INTRODUCTION

The prevalence of Class III malocclusion is only 1% to 3% in whites but can be as high as 4% to 14% in some Asian populations.^1,2 Limited and even short-lived success has been achieved using reverse-pull headgear with or without rapid palatal expansion and/or chin cup therapy in the early to mixed dentition.³ The main effects of these treatment modalities were more dentoalveolar than skeletal in nature, with a significant chance of relapse to reverse overjet once mandibular growth is completed. With the introduction of bone-anchored maxillary protraction (BAMP), both maxillary protraction and restraint of mandibular growth with minimal dentoalveolar change is now possible. BAMP restricts the forward growth of the mandible through a combination of closure of the gonial angle, distalization of the ramus, and posterior positioning of the condyles with corresponding remodeling of the glenoid fossa.^4–6

Recent studies have shown that mandibular set-back surgery to correct Class III malocclusion results in a significant decrease to oropharyngeal and hypopharyngeal airway volumes.^7,8 This is of major concern since narrowing of the oropharynx is major risk factor for the development of obstructive sleep apnea (OSA).⁸ Initial studies correlating airway space reduction with OSA used two-dimensional (2D) cephalometric films and were often considered controversial because the film captured only a small cross section of the overall airway volume. Recently, airway studies have incorporated the use of cone beam computed tomography (CBCT) to assess the upper airway anatomy because CBCT offers numerous advantages when compared with lateral cephalograms, including volumetric rather than linear measurements, distortion-free measurements, and measurements that are independent of head positioning.^9–11 Although the predictive factors for the development of OSA are multifactorial, 3D airway studies have supported the findings that a decrease of airway volume and minimum cross-sectional area (choke point) are key contributors in the development of OSA.^12–14

The prevalence of pediatric OSA has increased in recent years, with an estimated 2% to 3% of US children affected.^15,16 Because BAMP corrects Class III malocclusion partially by a mechanism of mandibular restraint, it is important to determine whether or not this treatment modality will affect airway development in growing children. The aim of this study is to compare airway volumes and minimum cross-section area changes of Class III patients treated with BAMP versus untreated Class III controls. The null hypothesis is that BAMP treatment decreases oropharyngeal airway.

MATERIALS AND METHODS

Subjects

Twenty-eight consecutive patients treated with BAMP (14 girls and 14 boys) were enrolled in the study. The untreated Class III control group consisted of 29 patients (16 girls, 12 boys). All subjects had Class III malocclusion in the mixed or permanent dentitions characterized by an anterior crossbite or incisor end-to-end relationship, Class III molar relationship, and Wits appraisal of −1 mm or less (Table 1). All patients were of white ancestry, with a prepubertal stage of skeletal maturity according to the cervical vertebral maturation method (CS1–CS3). The mean age at T1 for the BAMP sample was 11.9 ± 1.2 years, and it was 13.1 ± 1.1 years at T2. The mean duration of the T1–T2 interval was 1.2 years. The mean age for the control group was 12.4 ± 1.2 years. The study was approved by the University of North Carolina Committee for Research on Human Subjects (12-1496).

Table 1. Demographic and Statistical Comparison for BAMP and Untreated Class III Controls^a

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BAMP Orthopedic Protocol

BAMP subjects had four miniplates placed, two in the infra-zygomatic crest of the maxillary buttress and two between the mandibular lateral incisors and canines. Small mucoperiosteal flaps were elevated, and the modified miniplates (Bollard, Tita-Link, Brussels, Belgium) were secured to the bone by two (mandible) or three (maxilla) screws (2.3-mm diameter, 5-mm length). The extensions of the plates perforated the attached gingiva near the mucogingival junction. Three weeks after surgery, the miniplates were loaded using Class III elastics applied at an initial force of 100 g on each side (Figure 1). The force was increased to 200 g after 1 month of traction and to 250 g after 3 months. The patients were asked to replace the elastics at least once a day and to wear those 24 hours per day. In cases with increased overbite, a removable bite plate was inserted in the upper arch to eliminate occlusal interference in the incisor region until correction of the anterior crossbite was obtained.

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Image Analysis Protocol

CBCT scans were acquired using an iCAT machine at a resolution of 0.3 mm × 0.3 mm × 0.3 mm (Imaging Sciences International, Hatfield, Penn) with a 20-second scan time and a 16-cm × 22-cm field of view using software (Dolphin 3D 11.7, Dolphin Imaging, Chatsworth, Calif). The scans were taken in maximal intercuspation. Attempts were made to standardize inspiration/expiration capture on all subjects. The CBCT volume was oriented with the inferior orbital rim of the left and right orbits parallel to true horizontal in the frontal view and the porion to orbitale line (Frankfurt horizontal plane) parallel to true horizontal in the sagittal view (Figure 2). Two-dimensional cephalograms were generated from the CBCTs and digitized. Using the airway autosegmentation feature in the midsagittal view, a line connecting the most posterior point of the bony nasal spine to basion defined the superior border, and a line from the inferior edge of C3 to the base of the epiglottis defined the inferior border (Figure 3). Once the borders were defined, 3D models of the airway were constructed. The constructed airway volume, midsagittal airway area, and transverse minimum cross-sectional area were computed from the 3D models (Figure 4). The DICOM files were also used to create lateral cephalograms for both the T1 and T2 BAMP group using Dolphin Imaging (version 11.7, Dolphin Imaging). Cephalometric films were traced by one examiner. Ten randomly selected cephalograms were retraced 1 week later. Measurement accuracy was assessed by using intraclass correlation coefficients, which were between .90 and .98 for all measurements.

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RESULTS

Descriptive statistics for T1 BAMP subjects and untreated Class III control patients are summarized in Table 1. Patients were well matched with regard to age, skeletal classification (SNA, SNB, and ANB), incisal angulation (U1-SN) and (L1-MP), and mandibular plane angle (SN-GoMe). The average age of BAMP subjects at T1 was 11.9 years, compared with 12.4 years for untreated Class III controls. Table 2 shows skeletal changes resulting BAMP treatment. The SNA increased by 2.23°, SNB decreased by 0.97°, and the average Wits correction was 5.49 mm. In addition, the mandibular plane angle decreased by 0.94°.

Table 2. Descriptive Statistics for Bone-Anchored Maxillary Protraction T1–T2 Cephalometric Changes

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From our previous study, we have shown that BAMP was effective in restraining mandibular growth^4–6; however, the restraint of anterior-posterior growth of the mandible did not appear to affect the development of the oropharynx. The mean airway volume of the oropharynx showed a statistically significant increase from T1 (12,636.89 mm³) to T2 (14,136.61 mm³; Table 3). The midsagittal area showed a statistically significant increase, and the minimum cross-sectional area increased slightly from 148.21 mm² to 163.65 mm², although this was not statistically significant. When we compared the posttreatment BAMP airway volume, midsagittal area, and minimum cross-sectional area against untreated Class III controls, there was no statistical difference between the groups (BAMP  =  14,432.98 mm³, 674.36 mm², and 174.56 mm²; control  =  14,560.33 mm³, 643.67 mm², and 170.94 mm²; Table 4).

Table 3. Statistical Comparison of T1 and T2 Airway Measurements^a

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Table 4. Statistical Comparison of T2 BAMP vs Control Airway Measurements

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ICC values and 95% confidence intervals of the ICC for each linear and angular measurement are reported in Table 5. All variables had ICC values greater than .90, showing high levels of reliability. A one-sample t-test showed (1) no significant difference between the repeated measurements and (2) the within-subject error is small enough, relative to between-subject variability, indicating no systematic bias.

Table 5. Intraclass Correlation Coefficient (ICC) with 95% Confidence Interval (CI)

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DISCUSSION

The effects of Class III orthopedics on airway development have been extensively studied in the literature, with conflicting results. Some have reported short- and long-term improvements of nasopharyngeal and upper airway space following maxillary protraction.^17,18 Kaygisiz et al.¹⁸ found that the improved airway dimensions obtained from face mask treatment were retained 4 years posttreatment. Others, including Baccetti et al.,^19,20 have reported no difference in nasopharyngeal or oropharyngeal airway dimensions between face mask subjects and untreated controls. Studies on chin-cup therapy have also reported similar findings. Tuncer et al.²¹ examined airway dimensions in an adolescent population following chin-cup therapy. Although chin-cup subjects had significant downward and backward rotation of the mandible, the rotation had no impact on oropharyngeal airway dimensions. However, these studies had limitations because they use 2D lateral cephalograms for airway. CBCTs have been shown to be more accurate in measuring airway volume and are not prone to the distortion or position errors that can occur with 2D imaging.^22–24 Furthermore, CBCT scans report 3D volumes rather than 2D regions, which might not reflect the true anatomic structure of interest. Our 3D study showed an increase in airway volume, sagittal dimension, and minimum cross-sectional area (choke point) in patients treated with the BAMP protocol. Furthermore, posttreatment airway measurements from BAMP subjects were comparable with untreated Class III controls, indicating that the orthopedic growth modification to redirect mandibular growth in a posterior direction did not hinder airway development. A limitation of this study is the lack of T1 control samples. Ideally, comparing volumetric changes over the same treatment duration between BAMP-treated subjects and untreated Class III controls would provide a more meaningful comparison, but ethical considerations regarding additional radiation dosage to untreated subjects precluded this. However, comparing airway volumes between well-matched (age and skeletal classification) BAMP and untreated Class III controls allows us to evaluate if the increase in airway of the BAMP group was equivalent to natural growth in Class III subjects.

In addition to redirecting mandibular growth in a posterior direction, BAMP treatment produces significant protraction of the entire midface.²⁵ It is possible that midface protraction can increase airway volume, especially in the upper segment of the oropharynx. However, cephalometric studies have concluded that while maxillary protraction increased the nasopharyngeal airway, it did not significantly affect the oropharynx.^26,27 Mucedero et al.²⁰ compared Class III patients treated with protraction face mask, protraction face mask with rapid palatal expansion, and untreated controls. While there were statistically significant skeletal improvements with the protraction groups compared with the untreated Class III controls, there was no difference in nasopharyngeal and oropharyngeal dimension between the groups. A recent 3D CBCT study compared airway volumes of subjects treated with protraction face mask and untreated controls.²⁷ They reported no significant change in the oropharynx as a result of face mask treatment. Interestingly, the authors noted that the oropharyngeal volume in the face mask group was smaller than in untreated controls. This is the first study to compare 3D airway dimensions of BAMP-treated subjects and untreated Class III controls.

An important consideration in airway studies is that increases/decreases in airway dimension or 3D volumes do not necessarily correlate with physiologic function. Many factors during image acquisition can affect the recorded volume of the airway, including inspiration vs expiration, supine vs upright position, neck flexure, and scan time. Airflow monitors remain the gold standard for evaluating respiratory obstruction and breathing efficiency. However, recent studies have made strong correlations between the minimum cross-sectional area of the oropharynx and OSA.^12,13,25 Yucel et al.²⁸ reported that patients with severe OSA had the narrowest cross-sectional area at the level of the uvula in expiration. In our study, the choke point of the oropharynx was consistently located at or slightly above the level of the uvula. In addition, our study showed that BAMP-treated subjects exhibited an increased in choke point dimensions and that these measurements were comparable with those of untreated Class III controls.

CONCLUSIONS

• Subjects treated with BAMP showed an increase in airway volume and oropharyngeal dimensions.

• Furthermore, airway volume and minimum cross-sectional area were similar for BAMP subjects and untreated Class III controls.