Atlantoaxial Median Joint

articulatio atlantoaxialis mediana

The atlantoaxial median joint is the pivot joint formed between the anterior surface of the odontoid process (dens) of C2 and the posterior surface of the anterior arch of C1, stabilised by the transverse ligament of the atlas which presses the dens against the arch. There are actually two synovial cavities: one anterior (between dens and atlas anterior arch) and one posterior (between dens and transverse ligament). This joint allows approximately 50 degrees of head rotation.

Region: Neck

Clinical Relevance

Clinical Notes

The atlantoaxial median joint is the primary site of pathological instability at the craniovertebral junction. The atlanto-dental interval (ADI), normally 3 mm or less in adults, increases when the transverse ligament is disrupted in rheumatoid arthritis, trauma, or Down syndrome. Atlantoaxial fusion (Magerl C1-C2 transarticular screws or Harms C1 lateral mass-C2 pedicle screws) stabilises this joint when conservative management fails. The median joint is also the site of the os odontoideum anomaly where the dens fails to fuse to the C2 body.

Pathology

Common Injuries & Conditions

Transverse Ligament Rupture at Atlantoaxial Median Joint

Transverse ligament disruption from RA pannus erosion or trauma allows the dens to sublux posteriorly into the spinal canal; ADI above 3 mm on lateral flexion radiograph indicates instability requiring Harms or Magerl C1-C2 fixation before cord compression produces irreversible myelopathy.