Atlantoaxial

Atlantoaxial refers to the region and joints between the first cervical vertebra (atlas, C1) and the second cervical vertebra (axis, C2) in the upper spine. This area is crucial for permitting the majority of head rotation, with the atlas forming a ring around the odontoid process (dens) of the axis, which acts as a pivot for rotation.

Anatomy and ligaments: The atlantoaxial complex includes a median joint between the dens of C2 and the anterior arch of C1, and two lateral joints between the inferior facets of C1 and the superior facets of C2. The transverse ligament of the atlas holds the dens within the ring of C1, stabilizing the joint. The cruciform ligament—comprising the transverse part and vertical bands—adds stability to the dens. The alar ligaments extend from the dens to the occipital bones, helping limit excessive rotation, while the tectorial membrane provides a posterior extension of the longitudinal ligament.

Function: The atlantoaxial joints enable substantial rotational movement of the head, accounting for a large portion of cervical rotation. The design allows pivoting motion while protecting the spinal cord within the vertebral canal.

Clinical relevance: Instability at the atlantoaxial joint can result from trauma, inflammatory disease (such as rheumatoid arthritis), congenital conditions, or dens fractures, with potential risk of spinal cord compression. Conditions like Down syndrome increase the risk of atlantoaxial instability. Diagnosis relies on radiographs with dynamic views, CT for bone detail, and MRI for ligaments and neural structures. Treatment ranges from immobilization with cervical orthoses to surgical stabilization, typically through posterior C1–C2 fusion techniques (for example, transarticular or Goel–Harms fusion) in cases of persistent instability or neural compromise.