Dentofacial deformities may lead to compromised function, poor aesthetics, and possible narrowing of the upper airway.1Muto T. Yamazaki A. Takeda S. Kawakami J. Tsuji Y. Shibata T. Mizoguchi I. Relationship between the pharyngeal airway space and craniofacial morphology, taking into account head posture., 2Airway volume for different dentofacial skeletal patterns. Previous studies have demonstrated that skeletal deformity with maxillary deficiency results in smaller nasopharynx airway volumes, and that skeletal deformity with mandibular deficiency results in smaller hypopharynx airway volumes.3dos Santos L.F. Albright D.A. Dutra V. Bhamidipalli S.S. Stewart K.T. Polido W.D. Is there a correlation between airway volume and maximum constriction area location in different dentofacial deformities?. Orthognathic surgery, i.e. the correction of skeletal deformities by surgical displacement of the maxilla and/or the mandible, alters the relationship between the bony structures and the soft tissues, including those that are closely related to the anatomy of the upper airway. Hence, orthognathic surgery may lead to changes in the airway volume.4Effect of orthognathic surgery on the posterior airway space (PAS).The effect of orthognathic surgery on the airway has been described in various studies. A decrease in the airway volume in the oropharyngeal airway has been reported after mandibular setback in class III patients.5Choi S.K. Yoon J.E. Cho J.W. Kim J.W. Kim S.J. Kim M.R. Changes of the airway space and the position of hyoid bone after mandibular set back surgery using bilateral sagittal split ramus osteotomy technique. Mandibular setback combined with maxillary advancement appears to attenuate the reduction in the oropharyngeal airway volume.6Chen F. Terada K. Hua Y. Saito I. Effects of bimaxillary surgery and mandibular setback surgery on pharyngeal airway measurements in patients with Class III skeletal deformities., 7Pereira-Filho V.A. Monnazzi M.S. Gabrielli M.A.C. Spin-Neto R. Watanabe E.R. Gimenez C.M.M. Santos-Pinto A. Gabrielli M.F.R. Volumetric upper airway assessment in patients with transverse maxillary deficiency after surgically assisted rapid maxillary expansion. In contrast, advancement of the mandible has often been associated with an increase in the airway volume,8Nishanth R. Sinha R. Paul D. Uppada U.K. Rama Krishna B.V. Tiwari P. Evaluation of changes in the pharyngeal airway space as a sequele to mandibular advancement surgery: a cephalometric study. and similarly maxillomandibular advancement surgery.9Mattos C.T. Vilani G.N.L. Sant’Anna E.F. Ruellas A.C.O. Maia L.C. Effects of orthognathic surgery on oropharyngeal airway: a meta-analysis. However, as different definitions and anatomical borders have been applied for the airway and the airway segments in the current literature, direct comparisons of the results are not possible.Several systematic reviews on the effects of orthognathic surgery on the airway have been published in recent years. Christovam et al.10Christovam I.O. Lisboa C.O. Ferreira D.M.T.P. Cury-Saramago A.A. Mattos C.T. Upper airway dimensions in patients undergoing orthognathic surgery: a systematic review and meta-analysis. reported airway volume measurements obtained from computed tomography (CT) and magnetic resonance imaging (MRI) studies. Their results showed a significant increase in the airway volume after maxillomandibular advancement and a significant decrease after mandibular setback, isolated or in combination with maxillary advancement. He et al.11He J. Wang Y. Hu H. Liao Q. Zhang W. Xiang X. Fan X. Impact on the upper airway space of different types of orthognathic surgery for the correction of skeletal class III malocclusion: a systematic review and meta-analysis. investigated only patients with class III malocclusion and showed less reduction in the airway volume after bimaxillary surgery than after mandibular setback surgery only. More recently, Shokri et al.12Shokri A. Ramezani K. Afshar A. Poorolajal J. Ramezani N. Upper airway changes following different orthognathic surgeries, evaluated by cone beam computed tomography: a systematic review and meta-analysis. focused on mandibular advancement and demonstrated a significant volumetric increase in the upper airway. However, a comprehensive overview of the current literature on the effects of different types of orthognathic surgery on volumetric changes to the total airway and to the airway segments is still lacking.Over the last decade, cone beam computed tomography (CBCT) has become recognized as an accurate and reliable tool for the three-dimensional (3D) evaluation of the upper airway.13Guijarro-Martínez R. Swennen G.R.J. Cone-beam computerized tomography imaging and analysis of the upper airway: a systematic review of the literature., 14Accuracy and reliability of cone-beam computed tomography for airway volume analysis. In addition to the advantages of lower costs and a reduced radiation dose when compared to medical CT or MRI, CBCT allows imaging of the airway in a seated position and with shorter acquisition times, thereby reducing the opportunity for patient movement, which can affect volumetric measurements of the airway.15Obelenis Ryan D.P. Bianchi J. Ignácio J. Wolford L.M. Gonçalves J.R. Cone-beam computed tomography airway measurements: can we trust them?. For reasons of both anatomical location and clinical relevance, the upper airway is often divided into a number of segments: nasopharynx, upper and lower oropharynx, and hypopharynx.13Guijarro-Martínez R. Swennen G.R.J. Cone-beam computerized tomography imaging and analysis of the upper airway: a systematic review of the literature. However, no consensus exists regarding the nomenclature of the different airway segments, and the definitions of the borders and anatomical landmarks used to describe the airway and its segments are highly inconsistent.16Lenza M.G. Lenza M.M. Dalstra M. Melsen B. Cattaneo P.M. An analysis of different approaches to the assessment of upper airway morphology: a CBCT study.

Therefore, the aim of this systematic review was to provide a structured overview of 3D volumetric changes of the airway after various orthognathic surgeries. Airway volumes measured on CBCT scans were assessed in order to gain an insight into the relationship between specific types of surgery and the volumetric changes of the airway. To ensure the comparability of the outcomes between studies, the airway segments used in the studies that were finally included were matched to a predefined airway segments framework.

Materials and methodsProtocol and criteriaThe PRISMA protocol (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) was used to guide the conduct of this systematic review (http://prisma-statement.org/PRISMAStatement/). The following outcomes were assessed: (1) volumetric airway changes in cubic millimetres (mm3) or cubic centimetres (cm3), and (2) the type of orthognathic intervention.

A search was conducted in the PubMed, Embase, and Web of Science electronic databases. The time frame for the search was from database inception (as CBCT has only recently been introduced) to January 1, 2022. The following primary keywords were applied in the search: CBCT, cone-beam computed tomography (MeSH), three-dimensional imaging (MeSH), 3D CBCT, CBCT, digital volume tomography (MeSH), tomography (MeSH), compact computed tomography. The following were used as secondary keywords: airway, pharynx (MeSH), nasopharynx (MeSH), and oropharynx (MeSH). The screening of titles, abstracts, and full texts was performed by two authors (FH and RS). The title and abstract of each article retrieved in the search were scanned for all of the exclusion criteria; if this was insufficient to determine inclusion, the full text was screened only against the exclusion criteria. The full text of remaining articles was screened by the same two authors against the inclusion criteria. To be included, all inclusion criteria had to be met. Any uncertainty or disagreement was resolved by discussion with a third author (YR).

Inclusion and exclusion criteria

The inclusion criteria were English or Dutch as the language, prospective or retrospective clinical studies, orthognathic surgery as the intervention, pre- and post-treatment CBCT 3D volumetric assessments available, CBCT acquisition with the patient in an upright position, airway definition clearly described or illustrated, and treatment group with 10 or more patients. Exclusion criteria were animal studies, patients with syndromes and non-healthy patients, patients with obstructive sleep apnoea syndrome (OSAS), patients with any airway disease, expiratory flow studies, aerodynamics or airway pressure studies, two-dimensional (2D) volumetric assessments, case series with fewer than 10 patients in the treatment group, age younger than 16 years, and systematic reviews.

Quality assessmentAll studies were rated once by two authors for risk of bias (FH, RS), according to the National Heart, Lung, and Blood Institute quality assessment tools for case–control studies and before–after studies with no control group (https://www.nhlbi.nih.gov/health-topics/study-quality-assessment-tools). This tool is designed to assess studies for risk of bias and consists of 12 yes/no questions concerning the research question, study population, sample size justification, recruitment of the cases and controls from the same population, the use of pre-specified inclusion and exclusion criteria applied uniformly, the definitions of cases and controls, random selection of study participants, the use of concurrent controls, exposure assessed prior to outcome measurement, exposure measures and assessment, blinding of exposure assessors, and statistical analysis. A ‘yes’ answer scores 1 point, while a ‘no’ answer scores 0 points.

If risk of bias is significant, the study is deemed to be of poor quality. A maximum of 12 points could be obtained, with a score of 1–4 indicating ‘low’ quality, a score of 5–9 ‘medium’ quality, and a score of 10–12 ‘high’ quality. In the case of disagreement regarding the rating, a consensus was reached through discussion with a third author (YR).

Anatomical landmarks, borders, and reference planes of the airwayConsidering the inconsistency in the definitions of the upper airway and the upper airway segments, the method outlined below was proposed for the data analysis in this review, based on five easy-to-determine soft and hard tissue anatomical landmarks in the midsagittal plane and five cross-sectional planes (two frontal and three axial planes). A detailed description and definition of the borders and reference planes is provided in Table 1. Fig. 1 illustrates the five reference planes in the sagittal view in a 2D lateral cephalogram and in a 3D CBCT surface model.

Table 1Description and definitions of the borders and reference planes to define the airway.

FH, Frankfort horizontal.

Fig. 1The total airway and airway segments evaluated in this systematic review, for data analysis. (A) Schematic diagram of the reference points and planes for the airway segments in two-dimensional lateral cephalograms. (B) Reference points and planes for the airway segments in a CBCT surface model in sagittal view. The purple line indicates the most superior border of the airway; the red line indicates the lower border of the nasopharynx; the blue line indicates the lower border of the middle pharynx; the green line indicates the lower border of the inferior pharynx and oropharynx; and the yellow line indicates the most inferior border of the airway and the hypopharynx.

In short, the upper airway is divided into four segments: nasopharynx, middle pharynx, inferior pharynx, and the hypopharynx. The middle and inferior pharynx together form the oropharynx.

An airway segments frameworkDue to the high heterogeneity in the definition of the airway and its segments, the available data on the same airway segment from different studies were converted into comparable measurements following a strict protocol. First, the anatomical landmarks and reference planes used in the original studies were compared to the proposed borders (Table 1, Fig. 1). Considering the variations in landmark definition in the individual studies, the concept of a ‘reference field’ was proposed that accommodates more reference planes within a predefined range of variations. The reference field was primarily based on the proposed plane, with an extension to include a limited amount of deviation of the planes that were not exactly the same as the proposed one, but were anatomically close and were used in four or more of the finally included studies. Fig. 2 illustrates the ranges of the deviations in landmark positioning with the associated reference fields, which served as a predefined airway segments framework to enable volumetric data conversion from the finally included studies.

Fig. 2Predefined airway segments framework with reference fields. The thick lines refer to the proposed reference planes of the borders of the respective airway segments; the shaded boxes refer to the reference fields within which variations in the reference planes from different studies were accepted. Red indicates the ranges for the superior borders of the oropharynx (PNS plane to anterior superior part of C1) and green indicates the ranges for the inferior borders of the oropharynx (anterior superior part of C3 to anterior inferior part of C3). The yellow triangle indicates the accepted anterior borders (PNS to sella to the frontal plane perpendicular to FH passing through PNS).

Volumetric data interpretation and inclusionPre- and post-treatment volumetric data were extracted from all included studies. In the case of multiple post-treatment follow-ups, the latest available data were used, thereby providing the longest follow-up results. Volumetric data were extracted according to the protocol described below.(1)Direct data inclusion: originally reported data were included directly when the definition of the airway and its segments concurred with those proposed (Table 1, Fig. 1).(2)Data inclusion with note: when the reference planes used in an individual study were not the same as the proposed planes, but fell within the borders of the reference fields as described previously, the volumetric data were included. Specifically, for the anterior border of the total airway and nasopharynx, anatomical reference points were accepted when they were positioned between the frontal plane perpendicular to Frankfort horizontal (FH), passing through posterior nasal spine (PNS) and a plane from PNS to sella. For the superior border of the oropharynx and inferior border of the nasopharynx, reference planes through the PNS plane to the anterior superior part of C1 were accepted. For the inferior border of the oropharynx and superior border of the hypopharynx, reference planes passing through the most superior part of the epiglottis or between the anterior superior part of C3 and anterior inferior part of C3 were accepted. For the inferior border of the hypopharynx and total airway, reference planes passing through the bottom of the epiglottis to a plane passing through the superior anterior part of C4 were accepted (Fig. 2).(3)

Data exclusion: when deviations in landmark positioning or reference planes exceeded the borders of the reference fields, no data were included in the final analysis.

Discussion

The aim of this systematic review was to provide a structured overview of 3D volumetric changes of the airway after various orthognathic surgeries. The findings demonstrated a clear impact of orthognathic surgery on volumetric measurements of the airway. The impact appeared to be related to the type of orthognathic surgery, i.e. the direction of the jaw displacement. In other words, the magnitude of the jaw displacement did not show a consistent pattern of the effect. A meta-analysis, however, was not possible due to the high heterogeneity in different aspects of the study designs and outcome reporting of the included studies.

Only two systematic reviews on airway volumetric measurements obtained from CBCT have been published previously, with one demonstrating less reduction of the volume after bimaxillary surgery than mandibular setback only in patients with a class III malocclusion,11He J. Wang Y. Hu H. Liao Q. Zhang W. Xiang X. Fan X. Impact on the upper airway space of different types of orthognathic surgery for the correction of skeletal class III malocclusion: a systematic review and meta-analysis. and the other showing a significant volume increase after mandibular advancement in patients with mandibular deficiency.12Shokri A. Ramezani K. Afshar A. Poorolajal J. Ramezani N. Upper airway changes following different orthognathic surgeries, evaluated by cone beam computed tomography: a systematic review and meta-analysis. In comparison, the present study is novel in providing a comprehensive overview of the volumetric changes of both the total airway and the three airway segments, and including the different types of orthognathic surgery reported in the literature.It is well known that the upper airway anatomy can affect airway obstruction problems. In individuals with sleep apnoea, the size of the upper oropharyngeal airway is smaller compared to control subjects without sleeping disorders.57Schwab R.J. Pasirstein M. Pierson R. Mackley A. Hachadoorian R. Arens R. Maislin G. Pack A.I. Identification of upper airway anatomic risk factors for obstructive sleep apnea with volumetric magnetic resonance imaging. The surrounding craniofacial structures or body fat can decrease the upper airway volume, leading to an increased likelihood of pharyngeal collapse.58Jordan A.S. McSharry D.G. Malhotra A. Adult obstructive sleep apnoea. As also shown in this review, isolated mandibular setback surgery resulted in a reduction in the volume of the total airway and the three airway segments in almost all of the included studies, which is the opposite of isolated maxillary advancement surgery, which resulted in an increase in the airway volume. Bimaxillary surgery including mandibular setback resulted in an increase in the airway volume in most of the studies. In those that showed a reduction in the airway volume, the magnitude of the reduction was less than isolated mandibular setback surgery. This implies that when a mandibular setback is indicated, bimaxillary surgery could be favoured over single mandibular setback surgery6Chen F. Terada K. Hua Y. Saito I. Effects of bimaxillary surgery and mandibular setback surgery on pharyngeal airway measurements in patients with Class III skeletal deformities., 11He J. Wang Y. Hu H. Liao Q. Zhang W. Xiang X. Fan X. Impact on the upper airway space of different types of orthognathic surgery for the correction of skeletal class III malocclusion: a systematic review and meta-analysis. in order to minimize the negative effect on the airway volume, and thereby reduce the risk of postoperative pharyngeal collapse.59Use of 3-dimensional computed tomography scan to evaluate upper airway patency for patients undergoing sleep-disordered breathing surgery., 60Fairburn S.C. Waite P.D. Vilos G. Harding S.M. Bernreuter W. Cure J. Cherala S. Three-dimensional changes in upper airways of patients with obstructive sleep apnea following maxillomandibular advancement., 61Degerliyurt K. Ueki K. Hashiba Y. Marukawa K. Nakagawa K. Yamamoto E. A comparative CT evaluation of pharyngeal airway changes in class III patients receiving bimaxillary surgery or mandibular setback surgery.Of the 41 included studies only three, all on patients with a class III malocclusion, looked into the effect on sleep after orthognathic surgery. Canellas et al.20Canellas J.V. Barros H.L. Medeiros P.J. Ritto F.G. Effects of surgical correction of class III malocclusion on the pharyngeal airway and its influence on sleep apnoea. performed a clinical assessment and used a questionnaire to screen for OSAS after mandibular setback only or Le Fort I maxilla advancement and mandibular setback. A significant reduction in the oropharyngeal airway volume was observed in patients undergoing mandibular setback alone. In other words, patients undergoing a double-jaw surgery did not show a reduction in the airway volume. Nevertheless, there were no signs or symptoms of OSAS in any of the patients. Uesugi et al.23Uesugi T. Kobayashi T. Hasebe D. Tanaka R. Ike M. Saito C. Effects of orthognathic surgery on pharyngeal airway and respiratory function during sleep in patients with mandibular prognathism. measured the apnoea–hypopnoea index (AHI) with a polysomnography system in patients after mandibular setback and Le Fort I advancement in addition to mandibular setback. The AHI did not change in either group. Lee et al.33Lee U.L. Oh H. Min S.K. Shin J.H. Kang Y.S. Lee W.W. Han Y.E. Choi Y.J. Kim H.J. The structural changes of upper airway and newly developed sleep breathing disorders after surgical treatment in class III malocclusion subjects. reported a significant decrease in oropharynx volume in 22 patients undergoing bimaxillary surgery including Le Fort I advancement and BSSO setback. The outcomes of endoscopic examination and a sleep study showed that, although none of the patients had sleep-related symptoms before surgery, three (13%) were newly diagnosed with mild or moderate obstructive sleep apnoea and six (27%) showed increased loudness of snoring 3 months after surgery.33Lee U.L. Oh H. Min S.K. Shin J.H. Kang Y.S. Lee W.W. Han Y.E. Choi Y.J. Kim H.J. The structural changes of upper airway and newly developed sleep breathing disorders after surgical treatment in class III malocclusion subjects.As expected, isolated mandibular advancement and bimaxillary advancement generally increased the volume of the oropharynx and of the total airway. Collectively, the postoperative airway volume, relative to the preoperative level as a percentage, increased more in patients with bimaxillary advancement than in those with mandibular advancement only; furthermore, the largest increases reported in the former category (85–90%) were twice those in the latter (32–42%). These results indicate a clear advantage of bimaxillary surgery in patients who are predisposed to sleep-related problems. Bimaxillary advancement has indeed been reported as an effective surgical treatment for sleep apnoea.62Holty J.E.C. Guilleminault C. Maxillomandibular advancement for the treatment of obstructive sleep apnea: a systematic review and meta-analysis., 63Faria A.C. da Silva-Junior S.N. Garcia L.V. dos Santos A.C. Fernandes M.R.F. de Mello-Filho F.V. Volumetric analysis of the pharynx in patients with obstructive sleep apnea (OSA) treated with maxillomandibular advancement (MMA)., 64Surgical management of obstructive sleep apnea. The mechanism is assumed to be that the anterior–posterior dimension of the pharyngeal airway is increased by forward traction of the maxilla, the mandible, and associated soft tissue structures, which may lead to a consequent reduction in pharyngeal collapse and improvement in the AHI.65Zaghi S. Holty J.E.C. Certal V. Abdullatif J. Guilleminault C. Powell N.B. Riley R.W. Camacho M. Maxillomandibular advancement for treatment of obstructive sleep apnea: a meta-analysis., 66Vinha P.P. Faria A.C. Xavier S.P. Christino M. de Mello-Filho F.V. Enlargement of the pharynx resulting from surgically assisted rapid maxillary expansion., 67Babacan H. Sokucu O. Doruk C. Ay S. Rapid maxillary expansion and surgically assisted rapid maxillary expansion effects on nasal volume.Although only four studies were eligible for final inclusion in the category of surgically assisted maxillary expansion, the results from these studies showed an evident tendency towards an increase in the volume of the total airway, nasopharynx, and oropharynx, regardless of the surgery procedure or the expansion protocol. This is in agreement with the outcomes of previous reports, where surgical maxillary expansion procedures demonstrated a positive effect on the function of the nasopharynx and improved sleep apnoea.68Buck L.M. Dalci O. Darendeliler M.A. Papadopoulou A.K. Effect of surgically assisted rapid maxillary expansion on upper airway volume: a systematic review., 69Vinha P.P. Thuler E.R. de Mello-Filho F.V. Effects of surgically assisted rapid maxillary expansion on the modification of the pharynx and hard palate and on obstructive sleep apnea, and their correlations., 70Vinha P.P. Eckeli A.L. Faria A.C. Xavier S.P. de Mello-Filho F.V. Effects of surgically assisted rapid maxillary expansion on obstructive sleep apnea and daytime sleepiness.Although CBCT shows significant advantages over conventional cephalograms to visualize different craniofacial structures and investigate volumetric airway changes, it is not without limitations. For example, non-standardized CBCT acquisition protocols including patient positioning and patient movement or swallowing during the scan, can all affect the airway volumetric measurement.15Obelenis Ryan D.P. Bianchi J. Ignácio J. Wolford L.M. Gonçalves J.R. Cone-beam computed tomography airway measurements: can we trust them?. In the present review, only studies with a standardized patient positioning protocol were included to reduce the risk of alterations between different scans. In addition, only studies with CBCT acquisition in an upright position were included, since differences are seen in the upper airway morphology between the supine and upright positions.71Van Holsbeke C.S. Verhulst S.L. Vos W.G. De Backer J.W. Vinchurkar S.C. Verdonck P.R. van Doorn J.W.D. Nadjmi N. De Backer W.A. Change in upper airway geometry between upright and supine position during tidal nasal breathing. Gravity can produce movements of the oropharyngeal soft tissue structures in response to postural changes between sitting upright and lying in the supine position.72Sutthiprapaporn P. Tanimoto K. Ohtsuka M. Nagasaki T. Iida Y. Katsumata A. Positional changes of oropharyngeal structures due to gravity in the upright and supine positions. Another limitation is the reliability of the airway assessment on CBCT scans. Nevertheless, the current literature shows that the intra- and inter-examiner reliability of volumetric airway measurements varies between moderate and excellent.73Zimmerman J.N. Lee J. Pliska B.T. Reliability of upper pharyngeal airway assessment using dental CBCT: a systematic review. In the present study, 12 of the 41 included studies were determined to have a risk of bias in the reliability of the airway measurements.

The quality of all of the included studies was determined to be moderate (medium risk of bias). This outcome is mainly related to the fact that no randomized clinical trials could be identified, that only two studies included a control group, and that a study power analysis and blinding was applied only in a few studies. Though one can argue that there are practical barriers, often related to ethical considerations or availability problems, to including these parameters, they remain important aspects in quality assessment.

Due to the high heterogeneity in the definition of the airway and the airway segments across the different studies, the available data on the same airway segment from different studies had to be converted for the measurements to be comparable. Another aspect of heterogeneity is the follow-up. When multiple postoperative CBCT sca