Three-dimensional Morphological Evaluation And Computational Fluid Dynamics Simulation Of The Upper Airway Response To Twin Block Treatment In Growing Patients Of Class â…¡ Division 1 Malocclusion With Mandibular Retrognathia

Objective:This study is based on three-dimensional (3D) reconstruction of CBCT data and computational fluid dynamics (CFD) simulation to evaluate the 3D morphological and CFD response of the upper airway to Twin Block (TB) functional appliance in growing patients with Class II Division 1 and mandibular retrognathia, to provide important theoretical basis for the upper airway respiratory function improvement through clinical treatment of malocclusion.Materials and Methods:Thirty growing patients (13 boys and 17 girls, mean age of 11.57±0.94) with Class Ⅱ Division 1 and mandibular retrognathia who have completed TB treatment were randomly recruited into TB group. The control group (13 boys and 17 girls, mean age of 11.72±0.86) was selected from the patients with the same diagnosis and without TB treatment. Patients of two groups were matched with each other based on age, sex and development condition. The CBCT scans of the pre-treatment (T1 data) and post-treatment (T2 data) of TB group and control group (control data) were collected, respectively.3D reconstruction and registration of T1 and T2 data were performed with MIMICS 16.0 and pharyngeal flow field characteristic were simulated by ANSYS 14.0. T-test was used to compare changes between T1, T2 and control data. The relationship between pharyngeal airflow resistance and morphological variables was analyzed by Pearson correlation coefficient.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. The greatest changes of volume (V) and mean cross-sectional area (Smean) occurred in the oropharynx. In addition, the nasopharynx turned more circular, and the oropharynx became more elliptic in transverse shape (LR/AP). The minimum cross section was located in the oropharynx in both groups. The oropharyngeal morphological uniformity (Smin/Smean) after treatment was superior to before significantly. After comparison between T2 and control data, only the horizontal movement of the hyoid bone, the expansion of the oropharynx and hypopharynx, and changes of LR/AP and Smin/Smean of oropharynx showed significant difference.After numerical simulation of pharyngeal airflow, we found that the minimum pressure occurs in hypopharynx, while the maximum pressure drop and velocity were located in oropharynx. The pressure drop and velocity of airflow in oropharynx and hypopharynx decreased significantly after TB treatment, and oropharynx still had the most significant change. With the improvement of pharyngeal morphology, the minimum pressure of oropharynx increased and hypopharynx showed no significant differences. No significant differences were observed in the flow field changes in nasopharynx. There was a significant negative correlation between the oropharyngeal pressure drop changes and morphological variables changes (V, Smean, Smin/Smean) of oropharynx.Conclusion:The 3D morphology and respiratory function of oropharynx improved significantly after TB functional treatment in growing patients with Class Ⅱ Division 1 and mandibular retrognathia.