Introduction
The primary aim of this study was to investigate the change in upper airway dimensions and in mandibular position after miniscrew-assisted treatment with premolar extractions in adult patients with Class II high-angle malocclusion. The secondary aim was to determine the correlation between changes in upper airway dimensions and changes in mandibular position in these patients.
Methods
Eighteen adult patients with Class II high-angle malocclusion (mean ± standard deviation age = 21.2 ± 2.9 years) were selected retrospectively. All patients underwent 4 premolar extractions, and 2 miniscrews were implanted in the maxilla to intrude molar height. Cone beam computed tomography images were taken pretreatment and posttreatment for every patient. The primary outcome variable for the upper airway was the minimal cross-sectional area of the upper airway (CSA min ), and the primary outcome variables for the mandible were mandibular rotation (Mp-SN angle), mandibular horizontal position (SNB angle), and mandibular vertical position (ANS-Me distance).
Results
The CSA min significantly increased by 47.2 mm 2 ( t = −2.26, P = 0.04) after orthodontic treatment. The mandible significantly rotated counterclockwise by 0.9° ( t = 2.20, P = 0.04) after treatment, which consisted of forward movement of 1.2° ( t = −4.30, P = 0.00) and upward movement of 1.3 mm (Z = −1.98, P = 0.05). Furthermore, the change of the CSA min showed a significant correlation with the change of the ANS-Me ( P = 0.01).
Conclusions
By using miniscrews to intrude maxillary molars, orthodontic premolar extraction treatment results in mandibular counterclockwise rotation, and upper airway dimensions increase in Class II high-angle young adult patients. The increase of the upper airway dimensions significantly correlates to the upward movement of the mandible.
Highlights
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Young adult patients with Class II high-angle malocclusion were studied.
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Treatment included miniscrew-assisted molar intrusion and premolar extractions.
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Treatment produced counterclockwise rotation of the mandible.
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Upper airway dimension was increased after treatment.
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Increase was related to the upward movement of the mandible.
Class II high-angle malocclusion combines Class II Division 1 malocclusion and mandibular vertical excess, which is one of the most complex malocclusions in the field of orthodontics. For dental malocclusion, orthodontic premolar extraction treatment is typically used to provide space to align crowded teeth, correct anteroposterior interarch discrepancies, and retract incisors. However, traditional orthodontic treatment has only a small effect on mandibular vertical excess. Orthognathic surgery can correct the mandibular position directly, but many patients do not accept this type of treatment. An alternative treatment, as reported in a few studies, is to use skeletal anchorage (miniplates, miniscrews) to intrude molars, which leads to a significant counterclockwise rotation of the mandible and shortening of the mandibular vertical excess in patients with an open bite. However, no previous studies were performed to study the effects of this treatment in patients with Class II high-angle malocclusion.
Patients with Class II high-angle malocclusion may tend to have a smaller upper airway , and a higher risk for obstructive sleep apnea (OSA). , OSA is characterized by recurrent obstruction of the upper airway, often resulting in oxygen desaturations and arousals from sleep. OSA is a common sleep-related disorder, affecting a mean of 22% (range, 9%-37%) men and 17% (range, 4%-50%) women. Besides suffering from the common complaints such as snoring and excessive daytime sleepiness, severe OSA patients also develop cardiovascular problems, such as myocardial infarction and stroke. As a constricted airway is one of the key factors to the pathogenesis of OSA, it is of importance to understand the effect of orthodontic treatment on upper airway dimensions of patients with Class II high-angle malocclusion.
Many studies , , have discussed the effects of orthodontic teeth extraction treatment on upper airway dimensions in adult patients. However, the results were not consistent. One study found the upper airway increased after extraction using lateral cephalograms, whereas other studies found the upper airway narrowed after treatment based on lateral cephalograms , or multislice computed tomography images. Besides, some studies reported that the upper airway remained unchanged after extraction based on cone beam computed tomography (CBCT) images , and lateral cephalograms. These conflicting results might be related to different malocclusion types, treatment strategies, imaging techniques (2-dimensional vs 3-dimensional imaging techniques), and upper airway variables. Besides, the mandibular position change during treatment was not taken into consideration in previous studies. Thus, the effects of mandibular position change on upper airway dimensions in adult patients with Class II high-angle malocclusion have not been determined yet.
As mandibular position is associated with upper airway dimensions via the lingual musculature (eg, genioglossus muscle), we hypothesized that the counterclockwise rotation of the mandible as a result of an orthodontic treatment with the assistance of skeletal anchorage and premolar extractions would result in an increase in the upper airway dimensions. Therefore, the aims of this study were to determine in adult patients with Class II high-angle malocclusion: (1) the effects of miniscrew-assisted orthodontic extraction treatment on mandibular position and on upper airway dimensions; and (2) the correlation between the changes of mandibular position and the changes of upper airway dimensions.
Material and methods
This retrospective study was approved by the Ethics Committee of the Stomatology Hospital of Shandong University, Jinan, Shandong, China (No. R20180706). Participants were selected from among all patients treated from January 2015 to July 2017 at the orthodontic clinic of the Stomatology Hospital of Shandong University.
The inclusion criteria were (1) adult patients (age ≥18 years old); (2) Class II malocclusion (ANB angle ≥4°) and high-angle pattern (Mp-SN angle ≥37°) ; (3) extraction of the maxillary first premolars and mandibular second premolars; (4) 2 miniscrews implanted bilaterally in the maxilla; and (5) available CBCT images before treatment and after treatment. The exclusion criteria were (1) temporomandibular joint disorders; (2) congenital absence of permanent teeth; (3) history of upper airway surgery; (4) previous orthodontic treatment and/or orthognathic surgery; (5) self-reported snoring and/or sleep apnea; and (6) impairment in the lip and/or palate function, such as a cleft lip and/or palate.
After extracting the maxillary first premolars and mandibular second premolars, all patients underwent a preadjusted edgewise appliance of 0.022-inch slot McLaughlin, Bennett, Trevisi brackets (Tomy International, Tokyo, Japan). Two miniscrews of 8 mm length (Vector TAS; Ormco, Orange, Calif) were implanted bilaterally in the maxilla between the second premolars and the first molars around the apical region through the buccal mucosa after local anesthesia by the same orthodontist. Four weeks after the miniscrew placement, orthodontic intrusion load, estimated as 150 g of force, was applied using an elastic chain. Moderate anchorage reinforcement was used in the maxilla to retract the maxillary anterior teeth, while minimal anchorage strategy was used in the mandible. The treatment goal was Class I canine and/or molar relationship, and the treatment time for each patient was approximately 2 and a half years.
Galileos CBCT scans (Sirona Dental Systems, Bensheim, Germany) were taken before and after treatment in all patients. During the scan, patients stood in the CBCT machine with a natural upright head position and maximum intercuspation. They were instructed to breath quietly through nose and to avoid swallowing and other movements. The fixed field of view size was 15 cm × 15 cm. All CBCT images were taken by the same operator. The images were saved in Digital Imaging and Communications in Medicine format for further analysis.
To measure the mandibular position, the maxillary position, incisors position, and the hyoid position, 15 landmarks were identified according to the guidelines of American Board of Orthodontics, yielding 6 linear and 5 angular measurements ( Table I ; Fig 1 ). All measurements were done before and after treatment based on CBCT images using the Dolphin Imaging software (version 11; Dolphin Imaging & Management Solutions, Chatsworth, Calif). Before the measurement, all images were standardized in orientation with the hard-palatal plane (PP plane) paralleled to the horizontal plane.
| Landmarks | Definition |
|---|---|
| N | Nasion: the anterior point of the intersection between the nasal and frontal bones. |
| S | Sella: the center of the hypophyseal fossa, determined by inspection. |
| SN Plane | SN Plane: the line connecting the point S to N. |
| ANS | Anterior nasal spine. |
| PNS | Posterior nasal spine. |
| PP plane | Palatal plane: the line joining anterior nasal spine with posterior nasal spine. |
| A | Subspinale: the most posterior point on the exterior ventral curve of the maxilla between the anterior nasal spine and Supradentale. |
| B | Supraemental: the most posterior point on the bony curvature of the mandible between Infradentale and Pogonion. |
| Pg | Pogonion: the most anterior point of mandibular symphysis. |
| Me | Menton: the most inferior point on the symphysis of the mandible. |
| Go | Constructed gonion: bisecting the angle formed by the tangents to the lower and the posterior borders of the mandible. |
| Mandibular plane (MP) | Mandibular plane: the line connecting the point Go to Me. |
| H | Hyoidale: the most superior and anterior point on the body of the hyoid bone. |
| U1 | Maxillary central incisor. |
| L1 | Mandibular central incisor. |
| Mandibular position | |
| Mp-SN angle, ° | Angle between MP plane and SN plane. |
| SNB angle, ° | Angle between point B and S at N, representing the position of the mandible in relation to the cranium. |
| ANS-Me, mm | Lower face height. Perpendicular distance between point ANS and Me to PP plane. |
| Maxillary position | |
| SNA angle, ° | Angle between point A and S at N, representing the position of the maxilla in relation to the cranium. |
| Dental structures | |
| U1-APg, mm | Perpendicular distance between from the tip of maxillary incisor to A-Pg line. |
| SN-U1 angle, ° | Angle between the long axis of U1 and SN plane. |
| L1-APg, mm | Perpendicular distance between from the tip of mandibular incisor to A-Pg line. |
| L1-Mp angle, ° | Angle between the long axis of L1 and MP plane. |
| Overjet, mm | Anteroposterior overlap of U1 and L1. |
| Overbite, mm | Superior-inferior overlap of U1 and L1. |
| Hyoid position | |
| H-MP, mm | Perpendicular distance from H to MP plane. |
Because the primary aim of this study was to investigate the changes of the mandibular position, 3 variables related to the mandible (Mp-SN angle, SNB angle, ANS-Me distance) were primary outcome variables. Mp-SN angle is the angle between the mandibular plane (MP plane) and sella-nasion plane (SN plane), which is typically used to represent the mandibular rotation. , , SNB angle is the angle between point supramental (B) and sella (S) at nasion (N) to represent the horizontal position of the mandible. ANS-Me distance is the perpendicular distance to PP plane between point anterior nasal spine (ANS) and Menton (Me) to present the vertical position of the mandible. Other craniofacial and dental measurements were the secondary outcome variables.
The upper airway dimensions were measured before and after treatment using the Dolphin Imaging software. Before the measurement, all images were also standardized in orientation with PP plane paralleled to the horizontal plane. All planes used to define the upper airway boundary were parallel to PP plane.
The upper airway was segmented into 2 parts according to the manual segmentation in midsagittal view: retropalatal airway and retroglossal airway. The volume of retropalatal airway and retroglossal airway was calculated automatically in cubic millimeters (mm 3 ) by Dolphin software after giving the boundaries. The retropalatal airway was limited superiorly by a horizontal plane crossing the PP plane, and inferiorly by a horizontal plane crossing the most posteroinferior point of the soft palate. The retroglossal airway was limited superiorly by a horizontal plane crossing the most posteroinferior point of the soft palate, and inferiorly by a horizontal plane crossing the most superior point of the epiglottis ( Fig 2 ).
The minimum cross-sectional area of the upper airway (CSA min ) is the most constricted axial area of the airway. By using the upper boundary of the retropalatal airway (PP plane) and the lower boundary of the retroglossal airway (the plane passing most superior point of the epiglottis), the CSA min can be automatically identified and calculated by Dolphin software in square millimeters (mm 2 ) ( Fig 2 ). According to a previous review, CSA min might be the most relevant anatomic characteristic of the upper airway related to the pathogenesis of OSA. Therefore, CSA min was the primary outcome variable for the upper airway dimensions and the volume of the retropalatal and retroglossal airway were the secondary outcome variables.
Whether the data was normally distributed or not was tested by the Shapiro-Wilk test. Pretreatment outcome variables were compared with posttreatment outcome variables using a paired t test for the normally distributed variables, and Wilcoxon signed rank test for the nonnormally distributed variables. The Bonferroni-Holm method was used to correct for the increased risk of type I error because of multiple statistical comparisons for the secondary outcome variables. Pearson correlation coefficient was calculated to determine the relation between the change in CSA min (ΔCSA min ) and changes in mandibular position (ΔMp-SN angle, ΔSNB angle, and ΔANS-Me distance). The significance level was set at P < 0.05. Statistical analysis was performed using SPSS software (version 24; IBM, Armonk, NY). A post-hoc power analysis was conducted to assess the power of the outcome variables using the software G∗power (version 3.1.9; University of Kiel, Kiel, Germany).
Results
Twenty-one patients who fulfilled the inclusion criteria were selected from approximately 400 orthodontic treatment recordings. Of those, 3 patients were excluded because of temporomandibular joint disorders (n = 2) and previous orthodontic treatment (n = 1). Finally, a total of 18 patients’ records (mean ± standard deviation age = 21.2 ± 2.9 years), including 11 females and 7 males, were enrolled in analysis. All patients achieved the treatment goal of Class I canine and/or molar relationship with improved vertical excess and convex facial profile.
The primary outcome variables of the upper airway dimensions and mandibular position at pretreatment and posttreatment are described in Table II . After treatment, the CSA min significantly increased by 47.2 mm 2 ( t = −2.26, P = 0.04) compared with baseline measurements. The Mp-SN angle significantly reduced by 0.9° ( t = 2.20, P = 0.04), which indicated the counterclockwise rotation of the mandible. Besides, the SNB angle significantly increased by 1.2° ( t = −4.30, P = 0.00), and ANS-Me significantly decreased by 1.3 mm (Z = −1.98, P = 0.05), which respectively represented the horizontal and vertical mandibular movement component of the counterclockwise rotation of the mandible.
