INTRODUCTION

Distalization of the maxillary dentition has become an important treatment modality for Class II and Class I malocclusion with bimaxillary dentoalveolar protrusion patients, especially when extraction treatment is not accepted by patients.^1,2 Recently, the modified C-palatal plate (MCPP) has been reported to achieve efficient and effective total arch distalization.³ Its treatment effects, as reported by the same research group, showed significant amounts of distalization and intrusion with minimal distal tipping of the first molar.^4,5 However, accurate evaluation of the root position is very difficult to perform on two-dimensional (2D) lateral cephalograms because of superimposition and projection factors.

Recently, airway analysis and the effect of different treatment modalities on the airway space have gained increased interest from researchers and have had conflicting results in the literature. Several studies showed a decrease in the pharyngeal airway size with extraction treatment.^6–8 Guilleminault et al.⁹ suggested that extraction treatment may predispose patients to obstructive sleep apnea (OSA). On the other hand, Larsen et al.¹⁰ demonstrated strong evidence using a large sample matched for age range, gender, and body mass index that there was no relationship between premolar extraction treatment and OSA. However, the effect of distalization of the maxillary dentition in nonextraction Class II patients on the airway has not been evaluated. Also, the three-dimensional (3D) evaluation of the position of the maxillary roots after distalization, especially changes in the transverse dimension and buccolingual axial inclinations, have not been assessed. The evaluation of changes in root position is essential for the prediction of the stability of the treatment outcome.

Therefore, the purposes of this study were to evaluate the changes in position of the maxillary dentition and the airway space after distalization using a MCPP in adult patients using cone-beam computed tomography (CBCT) images and to analyze the relationship between the amount of distalization and the changes in airway space.

MATERIALS AND METHODS

This study analyzed CBCT images of 33 consecutively treated Class II adult patients (27 women and 6 men), with an average age of 22.18 ± 3.99 years, who underwent bilateral total arch distalization of the maxillary dentition at the Department of Orthodontics Seoul St. Mary's Hospital, The Catholic University of Korea, Seoul, Korea. The inclusion criteria in this retrospective study were (1) dental Class II relationship, (2) 3D CBCT images taken immediately before (T1) and after (T2) distalization, (3) age was older than 18 years. Approval was obtained from the institutional review board of the Catholic University of Korea (KC11RASI0790), and informed consent was provided according to the Declaration of Helsinki.

The sample was divided into extraction and nonextraction groups. The extraction group (n = 16; 22.9 years old) consisted of those who were treated by premolar extraction and retraction of the anterior segment; they then received the MCPP for additional maxillary total arch distalization after extraction space closure. The nonextraction group (n = 17; 21.5 years old) was treated using MCPP appliances for distalization of the maxillary dentition (Figure 1). Table 1 shows the distribution of the severity of the Class II molar relationship in the extraction and nonextraction groups.

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Table 1. Distribution of the Severity of Class II Molar Relationship in Extraction and Nonextraction Groups

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All patients were treated by the same operator (Y-A K). The installation method of the MCPP appliance has been previously described.¹¹ The MCPP was fitted on the palatal surface of the dental cast with its arms extended between the first molar and second premolar, leaving 2 mm between the arms and the palatal slopes. A jig was used to transfer the MCPP to the patient's mouth. It was then installed by three 8-mm length/2.0-mm diameter miniscrews (Jeil Corporation, Seoul, Korea). A palatal bar with two hooks was extended along the gingival margins of the teeth and banded to the maxillary first molars. Immediately after placement, distalization was initiated by connecting elastics or nickel-titanium closed coil springs, applying approximately 300 g of force per side between the MCPP arms and the palatal bar hooks. Along with the MCPP appliance, 0.022-inch slot (Tomy Inc., Tokyo, Japan) brackets and bands were placed on the maxillary and mandibular teeth, including the second molars.

CBCT images (at T1 and T2) were taken using an iCAT scanner (Imaging Science International, Hatfield, Pa). The scanning parameters were 120 kV, 47.7 mAs, 20 seconds per revolution, a 170 × 130 mm field of view, and a voxel size of 0.4 mm. The head position was oriented so that the Frankfort plane was parallel to the floor in a seated position, and the images were taken at the intercuspal position.

The CBCT data were exported in a digital imaging and communications in medicine multifile format and imported into InVivo 5.2 software (Anatomage Inc, San Jose, Calif) for 3D volume rendering. Reorientation of the head position of each scan was performed as follows: the horizontal plane was defined through the right and left orbitales and the left porion, whereas the midsagittal plane was defined as the perpendicular plane passing through nasion and the anterior nasal spine. The vertical plane was perpendicular to both the horizontal and midsagittal planes. The CBCT images were digitized, and the software calculated 24 linear and angular measures between certain landmarks (Figure 2).

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Assessment of the Airway Space

The airway measurements of the pre- and postdistalization CBCT images were done by the same researcher using In vivo 5.2 software (Anatomage Inc). The oropharynx was divided into the following two areas: velopharynx and glossopharynx. The velopharynx was defined from the horizontal level of the palatal plane to the horizontal level of the end of the uvula, whereas the glossopharynx was defined from the horizontal level of the end of the uvula to the horizontal level of the C3 (the most anterior and inferior point of the third cervical vertebra).¹² The volume of the airway space and the minimum cross-sectional area (MCA) were computed automatically by the software (Figures 3 and 4). The amounts of change between pre- and postdistalization variables were calculated.

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Ten randomly selected cases were reprocessed 4 weeks later to evaluate intraoperator reliability. IntraClass correlation coefficient using a two-factor, mixed-effects model and type consistency showed that the measurements were reliable (intraClass correlation coefficient for all variables ranged between 0.90 and 0.99). It was not possible to blind the groups and the time point of each CBCT image.

RESULTS

There was no significant difference in the distribution of the severity of Class II relationships between the groups (P = .601; Table 1). Tables 2 and 3 demonstrate the before and after distalization comparisons between the extraction and nonextraction groups.

Table 2. Comparison of Predistalization Measurements Between Extraction and Nonextraction Groups

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Table 3. Comparison of Postdistalization Measurements Between Extraction and Nonextraction Groups

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After distalization, in the nonextraction group, the crown of the first molar showed 3.24 mm of distalization (P < .001). The palatal root had a nonsignificant 0.84-mm buccal movement, but significant 1.26-mm intrusion and 2.43-mm distalization (P < .001). The angulation of the long axis showed a nonsignificant 3.07° of distal tipping. The crown showed no significant rotation in the transverse plane (P = .387).

The root of the central incisor showed no significant changes, whereas the crown showed a significant retraction of 1.41 mm (P = .001), and a lingual inclination of 4.44° (P = .001). Also, the root of the canine demonstrated no significant changes, with a significant lingual crown tipping of 5.55° (Table 3).

In the extraction group, the first molar showed 3.41 mm of distalization at the crown level (P < .001). The root demonstrated 2.00 mm of distalization (P < .001), 1.07 mm of intrusion (P = .001), and a nonsignificant buccal movement of 0.41 mm. The inclination of the first molar had 3.68° of distal tipping (P < .001). Also, there was no significant rotation of the crown (P = .472).

The root of the central incisor in the extraction group showed no significant changes, whereas the crown showed a significant retraction of 3.26 mm (P < .001), and a lingual inclination of 7.46° (P < .001). The root of the canine showed a facial movement of 1.25 mm in the transverse plane (P = .001), with lingual crown tipping of 6.15°. There were no significant differences in the distalization effect between the nonextraction and extraction groups (Table 4).

Table 4. Comparison of Distalization Effect Measurements Between Extraction and Nonextraction Groups

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Regarding the airway space analysis, the nonextraction group showed a nonsignificant decrease of 0.92 cm³ in airway volume. Also, the MCA of the oropharynx was decreased by 7.82 mm², which was not statistically significant. Similarly, the extraction group demonstrated no significant changes in the airway volume or the MCA. There were no significant differences in the airway variables between the two groups (Table 5).

Table 5. Comparison of Airway Space Between Pre- and Postdistalization Measurements in Extraction and Nonextraction Groups

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When the correlation between the amount of distalization and airway space parameters were evaluated, the nonextraction group showed no significant correlation between the amount of distalization and the changes in the total volume or the MCA. However, in the extraction group, there were significant inverse moderate correlations (r = −0.58, P = .025; r = −0.53, P = .040, respectively).

DISCUSSION

The CBCT images allowed the clinicians to accurately evaluate the root position before and after treatment in this study. The analysis of changes in root position is crucial for the confirmation of achieving proper root movement to enhance the stability of treatment results after debonding. This study attempted to assess the positional changes in the maxillary dentition and the dimensional alterations in the airway after distalization using MCPP using 3D CBCT images.

A study recently evaluated the 3D changes in the maxillary tuberosity after distalization using MCPP in adults. A small amount of increase in the bone size was reported at the distal end of the tuberosity.¹² However, there was no assessment of the 3D changes in the molar root positions. Only a few studies evaluated the transverse changes after distalization. Fuziy et al.¹³ reported approximately 1.5 mm of buccal movement of the molars after distalization using the pendulum appliance. Varlik and Iscan¹⁴ showed an increase of about 1.0 mm in the intercanine width and 1.6 mm in the intermolar width after distalization using cervical headgear. In the current study, the roots of the canine and molar showed nonsignificant displacement (0.4 and 0.8 mm, respectively) in the buccal direction.

Another study evaluating the transverse effect of distalization demonstrated that the Frog appliance resulted in distobuccal rotation of the first molar, whereas “Karad's integrated distalizing system” resulted in greater rotation but in the distopalatal direction with greater expansion.¹⁵ Kang et al.¹⁶ reported, in a finite element study, a mesial-in rotation with distalization using MCPP that was decreased with the eruption of the second molar. The pendulum appliance also showed mesial-in rotation, whereas the headgear showed distal-in rotation. In the current study, the MCPP resulted in significant distalization and intrusion of the first molar with no significant rotation of the crown and no significant buccal displacement of its root in the transverse dimension. Therefore, the palatal approach method might be superior to the buccal approach in controlling the resultant tooth movements.

Sagitally, a recent study showed 4.2 mm of molar distalization and 3.2° of distal tipping accompanied by 0.3 mm of extrusion using a distal screw appliance placed on the palate.¹⁷ Another study reported 3.1 mm of molar distalization with 1.5° of distal tipping and 1.7 mm of extrusion using the MCPP.⁴ However, in the current study, the molars were distalized 3.2 mm with 3.1° of distal tipping and 1.3 mm of intrusion. These differences might be because the previous studies were performed on adolescents using a 2D evaluation method.

Regarding the airway analysis, Hsieh et al.¹⁸ reported an approximately 3-cm³ decrease in the airway volume after mandibular and maxillary set-back surgery. Wang et al.⁶ reported a decrease of 1 to 1.5 mm in the oropharyngeal airway anteroposterior width with extraction treatment. However, the decrease in the oropharyngeal airway anteroposterior width was 2.1 mm and 3.8 mm in patients treated with extraction and maximum anchorage in the study of Germec-Caken et al.⁸ Chen et al.⁷ demonstrated a 21.0% and 25.2% decrease in the cross-sectional areas of the palatopharynx and glossopharynx, respectively, after four first premolar extraction treatments. However, no previous study has evaluated the changes that may occur in the airway space after distalization of the maxillary dentition. In the current study, there was an approximately 0.5 to 1.5 cm³ decrease in the volume and about an 8% to 10% decrease in the MCA of the upper oropharynx; however, these decreases were not statistically significant.

In the extraction group, the amount of distalization had a significant inverse moderate correlation with the changes in airway volume and MCA. This suggests that an increase in distalization would cause some decrease in the airway volume. However, this was not found in the nonextraction group. This might be because the amount of incisor retraction was greater in the extraction group (3.26 mm) than in the nonextraction group (1.41 mm; P = .004), causing some decrease in the tongue space, and in turn decreasing the airway volume to a level that would cause further distalization to have an inverse correlation with the airway volume.

Several studies reported a relationship between the MCA of the oropharynx and the occurrence and severity of OSA.^19–21 Lowe et al.²¹ showed that the mean airway volume in OSA patients was 13.9 cm³. In the current study, the mean airway volume was 16.3 cm³ in the nonextraction group and 14.4 cm³ in the extraction group. After distalization, the volumes decreased to 15.3 cm³ and 13.8 cm³, respectively. However, these changes were not statistically significant.

There were no significant changes in the airway volume or the MCA of the oropharynx. However, because the patients in the current study were adults, the amount of distalization (mean 3.3 mm) might not be large enough to induce changes on the airway space. Therefore, future studies evaluating the airway changes in adolescent patients treated by MCPP with large amounts of distalization might be recommended.

In this study, the space of the nasopharynx was not evaluated because the treatment effect was related to the oral cavity and its effect on the airway space would be on the oropharynx and not the nasopharynx. Therefore, adding the nasopharynx space may have resulted in an increase of confounding factors. In addition, the CBCT images were acquired with the patient in a seated position, which resulted in different airway measurements than those taken in a supine position. Therefore, the dimensions of the airway space in this study should be interpreted with caution. Moreover, the body mass index was not reported and the mode of respiration was not controlled. Also, the shape of the palate was not included in the current study; therefore, an assessment of the relationship between palatal dimensions and the changes in root positions might be warranted.

CONCLUSIONS

• The use of the MCPP resulted in significant total arch distalization without significant displacement of the teeth in the transverse dimension or changes in the oropharynx airway space.

• There was a statistically significant moderate inverse correlation between the amount of distalization and the amount of change in the airway volume in the extraction group only.

• Based on the results observed, no patients in this study developed a significant risk of OSA after distalization of the maxillary dentition.