Radiological evaluation of the cervical sagittal alignment parameters. (1) White line: Cervical lordosis (CL), the angle between a line parallel to the inferior endplate of C2 and a line parallel to the inferior endplate of C7. (2) Yellow line: Segment alignment (SA), the angle between the lines parallel to the superior and inferior aspect of the surgical segmental vertebral bodies. (3) Red line: Sagittal vertical axis (SVA), the distance between a plumb line from the center of C2 and the superior posterior corner of C7; (4) Green line: T1-slope (T1s), the angle between a line parallel to the superior endplate of T1 and a horizontal line