Atlanto-Odontoid Joint Space

spatium articulationis atlantoodontoidea

The atlanto-odontoid space (atlantodental interval, ADI) is the distance between the posterior surface of the anterior arch of C1 and the anterior surface of the odontoid process, measured on a lateral cervical spine X-ray. Normal ADI is 3 mm or less in adults and 5 mm or less in children. The transverse atlantal ligament fills this space between the dens and the anterior arch.

Region: Neck

Clinical Relevance

Clinical Notes

The ADI is the primary radiographic measure of atlantoaxial instability. An ADI greater than 3 mm in adults indicates transverse ligament incompetence from trauma, rheumatoid arthritis, or Down syndrome. The posterior atlanto-odontoid interval (PADI — space available for the cord) is equally important: less than 14 mm indicates cord compromise risk. Flexion-extension lateral X-rays dynamically assess ADI change. Pre-operative ADI measurement is mandatory before any procedure requiring neck manipulation or positioning.

Pathology

Common Injuries & Conditions

Atlantoaxial Instability (ADI widening)

Atlantodental interval exceeding 3 mm from transverse ligament incompetence in rheumatoid arthritis, Down syndrome, or trauma, producing risk of spinal cord compression with neck movement requiring surgical C1-C2 fusion.