Changes in the Pharyngeal Airway and Position of the Hyoid Bone After Treatment With a Modified Bionator in Growing Patients With Retrognathia

Background: To understand changes in (1) the anteroposterior dimensions of the pharyngeal airway and (2) the position of the hyoid bone after newly modified bionator treatment in growing patients with retrognathia. Methods: Data of 86 patients (51 males aged 10.83 ± 1.40 years and 35 females aged 10.25 ± 1.47 years) with an Angle class II, Division I malocclusion, and a retruded mandible in mixed dentition were analyzed. All patients were treated using a modified bionator. The average treatment time was 1.86 years. After treatment, 56 patients were followed-up for approximately 2 years, and 22 patients were followed-up for 4 years. Lateral cephalometric radiographs were taken in four stages: before treatment (T0), at the time bionator treatment was stopped (T1), at 2 years of follow-up (T2), and at 4 years of follow-up (T3). Nine linear and 17 angular items of the craniofacial morphology, five linear items of the pharyngeal airway dimensions, and six linear items of the hyoid bone position were measured. Analysis of variance was used to compare measurements in the four stages. Results: The anteroposterior dimensions of the pharyngeal airway did not change after the modified bionator treatment, except for the nasopharynx area in both genders and the hypopharynx area in males. Vertically, the hyoid bone remained in a constant position relative to the mandible and third cervical vertebra through the four stages in both genders. Horizontally, the hyoid bone moved forward during treatment but returned to a posterior position after use of the modified bionator was stopped. Conclusions: Even with mandible advancement by the modified bionator in growing patients with retrognathia, there were no significant changes in the anteroposterior dimensions of the pharyngeal airway, and the hyoid bone remained in a vertical position relative to the mandible during the pubertal growth phase.