C1 Fractures

The craniocervical junction includes the occiput, C1 (atlas), and C2 (axis). C1, also known as the atlas, has a unique anatomy, located just inferior to the occiput, and connects the cranium to the cervical spine through the articulations with C2 (axis) and the occipital condyles. The occipito-atlantal and atlantoaxial articulations contribute significantly to flexion and rotation in the cervical spine, respectively, accounting for 50% of these movements. Due to their high degree of motion, these bony segments are the most commonly injured body parts in adults, particularly prone to fractures. Although C1 fractures rarely require surgery, they should be promptly identified and treated in all patients. Any fracture involving the atlas warrants a thorough examination of the ligamentous structures between C0 and C1 and C1 and C2.

The C1 (atlas) vertebra lacks a vertebral body and spinous process. Instead, it consists of anterior and posterior arches that encircle the spinal cord posteriorly and the odontoid process anteriorly. The lateral masses on either side join the arches, with their superior articular surfaces participating in the occipital-cervical joint and their inferior articular surfaces contributing to the highly mobile atlantoaxial joint. Stabilization of the atlantoaxial joint is achieved through the anterior atlantoaxial ligament (between the anterior ring of C1 and C2), the transverse ligament (posterior to the odontoid process), and the posterior atlantoaxial ligament (between the posterior ring of the atlas and C2). Among these, the transverse ligament plays the most significant role in C1-C2 articulation. C1 characteristically lacks a spinous process or vertebral body. C1 behaves as an intercalated segment cradling the base of the occiput at the atlanto-occipital articulation, allowing almost 50% of flexion and extension movements at the neck.