Fig. 6

Craniocervical reference planes used in this study 1.Nasion-sella line (NSL, plane through nasion and sella); 2. Frankfort horizontal plane (FH, plane through porion and orbitale); 3. Nasal line (NL, line through the posterior nasal spine and anterior nasal spine); 4.Mandibular line (ML, a line tangent to the lower edge of the mandible); 5. Ramal line (RL, tangent of posterior margin of mandibular ramus); 6. Odontoid process tangent (OPT, the tangent of posterior second cervical vertebral); 7. OP (Odontoid plane, the tangent of anterior second cervical vertebral); 8. CP (Cervical plane, vertically bisect the line of the second cervical vertebral body); 9. C2’ (The tangent of the lower edge of the second cervical vertebra); 10. MGP (McGregor’s line, the line from the posterior edge of the hard palate to the lowest point of the occipital squama)