# Application of Computational Fluid Dynamics Analysis after Bimaxillary Orthognathic Surgery
## 4 Discussion  
Bimaxillary orthognathic surgery significantly alters facial appearance [1]. Implants can also be used to address facial problems caused by hypogrowth of jaws, such as midfacial deficiency. The implantation of stem cells of dental pulp origin with decellularized bone extracellular matrix (bECM) can induce bone formation [14]. The new bone can be useful in sites where implants are insufficient [15]. Induction of bone formation can hopefully be performed more in the future [16]; however, the most effective method is bimaxillary orthognathic surgery. The maxillomandibular number complex can be repositioned up, down, left, right, or rotationally in this operation. As such, it is widely used in Asia where skeletal class III malocclusion is observed at high rates [2]. Changes in jaw position \begin{table}
\begin{tabular}{c c c c} \hline  & \(V_{max}\) **(m/s)** & \(\Delta P\) **(Pa)** & \(WSS_{max}\) **(Pa)** \\ \hline Pre-surgery (T0) & 7.038 & \(-\)7.723 & 4.214 \\ Immediately after surgery (T1) & 8.723 & \(-\)27.004 & 10.733 \\ Post-surgery (T2) & 12.054 & \(-\)53.739 & 14.323 \\ \hline \end{tabular}
\end{table} 
Table 2: Flow parameters in this study. Maximum velocity (\(V_{max}\)), pressure drop (\(\Delta P\)), and maximum wall shear stress (\(WSS_{max}\)). Figure 5: 3D distribution of WSS (wall shear stress) for pre-surgery, immediately after surgery, and post-surgery. also affect the shape of the soft tissue attached to it. According to [PERSON], the posterior movement of the mandible changes the position of the tongue base and hyoid bone rearward and downward [17]. The change in position of these structures ultimately affects the shape of the airway [3]. According to a meta-analysis by [PERSON] et al., the volume of the oropharynx and total airway volume decrease after bimaxillary setback orthognathic surgery [18]. Furthermore, [PERSON] et al., demonstrated that airway volume decreased by 29% after bimaxillary setback orthognathic surgery, especially in the oropharynx. Studies to observe changes in the upper respiratory tract after surgeries in various directions have been conducted. [PERSON] et al., performed a rapid maxillary expansion (RME) analysis using the three-dimensional stereophotogrammetrical method. The results of a meta-analysis confirmed that the effect of RME on the upper airway is not clear [19]. Reduction in airway volume is a risk factor for obstructive sleep apnea (OSA). [PERSON] et al., confirmed that children with OSA could undergo MRI assessment to effectively analyze the tonsil and airway patterns [20]. This is supported by the finding that airway stenosis is observed at the level of the velopharynx and oropharynx in many patients with OSA [21; 22; 23; 24; 25; 26]. We conducted this study to determine the effect of positional changes in the maxillomandibular complex on the airway. We performed bimaxillary orthognathic surgery in one patient and acquired CBCT images preoperatively (T0), postoperatively (T1), and 7 months postoperatively 7 (T2). CBCT images and CFD simulations were used to analyze airflow in the airway. The airway is a continuous organ consisting of soft tissue with irregular shapes and inner surfaces. To apply CFD simulation to such a \"flabby\" organ, it was reconstructed into a simple shape. CFD simulation applied to the reconstructed airway was helpful in effectively analyzing the airflow pattern according to the individual patient's airway type [10; 11; 25; 27; 28]. Analysis revealed that the velopharynx at T1 was narrowed, but the oropharynx was rather widened. This is assumed to be a result of the upward movement of the maxillomandibular complex because the position of the hyoid bone did not change immediately after the operation, and the distance from the mandible was increased. The direction of jaw movement in the patient is shown in Figure 6. Although the oropharynx is wider, the upper velopharynx is narrower, resulting in a decrease in the amount of incoming air, creating a strong vortex concentrated in front of the airway.