ABSTRACT
I ANATOMY The occipito-atlanto-axial region of the spine is uniquely complex in both its bony and ligamentous composition. The skull articulates with the cervical spine through congruent articulations of the occipitoatlantal joints, with stability conferred by strong and stout ligamentous structures. The occipitoatlantal joints are composed of the convex occipital condyles, which articulate with the reciprocally concave superior articular facets of the atlas. Because of the long anteroposterior diameter of this articulation and its radius of curvature, this joint provides up to 30 of cervical spine flexion and extension. The anterior occipitoatlantal membrane is the physical continuation of the anterior longitudinal ligament. It connects the anterior rim of the foramen magnum to the anterior arch of the atlas. Homologous to the ligamentum flavum, the posterior occipitoatlantal membrane unites the posterior rim of the foramen magnum to the posterior arch of the atlas. The tectorial membrane, running from the dorsal surface of the odontoid process to the ventral surface of the foramen magnum, is the structural continuation of the posterior longitudinal ligament and is believed to be the prime ligament responsible for stability of the occipitoatlantal articulation (1). Hyperextension is limited by the tectorial membrane and by contact between the posterior arch of the atlas and the occiput (Fig. 1).
