Fluid structure interaction simulations of the upper airway in obstructive sleep apnea patients before and after maxillomandibular advancement surgery Kwang K. Chang, a Ki Beom Kim, a Mark W. McQuilling, b and Reza Movahed c St. Louis, Mo Introduction: The purpose of this study was to analyze pharyngeal airflow using both computational fluid dy- namics (CFD) and fluid structure interactions (FSI) in obstructive sleep apnea patients before and after maxillo- mandibular advancement (MMA) surgery. The airflow characteristics before and after surgery were compared with both CFD and FSI. In addition, the presurgery and postsurgery deformations of the airway were evaluated using FSI. Methods: Digitized pharyngeal airway models of 2 obstructive sleep apnea patients were generated from cone-beam computed tomography scans before and after MMA surgery. CFD and FSI were used to evaluate the pharyngeal airflow at a maximum inspiration rate of 166 ml per second. Standard steady-state numeric formulations were used for airflow simulations. Results: Airway volume increased, pressure drop decreased, maximum airflow velocity decreased, and airway resistance dropped for both patients after the MMA surgery. These findings occurred in both the CFD and FSI simulations. The FSI simulations showed an area of marked airway deformation in both patients before surgery, but this deformation was negligible after surgery for both patients. Conclusions: Both CFD and FSI simulations produced airflow results that indicated less effort was needed to breathe after MMA surgery. The FSI simulations demonstrated a substantial decrease in airway deformation after surgery. These beneficial changes positively correlated with the large improvements in polysomnography outcomes after MMA surgery. (Am J Orthod Dentofacial Orthop 2018;153:895-904) O bstructive sleep apnea (OSA) is a condition char- acterized by recurring episodes of partial and complete pharyngeal airway collapse during sleep. OSA has been reported to affect 9% to 24% of the middle-aged adult population and 1% to 4% of chil- dren. 1-3 Severe cases of OSA can have many medical consequences due to the chronic reduction of air reaching the lungs. There is a wide range of side effects from OSA, from minor, such as dry mouth and fatigue, to life threatening, such as hypertension and stroke. 1-3 There are many treatment options for OSA. The use of the continuous positive airway pressure machine dur- ing sleep is considered the gold standard treatment for OSA. However, noninvasive, lifestyle modifications are often the first line of treatment used for OSA, especially in milder cases. Examples of treatment include weight management, cessation of smoking, exercise, sleep posi- tional changes, and avoiding alcohol or sedatives before bedtime. 4 Oral appliances are being used more and more to manage OSA because of their relative noninvasiveness and ease of use. 5 In some patients with OSA, surgical intervention may be the only effective way to treat the condition. Although there are many possible surgical treatments, maxillomandibular advancement (MMA) surgery has been shown in many studies to be an effec- tive, definitive treatment for OSA, leading to substantial reductions in apnea-hypopnea index numbers in sleep studies. 6-10 Studying the upper airway airflow is challenging due to the complexities of airway anatomy and physiology, the high cost of in-vitro studies, and the limitations of in-vivo studies. Because of this, fluid mechanics meth- odologies such as computational fluid dynamics (CFD) From Saint Louis University, St. Louis, Mo. a Center for Advanced Dental Education. b College of Engineering, Aviation and Technology. c Center for Advanced Dental Education; private practice, St. Louis, Mo. All authors have completed and submitted the ICMJE Form for Disclosure of Po- tential Conflicts of Interest, and none were reported. Address correspondence to: Kwang K. Chang, 3017 Burlington Ave, Lisle, IL 60532; e-mail, kkcman@gmail.com. Submitted, February 2017; revised, July 2017; accepted, August 2017. 0889-5406/$36.00 Ó 2018 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2017.08.027 895 TECHNO BYTES