Fig. 1

Diagram showing the correction of the CCA with interfacet distraction and fixation. A The clivus-canal angle (CCA) is the angle between the plane of the clivus and the posterior edge of the axial vertebrae. The CCA tends to be smaller in basilar invagination (BI) without atlantoaxial dislocation (AAD). The articular surface of BI without AAD tends to be parallel or only slightly tilted anteriorly and posteriorly in the sagittal position. B The lateral atlantoaxial articulation consists of the inferior C1 articular surface and the superior C2 articular surface. After the articular surface was released, bone blocks were inserted to assist in the correction of the CCA. It is equivalent to the descent of the odontoid process and the upward and backward rotation of C1. For atlantoaxial fixation, cantilever technology can be used to assist in increasing the CCA. After fixation and increase of the CCA, a trapezoid was formed between the upper articular surfaces in the sagittal position, with its lower base in front. The blue triangle area decreased after the operation, suggesting that the compression on the ventral side was reduced