Fig. 2

 A 51-year-old man presented with gait disturbance and dizziness for six months. The symptoms of the patient significantly improved postoperatively, and the JOA score increased from 13 before the operation to 16. A and B: Sagittal reconstructed computed tomography (CT) scan showing the presence of occipitalization of C1, basilar invagination (BI) without atlantoaxial dislocation (AAD), clivus-canal angle (CCA) = 128.3°, pB-C2 = 5.3 mm, and that the relationship of the two facets of the C1–C2 joint was relatively parallel (red lines). C: Preoperative magnetic resonance imaging (MRI) scan showing associated Chiari malformation and syringomyelia. D: Postoperative sagittal CT showed that the odontoid process decreased slightly and the CCA increased; CCA = 135.6° and pB-C2 = 2.7 mm. The two facets of the C1–C2 joint are not parallel E. F: The facets of C1 and C2 were released, and an autologous bone block was implanted. G: Sagittal MRI showed a slight decrease in the syringomyelia and a slight upward movement of the tonsillar herniation. The 30-month follow-up showed that the syringomyelia disappeared completely and the tonsillar herniation was reduced (H), and bony fusion occurred as confirmed on the CT scan I