Reference lines of the: (A) occiput; (B) C1; (C) C2; (D) C4; (E) C5. The reference lines of the occiput and C1 were defined as a line connecting the sellar base and the opisthion and a line connecting the inferior cortical margin of the C1 anterior arch and the inferior cortical margin of the C1 spinous process, respectively [15,16,27]. The reference lines of C2, C4, and C5 were defined as a line connecting the antero-inferior cortical margin of the C2 body and the inferior cortical margin of the C2 spinous process and lines parallel to the endplate of the C4 and C5 body, respectively [15,16,27].