Atlanto-axial instability


Atlanto-axial 2




Loss of ligamentous stability between atlas and axis

Can cause impingement of the spinal cord




Down's syndrome - combination of ligamentous laxity, ondontoid dysplasia & os odontoid

Rheumatoid arthritis - attrition of transverse ligament and ondontoid erosion

Ligamentous laxity - Marfan's, Larsen's

Os odontoid - failure of fusion of odontoid

Congenital - SED, Achondroplasia, Morquio's syndrome, Klippel Feil


Wang et al. Spine 2013

- 904 surgical cases

- os odontoid 429/904 (47%)

- atlas occipitalization 265/904 (29%)

- transverse ligament loose 84/904 (9%)

- rheumatoid arthritis 36/904 (4%)

- nonunion odontoid fracture 25/904 (3%)

- anklylosing spondylitis 11/904 (1%)

- Down syndrome 7/904 (1%)







Atlas - ring of anterior and posterior arches with lateral mass




Axis - odontoid, vertebral body, and articular facets



Structure Attachments Action
Cruciform ligament Transverse ligament - tubercles medial aspect lateral mass C1 Prevents anterior displacement of atlas
  Longitudinal bands - from transverse ligament

up to occiput and down to C2


Alar ligament Side of the dens up to the lateral margins foramen magnum Prevent excessive rotation
Apical ligament Tip of dens to anterior foramen magnum  
Tectorial membrane

Extension of PLL behind transverse and alar ligaments



C0 C1 C2C1 C2




C1/C2 - 50% of rotation of the cervical spine




Neck pain

Neurological symptoms





- abnormal gait

- hyper-reflexia


Boneschool / myelopathy examination




Atlanto-dens interval (ADI)

- > 5 mm indicates instability


Space available for Cord (SAC)

- posterior atlanto-dens interval (PADI)

- < 13 mm concerning


Atlanto-axial 2Downs ADI SAC

Increased ADI in patient with Down's syndrome


Flexion / Extension views


Atlanto-axial flexionAtlanto-axial extension

Down's syndrome flexion xray                                               Down's syndrome extension xray




Downs neck CT




Assess transverse ligament

Assess space available for cord (SAC)


Atlanto axial MRI

Intact transverse ligament on MRI




Wang classification Spine 2013


904 cases

Type I Instability Reduced on flexion or extension xray 472 (52%) Reduce and posterior fusion
Type II Reducible dislocation

No bony fusion on CT

Reducible under GA with skeletal traction

160 (18%) Reduce and posterior fusion
Type III Irreducible dislocation

No bony fusion on CT

Not reducible under GA with skeletal traction

268 (30%)

Trans-oral anterior release

Posterior fusion

Type IV Bony dislocation C1/C2 bony fusion on CT 4 (<0.5%)

Trans-oral anterior osseous decompression

Posterior fusion


Criteria for diagnosis of AAI


1.  ADI > 5 mm

2.  Overriding of the anterior arch of the atlas over the odontoid

3.  Space available for the cord (SAC) of less than 13 mm

4.  Violation of the Steel’s rule of thirds (one-third cord, one-third odontoid, and one-third safe space)

5.  Translation of the tip of the odontoid of more than 4 mm of the basion


Surgical technique


Wang et al. Spine 2013



- cranial traction with Gardner-Wells tongs

- GA and muscle paralysis

- gradual increased in traction weight up to maximum 1/6 patient body weight

- fluoroscopic assessment after 10 minutes


Type I / II

- anatomic reduction with skeletal traction

- posterior fusion

- transarticular C1/C2 (Mageryl)

- C1 lateral mass / C2 pedicle screw (Goel and Harms)

- occasional occiput - C2 fusion


Type III

- irreducible with skeletal traction

- trans oral release / facet joint release

- posterior fusion


Type IV

- bony fusion on CT

- anterior release +/- trans oral odontoidectomy

- posterior fusion




Wang et al. Spine 2013

- 904 cases of AAI treated surgically

- anatomic reduction in 99%

- 99% solid fusion

- neurological improvement in 84.1% (512/609) of the patients with myelopathy.

- 2 vertebral artery injuries with trans-oral approach (1 death)

- 4 vertebral artery injuries with posterior approach

- 1 death from PE

- 3 deaths from respiratory distress


Down's Syndrome




Machida et al Cureus 2022

- up to 60% develop AAI

- 1% neurological symptoms and signs




Bouchard et al Spine Deform 2019

- ADI > 6mm

- SAC < 14 mm


Natural history


Cremers et al Lancet 1993

- 91 Down's syndrome children with AAI > 4 mm

- half randomly allowed to play sport, half restricted

- at one year, no difference in neurological outcomes


Management guidelines


ADI > 5mm & asymptomatic: ?avoid contact sports

ADI > 5mm & symptomatic: fusion

ADI > 10 mm: C1/C2 fusion


Tomlinson et al Clin J Sports Med 2020

- screen for myelopathy

- ensure good neck control / encourage regular neck strengthening

- ensure good neck ROM

- allow to play sports if above 3 categories met

- monitor for any neurological symptoms


Boneschool / Down's syndrome


Rheumatoid Arthritis



- transverse ligament incompetent

- bony erosions


Boneschool / Rheumatoid neck


Os Odontoid


Boneschool / os odontoid