Simulations Of The Upper Airway In A Growing Skeletal Class ? Maxillary Retrognathic Patient With Obstructive Sleep Apnea Before And After PE Treatment Using Fluid Structure Interaction

Objective:This study is based on the 3D models of upper airway in a growing skeletal Class ? maxillary retrognathic patient with obstructive sleep apnea and fluid structure interaction to simulate the dynamic respiration,to evaluate the upper airway and soft palate characteristics before and after protraction headgear and rapid maxillary expansion(PE),presenting the mechanism about how this therapy treat these patients and providing guidelines when doctors develop treatment protocols for them.Material and methods:A skeletal Class ? maxillary retrognathic patient who had OSAHS and was in the period before the peak of growth was selected in this research.This patient was treated by PE and New Tom CBCT data was collected before and after the treatment.The original DICOM data was then imported into MIMICS 19.0 for 3D reconstruction of the upper airway and surrounding soft tissues.After the process of the model in Geomagic Studio 12.0 and NX 9.0,the suited model was imported into ANSYS Workbench 18.2 for model combination,material assignment,mesh division and fluid structure interaction(FSI).Finally,the simulation results were compared and analyzed.Results:(1)Morphological changes of the upper airway after PE:CBCT data showed that after PE,the volume of upper airway increased.The growth in the nasopharyngnx and velophaynx was significant,while the hypophaynx saw no change.The minimal plane before and after treatment was located in velophaynx,close to uvula.After PE,the area of minimal cross section rose,no matter in the quiet,in the maximal inspiratory or in the maximal expiratory,and the change in the maximal inspiratory was most important.(2)Flow field characteristics of the upper airway before and after PE:After the simulation of the dynamic breathing process,it was found that in the inspiratory phase and the expiratory phase,the upper airway was under negative pressure and positive pressure respectively,and pressure in the upper airway was more evenly distributed after PE.In the maximal inspiratory,the pressure drop in the velopharynx was significant(12.36 Pa)and the maximal negative pressure occurred in the minimal cross section where area-averaged pressure was-44.76 Pa.Velopharyngeal pressure drop decreased after treatment(1.74 Pa),and the area-averaged pressure in the minimal cross section was-39.78 Pa,with an increase of 4.98 Pa.In the maximal expiratory,the area-averaged pressure in the minimal cross section was 9.82 Pa,which increased to 15.94 Pa after treatment,showing a significant change.During the maximal inspiratory phase,the maximal velocity appeared in the velopharynx and the velocity change was the largest in the same position before PE;after this treatment,the area-averaged velocity at the minimal cross section decreased from 4.72m/s to 2.25m/s,which changed by 2.47m/s,and the speed distribution in the upper airway was more uniform.During the maximal expiratory phase,the area-averaged velocity at the minimal cross section in the velopharynx decreased from 2.62m/s to 1.24m/s after treatment,and the drop was lower than that in the maximal inspiratory phase.At the peak of inspiratory,the total upper airway resistance reached 6.95*105,and decreased to 6.54*105 after PE.The upper airway resistance in the expiratory process was lower than that in the inspiratory process.At the peak of expiratory,it reduced from 4.45*105 before treatment to 4.20*105 after treatment,and the decrease was not as obvious as that at the peak of inspiratory.(3)Soft palate characteristics before and after PE:During dynamic respiration,the tip of the soft palate,which was called uvula,had the largest deformation.In the maximal inspiratory phase,the maximal displacement of the soft palate decreased from 0.78mm to 0.43mm after the treatment.At the maximal expiratory phase,the maximal displacements of the soft palate before and after PE were 0.18mm and 0.16mm respectively,with no significant change.The stress of the soft palate was concentrated in the connection between the hard palate and the soft palate as well as in the uvula.In the maximal inspiratory,the maximal stress decreased from 4.39*103Pa to 5.53*102Pa after treatment,showing a significant drop.In the maximal expiratory,the maximal stress decreased from 1.37*103 Pa to 2.69*102Pa,and the change was not as obvious as that in the maximal inspiratory.Conclusion:PE treatment can improve ventilative function effectively and reduce risk of soft palate tissue collapse in growing skeletal Class III maxillary retrognathic patients with obstructive sleep apnea.