Airflow Response To Twin Block Treatment In Growing Patients With Class Ⅱ~1 And Mandibular Retrognathia:A Simulation Of Fluid-structure Interaction

ObjectiveThis study is based on the fluid-structure interaction(FSI)simulation to evaluate the response of the upper aiway to Twin Block(TB)functional appliance in growing patients with Class Ⅱ1 and mandibular retrognathia,and explore the effect of soft palate’s elastic modulus on the collapsibity of upper airway,to provide important theoretical basis for the upper airway respiratory function improvement.MethodsOne growing patients with Class Ⅱ1 and mandibular retrognathia who had completed TB treatment was recruited into this study.The CBCT data of pre-treatment and post-treatment was collected.The data was imported into the MIMICS 19.0 to build three dimensional models of upper airway and soft palate,and then the models were repaired and Boolean caculated in the Geomagic studio 12.0 and NX9.0 software to acquire the combined models.Subsequently,the combined models were imported into the ANSYS Workbench 16.0 to carry the FSI simulation(the soft palate elastic modulus of FSI 1 was 7540Pa and that of FSI2 was 2000Pa).At last,the results of the simulation of FSI were analysed.Results(1)Morphology:the minimal cross sectional area of pharynx increased from 1.10cm2 to 3.03cm2 after TB treatment,and Amin/Amean increased from 0.59 to 0.84.In the FSI1 simulation,minimal cross sectional area of velopharynx decreased during inspiratory compared with expiratory,and that of FSI2 changed more,while the changes were smaller after TB treatment.(2)Pressure:In the inspiratory phase,and the minimal area-averaged negative pressure was near the posterior of epiglottis,but area-averaged negative pressure in velopharynx changed the fastest,which increased from-39.17Pa to-13.34Pa in the velopaharynx after treatment.The pressure distribution was more uniform.In the expiratory phase,positive pressure occured in the upper airway,and the minimal area-averaged positive pressure was in velopharynx before treatment,which increased from9.54Pa to 14.48Pa after treatment compared with pre-treatment.(3)Velocity:In FSI1 simulation,the maxmum area-averaged velocity happened in velopharynx,which changed most after treatment.While the maxmum area-averaged velocity occured near the posterior of epiglottis after treatment.The maxmum area-averaged velocity of velopharynx decreased from 5.64m/s to 1.26m/s in the maximum inspiratory phase,and the area-weighted average velocity became more uniform after treatment.In the maximum expiratory phase,compared with pre-treatment,maxmum area-averaged velocity decreased from 4.53m/s to 1.20m/s,but not as obvious as that in the maximum inspiratory phase.(4)Wall shear:In FSI1 simulation,maxmum wall shear decreased from 0.38Pa to 0.06Pa in the maxmum inspiratory phase.(5)Displacement:In the maximum inpiratory phase,the displacement of uvula minished after TB treatment,while there wasn t evident changes in the maximum expiratory phase.The changes of FSI1 were 0.80mm before TB treatment and 0.35mm after TB treatment,those of FSI2 were 1.42mm before TB treatment and 0.51 mm after TB treatment.Conclusion(1)After TB treatment in growing patients with Class Ⅱ1 and mandibular retrognathia,the risk of collapse of upper airway decreased.(2)With the elasticity modulus of soft palate decreasing,the displacement increased,which may aggravate the risk of upper airway collapse.