C1C2

C1C2 refers to the joint and adjacent structures formed by the first and second cervical vertebrae, C1 (atlas) and C2 (axis). The C1–C2 region contains the atlantoaxial joints, consisting of a median joint between the dens of C2 and the anterior arch of C1, and two lateral (facet) joints between the inferior articular facets of C1 and the superior articular facets of C2. The dens, or odontoid process, of C2 acts as a pivot that enables rotation of the head. The stability of this region is maintained by the transverse ligament of the atlas, which holds the dens against the anterior arch of C1, and by a set of cruciform and alar ligaments that limit excessive movement and protect the spinal cord and medulla.

C1 has no vertebral body and forms a ring around the dens; C2 provides the upward peg around which C1 rotates. Together with the atlantooccipital joints, the C1–C2 complex allows the majority of cervical rotation, with nodding primarily produced by the atlantooccipital joints.

Clinically, the C1C2 area is important because injuries such as odontoid fractures (classified I–III) and atlantoaxial dislocation can threaten spinal cord integrity. Imaging commonly includes X-ray, CT, and MRI. Management ranges from immobilization with a cervical collar to surgical stabilization with posterior atlantoaxial fusion, such as Harms-Goel techniques, particularly in cases of instability or recurrent dislocation. Variants include os odontoideum or a high-riding dens, which may predispose to abnormal motion or instability.