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Reference planes and pharyngeal airway measurements used in this study. (1) FH plane: the plane including porion (Po) and orbitale (Or). (1′) FH′ plane: the plane that is parallel to the FH plane and passing through the posterior nasal spine (PNS). (2) The anterior border of the nasopharyngeal airway, which is the coronal plane passing through the posterior nasal spine (PNS) and superior tip of the nasopharyngeal airway. (3) ang-PA. (4) ang-OA. (5) Vol-NA. (6) Vol-OA.
