Does an alar ligament tubercle exist? Anatomical study with application to imaging interpretation and skull base approaches.

Although numerous anatomical and radiological studies describe a "tubercle" of the alar ligaments at the medial OC as the alar ligament insertion, no systematic osteological investigation has evaluated the prevalence, morphology, and variability of this structure. Therefore, this study aimed to determine whether an alar ligament tubercle (ALT) is a consistent anatomical feature or a morphological variant. Two hundred fifty adult skulls were examined. The presence, morphology, and laterality of the ALTs were documented. Tubercle dimensions were measured using microcalipers, and a classification system was developed to detail their anatomy. High-resolution inferior-view photographs were used to outline the OCs and corresponding ALTs, and ImageJ was used to calculate the proportion of OC surface area occupied by the ALT. An ALT was identified in eight (3.2%), occurring bilaterally in seven specimens and unilaterally in one. Three morphological types were observed: small (≤ 3 mm), large (> 3 mm), and irregular/spicule-like forms. The tubercles occasionally extended to the anterior foramen magnum margin. The mean ALT dimensions were length 4.1 mm, width 3.4 mm, and thickness 4.0 mm. Five tubercles were classified as type 1 (33.3%), four as type 2 (26.7%), and six as type 3 (40.0%). The ALT occupied 10-24% (mean 15%) of the OC surface area. The ALT is a rare anatomical variant rather than a consistent OC landmark. Awareness of this variation could be useful for accurate radiological interpretation, avoidance of diagnostic pitfalls, and safe planning of skull base approaches involving the OC.