-extension radiographs |
| Crowned dens syndrome (CPPD) | Acute severe neck pain, fever, raised inflammatory markers in the elderly | CT showing calcification around the dens; responds to NSAIDs/colchicine |
| Cervicogenic / upper cervical degenerative headache | Subaxial degeneration, no instability, normal ADI | MRI/CT showing subaxial rather than C1-C2 pathology |
| Craniovertebral junction tumour / metastasis | Night pain, constitutional symptoms, destructive lesion | MRI with contrast, CT for bony destruction |
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
| 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 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 |
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 |
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.
Controversies and Areas of Uncertainty
The anterior ADI is the classic teaching threshold, yet Boden showed the posterior interval (PADI/SAC) predicts paralysis and recovery far better and the anterior ADI did not correlate. Modern practice increasingly weights PADI/SAC and MRI cord signal over a single ADI number.
Whether to fuse a radiographically unstable but neurologically intact RA patient is unsettled. Arguments for early fusion (prevent irreversible myelopathy, lower operative risk before vertical migration) compete with the morbidity of fusing a patient who may never deteriorate, especially now disease control is better.
Both give ~95-98% fusion. Harms screw-rod allows intraoperative reduction and tolerates an aberrant vertebral artery on one side; transarticular needs an unobstructed trajectory bilaterally. No randomised trial settles superiority - choice is anatomy- and surgeon-driven.
Deliberate C2 ganglion sacrifice improves joint exposure and fusion-bed access and reduces venous bleeding, at the cost of occipital numbness; preservation avoids the sensory deficit but risks more bleeding and screw malposition. Evidence is mixed and practice varies.
Whether vertical migration should be actively reduced (traction, intraoperative distraction, anterior release) or simply stabilised in situ with occipitocervical fusion remains debated, balancing decompression against neurovascular risk at the craniovertebral junction.
rhBMP-2 and intraoperative navigation/robotics may raise fusion and screw-accuracy rates but add cost and (for BMP) off-label use and swelling concerns at this level - their routine role is not established.
Evidence Base and Key Trials
Harms Technique: Polyaxial Screw-Rod C1-C2 Fusion (Original Description)
- Original description of individual C1 lateral mass + C2 pars/pedicle polyaxial screws connected by rods
- 37 patients: solid fusion in all on early clinical and radiographic follow-up
- No neural or vascular injury attributable to the technique
- Permits intraoperative reduction of fixed subluxation and avoids the transarticular vertebral-artery-at-risk trajectory
PADI / SAC as Predictor of Paralysis and Recovery (Landmark)
- 73 RA cervical patients followed a mean of 7 years (minimum 2)
- Posterior atlanto-odontoid interval (PADI, equivalent to space available for cord) under 14mm correlated with presence and severity of paralysis; the traditional anterior ADI did not
- No neurological recovery occurred when PADI was under 10mm; recovery of at least one Ranawat class always occurred when PADI was 10mm or more
- Recovery potential depended on severity of paralysis at operation, not its duration
Ranawat Classification: Cervical Spine Fusion in RA (Original Description)
- 33 RA patients undergoing cervical fusion; introduced the still-used Ranawat classification of pain and neural involvement (Class I-IIIB)
- Defined the surgical pattern logic: Gallie C1-C2 fusion for atlantoaxial subluxation, occiput-C2 for superior odontoid migration, posterior fusion for subaxial subluxation
- Described a reproducible method for measuring superior odontoid migration
- Posterior fusion improved most patients; anterior fusion performed poorly (4 of 5 unimproved)
Transarticular (Magerl) Screw Fixation: Fusion Rate and Vertebral Artery Risk
- 191 consecutive adults, 353 C1-C2 transarticular screws (indications included trauma, rheumatoid arthritis, congenital anomaly and chronic instability)
- Fusion achieved in 98% of cases followed to fusion or for at least 24 months (mean time to fusion 9.5 months)
- Five vertebral artery injuries; one bilateral injury was fatal, the remainder caused no permanent neurological deficit
- Defined positioning, instrumentation and the role of navigation to improve safety
Neurological Improvement After Fusion in Rheumatoid Cervical Disease
- 110 consecutively operated rheumatoid cervical patients; 90 had a neurological deficit and 55 had atlantoaxial subluxation
- After C1-C2 stabilisation, 94.8% improved by at least one Ranawat class - even severely affected (Class IIIB) patients recovered
- Patients with superior odontoid migration (SMO) had worse deficits and poorer results; all four postoperative deaths were Class IIIB
- Late symptom recurrence was usually due to new subaxial subluxation
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“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?”
“Walk me through your technique for C1-C2 fusion using the Harms method. Focus on screw trajectories and danger structures.”
“During C2 pedicle screw placement, you encounter brisk arterial bleeding from the drill hole. How do you manage this intraoperatively?”
MCQ Practice Points
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.
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.
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.
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.
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.
Guidelines, Registries & Global Practice
Global epidemiology
- Rheumatoid arthritis affects roughly 0.5-1% of adults worldwide (female:male ~2-3:1); historically 30-40% of established RA developed atlantoaxial subluxation, though the incidence has fallen markedly in the biologic-DMARD era due to earlier, tighter disease control.
- Primary degenerative C1-C2 osteoarthritis is uncommon but rises with age; secondary degeneration follows odontoid nonunion, os odontoideum and prior trauma. Type II odontoid fracture nonunion is a leading cause in the elderly.
- Reduced operative demand for rheumatoid cervical disease in high-income settings contrasts with later presentation (and more vertical migration/myelopathy at first contact) where access to early rheumatology and screening imaging is limited.
Side-by-side guidance
| Body | Emphasis | Practical recommendation |
|---|---|---|
| ACR / EULAR (rheumatology) | Treat-to-target to suppress synovitis and limit structural cervical damage | Tight inflammatory control reduces atlantoaxial instability; image the cervical spine before any general anaesthetic in established RA |
| AAOS / NASS (North America) | Operative thresholds driven by neural compromise, not pain alone | PADI/SAC under 14mm and myelopathy are operative; favour rigid screw-rod constructs |
| BOA / British (UK) | Pre-anaesthetic cervical assessment and MDT decision-making in RA | Lateral flexion-extension radiographs before intubation; refer instability to a spinal unit |
| AO Spine (global) | Standardised craniovertebral junction surgical technique and training | Preoperative CT angiography of the vertebral artery before C1-C2 instrumentation; navigation for aberrant anatomy |
Registries and resource variation
- There is no large dedicated atlantoaxial-fusion registry comparable to arthroplasty registries; evidence rests on institutional series and the upper-cervical literature, so individual surgeon CT-angiographic planning matters more than registry implant data here.
- High-resource settings: routine preoperative CT angiography, intraoperative neuromonitoring and image guidance/navigation; biologic DMARDs have shifted RA cervical disease from a common to an uncommon operative problem.
- Limited-resource settings: later presentation with established vertical migration/myelopathy; transarticular or wiring constructs may predominate where polyaxial screw-rod systems or navigation are unavailable - careful patient selection and preoperative imaging remain the key safety levers.
- RA cervical screening: Flexion-extension lateral radiographs in symptomatic or long-standing RA, and before any general anaesthetic
- Surgical planning: Preoperative CT (and CT angiography) of the C2 isthmus and vertebral artery before C1-C2 instrumentation
- Intraoperative: Neuromonitoring (SSEP/MEP) and fluoroscopic/navigated screw confirmation
- Consent: Document the expected 50% rotation loss, fusion rate and pseudarthrosis, neurological and vertebral artery risk
- Delayed diagnosis: Failure to obtain dynamic views in RA with new neck pain or pre-intubation
- Anatomy ignored: Instrumenting without assessing vertebral artery course or a high-riding/aberrant VA
- Operating too late: Allowing superior odontoid migration or PADI under 10mm before fusion, when recovery potential is lost
- Inadequate consent: Not explaining functional rotation loss and the body-turn it requires
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
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