Case 2: cervical CT. C1 congenital partial assimilation to the skull base. (a) The right C1 lateral mass is partially fused with the right occipital condyle. The left C1 lateral mass is completely fused to the left condyle. The tip of the dens is dysplastic and asymmetrically positioned towards the left lateral mass. (b) The rachischisis of the C1 posterior arch. (d, e) Acute fracture dislocation of the previously right and left lateral mass assimilation, respectively.