Fig. 1

The measurement method for radiological parameters. The angle between the inferior margin of the C2 vertebrae and the inferior margin of the C7 vertebrae was measured as cervical lordosis, while the lines between the superior endplate of the cephalad vertebral body and inferior endplate of the caudal vertebral body were measured as the FSU angle. The FSU height was measured as the distance from the highest portion of the upper end plate of the cephalad vertebra to the lowest portion of the lower end plate of the caudal vertebra in the anterior or posterior FSU. The ROM was the difference in CL and FSU angle between the full flexion and extension radiographs. CL, cervical lordosis; FSU, functional spinal unit; AFH, anterior FSU height; PFH, posterior FSU height