Fig. 1

a The crossing angle of the line of inferior 1/3 clivus (line a) and the line extending from the posterior border of the dens to the posterior-inferior border of the axis body (line a’) was defined as clivus-canal angle (CCA). The distance from the posterior border of the anterior C1 arch to the anterior border of the odontoid process (line b) was measured as anterior atlantodental interval (ADI). b The crossing angle of the line extending the anterior border of the ventral medulla (line c) and the line extending the anterior border of the ventral upper cervical spinal cord (line d) in midline sagittal T2 MRI was defined as cervicomedullary angle (CMA). c and d The middle line of the lateral mass in the cross-sectional slice was defined as line e. The corresponding sagittal reconstruction CT based on line e was seen in D. We measured the anterior part, middle part, and posterior part of the lateral mass interspace, and the average value was defined as the lateral mass interspace (LMI). The combined bilateral LMI was defined as sum of LMI (SLMI)