LAUSR

STUDY DESIGN Retrospective radiographic analysis. OBJECTIVES Evaluation of the anatomical features of the craniovertebral junction in patients with occipitalization with and without atlantoaxial dislocation (AAD). SUMMARY OF BACKGROUND DATA Atlas occipitalization is a common feature of congenital AAD and usually requires surgical intervention. However, not all instances of occipitalization necessarily lead to AAD. No study has specifically examined and compared the craniovertebral bony morphology in occipitalization with, and without, AAD. METHOD We reviewed computed tomography (CT) scans of 2500 adult outpatients. Occipitalization cases without AAD (ON) were selected. Meanwhile, a series of 20 in-patient occipitalization cases with AAD (OD) were obtained in parallel. Another 20 control cases without occipitalization were also included. Multi-directional reconstructed CT images of all cases were analyzed. RESULTS A total of 18 adults with ON were found in all 2500 outpatients (0.7%). Both anterior height (AH) and posterior height (PH) of C1 lateral mass (C1LM) in the control group were significantly larger than those in both the ON and OD groups, whereas PH in the OD group was significantly less than that in the ON group. Three morphological types of the occipitalized atlas posterior arch were identified: Type I, bilateral sides were unfused with opisthion; Type II, unilateral side was unfused with opisthion, while the other side was fused; and Type III, bilateral sides were fused with opisthion. In the ON group, 3 cases were type I (17%), 6 cases were type II (33%), and 9 cases were type III (50%). In the OD group, all 20 cases were type III (100%). CONCLUSION Atlas occipitalization with, and without, AAD results from a distinctly different bony morphology at the craniovertebral junction. The novel classification system based on reconstructed CT images may be useful in prognosticating AAD in the setting of atlas occipitalization.