Axis or Second Cervical Vertebra

Description

Oriented in the upper cervical spine, the atlas (C1) sits superior to the axis (C2), with the dens (odontoid process) of C2 projecting cranially toward the anterior arch of C1. Short, paired transverse processes extend laterally from the C2 vertebral arch, while the superior articular facets lie posterolateral to the dens and meet the inferior facets of the atlas at the atlantoaxial joints. Inferiorly, the C2 vertebral body continues into the remainder of the cervical column, separated by an intervertebral disc. Vertebral foramina align posterior to the vertebral bodies to form the cervical spinal canal. Clinically, this anatomy is the mechanical basis for atlantoaxial rotation, and it is also where stability can fail. The dens is restrained against the anterior arch of C1 by the transverse ligament of the atlas, and disruption from rheumatoid arthritis, trauma, or congenital laxity can permit atlantoaxial subluxation with spinal cord compromise. Fractures of the odontoid process (Anderson and D’Alonzo types) and traumatic spondylolisthesis of C2 (hangman’s fracture through the pars interarticularis) are common board and on call diagnoses, and appreciating the relationship of the transverse processes and foramina helps frame the nearby course of the vertebral artery at this level. A hard junction. Use this illustration in gross anatomy or neuroanatomy teaching to anchor discussions of the craniovertebral junction, cervical spine biomechanics, and the difference between the atlas and typical cervical vertebrae. It also fits well in radiology and orthopedic or neurosurgical materials paired with CT sagittal and axial correlations for dens fractures and C2 injury patterns. Anatomical accuracy verified by SciePro's Medical Advisory Board.