Objectives:To investigate the differences of the CT images between normal people and patients with atlanto-axial dislocation

and to scree the most effective radiographic diagnostic indexes.Methods:The CT images of the normal people（n=501）and the atlanto-axial dislocation patients were collected.The anterior atlantodental interval（AADI）

posterior atlantodental interval（PADI）

vertical atlantoaxial index（VAAI）

lateral mass interval（LMI）

rotating angle of atlas on dentata（RAAD）

lateral atlanto-dental interval（LADI）

lateral mass overhang distance

atlanto-axial lateral mass rotating angle of two groups were measured.The various types of the altanto-axial joint dislocation were counted and the statistical analysis was made.Results:AADI

LADI

atlanto-axial lateral mass rotating angles were higher in atlanto-axial dislocation group than in normal group.PADI

VAAI

LMI were lower in atlanto-axial dislocation group than in normal group with significant differences（P<0.01）.The area under ROC curve of AADI（0.795）

PADI（0.825）

VAAI（0.821）

LADI（0.890） were high indicating high diagnostic value.If measured value met the following items

it prompted atlanto-axial dislocation:AADI≥2.2 mm

PADI≤15.4 mm

VAAI≤0.65

LADI≥4.4 mm.Conclusion:AADI

PADI

VAAI

LADI

atlanto-axial lateral mass rotating angles are the most effective diagnostic indexes of atlanto-axial dislocation.According the measured diagnostic results

it is important for a surgeon to chose the reasonable treatment.