Morphometric Study of the Atlas

 To determine the anatomical parameters of the atlas (C1) in a sample of the Brazilian population and compare them with the results already presented in the literature for other populations and, thus, to identify and change the indications of implants used in the treatment of the upper cervical spine.  We performed a retrospective observational study of a prospective database, including 100 patients seen between January 2012 and December 2013. We used axial and sagittal sections of computed tomography (CT) scans. The parameters studied were; axial: distance from posterior tubercle to the screw entry point (DPT_EP), safety angle of the axial screw (SA_AS), ideal axial screw length (_ASL) and distance of the spinal canal from the transverse foramen (DSC_TF); Sagittal: ideal sagittal screw length (_SSL), safety angle of the sagittal screw (SA_SS) and posterior arch thickness (_PAT). All of the parameters were divided according to age, gender, and left and right sides.  The minimum mean point of entry ranged from 20 mm to 22.7 mm. The average security zone ranged from 18.09° to 23.68°. The mean posterior arch thickness ranged from 7.21 mm to 8.95 mm. The lowest averages were found in females. Using an original and reproducible technique of CT evaluation.  The anatomical parameters of the atlas found in the sample of the studied population were similar to those previously presented in the literature. However, differences between the sexes were observed.