Measurement of intervertebral space and cervical curvature. H0, the preoperative height of surgical segment intervertebral disk space, the distance measured from the middle of an inferior endplate of the upper vertebral body to the superior endplate of the lower vertebral body. H1, upper intervertebral disk space height before the operation. H2, inferior intervertebral disk space height before the operation. α1, surgical segmental curvature, angle between two lines perpendicular to the superior endplate of the upper vertebral body and the inferior endplate of the lower vertebral body in the surgical segment. α2, global curvature, angle between two lines perpendicular to the inferior endplate of the C2 and the inferior endplate of the C7