Effect of large incisor retraction on upper airway morphology in adult bimaxillary protrusion patients

Abstract

Objective: To evaluate, using multislice computed tomography (MSCT), the morphologic changes in the upper airway after large incisor retraction in adult bimaxillary protrusion patients.

Materials 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. A paired t-test was used to compare changes from T1 to T2. The relationship among the three variables (upper incisor retraction amount, upper airway size, and hyoid position) was analyzed by Pearson correlation coefficient.

Results: The amounts of upper incisor retraction at the incisal edge and apex were 7.64 ± 1.68 mm and 3.91 ± 2.10 mm, respectively. The hyoid was retracted 2.96 ± 0.54 mm and 9.87 ± 2.92 mm, respectively, in the horizontal and vertical directions. No significant difference was observed in the mean cross-sectional area of the nasopharynx (P > .05) between T1 and T2, while significant differences between T1 and T2 were found in the mean cross-sectional areas of the palatopharynx, glossopharynx, and hypopharynx (P < .05); these mean cross-sectional areas were decreased by 21.02% ± 7.89%, 25.18% ± 13.51%, and 38.19% ± 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.

Conclusion: Large incisor retraction leads to narrowing of the upper airway in adult bimaxillary protrusion patients.

Figure 1

Miniscrews were placed in the interradicular locations between the first molar and second premolar and were attached at gingival-level height: pretreatment (A) and posttreatment (B).

Figure 2

CT scan in sagittal plane after point registration and STL registration; the pharynx was subdivided into four parts by three planes perpendicular to the sagittal plane.

Figure 3

The 3D model of four parts of the pharynx, hyoid, and cervical vertebra after point registration and STL registration and the distance between T1 and T2 models were measured.

Figure 4

The effect of point registration (A, B) and STL registration (C, D, E). The teeth and maxilla were registered with the surface points that did not change after orthodontic treatment: (A) the most protruding points on the inferior margin of the zygomatic arch; (B) registration of pre- and posttreatment models; (C) the pre- and posttreatment models of the teeth were matched to each other after the registration of maxillary; (D) STL registration with cranial base; and (E) STL model occlusal view.

Figure 5

Changes in the teeth between the T1 and T2 models were measured.

Figure 6

Changes in the cross-sectional areas of the pharynx between T1 and T2 models were measured.