The role of Atlas assimilation in the context of craniocervical junction anomalies

Introduction Atlas assimilation (AA) may be associated with atlanto-axial dislocation (AAD), Chiari malformation (CM) and Basilar Invagination (BI). The importance of AA in the context of CVJ anomalies is unclear. Considering this context, this study’s objective is to discuss the role of AA in the management of CVJ anomalies, especially in CM. Methods A comprehensive literature review was performed. Additionally, some illustrative cases were discussed based on our review and we finally propose a theoretic algorithm to evaluate patients with AA and CM. Results AA is a proatlas segmentation anomaly that may be complete or incomplete. It may be totally asymptomatic or symptomatic as the result of transferred shifted forces onto the C1-2 joints leading to clear instability (AAD) or mild C1-2 instability. Cautious surgical planning may be required due to associated vertebral artery (VA) anomalies. AA with concomitant C2-C3 segmentation failure is highly associated with late C1-C2 instability. CVJ decompression failure was reported in patients with CM and a low Clivus Canal Angle (CCA) ( Conclusions Although AA per si it is not considered an unstable configuration, further and detailed evaluations of patients with CM associated with AA are necessary. Some associated unstable configurations required concomitant CVJ fixation.