| Background and objectiveIt is common to see Class III patient combined with maxillary skeletal deficiency in the clinical setting, whose incidence rate can be as high as 42% to 63% in the skeletal class III malocclusion. can always manifest itself as maxillary skeletal deficiency with normal or excessive mandible. The malocclusion can pose various form of negative influence on people's health, including dental and systematic, specifically tempromandibular joint disorder, caries caused by difficulty of cleaning, decreased chewing function, malnutrition as a subsequent result of insufficient chewing, negative impact on patients'facial aesthetics and even psychological wellbeing. Severe form of maxillary skeletal deficiency can also lead to constriction of the the nasal cavity and airway of the palatal and pharyngeal regions. The constricted nasal cavity can cause nasal congestion of various degree. Narrowed upper airway can influence the dynamic of the airflow inside the airway and the aspiration ability of the patients, which can lead to more serious complications. So, it is very important to treat these patient in an orthopedic way early in their growing period.Protraction headgear and rapid maxillary expansion (PE) is an effective way to treat growing patients with skeletal Class ? malocclusion. Rapid maxillary expansion employs orthopedic force to horizontally expand the open metopism, stimulating the new bone formation in the newly formed bony fissure. Protraction headgear employs orthopedic forces to advance the maxilla, stimulating the growth of maxilla in the sagittal and vertical dimension. At the same time, it can stimulate the periosteum to acquire bony deposition on the surface of maxilla; it also employs the chin cup to restrict the growth of the mandible and to rotate the mandible in a clockwise direction.Horizontal component of the force of the protraction headgear gives tension in the metopism, causing the condensation of the anterior part of the palate. When combined with the RME, it can open the fissure between maxilla and other facial bones, activate cells inside the fissure, increase the effect of protraction. Previous evaluations of upper airway after orthodontic treatment mostly focus changes in the two-dimensional manner such as linear measurement of the upper airway changes, which led to the loss of some information. With the development of computer technology and the wider application in the clinical setting, researchers can utilize imaging like CBCT and MRI to evaluate the morphological changes in a three-dimensional way, and employ multiple upper airway biomechanical model to study the features of air flow field and the influence of different treatment methods on the air flow field. However, there has been no such research on the fluid dynamic changes of the upper airway of growing patients with Class ? malocclusion combining with maxillary insufficiency after PE treatment. the influence of the PE treatment on the morphology and fluid dynamic of the upper airway is still to be fully investigated.Computational fluid dynamics is a rapid developing technology in the field of fluid dynamics. Its basic method is building an accurate 3D model using the upper airway imaging acquired by CT or MRI, and finite element analysis, finding the solution for parameters concerning fluid dynamics via applied mechanics equation, such as velocity, pressure distribution, shearing force, flow field, etc. The method also divides the whole computational field into hundreds of thousands of connected controlled volume, and then integrate and solve the simultaneous equations. CFD has eliminated the uncertain interfering factors in the simulation experiment, and become an effective way to diagnose diseases and evaluate upper airway treatment as it has features like numerical simulation of airflow, delicate unit division and high precision of measurement.Our study evaluated the morphological and fluid dynamical changes of upper airways after PE treatment in Class III growing patients with maxillary insufficiency. Our study can be an important theoretical basis for the clinical practice to improve the aspirational function of the upper airway by orthodontic or orthopedic treatment of the malocclusion.Materials and methodsThe PE group was randomly selected 30 Class III growing patients with maxillary insufficiency who have been treated with PE treatment, among whom there are 16 males and 14 females with the pre-treatment age of 9.42±0.35 and post-treatment age of 10.19±0.25. In the PE group, the practitioner used Hyrax rapid maxillary expansion appliances for 2-3 weeks (expand 0.5 mm to 1.0 mm per day). After expansion, the screw holes were sealed to maintain the expansion result. The fully adjustable protraction headgears from Shinye Co. were prescribed with 500g forces on each side, the direction of the protraction is 30 degrees in relation with the occlusal plane. The detailed instructions were that the minimum protraction time is 14 hour and reevaluation time was scheduled in a 3-week interval until the overjet of the anterior teeth is 3 to 5 mm with a Angle Class ? or Class ? relationship. The average treatment period is 7.62±1.27 months.The research subject in the PE group is patients in their growing period, so there is a considerable growth influence of the upper airway in the treatment period.The control group consists of 30 pre-treatment Class ? growing patients with maxillary insufficiency (age 10.25±0.37,16 males & 14 females). The patients within the control group and the patients within the PE group were one-to-one paired according to their individual age, gender, and development status. the BMI of two groups has no statistical significant differences.The CBCT imaging of pre-(T1) and post-treatment (T2) of the PE group and the control group was delivered by the same investigator using NewTom 5G focusing on the head and neck region with the patients in spine position. The acquired data were stored and exported in DICOM format. the data were imported into Mimics 16.0 software for reconstruction and registration of the pre- and post-treatment models. Craniobase was used to roughly register the two models, then STL registration was used to improve the accuracy of the registration. The three-dimensional changes of the airway is reflected by the changes of maxilla, mandible, hyoid bone, upper and lower incisors and arch form, as well as the volume, length, minimum cross sectional area, the maximum horizontal dimension (LR) and the maximum anteroposterior dimension (AP) of the airway cross sectional area. When we performed the CFD, reconstructed 3D models were imported into ICEM software to divide the mesh, then Fluent 16.0 was used to further simulate the flow field of the airway in order to get the CFD related parameters such as flow velocity, pressure, and pressure drop of different parts, which is the change of resistance of the airway.All the statistical analysis are delivered via SPSS 19.0, and all the measurements are shown in mean ± standard deviation (mean ± SD). t test was used to compare the 3D morphological changes and characteristical changes of the flow field. Specifically, the changes of T1 and T2 of the PE group were evaluated using paired t test and the difference of the T2 and the control groups were evaluated using independent t test. Pearson correlation analysis was used to analyze the relationship between pressure drop of the pharynx and the parameters reflecting the morphological changes.Result1. Three-dimensional morphological changes of the upper airway after PE treatment for Class ? growing patient with maxillary insufficiencyComparing the T1 and T2 group, the changes of PE treatment are as follows: maxilla and incisal tip of the upper incisor move upward and anteriorly; mandible and hyoid bone move downward and posteriorly; both the width and length of the upper arch increase. The total volume of the T2 group increases, the nasopharynx, velophaynx, glossopharynx part of the upper airway and minimum cross sectional area also increase. There is statistically significant difference between T1 and T2. among then, the most significant increase of minimum cross sectional area and upper airway volume appears in the velophaynx part (P<0.01), which increase 20.38±6.45% and 11.67±3.27% respectively, the ratio between the maximum anteroposterior dimension and the maximum horizontal dimension increase in velophaynx and glossopharynx (AP/LR) increases. When compared with T1 the shape of the airway of T2 is rounder (P<0.05). however, the AP/LR ratio decreases significantly in the hypopharynx. The minimum cross sectional area of the patients' airway in located in the lower border of the velophaynx. The consistency of the morphology of the anterior palate Smin/Smean are 0.35±0.15 for T1 and 0.55±0.23 for T2 with a significant improvement.Comparing the T2 group with the control group, we discover that upper airway volume of the nasopharynx and velophaynx and the minimum cross sectional area all significantly increase. The upper airway volume of the glossopharynx and hypopharynx and the minimum cross sectional area show no significant changes. The AP/LR of the velophaynx airway increase and become rounder compared with the pre-treatment group (P<0.05). There is significant decrease of the AP/LR ratio in the glossopharynx, while there is no significant change in the nasopharynx and hypopharynx. After comparing group T2 and the control group and rule out the growth factor, the width and length of maxillary arch, and the horizontal and vertical movement of the hyoid bone has significant differences. In the nasopharynx and velophaynx, there is significant difference in airway volume and minimum cross sectional area, the AP/LR and morphological consistency Smin/Smean of the velophaynx show significant changes. There is also significant changes of AP/LR in the laryngopharynx.2. Flow field changes of the upper airway of Class ? growing patients with maxillary insufficiency after PE treatmentAfter simulating the flow field of the patient upper airway, we discover that before PE treatment, pressure continues to drop with the air going from nasopharynx to hypopharynx. The maximum negative pressure is located in the hypopharynx and the maximum pressure drop in velophaynx. With the morphological changes of the airway, the maximum and minimum pressure and pressure drop in naso-, velo-and glossopharynx all decrease after treatment, among which the most significant drop is located at the velophaynx. In the PE treatment group, the maximum negative pressure and pressure drop decrease 51.11%±16.49% and 39.41%±11.33% respectively, while in the control group, they respectively decrease 34.91%±19.36% and 20.54%±11.24%.Comparing with the T1 and T2 group, with the changes of the distribution of air flow pressure in the upper airway, the velocity of the air flow decreases. It is most obvious in velophaynx, which decreases 37.96±3.57% from 4.25±2.59 m/s pre-treatment to 8.84±1.74 m/s post-treatment with relative even cross sectional velocity distribution. The velocity in nasopharynx decreases 23.92±3,51% from 5.35±1.19m/s pre-treatment to 4.07±1.07m/s post-treatment. Comparing T2 group and control group, there is significant decrease of air flow velocity.After correlation analysis, there is a negative correlation between the pressure drop of the velophaynx and volume, minimum cross sectional area and morphological consistency of the velophaynx.Conclusion1. The volume and minimum cross sectional area of the naso- and velophaynx of upper airway of Class ? growing patients with maxillary insufficiency after PE treatment both increase. The morphological distribution of the velophaynx area become more even.2. With the improvement of the velophaynx morphology, the airflow resistance of the upper airway decreases, and the aspiration ability of the patients improves dramatically. |
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