Mechanical mechanism study of upper airway collapse and twin block treatment in a patient with mandibular retrognathia using fluid-structure interaction simulation

The purpose of this study was to investigate the mechanical mechanism of upper airway collapse and orthodontic treatment in a child with mandibular retrognathia. Fluid-structure interaction (FSI) simulation was used to evaluate the collapse mechanism of upper airway and orthodontic mechanism of Twin Block (TB) in a patient with mandibular retrognathia. The upper airway model was 3D printed by lithography technology, and the FSI boundary conditions of airway soft tissue collapse and biomechanical parameters of oropharynx were obtained by in vitro experiments. The results showed that the maximum negative pressure of oropharynx before treatment was located in the posterior wall, and the pressure gradient was larger than that of other parts. After treatment, the maximum negative pressure was limited to a small area of the anterior oropharynx wall, which decreased from − 2741.81 Pa to -1767.54 Pa, and the pressure gradient also decreased significantly. And the maximum deformation was reduced from 3.44 mm to less than 1 mm, which was reduced by more than 70%. Pearson correlation test showed that the change rate of the cross section area (α) was positively correlated with pressure drop (P < 0.05), and the closer to 1 the α value reached, the smaller the oropharynx pressure drop was. The larger the aspect ratio was, the smaller the maximum negative pressure was (P < 0.05). The collapse site of the upper airway in the presented child with mandibular retrognathia was not necessarily consistent with the narrowest part of the upper airway, and the cross-section shape and minimum pressure of the airway played a crucial role in affecting the collapse of the upper airway.