Anterior and inferior regions of the 4th cervical vertebra were set as the origin “O” of the coordinate, and starting from the origin, the straight line connecting the anterior and inferior regions of the 2nd cervical vertebra was set as the y-axis. With the y-axis passing the origin as the center, the perpendicular line was set as the x-axis vertebra, and the x-axis was the line perpendicular to the y-axis.