Occipitalisation of the atlas. Museological case-studies and review of population-based studies

Background: Occipitalisation of the atlas, or atlanto-occipital fusion (AOF), is an anomaly of the craniovertebral junction (CVJ). It may be either congenital or acquired and can result in marked asymmetry and stenosis of the foramen magnum. Additionally, it may contribute to spinal instability and the development of neurological symptoms. The study aims to investigate the presence of this condition in a museological collection from Italy, reaffirming the importance of museum collections in the study of human variability. It also compiles previously reported cases of AOF to provide a comparative framework across modern and ancient populations. Materials and methods: We analysed 126 human crania housed at the Museum of Anthropology “Giuseppe Sergi”, Sapienza University of Rome. Of these, 18 specimens were excluded due to taphonomic fractures involving the foramen magnum region. Consequently, 108 crania were deemed suitable for analysis. Standard anthropological techniques were employed to assess biological profiles, including sex and age estimation. Macroscopic analysis specifically targeted the identification of AOF and potential comorbidities. Results: In this museological sample, we identified two cases of occipitalization of the atlas, both exhibiting partial fusion. The first case (Cranium 5293), a late adult male, presented a bifid posterior arch and a paracondylar tubercle. The second case (Cranium 5309), a young adult male, presented a well-formed atlas and showed the fusion of the right transverse process with a paracondylar process. Conclusions: The two cases of AOF identified are consistent with most known morphological patterns of this condition. While acknowledging the inherent limitations of skeletal samples for epidemiological interpretations, our findings contribute to the most comprehensive review of AOF prevalence records to date. We emphasise the need for improved methodological transparency and broader geographical representation in future research to better understand AOF's anatomical and anthropological significance.