In this study, we performed CBCT to measure three-dimensional changes in the upper airway and craniomaxillofacial morphology to obtain more comprehensive and accurate measurements than those reported previously. CBCT has several advantages over 2D imaging, such as improved image quality, three-dimensional reconstructions, and a 1 : 1 ratio, which allowed reliable measurements. Moreover, CBCT provided the ability to visualize craniofacial structures with a short exposure time and a lower radiation dose, compared to traditional computed tomography.Once a CBCT is acquired, there is no need for lateral, frontal, and curvilinear cephalometric radiographs, due to the three-dimensional nature of the CBCT. In this study, the two CBCTs were performed one year apart; therefore, they caused almost no radiation damage to patients. Indeed, the American Dental Association recommended the following indications for CBCT in orthodontic treatments: observation of tooth development, limitation of tooth movement, airway evaluation, craniofacial morphology etc.
The nasopharynx is a musculomembranous tube that serves as a portal between the nasal chamber, anteriorly, and the oral pharynx, inferiorly. The nasopharynx is one of the most important areas of the upper airway, due to its close relationship to the occurrence of obstructive sleep apnea hypopnea syndrome.The shape and size of the nasopharyngeal cavity can be defined in terms of its depth and height, in the median sagittal plane, and its width, in the frontal plane. Brodie and King stated that the total depth of the nasopharynx is established in the first or second years of life.[15, 16]King also showed that, with growth, increases in the depth of the nasopharynx at the spheno-occipital junction are minimized by the forward growth of the anterior arch of the atlas. Furthermore, there is a positive correlation between the cranial base and the nasopharyngeal depth; thus, the more obtuse the base, the greater the depth, as mentioned by Rickettsand Bergland.[17, 18]In contrast to the early establishment of the nasopharyngeal depth, King demonstrated that the nasopharyngeal height continued to increase until maturity. King accounted for this increase by the descent of the hard palate and cervical vertebrae from the cranium. Bergland found that nasopharyngeal height increased by 38%,from six years of age to maturity. Similarly, in this study, during the one year of treatment, the nasopharynx height increased by 11.67%, and its volume increased by 22.75%. However, the average cross-section of nasopharynx did not increase significantly, which may be due to the fact that the nasopharynx depth had been determined in early childhood.
The main feature of the Frankel III appliance is the lip pad, which eliminates the restrictive pressure of the upper lip on the underdeveloped maxilla, exerts tension on the tissue and periosteal attachments to stimulate bone growth, and forces the mandible to grow in the backward and downward directions, by delivering upper lip pressure to the lower labial wire. Three-dimensional assessments of changes in the upper airway with age have shown that airway dimensions consistently increase, until about 20 years of age.[19, 20] In addition, craniofacial morphology has been associated with the upper airway structure in children.
It also has been reported that there was a countless relationship between the positions of the maxilla and nasopharynx.[21, 22] Indeed, maxillary protraction significantly increased the dimensions of both the naso- and oro-pharyngeal airways. In this study, the distance between the posterior nasal spine and the cervical spine (PNS-CVP, Table 1) did not significantly change, which indicated that the distance from the back of the maxilla to the upper airway did not change. Consequently, there was no significant change in the depth of the nasopharynx. In other words, the Frankel III appliance increased the development of the anterior maxilla without changing the posterior maxilla.
The velopharynx is a muscular valve that extends from the posterior surface of the hard palate (roof of the mouth) to the posterior pharyngeal wall.The velopharynx and surrounding oral and pharyngeal structures change rapidly during early development. The velum and epiglottis separate at about 4 to 6 months of age,as the larynx moves from the level of the first cervical vertebra to the level of the third cervical vertebra. This movement is accompanied by rapid growth of the pharynx, in the vertical dimension, from its newborn length, of about 4 cmto its adult length of approximately 12 cm.The rate of laryngeal descent is accelerated during the first 2 years of life, when the pharyngeal length increases by up to 2 cm; this growth period is followed by more gradual lengthening,In contrast, the anteroposterior dimension of the pharynx changes little from infancy to adulthood.[16–17, 27, 26–28]
In this study, we observed clear increases in the height (6.14%) and volume (24.56%) of the velopharynx, during the one year of treatment. This finding indicated that increases in the velopharyngeal height and its average cross-sectional area (Ve-CSAavg) during this period led to a corresponding increase in its volume. However, the velopharyngeal anterior-posterior diameter remained almost unchanged, consistent with the above study, which showed that little change occurred in the anteroposterior dimension of velopharynx.
Levin et al. reported that the Frankel III appliance induced significant improvements in maxillary size. Similarly, in this study, the sagittal position of the maxilla significantly increased, by 1.74°, after one year of treatment. This finding verified the effects of Frankel III treatment on the maxilla. Moreover, Ceylan and Oktay reported that the oropharynx area was influenced by changes in the ANB angle.In the present study, the ANB increased significantly, and we deduced that this could be one reason why we observed significant increases in the velopharyngeal height, volume, and cross-sectional area.
In this study, the maxilla widths in both the dental and alveolar areas increased significantly. These increases may have been caused by the vestibular shields of the Frankel III device, which eliminated the restrictive pressure of the buccinator on the maxilla bone and dental arch. This result was consistent with those reported by Tollaro et al.Finally, although the lateral velopharyngeal diameter increase was insignificant, it led to an increase in maxilla width, and it might have contributed to increases in the velopharyngeal cross-sectional area and volume.
Few previous studies have investigated the development of the glossopharynx. As a part of oropharynx, the glossopharynx develops mainly in the vertical direction , and mainly due to increases in the height of the cervical vertebra. This process continues until adulthood, and two rapid growth periods are observed: one at 5-7 years old and the other at 12-15 years old. In this study, the glossopharynx height also increased, although not significantly. Previous studies also showed that the hyoid position could affect the size of the upper airway. Indeed, hyoid retrogression, caused by mandible retrogression, could cause narrowing of the upper airway .In this study, although the mandible was guided backward for bite construction, the sagittal position of the mandible did not decrease significantly, due to growth. In contrast, the distances between the hyoid and the cervical vertebrae and between the hyoid and the Frankfort plane increased significantly. These findings indicated that the hyoid bone had moved forward and downward, which may have contributed to the profound increases in the minimum cross-sectional area and volume of the glossopharynx.
Lieberman et al. studied the growth and development of the laryngopharynx. They found that the height of the laryngopharynx increased significantly from birth to 6-8 years old, and after that, it remained stable. King also found that, at the peak of growth and development, the depth of the laryngopharynx increased little, due to the forward movements of the hyoid and mandible. In the present study, the height of the laryngopharynx increased significantly, by 10.53% .
Additionally, we observed a significant change in the length of the ramus of the mandible and the distance between the menton and the cervical vertebrae. This finding indicated that the growth of mandible was not inhibited. Although the hyoid bone moved forward, the laryngopharynx showed increases in the minimum cross-sectional area and the lateral and anterior-posterior diameters, but the increases were not significant. Overall, the total volume, height, and average cross-sectional area of the upper airway increased significantly. Thus, the total upper airway was larger after treatment than before treatment with the Frankel III appliance.