## 2 Materials and Methods
### Radiographic Analysis
#### 2.2.2 Cervical Curvature  
Classification  
For our study, we classified cervical curvature into four categories suggested by [PERSON] et al. [4]. One linear measurement method and two angular measurement methods were used to evaluate cervical curvature. 1. C2-C6 Ohara method (Figure 2) [27]: A line was constructed to connect the midpoint of the C2 inferior endplate and the C6 superior endplate. The centroids of C3-C5 were defined as the points of intersection of lines drawn from opposite corners of the vertebral body. The four types of cervical curvature were defined based on the relative positions of the C3-C5 centroids to line AB. If all centroids were anterior to line AB and the maximum distance was >1 mm but <2 mm, it was classified as 'lordotic'. If the distance between line AB and each centroid was <1 mm, the line was classified as'straight'. If all centroids were posterior to line AB and the maximum distance was >1 mm, it was classified as 'kyphotic'. If some centroids were anterior to and some posterior to line AB but the maximum distance was >1 mm, it was classified as'sigmoid'. 2. C2-C6 Cobb method (Figure 3) [28]: The Cobb angle is the angle between the two perpendicular lines made from the inferior margin of C2 and the superior margin of C6. The angle is considered to be positive when the superior margin of C6 was more clockwise than the inferior margin of C2. The cervical spine types are classified according to the following criteria: If the Cobb angle is >7\({}^{\circ}\) but <20\({}^{\circ}\), it is a 'lordotic' curvature type; if it is >-7\({}^{\circ}\) but <7\({}^{\circ}\), it is a'straight' curvature type; and if it is \(-7^{\circ}\) or less, it is a 'kyphotic' curvature type. Figure 1: (**a**) Sagittal vertical axis. This is the horizontal distance between a plumb line dropped from the anterior margin of the external auditory meatus (CGH) and the posterior–superior corner of C6. (**b**) Cervical inclination angle. This is the angle formed by the line connecting the posterior superior corner of the sixth cervical vertebra with the centroid of the second cervical vertebra (C2) and the horizontal line. The centroid of C2 is the point where the lines drawn between the opposing corners within the C2 intersect. SVA, sagittal vertical axis; CGH, the center of gravity of the head; C6, sixth cervical vertebra. C2-C5 Harrison posterior tangent method (Figure 4) [28]: The lines are drawn parallel to the posterior surface of each cervical vertebral body from C2 to C5, and angles from C2 to C3, C3 to the fourth cervical vertebra (C4), and C4 to C5 is added. When the posterior surface line of the lower vertebral body opens more clockwise than the upper vertebral body, the angle is considered positive. In this study, if the summed angle was >10\({}^{\circ}\) but <30\({}^{\circ}\), it was classified as a 'lordotic.' If the summed angle was <10\({}^{\circ}\) but >\(-\)5\({}^{\circ}\), it was classified as'straight.' If the summed angle was <\(-\)5\({}^{\circ}\), it was classified as 'kyphotic'. Because this study analyzed data from the middle of C6 to the upper part of the cervical vertebral bodies, measurements were extended only up to C5 in this method. Figure 3: Cobb angle. This is the angle between the two perpendicular lines made from the inferior margin of C2 and the superior margin of C6. When the superior margin of C6 is rotated more clockwise than the inferior margin of C2, the angle is positive. Figure 2: Ohara method. A line is constructed to connect the midpoint of the inferior endplate of the second cervical vertebra (C2) (A point) and the midpoint of the superior endplate of the sixth cervical vertebra (C6) (B point). The centroids of the third, fourth, and fifth cervical vertebrae (C3, C4, and C5) are the points of intersection of lines drawn from opposite corners within the vertebral body. The four types of cervical curvature are defined based on the relative positions of the C3–C5 centroids to line AB. Lordotic: All centroids are anterior to line AB, and the maximum distance is >1 mm but <2 mm.