Morphologic And Aerodynamic Changes Of Upper Airway In Paediatric Hemifacial Microsomia Patients Undergoing Distraction Osteogenesis

Background and objective:Hemifacial microsomia(HFM)is a progressive,asymmetrical craniofacial malformation mainly manifested by unilateral mandibular dysplasia.In HFM patient,the growth of the mandible on the affected side is impaired,leading to the shortening and narrowing of the mandible that not only leading to asymmetric appearances,but also stenosis of the upper airway.Maxillofacial dysplasia can cause anatomical stenosis of the upper airway and sleep related diseases,such as obstructive sleep apnea(OSA),The changes of the respiratory pattern will also affect the growth and development of the maxillofacial region,induce or aggravate malocclusion,and form a vicious circle.Mandibular distraction osteogenesis(MDO)is the main treatment means for HFM children so far.However,the evaluation of postoperative curative effects lacks the consideration of its influence on the upper airway.The changes of the volume,the cross-sectional area and the shape of the upper airway,might change the characteristics of the flow field,such as the pressure distribution and air flow velocity,thus further affect the ventilation function.Studies on the morphologic and aerodynamic characteristics are very important to understand the relationship between the structure and function of the upper airway.However,up to date,few studies were conducted in HFM patients to investigate the extent of the increase in upper airway and the aerodynamic changes as a result of unilateral MDO.The geometric structure of the upper airway is complex and difficult to be checked in the inner part,so it is very difficult to quantitatively analyze the three-dimensional(3D)structure and the inner flow field of the upper airway in detail.With the development of science and technology,especially the use of imaging technology and high-performance computer,it is possible to construct a physical and digital model of the upper airway,and great progress has been made in the study of the internal flow field of the upper airway by means of biomechanics.In recent years,the application of computational fluid dynamics(CFD)in the study of the aerodynamic characteristics has been a hot spot in airway research.Through CFD,the flow movement inner the complex structure of the upper airway can be accurately predicted,and the aerodynamic parameters such as velocity and pressure that cannot be measured by clinical means can be obtained.Through the comparative study of the morphologic and aerodynamic characteristics of the upper airway before and after surgery,we can noninvasively and quantitatively analyze the changes of the internal flow field,so as to provide accurate reference for the evaluation of the postoperative efficacy.The present study was designed to establish 3D models of the upper airway based on the computed tomography(CT)datasets of the pediatric HFM patients,evaluate the morphologic changes of the upper airway before and after unilateral MDO by using 3D measurements,and simulate the airflow inner the upper airway with CFD,quantitatively investigate the effects of unilateral MDO on the upper airway,and further from the perspective of biomechanics to explore the mechanical mechanism of the aerodynamic changes caused by the morphologic changes.Materials and methods:1.Reconstruction of the upper airway in pediatric HFM patientsWith the application of MIMICS software,twenty CT datasets of the HFM patients(age 9.52±2.47 y;11 males,9 females)before unilateral MDO were selected and 3D reconstruction of upper airway were performed.The area of the boundaries and the volume of each part of the upper airway were measured.The same and another examiner repeated all the procedures such as the identification of landmarks and the measurements two weeks later.Intraclass correlation coefficients(ICC)were performed to evaluate the reliability of this 3D reconstruction method,with a value>0.75 considered a good reliability.Additionally,the differences between the two readings for both area and volumetric measurements by the same examiner were tested to estimate the systematic error by means of paired t-test.Statistical significance was set at p<0.05.For accuracy testing,five models of the upper airway were 3D printed to form hollow shell models and then were scanned using the same CT scanner and parameters as in this study,and the obtained CT datasets saved in digital imaging and communications in medicine(DICOM)formats were transferred to MIMICS to carry out the 3D reconstruction of the upper airway with the same method.2.Morphologic changes of the upper airway in pediatric HFM patients undergoing MDOThe CT datasets of the previous mentioned twenty HFM patients,which were taken before and within six months after unilateral MDO were selected.CT datasets saved as DICOM formats were imported into MIMICS for 3D reconstruction of the upper airway.The upper airway was further divided into four parts:nasal cavity,nasopharynx,oropharynx,and hypopharynx by the corresponding transversal planes(A,B,D,and E).Plane C was the minimum cross-sectional area plane.The area of planes A?E,the length of the nasopharynx,oropharynx and hypopharynx,and the volume of the nasal cavity,nasopharynx,oropharynx and hypopharynx were calculated to assess the size changes of the upper airway.All procedures,such as landmark identification and measurements,were repeated with a two-week interval by the same examiner.ICC was performed to assess the intra-examiner reliability of the measurements,with a value>0.75 considered a good reliability.Paired t-tests were used to compare the differences between the pretreatment and posttreatment parameters with the significance at p<0.05.3.Aerodynamic changes of the upper airway in pediatric HFM patients undergoing MDOAfter the reconstruction of the upper airway of the previous selected twenty HFM patients before and within six months after unilateral MDO,the 3D geometric airway models were converted into non-uniform rational B-splines surfaces,and generated unstructured tetrahedral volume meshes.Standard ?-? turbulence model was applied in this study to simulate a complete respiratory cycle of the airflow in the upper airway.An in vitro experimental sy stem was designed and constructed for the validation of the CFD simulation.The data files generated from Fluent were processed in CFD-Post(ANSYS,Canonsburg,PA)to calculate the average pressure,the average velocity and the pressure drop(?P)of the cross-sections A?E,the maximum velocity(vmax)and the effective resistance(R)of the nasal cavity,nasopharynx,oropharynx and hypopharynx at peak inspiration.The differences between the pretreatment and posttreatment parameters were tested by means of paired t-tests with the significance at p<0.05.Results:1.Reconstruction of the upper airway in pediatric HFM patientsThe results of the paired t-test showed no significant difference between the first and the second examinations by the same examiners(p>0.05).Both the intra-observer and inter-observer reliabilities were very high(ICC,0.976 to 0.995).The results of the accuracy testing showed that the inner volume of upper airway was slightly enlarged after 3D reconstruction(102.4%?107.4%).2.Morphologic changes of the upper airway in pediatric HFM patients undergoing MDOBefore treatment,the affected side of the upper airway in the HFM patients was compressed and the stenosis was mainly located in oropharynx.The appearance of upper airway after 3D reconstruction was coronally narrowed.After MDO,the area of the plane B(+147.75%)and plane C(+52.71%),the length of the oropharynx(+23.86%),the volume of the oropharynx(+53.74%)and nasopharynx(+42.8*%)increased significantly(p<0.05).The sagittal caliber of the upper airway was enlarged postoperatively.The coronal constriction area of the affected side was expanded after treatment.The appearance of the upper airway tended to be symmetrical.3.Aerodynamic changes of the upper airway in pediatric HFM patients undergoing MDOThe CFD results of the standard k-? turbulence model were in good agreement with the experimental data,the difference varied from 5.21%to 14.75%,After MDO,the decrease of average velocity in sections A,B,C and E(-25.76%?-53.45%?-39.73%?-21,64%respectively)was statistically significant(p<0.05),especially in section B and C;the increase of average pressure in all cross-sections A?E(+44.24%?+41.28%?+45.28%?+40.22%?+41.24%respectively)was statistically significant(p<0.05);?P showed significant reduction in planes C,D and E(-66.59%?-63.69%?-50.07%respectively).Compared with pre-treatment,the airway resistance of the nasal cavity,nasopharynx,oropharynx,and hypopharynx decreased(-38.31%?-27.64%?-57.52%?-51.54%respectively)significantly(p<0.05).Except for the nasal cavity,the vmax of other parts of the upper airway decreased(-1 8.61%?-28.99%?-20.56%respectively)significantly(p<0.05).Conclusions:1.In this study,we successfully constructed the models of the upper airway in HFM children,and the 3D reconstruction method of the upper airway showed high reliability and accuracy.2.MDO may enlarge the nasopharynx and orophary nx of the HFM patients,relieve the stenosis of the oropharynx,expand the constriction of the affected side and obtain a relatively symmetrical airway shape.3.The abnormal stenosis of the anatomical structure could affect the airflow distribution in the upper airway,and the relief of the stenosis could also change the airflow pattern conversely.After distraction,along with the upper airway was effectively expanded,the air flow velocity reduced,and the pressure increased;along with the volume of the upper airway increased and the pressure drop decreased,the compliance increased,so greatly reduced the airway resistance,which was consequently reduced the workload necessary for breathing and facilitate inspiration.