Evaluation of Hyoid Bone Position and Pharyngeal Airway Morphology before and after Orthognathic Surgery for Mandibular Prognathism

Abstract

Treatment to improve malocclusion in patients with skeletal mandibular protrusion often involves the combined use of surgical treatment in orthodontic treatment to improve not only occlusion but also facial appearance. Mandibular setback surgery is known to cause displacement of the hyoid bone and affect the morphology of the respiratory tract. However, although there have been previous reports regarding displacement of the body of the hyoid bone, few reports have verified hyoid bone rotation taking complex muscle adhesion into consideration. Therefore, the aim of the present study was to comparatively investigate the position of the hyoid bone and morphological changes of pharyngeal airway taking preoperative and postoperative inclination into consideration in patients who underwent setback of the mandible with orthognathic surgery. The subjects comprised 14 patients who underwent sagittal split ramus osteotomy to treat skeletal mandibular protrusion, exhibited at least 8.0 mm (mean : 9.0mm) of mandibular retraction with model surgery directly before surgical treatment and consented to participate after being given an outline of this study. Data comprising lateral roentgenographic cephalograms taken directly before and one year after orthognathic surgery were used to measure reference points before and after surgery and measurement items determined with the following method. When analyzing hyoid bone displacement, the hyoid bone upper projection (H1) , lower projection (H2) and lowest point of the third cervical vertebra (C3) were set as reference points. Measurement items were ∠ NSH1, ∠ NSH2, ∠ H1C3H2, C3-H1 and C3-H2. Hyoid bone rotation and displacement of the body of the hyoid bone were then evaluated. Morphology of the pharyngeal airway was analyzed by measuring respiratory tract diameter before and after surgery in the nasopharynx region, soft palate area, uvula area, angle of mandible area and epiglottis area. Results indicated that ∠ NSH1 and ∠ NSH2 increased significantly.However, no differences were observed in ∠ H1C3H2, C3-H1 or C3-H2, with the results clarifying that the hyoid bone had moved downward and backward without rotating. Evaluation of pharyngeal airway morphology found no differences in APW1-PPW1 and APW2-PPW2, which correspond to the upper section of the pharyngeal airway but APW3-PPW3 and APW4-PPW4, which correspond to the middle section, and APW5-PPW5, which corresponds to the lower section, were found to have 1)

The base of the tongue regulates the main occluding area in the patients with mandibular prognathism

“…The width of the airway decreases after mandibular setback in the area from the intermediate part of the soft palate to the posterior border of the base of the tongue, though there is no significant difference in the decrease of that in the area of the upper part the soft palate and the epiglottic vallecular [20]. The airway was decreased 2.24 mm around the uvula, 2.5 mm at the angle of the mandible, and 1.05 mm around the epiglottis after mandibular setback more than 8 mm [21]. As for the epiglottis, it is reported that the swallowing triangle, that composed of the top of the epiglottis, the posterior pharynx, and hyoid bone, moves postero-inferiorly, just after mandibular setback, though it returns at certain extent overtime [22].…”