Three-dimensional changes of the hyoid bone and airway volumes related to its relationship with horizontal anatomic planes after bimaxillary surgery in skeletal Class III patients

Abstract

Objective: To evaluate longitudinal changes of the hyoid bone position and pharyngeal airway space after bimaxillary surgery in mandibular prognathism patients.

Materials and methods: Cone-beam computed tomography scans were taken for 25 mandibular prognathism patients before surgery (T0), 2 months after surgery (T1), and 6 months after surgery (T2). The positional displacement of the hyoid bone was assessed using the coordinates at T0, T1, and T2. Additionally, the volume of each subject's pharyngeal airway was measured.

Results: The mean amount of posterior maxilla impaction was 3.76 ± 1.33 mm as the palatal plane rotated 2.04° ± 2.28° in a clockwise direction as a result of bimaxillary surgery. The hyoid bone moved backward (P < .05, P < .001) and downward (P > .05, P < .05) at 2 months and 6 months after surgery, while the total volume of the pharyngeal airway significantly decreased at the same time points (P < .001, P < .001). There was significant relationship between the changes of the hyoid bone position and airway volume at 2 months after surgery (P < .05). The change of the palatal plane angle was positively correlated to the decrease in the total airway volume (P < .001).

Conclusions: The null hypothesis was rejected. The hyoid bone moved inferoposteriorly, and the pharyngeal airway volume decreased for up to 6 months after bimaxillary surgery. The decrease in the pharyngeal airway volume was correlated to the changes in the palatal plane inclination and the positional change of the hyoid bone.

Figure 1.

Measurement of hyoid bone position. Nasion was set up as a reference point. The coordinate of the most anterosuperior point of the hyoid bone (Hb) was marked.

Figure 2.

Volumetric measurements of pharyngeal airway space. (A) Reference planes: A, posterior nasal spine (PNS) plane (axial plane passing through PNS); B, soft palate plane (axial plane passing through most inferior point of soft palate); and C, epiglottis plane (axial plane passing through base of epiglottis on anterior pharyngeal wall). (B) ‘a’ indicates the naso-pharyngeal airway; ‘b’ indicates the oro-pharyngeal airway; and; ‘c’ indicates the hypo-pharyngeal airway. The red arrows indicating the borders were planes through the PNS and the most inferior point of the soft palate parallel to the Frankfort Horizontal plane.