| Background and purpose: Craniovertebral junction abnormalities are divided into congenital and acquired malformation according to its original formation. The former is the congenital malformation of occipital bone, atlas, axis and relative soft tissues such as ligaments, and so on. The occipital foramen magnum, atlas and axis consist of the region of craniocervical junction. During the development process of embryogenesis a great variety of malformations caused by some harmful factors are named congenital craniocervical malformation, also called congenital anomalies of atlanto-occipital region or congenital malformation of foramen magnum region. It is a kind of congenital malformation affected the health of peopleseriously. So far, there has not been an ideal treatment method for the disease especially in the prevention and treatment of postoperative instabilization. The structural stability of occipital bone, atlas and axis maintain the stability of the craniocervical junction together with pertaining muscles, ligaments, fascias as well as joint capsules. The clinical manifestations of the instability of the craniocervical junction appear when the function and structure of atlanto-occipital joint and antalto-axial joint are destroyed, and then cause abnormal or excess movement in physiological state. With the further research on biomechanics, people know deeply about the instability of the craniocervical junction. There many factors affect the stability of the superior cervical vertebrae including congenital malformation, trauma, inflammation, rheumatoid arthritis, tumor, tuberculosis and medical injury. Presently, the research on the factors about the instability of congenital craniocervical malformation is still scarcely reported systematically at home and abroad. The aim of this research is to provide the theory for enhancing the clinical treatment effects of the disease through the retrospectively analysis of the image measurement, the clinical data, follow-up survey of patients with craniocervical malformation.Materials and Methods: Select 40 cases diagnosed as congenital craniocervical malformation, with complete clinical and imaging data. According to post-operative clinical symptoms and signs and/or imaging data, 40 cases are divided into different groups - stable group (Scases) and instablegroup (32cases). As a control, 20 normal subjects were also investigated by the means of the same image processing software. The structures of the brain and skull were investigated on mid-sagittal Tl-weighted MR image. The nonparametric statistics (Kruskal-Wallis test) and Chi-Square test was used for statistical analysis by the software of SSPS10.0 version. Take ". =0.05" as test standard.Results: Comparing to the control group, there was a significantly larger distance between atlas and odontoid process of axis ratio (jP<0.05); brainstem-spine angle was a significantly smaller ratio (.P<0.05) among the patients who suffered from craniovertebral congenital abnormalities. The incidence of basilar invagination, atlanto-occipital fusion between the patient group and control group has a significantly ratio (P<0.05). Segmentation failure of C2-3 between the patient group and control group has not a significantly ratio (P>0.05).Conclusion: (1) Comparing to the control group, there was a significantly larger distance between atlas and odontoid process and a significantly smaller brainstem-spine angle among the patients who suffered from craniovertebral congenital abnormalities. Author presume that the more serious, the more probable the instability of craniovertebral juntion. (2) The incidence of basilar invagination, atlantooccipital fusion among the patient group has a higher ratio than control and stable groups, and segmentation failure of C2-3 has no obvious difference. Author deduce that basilarinvagination, atlantooccipital fusion which caused atalanto-occipital subluxation are the main factors caused the instability of craniovertebral juntion. (3) The incidence of instabilities among the patients...
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