Atlantoaxial Instability
Excessive C1–C2 joint motion creating risk of spinal cord compression
ICD-10: M43.5 · cervical condition
Atlantoaxial instability (AAI) refers to abnormal, excessive movement at the joint between the first and second cervical vertebrae (C1 atlas and C2 axis). The stability of this joint depends on the integrity of the transverse atlantal ligament, alar ligaments, and the odontoid process itself. When any of these restraints fail — due to ligamentous laxity, bony abnormality, or trauma — the atlas can slide forward (anterior subluxation) or rotate excessively on the axis, placing the spinal cord at risk. AAI has many causes. Down syndrome (trisomy 21) is the most common pediatric cause, occurring in 10–30% of individuals due to congenital ligamentous laxity. Rheumatoid arthritis is the most common adult cause, as pannus formation erodes the transverse ligament and odontoid. Traumatic odontoid fractures, os odontoideum, and congenital conditions such as Morquio syndrome also produce AAI. Treatment is guided by the degree of instability and neurological status. Asymptomatic ligamentous laxity in Down syndrome may be observed. Symptomatic instability — with myelopathy, intractable pain, or high-grade subluxation — is treated with posterior C1–C2 fusion, which restores stability and prevents cord injury.
Anatomy & Pathology
The transverse atlantal ligament (TAL) crosses posterior to the dens from one C1 lateral mass to the other, preventing anterior translation of C1 on C2. It is the single most important stabilizer of the atlantoaxial joint. The alar ligaments run from the dens tip to the medial occipital condyles, limiting rotation and lateral bending. Together these structures define a "ring" at C1 whose anterior half contains the dens and whose posterior half accommodates the spinal cord. Steel's Rule of Thirds states that the dens, cord, and "free space" each occupy approximately one-third of the C1 ring — a margin that disappears when the dens migrates posteriorly due to instability.
Symptoms
- Neck pain and stiffness, especially with rotation
- Occipital headaches radiating to the back of the skull
- Myelopathic signs: hand weakness, spasticity, gait ataxia
- Transient quadriparesis with sudden head movements
- Vertebrobasilar symptoms: dizziness, tinnitus, visual disturbance
- Torticollis or head tilt
- Bowel or bladder dysfunction in severe cases
Causes & Risk Factors
- Down syndrome (trisomy 21) — ligamentous laxity
- Rheumatoid arthritis — pannus erosion of transverse ligament and odontoid
- Odontoid fracture (Type II) — bony instability
- Os odontoideum — absent or detached odontoid support
- Congenital ligament laxity syndromes (Marfan, Ehlers-Danlos, Morquio)
Treatment Options
Conservative
- Observation with periodic flexion-extension imaging for mild, asymptomatic cases
- Cervical collar or halo orthosis for temporary stabilization
- Activity restriction: no contact sports or positions requiring extreme neck movement
Surgical
- Posterior C1–C2 arthrodesis (Harms technique with polyaxial screws and rods) — standard of care for unstable AAI
- Occipitocervical fusion (occiput–C2 or C3) for cases with occipital involvement or insufficient C1 purchase
- Transoral odontoidectomy followed by posterior fusion for irreducible ventral cord compression
When to see a spine specialist
Patients with Down syndrome, rheumatoid arthritis, or connective tissue disorders should be screened for AAI before any procedure requiring general anesthesia or neck manipulation. Seek urgent evaluation for new myelopathic symptoms, sudden onset of severe neck pain after trauma, or transient limb weakness — these may indicate acute subluxation.
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Search spine specialists →Frequently Asked Questions
Should all children with Down syndrome be screened for atlantoaxial instability?
Yes. Screening plain radiographs with flexion-extension views are recommended before participation in contact sports or activities with fall risk (required by Special Olympics). An atlantodens interval (ADI) greater than 4–5 mm in children indicates instability. Neurologically asymptomatic children with ADI under 10 mm and preserved space for the cord are often cleared with activity restrictions and annual monitoring.
What is the atlantodens interval and what values are concerning?
The atlantodens interval (ADI) is the distance between the posterior surface of the anterior arch of C1 and the anterior surface of the odontoid on a lateral radiograph in flexion. Normal is ≤3 mm in adults and ≤4 mm in children. An ADI of 5–10 mm suggests ligamentous injury; above 10 mm indicates complete transverse ligament disruption and very high cord injury risk.
How successful is C1–C2 fusion for atlantoaxial instability?
Posterior C1–C2 screw-rod fusion (Harms technique) achieves fusion rates above 95% in most series. Patients with myelopathy typically stabilize and partially recover. The main trade-off is permanent loss of approximately 50% of cervical rotation, since the atlantoaxial joint normally provides the majority of head rotation.