Fig. 1

A 50-year-old male patient was admitted for numbness of limbs with walking instability for 6 months, aggravated for 2 months. The ADI was 5.83 mm in preoperative hyperflexion X-ray (a), but the atlantoaxial dislocation could be completely reduced in the hyperextension X-ray (b, white arrow), So it was classified as type I. Computed tomography (CT) coronal reconstruction presented the OO (c, black arrow). The upper cervical spinal cord was compressed by the superior posterior part of axis based on magnetic resonance imaging (MRI), with a cervicomedullary angle of 143° (d). Posterior fixation and fusion were performed. Postoperative X-ray showed atlantoaxial fixation by screw and rod, and anatomical reduction was observed (e, blue arrow). At 6 months after operation, CT reexamination showed posterior bone graft healing (f, red arrow)