Table 2 Definitions of tongue dimension and position and oropharyngeal airway space measurements used in this study

Tongue length (TGL) (mm)

Length of tongue between epiglottis base and tongue tip (epiglottis base, the point located at the intersection of the epiglottis and the base of tongue)

Tongue height (TGH) (mm)

The length of the vertical bisector from the dorsal tongue surface to a line connecting between epiglottis base and tongue tip

Palatal height (PH) (mm)

The distance between the tongue’s highest point and the palate

Intraoral airway volume (IAV) (mm³)

The empty intraoral space between the tongue and palatal section

PNS_perp-TT/PNS_perp-U1

The ratio of the distance from TT and U1 to a PNS perpendicular line to the FH plane

Velopharynx (VP)

The area between a plane parallel to the Frankfort horizontal passing through posterior nasal spine (PNS) and a plane parallel to the Frankfort horizontal passing through the end of the soft palate

Glossopharynx (GP)

The area bounded superiorly by the inferior border of velopharynx and inferiorly by a plane parallel to the Frankfort horizontal passing through the tip of the epiglottis