CBCT Evaluation of the Upper Airway Morphological Changes in Growing Patients of Class II Division 1 Malocclusion with Mandibular Retrusion Using Twin Block Appliance: A Comparative Research

OBJECTIVE: The purpose of this study was to evaluate the morphological changes of upper airway after Twin Block (TB) treatment in growing patients with Class II division 1 malocclusion and mandibular retrusion compared with untreated Class II patients by cone beam computed tomography (CBCT). MATERIALS AND METHODS: Thirty growing patients who have completed TB treatment were recruited into TB group. The control group (n = 30) was selected from the patients with the same diagnosis and without TB treatment. CBCT scans of the pre-treatment (T1) and post-treatment (T2) data of TB group and control data were collected. After three-dimensional (3D) reconstruction and registration of T1 and T2 data, the morphological changes of upper airway during TB treatment were measured. The statistical differences between T1 and T2 data of TB group as well as T2 and control data were accessed by t-test. RESULTS: During the TB treatment, the mandible moved advanced by 3.52±2.14 mm in the horizontal direction and 3.77±2.10 mm in the vertical direction. The hyoid bone was in a more forward and inferior place. The upper airway showed a significant enlargement in nasopharynx, oropharynx and hypopharynx. In addition, the nasopharynx turned more circular, and the oropharynx became more elliptic in transverse shape. However, the transverse shape of the hypopharynx showed no significant difference. After comparison between T2 and control data, only the horizontal movement of the hyoid bone, the volumetric expansion of the oropharynx and hypopharynx, and changes of the oropharyngeal transverse shape showed significant difference. CONCLUSION: Compared to the untreated Class II patients, the upper airway of growing patients with Class II division 1 malocclusion and mandibular retrusion showed a significant enlargement in the oropharynx and hypopharynx as well as a more elliptic transverse shape in the oropharynx, and the hyoid bone moved to an anterior position after TB treatment.