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Atlanto-axial dislocation (AAD) refers to abnormal instability or displacement between the atlas (C1) and axis (C2) vertebrae, which can result in compression of the spinal cord or brainstem at the craniovertebral junction (CVJ).

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It may be congenital, acquired, or traumatic, and carries significant neurological risk if left untreated.

Anatomy refresher:

Structure	Role
Atlas (C1)	Supports the skull, articulates with occiput and C2
Axis (C2)	Contains the dens (odontoid process) which acts as a pivot for rotation
Transverse ligament of atlas	Secures the dens against the anterior arch of C1 and is critical for stability

Etiology

Congenital:

Condition	Mechanism
‣	Hypoplastic or detached dens allows mobility
‣	Ligamentous laxity, especially of the transverse ligament
‣	Odontoid hypoplasia with instability

Acquired:

Cause	Notes
‣	Pannus formation erodes ligaments and odontoid process
Trauma	Fracture of dens or transverse ligament injury
Infection (TB, osteomyelitis)	Destroys ligamentous and bony integrity
Neoplastic	Primary or metastatic destruction of C1/C2 structures

Radiology

Atlanto-Dental Interval (ADI):

Measured on lateral cervical spine X-ray (neutral/flexion view)
Normal values:
- Adults: <3 mm
- Children: <5 mm
Increased ADI suggests instability due to ligamentous injury or dens hypoplasia

Posterior Atlanto-Dental Interval (PADI):

Distance between posterior aspect of dens and anterior surface of spinal cord
<14 mm indicates high risk of cord compression