Effect Of Large Incisor Retraction On Upper Airway Morphology And Root Resorption In Adult Bimaxillary Protrusion Patients:3D CBCT Registration Evaluation

Objective:Bimaxillary protrusion is a common disease in China and is commonly characterized by protrusive and prominent upper and lower incisors and an increased procumbency of the lips.The goal of orthodontic treatment is not only the coordination and stability of dentofacial structure and facial appearance but also the normal functioning of the stomatognathic system and airway. Typical orthodontic treatment includes extraction of the bimaxillary premolars and anterior tooth retraction, with maximum anchorage achieved through the placement of miniscrews, which enable the practitioners to achieve maximum anterior tooth retraction without undesirable movements of the posterior teeth. This study is based on CBCT measurement to evaluate the effect of large incisor retraction on upper airway morphology and root resorption in adult bimaxillary protrusion patientsMaterials and Methods:Thirty adult patients with bimaxillary protrusion had four first premolars extracted, and then miniscrews were placed to provide anchorage. A CT scan was performed before incisor retraction and again posttreatment. Three-dimensional (3D) reconstruction of the pre-(T1) and post-(T2) CT data was used to assess for morphological changes of the upper airway and teeth roots. A paired t-test was used to compare changes from T1to T2. The relationship among variables was analyzed by Pearson correlation coefficient. Results:1. The amounts of upper incisor retraction at the incisal edge and apex were7.64±1.68mm and3.91±2.10mm, respectively. The hyoid was retracted9.87±2.92mm and2.96±0.54mm,respectively, in the horizontal and vertical directions. No significant difference was observed in the mean cross-sectional area of the nasopharynx (P>0.05) between T1and T2, while significant differences between T1and T2were found in the mean cross-sectional areas of the palatopharynx, glossopharynx, and hypopharynx (P<0.05);these mean cross-sectional areas were decreased by21.02%±7.89%,25.18%±13.51%, and38.19%±5.51%, respectively. The largest change in the cross-sectional area is always noted in the hypopharynx. There was a significant correlation among the retraction distance of the upper incisor at its edge, the retraction distance of the hyoid in the horizontal direction, and the decrease of the hypopharynx.2. No significant difference was observed in the cervica and middle thirds of root (P>0.05) between T1and T2, while significant differences were found in apical third of root (P<0.05) by using the paired t-test. In apical third, the largest resorption in apical third of the root is always noted in the palatal and distal sectors. In addition, the largest change of root length in the anterior teeth is always occurred in the lateral incisors. A significant correlation was observed in the amount of anterior teeth retraction at the edge and the loss of root in distal and palatal sectors. A significant correlation was in the amount of anterior teeth intrusion at the root apex and the loss of root length at the apex. Conclusion:Large ATRI leads to obvious anterior teeth root resorption in the adult bimaxillary protrusion patients.Conclusion:Large incisor retraction may leads to narrowing of the upper airway and the root resorption in adult bimaxillary protrusion patients.