Morphometric Measurements of the C1 Lateral Mass with Congenital Occipitalization of the Atlas

OBJECTIVE: Unfamiliarity with the morphometry of the assimilated C1 lateral mass (C1LM) could make screw placement dangerous. In the present study, we defined the morphometric dimensions of the occipitalized C1LM to provide surgeons with valuable information for preoperative planning. METHODS: Thin-slice computed tomography scanning data from 131 patients with occipitalization of the atlas (OA) and 50 control cases were imported into Mimics software for analysis. The widths and heights of the C1LM were fully measured in the different planes. The ideal inward angulation and the safe maximum cephalic angulation of C1 screw trajectory were evaluated. RESULTS: Except for the medial height, all the widths and heights of C1LM were significantly shorter in the OA group than those in the control group. The ideal inward angle (alpha) was significantly larger in the OA group (23.8 degrees +/- 8.3 degrees) than that (15.3 degrees +/- 3.8 degrees) in the control group; the corresponding screw length was also significantly longer in the OA group (20.9 +/- 2.9 mm). The safe maximum cephalic angles (beta) of the screw trajectory did not reach a significant difference between the 2 groups. All the widths and heights were shorter in the females than those in the males. The a angle also did not reach a significant difference between the sexes; however, the b angles in the males (35.9 degrees +/- 10.4 degrees) was significantly larger than that in the females (32.0 degrees +/- 9.4 degrees). CONCLUSIONS: Although the hypoplastic C1LM brings limitations to screw insertion to some extent, it is still broad enough to accommodate a screw safely in both female and male patients. Considering the irregularity of the C1LM in patients with OA, the preoperative imaging assessment is critical, and C1LM screw placement should be performed individually.