C1 Fractures

Atlas has unique anatomy in that it sits just inferior to the occiput and, through the articulations with C2 and condyles of the occipital bone, joins the skull with the cervical spine. C1 is devoid of a vertebral body. It contains the anterior and posterior arches, which encircle the spinal cord (posteriorly) and the odontoid process (anteriorly). The arches are joined by lateral masses on either side, whose superior and inferior articular surfaces take part in the occipito-cervical and atlantoaxial joints, respectively. The atlantoaxial joint is highly mobile and is stabilized by the anterior atlantoaxial ligament (between the anterior ring of atlas and C2), transverse ligament (posterior to odontoid process), and posterior atlantoaxial ligament (between the posterior ring of atlas and C2). The transverse ligament contributes the most to the C1-C2 articulation among these ligaments. The atlas characteristically lacks a spinous process or vertebral body. It behaves as an intercalated segment cradling the base of the occiput at the atlanto-occipital articulation allowing almost fifty percent of flexion-extension to occur at the neck. The craniocervical junction is comprised of the occiput, C1 (atlas), and C2 (axis). The occipito-atlantal and atlantoaxial articulations provide 50% of the flexion and rotation in the cervical spine, respectively. Due to their high degree of motion, these bony segments are also the most often injured in adults. C1 fractures should be promptly identified and treated in all patients, although they rarely require surgery. Any bony fracture of the atlas merits a thorough examination of the ligamentous structures between O-C1 and C1-C2.
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