Atlantoaxial Instability

Atlantoaxial instability (AAI) refers to excessive movement at the C1-C2 articulation, most commonly anterior subluxation of C1 on C2. This can lead to spinal cord compression and potentially catastrophic neurological injury.

Epidemiology:

• Down syndrome: 10-30% radiographic AAI, 1-2% symptomatic
• Rheumatoid arthritis: Common in adults (25-80% radiographic)
• Os odontoideum: Associated with AAI in 60% of cases
• Trauma: Transverse ligament rupture with Jefferson fractures
• Other conditions: Morquio syndrome, Grisel syndrome, Klippel-Feil

Causes by age group:

C1-C2 articulation: The atlantoaxial complex is unique in the cervical spine - it has:

• No intervertebral disc between C1 and C2
• Specialized ligamentous stability rather than bony congruence
• Allows 50% of cervical rotation (approximately 45 degrees each side)

Key stabilizing ligaments:

Biomechanics of instability:

In flexion:

• Normal: Transverse ligament prevents anterior C1 translation
• AAI: C1 slides forward on C2, narrowing SAC
• Severe: Cord compression between posterior C1 arch and dens

In extension:

• May allow some reduction of subluxation
• Less dangerous position for cord

Steel's Rule of Thirds:

• Anterior 1/3: Odontoid process
• Middle 1/3: Spinal cord
• Posterior 1/3: Safety space (allows some subluxation before cord compression)

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History:

• Neck pain (may be occipital headache)
• Torticollis (especially rotatory subluxation)
• Weakness, clumsiness, gait disturbance
• Bowel/bladder dysfunction (late sign)
• History of Down syndrome, RA, or other predisposing condition
• Precipitating trauma or infection

Physical examination:

Red flags requiring urgent workup:

• Progressive weakness or numbness
• Gait deterioration
• Bowel or bladder dysfunction
• Respiratory compromise (high cord compression)
• Worsening headache with neck movement

Imaging protocol:

1. Plain radiographs:

• Lateral flexion-extension views - KEY for assessing instability
• Measure ADI in flexion (should be less than 3mm adult, less than 5mm child)
• Measure SAC (should be greater than 14mm)
• Open-mouth (odontoid) view - assess dens, lateral masses
• Standard AP and lateral views

2. CT scan:

• Best for bony anatomy
• Identifies os odontoideum, fractures, bony anomalies
• CT with 3D reconstruction for surgical planning
• Dynamic CT (flexion-extension) in select cases

3. MRI:

• Essential if neurological symptoms
• Shows cord compression, signal change (myelomalacia)
• Identifies transverse ligament integrity
• Shows pannus (in RA)
• Rule out other cord pathology

Special measurements:

Vertebral artery injury:

• Most serious complication
• Can cause stroke, death
• Prevention: Preoperative CT/CTA if anatomy abnormal, careful screw placement
• Management: Control bleeding (bone wax, hemostatic agents), consider endovascular if ongoing

Nonunion:

• More common with wire techniques than screw fixation
• Check with CT at 3-6 months
• May require revision with more rigid fixation

Immediate postoperative:

Collar use:

• Rigid collar (Miami J or Philadelphia) for 6-12 weeks
• Halo vest if very unstable or poor bone quality
• Soft collar for comfort transition

Activity restrictions:

• No contact sports permanently (fused motion segment)
• No high-risk activities for 6-12 months
• Full activities (non-contact) when fusion confirmed

Understanding these activity limitations is important for patient counseling.

Fusion rates:

• Screw fixation (Harms, Magerl): 95-100%
• Wire techniques (Gallie, Brooks): 85-90%

Neurological outcomes:

• Patients with myelopathy: 60-80% improve, 10-20% stable, 5-10% deteriorate
• Prophylactic surgery in asymptomatic: Prevents deterioration in vast majority
• Established myelomalacia on MRI: Less likely to fully recover

Functional outcomes:

Loss of motion:

• C1-C2 provides 50% of cervical rotation
• Fusion results in 50% rotation loss
• Well compensated in most patients
• May cause some adjacent segment stress long-term

High-yield MCQ facts:

1. Normal ADI: Less than 3mm adults, less than 5mm children

2. SAC less than 14mm indicates cord at risk - surgical consideration

3. Transverse ligament is the PRIMARY restraint to anterior C1 translation

4. Down syndrome has 10-30% radiographic AAI but only 1-2% symptomatic

5. Steel's Rule of Thirds: 1/3 dens, 1/3 cord, 1/3 safety space

6. Harms technique (C1 lateral mass + C2 pedicle screws) is current gold standard for C1-C2 fusion

7. Os odontoideum - 60% have associated AAI, treat if unstable

8. Myelomalacia on MRI (T2 hyperintensity) indicates established cord injury - limited recovery potential

9. Vertebral artery at risk during C1 lateral mass screw placement - stay within 15mm of midline

10. Fusion rate with screws is 95-100% vs 85-90% with wires

Australian practice points:

Screening:

• Royal Australasian College of Physicians recommends symptom-based screening for Down syndrome
• Many Australian anaesthetic departments still require cervical spine clearance before GA in Down syndrome
• Special Olympics Australasia requires cervical spine clearance for certain sports

Surgical considerations:

• Tertiary pediatric hospitals perform most pediatric C1-C2 fusions
• Major centers: Royal Children's Hospital Melbourne, Children's Hospital Westmead, Queensland Children's Hospital
• Adult cases often managed at major spine units

Follow-up:

• Long-term follow-up through pediatric orthopaedic clinics until skeletal maturity
• Transition to adult spine services at age 16-18
• Lifelong activity restrictions post-fusion

Medicare and insurance:

• Cervical fusion generally covered under Medicare
• Extended rehabilitation may require supplementary coverage
• Equipment (halo, collars) often provided through hospital

Rural and remote:

• Screening and initial assessment in regional centers
• Surgical management at tertiary centers
• Telemedicine follow-up increasingly used

Understanding the local healthcare context helps guide appropriate referral and management pathways.