Abstract
The purpose of this study was to evaluate the volumetric change of the upper airway space in 36 Class III patients who had undergone bimaxillary surgery or isolated mandibular setback, and, further, to analyse the relation between post-surgical stability and airway change using cone-beam computed tomography (CBCT). A three-dimensional (3D) CBCT examination was performed at three stages: T0 (before surgery), T1 (an average of 4.6 months after surgery), and T2 (an average of 1.4 years after surgery). The results showed that the volumes of the oropharyngeal and hypopharyngeal airways decreased significantly 4.6 months post-surgery in the mandibular setback group ( p < 0.05), and these diminished airways had not recovered 1.4 years post-surgery. In the bimaxillary surgery group, the volume of the oropharyngeal airway also decreased. A Spearman correlation analysis showed that the anteroposterior length of the hypopharyngeal area had a correlation with post-surgical stability in the isolated mandibular surgery group, and that the cross-sectional area of the nasopharynx was correlated with maxillary relapse only in the bimaxillary surgery group ( p < 0.05).
The pharyngeal upper airway has attracted much attention because snoring and sleep apnea are known to be closely linked to its size. If the airway is or becomes narrow, the airflow resistance increases, heightening the risks of snoring and sleep apnea. In facial growth and development, there are important relationships between the pharyngeal structures and the development of the face and occlusion. Orthognathic surgery for skeletal deformity alters the skeletal and soft-tissue components.
Many studies on mandibular setback surgery for skeletal Class III malocclusion, for example, have found that the positions of the hyoid bone and the tongue are changed . The position of the hyoid bone after surgery can reflect stretching of the suprahyoid musculature, which plays an important role in maintaining the oropharyngeal airway. An increase in potential muscle tension might be related to skeletal relapse. Many studies have investigated the effect of orthognathic surgery on the pharyngeal airway space in Class III skeletal deformities . Some studies found that with time and physiologic adaptation of the soft tissues, the airway was restored to its original condition .
Most reported that the upper airway narrowed immediately after surgery and did so consistently with time . In the latter studies, postoperative changes in the upper airway were analysed using two-dimensional lateral cephalograms . Although lateral cephalometric measurements are useful for analysing airway size on the sagittal plane, they do not depict the three-dimensional (3D) airway anatomy accurately. Conventional computed tomography (CT) and magnetic resonance imaging (MRI) have been the methods of choice for obtaining 3D morphologic information on the upper airway, but their use is limited. Cone-beam computed tomography (CBCT), even if it cannot discriminate the various soft tissues, can at least distinguish the boundaries between soft tissues and the airway space. Given the advantages of lower costs and lower radiation doses for patients, CBCT is widely employed. L enza et al. demonstrated that CBCT-based 3D analysis can provide a clearer picture of the anatomical characteristics of the upper airways than single linear measurements performed on cephalograms. Apart from this study, there have been few studies on the application of 3D images of the upper airway; there are fewer still on the relation between upper airway changes and post-surgery stability. The aim of the present study was to use 3D CBCT to evaluate how the upper airway changed after orthognathic surgery in patients with skeletal Class III deformities and to analyse the relation between upper airway change and post-surgical stability.
Materials and method
The subjects included 36 adults (23 men, 13 women; mean age 22.97 ± 3.01 years; range 19–29 years) who had been diagnosed with Class III skeletal deformities and had undergone surgical orthodontic treatment. They were divided according to the type of orthognathic surgery received: group A (20 patients) had undergone mandibular setback sagittal split ramus osteotomy (SSRO with rigid fixation), and group B (16 patients), LeFort I osteotomy with advancement and mandibular setback SSRO. All of the operations were performed by the same surgical team. Rigid internal fixation was achieved with plates and screws. After a week of maxillo-mandibular fixation (MMF), the patients received physiotherapy, which involved mouth-opening exercises. The bicortical screws and plates were removed 6 months after orthognathic surgery. The criteria governing subject selection were: no severe facial asymmetry or presence of syndromes; no symptoms of temporomandibular disorders or degenerative joint disease on examination; no respiratory disease; Skeletal Class III. This study was reviewed and approved by the Ethics Committee of Pusan National University Hospital.
CBCT assessment
All the patients underwent a CBCT examination (DCT pro, Vatech, Seoul, Korea) for assessment of the upper airway volume changes and skeletal changes within a month prior to surgery (T0), 4.6 months after surgery (average: 4.6 ± 1.8 months) (T1) and 1.4 years after surgery (average: 16.6 ± 3.4 months) (T2) under the same conditions. The patients were seated in the upright position with maximum intercuspation. The Frankfort horizontal plane of the patients was parallel to the floor. The maxillofacial regions were scanned for 24 s using a CBCT machine with a field of view of 20 cm × 19 cm, a tube voltage of 90 kVp, and a tube current of 4.0 mA. The CBCT data were converted into DICOM (Digital Imaging and Communication in Medicine) format, after which the images were reconstructed for 3D with OnDemand3D™ (Cybermed Inc., Seoul, Korea).
Upper airway measurements
Preparatory to upper airway analysis, the relevant reference planes and points were determined ( Fig. 1 and Table 1 ). The reference planes as based on the midsagittal plane are shown in Fig. 2 . To evaluate the anatomic characteristics of the upper airway, the largest transverse width (LTW), anteroposterior length (APL) and cross-sectional area (CSA) on axial plane (the PNS–V p , CV 1 , CV 2 , CV 3 and CV 4 planes) were computed at T0, T1 and T2 ( Fig. 2 ).
| Definition | Explanation |
|---|---|
| Reference points | |
| Na (Nasion) | The most anterior point of the frontonasal suture in the midsagittal plane. |
| Ba (Basion) | The most posterior inferior point of the occipital bone at the anterior margin of the foramen magnum. |
| Or (Orbitale) | The most inferior point of the orbital margin. |
| Po (Porion) | The most superior point of the external auditory meatus. |
| A-point | The deepest anterior point in the concavity of the anterior maxilla. |
| B-point | The deepest anterior point in the concavity of the anterior mandible. |
| PNS (Posterior nasal spine) | The most posterior point of the hard palate. |
| CV 1 | The most anterior inferior point of the anterior arch of the atlas. |
| CV 2 | The most anterior inferior point of the body of the 2nd cervical vertebra. |
| CV 3 | The most anterior inferior point of the body of the 3rd cervical vertebra. |
| CV 4 | The most anterior inferior point of the body of the 4th cervical vertebra. |
| V p | The most posterior point of the ala of the vomer. |
| Reference planes | |
| FH plane (Frankfort Horizontal plane) | The plane was constructed on both side of Po and right of Or. |
| Midsagittal reference plane | The plane was perpendicular to the FH plane passing through Na and Ba. |
| Na-perpendicular plane | The plane was perpendicular to the FH and the midsagittal planes passing through Na. |
| PNS–V p plane | The plane was perpendicular to the midsagittal plane passing through PNS and Vp. |
| CV 1 plane | The plane was parallel to the FH plane passing through CV 1. |
| CV 2 plane | The plane was parallel to the FH plane passing through CV 2. |
| CV 3 plane | The plane was parallel to the FH plane passing through CV 3. |
| CV 4 plane | The plane was parallel to the FH plane passing through CV 4. |
The upper airway was divided into three regions relative to the reference planes: the nasopharynx (between the PNS–V p and CV 1 planes), the oropharynx (between the CV 1 and CV 2 planes), and the hypopharynx (between the CV 2 and CV 4 planes) ( Fig. 3 ). The three airway volumes were measured at T0, T1 and T2.
Measurements of skeletal changes and relapse
To assess the movement of the maxilla and mandible, the distances from the Na-perpendicular plane to A-point and B-point were evaluated under the same conditions (window width: 4000, window level: 1000) at T0, T1 and T2 ( Fig. 1 ).
Statistical analysis
To determine the measurement errors, 10 randomly chosen subjects were measured twice; the errors were calculated according to Dahlberg’s formula. The data were statistically analysed using SPSS (ver 12.0 for Windows, Chicago, IL, USA). Friedman tests were conducted to determine whether the changes in each parameter had significance in the pertinent groups. After the significance of the parameters was proved, a Wilcoxon signed-rank test was employed to evaluate the upper airway changes and skeletal stability. A Spearman’s correlation analysis was performed to obtain the correlation between the skeletal movements and upper airway changes ( p < 0.05).
Results
In group A, the mandibular setback movement at B-point was 7.87 ± 3.58 mm at T1. At T2, the mandible relapse was 1.73 ± 1.76 mm. The mandibular setback movement affected the oropharyngeal and hypopharyngeal regions. The LTW on the CV 1 plane decreased after surgery and was maintained at T2. The APL on the CV 2 , CV 3 and CV 4 planes decreased after surgery and was maintained through follow-up. The decreased CSA on the CV 1 , CV 2 , CV 3 and CV 4 planes was recorded after surgery. The orophaynx, hypopharynx and total airway volumes were reduced from T0 to T1. At T2, the reduced volume was maintained, without any recovery tendency shown ( Table 2 ).
| T0 | T1 | T2 | Friedman p -value | ||||
|---|---|---|---|---|---|---|---|
| Average | SD | Average | SD | Average | SD | ||
| PNS–V p plane | |||||||
| APL | 23.97 | 2.46 | 24.04 | 3.21 | 24.84 | 2.73 | 0.153 |
| LTW | 30.81 | 3.69 | 30.61 | 3.72 | 30.58 | 3.99 | 0.674 |
| CSA | 1265.55 | 324.91 | 1314.10 | 385.43 | 1309.65 | 299.66 | 0.861 |
| CV 1 plane | |||||||
| APL | 17.39 | 3.267 | 16.55 | 4.45 | 16.78 | 3.90 | 0.336 |
| LTW | 37.06 a | 5.26 | 32.60 b | 5.77 | 33.68 b | 5.89 | 0.002 ** |
| CSA | 1233.90 a | 354.42 | 1105.05 b | 476.16 | 1105.45 b | 392.99 | 0.035 * |
| CV 2 plane | |||||||
| APL | 18.15 a | 5.26 | 16.54 ab | 5.67 | 15.02 b | 2.82 | 0.015 * |
| LTW | 59.87 a | 19.56 | 30.08 b | 6.19 | 31.27 b | 5.31 | 0.024 * |
| CSA | 1113.55 a | 400.77 | 976.60 ab | 460.72 | 921.10 b | 309.18 | 0.022 * |
| CV 3 plane | |||||||
| APL | 18.20 a | 5.03 | 15.47 b | 4.44 | 14.98 b | 3.49 | 0.004 ** |
| LTW | 36.49 | 5.40 | 35.25 | 5.19 | 35.54 | 4.85 | 0.841 |
| CSA | 1079.00 a | 379.49 | 905.20 ab | 433.07 | 819.70 b | 361.06 | 0.004 ** |
| CV 4 plane | |||||||
| APL | 21.39 a | 4.74 | 18.59 b | 3.95 | 18.87 b | 3.39 | 0.006 ** |
| LTW | 36.98 | 7.88 | 36.22 | 7.96 | 38.76 | 8.06 | 0.280 |
| CSA | 1168.70 a | 451.11 | 911.20 b | 393.76 | 899.45 b | 395.12 | 0.000 ** |
| Volume | |||||||
| Nasopharynx | 12,395.78 | 3908.88 | 11,452.12 | 4072.93 | 11,644.66 | 3293.12 | 0.086 |
| Oropharynx | 11,465.89 a | 6115.06 | 9093.53 b | 5439.24 | 8671.94 b | 3849.82 | 0.000 ** |
| Hypopharynx | 12,788.23 a | 5976.67 | 11,840.99 b | 5702.02 | 11,803.67 ab | 4856.38 | 0.043 * |
| Total | 36,649.90 a | 14,569.73 | 32,386.65 b | 13,862.00 | 32,120.27 b | 10,648.96 | 0.002 ** |
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