Unilateral arcuate foramen with a posteriorly projecting vertebral artery loop between C1-C2: a rare anatomical variation.

PURPOSE: To describe the morphological features of this rare anatomical variation and to evaluate its significance in relation to embryological development, potential pathological consequences, and surgical considerations. METHODS: A detailed anatomical dissection of the craniovertebral junction was conducted. Morphometric analysis was conducted to document the characteristics of the arcuate foramen and the course of the vertebral artery, with emphasis on osseous bridging and vascular loop. RESULTS: On the right side, two osseous bridges (ponticulus posticus and ponticulus lateralis) were identified between the lateral mass of the atlas and both its posterior arch and transverse process. The ipsilateral vertebral artery demonstrated a pronounced posterior loop-like tortuosity in its course between the transverse foramen of the axis and that of the atlas. CONCLUSION: This first reported case of a concurrent arcuate foramen and posterior vertebral artery loop at C1-C2 level significant challenges for surgical planning and procedures. It emphasizes that recognizing such complex variations through careful imaging is essential for safe intervention at the craniovertebral junction.