Atlantoaxial Arthritis

Atlantoaxial arthritis is degenerative or inflammatory change in the C1-C2 articulation. The joint is unusual: it provides approximately 50% of total cervical rotation and 10% of flexion-extension, yet bears minimal axial load. That high-mobility, low-load environment makes primary osteoarthritis uncommon, and inflammatory arthritis, especially rheumatoid, is the predominant aetiology.

Deep dive. This page covers the atlantoaxial pathology (degenerative and rheumatoid involvement, instability, Crowned Dens, the Ranawat classification). For the operative technique — indications, vertebral-artery anatomy, and the Magerl-versus-Goel-Harms decision — see atlantoaxial C1-C2 fusion.

Who gets it. Rheumatoid arthritis accounts for 80% of inflammatory atlantoaxial disease, and 30-40% of patients with RA develop C1-C2 involvement. Rheumatoid cases follow the onset of systemic disease and affect women twice as often as men (2:1). Degenerative disease peaks at 60-70 years; primary osteoarthritis is uncommon, and secondary degeneration follows odontoid fracture nonunion or os odontoideum.

What it costs the patient. A deficit of up to 50% of cervical rotation impairs daily activities such as driving and checking blind spots. Three further figures are conventional teaching estimates rather than measured rates from a single defined cohort:

• Chronic occipitocervical pain in 90% of symptomatic cases
• Spinal cord compression (myelopathy) in 5-10% without treatment
• Fusion required in 15-20% of patients with rheumatoid atlantoaxial disease

The measured figures on this page are the 30-40% rate of rheumatoid atlantoaxial involvement and the series outcomes in the Evidence section.

The joints. C1 and C2 meet at three synovial joints: the median atlantoaxial joint, between the odontoid process (the dens of C2) and the anterior arch of C1, and the paired lateral atlantoaxial joints, between the inferior facets of C1 and the superior facets of C2. Posteriorly, the posterior membrane connects the posterior arch of C1 to the C2 lamina.

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The ligaments. The transverse ligament is the primary stabiliser. It spans the C1 lateral masses behind the dens and prevents anterior translation of C1. The paired alar ligaments connect the dens to the occipital condyles and limit rotation and lateral flexion; the apical ligament runs from the dens tip to the foramen magnum and has only a minimal stabilising role.

Motion. The odontoid process is the central axis about which C1 rotates, 45 degrees to each side. Flexion-extension totals 10 degrees, and lateral flexion is minimal and coupled with rotation.

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The vertebral artery. The vertebral arteries run lateral to the C1 lateral masses and through the C2 pedicles, so screw fixation requires precise trajectories.

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Degenerative disease. When the joint does degenerate, the change typically affects the lateral atlantoaxial facet joints, where rotatory shear erodes the cartilage. The sequence:

1. Cartilage degradation: repetitive rotatory shear causes chondrocyte dysfunction and matrix breakdown
2. Subchondral sclerosis: loss of cartilage protection leads to bony eburnation
3. Marginal osteophytes at the joint margins
4. Joint space narrowing as cartilage loss progresses
5. Secondary instability: advanced degeneration may lead to ligamentous laxity

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Rheumatoid disease. Pannus forms within the atlantoaxial synovial joints and erodes the transverse ligament directly, along with the cartilage and subchondral bone of the lateral atlantoaxial facets. Failure of the transverse ligament allows C1 to translate anteriorly and the ADI to widen. Collapse of the lateral masses allows the odontoid to migrate superiorly into the foramen magnum, and either path can end in neural compression.

The rheumatoid atlantoaxial patterns:

• Anterior subluxation (60%) - transverse ligament erosion; the most common presentation
• Vertical migration (30%) - lateral mass collapse allows the odontoid to migrate into the foramen magnum (basilar invagination)
• Posterior subluxation (10%) - odontoid erosion allows C1 to translate posteriorly
• Lateral subluxation - asymmetric lateral mass erosion causes rotatory deformity

Below the axis, subaxial subluxation produces the C3-C7 staircase deformity.

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Neurological consequences. Instability threatens the cord in two ways. Compression is dynamic when flexion increases the ADI and compresses the cord intermittently, and static when vertical migration compresses the brainstem persistently. The cervicomedullary junction is particularly vulnerable: compression there affects the respiratory centres, upper motor neuron pathways and posterior column function.

History. Occipitocervical pain radiates to the vertex and is worse with rotation, and the loss of rotation is progressive: the patient has difficulty turning the head to look sideways. Look also for:

• Clicking or clunking with head rotation
• Paraesthesia in the C2 distribution
• Gait instability if myelopathic
• Red flags of brainstem compression: bowel or bladder dysfunction, drop attacks, dysphagia

Examination.

• Look: head fixed in rotation, the cock-robin position if there is rotatory subluxation
• Move: rotation reduced to under 45 degrees in each direction, with pain at the extremes
• Feel: tenderness over C1-C2, 2cm below the occiput
• Neurology: upper motor neuron signs if myelopathic (hyperreflexia, Hoffmann sign, Babinski, gait ataxia)
• Sharp-Purser test: anterior C1 translation with head flexion; now rarely used

Crowned dens syndrome is the crystalline form of atlantoaxial arthritis and the classic acute mimic that catches the unwary. Calcium pyrophosphate dihydrate (CPPD), and less commonly basic calcium phosphate or hydroxyapatite, is deposited in the ligaments encircling the odontoid (the transverse and alar ligaments and the peri-odontoid tissues), producing a "crown" or "halo" of calcification around the dens on CT.

The patient. Typically an older adult, often female, with acute or subacute severe upper neck and occipital pain, marked stiffness and painfully restricted rotation. Low-grade fever and markedly raised CRP and ESR make it a convincing mimic of meningitis, giant cell arteritis, polymyalgia rheumatica, pseudogout elsewhere, or septic or infective arthritis of the craniovertebral junction. There is often a history of CPPD or chondrocalcinosis at other joints (knee, wrist, symphysis).

Diagnosis. CT is the key investigation, showing crown- or halo-shaped calcification around the odontoid, best appreciated on axial and coronal reconstructions. Plain radiographs and MRI frequently miss the calcification; MRI shows non-specific peri-odontoid soft-tissue change or oedema, so a normal MRI does not exclude the diagnosis. There is no true instability: the ADI is normal and dynamic films are stable, which distinguishes it from rheumatoid or degenerative atlantoaxial disease.

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Management is medical, not surgical. The syndrome is self-limiting and responds rapidly to NSAIDs, colchicine, or a short course of low-dose corticosteroids, usually settling within days to a couple of weeks. Fusion has no role; the value of the diagnosis is precisely that it converts an alarming surgical or infective presentation into a treatable crystal arthropathy.

Radiographs come first:

• AP and lateral cervical spine - atlantoaxial alignment and the ADI
• Flexion-extension laterals - dynamic instability
• Open-mouth odontoid - lateral mass symmetry and dens integrity

In rheumatoid arthritis, annual flexion-extension cervical radiographs are mandatory for screening.

Measuring the ADI. The ADI is measured on the lateral radiograph from the posterior cortex of the anterior C1 arch to the anterior cortex of the odontoid, in neutral, flexion and extension. An increase of over 2mm between positions indicates dynamic instability and ligament failure.

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CT is the gold standard for bony anatomy: odontoid morphology, lateral mass dimensions, and C2 pedicle anatomy for surgical planning. Sagittal and coronal reconstructions show vertical migration and basilar invagination.

MRI assesses the cord: the SAC, and T2 signal within the cord, where hyperintensity indicates myelomalacia. It also shows the soft tissues, the pannus (which enhances with gadolinium in RA) and the integrity of the transverse ligament.

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ADI or PADI. Boden showed that the posterior interval, the PADI, predicts paralysis and recovery far better than the anterior ADI, which did not correlate. The PADI and the SAC are the same space, so the two cord-space figures on this page sit side by side: an SAC under 13mm is quoted as the myelopathy-risk and surgical threshold, and a PADI under 14mm is Boden's operative threshold.

The incidence ranges are conventional teaching figures consistent with the cited series (for example Gluf: 5 vertebral artery injuries in 191 patients, about 2.6%); they are not measured rates from a single defined cohort.

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Restrictions after fusion:

• Rigid collar full-time for 12 weeks, except for showering (with assistance)
• Lifting under 2kg for 6 weeks and under 5kg for 12 weeks
• No driving until off the collar, because of the vision impairment from the collar and the rotation loss
• Return to sedentary work at 6-8 weeks and to manual labour at 3-6 months
• Swimming at 3 months and non-contact sport at 6 months; no contact sport ever (risk of hardware failure)

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

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.

ADI or PADI as the decisive measure. The anterior ADI is the classic teaching threshold, but modern practice increasingly weights the PADI or SAC and MRI cord signal over a single ADI number.

Asymptomatic instability: when to fix. Whether to fuse a radiographically unstable but neurologically intact RA patient is unsettled. The arguments for early fusion, preventing irreversible myelopathy and operating at lower risk before vertical migration, compete with the morbidity of fusing a patient who may never deteriorate, especially now that disease control is better.

Harms or transarticular (Magerl). Both give about 95-98% fusion. The Harms screw-rod construct allows intraoperative reduction and tolerates an aberrant vertebral artery on one side; transarticular screws need an unobstructed trajectory bilaterally. No randomised trial settles superiority, and the choice is driven by anatomy and surgeon.

C2 nerve root: sacrifice or preserve. Deliberate sacrifice of the C2 ganglion improves joint exposure and access to the fusion bed and reduces venous bleeding, at the cost of occipital numbness. Preservation avoids the sensory deficit but risks more bleeding and screw malposition. The evidence is mixed and practice varies.

Reducing basilar invagination. 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.

BMP and navigation. rhBMP-2 and intraoperative navigation or robotics may raise fusion and screw-accuracy rates, but they add cost and, for BMP, off-label use and concerns about swelling at this level; their routine role is not established.