ABSTRACT
I INTRODUCTION The atlantoaxial region of the spine is a transition zone that helps allow the head to properly function on top of the spine. Specifically, the atlantoaxial joint allows a large degree of head rotation. Its stability is provided by both bony and soft tissue structures. As opposed to the well-fitting, convex-concave design of the occipitalatlantal joints, which provide for weight bearing and some flexion extension, the corresponding facet joints of C1 and C2 are both convex in design. This allows for approximately 50% of the axial rotation of the entire cervical spine. Flexion and extension are approximately 13 to 15 and rotation is 45 to each side at this articulation. This biconvexity also explains the vertical approximation seen with rotation. The low point of each joint surface is at maximal rotation and the highest point is at neutral rotation. Rotation past 63 to 65at the C1-C2 articulation will start to result in dislocation of the C1-C2 facet joint, with upper cervical canal narrowing down to 7 mm. Rotatory injuries of the atlantoaxial joint thereupon range from a
minor subluxation that preserves normal C1-C2 motion correctable with collar immobilization to frank dislocation with an attendant dense neurological deficit (1-4). A Anatomy The cruciate ligament stabilizes the atlantoaxial complex. The horizontal limb of the cruciate ligament is the thick and sturdy transverse atlantal ligament, which attaches to the condyles of the axis. The ascending limb of the cruciate ligament attaches to the anterior margin of the foramen magnum while the descending limb attaches to the body of the axis. Just posterior to the cruciate ligament is the tectoral membrane. Anterior to the cruciate ligament are the paired alar ligaments and the single apical ligament. The alar ligaments attach from each side of the odontoid process to the medial aspect of each occipital condyle. The apical ligament attaches from the apex of the odontoid process to the anterior rim of the foramen magnum.
