Establishment of CFD models of the upper airway based on HFM patient with OSA to quantitatively estimate the changes of the airway after unilateral DO, might be helpful to explain the relationship between the airway morphology and function and improve the understanding of pathogenesis and treatment strategy of OSA. PSG tests were used to verify the effectiveness of CFD.
OSA is a disease characterized by recurrent collapse of the pharyngeal soft tissues during sleep , which can lead to hypoxemia and sleep-disordered breathing, and cause functional damage of multiple systems. The researches on the relationship between OSA and the complications of various systems have also attracted the attention of many scholars. Effective researches and treatments of this disease require quantitative analysis of the airway morphology and function. With the development of the biomechanical technique and on the basis of studies on the airflow characteristics in normal airway, there are more and more researches on the internal flow field of upper airway in OSA patients. In order to meet the needs of clinical diagnosis and treatment, scholars have carried out a lot of CFD studies on the biomechanical model of upper airway in OSA patients, from simulating various virtual surgeries  to evaluating the real surgical effects [10–12]. Based on the previous studies, CFD has been considered to have high reliability and accuracy, can be used to evaluate the changes of airflow in the upper airway caused by stenosis in OSA patients, also can be used to quantitatively analyze the influence of the changes caused by surgical treatments on airflow characteristics, which are helpful to objectively evaluate the curative effects after surgical treatment.
Katz et al.  used PSG results to show that the duration of apnea-hypopnea > 2 breathing cycles, AHI ≥ 5 times/h, and LSR < 92% were the diagnostic criteria for children with OSA. OSA is a disease with high incidence rate, many etiologies and complicated pathogenesis. Because of its characteristic repeated or incomplete obstruction of upper airway and intermittent hypoxemia that occur during sleep, OSA is considered as one of the important risk factors for hypertension, angina pectoris and cerebral vascular embolism . At present, it is considered that the stenosis and dysfunction of the upper airway are the most important causes of OSA . In terms of treatment, OSA is a sleep disorder characterized by obstruction of the upper airway, and surgical removal of its anatomical stenosis is an important principle for the treatment of OSA . Among the surgical treatments, tracheotomy is the earliest, but it is only used in palliative or emergency situations. Uvulopalatopharyngoplasty only relieves the problem of soft tissue obstruction around the upper airway, and has no obvious effect on posterior lingual stenosis caused by mandibular retrusion, and the total effective rate is less than 50% . Because of the traction of the surrounding soft tissues, the potential risk of bone recurrence and neurosensory impairment would be increased along with larger mandibular advancement. In patients with severe retrusive and narrow mandible, the pharyngeal cavity behind the tongue may collapse and lead to OSA. Mandibular DO can make the mandible move forward and increase the tension of soft tissues such as mandible hyoid muscle, genioglossus muscle, and genioglossus muscle, so as to make the tongue root move forward, which can expand the upper airway, and fundamentally relieve the stenosis of the upper airway. The accompanying insufficient lower 1/3 facial height in patients with micrognathia can also be improved.
At present, many studies have shown that pediatric OSA involves multiple occlusive planes of upper airway. Therefore, acquisition of more accurate morphological data of the upper airway is necessary to locate the stenosis sites. Major et al.  pointed out that the upper airway had complex 3D geometric structures. Due to the influence of OSA, the shape of the upper airway would generally have a certain degree of variation, and some information would be lost when it was transformed into two-dimensional images. In this study, the 3D reconstruction could obtain more accurate morphology of the upper airway. In addition, the pathophysiological process of OSA is greatly affected by the flow field inner the airway. The investigation and analysis of the airflow is helpful for us to further understand the connection between the anatomical structure and function of the upper airway, and facilitate to explore the pathogenesis of OSA and the implementation of clinical diagnosis and treatment measures. However, due to the lack of direct examination methods and uniform standards, the researches on OSA and the related problems are greatly limited. With the development of medical imaging and 3D reconstruction technology, we can obtain more information about the anatomical structure of the upper airway more easily and intuitively. The interdisciplinary development has prompted many scholars to establish biomechanical models of the upper airway in order to analyze the connection between the morphological changes and function by CFD, which bring a new way for clinical researches on OSA. The establishment of biomechanical models of the upper airway and analysis of the internal airflow characteristics have become a research hotspot, aiming to provide theoretical basis for the pathogenesis research, clinical diagnosis, and treatment strategy of OSA.
In this case, the area of planes II and III, and the volume of oropharynx and nasopharynx showed obvious increase after distraction. As shown in Fig. 1, we can see that the upper airway before treatment was narrowed in the sagittal and coronal directions, especially in oropharynx. After treatment, with the mandible moving forward, the nasopharynx and oropharynx were expanded in the sagittal direction; in the coronal direction, the narrowing area of the affected side was expanded, and from the back view, the shape of the upper airway was more symmetrical. The average velocity achieved the peak at the narrowest part of the oropharynx before DO. The airflow velocity would decrease while the diameter of the airway would increase because of their proportional relationship . Therefore, the average velocity of planes II and III was apparently reduced and νmax in oropharynx appeared maximum decrease after distraction.
After DO, the pharyngeal stenosis was greatly improved by skeletal expansion. Nevertheless, the area of plane Ⅳ and the volume of hypopharynx were slightly reduced unexpectedly. This probably on account of the posterior shift of the tongue root, which might be resulted from the premature contact of the left second primary molar after DO and hypotonia of tongue muscles in the supine position during CT scanning (Fig. 4). However, we observed increased average pressure and decreased average velocity in plane Ⅳ, greatly decreased resistance and νmax in hypopharynx, which were consistent with the performance of the expanded regions.
According to Bernoulli's principle, the pressure would increase when the airflow slowed down, which is consistent with our results of the CFD analysis. We observed greatly increased negative pressure and decreased ΔP in all selected planes and each part of the upper airway after distraction. Upon dilation of the stenosis by surgeries, airflow resistance usually decreases with reduction of the required pressure during inspiration. Similarly, the present case indicated that unilateral DO improved OSA by reducing resistance of the whole upper airway. The markedly improved AHI and LSR after distraction confirm the efficacy of unilateral DO on expanding the upper airway.
The establishment of the biomechanical upper airway models in OSA children can help us better understanding the pathogenesis and evaluating the therapeutic effect of DO on OSA. In the past, the clinical diagnosis of OSA mainly depended on their clinical symptoms and PSG examination, but there was no better method to predict the actual stenosis site. The assessment of the morphology and internal flow field of the whole upper airway by computational modeling is helpful to make a more objective diagnosis of pediatric OSA. The 3D computer numerical simulation of this study was based on the established 3D accurate models of the upper airway according to the individualized CT datasets, and obtained the aerodynamic parameters by non-invasive CFD technology, and provided a theoretical basis for the evaluation of curative effects after DO.