ORIGINAL ARTICLE
Postsurgical volumetric airway changes in 2-jaw orthognathic surgery patients €ns Currier,d Steven M. Sullivan,e Ji Li,f P. Sheamus Hart,a Brian P. McIntyre,b Onur Kadioglu,c G. Fra and Christina Shayg Woodridge, Va, Omaha, Neb, and Oklahoma City, Okla
Introduction: Findings from early cephalometric studies on airway changes after 2-jaw orthognathic surgery have been challenged because the previous anteroposterior interpretation of airway changes can now be evaluated in 3 dimensions. The aims of this study were to use cone-beam computed tomography to quantify the nasopharynx, oropharynx, and total airway volume changes associated with skeletal movements of the maxilla and mandible in a sample of patients undergoing 2-jaw orthognathic surgery for correction of skeletal malocclusion. Methods: Skeletal movements and airway volumes of 71 postpubertal patients (31 male, 40 female; mean age, 18.8 years) were measured. They were divided into 2 groups based on ANB angle, overjet, and occlusion (Class II: ANB, .2 ; overjet, .1 mm; total, 35 subjects; and Class III: ANB, \1 ; overjet, \1 mm; total, 36 subjects). Presurgical and postsurgical measurements were collected for horizontal, vertical, and transverse movements of the maxilla and the mandible, along with changes in the nasopharynx, oropharynx, and total airways. Associations between the directional movements of skeletal structures and the regional changes in airway volume were quantified. Changes in the most constricted area were also noted. Results: Horizontal movements of D-point were significantly associated with increases in both total airway (403.6 6 138.6 mm3; P \0.01) and oropharynx (383.9 6 127.9 mm3; P\0.01) volumes. Vertical movements of the posterior nasal spine were significantly associated with decreases in total airway volume ( 459.2 6 219.9 mm3; P 5 0.04) and oropharynx volume ( 639.7 6 195.3 mm3; P \0.01), increases in nasopharynx (187.2 6 47.1 mm3; P \0.01) volume, and decreases in the most constricted area ( 10.63 6 3.69 mm2; P \0.01). In the Class III patients only, the vertical movement of D-point was significantly associated with decreases in both total airway ( 724.0 6 284.4 mm3; P 5 0.02) and oropharynx ( 648.2 6 270.4 mm3; P 5 0.02) volumes. A similar negative association was observed for the most constricted area for the vertical movement of D-point ( 15.45 6 4.91 mm2; P \0.01). Conclusions: Optimal control of airway volume is through management of the mandible in the horizontal direction and the vertical movement of the posterior maxilla for all patients. The surgeon and the orthodontist should optimally plan these movements to control gains or losses in airway volume as a result of orthognathic surgery. (Am J Orthod Dentofacial Orthop 2015;147:536-46)
a
Private practice, Woodridge, Va. Private practice, Omaha, Neb. c Assistant professor and program director, Department of Orthodontics, University of Oklahoma, Oklahoma City, Okla. d Professor and chair, Department of Orthodontics, University of Oklahoma, Oklahoma City, Okla. e Professor and chair, Department of Oral and Maxillofacial Surgery, University of Oklahoma, Oklahoma City, Okla. f Biostatistician, Department of Biostatistics and Epidemiology, University of Oklahoma, Oklahoma City, Okla. g Assistant professor, Department of Biostatistics and Epidemiology, University of Oklahoma, Oklahoma City, Okla. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Onur Kadioglu, Department of Orthodontics, College of Dentistry, University of Oklahoma, 1201 N. Stonewall Ave, Room 400; Oklahoma City, OK 73117; e-mail, [email protected]. Submitted, February 2014; revised and accepted, December 2014. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2014.12.023 b
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ephalometric studies of the airway have aided oral surgeons for many years in making predictions about the potential and realized airway changes that occur during surgery. A meta-analysis by Mattos et al1 provided evidence that mandibular setback surgery alone was associated with notable decreases in the oropharyngeal airway at the level of the soft palate and at the base of the tongue. Specifically, in 2-jaw surgery including maxillary advancement and mandibular setback, an increase in airway volume was seen at the level of the posterior nasal spine (PNS), and significant decreases were seen at the level of the soft palate and at the base of the tongue. Their results also indicated that 2-jaw advancement showed a highly significant increase in oropharyngeal airway at the level of the soft palate.1 However, the long-term stability of these changes is controversial, with some studies claiming that these gains and losses were
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stable in the long term, whereas others reported that the airway adapts to these change and returns to presurgical values.2-6 Since its introduction in 1998, cone-beam computed tomography (CBCT) has been improved, with lower costs, less radiation exposure to patients, and better accuracy in identifying the boundaries of soft tissues and air spaces.7 CBCT allows for segmentation and visualization of hollow structures, such as the airway in 3 dimensions, permitting the transition from lengths and angles to volumes and cross-sectional areas.8 Consequently, findings from early 2-dimensional evaluations on the influence of skeletal repositioning on the structures of the head and neck have been challenged with the introduction of these 3-dimensional (3D) tomographic evaluations. Although earlier studies focused on the reliability of the new technique8 as well as defining normative values,9 changes in the airway after orthognathic surgery have become important in the comprehensive evaluation of patients. In a study evaluating pretreatment CBCT images of 140 patients, El and Palomo9 reported that skeletal Class II patients have smaller airway volumes when compared with Class I and Class III patients. Two-dimensional studies have reported decreases in oropharyngeal dimensions as a result of mandibular setback surgery. However, using 3D methods, Park et al10 indicated no significant changes in linear, area, or volumetric measurements of the nasopharyngeal or oropharyngeal airway in patients undergoing setback surgery. Conversely, linear assessments using cephalometric radiographs of the same patients at the same times indicated decreases in all measurements of pharyngeal depth and airway space, as well as posterior movements of the soft palate, tongue, and hyoid bone. These conflicting results between the CBCT and the cephalometric analyses suggest that the absolute volume of the airway may not be significantly reduced by setback surgery, with the soft tissue pharynx expanding laterally to preserve its volume.10 CBCT technology has been found to be accurate and repeatable in making skeletal measurements and measuring volumes of known value. Linear measurements are accurate to the submillimeter level, and the volumes acquired from CBCT are a near 1:1 representation of the known volume.11-14 Software programs such as Dolphin (Dolphin Imaging & Management Solutions, Chatsworth, Calif), Anatomage (San Jose, Calif), and Osirix (Pixeo, Geneva, Switzerland) have also been tested for accuracy and reliability in making volumetric predictions.7,8 Although the comparative accuracy for the programs was found to be low, the programs are reliable for repeated measurements of a given volume; thus, serial measurements of the same
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patient should give a reliable assessment of the changes in airway volumes. Additionally, studies on airway changes in orthognathic surgery patients have generally focused on the surgical correction of the Class III phenotype with either 1-jaw or 2-jaw correction. The results have shown that single-jaw mandibular setback constricts total, oropharyngeal, and hypopharyngeal airways and are maintained in the long term. In patients undergoing 2-jaw mandibular setback with maxillary advancement, loss in regional airway volume was reduced, with gains seen in some regions that ameliorated the total airway losses.15,16 The evidence of an association between the surgical movements and the resultant changes in airway volume includes conflicting views seen both within and between the cephalogram and the CBCT investigations. It is also readily apparent from a review of the literature that few data exist concerning CBCT analysis of volumetric changes in mandibular advancement groups. Further 3D studies on the volumetric changes resulting from the surgical correction of anteroposterior skeletal discrepancies are needed to confirm the results obtained thus far.1 Therefore, the purpose of this study was to obtain quantitative evidence of the volumetric changes occurring in the airway as a result of 2-jaw orthognathic surgery using CBCT. Specifically, we aimed to quantify the associations between the amount of surgical movement of the maxilla or the mandible in all 3 planes of space and the volumetric airway changes that occur. MATERIAL AND METHODS
Institutional review board approval from the University of Oklahoma was obtained before this study (#15519). Records of 71 subjects who were treated for orthognathic surgery between May 2009 and June 2013 were selected. All patients had 2-jaw orthognathic surgery in conjunction with orthodontic treatment. Thirty-five patients (15 male, 20 female) were treated for correction of Class II skeletal patterns, and 36 patients (16 male, 20 female) were treated for correction of Class III skeletal patterns. The skeletal classification was preassigned by the surgeon; however, the presurgical cephalographs of all subjects were reviewed for agreement. The prerequisites for receiving a Class II assignment were a positive ANB value, a positive overjet, and Class II posterior dental segments. The prerequisites for receiving a Class III assignment were a negative ANB value, a negative overjet, and Class III posterior dental segments. The average age of subjects in this study was 18.8 years (range, 13.5-38.7 years). The orthodontic treatment was completed by different practitioners in and around Oklahoma City. The surgical procedures
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were all completed by the same oral surgeon (S.M.S.), who used the same surgical and fixation protocol for each patient. To be eligible for this study, all patients had to have a CBCT scan taken before surgery (T0) and a second scan taken after the surgery (T1). The postsurgical CBCT images were taken a minimum of 4 months postsurgery to allow for reduction of tissue inflammation. The duration between T0 and T1 was, on average, 9.1 months and ranged from 4 to 14 months. All patients were operated based on the assessment of growth cessation by their orthodontists. Patients treated for the surgical correction of skeletal open bites and craniofacial deformities were excluded from this study. No patients received treatment for the correction of sleep apnea. The T0 and T1 CBCT images were taken on either an Iluma Ultra Cone Beam CT scanner (IMTEC, Ardmore, Okla) with a field of view of 19 3 22 cm and a voxel size of 0.3 mm, or a ProMax 3D CT scanner (Planmeca, Roselle, Ill) with a field of view of 17 3 20 cm and a voxel size of 0.2 mm. All subjects had their CBCT scans taken before and after surgery on the same machine. All scans were performed at 3.8 mA for 40 seconds at 120 kV (Iluma), or variable 1 to 14 mA for 27 seconds at 90 kV (Planmeca), and the patients were instructed to breathe lightly without swallowing while in maximum intercuspation with head positions standardized by holding the Frankfort horizontal parallel to the floor while either sitting (Iluma) or standing (Planmeca). The scans were reconstructed at 0.3 mm and exported in DICOM format. All CBCT analyses were performed by 1 examiner (P.S.H.). The DICOM files were imported into Invivo5 (Anatomage), and the orientation widget was performed in volume rendering to orient all images to the Frankfort horizontal, making it the horizontal reference plane to be used for skeletal and airway measurements (Fig 1). Skeletal measurements were performed according to the methods of Park et al10 and McIntyre17 using a generated lateral cephalometric image with the Frankfort horizontal as the horizontal reference plane and a line perpendicular to the Frankfort horizontal at porion as the vertical reference plane. Changes in the horizontal and vertical positions of A-point and D-point (midpoint of the internal symphysis) were recorded with linear measurements that were perpendicular to these 2 reference planes (Fig 2). Changes in the transverse dimension of the maxilla were also measured using the frontal view of the oriented cephalometric image. Vertical lines perpendicular to the Frankfort reference plane were directed to the innermost curvature on the lateral surface of the right and left sides of the maxilla (Fig 3). The distance between these 2 lines was recorded as the transverse dimension of the maxilla.
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The volume-rendering tab was then selected to measure changes in the position of the posterior maxilla. PNS was used as the posterior reference point and was better visualized in gray scale with 3D image with clipping enabled. The Frankfort horizontal was used as the horizontal reference plane, and a line perpendicular to the Frankfort horizontal through the PNS was used to measure the changes before and after surgery (Fig 4). For the horizontal and vertical measurements, anterior and inferior projections of either the maxilla or the mandible received a positive value (T1-T0), and posterior and superior projections a negative value (T1-T0). For maxillary expansion, T1 to T0 measurements were taken, and a positive value indicated that the maxilla was expanded as part of the surgical correction. If a negative value was recorded, it was assumed to be the result of user error, since maxillary constriction was unlikely. All skeletal movements were measured in millimeters. If a movement was less than 1 mm in any plane, it was recorded as no change. Table I presents the directions and occurrences of skeletal movements in the Class II and Class III subjects. After the horizontal and vertical positions of the maxilla and the mandible were recorded, the airway analysis was performed. A plane between PNS and sella formed the superior limit of the airway, and a plane parallel to the Frankfort horizontal at vertebra C3 formed the inferior limit (Fig 5). These limits were modified from the study of Park et al,10 who used the same inferior border but used a plane between PNS and pterygoid (PT) point as the superior plane. The total airway (pharynx) was defined as the sum of the nasopharynx and the oropharynx. The nasopharynx was defined as the area between a plane parallel to the Frankfort horizontal through PNS and a plane passing between PNS and sella. The oropharynx was defined as the airway bounded superiorly by the plane parallel to the Frankfort horizontal passing through PNS and bounded inferiorly by a plane parallel to the Frankfort horizontal passing through vertebra C3. The most constricted area was defined as the narrowest part of the airway. The volume of each segment of the airway was calculated similarly to the study of Kim et al.18 Sculpting and clipping were then done to isolate the desired airway sections by removing unnecessary structures (Fig 6). To maintain consistency, threshold values of the image were adjusted to 1000 and 604.3 Hounsfield units (HU) to exclude any possible hard or soft tissue structures that were retained in the image. Park et al10 used a range of 1024 to 600 HU for airway volume measurements. The selected range in this study was as close to those values as the Anatomage software would allow. Volume measurements were recorded in cubic millimeters. Area changes
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Fig 1. Volume rendering of image with head position orientation using the widget.
Fig 2. Oriented lateral cephalogram with horizontal and vertical measurements of A-point and D-point that extend to the Frankfort horizontal and the vertical reference planes.
were recorded in square millimeters. Changes in airway volume for all 3 regions as well as the most constricted area were calculated as T1 minus T0. A positive value indicated that an increase in airway volume occurred; a negative value indicated a decrease in airway volume. Changes in volume less than 100 mm3 were recorded as no change. See Table I for the occurrences and directions of the volumetric changes in the Class II and Class III patients.
Statistical analyses
A paired t test was used to compare the means between the T0 and T1 skeletal movements and airway changes. Multivariate factor analysis on all subjects checking for interactions by class was used to analyze the association between linear changes in skeletal position and changes in regional airway volumes. Multivariate analysis adjusting for classification and holding
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Fig 3. Oriented posteroanterior cephalogram with transverse measurements taken of the innermost curvature of the maxilla.
Fig 4. Vertical measurement of PNS oriented to the Frankfort horizontal reference line.
D-point vertical constant (see “Results”) was performed on all linear skeletal movements to determine the effect on each region of the airway. A univariate analysis
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stratified by class was then performed on the D-point vertical changes. A 2-sided 0.05 alpha level was used to define significance. Dahlberg's formula19
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Table I. Frequency of directional changes for skeletal
linear measurements and regional airway volumes postsurgery according to classification Class II
Class III
Direction of change
Direction of change
Variable 1 Facial linear measurements (mm) A-point horizontal 6 20 A-point vertical 8 13 D-point horizontal 30 2 D-point vertical 13 11 Transverse maxilla 9 XX PNS vertical 22 5 Pharyngeal volumes (mm3) Total pharynx 16 19 Nasopharynx 15 20 Oropharynx 17 18
No change
1
No change
9 14 3 11 26 8
18 11 8 12 10 17
6 9 20 15 XX 9
12 16 8 9 26 10
0 0 0
14 20 14
21 13 22
1 3 0
Positive change denotes forward or downward skeletal movement or an increase in airway volume. Negative change denotes upward or backward movement or decrease in airway volume. No change denotes less than 1 mm or 100 mm3 of change in either direction.
rffiffiffiffiffiffiffiffiffiffiffi! P d2 s5 2n was used to calculate the error of measurements; d is the difference between the first and second measurements. The errors ranged from 0.5 to 1.2 mm for linear measurements and from 150.6 to 3 352.9 mm for volumetric measurements. The measure2 ment error for the area was 2.37 mm . The intrarater correlation ranged from 0.77 to 0.99.
region for either the Class II or Class III group were considered significant (Table II). No significant interactions were observed in the association between any linear measurements and the changes in total, nasopharyngeal, and oropharyngeal airway by skeletal classification, except for the D-point vertical and total airway as well as the most constricted area (P \0.05), probably because of the variations in skeletal movements for both classes (Table I). Therefore, the subjects were pooled to determine what movements, if any, could be applied to all patients with a significant effect on the airway. Linear regression analyses quantifying the association between individual linear measurements, airway volumetric, and area changes adjusted for skeletal classification are given in Table III. After accounting for the skeletal classification, per-unit changes for D-point horizontal were 1403.6 mm3 (P \0.01) for the total airway and 1383.9 mm3 (P \0.01) for the oropharynx, and the changes related to the vertical movement of PNS were 459.2 mm3 (P 5 0.04) for the total airway volume, 1187.2 mm3 (P \0.01) for the nasopharynx, 639.7 mm3 (P \0.01) for the oropharynx, and 10.63 mm2 (P \0.05) for the most constricted area in all subjects (Table III). Univariate analysis of the effect of the D-point vertical change on regional airway volume in the Class II and Class III subjects showed a significant effect on total airway (P 5 0.02) and oropharynx (P 5 0.02) volumes and airway area (P 5 0.00) in the Class III subjects only (Table IV).
RESULTS
DISCUSSION
When examined by skeletal classification, the Class II (n 5 35) patients had a mean age of 18.8 (SD, 5.8) years and a mean postsurgery duration of 9.4 (SD, 3.1) months. The Class III patients (n 5 36) had a mean age of 18.6 (SD, 5.4) years and a mean postsurgery duration of 8.8 (SD, 3.1) months. The descriptive statistics for all skeletal landmark changes, regional airway changes, and changes in the most constricted area are shown in Table II. For the Class II patients, significant skeletal movements were seen in the setback of the maxilla (P \0.01), the advancement of the mandible (P \0.01), the expansion of the maxilla (P \0.05), and the downward movement of the posterior maxilla (P \0.01). For the Class III patients, significant skeletal movements were seen in the advancement of the maxilla (P \0.01), the setback of the mandible (P \0.05), and the downward movement of the posterior maxilla (P \0.05). No airway volume or area changes in any
It is widely accepted that orthognathic surgery has an effect on the upper airway, and our findings have confirmed that. The interrelationship between the skeletal position of the maxilla and the mandible, the connecting soft tissues, and the musculature that anchors and stabilizes the airway explains how skeletal movements can influence the position, shape, and size of the air tube.20 When serial cephalographs were evaluated for changes in airway volume after orthognathic surgery, controversy existed over the occurrence and the longterm stability of the airway changes.6,20 However, many studies agreed that Class III patients treated with 2-jaw orthognathic surgery showed increases in the nasopharyngeal airways and the regions above the soft palate, whereas reductions were seen in the oropharynx and the regions behind the tongue.1,6,19 Our findings were consistent with these early reports. The CBCT analysis used in this study followed the guidelines of Park et al,10 with the exception of the
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Fig 5. Clipping and sculpting to isolate the region of the airway according to the superior and inferior boundaries for volumetric analysis.
cranial limit of the nasopharynx. The PNS-sella plane was used in this study because sella was more accurately and consistently identified than the PT point, which was used by Park et al.10 A slight reduction in the total volume of the nasopharynx may have resulted when volumes are compared between the 2 studies. The anatomic constraints of the CBCT image prevented measurement of the laryngopharynx. Extension of the image below the epiglottis was not consistent in all images; thus, measurements of the laryngopharynx were not included in this study. Changes in this region have been reported in previous studies after orthognathic surgery. Most CBCT studies have looked at the effects on the airway of Class III 1-jaw mandibular setback, and 2-jaw mandibular setback with maxillary advancement surgeries.15,16 Studies with CBCT images have been conflicting; however, a common trend was shown: mandibular setbacks constrict the oropharyngeal and hypopharyngeal airways. The concurrent advancement of the maxilla limits the constriction of the total airway and causes increases in nasopharyngeal volume.15,16
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In this study, not all Class II and Class III subjects were treated with conventional Class II and Class III surgical movements, especially with regard to the horizontal movements of the maxilla and mandible (Table I). The ability to compensate for skeletal discrepancies by moving both jaws simultaneously allows for reduced total movement of any 1 arch in the hope to promote greater stability, more precise occlusion, and greater control of facial balance.21 All patients required unique surgical setups, but not all Class II patients were treated the same, and not all Class III patients were treated the same. This prompted an evaluation of the sample as a whole. The insignificant amount of transverse change of the maxilla in the Class III sample was also surprising, given the penchant for this group to have maxillary deficiency in all 3 planes of space. This may be attributed to poor stability of the surgical movement or simply minimal expansion required once the arches were properly coordinated.22 When viewing the sample as a whole, maxillary expansion was not a significant surgical movement and resulted in
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Fig 6. Volume rendering of the airway with the image inverted to isolate areas of density that range from 1000 to 603 HU.
minimal changes in total and nasopharyngeal airway volumes. A clinically insignificant, submillimeter constriction was also observed in the Class II group. However, this was probably due to measurement error (Table II). Pooling the Class II and Class III subjects into 1 large sample allowed us to use linear regression analysis to examine any associations between skeletal movements and changes in airway, regardless of the skeletal classification. To justifiably pool the sample, it was necessary to determine whether the effects of each skeletal movement on airway volume were the same for both classifications. The vertical movement of the anterior mandible was shown to have a significant effect on total airway in the Class III subjects but was not seen in the Class II subjects; this required adjusting for this movement. For the univariate and multivariate regression analyses, a given increase or decrease in volume or area, depending on direction, will occur for each 1 mm
of movement of a point. For instance, if a region of the airway increases by 100 mm3 or 10 mm2 for every 1 mm of forward movement of D-point, it would also decrease by 100 mm3 or 10 mm2 for every 1 mm of backward movement of D-point. This principle can be applied to all horizontal and vertical movements of all points. Downward and forward skeletal movements were positive, and upward and backward movements were negative. All univariate and multivariate values assume a positive skeletal movement. An airway increase was positive, and a decrease was negative. When looking at the sample as a whole, regardless of classification and maintaining the vertical position of the anterior mandible in a constant position, we found that the horizontal movement of the mandible had a significant effect on total airway and oropharynx volumes in all patients. One millimeter of forward or backward movement could account for 403.6 mm3 and
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204.41 (119.41) 10.13 (88.80) 193.91 (114.77) 9.34 (85.10) 199.10 (119.68) 207.92 (133.77)
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All data presented as mean (SD). T0, Presurgery measurements; T1, postsurgery measurements. *Significantly different within class or group, P \0.05.
30.15 (89.26)
154.4 (4731.5) 20457.0 (8127.8) 20917.0 (8319.4) 459.4 (5059.7) 296.9 (1310.5) 4947.4 (1999.9) 5043.7 (1952.0) 96.8 (1175.3) 493.0 (4457.6) 15513.0 (7022.6) 15852.0 (7313.0) 338.8 (4727.5) 1090.8 (5371.5) 20693.0 (9770.4) 20539.0 (9535.9) 110.1 (994.7) 4656.7 (2256.2) 4953.6 (2279.7) 1194.4 (4906.4) 16037.0 (8304.4) 15544.0 (8306.6)
0.1 (2.6) 0.1 (1.9) 2.02 (5.6) 0.1 (2.9) 0.2 (2.2) 1.3 (2.7) 91.2 (6.0) 27.8 (2.8) 87.6 (7.0) 78.9 (7.2) 60.5 (3.4) 54.7 (6.1) 91.3 (5.7) 27.9 (3.2) 85.5 (9.0) 78.8 (7.7) 60.6 (3.3) 53.4 (6.4) 1.2 (2.5)* 0.3 (2.0) 1.6 (4.1)* 0.2 (2.6) 0.5 (2.0) 1.0 (2.6)* 89.0 (6.0) 27.6 (3.7) 90.7 (8.3) 79.6 (8.1) 60.6 (3.3) 53.2 (7.2) 1.4 (2.0)* 0.4 (1.8) 5.7 (4.2)* 0.4 (3.2) 0.8 (2.3)* 1.7 (2.8)*
90.2 (7.0) 27.8 (2.9) 89.1 (7.5) 79.5 (7.5) 61.0 (3.0) 54.1 (6.8)
T1 T0
T0 T1 Facial linear measurements (mm) A-point horizontal 93.7 (4.4) 92.3 (4.6) A-point vertical 28.1 (2.6) 27.8 (2.7) D-point horizontal 80.2 (6.1) 85.9 (6.2) D-point vertical 78.0 (7.4) 78.4 (7.0) Transverse maxilla 60.7 (3.4) 59.9 (3.7) PNS vertical 53.6 (5.5) 55.3 (5.4) Pharyngeal volumes (mm3) Total pharynx 20214.0 (6133.6) 21305.0 (6970.2) Nasopharynx 5246.5 (1677.1) 5136.3 (1573.9) Oropharynx 14974.0 (5473.0) 16169.0 (6234.8) Most constricted area (mm2) Area 179.51 (90.97) 209.88 (120.63)
T1-T0
T1
T1-T0
T0
Overall Class III Class II
Table II. Distribution of skeletal linear measurements and regional airway volumes before and after surgery according to classification
T1-T0
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383.9 mm3 of total airway and oropharyngeal volume increases or decreases, respectively. This makes sense, given the number of soft tissue attachments and the close relationship of the mandible to the supporting structures of the airway. Although statistically insignificant, the effect on the most constricted area was positive with this movement and was found to be 3.95 mm2. This needs to be noted because it may have clinical significance when considering the profound inverse relation of the most constricted area and the resistance to flow. The vertical movements of the posterior maxilla also had a significant effect on all regions of the airway. For every millimeter of downward movement of the posterior maxilla, 459.2 mm3 of total airway and 639.7 mm3 of oropharynx were lost. This movement negatively affected the most constricted area by 10.63 mm2. Upward movement results in airway volume and area increases of the same values. Although downward movement of the posterior maxilla was seen to increase nasopharyngeal volume, its negative influence on the total airway should limit this movement as a means of increasing airway volume. These findings suggest that impaction or vertical maintenance of the posterior maxilla coupled with proper horizontal control of the mandible are the keys to successful airway management in all patients. One can predict that if a Class II patient is planned for a large mandibular advancement and the vertical position of the maxilla is controlled, increases in total airway and oropharyngeal volumes should be seen. Similarly, in Class III patients requiring large mandibular setbacks, volume losses will occur unless maxillary movements are used to help offset the decrease. These findings support many claims made in earlier studies about the negative effect of mandibular setback on total airway volume in Class III patients, but also provide an estimate of the amount of volume change per millimeter of setback to guide surgeons in treatment planning. These findings are also parallel with the published information on the pretreatment volumetric values of different skeletal malocclusions and the types of surgical movements needed for their correction.9 By checking for interactions between skeletal classifications, we determined that the preceding measurements could be confidently applied to most surgical patients. However, muscle adaptation takes place during the first 6 months after surgery, and it may limit the ability to accurately apply these recommended guidelines.23 In our study, the mean difference between the 2 time points was 9.1 months, which should have allowed adequate time for muscle adaptation. Additionally, the vertical movement of the anterior mandible may have differing effects on the total airway between the Class
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Table III. Linear regression coefficients (ß*, SE) adjusted for skeletal classification for the association between post-
surgical changes in individual linear measurements with changes in airway volume (n 5 71) Total airway change (mm3) Parameter A-point horizontal A-point vertical D-point horizontal Transverse maxilla PNS vertical
ß* 181.5 525.5 403.6* 453.1 459.2*
SE 267.9 312.2 138.6* 283.1 219.9*
P value 0.5005 0.0969 0.0049* 0.1142 0.0406*
Nasopharynx change (mm3) ß* 9.4 0.5 13.8 57.2 187.2*
SE 61.9 73.4 33.8 66.1 47.1*
P value 0.8801 0.9947 0.6852 0.3901 0.0002*
Oropharynx change (mm3) ß* 165.1 506.6 383.9* 407.1 639.7*
SE 248.2 288.7 127.9* 262.6 195.3*
P value 0.5080 0.0837 0.0038* 0.1256 0.0017*
Area change (mm2) ß* 0.19 5.62 3.95 4.92 10.63*
SE 4.63 5.45 2.49 4.94 3.69*
P value 0.9675 0.3065 0.1171 0.3228 0.0053*
All models presented were adjusted for skeletal class. *Significantly different, P \0.05.
Table IV. Linear regression coefficients stratified by skeletal classification for the association between D-point ver-
tical change and postsurgery airway volume changes Class II Total airway change (mm3) Nasopharynx change (mm3) Oropharynx change (mm3) Area change (mm2)
Parameter D-point vertical D-point vertical D-point vertical D-point vertical
ß 151.4 52.5 100.9 1.12
SE 292.9 53.7 268.1 4.88
Class III P value 0.6087 0.3353 0.7089 0.8201
ß 724.0* 49.4 648.2* 15.45*
SE 284.4* 85.5 270.4* 4.91*
P value 0.0156* 0.5672 0.0221* 0.0034*
*Significantly different, P \0.05.
II and Class III patients. Table IV shows that downward movement of the anterior mandible in the Class III subjects led to a significant reduction in total and oropharyngeal airway volumes and a significant reduction in the most constricted area. No such effect was seen in the Class II subjects. In the evaluation of these results, posterior impaction on Class III patients in this study was not a common approach to prevent upward movement of the PNS. With patients who had lower occlusal planes, and thus lower palatal planes, a more common technique used by our surgeon was to move the PNS inferiorly, out of concern that superior movements would result in a narrowing effect on the nasopharyngeal space and compromise the airway. In general, the Class III patients in this study underwent counterclockwise rotational advancements of the maxillomandibular complex, with the maxillary incisors moving forward, the posterior teeth moving slightly downward, and the mandibular incisors rotating back, but with the chin actually staying the same or moving slightly forward and upward to minimize the impact on the airway. These data have shown that surgical movements may not always be predictable for Class II and Class III patients. If the surgeons wish to predictably alter the airway, this study should serve as a guideline for properly planning the movements and relaying their desires to the orthodontists. Volumetric gains or losses can be estimated
from the surgical plan, and decisions on extraction protocol may be influenced by airway demands. Surgical planning, however, takes into account not only the airway, but also masticatory function, occlusion, and esthetics. The proper management of all 4 variables leads to success. This study and most other studies examining the surgical effect on changes in airway provide quantitative information without addressing the qualitative effects experienced by patients as a result of the change. Poiseuille's law24 for flow rate demonstrates that even a modest increase in the radius of a tube (or an airway) will result in a significant decrease in airway resistance (doubling the radius results in resistance decreased by a factor of 16). Significant changes of the most constricted area were noted for the vertical movements of the posterior maxilla and the anterior mandible, with the latter seen in Class III patients only. What effect this has on patient quality of life needs to be determined, and further work is underway focusing on this aspect of the airway. CONCLUSIONS
Separating patients by skeletal classification showed that predictable movements in the treatment of Class II and Class III patients did not always occur. For all patients, horizontal movements of the mandible were
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found to significantly change total airway volume, with anterior movement increasing and posterior movement decreasing volume. Posterior maxillary vertical movements were found to have significant effects. Downward displacement of the PNS significantly increased nasopharynx volume while significantly decreasing the oropharynx and total airway volumes and the most constricted area. Downward movement of the anterior mandible in the Class III patients led to a decrease in total airway and oropharyngeal volumes as well as the most constricted area; this was not seen in Class II patients.
11.
12.
13.
14.
ACKNOWLEDGMENT
We thank Jarom Maurer for his kind assistance in gathering information and data. 15.
REFERENCES 1. Mattos CT, Vilani GNL, SantAnna EF, Ruellas ACO, Maia LC. Effects of orthognathic surgery on oropharyngeal airway: a meta analysis. Int J Oral Maxillfac Surg 2011;40:1347-56. 2. Enacar A, Aksoy AU, Sencift Y, Haydar B, Aras K. Changes in hypopharyngeal airway space and in tongue and hyoid bone positions following the surgical correction of mandibular prognathism. Int J Adult Orthod Orthognath Surg 1994;9:285-90. 3. Samman N, Tang SS, Xia J. Cephalometric study of the upper airway in surgically corrected Class III skeletal deformity. Int J Adult Orthod Orthognath Surg 2002;17:180-90. 4. Guven O, Saracoglu U. Changes in pharyngeal airway space and hyoid bone positions after body ostectomies and sagittal split ramus osteotomies. J Craniofac Surg 2005;16:23-30. 5. Tselnik M, Pogrel MA. Assessment of the pharyngeal airway space after mandibular setback surgery. J Oral Maxillofac Surg 2000;58: 282-5. 6. Gokce SM, Gorgulu S, Gokce HS, Bengi O, Sabuncuoglu F, Ozgen F, et al. Changes in posterior airway space, pulmonary function and sleep quality, following bimaxillary orthognathic surgery. Int J Oral Maxillfac Surg 2012;41:820-9. 7. Weissheimer A, Macedo de Menezes L, Sameshima GT, Enciso R, Pham J, Grauer D. Imaging software accuracy for 3-dimensional analysis of the upper airway. Am J Orthod Dentofacial Orthop 2012;142:801-13. 8. El H, Palomo JM. Measuring the airway in 3 dimensions: a reliability and accuracy study. Am J Orthod Dentofacial Orthop 2010;137(4 Suppl):S50.e1-9. 9. El H, Palomo JM. Airway volume for different dentofacial skeletal patterns. Am J Orthod Dentofacial Orthop 2011;139:e511-21. 10. Park JW, Kim NK, Kim JW, Kim MJ, Chang YI. Volumetric, planar, and linear analyses of pharyngeal airway change on computed to-
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mography and cephalometry after mandibular setback surgery. Am J Orthod Dentofacial Orthop 2010;138:292-9. Kobayashi K, Shimoda S, Nakagawa Y, Yamamoto A. Accuracy in measurement of distance using limited cone-beam computerized tomography. Int J Oral Maxillofac Implants 2004;19: 228-31. Loubele M, Guerrero ME, Jacobs R, Suetens P, van Steenberghe D. A comparison of jaw dimensional and quality assessments of bone characteristics with cone-beam CT, spiral tomography, and multi-slice spiral CT. Int J Oral Maxillofac Implants 2007;22: 446-54. Aboudara C, Nielsen I, Huang JC, Maki K, Miller AJ, Hatcher D. Comparison of airway space with conventional lateral headfilms and 3-dimensional reconstruction from cone-beam computed tomography. Am J Orthod Dentofacial Orthop 2009;135:468-79. Yamashina A, Tanimoto K, Sutthiprapaporn P, Hayakawa Y. The reliability of computed tomography values and dimensional measurements of the oropharyngeal region using cone beam CT: comparison with multidector CT. Dentomaxillofac Radiol 2008;37: 245-51. Hong JS, Oh KM, Kim BR, Kim YJ, Park YH. Three-dimensional analysis of pharyngeal airway volume in adults with anterior position of the mandible. Am J Orthod Dentofacial Orthop 2011;140: e161-9. Park SB, Kim YI, Son WS, Hwang DS, Cho BH. Cone-beam computed tomography evaluation of short and long term airway change and stability after orthognathic surgery in patients with Class III skeletal deformities: bimaxillary surgery and mandibular setback surgery. Int J Oral Maxillfac Surg 2012;41:87-93. McIntyre BP. Volumetric airway changes in patients undergoing orthognathic surgery: a conebeam CT evaluation [thesis]. Oklahoma City, Okla: University of Oklahoma; 2011. Kim YJ, Hong JS, Hwang YI, Park YH. Three-dimensional analysis of pharyngeal airway in preadolescent children with different anteroposterior skeletal patterns. Am J Orthod Dentofacial Orthop 2010;137:306-7. Dahlberg G. Statistical methods for medical and biological students. New York: Interscience Pulblications; 1940. Saitoh K. Long-term changes in pharyngeal airway morphology after mandibular setback surgery. Am J Orthod Dentofacial Orthop 2004;125:556-61. Chen 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. Am J Orthod Dentofacial Orthop 2007;131:373-7. Proffit WR, Turvey TA, Phillips C. The hierarchy of stability and predictability in orthognathic surgery with rigid fixation: an update and extension. Head Face Med 2007;3:21. Carlson DS, Ellis E 3rd, Dechow PC. Adaptation of the suprahyoid muscle complex to mandibular advancement surgery. Am J Orthod Dentofacial Orthop 1987;92:134-43. Bird RB, Stewart WE, Lightfoot EN. Transport phenomena, 2nd edition. New York: John Wiley and Sons, Inc; 2002.
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Postsurgical volumetric airway changes in 2-jaw orthognathic surgery patients €ns Currier,d Steven M. Sullivan,e Ji Li,f P. Sheamus Hart,a Brian P. McIntyre,b Onur Kadioglu,c G. Fra and Christina Shayg Woodridge, Va, Omaha, Neb, and Oklahoma City, Okla
Introduction: Findings from early cephalometric studies on airway changes after 2-jaw orthognathic surgery have been challenged because the previous anteroposterior interpretation of airway changes can now be evaluated in 3 dimensions. The aims of this study were to use cone-beam computed tomography to quantify the nasopharynx, oropharynx, and total airway volume changes associated with skeletal movements of the maxilla and mandible in a sample of patients undergoing 2-jaw orthognathic surgery for correction of skeletal malocclusion. Methods: Skeletal movements and airway volumes of 71 postpubertal patients (31 male, 40 female; mean age, 18.8 years) were measured. They were divided into 2 groups based on ANB angle, overjet, and occlusion (Class II: ANB, .2 ; overjet, .1 mm; total, 35 subjects; and Class III: ANB, \1 ; overjet, \1 mm; total, 36 subjects). Presurgical and postsurgical measurements were collected for horizontal, vertical, and transverse movements of the maxilla and the mandible, along with changes in the nasopharynx, oropharynx, and total airways. Associations between the directional movements of skeletal structures and the regional changes in airway volume were quantified. Changes in the most constricted area were also noted. Results: Horizontal movements of D-point were significantly associated with increases in both total airway (403.6 6 138.6 mm3; P \0.01) and oropharynx (383.9 6 127.9 mm3; P\0.01) volumes. Vertical movements of the posterior nasal spine were significantly associated with decreases in total airway volume ( 459.2 6 219.9 mm3; P 5 0.04) and oropharynx volume ( 639.7 6 195.3 mm3; P \0.01), increases in nasopharynx (187.2 6 47.1 mm3; P \0.01) volume, and decreases in the most constricted area ( 10.63 6 3.69 mm2; P \0.01). In the Class III patients only, the vertical movement of D-point was significantly associated with decreases in both total airway ( 724.0 6 284.4 mm3; P 5 0.02) and oropharynx ( 648.2 6 270.4 mm3; P 5 0.02) volumes. A similar negative association was observed for the most constricted area for the vertical movement of D-point ( 15.45 6 4.91 mm2; P \0.01). Conclusions: Optimal control of airway volume is through management of the mandible in the horizontal direction and the vertical movement of the posterior maxilla for all patients. The surgeon and the orthodontist should optimally plan these movements to control gains or losses in airway volume as a result of orthognathic surgery. (Am J Orthod Dentofacial Orthop 2015;147:536-46)
a
Private practice, Woodridge, Va. Private practice, Omaha, Neb. c Assistant professor and program director, Department of Orthodontics, University of Oklahoma, Oklahoma City, Okla. d Professor and chair, Department of Orthodontics, University of Oklahoma, Oklahoma City, Okla. e Professor and chair, Department of Oral and Maxillofacial Surgery, University of Oklahoma, Oklahoma City, Okla. f Biostatistician, Department of Biostatistics and Epidemiology, University of Oklahoma, Oklahoma City, Okla. g Assistant professor, Department of Biostatistics and Epidemiology, University of Oklahoma, Oklahoma City, Okla. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Onur Kadioglu, Department of Orthodontics, College of Dentistry, University of Oklahoma, 1201 N. Stonewall Ave, Room 400; Oklahoma City, OK 73117; e-mail, [email protected]. Submitted, February 2014; revised and accepted, December 2014. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2014.12.023 b
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C
ephalometric studies of the airway have aided oral surgeons for many years in making predictions about the potential and realized airway changes that occur during surgery. A meta-analysis by Mattos et al1 provided evidence that mandibular setback surgery alone was associated with notable decreases in the oropharyngeal airway at the level of the soft palate and at the base of the tongue. Specifically, in 2-jaw surgery including maxillary advancement and mandibular setback, an increase in airway volume was seen at the level of the posterior nasal spine (PNS), and significant decreases were seen at the level of the soft palate and at the base of the tongue. Their results also indicated that 2-jaw advancement showed a highly significant increase in oropharyngeal airway at the level of the soft palate.1 However, the long-term stability of these changes is controversial, with some studies claiming that these gains and losses were
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stable in the long term, whereas others reported that the airway adapts to these change and returns to presurgical values.2-6 Since its introduction in 1998, cone-beam computed tomography (CBCT) has been improved, with lower costs, less radiation exposure to patients, and better accuracy in identifying the boundaries of soft tissues and air spaces.7 CBCT allows for segmentation and visualization of hollow structures, such as the airway in 3 dimensions, permitting the transition from lengths and angles to volumes and cross-sectional areas.8 Consequently, findings from early 2-dimensional evaluations on the influence of skeletal repositioning on the structures of the head and neck have been challenged with the introduction of these 3-dimensional (3D) tomographic evaluations. Although earlier studies focused on the reliability of the new technique8 as well as defining normative values,9 changes in the airway after orthognathic surgery have become important in the comprehensive evaluation of patients. In a study evaluating pretreatment CBCT images of 140 patients, El and Palomo9 reported that skeletal Class II patients have smaller airway volumes when compared with Class I and Class III patients. Two-dimensional studies have reported decreases in oropharyngeal dimensions as a result of mandibular setback surgery. However, using 3D methods, Park et al10 indicated no significant changes in linear, area, or volumetric measurements of the nasopharyngeal or oropharyngeal airway in patients undergoing setback surgery. Conversely, linear assessments using cephalometric radiographs of the same patients at the same times indicated decreases in all measurements of pharyngeal depth and airway space, as well as posterior movements of the soft palate, tongue, and hyoid bone. These conflicting results between the CBCT and the cephalometric analyses suggest that the absolute volume of the airway may not be significantly reduced by setback surgery, with the soft tissue pharynx expanding laterally to preserve its volume.10 CBCT technology has been found to be accurate and repeatable in making skeletal measurements and measuring volumes of known value. Linear measurements are accurate to the submillimeter level, and the volumes acquired from CBCT are a near 1:1 representation of the known volume.11-14 Software programs such as Dolphin (Dolphin Imaging & Management Solutions, Chatsworth, Calif), Anatomage (San Jose, Calif), and Osirix (Pixeo, Geneva, Switzerland) have also been tested for accuracy and reliability in making volumetric predictions.7,8 Although the comparative accuracy for the programs was found to be low, the programs are reliable for repeated measurements of a given volume; thus, serial measurements of the same
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patient should give a reliable assessment of the changes in airway volumes. Additionally, studies on airway changes in orthognathic surgery patients have generally focused on the surgical correction of the Class III phenotype with either 1-jaw or 2-jaw correction. The results have shown that single-jaw mandibular setback constricts total, oropharyngeal, and hypopharyngeal airways and are maintained in the long term. In patients undergoing 2-jaw mandibular setback with maxillary advancement, loss in regional airway volume was reduced, with gains seen in some regions that ameliorated the total airway losses.15,16 The evidence of an association between the surgical movements and the resultant changes in airway volume includes conflicting views seen both within and between the cephalogram and the CBCT investigations. It is also readily apparent from a review of the literature that few data exist concerning CBCT analysis of volumetric changes in mandibular advancement groups. Further 3D studies on the volumetric changes resulting from the surgical correction of anteroposterior skeletal discrepancies are needed to confirm the results obtained thus far.1 Therefore, the purpose of this study was to obtain quantitative evidence of the volumetric changes occurring in the airway as a result of 2-jaw orthognathic surgery using CBCT. Specifically, we aimed to quantify the associations between the amount of surgical movement of the maxilla or the mandible in all 3 planes of space and the volumetric airway changes that occur. MATERIAL AND METHODS
Institutional review board approval from the University of Oklahoma was obtained before this study (#15519). Records of 71 subjects who were treated for orthognathic surgery between May 2009 and June 2013 were selected. All patients had 2-jaw orthognathic surgery in conjunction with orthodontic treatment. Thirty-five patients (15 male, 20 female) were treated for correction of Class II skeletal patterns, and 36 patients (16 male, 20 female) were treated for correction of Class III skeletal patterns. The skeletal classification was preassigned by the surgeon; however, the presurgical cephalographs of all subjects were reviewed for agreement. The prerequisites for receiving a Class II assignment were a positive ANB value, a positive overjet, and Class II posterior dental segments. The prerequisites for receiving a Class III assignment were a negative ANB value, a negative overjet, and Class III posterior dental segments. The average age of subjects in this study was 18.8 years (range, 13.5-38.7 years). The orthodontic treatment was completed by different practitioners in and around Oklahoma City. The surgical procedures
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were all completed by the same oral surgeon (S.M.S.), who used the same surgical and fixation protocol for each patient. To be eligible for this study, all patients had to have a CBCT scan taken before surgery (T0) and a second scan taken after the surgery (T1). The postsurgical CBCT images were taken a minimum of 4 months postsurgery to allow for reduction of tissue inflammation. The duration between T0 and T1 was, on average, 9.1 months and ranged from 4 to 14 months. All patients were operated based on the assessment of growth cessation by their orthodontists. Patients treated for the surgical correction of skeletal open bites and craniofacial deformities were excluded from this study. No patients received treatment for the correction of sleep apnea. The T0 and T1 CBCT images were taken on either an Iluma Ultra Cone Beam CT scanner (IMTEC, Ardmore, Okla) with a field of view of 19 3 22 cm and a voxel size of 0.3 mm, or a ProMax 3D CT scanner (Planmeca, Roselle, Ill) with a field of view of 17 3 20 cm and a voxel size of 0.2 mm. All subjects had their CBCT scans taken before and after surgery on the same machine. All scans were performed at 3.8 mA for 40 seconds at 120 kV (Iluma), or variable 1 to 14 mA for 27 seconds at 90 kV (Planmeca), and the patients were instructed to breathe lightly without swallowing while in maximum intercuspation with head positions standardized by holding the Frankfort horizontal parallel to the floor while either sitting (Iluma) or standing (Planmeca). The scans were reconstructed at 0.3 mm and exported in DICOM format. All CBCT analyses were performed by 1 examiner (P.S.H.). The DICOM files were imported into Invivo5 (Anatomage), and the orientation widget was performed in volume rendering to orient all images to the Frankfort horizontal, making it the horizontal reference plane to be used for skeletal and airway measurements (Fig 1). Skeletal measurements were performed according to the methods of Park et al10 and McIntyre17 using a generated lateral cephalometric image with the Frankfort horizontal as the horizontal reference plane and a line perpendicular to the Frankfort horizontal at porion as the vertical reference plane. Changes in the horizontal and vertical positions of A-point and D-point (midpoint of the internal symphysis) were recorded with linear measurements that were perpendicular to these 2 reference planes (Fig 2). Changes in the transverse dimension of the maxilla were also measured using the frontal view of the oriented cephalometric image. Vertical lines perpendicular to the Frankfort reference plane were directed to the innermost curvature on the lateral surface of the right and left sides of the maxilla (Fig 3). The distance between these 2 lines was recorded as the transverse dimension of the maxilla.
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The volume-rendering tab was then selected to measure changes in the position of the posterior maxilla. PNS was used as the posterior reference point and was better visualized in gray scale with 3D image with clipping enabled. The Frankfort horizontal was used as the horizontal reference plane, and a line perpendicular to the Frankfort horizontal through the PNS was used to measure the changes before and after surgery (Fig 4). For the horizontal and vertical measurements, anterior and inferior projections of either the maxilla or the mandible received a positive value (T1-T0), and posterior and superior projections a negative value (T1-T0). For maxillary expansion, T1 to T0 measurements were taken, and a positive value indicated that the maxilla was expanded as part of the surgical correction. If a negative value was recorded, it was assumed to be the result of user error, since maxillary constriction was unlikely. All skeletal movements were measured in millimeters. If a movement was less than 1 mm in any plane, it was recorded as no change. Table I presents the directions and occurrences of skeletal movements in the Class II and Class III subjects. After the horizontal and vertical positions of the maxilla and the mandible were recorded, the airway analysis was performed. A plane between PNS and sella formed the superior limit of the airway, and a plane parallel to the Frankfort horizontal at vertebra C3 formed the inferior limit (Fig 5). These limits were modified from the study of Park et al,10 who used the same inferior border but used a plane between PNS and pterygoid (PT) point as the superior plane. The total airway (pharynx) was defined as the sum of the nasopharynx and the oropharynx. The nasopharynx was defined as the area between a plane parallel to the Frankfort horizontal through PNS and a plane passing between PNS and sella. The oropharynx was defined as the airway bounded superiorly by the plane parallel to the Frankfort horizontal passing through PNS and bounded inferiorly by a plane parallel to the Frankfort horizontal passing through vertebra C3. The most constricted area was defined as the narrowest part of the airway. The volume of each segment of the airway was calculated similarly to the study of Kim et al.18 Sculpting and clipping were then done to isolate the desired airway sections by removing unnecessary structures (Fig 6). To maintain consistency, threshold values of the image were adjusted to 1000 and 604.3 Hounsfield units (HU) to exclude any possible hard or soft tissue structures that were retained in the image. Park et al10 used a range of 1024 to 600 HU for airway volume measurements. The selected range in this study was as close to those values as the Anatomage software would allow. Volume measurements were recorded in cubic millimeters. Area changes
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Fig 1. Volume rendering of image with head position orientation using the widget.
Fig 2. Oriented lateral cephalogram with horizontal and vertical measurements of A-point and D-point that extend to the Frankfort horizontal and the vertical reference planes.
were recorded in square millimeters. Changes in airway volume for all 3 regions as well as the most constricted area were calculated as T1 minus T0. A positive value indicated that an increase in airway volume occurred; a negative value indicated a decrease in airway volume. Changes in volume less than 100 mm3 were recorded as no change. See Table I for the occurrences and directions of the volumetric changes in the Class II and Class III patients.
Statistical analyses
A paired t test was used to compare the means between the T0 and T1 skeletal movements and airway changes. Multivariate factor analysis on all subjects checking for interactions by class was used to analyze the association between linear changes in skeletal position and changes in regional airway volumes. Multivariate analysis adjusting for classification and holding
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Fig 3. Oriented posteroanterior cephalogram with transverse measurements taken of the innermost curvature of the maxilla.
Fig 4. Vertical measurement of PNS oriented to the Frankfort horizontal reference line.
D-point vertical constant (see “Results”) was performed on all linear skeletal movements to determine the effect on each region of the airway. A univariate analysis
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stratified by class was then performed on the D-point vertical changes. A 2-sided 0.05 alpha level was used to define significance. Dahlberg's formula19
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Table I. Frequency of directional changes for skeletal
linear measurements and regional airway volumes postsurgery according to classification Class II
Class III
Direction of change
Direction of change
Variable 1 Facial linear measurements (mm) A-point horizontal 6 20 A-point vertical 8 13 D-point horizontal 30 2 D-point vertical 13 11 Transverse maxilla 9 XX PNS vertical 22 5 Pharyngeal volumes (mm3) Total pharynx 16 19 Nasopharynx 15 20 Oropharynx 17 18
No change
1
No change
9 14 3 11 26 8
18 11 8 12 10 17
6 9 20 15 XX 9
12 16 8 9 26 10
0 0 0
14 20 14
21 13 22
1 3 0
Positive change denotes forward or downward skeletal movement or an increase in airway volume. Negative change denotes upward or backward movement or decrease in airway volume. No change denotes less than 1 mm or 100 mm3 of change in either direction.
rffiffiffiffiffiffiffiffiffiffiffi! P d2 s5 2n was used to calculate the error of measurements; d is the difference between the first and second measurements. The errors ranged from 0.5 to 1.2 mm for linear measurements and from 150.6 to 3 352.9 mm for volumetric measurements. The measure2 ment error for the area was 2.37 mm . The intrarater correlation ranged from 0.77 to 0.99.
region for either the Class II or Class III group were considered significant (Table II). No significant interactions were observed in the association between any linear measurements and the changes in total, nasopharyngeal, and oropharyngeal airway by skeletal classification, except for the D-point vertical and total airway as well as the most constricted area (P \0.05), probably because of the variations in skeletal movements for both classes (Table I). Therefore, the subjects were pooled to determine what movements, if any, could be applied to all patients with a significant effect on the airway. Linear regression analyses quantifying the association between individual linear measurements, airway volumetric, and area changes adjusted for skeletal classification are given in Table III. After accounting for the skeletal classification, per-unit changes for D-point horizontal were 1403.6 mm3 (P \0.01) for the total airway and 1383.9 mm3 (P \0.01) for the oropharynx, and the changes related to the vertical movement of PNS were 459.2 mm3 (P 5 0.04) for the total airway volume, 1187.2 mm3 (P \0.01) for the nasopharynx, 639.7 mm3 (P \0.01) for the oropharynx, and 10.63 mm2 (P \0.05) for the most constricted area in all subjects (Table III). Univariate analysis of the effect of the D-point vertical change on regional airway volume in the Class II and Class III subjects showed a significant effect on total airway (P 5 0.02) and oropharynx (P 5 0.02) volumes and airway area (P 5 0.00) in the Class III subjects only (Table IV).
RESULTS
DISCUSSION
When examined by skeletal classification, the Class II (n 5 35) patients had a mean age of 18.8 (SD, 5.8) years and a mean postsurgery duration of 9.4 (SD, 3.1) months. The Class III patients (n 5 36) had a mean age of 18.6 (SD, 5.4) years and a mean postsurgery duration of 8.8 (SD, 3.1) months. The descriptive statistics for all skeletal landmark changes, regional airway changes, and changes in the most constricted area are shown in Table II. For the Class II patients, significant skeletal movements were seen in the setback of the maxilla (P \0.01), the advancement of the mandible (P \0.01), the expansion of the maxilla (P \0.05), and the downward movement of the posterior maxilla (P \0.01). For the Class III patients, significant skeletal movements were seen in the advancement of the maxilla (P \0.01), the setback of the mandible (P \0.05), and the downward movement of the posterior maxilla (P \0.05). No airway volume or area changes in any
It is widely accepted that orthognathic surgery has an effect on the upper airway, and our findings have confirmed that. The interrelationship between the skeletal position of the maxilla and the mandible, the connecting soft tissues, and the musculature that anchors and stabilizes the airway explains how skeletal movements can influence the position, shape, and size of the air tube.20 When serial cephalographs were evaluated for changes in airway volume after orthognathic surgery, controversy existed over the occurrence and the longterm stability of the airway changes.6,20 However, many studies agreed that Class III patients treated with 2-jaw orthognathic surgery showed increases in the nasopharyngeal airways and the regions above the soft palate, whereas reductions were seen in the oropharynx and the regions behind the tongue.1,6,19 Our findings were consistent with these early reports. The CBCT analysis used in this study followed the guidelines of Park et al,10 with the exception of the
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Fig 5. Clipping and sculpting to isolate the region of the airway according to the superior and inferior boundaries for volumetric analysis.
cranial limit of the nasopharynx. The PNS-sella plane was used in this study because sella was more accurately and consistently identified than the PT point, which was used by Park et al.10 A slight reduction in the total volume of the nasopharynx may have resulted when volumes are compared between the 2 studies. The anatomic constraints of the CBCT image prevented measurement of the laryngopharynx. Extension of the image below the epiglottis was not consistent in all images; thus, measurements of the laryngopharynx were not included in this study. Changes in this region have been reported in previous studies after orthognathic surgery. Most CBCT studies have looked at the effects on the airway of Class III 1-jaw mandibular setback, and 2-jaw mandibular setback with maxillary advancement surgeries.15,16 Studies with CBCT images have been conflicting; however, a common trend was shown: mandibular setbacks constrict the oropharyngeal and hypopharyngeal airways. The concurrent advancement of the maxilla limits the constriction of the total airway and causes increases in nasopharyngeal volume.15,16
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In this study, not all Class II and Class III subjects were treated with conventional Class II and Class III surgical movements, especially with regard to the horizontal movements of the maxilla and mandible (Table I). The ability to compensate for skeletal discrepancies by moving both jaws simultaneously allows for reduced total movement of any 1 arch in the hope to promote greater stability, more precise occlusion, and greater control of facial balance.21 All patients required unique surgical setups, but not all Class II patients were treated the same, and not all Class III patients were treated the same. This prompted an evaluation of the sample as a whole. The insignificant amount of transverse change of the maxilla in the Class III sample was also surprising, given the penchant for this group to have maxillary deficiency in all 3 planes of space. This may be attributed to poor stability of the surgical movement or simply minimal expansion required once the arches were properly coordinated.22 When viewing the sample as a whole, maxillary expansion was not a significant surgical movement and resulted in
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Fig 6. Volume rendering of the airway with the image inverted to isolate areas of density that range from 1000 to 603 HU.
minimal changes in total and nasopharyngeal airway volumes. A clinically insignificant, submillimeter constriction was also observed in the Class II group. However, this was probably due to measurement error (Table II). Pooling the Class II and Class III subjects into 1 large sample allowed us to use linear regression analysis to examine any associations between skeletal movements and changes in airway, regardless of the skeletal classification. To justifiably pool the sample, it was necessary to determine whether the effects of each skeletal movement on airway volume were the same for both classifications. The vertical movement of the anterior mandible was shown to have a significant effect on total airway in the Class III subjects but was not seen in the Class II subjects; this required adjusting for this movement. For the univariate and multivariate regression analyses, a given increase or decrease in volume or area, depending on direction, will occur for each 1 mm
of movement of a point. For instance, if a region of the airway increases by 100 mm3 or 10 mm2 for every 1 mm of forward movement of D-point, it would also decrease by 100 mm3 or 10 mm2 for every 1 mm of backward movement of D-point. This principle can be applied to all horizontal and vertical movements of all points. Downward and forward skeletal movements were positive, and upward and backward movements were negative. All univariate and multivariate values assume a positive skeletal movement. An airway increase was positive, and a decrease was negative. When looking at the sample as a whole, regardless of classification and maintaining the vertical position of the anterior mandible in a constant position, we found that the horizontal movement of the mandible had a significant effect on total airway and oropharynx volumes in all patients. One millimeter of forward or backward movement could account for 403.6 mm3 and
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204.41 (119.41) 10.13 (88.80) 193.91 (114.77) 9.34 (85.10) 199.10 (119.68) 207.92 (133.77)
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All data presented as mean (SD). T0, Presurgery measurements; T1, postsurgery measurements. *Significantly different within class or group, P \0.05.
30.15 (89.26)
154.4 (4731.5) 20457.0 (8127.8) 20917.0 (8319.4) 459.4 (5059.7) 296.9 (1310.5) 4947.4 (1999.9) 5043.7 (1952.0) 96.8 (1175.3) 493.0 (4457.6) 15513.0 (7022.6) 15852.0 (7313.0) 338.8 (4727.5) 1090.8 (5371.5) 20693.0 (9770.4) 20539.0 (9535.9) 110.1 (994.7) 4656.7 (2256.2) 4953.6 (2279.7) 1194.4 (4906.4) 16037.0 (8304.4) 15544.0 (8306.6)
0.1 (2.6) 0.1 (1.9) 2.02 (5.6) 0.1 (2.9) 0.2 (2.2) 1.3 (2.7) 91.2 (6.0) 27.8 (2.8) 87.6 (7.0) 78.9 (7.2) 60.5 (3.4) 54.7 (6.1) 91.3 (5.7) 27.9 (3.2) 85.5 (9.0) 78.8 (7.7) 60.6 (3.3) 53.4 (6.4) 1.2 (2.5)* 0.3 (2.0) 1.6 (4.1)* 0.2 (2.6) 0.5 (2.0) 1.0 (2.6)* 89.0 (6.0) 27.6 (3.7) 90.7 (8.3) 79.6 (8.1) 60.6 (3.3) 53.2 (7.2) 1.4 (2.0)* 0.4 (1.8) 5.7 (4.2)* 0.4 (3.2) 0.8 (2.3)* 1.7 (2.8)*
90.2 (7.0) 27.8 (2.9) 89.1 (7.5) 79.5 (7.5) 61.0 (3.0) 54.1 (6.8)
T1 T0
T0 T1 Facial linear measurements (mm) A-point horizontal 93.7 (4.4) 92.3 (4.6) A-point vertical 28.1 (2.6) 27.8 (2.7) D-point horizontal 80.2 (6.1) 85.9 (6.2) D-point vertical 78.0 (7.4) 78.4 (7.0) Transverse maxilla 60.7 (3.4) 59.9 (3.7) PNS vertical 53.6 (5.5) 55.3 (5.4) Pharyngeal volumes (mm3) Total pharynx 20214.0 (6133.6) 21305.0 (6970.2) Nasopharynx 5246.5 (1677.1) 5136.3 (1573.9) Oropharynx 14974.0 (5473.0) 16169.0 (6234.8) Most constricted area (mm2) Area 179.51 (90.97) 209.88 (120.63)
T1-T0
T1
T1-T0
T0
Overall Class III Class II
Table II. Distribution of skeletal linear measurements and regional airway volumes before and after surgery according to classification
T1-T0
544
383.9 mm3 of total airway and oropharyngeal volume increases or decreases, respectively. This makes sense, given the number of soft tissue attachments and the close relationship of the mandible to the supporting structures of the airway. Although statistically insignificant, the effect on the most constricted area was positive with this movement and was found to be 3.95 mm2. This needs to be noted because it may have clinical significance when considering the profound inverse relation of the most constricted area and the resistance to flow. The vertical movements of the posterior maxilla also had a significant effect on all regions of the airway. For every millimeter of downward movement of the posterior maxilla, 459.2 mm3 of total airway and 639.7 mm3 of oropharynx were lost. This movement negatively affected the most constricted area by 10.63 mm2. Upward movement results in airway volume and area increases of the same values. Although downward movement of the posterior maxilla was seen to increase nasopharyngeal volume, its negative influence on the total airway should limit this movement as a means of increasing airway volume. These findings suggest that impaction or vertical maintenance of the posterior maxilla coupled with proper horizontal control of the mandible are the keys to successful airway management in all patients. One can predict that if a Class II patient is planned for a large mandibular advancement and the vertical position of the maxilla is controlled, increases in total airway and oropharyngeal volumes should be seen. Similarly, in Class III patients requiring large mandibular setbacks, volume losses will occur unless maxillary movements are used to help offset the decrease. These findings support many claims made in earlier studies about the negative effect of mandibular setback on total airway volume in Class III patients, but also provide an estimate of the amount of volume change per millimeter of setback to guide surgeons in treatment planning. These findings are also parallel with the published information on the pretreatment volumetric values of different skeletal malocclusions and the types of surgical movements needed for their correction.9 By checking for interactions between skeletal classifications, we determined that the preceding measurements could be confidently applied to most surgical patients. However, muscle adaptation takes place during the first 6 months after surgery, and it may limit the ability to accurately apply these recommended guidelines.23 In our study, the mean difference between the 2 time points was 9.1 months, which should have allowed adequate time for muscle adaptation. Additionally, the vertical movement of the anterior mandible may have differing effects on the total airway between the Class
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Table III. Linear regression coefficients (ß*, SE) adjusted for skeletal classification for the association between post-
surgical changes in individual linear measurements with changes in airway volume (n 5 71) Total airway change (mm3) Parameter A-point horizontal A-point vertical D-point horizontal Transverse maxilla PNS vertical
ß* 181.5 525.5 403.6* 453.1 459.2*
SE 267.9 312.2 138.6* 283.1 219.9*
P value 0.5005 0.0969 0.0049* 0.1142 0.0406*
Nasopharynx change (mm3) ß* 9.4 0.5 13.8 57.2 187.2*
SE 61.9 73.4 33.8 66.1 47.1*
P value 0.8801 0.9947 0.6852 0.3901 0.0002*
Oropharynx change (mm3) ß* 165.1 506.6 383.9* 407.1 639.7*
SE 248.2 288.7 127.9* 262.6 195.3*
P value 0.5080 0.0837 0.0038* 0.1256 0.0017*
Area change (mm2) ß* 0.19 5.62 3.95 4.92 10.63*
SE 4.63 5.45 2.49 4.94 3.69*
P value 0.9675 0.3065 0.1171 0.3228 0.0053*
All models presented were adjusted for skeletal class. *Significantly different, P \0.05.
Table IV. Linear regression coefficients stratified by skeletal classification for the association between D-point ver-
tical change and postsurgery airway volume changes Class II Total airway change (mm3) Nasopharynx change (mm3) Oropharynx change (mm3) Area change (mm2)
Parameter D-point vertical D-point vertical D-point vertical D-point vertical
ß 151.4 52.5 100.9 1.12
SE 292.9 53.7 268.1 4.88
Class III P value 0.6087 0.3353 0.7089 0.8201
ß 724.0* 49.4 648.2* 15.45*
SE 284.4* 85.5 270.4* 4.91*
P value 0.0156* 0.5672 0.0221* 0.0034*
*Significantly different, P \0.05.
II and Class III patients. Table IV shows that downward movement of the anterior mandible in the Class III subjects led to a significant reduction in total and oropharyngeal airway volumes and a significant reduction in the most constricted area. No such effect was seen in the Class II subjects. In the evaluation of these results, posterior impaction on Class III patients in this study was not a common approach to prevent upward movement of the PNS. With patients who had lower occlusal planes, and thus lower palatal planes, a more common technique used by our surgeon was to move the PNS inferiorly, out of concern that superior movements would result in a narrowing effect on the nasopharyngeal space and compromise the airway. In general, the Class III patients in this study underwent counterclockwise rotational advancements of the maxillomandibular complex, with the maxillary incisors moving forward, the posterior teeth moving slightly downward, and the mandibular incisors rotating back, but with the chin actually staying the same or moving slightly forward and upward to minimize the impact on the airway. These data have shown that surgical movements may not always be predictable for Class II and Class III patients. If the surgeons wish to predictably alter the airway, this study should serve as a guideline for properly planning the movements and relaying their desires to the orthodontists. Volumetric gains or losses can be estimated
from the surgical plan, and decisions on extraction protocol may be influenced by airway demands. Surgical planning, however, takes into account not only the airway, but also masticatory function, occlusion, and esthetics. The proper management of all 4 variables leads to success. This study and most other studies examining the surgical effect on changes in airway provide quantitative information without addressing the qualitative effects experienced by patients as a result of the change. Poiseuille's law24 for flow rate demonstrates that even a modest increase in the radius of a tube (or an airway) will result in a significant decrease in airway resistance (doubling the radius results in resistance decreased by a factor of 16). Significant changes of the most constricted area were noted for the vertical movements of the posterior maxilla and the anterior mandible, with the latter seen in Class III patients only. What effect this has on patient quality of life needs to be determined, and further work is underway focusing on this aspect of the airway. CONCLUSIONS
Separating patients by skeletal classification showed that predictable movements in the treatment of Class II and Class III patients did not always occur. For all patients, horizontal movements of the mandible were
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found to significantly change total airway volume, with anterior movement increasing and posterior movement decreasing volume. Posterior maxillary vertical movements were found to have significant effects. Downward displacement of the PNS significantly increased nasopharynx volume while significantly decreasing the oropharynx and total airway volumes and the most constricted area. Downward movement of the anterior mandible in the Class III patients led to a decrease in total airway and oropharyngeal volumes as well as the most constricted area; this was not seen in Class II patients.
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ACKNOWLEDGMENT
We thank Jarom Maurer for his kind assistance in gathering information and data. 15.
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