C1 Assimilation and Basilar Invagination

3D Model

Three-dimensional reconstruction of the craniovertebral junction demonstrating a rare constellation of congenital anomalies: C1 assimilation, platybasia, and basilar invagination, resulting in superior migration of the odontoid process and anterior brainstem compression. The occipital condyles and atlas are fused, producing a rigid cranial–C1 block, while the flattened skull base (platybasia) reduces the clivus–axial angle and facilitates upward displacement of the dens. A high-riding vertebral artery entering the transverse foramen at C5 is present. The upward projection of the dens impinges on the ventral medulla and lower pons, illustrating the mechanism of cervicomedullary compression characteristic of basilar invagination.