Unclosed transverse foramen of the atlas: A systematic review and meta-analysis of prevalence and translational anatomical relevance

Background The atlas vertebra (C1) has a close anatomical relationship with the vertebral artery through the transverse foramen. Incomplete osseous closure of this canal, described as an unclosed transverse foramen (UTF), may modify the expected bony boundaries around the vertebral artery. Objective This systematic review and meta-analysis estimated the prevalence of UTF in anatomically modern humans and summarized its anatomical and potential clinical implications. Methods Searches were conducted in MEDLINE, CINAHL, Web of Science, Scopus and complementary sources following PRISMA 2020 and evidence-based anatomy recommendations. Eligible studies were original human anatomical, osteological, cadaveric, skeletal or imaging-based studies that reported C1-specific UTF with an extractable numerator and denominator. Methodological quality was assessed using the AQUA tool. Pooled prevalence was estimated using a random-effects model with logit-transformed proportions. Results Six studies were included in the quantitative synthesis. The pooled prevalence of UTF was 8.8%, with negligible between-study heterogeneity. Methodological quality was predominantly favorable. Conclusions UTF is an uncommon but recurrent anatomical variant of the atlas. Current evidence does not establish isolated UTF as an independent cause of symptoms. The variant should nevertheless be recognized in anatomical description, radiological reporting and preoperative planning for upper cervical procedures.