Representation of radiographic measurements: IH (intervertebral height), the distance from the midpoint of the upper endplate of the upper vertebral body to the midpoint of the lower endplate of the lower vertebral body; C2-7a (C2-7 angle), the Cobb angle between the lower endplate of C2 and C7; SA (segmental alignment), the Cobb angle between the superior endplate of the upper vertebra and the inferior endplate of the lower vertebra of the implanted level