Case ReportA fourth atlantoaxial joint: an initial description
Introduction
The C1–C2 (atlantoaxial) articulation is the most complex articulation in the spine. The first two vertebrae are structurally different from the other vertebrae and together form an articular system whose primary function is to allow rotation and flexion/extension of the head. The concave inferior facet surfaces of the paired lateral masses of C1 are angled internally and articulate with the convex laterally angled superior C2 facets. A semicircular depression on the posterior surface of the anterior arch marks the third atlantoaxial synovial articulation between the anterior arch and the odontoid process.
The development of the atlantoaxial joint is complex. It begins in utero and is not typically complete until the eleventh year of life [1], [2]. This complexity allows for the possibility of congenital anomalies, and, although many have been documented, to our knowledge there have been no reports of the formation of a supernumerary (fourth) atlantoaxial synovial facet joint. In this report, we present the case of a 35-year-old woman who presented with a unilateral anomalous posterior facet joint, and we discuss its possible embryological origin.
Section snippets
Case report
A 35-year-old woman presented with an 18-month history of posterior neck pain and headaches. Her neck pain was significantly worse with flexion or extension and better at rest. Additionally, she was experiencing tingling and numbness of the left arm and hand.
On neurological examination, the patient had full strength in all muscle groups. Sensation was intact to light touch and pinprick, and joint position sense was intact. Her reflexes were two plus in both her upper and lower extremities.
Plain
Discussion
Several congenital anomalies occur in the occipitocervical region (occiput–C1–C2), including occipitalization of the atlas, defects of the anterior or posterior rings of the atlas, basilar invagination, posterior arch defects of the axis, and anomalies of the odontoid [3]. A rare anomaly described by Rao [4], termed the median occipital condyle or condylus tertius, is analogous to the anomaly found in our case in that there is an extra joint at the atlanto-occipital articulation. In that case,
Acknowledgments
We thank Kristin Kraus, MSc, for editorial assistance in preparing this article.
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Author disclosures: RIR (grants, Stryker Spine).