Partial Atlanto-Occipital Assimilation in a Dry Human Skull: An Osteological Case Report and Clinical Implication

Abstract

Background: Atlanto-occipital assimilation, also termed occipitalization of the atlas, is a congenital craniovertebral junction anomaly characterized by partial or complete bony fusion of the atlas (C1) with the occipital bone. Although many individuals remain asymptomatic, such fusion can alter the anatomy of the foramen magnum and predispose to atlantoaxial instability and neurovascular compromise in some patients.

Case description: During routine osteology teaching in the Department of Anatomy, AIIMS Guwahati, a dry adult human skull with an attached atlas vertebra was found to exhibit partial fusion between the atlas and the occipital bone. Fusion involved the anterior arch of the atlas and the anterior and lateral margins of the foramen magnum, while the posterior margin remained unfused. The anteroposterior and transverse diameters of the foramen magnum measured 3.2 cm and 3.0 cm respectively, lying toward the lower end of reported normal adult ranges. Three distinct foramina were observed: a right posterior condylar foramen, an accessory anterior tunnel-like foramen between the fused anterior arch and the anterior margin of the foramen magnum, and bilateral intervertebral foramina for the first cervical nerve.

Conclusion: This specimen represents a combined anterior and lateral partial occipitalization. Despite near-normal linear dimensions of the foramen magnum, the altered bony configuration around the craniovertebral junction could have important clinical implications in vivo, particularly for radiological interpretation and surgical planning.