Fig. (1) Cephalometric indexes used in two articles [13, 15]. CVT: the upper part of the cervical spine. A line through cv2tg and cv4ip. EVT: the lower part of the cervical spine. A line through cv4ip and cv6ip. OPT: odontoid line. A line through cv2tg and cv2ip. SN: anterior cranial base. A line through point S and point N. PP: palatal plane. A line through posterior nasal spine (pns) and anterior nasal spine (ans). MP: mandibular plane. A tangent line to the inferior border of the mandible. Variables: CVT/EVT: cervical lordosis angle. The downward opening angle between the CVT and EVT line. OPT/Ver: odontoid angle. The downward opening angle between the OPT and true vertical lines. EVT/Ver: lower cervical column angle. The downward opening angle between the EVT and true vertical lines. CVT/ Ver: upper cervical column angle. The downward opening angle between the CVT and true vertical lines. SN/Ver: the downward opening angle between the SN and true vertical lines. PP/Ver: the downward opening angle between the palatal and true vertical lines. MP/Ver: the downward opening angle between the mandibular and true vertical lines. SN/OPT: the downward opening angle between OPT and SN lines. SN/CVT: the downward opening angle between CVT and SN lines. PP/OPT: the downward opening angle between OPT and palatal lines. PP/CVT: the downward opening angle between CVT and palatal lines. MP/OPT: the downward opening angle between OPT and mandibular lines. MP/CVT: the downward opening angle between CVT and mandibular lines.