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orthognathic surgery can affect the oropharyngeal airway through skeletal displacement and changes in the positions of the hyoid bone and tongue1 .
the main concern regarding pharyngeal dimensional changes caused by orthognathic surgery is patient sleep quality1 .
up to 25% of adults suffer from obstructive sleep apnea ( osa ) ( apnea - hypopnea index [ ahi ] 5/h ) , and 10% of patients have moderate to severe osa ( ahi 15/hr)23 .
maxillomandibular advancement surgery has been shown to safely and effectively treat osa4 , but the efficacy of other kinds of orthognathic surgery on the upper airway is not well understood .
two - dimensional ( 2d ) radiograph - based evaluations of the upper airway have revealed that mandibular setback surgery narrows the pharyngeal airway , whereas bimaxillary surgery has a milder or non - narrowing effect156 .
one study reported a significant decrease in the pharyngeal airway six months after bimaxillary surgery7 .
others used computed tomography ( ct ) to evaluate the effect of bimaxillary surgery on class iii patients and found no significant decrease in the pharyngeal airway89 .
cone - beam ct ( cbct ) is an accurate and reliable tool for upper - airway evaluation10 . in this study
, we aimed to assess changes in the upper airway space following bimaxillary orthognathic surgery to correct skeletal class iii malocclusion and to determine whether changes in 2d measurements correlate with three - dimensional ( 3d ) measurements on cbct images .
this was a prospective cohort study conducted at mashhad university of medical sciences ( mashhad , iran ) from april to june 2012 .
all study activities were performed in accordance with the ethical guidelines set by the ethical committee of mashhad university of medical sciences , which conform to the declaration of helsinki . the aims and procedures of the study
we included healthy subjects with no previous medical problems , anteroposterior discrepancy > 10 mm , and a need for bimaxillary orthognathic surgery to correct a class iii malocclusion .
patients were excluded if they had any previous craniofacial syndrome or dentofacial trauma or if they did not require maxillary impaction as part of their surgical plan .
twelve patients participated in the study ( five males and seven females ) with an age range of 16 - 35 years ( mean age , 20.4 years ) and normal body mass index ( bmi ) of 18 - 25 kg / m .
two patients were excluded during the experiment because one did not complete follow - up , and the medical documents of the other were lost ( one male and one female ) .
all patients underwent bimaxillary orthognathic surgery ( maxillary advancement by lefort i osteotomy and mandibular setback by bilateral sagittal split ramus osteotomy [ bssro ] ) and rigid fixation with titanium miniplates and screws .
the mean amounts of maxillary advancement and mandibular setback were 4.4 mm and 6.55 mm , respectively .
maxillomandibular fixation was applied for a two - week period for all patients and was followed by elastic therapy to establish optimal occlusion .
two upper airway cbct scans were performed for all patients one week before and two to four months after surgery .
all cbcts were performed by the same technician at the same clinic , using the same machine and the same settings ( 80 kvp with 6 ma and 0.32 mm slice thickness , planmeca promax 3d s ; planmeca oy , helsinki , finland ) . to provide a reproducible cbct image before and after surgery
all cbct segmentations and measurements were performed with the same workstation ( hp xw9400 ; hewlett - packard , palo alto , ca , usa ) and same software ( planmeca romexis 2.8.1.r ; planmeca oy ) .
the anterior boundaries of the volume space of interest were the vomer bone , soft palate , base of the tongue , and anterior wall of the pharynx .
the posterior , lateral , superior , and inferior boundaries were the posterior wall of the pharynx , the lateral walls of the pharynx , the roof of the nasopharynx , and the plane crossing the vallecula perpendicular to the posterior wall of the pharynx , respectively .
1 , table 1 ) using the mid - sagittal plane view , a mid - sagittal slice was selected and used as a 2d measurement reference ; the landmarks , which are described in fig .
2 and table 2 , were used to perform 2d measurements as previously described by jakobsone et al.9 the wilcoxon one - sample test was used to compare the mean measurements of the airway pre- and post - surgery .
spearman 's rank correlation coefficient was used to test associations between cbct volumetric and linear measurements .
this was a prospective cohort study conducted at mashhad university of medical sciences ( mashhad , iran ) from april to june 2012 .
all study activities were performed in accordance with the ethical guidelines set by the ethical committee of mashhad university of medical sciences , which conform to the declaration of helsinki . the aims and procedures of the study
we included healthy subjects with no previous medical problems , anteroposterior discrepancy > 10 mm , and a need for bimaxillary orthognathic surgery to correct a class iii malocclusion .
patients were excluded if they had any previous craniofacial syndrome or dentofacial trauma or if they did not require maxillary impaction as part of their surgical plan .
twelve patients participated in the study ( five males and seven females ) with an age range of 16 - 35 years ( mean age , 20.4 years ) and normal body mass index ( bmi ) of 18 - 25 kg / m .
two patients were excluded during the experiment because one did not complete follow - up , and the medical documents of the other were lost ( one male and one female ) .
all patients underwent bimaxillary orthognathic surgery ( maxillary advancement by lefort i osteotomy and mandibular setback by bilateral sagittal split ramus osteotomy [ bssro ] ) and rigid fixation with titanium miniplates and screws .
the mean amounts of maxillary advancement and mandibular setback were 4.4 mm and 6.55 mm , respectively .
maxillomandibular fixation was applied for a two - week period for all patients and was followed by elastic therapy to establish optimal occlusion .
two upper airway cbct scans were performed for all patients one week before and two to four months after surgery .
all cbcts were performed by the same technician at the same clinic , using the same machine and the same settings ( 80 kvp with 6 ma and 0.32 mm slice thickness , planmeca promax 3d s ; planmeca oy , helsinki , finland ) . to provide a reproducible cbct image before and after surgery
all cbct segmentations and measurements were performed with the same workstation ( hp xw9400 ; hewlett - packard , palo alto , ca , usa ) and same software ( planmeca romexis 2.8.1.r ; planmeca oy ) .
the anterior boundaries of the volume space of interest were the vomer bone , soft palate , base of the tongue , and anterior wall of the pharynx .
the posterior , lateral , superior , and inferior boundaries were the posterior wall of the pharynx , the lateral walls of the pharynx , the roof of the nasopharynx , and the plane crossing the vallecula perpendicular to the posterior wall of the pharynx , respectively .
1 , table 1 ) using the mid - sagittal plane view , a mid - sagittal slice was selected and used as a 2d measurement reference ; the landmarks , which are described in fig .
2 and table 2 , were used to perform 2d measurements as previously described by jakobsone et al.9
the wilcoxon one - sample test was used to compare the mean measurements of the airway pre- and post - surgery .
spearman 's rank correlation coefficient was used to test associations between cbct volumetric and linear measurements .
the 2d measurements showed no significant decrease in airway space , whereas there was a significant increase in distance between posterior nasal spine ( pns ) and the posterior wall of the pharynx ( p=0.004 ) .
the volumetric measurements of the upper airway space increased in total volume , but the change was not statistically significant .
hypopharyngeal and oropharyngeal areas decreased , and nasopharyngeal area increased , but none of these changes were significant .
we found that 2d and 3d measurements were significantly correlated with changes in u - mpw ( distance between the uvula [ u ] and the middle pharyngeal wall [ mpw ] , which represents the oropharyngeal airway space ) and oropharyngeal volume ( opv ) ( r=0.83 , p=0.004 ) , as well as with changes in p - upw and degree of maxillary advancement ( r=0.72 , p=0.02 ) .
the 2d and 3d changes in the upper airway space that were estimated from cbct scans are shown in tables 3 and 4 .
our results indicated that there were no significant decreases in the upper airway , but a significant increase in pns - upw ( distance between the pns and the upw , which represents the nasopharyngeal airway space ) in 2d measurements was noted . although total and oropharyngeal volumes increased , and nasopharyngeal and hypopharyngeal volumes decreased , these changes were not significant .
we found correlations between changes in u - mpw and opv ( r=0.83 , p=0.004 ) as well as between changes in p - upw and amount of maxillary advancement ( r=0.72 , p=0.02 ) .
one of the advantages of ct scanning is that it offers better delineation between soft tissue and air based on different hounsfield unit ( hu ) densities9 , allowing more accurate measurement of the upper airway .
herein , we used the categorizations and boundaries of upper airway levels described by jakobsone et al.9 and lowe et al.11 .
our upper airway volume calculations were performed automatically using software that calculated airway volume in a segmented manner based on hu density differences .
several authors have demonstrated the feasibility of automatic segmentation and dimension estimation from cbct images for upper airway volume measurement12 .
other researchers have used ct , cbct , and cephalometric imaging techniques to assess correlations between 2d and 3d measures and volumetric measurements , but they did not find any statistically significant results913 .
the cephalograms were taken in an upright position , and no special measures were performed to obtain standardized radiographs in relation to respiration phase9 . in all cases , the acquisition time for the ct scans was too long for patients to hold their breath .
the cephalograms were performed with the patient in an upright position , while ct scans were performed in a supine position9 .
several studies found that changes in posterior airway space are associated with changes in body position among osa patients1415 and healthy individuals1516 .
furthermore muto et al.17 showed that a 10-degree increase in craniocervical inclination enlarged the posterior airway space by approximately 4 mm .
therefore , any increase in craniocervical inclination following mandibular setback17 should be considered during cephalometric imaging with a natural head position .
our study was not affected by these limitations because all 2d and 3d measurements were conducted on cbct scans that were performed by the same radiologist , using the same machine , with the patients in the same position .
however , the main limitation of our study was the small number of patients . although the only procedure we performed on the mandible for this study was bssro , another common procedure in other studies is intraoral vertical ramus osteotomy ( ivro ) , which has been evaluated for its effect on the upper airway .
kawamata et al.18 did not find significant differences between the effects of bssro and ivro on posterior airway space .
the only significant change in the 2d measurements was the distance between the pns and the upper pharyngeal wall , which was consistent with previous studies919 .
the volumetric measurements showed increases in total and oropharyngeal volumes and decreases in nasopharyngeal and hypopharyngeal volumes , but none of these changes were significant .
these findings are in contrast with those of kim et al.20 , who found a significant decrease in total pharyngeal airway volume after bimaxillary surgery in class iii patients .
this discrepancy can be attributed to the surgery procedure , in which they performed posterior maxillary impaction and rotated the occlusal plane . however , our results supported the findings of jakobsone et al.9 because of the similar boundaries and areas used to measure the volumes .
we found a correlation between change in pns - upw and the amount of maxillary advancement in 2d measurements .
the only significant correlation between 2d and volumetric measurements was between changes in u - mpw and oropharyngeal volume .
kim et al.20 found correlation between hyoid bone position and airway volume and between changes in palatal plane angle and decrease in total airway space .
postoperative pharyngeal airway space ( pas ) values should be compared with osa development values in order to assess osa risk , particularly among patients with osa syndrome9 . a cephalometric - based study conducted by kollias and
krogstad21 reported that the normal range of pas values was 14.84.4 mm for males and 12.13.7 mm for females .
pas values < 5 mm were reported to correlate with a high apnea index , independent of bmi22 .
only one patient , whose preoperative pas minimum was 7.37 mm , had a pas < 5 mm ( 4.58 mm ) three months after surgery , which might be clinically important .
there are few studies on the evaluation of polysomnographic value changes after orthognathic surgery for treatment of class iii patients , and the findings from these studies are controversial232425 .
gokce et al.23 reported an improvement in sleep quality among class iii patients after bimaxillary orthognathic surgery for treatment .
in contrast , hasebe et al.24 reported two cases of mild osa after mandibular setback surgery , and foltn et al.25 concluded that bimaxillary surgery for correction of class iii malocclusion increased upper airway resistance .
upper airway changes are not statistically significant but are nevertheless clinically important in some patients because of their associated risk with osa .
objectivesany change in maxilla and mandible position can alter the upper airway , and any decrease in the upper airway can cause sleep disorders . 
 thus , it is necessary to assess airway changes after repositioning of the maxilla and mandible during orthognathic surgery . 
 the purpose of this study was to evaluate linear and volumetric changes in the upper airway after bimaxillary surgery to correct class iii malocclusion via cone - beam computed tomography ( cbct ) and to identify correlations between linear and volumetric changes.materials and methodsthis was a prospective cohort study . 
 cbcts from 10 class iii patients were evaluated before surgery and three months after . 
 the wilcoxon one - sample test was used to evaluate the differences in measurements before and after surgery . 
 spearman 's rank correlation coefficient was used to test the correlation between linear and volumetric changes.resultsthe results show that the nasopharyngeal space increased significantly , and that this increase correlated with degree of maxillary advancement . 
 no significant changes were found in volumes before and after surgery . 
 a correlation was found between linear and volumetric oropharyngeal changes.conclusionbimaxillary surgical correction of class iii malocclusion did not cause statistically significant changes in the posterior airway space .