Change of the airway space in mandibular prognathism after bimaxillary surgery involving maxillary posterior impaction

In the patients of skeletal class III malocclusion who were treated with the surgical procedure, it causes a change in the position of the tongue and hyoid bone and the tongue base moved to the posterior [12, 13]. As a result, the contact surface would increase between the soft palate and the tongue and consequently decrease the pharyngeal airway space [14]. This mechanism brings about morphologic change of oro-pharyngeal area and causes problems such as sleep apnea and snoring [1]. Riley et al. reported that if the pharyngeal airway space is less than 11 mm, the distance from the mandibular plane to the hyoid bone is more than 15 mm, it can cause sleep apnea or snoring [15]. Partinen et al. reported that if the pharyngeal airway space on the tongue base is less than 5 mm and the distance from the mandibular plane to the hyoid bone is more than 24 mm, it can bring about severe respiratory disorder [16].

Evaluation of the airway through the CBCT is reliable. In the airway analysis, many studies have used CBCT than conventional CT [17, 18]. CBCT is equipped with fast scanning equipment and low exposure to radiation as compared to conventional CT [8–10]. Mattos CT et al. reported that CBCT evaluation of the upper airway was reliable. In the study, the authors evaluated linear and volumetric measurements by CBCT [19]. Vizzotto MB et al. reported that both lateral cephalogram and 3D reconstruction by CBCT were suitable for evaluation of airway space [20]. In our study, there were minimal linear difference on midsagittal plane and axial plane. And patient position was reproducible. So, CBCT evaluation of this study was reliable.

In this study, we compared the changes in airway volume and diameter of the patients who underwent maxillary posterior impaction surgery. All patients were accompanied mandibular set back surgery via SSRO or VRO. Although total airway volume was reduced significantly, the changes in volume and diameter of nasopharynx were not statistically significant. The maxillary posterior impaction affected minimally on the nasopharyngeal airway. The average movement of 4 mm is very small so that it could not result in airway change. However, oropharynx and hypopharynx showed statistically significant differences between preoperatively and postoperatively. In the author’s opinion, this was due to mandibular set back movement. The amount of movement of the mandible is usually larger than that of the maxilla. And this mandibular movement is granted a direct effect on the position of the tongue and hyoid bone. Therefore, the diameter and volume of the oropharynx and the hypopharynx are reduced, and total airway volume was also decreased.

Female patients showed the narrower airway after surgery compared to male patients. The reduction of the airway was related not only to the position of the tongue and hyoid bone but also to the edema of the respiratory mucosa. Articles which analyzed the changes in the edema by gender are rare. It was believed that this was the result of the difference in the hormone between the male and female. More research is needed.

Snoring occurred in one patient who underwent both SSRO and maxillary posterior impaction. She had narrow airway before the surgery. Comparing to preoperative CBCT measurements, we could find reduction of the airway, and in the postoperative 6-month CBCT evaluation, the airway did not returned to the preoperative. However, in the postoperative 6-month CBCT evaluation, the patient did not complain about her snoring anymore (Fig. 3a). Another patient had narrower airway than her before surgery and in postoperative CBCT. However, she did not present snoring and any other airway compromises (Fig. 3b). Therefore, snoring did not always occur because of airway reduction due to orthognathic surgery [21]. There are several factors correlated with snoring such as head posture, respiratory habit, and tongue position [1, 3, 13, 16]. However, orthognathic surgery is not directly related to snoring and more research is needed.

There are many reports of airway reduction after orthognathic surgery [4], and some patients appear to have respiratory disorder or snoring [5, 12, 22]. If the surgeons understand the volume and width change of the airway between preoperative and postoperative, respiratory complications such as sleep apnea and snoring can be prevented [23]. Withal, if the evaluation of upper airway change in maxillary posterior impaction is possible, the surgeons would perform additional surgical procedures to prevent nasal airway problems [21].

There are many articles dealing with airway change after orthognathic surgery. Pereira-Filho VA et al. evaluated the correlation of maxillary advancement surgery and airway change. In this study, maxillary advancement surgery could dilate the volumes of nasopharynx and oropharynx but not hypopharynx [22]. R. Foltan et al. reported that maxillary advancement surgery could improve the respiratory condition of the upper airway [23]. But the relationship between maxillary posterior impaction surgery and upper airway change has not had been sufficient research. In the total volume of the airway, Lee Y et al. reported that bimaxillary orthognathic surgery for the correction of class III malocclusion affected the morphology by increasing the upper part and decreasing the lower part of the airway, but not the total volume [24]. However, Choi SK et al. reported that mandibular set back surgery was significantly associated with postoperative reduction of airway space [25]. In this study, we found that bimaxillary surgery involving maxillary posterior impaction could highly affect airway space of mandibular prognathism patients. So, the airway change due to bimaxillary surgery should be considered.