Abstract: BACKGROUND: The development and evolution of atlas are complex, and there are a few research reports. CT imaging can show the normal development process, anatomical structure, developmental variation and deformity of the axis. It has important clinical value to clarify the time of occurrence of ossification center of axis and closure of epiphyseal plate and its evolution process and law. 
OBJECTIVE: To present the anatomical structure of the normal development and variation of children’s axis based on CT images.
METHODS: CT images obtained from 732 children aged 0 to 15 years who underwent neck scans between June 2016 and November 2019 were retrospectively analyzed. The observation indicators encompassed the axis odontoid, bilateral pedicle, vertebral ossification center, secondary ossification center at the tip of odontoid, pedicle, base of odontoid, and posterior median epiphyseal plate, as well as any variations or deformities in axis development. The changes in these indicators were analyzed and compared across different age groups. SPSS 17.0 statistical software package was utilized for data classification and statistical analysis. 
RESULTS AND CONCLUSION: (1) A total of 732 subjects were examined, comprising 718 cases (98.1%) with normal development of the axis and 14 cases (1.9%) exhibiting deformity or dysplasia. (2) The axis demonstrated the presence of five ossification centers, including those of the bilateral pedicles, odontoid process, and vertebral body, which were observed at birth. The median age for the secondary ossification center located at the tip of the odontoid process was determined to be 5.7 years, with an interquartile range of 4.1 to 7 years. The earliest recorded occurrence was observed at 8 months and 22 days, while the latest occurrence was noted at 12 years and 10 months. (3) The median age at which fusion took place was 6 years, with an interquartile range of 5-8 years. The maximum age at which non-fusion was observed was 8 years and 9 months, while the minimum age at which fusion occurred was 4 years and 3 months. (4) The median age at which bilateral epiphyseal plate closure occurred was approximately 3.8 years, with an interquartile range of about 2.9-4.6 years. The earliest observed closure was at 2 years and 3 months, while the latest observed unclosure was at 6 years old. (5) The median age at which odontoid base epiphyseal plate closure occurred was 5.2 years, with an interquartile range of 3.5-6.8 years. The minimum age for closure was 2 years and 6 months, and the latest age for non-closure was 9 years and 6 months. (6) The posterior median epiphyseal plate typically closed at a median age of 1.5 years, with an interquartile range of 1.0-2.1 years. However, two cases exhibited delayed closure, occurring at ages 2 years and 5 months, and 14 years, respectively. The earliest closure observed was at 6 months and 20 days. (7) Axis malformation or developmental abnormalities, including 7 cases of accessory ossification center and accessory epiphyseal plate, 3 cases of free ossified small bones in the axis, 2 cases of posterior median epiphyseal plate failure, 2 cases of secondary ossification centers in the absence of the apex of odontosis, and 1 case of absence at ossification center in the odontoid of the armature vertebrae. (8) It is concluded that the utilization of multi-slice spiral CT scanning in conjunction with the multi-plane reconstruction technique enables comprehensive visualization of the anatomical structure of the axis, facilitating precise assessment of both its typical developmental variations and deformities.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: ">axis, ossification center, epiphyseal plate, development and variation, CT image