Fig. 3

a Cervical alignment was formed by the lines along the lower endplate of C2 to the lower endplate of C7 in the neutral position. The segmental angle was assessed using the Cobb angle of the vertebral bodies adjacent to the involved disc. b The disk height was the distance from the lower endplate of the cephalad vertebral body to the upper endplate of the caudal vertebral body of the fusion segment was measured. And disc height was measured as the average value of the height of the anterior portion and posterior. c The cephalad and caudal plate-to-disc distance (PDD) and the PDD/anterior body height (ABH) ratio were measured. d The segmental angle was measured using the Cobb angle on flexion X-ray. The interspinous distance was measured on flexion X-ray across the fusion segment. e The segmental angle was measured using the Cobb angle on extension X-ray. The interspinous distance was measured on extension X-ray across the fusion segment