Occipitalization of the atlas: prevalence, functional and anatomical considerations. A systematic review and meta-analysis

Background Occipitalization of the atlas, defined as a congenital fusion between the first cervical vertebra (C1) and the occipital bone, is an uncommon anatomical variant of the craniovertebral junction. Reported prevalence in the general population varies widely, and the condition is often identified incidentally during imaging or anatomical assessment. Objective To synthesize available evidence on the prevalence of atlas occipitalization and to describe its anatomical characteristics across different populations and study designs. Methods A comprehensive literature search was conducted in MEDLINE, Scopus, Web of Science, Google Scholar, CINAHL, and LILACS from inception to January 2025. Study selection and data extraction were performed independently by four reviewers. Methodological quality was assessed using the Anatomical Quality Assessment (AQUA) tool. A random-effects meta-analysis was applied to estimate pooled prevalence values and explore predefined subgroups. Results Twenty-five studies met the inclusion criteria for qualitative synthesis, of which eleven were included in the meta-analysis, encompassing a total of 4219 subjects. The pooled prevalence of atlas occipitalization was 0.64 % (95 % confidence interval: 0.00-1.00 %). Variability in prevalence estimates was observed across populations and assessment methods. Conclusion Atlas occipitalization is a rare congenital anatomical variant of the craniovertebral junction. Although often asymptomatic, its identification is anatomically relevant due to potential associations with other craniovertebral anomalies. Awareness of this variant is important for accurate anatomical interpretation and for planning procedures involving the craniovertebral junction.