Image-based analyses of morphology and function in the upper airway of orthodontic patients

Abstract

Adenoid hypertrophy (AH) is one of the most common causes of nasal obstruction in children and adolescents. This may lead to breathing-related symptoms such as mouth breathing, snoring, asthma, speech problems, and obstructive sleep apnea (OSA). An association between AH and craniofacial abnormalities implies that the initial identification of AH should be an integral part of orthodontic treatment. Moreover, rapid maxillary expansion (RME) has been considered a beneficial tool for reducing nasal obstruction. However, there is currently no consensus on the imaging part of AH diagnosis and how geometrical obstruction due to AH is associated with breathing characteristics. Evidence of both morphological and aerodynamical characteristics of upper airway (UA) is warranted for verifying the effect of RME on UA. In this thesis, we applied a novel technique, computational fluid dynamic (CFD), to simulate airflow characteristics in orthodontic patients. CFD simulation is a well-established method that uses numerical analysis and data structures to precisely evaluate aerodynamic characteristics of the fluid and their interactions with the surrounding surfaces, as defined by boundary conditions. CFD is widely applied to solve engineering problems, such as in aerospace analysis, weather simulation, and industrial system design, but sparsely used in UA research. This thesis aimed to validate lateral cephalogram in UA examination using Cone Beam Computed Tomography (CBCT) and CFD simulations. Furthermore, the effect of RME on UA was investigated by CBCT and CFD. The adenoidal nasopharyngeal (AN) ratios measured on the lateral cephalograms were applied to express the nasopharyngeal airway’s adenoidal size and patency. We found a notable correlation (r = -0.78) between the AN ratios and the nasopharynx volumes, indicating the higher AN ratio and smaller nasal space in patients 15 years or younger. The CFD simulation demonstrated that the maximal airflow velocity at both inspiration and expiration significantly increased, nearly 30%, once AN ratios were more than 0.6. With respect to the effect of RME on UA, neither the morphological nor aerodynamic characteristics were significantly changed after RME. Based on the four studies, we conclude that AN ratios measured on lateral cephalograms could be a feasible method to initially estimate the nasopharynx’s patency in children. An AN ratio of more than 0.6 may induce potential alteration in airflow’s characters. The utilisation of AN ratios would assist dentists better recognising patients who are at risk of AH. The morphological and aerodynamic changes obtained from CBCT assessment and CFD simulation could not verify the positive effect of RME on UA.