Atlanto-occipital assimilation: the role of ex-, basi-, and suboccipital bones in the physiopathology of type I basilar invagination. Patient series.

backgroundAtlas assimilation is the most common congenital anomaly of the craniocervical junction and is consistently associated with type I basilar invagination (BI1). Its precise bone composition and distribution, however, remain poorly defined. OBSERVATIONS: All BI1 patients demonstrated atlas assimilation, which was absent in controls. The condyle-mass complex was the most consistent pattern, forming a continuous monoblock in all cases. Atlas anterior arch-basion fusion was universal but asymmetrical in 2 patients, while posterior arch fusion was complete in 5 patients and partial in 3. Compared with controls, BI1 patients exhibited elongated suboccipital, shortened clivus, shortened exoccipital, and widened lateral masses. LESSONS: CT with multiplanar and 3D reconstructions provides a comprehensive evaluation of atlanto-occipital assimilation in BI1. The condyle-mass complex was the most consistent assimilation site, while anterior and posterior arch fusions were variable. Atlanto-occipital assimilation in BI1 appears to extend beyond midline structures, involving consistent exoccipital-atlas lateral mass nonsegmentation. https://thejns.org/doi/10.3171/CASE25765.