Vertebral reference lines. The reference line for the occiput was defined as a line between the base of the sella and the opisthion (line a), and the C1 reference line as a line between the lower cortical margin of the anterior arch of C1 and the lower cortical margin of the C1 spinous process (line b). The C2 reference line was defined as a line between the anterior, inferior margin of the C2 body and the lower cortical margin of the C2 spinous process (line c). The C5 reference line was a tangent along the superior end-plate of the C5 vertebral body (line d)