Three bones of the occipital cervical junction, occiput, C1, and C2, join together to form a single, complex, highly mobile articulation. The structural ligaments of this joint complex span from the occiput to C2 with C1 acting as a spacer or a cushion. The convex surfaces of the occipital condyles join matching concave surfaces on the cephalad portion of C1. This joint provides half of the overall cervical flexion / extension motion (45 degrees). The transverse ligament attaches to the anterior arch of C1 on both sides and wraps around the odontoid process of C2. The flat articulating surfaces between the lateral joints of C1 and C2 provide rotation along the axis of the odontoid process. This joint provides half of the normal rotation of the cervical spine (45 degrees to each side). The principal ligaments joining these three bones together are the two alar ligaments extending from the base of the occipital condyles to the lateral top surfaces of the odontoid process and the central apical ligament. Additional secondary restraints include the cephalad portion of the anterior longitudinal ligament (anterior atlanto occipital ligament) cephalad extension of the posterior longitudinal ligament (tectorial membrane), and cephalad extension of the ligamentum flavum (posterior atlanto occipital membrane).