INTRODUCTION

Posterior crossbite is a commonly occurring malocclusion observed in the primary and early mixed dentition as unilateral or bilateral.¹ It is defined as any abnormal transverse relation between the upper and lower posterior teeth in centric occlusion.¹ Posterior crossbite could be due to dental, skeletal, and neuromuscular functional components, but the most common cause is transverse maxillary deficiency. Such a deficiency can be induced by sucking habits, certain swallowing habits, or upper airway obstruction caused by nasal allergies or adenoid tissues.^2–5

In the treatment of bilateral posterior crossbite, symmetric rapid maxillary expansion (RME) has been commonly used to correct maxillary constriction. In the treatment of unilateral posterior crossbite, RME or asymmetric rapid maxillary expansion (ARME) have been frequently used, depending on whether the crossbite was functional or not. To distinguish between a true unilateral posterior crossbite and a functional posterior crossbite, the mandible can be observed along the closing path. In a true unilateral posterior crossbite, the unilateral crossbite relationship is seen both in centric relation and centric occlusion without a functional shift of the mandible. In the functional posterior crossbite, bilateral constriction of the maxillary arch causes occlusal interferences, leading to a functional shift of the mandible toward the crossbite side along the closing path. While functional posterior crossbite patients should be treated with RME, true unilateral posterior crossbite patients should be treated with ARME.^5–7 If true unilateral posterior crossbite is treated with RME, undesirable expansion and buccal crossbite malocclusion can occur on the nonaffected side.^6,7 The skeletal changes with RME and ARME appliances include maxillary sutural expansion, widening of the nasal cavity, and changes in the upper airway and maxillary sinus.^8–11

Two-dimensional imaging (cephalometric radiographs), three-dimensional imaging (magnetic resonance, computed tomography [CT], cone-beam computed tomography [CBCT]), and rhinomanometry have been used in the evaluation of RME treatment and its effects on facial structures.^8–13 Because of structural superimposition and the presence of complex bony structures, it is difficult to obtain definitive, accurate results with two-dimensional imaging techniques.⁸ Three-dimensional evaluation with CBCT allows quantitative airway volume measurement with many advantages compared with conventional CT, such as lower cost and lower radiation dose with minimal distortion.^5,12–17

Some previous studies evaluated changes in the pharyngeal airway and maxillary sinus volumes (MSVs) with RME treatment, but there was no consensus among those studies.^11,17,18 No studies were found that evaluated airway changes and sinus volumes with ARME. The aim of this study was to evaluate the changes in pharyngeal airway and MSVs after RME and ARME treatment in growing patients using three-dimensional images obtained from CBCT.

MATERIALS AND METHODS

This was a retrospective study, and the images used were prescribed diagnostic records collected due to dental treatment needs. The patients had signed an informed consent form allowing the use of their data for any scientific purposes. The study was approved by the Regional Ethical Committee of Necmettin Erbakan University, Faculty of Dentistry. The sample size for the groups was calculated based on a significance level of .05 and a power of 90% to detect a clinically meaningful difference of between 833 and 1719 mm³ (±1032 mm³) for the difference in the nasoalveolar airway volume between the two groups. The power analysis showed that 29 patients in each group were required. The clinical meaningful value was determined according to a study by El and Palomo.¹⁸

Individuals, aged 12–15 years, previously treated with symmetric expansion (RME group; 14 girls, 16 boys; mean age, 14.04 years) or asymmetric expansion (ARME group; 16 girls, 14 boys; mean age, 13.75 years) were included in the study. The inclusion criteria were posterior crossbite; no previous orthodontic or orthopedic treatment; all permanent teeth up to the first molars present; no carious lesions, gingival lesions, or periodontal lesions; no previous tonsillar, nasal, adenoid, or head or neck surgery; no history of facial trauma; no systemic diseases; no craniofacial anomalies or temporomandibular joint disorders; beginning and progress CBCT scans of the treatment; and no major variation in the head or craniocervical orientation (>5°) between the pretreatment (T1) and postexpansion or posttreatment (T2) CBCT scans. CBCT images at pretreatment and after 3 months of retention (Kodak Model CS 9300, Carestream Health Inc, Rochester, NY) of the 60 patients were compared.

All patients in the RME group were treated with acrylic bonded appliances (Figure 1). In the ARME group, patients were treated with modified acrylic bonded appliances, which were built by adding an occlusal-lock mechanism on the nonaffected side (Figure 2). The occlusal-lock mechanism on the nonaffected side was designed so that the vestibular surface of the appliance extended to the middle third of the mandibular teeth, and the inner surface of the appliance extended along the lingual surface of the mandibular teeth vertically.⁷ In both appliances, a hyrax screw (Dentaurum, Pforzheim, Germany) was placed in the acrylic plate parallel to the first molars and as close to the palate as possible. After occlusal adjustments were made, the appliances were bonded. The expansion protocol consisted of turning the screw twice every day for the first week and then turning once a day until a slight overcorrection of the crossbite was achieved. The screw was stabilized in both groups, and the occlusal lock was removed in the ARME group during the retention period. The appliances were left in place passively for 3 months.

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The CBCT scans were made at 70 kV and 8.0 mA with the following protocol: 25-cm field of view, 0.18-mm voxel size, and 6.15 seconds per section. The scans were taken with the patients in a supine position and the palatal plane perpendicular to the floor. The data obtained from CBCT images were transferred to a network computer workstation, where volumetric changes of the pharyngeal airway and maxillary sinus were measured using MIMICS Software (14.01 version, Materialise, Leuven, Belgium). All measurements were made by the same orthodontist with 10 years of experience (M.A.).

Quantitative measurement of the upper and lower pharyngeal airway volumes was determined according to the following defined borders: anterior border, the vertical plane passing through the posterior nasal spine (PNS); superior border, the roof of the pharynx; posterior border, posterior pharyngeal wall; and inferior border, horizontal plane passing from the most anterior and inferior point of the third vertebra (Figure 3). Then, the connection with the outer air was cropped slice by slice. The structures that failed to connect with the outer airway were separated, and the three-dimensional image of the pharyngeal airway was constructed and calculated in mm³. The three-dimensional image of the pharyngeal airway was divided into upper and lower parts by a plane drawn from the PNS to the most anterior and inferior point of the first vertebra.

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The MSVs of the affected and nonaffected sides for ARME and the right and left side MSVs for RME were determined. For the assessment of MSV, the coronal image was selected. The same thresholding limits were applied as in the pharyngeal airway assessment, and the sinus was cropped in the slice in which the greatest area was apparent. Cropping was also done in the axial and sagittal views. Any connection with the outer air was eliminated, three-dimensional images of the left and right sinus were constructed, and their volumes were calculated (Figure 4).

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RESULTS

Descriptive statistics and comparisons of MSVs between the RME and ARME groups before treatment are presented in Table 1. Pretreatment, there were significant differences between the affected and nonaffected sides in MSV in the ARME group (P < .05). However, there was no difference between the right- and left-side MSVs in the RME group (P > .05).

Table 1.  Results of the Independent-Sample t-Test Used to Compare the Right and the Left Side of Maxillary Sinus Volume Before Expansion (T1)

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Comparisons of the airway and MSVs before and after ARME treatment are presented in Table 2. The ARME treatment affected the upper and total pharyngeal airway volumes and the affected side and total MSVs (P < .05). Comparisons of the airway and MSV before and after RME treatment are presented in Table 3. All parameters except lower pharyngeal airway volume were significantly increased by RME treatment (P < .05).

Table 2.  Results of the Paired-Sample t-Test Used to Compare Expansion Volumes Before (T1) and After (T2) Asymmetric Rapid Maxillary Expansion Treatment

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Table 3.  Results of the Paired-Sample t-Test Used to Compare Expansion Volumes Before (T1) and After (T2) Rapid Maxillary Expansion Treatment

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Treatment comparisons between the ARME and RME groups of the airway and MSVs are presented in Table 4. There was a significantly greater change in upper pharyngeal airway volume and all parameters of MSV for the RME group (P < .05).

Table 4.  Results of the Paired-Sample t-Test Used to Compare Expansion Volumes Before (T1) and After (T2) Rapid Maxillary Expansion (RME) and Asymmetric Rapid Maxillary Expansion (ARME) Treatment

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DISCUSSION

The aim of the study was to evaluate the changes in pharyngeal airway (upper, lower, and total; mm³) and MSVs quantitatively via CBCT that occur with RME and ARME treatment. RME is a treatment method used to correct transverse deficiency in the maxillary arch. RME treatment also affects other structures such as the nasal cavity, maxillary sinus, and pharyngeal airways.^5,19–21 Although the effects of RME on the nasal cavity, maxillary sinus, or pharyngeal airways have been investigated in previous studies^22–24 two-dimensionally with cephalometric radiographs²⁵ and three-dimensionally with CBCT,^21,26,27 no previous study was found that evaluated the effects of ARME on these structures.

In the current study, a significant increase in upper and total pharyngeal airway volume after ARME and RME treatment was found, but the observed increase in the lower pharyngeal airway volume was insignificant. When ARME and RME treatment were compared, there was a significantly greater increase in upper pharyngeal airway volume in the RME group. Although many studies have been done, the effect of RME on pharyngeal airway volume is still under debate. Smith et al.¹¹ evaluated the pharyngeal airway volume of adolescent RME patients with CBCT. Similar to the current study, they found significant increases in nasopharyngeal airway volume after RME treatment. Conversely, Ribeiro et al.²⁸ used CBCT to evaluate the effect of RME treatment on nasopharyngeal dimensions. They reported that the oropharyngeal airway experienced increased volume, while the nasopharyngeal airway did not have the same effect. Zhao et al.¹⁷ analyzed the same area with CBCT before and 15 months after RME and found no significant differences in the volume of the oropharynx and nasopharynx. Chang et al.²⁹ analyzed the oropharyngeal airway volume via CBCT and reported that there was no significant change in total airway volume after RME treatment. The differences between the studies might be due to the lack of standardized positioning of the head and tongue, different reference points for the pharynx, and different imaging techniques.^17,29

In the present study, the changes in MSV as the affected side for ARME, right side for RME, nonaffected side for ARME, and left side for RME were also evaluated. Since there was no difference between the right and left sides in the MSV in the RME group before treatment, it was logical to believe that there would be no difference between choosing the right or left sides to compare the affected and nonaffected sides in MSV in the ARME group. In the pretreatment comparison, there were significant differences in MSV between the affected and nonaffected sides in the ARME group. The volume of the affected side was found to be significantly less than the nonaffected side. This may mean that, in the ARME group, maxillary sinus development might be decreased on the affected side. In the ARME results, MSV significantly increased only on the affected side after treatment. In the RME results, MSV significantly increased after treatment on both sides. When the ARME and RME treatment effects were compared, there were significant differences on both sides. The increase in MSV was higher on the affected side of the ARME group than the right side of the RME group, and the other parameters showed greater increases in the RME group than the ARME group. These results showed that the ARME treatment provided successful skeletal expansion on the affected side.

Darsey et al.²¹ examined the MSV of RME patients via CBCT and found that MSV did not change after RME treatment. Smith et al.¹¹ showed that MSV was not affected by RME treatment. On the other hand, Garrett et al.⁵ observed that maxillary sinus width decreased with RME treatment, which was a possible cause of decreased MSV. Pangrazio-Kulbersh et al.³⁰ evaluated the changes in nasopharyngeal airway and MSV after RME treatment with CBCT. Similar to the current study, they reported that MSV increased after treatment. Adkins et al.³¹ stated that alveolar bending in the molar and premolar regions could cause changes in the inferior border of the sinus; in the current study, increased MSV might have been related to changes in the inferior border of the sinus.

In patients treated with ARME in this study, an occlusal-lock mechanism was used with anchorage from the mandible formed on the nonaffected side. Therefore, no significant expansion occurred on the nonaffected side. It could be assumed that the occlusal-lock mechanism caused displacement of the condyles or impaired occlusal stability. In the patients treated with ARME, the occlusal lock was removed from the appliance during the retention period. There were no temporomandibular joint complaints after treatment from the patients treated with ARME. Further studies of the ARME appliance are needed to confirm this finding. Unfortunately, there were no previously published data evaluating pharyngeal airway and MSV in patients with true unilateral posterior crossbite. Future studies are needed to determine the effect of RME and ARME appliances on the pharyngeal airway and MSV.

CONCLUSIONS

• RME treatment was found to be effective in increasing pharyngeal airway and MSV in patients with bilateral maxillary deficiency.

• ARME treatment was found to be effective for treating true unilateral posterior crossbite and also for increasing pharyngeal airway and MSV.