The Effect Of Sagittal Changes In Incisors And Molars On The Upper Airway In Extraction Orthodontics

Objectives:1.By comparing the morphology and position of dento-maxillofacial structure and upper airway size of Class I patients with mesofacial vertical growth pattern before and after extraction,the response of adult upper airway to extraction was studied.2.Through the correlation analysis and regression analysis between sagittal movement of upper and lower incisors and molars and airway dimension,the key factors affecting airway size in orthodontic extraction were explored.3.By comparing the sagittal diameter of the pharyngeal airway with different teeth movement distances,the teeth movement that can cause significant changes in airway size was evaluated.Methods:43 subjects were selected from the patients who received extraction orthodontic treatment in the Department of Orthodontics,Stomatology Hospital,Jilin University from March 2012 to November 2019.The age of the subjects ranged from 19 to 33 years old（average 2.37±3.44 years）.All patients were treated with 0.028×0.022-inch slot fixed preadjusted edgewise appliances,and the average duration of treatment was 27.49±6.98 months.The lateral cephalometric radiographs（LCRs）before and after treatment were collected and imported into Dolphin software to measure and analyze the dental and maxillofacial and upper airway indices.Adobe Photoshop software was used to draw the contour of upper airway on all LCRs,and the processed images were imported into MATLAB software to calculate the minimum sagittal linear dimension（MSLD）and its vertical position.All data were statistically analyzed using SPSS 24.0.The upper airway size before and after treatment were compared using paired t-test.Evaluation of the relationship between the change of sagittal position of teeth and the upper airway size by simple linear correlation and regression analysis.Using independent t-test to compare the changes in airway size at different movement distances of incisors and molars.Results:1.The labial inclination of upper and lower incisors decreased by 8.47°（P<0.01）and 4.85°（P<0.01）,respectively.The tip of the upper incisors was retracted by 4.63mm（P<0.01）,and the tip of the lower incisors was retracted by 3.80 mm（P<0.01）.The mesial points of the upper and lower first molars crown were mesially moved by 2.09mm（P<0.01）and 1.93mm（P<0.01）,respectively.The occlusal plane angle increased significantly（P<0.01）,but the changes of ANB,mandibular plane angle and palatal plane angle were not obvious（P>0.05）.2.There were no significant changes in nasopharynx and the position of MSLD（P>0.05）.The palatopharynx,glossopharynx,hypopharynx and MSLD decreased significantly（P<0.05 or P<0.01）.3.The size of velopharynx,glossopharynx and MSLD were significantly correlated with the sagittal movement of upper and lower central incisors and first molars（P<0.05 or P<0.01）.The hypopharynx was significantly correlated with the sagittal movement of lower central incisors（P<0.05）.The airway decreased with the retraction of incisors,and increased with the proximal movement of molars.Among them,the sagittal movement of the lower central incisors was an independent influencing factor of hypopharynx,and the sagittal movement of the mandibular first molars was an independent influencing factor of velopharynx,glossopharynx and MSLD.4.When the mesial movement of the mandibular first molars was less than 4mm,the decrease of the length of the pharyngeal cavity after the soft palate reduced obviously with the mesial movement of the mandibular first molars（P<0.05 or P<0.01）.The length of the pharynx after soft palate tip and glossopharynx reduced obviously（P<0.05 or P<0.01）with the mesial movement of the mandibular first molars when the movement within 2.5-3mm,but there was no significant change with the further advancement of the mandibular first molars（P>0.05）.Similarly,the reduction of MSLDwas no longer changed significantly with the mesial movement of the mandibular first molars reaching about 1.5-2mm（P>0.05）.Nevertheless,with the anterior movement of molars,the decrease of the length of the pharynx after soft palate tip,glossopharynx and MSLD still tends to lessen.Besides,when the retraction of the lower incisors was greater than about 4.5 mm,the reduce of hypopharyngeal increased significantly with the retraction of the lower incisors（P<0.05）.Conclusions:1.Sagittal changes of incisors and molars were associated with upper airway size in adult patients with Class I with mesofacial vertical pattern.The upper respiratory airway dimensions decreased with the incisors retraction and increased with the mesial movement of the molars.2.The sagittal change of the lower first incisors was an independent influencing factor of hypopharynx,and the change of mandibular first molars was an independent influencing factor of velopharynx,glossopharynx and MSLD.Airway size is more related to the change of mandibular arch length.3.When the retraction of mandibular incisors reached about 4.5 mm,the impact on the airway was significantly increased.Moreover,the effect of lower molar mesial displacement on airway size should not be underestimated.4.Sagittal changes of maxillary and mandibular incisors and molars should be considered comprehensively in assessing adult airway changes after extraction orthodontics.