A new variant ligament of the atlantooccipital joint: the lateral oblique atlantooccipital ligament

Purpose: During routine dissection of the anterior craniocervical junction (CCJ), a variant ligament just anterior to the articular capsule of the atlantooccipital joint was observed. To our knowledge, no literature has previously described this ligament. Therefore, the aim of this study was to clarify the anatomy, incidence, and biomechanics of this undescribed structure of the anterior atlantooccipital joint. Methods: Twenty-six sides from 13 fresh-frozen adult cadavers were used for this study and the morphology of the variant ligament examined. When present, its length, width, thickness, and the angle from the midline of the CCJ were measured. Results: The variant ligament identified, when present, is distinct and located anterior to the atlantooccipital joint capsule traveling between the occipital bone and the transverse process of the atlas. The ligament was found on 12 of 26 sides (46.2%). The mean length of the ligament was 32.0 ± 5.5 mm. The ligament became taut with contralateral lateral flexion and the ipsilateral rotation of the atlantooccipital joint. Conclusions: We propose that this ligament may be termed the lateral oblique atlantooccipital ligament. To date, this structure has not been described in any textbooks or reports in the extant medical literature. Although its function is not clear, based on its course and connections, it might function as a secondary stabilizer of the atlantooccipital joint. As the stability of the craniocervical junction is of paramount importance, knowledge of normal and variant anatomical structures in this region is important for the surgeon treating patients with pathology of this region. Graphical abstract: These slides can be retrieved under Electronic Supplementary Material.[Figure not available: see fulltext.].