This study aims to determine the prevalence and types of the arcuate foramen (AF), an anatomical variation of the first cervical vertebra (atlas), using three-dimensional computed tomography (3D CT). The arcuate foramen is formed when a bony bridge develops over the groove for the vertebral artery. Although often asymptomatic, it may increase the risk of vascular or nerve injury during surgery in the craniovertebral junction region.

In this retrospective observational study, 3D CT scans of 200 adult patients obtained between January 2023 and July 2025 at Giresun University Faculty of Medicine were reviewed. The presence of AF was classified as complete or incomplete, and also categorized according to its location (right, left, or bilateral). Associations with demographic variables (age, sex) were analyzed.

The results will help improve awareness of AF in preoperative planning for neurosurgical and spinal procedures, potentially reducing the risk of complications involving the vertebral artery and surrounding neurovascular structures.

The arcuate foramen (AF) is an anatomical variation of the atlas (C1 vertebra) that occurs when a bony bridge forms over the groove for the vertebral artery, creating a complete or partial tunnel. While frequently asymptomatic, its presence has been associated with conditions such as vertebrobasilar insufficiency, cervicogenic headache, and chronic neck pain. Importantly, AF may alter the course of the vertebral artery and suboccipital nerve, increasing the risk of injury during craniovertebral junction surgery, C1-C2 instrumentation, and posterior fossa approaches.

This retrospective cross-sectional study reviewed 3D computed tomography (CT) images of 200 adult patients obtained between January 2023 and July 2025 at Giresun University Faculty of Medicine. Images were reconstructed and analyzed using OsiriX MD software with a 3D volume rendering technique. AF was classified as complete or incomplete and further categorized by side (right, left, or bilateral). Associations with demographic variables, including age and sex, were examined.

Prevalence estimates were calculated with 95% confidence intervals. Interobserver agreement between two experienced neurosurgeons was assessed using Cohen's Kappa statistic. The findings aim to provide a clearer understanding of AF prevalence in the Turkish population and its potential surgical relevance. By identifying these variations preoperatively, surgeons can better plan instrumentation trajectories and minimize neurovascular complications in craniovertebral and upper cervical procedures.

Inclusion Criteria:

• Age ≥ 18 years at the time of imaging
• High-quality CT or CTA images including the atlas (C1) vertebra and posterior arch
• Imaging obtained between January 2023 and July 2025 at Giresun University Faculty of Medicine
• Images suitable for 3D volume rendering analysis without significant artifacts

Exclusion Criteria:

• Motion or metallic artifact preventing adequate evaluation of atlas anatomy
• Prior cervical spine surgery or trauma affecting the atlas
• Congenital anomalies or severe deformity of the atlas obscuring posterior arch assessment
• Incomplete demographic or imaging data

Adult patients (≥18 years) who underwent head computed tomography (CT) or computed tomography angiography (CTA) at Giresun University Faculty of Medicine between January 2023 and July 2025, whose images included the atlas (C1) vertebra and met quality criteria for three-dimensional reconstruction.

This study involves retrospective analysis of de-identified radiological images. Individual participant data will not be shared to protect patient confidentiality.