This prospective study found that bimaxillary surgery could lead to the narrowing of upper airway at the retropalatal or retroglossal level as well as triggering snoring in subjects with class III malocclusion.
Based on the current clinical findings, we also found that upper airway narrowing at retropalatal level may contribute to increasing the probability of snoring and airway configuration and that sleep study may need to be conducted before orthognathic surgery in subjects with class III malocclusion.
The exact cause of OSA is unknown, yet it seems to be originated from multifactorial origin. Possible etiologies include neurologic control of upper airway, pharyngeal structures , and obesity , and possibly others [14, 15]. Though the cause of OSA is still unknown, it is known that pharyngeal airway volume decreases while airway resistances increase in apneic patients compared to normal population .
There were many previous studies on mandicular setback surgery and airway changes in skeletal class III malocclusion patients. Tselnik and Pogrel reported that airway narrows at oropharyngeal level when mandibular setback surgery is performed . Liukkonen et al. also reported that airway size decreased at oropharyngeal and hypopharyngeal levels when mandibular setback surgery was performed . Though there are not many reports of clinical respiratory disturbance caused by airway size decrease, Riley and Powell reported when patients with mandibular prognatism receives maxillary retrusion surgery, there is a possibility of developing OSA due to the airway size decrease . On the other hand, Wenzel et al. reported that pharyngeal airway does not necessarily increase the airway resistance [19, 20].
Upper airway narrowing following orthognathic surgeries has recently been attracting attention from orthognathic researches, focused on patients who undergo bimaxillary surgery and can develop sleep-breathing disorders including OSA according to structural alterations of bone, muscle, and soft tissue around the pharynx.
Other studies prior to the study measured the changes of anteroposterior of airway using two-dimensional images obtained from cephalogram. However, a setback for such two-dimensional evaluations is that it can be affected by the repositioning of maxillofacial structures. Park et al. showed that linear analysis showed the decrease of pharyngeal depth and airway space when mandibular setback surgery is performed, but there were no significant decrease in linear, area, or volumetric measurements of the nasopharyngeal or oropharyngeal airway when volumetric analysis was performed .
It was noted that such results may have been caused by a physiological deformation, which is caused by the effort of maintaining airway volume upon the sagittal compression. Since there is a clear limitation on the two-dimensional measurement, this study is thought to have accurately analyzed the change in airway after surgery by measuring the airway change using superimposition of 3D video through CBCT.
The result of this study shows that if CSA of CV1 decreases after a bimaxillary surgery, snoring significantly increases. Such decrease of APL and LTW in CV1 is thought to be caused by the retrusion of soft palate caused by retrusion of maxilla. The decrease of APL and LTW in CV2 and CV3, when compared with the influence of decrease of APL and LTW in CV1, does not influence on the snoring as much. In other words, the retrusion of tongue and epiglottis does not have much effect on snoring compared to the retrusion of soft palate.
In most cases of orthognathic surgery of skeletal class III malocclusion, the surgery aims mandibular retrusion by causing maxillary retrusion and posterosuperior rotation. In conclusion, it is possible that the involvement of maxillary retrusion when performing a corrective orthognathic surgery for mandibular retrusion would cause retrusion of soft palate, and thus, causing snoring. Therefore, it is crucial to screen patients with airway APL at the level of soft palate, have narrow LTW, or patients who already snores before performing the surgery. If a retrusion, which does not involve posterosuperior rotation of maxilla in STO, is planned, it would be necessary to sufficiently inform the patient of the possibility of snoring. Moreover, it would be necessary to reconsider anterior segmental osteotomy to decrease the shift of maxilla.
This research has many limitations, as it is difficult to measure continuous, actual respiratory functional change by conducting a survey on snoring. Also, this research evaluated the patients up to 6 months after the surgery, but there was no evaluation of airway change that can happen due to the physiological adaptation 6 months after surgery. Therefore, there should be an addition research on analyzing the dynamics of actual respiratory process as well as the physiological adaptation of the airway after the surgery.
As some of the researches performed to standardize the normal value for the pharyngeal space, Samman et al. and Hochban et al. reported that since skeletal class III malocclusion patients have wider upper airway compared to the normal population, the decrease caused by surgery would still put them in normal range [22,23,24]. Therefore, the occurrence of OSA is rare after corrective surgery of skeletal class III malocclusion patients, and if such clinical problems as OSA occurred, then the sudden decrease of airway volume would not be the cause, while it is probable that other factors of the patient would have caused such symptoms. Therefore, if any clinical symptoms persist, objective examinations of respiratory resistance while sleeping, such as polysomnography, would be needed, and subjective tests including survey would be necessary [25,26,27,28].
The change in dentofacial morphology is reflected closely in the reduction of upper airway induced by surgery and a decrease in the upper airway can cause changes in sleep architectures of subjects with class III malocclusion.
In particular, the reduced dimension at the retropalatal level was found to be more extensive in subjects who showed advanced symptom of snoring after surgery. Furthermore, there were larger horizontal movements of maxilla observed in subjects with class III malocclusion who developed snoring following orthognathic surgery. The present study showed that the pharyngeal airway was narrowed by the posterior movement of the maxilla. A greater magnitude of maxilla or mandibular backward positioning may also have influenced six of the studied subjects presenting with postoperative snoring. Therefore, in advance of surgical treatment to correct class III malocclusion for minimizing risk of postoperative sleep-breathing disorders, the careful evaluation on the cephalometric analysis should be conducted and a risk of excessive setback of the maxilla and mandible which may elevate the probability of sleep breathing disorders is high.