Fig. 4

 A 66-year-old woman presented with sensory disturbance and unsteady walking for more than 4 years. After the operation, the patient’s unsteady walking was significantly relieved, and the Japanese Orthopedic Association (JOA) score increased from 11 to 15. A: Preoperative sagittal magnetic resonance imaging (MRI) showed tonsillar hernia, large soft tissue volume behind the odontoid process, and ventral compression of the brain stem. B: Preoperative sagittal computed tomography (CT) showed basilar invagination (BI), occipitalization of C1, clivus-canal angle (CCA) = 130.6°, pB-C2 = 6.5 mm, and no deformity of the bilateral lateral atlantoaxial joint C, D. E: During the operation, the cage was placed between the joints and then screws were placed. F: Sagittal MRI on the third day after operation showed that the compression on the ventral side of the brain stem was reduced compared with that before operation, and the descending degree of the tonsillar hernia had no significant change compared with that before operation. G: Three months after operation, MRI showed that the descending degree of tonsillar hernia was significantly reduced, the volume of soft tissue behind the odontoid process was smaller than that before operation, and the ventral compression of brain stem was relieved. H: Six months after operation, CT scan showed increased CCA, CCA = 137.5°, pB-C2 = 4.4 mm, and bone fusion can be seen between the joints of the bilateral lateral atlantoaxial joint I, J, K