To evaluate the changes in pharyngeal airway dimensions and the position of the hyoid bone after maxillary protraction with different alternate rapid maxillary expansion and construction (Alt-RAMEC) protocols in patients with skeletal class III malocclusion as a result of maxillary retrusion.
The patients with skeletal class III malocclusions were consecutively divided into two groups. Group 1 consisted of 17 patients (11 boys and 6 girls, mean age 11.31 ± 1.71 years) who had the Alt-RAMEC protocol for 5 weeks, and group 2 consisted of 17 patients (10 boys and 7 girls, mean age 11.64 ± 1.24 years) who had the Alt-RAMEC procedure for 9 weeks. In this study, 4 angular and 13 linear measurements were performed to evaluate the skeletal and pharyngeal airway changes that occurred after maxillary protraction.
A significant increase in the maxillary growth, inhabitation of mandibular growth, and clockwise rotation of the mandible caused the improvement of the maxillo-mandibular relationship in both groups. Those changes caused a significant increase in the upper pharyngeal airway dimension (P < .01) and affected the vertical position of the hyoid bone in both groups (P < .05 and P < .01, respectively). However, changes that occurred in both groups were found to be similar for all airway variables (P > .05).
Upper pharyngeal dimension and vertical position of the hyoid bone were affected by the maxillary protraction with different Alt-RAMEC protocols. There were no statistically significant differences between the groups.
Skeletal class III malocclusion is characterized by maxillary retrusion, mandibular protrusion, or the combination of both components,1 and its prevalence was reported to be as high as 14% for Asian populations2 and 16.7% for orthodontic populations.3 Maxillary protraction with or without rapid maxillary expansion has been considered as a major treatment option in growing patients with skeletal class III malocclusion.
The effects of maxillary protraction with or without rapid maxillary expansion have been well documented and were mainly the forward displacement of the maxilla and maxillary dentition, inhabitation of the mandibular growth and clockwise rotation of the mandible, protrusion of the maxillary incisors, and retrusion of the mandibular incisors.4–9 Although a number of studies10–17 have been published about the pharyngeal airway effects of maxillary protraction, there were conflicting findings. Significant changes for nasopharyngeal10–12,14,15 and oropharyngeal10,12 airway dimensions after maxillary protraction were reported by some clinicians, whereas the studies by Hiyama et al.,13 Mucedero et al.,17 and Baccetti et al.16 did not show any significant nasopharyngeal and oropharyngeal airway dimension changes.
Liou18 introduced a method called alternate rapid maxillary expansion (Alt-RAMEC) protocol to increase the skeletal effects of maxillary protraction, and several clinicians19–21 reported the increased skeletal effects of this method when performed prior to maxillary protraction. To our knowledge, none of the previous studies evaluated the effects of maxillary protraction with Alt-RAMEC protocol on pharyngeal airway dimensions and hyoid bone position. Thus, the aim of the present study was to evaluate the changes in pharyngeal airway dimensions and the position of the hyoid bone after maxillary protraction with different Alt-RAMEC protocols in patients with skeletal class III malocclusion as a result of maxillary retrusion.
MATERIALS AND METHODS
Ethical approval for this prospective study was obtained from the local ethics committee of Akdeniz University, and informed consent was obtained from the parents of the patients included in the study.
The sample size was calculated based on a formula22 using a significance level of .05 and a power of 80% to detect a clinically meaningful difference of 2.0 mm (±2 mm) for the upper pharyngeal dimension between the groups. The samples comprised 34 patients (21 boys and 13 girls) with skeletal class III malocclusion as a result of maxillary retrusion, and they were divided into one of the following two groups: group 1 consisted of 17 patients (11 boys and 6 girls, mean age 11.31 ± 1.71 years) who were treated with an Alt-RAMEC protocol of 5 weeks followed by maxillary protraction, and group 2 consisted of 17 patients (10 boys and 7 girls, mean age 11.64 ± 1.24 years) who were treated with an Alt-RAMEC protocol of 9 weeks followed by maxillary protraction. The patients in both groups had dental and skeletal class III malocclusion with maxillary retrusion, normal vertical growth pattern, anterior crossbite with no functional shift, and no systemic disease, congenital anomalies, or temporomandibular joint disorders.
The parents of the patients were instructed to open the screw (Leone A0620-19, Florence, Italy) twice per day for 1 week and to close it twice per day for the following week (0.20 mm per turn). This protocol was repeated for 5 consecutive weeks in group 1 and 9 consecutive weeks in group 2. After the Alt-RAMEC protocols were completed in both groups, the activation of the screw was continued until the crossbite was overcorrected for the patients with a posterior crossbite. Then, a Petit-type maxillary protraction appliance was used with an approximately 500 g force applied bilaterally with an anteroinferior force vector of approximately 30° to the occlusal plane from the hooks placed in the canine region on the buccal sides of the full coverage expander (Figures 1 and 2). The patients were checked at four weekly intervals, and they were instructed to wear the appliances for at least 20 hours per day until at least a 2-mm positive overjet was achieved. All patients included in the study were treated in the same clinic by two clinicians between June 2015 and March 2016.
Cephalometric radiographs were obtained in standard positions by an experienced radiology technician just before (within 2 weeks) and after (within a week) the maxillary protraction using the same cephalostat (Planmeca Promax, Helsinki, Finland). The patients were requested to keep their teeth in centric occlusion without swallowing and breathing during the exposure. Four angular and 13 linear measurements (Figures 3–5) were blindly performed to evaluate the skeletal and pharyngeal airway effects of the maxillary protraction in both groups by an experienced orthodontist (Dr Buyukcavus) using cephalometric lateral films and Dolphin Imaging Version 11.8.06.24 Premium software (Dolphin Imaging and Management Solutions, Chatsworth, Calif).
To determine the reliability of the measurements, the same orthodontist repeated all tracings and measurements. The differences between the two readings were between 0.1 and 0.4 mm for linear measurements and 0.1 and 0.5° for the angular measurements. The Houston test also confirmed the reliability of the data (r > 0.961).
Parametric tests were performed for data analysis because a Shapiro-Wilks test showed normal distribution. Gender distribution was tested by means of a Pearson chi-square test. The mean changes observed in each group were evaluated using a paired t-test. The chronological ages, treatment durations, initial cephalometric and pharyngeal airway values, and mean changes that occurred in each group were compared using Student's t-test. All statistical analyses were performed using the SPSS software package program (SPSS for Windows 98, version 10.0; SPSS Inc, Chicago, Ill). The significance level was set at P < .05 for all statistical tests.
Patients in both groups were well matched according to gender distribution, chronological ages, and treatment durations (Table 1). The initial values of craniofacial and pharyngeal airway measurements of the patients in both groups showed similar values with no statistically significant differences (P > .05; Table 2).
In both groups, the skeletal class III relationships and anterior crossbites were improved; SNA significantly increased (2.80 ± 1.55° and 3.34 ± 1.40°, respectively; P < .001), SNB decreased (1.88 ± 1.34° and 1.53 ± 0.98°, respectively; P < .001), ANB increased (4.60 ± 1.29° and 4.90 ± 1.45°, respectively; P < .001), and SN-MP increased (2.09 ± 2.13° and 1.90 ± 1.60°, respectively; P < .01 and P < .001, respectively), which resulted in forward displacement of the maxilla, clockwise rotation of the mandible, and a significant improvement in the intermaxillary sagittal relationship (P < .001). The mean increases for PNS-AD1, PNS-AD2, PNS-Ba, and PNS-H were found to be 2.08 ± 1.82 mm (P < .001), 2.16 ± 1.95 mm (P < .001), 1.44 ± 2.08 mm (P < .05), and 2.42 ± 2.32 mm (P < .01) for the patients in group 1, and 2.06 ± 2.10 mm (P < .01), 2.06 ± 1.60 mm (P < .001), 1.82 ± 2.13 mm (P < .01), and 2.14 ± 1.67 mm (P < .001) for the patients in group 2, respectively. Upper pharyngeal airway dimensions were significantly increased in both groups (P < .001), whereas the changes for lower pharyngeal dimension were found to be statistically insignificant (P > .05). The vertical position of the hyoid bone was significantly affected in both groups (1.81 ± 2.67 mm, P < .05; and 2.36 ± 2.36 mm, P < .01, respectively; Table 3).
Both groups had similar changes in craniofacial and pharyngeal airway measurements with no statistically significant differences between the groups (P > .05), except for SNA (P < .001; Table 4).
In the present study, we evaluated the changes in pharyngeal airway dimensions and the position of the hyoid bone after maxillary protraction with different Alt-RAMEC protocols. Although the efficiency of maxillary protraction with the Alt-RAMEC protocol has been widely investigated in the literature,18–21,23,24 none of the previous studies have focused on pharyngeal effects of this method when used for the correction of maxillary retrusion. The groups were well matched for gender distribution, chronological ages, treatment durations, and initial craniofacial and pharyngeal airway measurements as statistically tested, and thus the effects of those factors on findings were eliminated.
Previous studies10,11,13–17 reported contrasting findings with regard to the possibility of improving the sagittal airway dimensions by means of maxillary protraction. The results of the present study showed significant increases for the variables PNS-AD1, PNS-AD2, PNS-Ba, and PNS-H, probably because of the anterior movement of the maxilla (points A and PNS) as supported by the increase in SNA variable. However, Baccetti et al.4 and Mucedero et al.17 reported insignificant increases in those variables. Those authors reported approximately 1-mm anterior movement of the maxilla, and this might be the main reason of their insignificant findings for airway measurements. Increased skeletal effects of maxillary protraction with the Alt-RAMEC protocol can explain the significant increases of those variables in our study. In agreement with our findings, previous studies19,23 reported increased skeletal effects of maxillary protraction with Alt-RAMEC in the maxilla when compared with those without Alt-RAMEC.
Our findings showed significant increases in the upper pharyngeal dimension and insignificant changes in the lower pharyngeal dimensions in both groups. This finding was in agreement with those of previous studies.11,14,15,25 One of the main effects of maxillary protraction is clockwise rotation of the mandible (shown as a decrease in SNB and an increase in SN-MP variables) as shown in this study, and the slightly posterior movement of the mandible did not appear to cause any change on oropharyngeal dimensions.
In the present study, the antero-posterior position of the hyoid bone did not change, but its vertical position (Hy-HRL; Figure 5) significantly increased. It is difficult to discuss this finding with previous findings because few of them evaluated the vertical position of the hyoid bone. Lee et al.15 reported an increase of 1.23 ± 2.77 mm for the vertical position of hyoid (Hy-MP variable). However, it should be noted that the authors used the mandibular plane as a reference line, and it is significantly affected by the maxillary protraction shown as an increase in the SN-MP variable.7,8 In this study, we used the T–W line as the horizontal reference line, which was reported to be the most similar superimposition method to Björk's structural method; thus, a reliable method for examining overall facial changes.26
One limitation of the present study is that no control group of untreated class III patients was used. According to Taylor et al.,27 who performed a longitudinal study evaluating the growth of the oropharyngeal tissues, the growth increments between the ages of 9 to 12 years were very small. Oktay and Ulukaya10 also reported that the changes in the pharyngeal measurements of patients aged 11.7 years for a duration of 8 months were at a negligible level. In addition, future prospective studies using cone-beam computed tomography images with larger study samples could provide accurate findings and provide an opportunity for clinicians to compare their findings with ours because those images were reported to have several advantages28–31 when compared with the conventional cephalometric films. Thus, the findings of the present prospective clinical study should be assessed within the limitations of the two-dimensional radiography design used for the assessment of the pharyngeal airway.
A significant increase in the maxillary growth, inhabitation of mandibular growth, and clockwise rotation of the mandible caused the improvement of the maxillo-mandibular relationship in both groups.
Both groups showed significant increases in the upper pharyngeal airway dimension (P < .01) and the vertical position of the hyoid bone (P < .05 and P < .01, respectively).
Changes in pharyngeal airway dimensions and position of the hyoid bone after maxillary protraction with different Alt-RAMEC protocols were found to be similar (P > .05).
A future study using cone-beam computed tomography images for the evaluation of airway changes after Alt-RAMEC and maxillary protraction would be welcome.
This study was part of a work that was supported by a research grant from Akdeniz University, Scientific Research Projects Unit (TDH-2016-1352).