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Volumetric, planar, and linear analyses of pharyngeal airway change on computed tomography and cephalometry after mandibular setback surgery
ORIGINAL ARTICLE
Volumetric, planar, and linear analyses of pharyngeal airway change on computed tomography and cephalometry after mandibular setback surgery Jae-Woo Park,a Nam-Kug Kim,b Jong-Wan Kim,c Myung-Jin Kim,d and Young-Il Change Seoul, Korea Introduction: The purpose of this study was to use lateral cephalometry and computed tomography (CT) to examine the volumetric, planar, and linear changes in the pharyngeal airway after mandibular setback surgery. Methods: The pharyngeal airways of 12 subjects who underwent mandibular setback surgery at Seoul National University Dental Hospital were assessed linearly and volumetrically on lateral cephalometric radiographs and CT before surgery and 6 months after surgery. The pharynx, nasopharynx, and oropharynx were evaluated by volumetric analysis. Pharyngeal depth, airway space, pharyngeal soft-tissue thickness, and hyoid bone position were measured by linear analysis. The axial section area of the airway was measured by area analysis. Results: From the linear analysis, a significant decrease in pharyngeal depth and a significant posterior movement of the hyoid bone (P \0.05) were noted. Volumetric analysis by CT showed that the oropharynx decreased after mandibular setback surgery. However, the volume and the axial section area of the airway in the CT images did not change significantly after mandibular setback surgery (P .0.05). Conclusions: Although the structures around the mandible inevitably moved backward after mandibular setback surgery on linear analysis, physiologic deformation could occur to preserve the airway capacity after sagittal compression. (Am J Orthod Dentofacial Orthop 2010;138:292-9)
T
he relationship between respiration and craniofacial morphology—airway anatomy, nasal flow, and orthognathic surgery—has been the subject of many studies.1-5 Mandibular setback surgery can reduce the volume of the pharyngeal airway because of the positional changes of the tongue.2,6 Pharyngeal airway constriction after surgery might lead to sleep apnea syndrome.6-8 The results from a 12-year postoperative observation suggested continuous narrowing of the airway.2 a
Chairman, Department of Orthodontics, Kooalldam Dental Hospital, Incheon, Korea. Postgraduate student, Department of Industrial Engineering, Seoul National University, Seoul, Korea. c Assistant professor, Department of Orthodontics, Section of Dentistry, Seoul National University Bundang Hospital, Seoul, Korea. d Professor, Department of Oral & Maxillofacial Surgery, School of Dentistry and Dental Research Institute, Seoul National University, Seoul, Korea. e Professor, Department of Orthodontics, School of Dentistry and Dental Research Institute, Seoul National University, Seoul, Korea. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Jong-Wan Kim, Department of Orthodontics, Seoul National University Bundang Hospital, 300 Gumi-dong, Bundang-gu, Sungnam-si, Gyunggi-do, 110-749, South Korea; e-mail, [email protected]. Submitted, March 2009; revised and accepted, October 2009. 0889-5406/$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2009.10.036 b
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Therefore, the positional changes in the tongue after mandibular setback surgery might cause postural adaptation of the tongue to preserve the airway,9 which can influence surgical stability.7 Another study reported that the hyoid bone finally returned to its original position after an immediate backward, downward movement by surgery, and the position of the hyoid bone might be related to the stability of the surgical results.10 On the other hand, Takagi et al11 reported that the size of the oropharyngeal airway did not change after mandibular setback surgery, even though there was downward movement of the hyoid bone. They suggested that physiologic mechanisms to maintain the pharyngeal airway can occur after the posterior location of the tongue through surgery. A long-term study on the changes in the airway after the mandibular setback surgery showed a gradual physiologic readaptation of the pharyngeal airway morphology to its original dimensions,12 even though the lower facial morphology and pharyngeal airway morphology had changed after surgery.13 Because most evaluations of the pharyngeal airway were carried out with cephalometric radiographs, there were some limitations, such as the lack of soft-tissue detail and the difficulty of 3-dimensional (3D) analysis.5,14
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Fig 1. Airway acquisition of the projected images from the CT data with V-works.
Computerized tomography (CT) can provide accurate anatomic information of the soft and hard tissues.15-17 Montgomery et al14 reported that the nasopharynx volume could be calculated by CT rather than with a silicone rubber cast. CT can allow an accurate invivo assessment of the 3D relationship of the facial structures. In this study, we evaluated the volumetric, planar, and linear changes in the pharyngeal airway after mandibular setback surgery on CT and lateral cephalometry, and compared the results from the volumetric and linear data. MATERIAL AND METHODS
Fig 2. Landmarks, reference planes, and measurements for the volumetric analysis on the CT: 1, FH plane; 2, PT point; 3, PNS; 4, C3 (the inferior and anterior point of the third cervical vertebra); 5, plane passing through the right and left PT points and PNS; 6, plane parallel to FH passing through PNS; 7, plane parallel to FH passing through C3 point; 8, nasopharyx, the airway space between planes 5 and 6; 9, oropharynx, the airway space between planes 6 and 7.
Twelve patients (5 men, 7 women; mean age, 25.5 years) who underwent mandibular setback surgery at Seoul National University Dental Hospital in Korea were enrolled in this study. All subjects had a bilateral sagittal split ramus osteotomy for the correction of mandibular prognathism by 1 surgeon (M.J.K.). The facial CT scan and lateral cephalometric radiograph of each subject in the supine position were taken preoperatively within 1 month (T0) and postoperatively about 6 months (T1). The CT scans were made by using a Somatom Plus 4 (Siemens, Erlangen, Germany) with 1.5-mm section intervals, 1.0-mm slice thickness, and 512 3 512
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Fig 3. Procedure for pharyngeal volumetric analysis with the CT images: A, 3D thresholding CT image of the airway and oral cavity; B, nasopharynx, oropharynx, and oral cavity on the sagittal CT image; C, pharynx and oral cavity range before analysis; D, the treated oropharynx image for volumetric analysis.
matrix. The head and neck were supported and fixed with a hard pillow while taking the CT. The resulting 2-dimensional image data were stored in Digital Imaging and Communications in Medicine (DICOM) format. Each subject was scanned from the cranial vault to the chin. Lateral cephalometric radiographs were taken (CX90SP, Ashahi, Tokyo, Japan). The data were analyzed with V-Works software (version 4.0, CyberMed, Seoul, Korea) for CT (Fig 1) and V-Ceph software (version 4.5, CyberMed) for cephalometry. The pharynx is located behind the nasal and oral cavities and the larynx, extending from the cranial base to the level of the sixth cervical vertebra and the lower border of the cricoid cartilage. It can be divided into 3 parts: nasopharynx, oropharynx, and laryngopharynx.18 The nasopharynx is located behind the nasal cavity and above the soft palate. Anteriorly, it is connected to the nasal cavity. Posteriorly, it continues downward to the oropharynx. The oropharynx is the opening into the oral cavity by an isthmus and extends from the second to the fourth cervical vertebrae. The laryngopharynx
joins the oropharynx at the level of the pharyngoepiglottic fold and hyoid, and then continues to the level of the sixth cervical vertebra. For volumetric analysis, the pharynx, nasopharynx, and oropharynx were defined as follows. For the volumetric analysis of the airway on CT, the volume of the pharynx, which consisted of the nasopharynx and the oropharynx, was assessed. The landmarks and planes were defined as the Frankfort horizontal (FH) plane, the most posterior point of the pterygomaxillary fissure (PT point),19 posterior nasal spine (PNS), and the third cervical vertebra (C3)20 (Fig 2). The nasopharyx was defined as the airway space between a plane parallel to FH passing through PNS and a plane passing through the right and left PT points, and the PNS. The oropharynx was defined as the airway space between a plane parallel to FH passing through PNS and a plane parallel to FH passing through C3. The pharynx was defined as the sum of the nasopharynx and theoropharynx. V-works software was used to reconstruct the airway space from the CT slice images. The airway volume in the area of interest was calculated
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Fig 4. Landmarks, planes, and measurements for pharyngeal linear analysis on the cephalometric radiographs. A, Landmarks and reference planes for linear analysis: 1, FH plane; 2, porion (Por); 3, posterior reference line (PRL) (FH-perpendicular plane passing through Por); 4, PNS; 5, most posterior point of soft palate (PSP); 6, PSP plane (FH-parallel plane passing through PSP); 7, PSP0 (intersecting point of the posterior pharyngeal wall [PPW] and PSP plane); 8, PSP00 (intersecting point of PRL and the PSP plane); 9, intersecting point of the mandibular inferior border and posterior area of the tongue (PTO); 10, PTO plane (FH-parallel plane passing through PTO); 11, PTO0 (intersecting point of the PPW and PTO plane); 12, PTO00 (intersecting point of the PRL and PTO plane). B, Measurements for linear analysis: 1, soft palate length (distance PNS-PSP); 2, middle airway space (distance PSP-PSP0 ); 3, middle pharyngeal depth (distance PSP-PSP00 ); 4, middle soft-tissue thickness (distance PSP0 -PSP00 ); 5, inferior airway space (distance PTO-PTO0 ); 6, inferior pharyngeal depth (distance PTO-PTO00 ); 7, inferior soft-tissue thickness (distance PTO0 -PTO00 ).
from the number of voxels ranging from the minimum value (–1024 HU) to –600 HU to exclude the soft and hard tissues (Fig 3, A).21-23 The airway selected by the threshold value was divided into the planes used to the define the airway spaces (Fig 3, B and C). The volume of each part was measured by exporting the divided spaces for volumetric analysis to the surface on demand model (Fig 3, D). Linear analysis of the pharyngeal airway morphology on the lateral cephalometric radiographs was based on the method of Pae et al24 and Saitoh.13 The landmarks and reference planes for linear analysis are shown in Figure 4, A. The following 7 measurements were defined for linear analysis on the cephalometric radiograph: soft palate length, middle pharyngeal depth, middle airway space, middle soft-tissue thickness, inferior pharyngeal depth, inferior airway space, and inferior soft-tissue thickness (Fig 4, B). The position of the hyoid bone was analyzed by CT to determine the effects of mandibular setback
surgery. The landmarks, planes, and measurements for the linear analysis of the hyoid bone on CT are shown in Figure 5. The axial section area of the airway was analyzed by CT to evaluate the changes. For area analysis of the axial section on CT, the landmarks and planes are shown in Figure 6. The measurements were defined as the area on the PNS (area of the airway on the PNS plane), area on C2 (area of the airway on the C2 plane), and area on C3 (area of the airway on the C3 plane). Statistical analysis
The airway volume was calculated twice 3 weeks apart to evaluate the reliability of the measuring method. The surgical changes in the pharyngeal airway and hyoid bone position were evaluated by comparing the volumetric and linear measurements and the axial section area of the airway at T0 and T1 by using a paired t test. P \0.05 was considered significant.
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Fig 5. Landmarks, planes, and measurements for the linear analysis of the hyoid bone position on CT: 1, sella; 2, sella line (SL) (FH-perpendicular plane passing through sella); 3, FH plane; 4, H point (the most anterior and superior point on the hyoid bone body); 5, GH point (midpoint between both ends of the greater horn of the hyoid bone); 6, H-X (distance H-SL); 7, H-Y (distance H-FH plane); 8, GH-X (distance GH-SL); 9, GH-Y (distance GH-FH plane). H-X and GH-X were defined as positive when anterior to the SL and as negative when posterior to the SL.
RESULTS
The reliability of the method for measuring the airway volume was high, with an error variance from 0.22% to 1.04% of the total mean value (Table I). The errors in the estimation ranged from 78.15 to 157.40 mm3. The results of the volumetric analysis from the CT data showed no significant difference between the T0 and T1 images of the nasopharynx, oropharynx, and pharynx (P .0.05; Table II). The volume of the pharynx, including the nasopharynx and oropharynx, changed from 17.63 3 103 mm3 at T0 to 16.09 3 103 mm3 at T1. There was little change in the nasopharynx volume: 6.70 3 103 mm3 and 6.69 3 103 mm3 at T0 and T1, respectively. The oropharynx volume decreased from 10.92 3 103 mm3 to 9.40 3 103 mm3. Linear analysis of the cephalometric radiographs showed decreases in all measurements after surgery except for soft-palate length (Table III). Some measurements decreased significantly from T0 to T1, such as middle pharyngeal depth (P \0.01) and inferior pharyngeal depth
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Fig 6. Landmarks and planes for the axial section of the airway: 1, FH plane; 2, PNS; 3, C2 (inferior and anterior point of the second cervical vertebra); 4, C3 (inferior and anterior point of the third cervical vertebra); 5, PNS plane (FH-parallel plane passing through PNS); 6, C2 plane (FH-parallel plane passing through C2); 6, C3 plane (FH-parallel plane passing through C3).
Table I. Reliability of the methods for the volumetric measurements of airway morphology in CT Measurement
Difference, mean 6 SD (mm3)
SE (mm3)
C (%)
Nasopharynx Oropharynx Pharynx
166.15 6 121.09 171.05 6 145.52 88.65 6 67.41
144.32 157.40 78.15
1.04 0.75 0.22
Difference, Absolute difference between double measurements; SE, standard error; C, error variance as a percentage of the total mean (SE 3 100/[mean 1 1 mean 2]).5
(P \0.05). Although the airway space (middle and inferior) and soft-tissue thickness (middle and inferior) were reduced, they showed no significant change (P .0.05). The position of the hyoid bone in the CT images changed posteriorly (Table IV). The horizontal position of the hyoid bone body and the horizontal position of the greater horn of the hyoid bone decreased significantly (P \0.05), but there were no significant changes in their vertical positions (P .0.05). Although the axial section areas of the airway (areas on PNS, C2, and C3) were reduced, there was no significant difference between T0 and T1 (P .0.05) (Table V).
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Table II.
Volumetric changes in the airway on the CT T0
T1
Measurement
Mean 6 SD (1 5 1000 mm3)
Mean 6 SD (1 5 1000 mm3)
Sig
Nasopharynx Oropharynx Pharynx
6.70 6 1.86 10.92 6 3.15 17.63 6 3.86
6.69 6 2.17 9.40 6 3.09 16.09 6 4.54
NS NS NS
Table III. Linear measurements of airway morphology in the cephalometric radiographs
Measurement Soft-palate length Middle pharyngeal depth Middle airway space Middle soft-tissue thickness Inferior pharyngeal depth Inferior airway space Inferior soft-tissue thickness
T1
Mean 6 SD (mm) Mean 6 SD (mm) Sig 35.55 6 3.51 26.36 6 5.07 15.15 6 3.79 12.59 6 7.13 24.64 6 4.70 14.15 6 3.51 11.77 6 7.20
36.45 6 4.41 23.95 6 4.38 13.25 6 3.49 11.91 6 7.53 23.05 6 4.53 13.50 6 3.59 10.77 6 7.06
Position of the hyoid bone in CT
Measurement
Sig, Significance of paired t test; NS, not significant (P .0.05).
T0
Table IV.
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HB-X HB-Y GH-X GH-Y
T0
T1
Mean 6 SD (mm)
Mean 6 SD (mm)
Sig
23.00 6 8.62 94.50 6 11.07 –14.77 6 5.55 81.86 6 8.99
19.27 6 9.31 95.36 6 10.55 –18.73 6 7.47 82.64 6 9.17
* NS * NS
Sig, Significance of paired t test; NS, not significant (P .0.05); *P \0.05. H-X, Horizontal position of the hyoid bone body (distance H-SL); H-Y, vertical position of the hyoid bone body (distance H-FH plane); GH-X, horizontal position of the greater horn of hyoid bone (distance GHSL); GH-Y, vertical position of the greater horn of the hyoid bone (distance GH-FH plance). H-X and GH-X were defined as positive when anterior to the SL and as negative when posterior to the SL.
NS †
NS NS * NS NS
Axial section area, horizontal to the FH plane at the level of PNS, C2, and C3 in the CT
Table V.
T0
T1
Measurement
Mean 6 SD (mm)
Mean 6 SD (mm)
Sig
Sig, Significance of paired t test; NS, not significant (P .0.05); *P \0.05; †P \0.01.
Area on PNS Area on C2 Area on C3
339.58 6 68.27 232.95 6 65.71 231.58 6 54.53
316.14 6 110.33 206.31 6 69.78 189.92 6 76.18
NS NS NS
DISCUSSION
Sig, Significance of paired t test; NS, not significant (P .0.05).
The reliability of the method used to measure the airway volume was high. The errors in the estimation ranged from 78.15 to 157.40 mm3, and the error variances were from 0.22% to 1.04% of the total mean value. A comparison of these values with the airway volume showed low error in the airway calculation. CT can provide reliable data for analyzing the airway. Mandibular setback surgery can affect cranial posture, and a changed cranial posture might affect the hyoid bone position. However, Bibby and Preston20 and Bibby25 reported that the hyoid triangle was maintained regardless of cranial posture, and the position of the hyoid bone was independent of cranial posture. Therefore, a change in hyoid bone position can only indicate the effects of posterior movement of the mandible, not a change in cranial posture. Some studies suggested that there is constriction of the airway after mandibular setback surgery.6,7 In this study, the results from linear analysis on cephalometry showed posterior movement of the oral structures: soft palate, tongue, and hyoid bone (Table III). Although middle and inferior airway spaces, and middle and inferior soft-tissue thicknesses on the cephalometric analysis did not change significantly from T0 to T1, the measurements decreased after the mandibular setback surgery.
The significant decreases in middle and inferior pharyngeal depths meant that the soft palate and tongue were positioned posteriorly by mandibular repositioning. In the linear analysis on CT, the hyoid bone was moved posteriorly by approximately 4 mm (Table IV). These results suggest that mandibular setback can affect the positions of the soft palate, tongue, and hyoid bone. The hyoid bone can affect the pharynx after mandibular setback surgery via muscle attachments.2 However, middle and inferior airway spaces and middle and inferior soft-tissue thicknesses showed no significant differences because the amounts of reduced middle and inferior pharyngeal depths were dispersed to the airway space and soft-tissue thickness. The results from volumetric analysis on CT showed that the airway volume did not change significantly after mandibular setback surgery, even though there was a small decrease in each measurement with the exception of the nasopharynx (Table II). This suggests that the airway might transform to preserve the airway capacity in the changed environment after mandibular setback surgery. The airway volume could be conserved with some deformation capability.26,27 It appears that the absolute volume of the airway might be not be
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Fig 7. Axial sections of the airway in the CT images: A, axial section image at T0 showed that the airway was a circle; B, at T1, the airway was constricted sagittally with lateral expansion.
reduced significantly by mandibular setback surgery according to CT. This suggests that the airway might be deformed both anteroposteriorly and laterally to preserve its volume. Although volumetric analysis in the CT showed no significant difference between T0 and T1 (P .0.05), the oropharynx had a tendency to decrease after mandibular setback (Table II). A slight decrease in the oropharynx suggests that mandibular setback surgery might affect the lower part, such as the oropharynx. Linear analysis showed changes in the lower part, such as decreased pharyngeal depth and distal movement of the hyoid bone. The axial section areas decreased after mandibular setback surgery (Table V). However, there was no significant difference in axial section area of the airway between T0 and T1. Distal movement of the posterior structures after mandibular setback surgery appears to be inevitable. However, the axial section views at T0 and T1 showed that the airway was constricted in the sagittal direction and expanded laterally (Fig 7). These results suggest the possibility of other physiologic deformations of the airway to preserve the airway volume under sagittal compression of the airway. There were some limitations in this study, such as the small sample size and accurate measurements of the images. Cone-beam CT can provide a short taking time and reduce the breathing effects on airway volume. In addition, the calibrating apparatus, such as airway phantoms, could make the airway volume more accurate and more valid.28 Other methods, such as magnetic resonance imaging, can be effective in analyzing the changes in soft tissues.29
CONCLUSIONS
The structures around the mandible inevitably moved posteriorly after mandibular setback surgery according to linear analysis. However, according to volumetric analysis, the airway was not reduced significantly by mandibular setback surgery. It is possible that physiologic deformation occurs to preserve the airway capacity after sagittal compression. REFERENCES 1. Ceylan I, Oktay H. A study on the pharyngeal size in different skeletal patterns. Am J Orthod Dentofacial Orthop 1995;108: 69-75. 2. Eggensperger N, Smolka W, Iizuka T. Long-term changes of hyoid bone position and pharyngeal airway size following mandibular setback by sagittal split ramus osteotomy. J Craniomaxillofac Surg 2005;33:111-7. 3. Gross AM, Kellum GD, Morris T, Franz D, Michas C, Foster ML, et al. Rhinometry and open-mouth posture in young children. Am J Orthod Dentofacial Orthop 1993;103:526-9. 4. Hellsing E. Changes in the pharyngeal airway in relation to extension of the head. Eur J Orthod 1989;11:359-65. 5. Kluemper GT, Vig PS, Vig KW. Nasorespiratory characteristics and craniofacial morphology. Eur J Orthod 1995;17:491-5. 6. Hochban W, Schu¨rmann R, Brandenburg U, Conradt R. Mandibular setback for surgical correction of mandibular hyperplasia— does it provoke sleep-related breathing disorders? Int J Oral Maxillofac Surg 1996;25:333-8. 7. Tselnik M, Pogrel MA. Assessment of the pharyngeal airway space after mandibular setback surgery. J Oral Maxillofac Surg 2000;58:282-5. 8. Riley RW, Powell NB, Guilleminault C, Ware W. Obstructive sleep apnea syndrome following surgery for mandibular prognathism. J Oral Maxillofac Surg 1987;45:450-2. 9. Chen F, Terada K, Hua Y, Saito I. Effects of bimaxillary surgery and mandibular setback surgery on pharyngeal airway
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measurements in patients with Class III skeletal deformities. Am J Orthod Dentofacial Orthop 2007;131:372-7. Wickwire NA, White RP Jr, Proffit WR. The effect of mandibular osteotomy on tongue position. J Oral Surg 1972;30:184-90. Takagi Y, Gamble JW, Proffit WR, Christensen RL. Postural change of the hyoid bone following osteotomy of the mandible. Oral Surg Oral Med Oral Pathol 1967;23:688-92. Athanasiou AE, Toutountzakis N, Mavreas D, Ritzau M, Wenzel A. Alterations of hyoid bone position and pharyngeal depth and their relationship after surgical correction of mandibular prognathism. Am J Orthod Dentofacial Orthop 1991;100:259-65. Saitoh K. Long-term changes in pharyngeal airway morphology after mandibular setback surgery. Am J Orthod Dentofacial Orthop 2004;125:556-61. Montgomery WM, Vig PS, Staab EV, Matteson SR. Computed tomography: a three-dimensional study of the nasal airway. Am J Orthod 1979;76:363-75. Lipton MJ, Hayashi TT, Boyd D, Carlsson E. Measurement of left ventricular cast volume by computed tomography. Radiology 1978;127:419-23. Miller SW, Dinsmore RE, Wittenberg J, Maturi RA, Powell WJ Jr. Right and left ventricular volumes and wall measurements: determination by computed tomography in arrested canine hearts. Am J Roentgenol 1977;129:257-61. Ryan CF, Lowe AA, Li D, Fleetham JA. Three-dimensional upper airway computed tomography in obstructive sleep apnea. A perspective study in patients treated by uvulopalatopharyngoplasty. Am Rev Respir Dis 1991;144:428-32. . Willans PL, Warwick R, Dyson M, Bannister LH, editors. Gray’s anatomy. 37th ed. Edinburgh, United Kingdom: Churchill Livingstone; 1989. p. 1323-5. Ricketts RM. Perspectives in the clinical application of cephalometrics. The first fifty years. Angle Orthod 1981;51:115-50.
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20. Bibby RE, Preston CB. The hyoid triangle. Am J Orthod 1981;80: 92-7. 21. Glover GH, Pelc NJ. Nonlinear partial volume artifacts in x-ray computed tomography. Med Phys 1980;7:238-48. 22. Kragskov J, Bosch C, Gyldensted C, Sindet-Pedersen S. Comparison of the reliability of craniofacial anatomic landmarks based on cephalometric radiographs and three-dimensional CT scans. Cleft Palate Craniofac J 1997;34:111-6. 23. Shigeta Y, Ogawa T, Venturin J, Nguyen M, Clark GT, Enciso R. Gender- and age-based differences in computerized tomographic measurements of the orophaynx. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;106:563-70. 24. Pae EK, Lowe AA, Sasaki K, Price C, Tsuchiya M, Fleetham JA. A cephalometric and electromyographic study of upper airway structures in the upright and supine positions. Am J Orthod Dentofacial Orthop 1994;106:52-9. 25. Bibby RE. The hyoid bone position in mouth breathers and tongue-thrusters. Am J Orthod 1984;85:431-3. 26. Liukkonen M, Va¨ha¨talo K, Peltoma¨ki T, Tiekso J, Happonen RP. Effect of mandibular setback surgery on the posterior airway size. Int J Adult Orthod Orthognath Surg 2002;17:41-6. 27. Winnberg A, Pancherz H, Westesson PL. Head posture and hyomandibular function in man. A synchronized electromyographic and videofluorographic study of the open-close-clench cycle. Am J Orthod Dentofacial Orthop 1988;94:393-404. 28. 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. 29. Zhao X, Liu Y, Gao Y. Three-dimensional upper-airway changes associated with various amounts of mandibular advancement in awake apnea patients. Am J Orthod Dentofacial Orthop 2008; 133:661-8.
Volumetric, planar, and linear analyses of pharyngeal airway change on computed tomography and cephalometry after mandibular setback surgery Jae-Woo Park,a Nam-Kug Kim,b Jong-Wan Kim,c Myung-Jin Kim,d and Young-Il Change Seoul, Korea Introduction: The purpose of this study was to use lateral cephalometry and computed tomography (CT) to examine the volumetric, planar, and linear changes in the pharyngeal airway after mandibular setback surgery. Methods: The pharyngeal airways of 12 subjects who underwent mandibular setback surgery at Seoul National University Dental Hospital were assessed linearly and volumetrically on lateral cephalometric radiographs and CT before surgery and 6 months after surgery. The pharynx, nasopharynx, and oropharynx were evaluated by volumetric analysis. Pharyngeal depth, airway space, pharyngeal soft-tissue thickness, and hyoid bone position were measured by linear analysis. The axial section area of the airway was measured by area analysis. Results: From the linear analysis, a significant decrease in pharyngeal depth and a significant posterior movement of the hyoid bone (P \0.05) were noted. Volumetric analysis by CT showed that the oropharynx decreased after mandibular setback surgery. However, the volume and the axial section area of the airway in the CT images did not change significantly after mandibular setback surgery (P .0.05). Conclusions: Although the structures around the mandible inevitably moved backward after mandibular setback surgery on linear analysis, physiologic deformation could occur to preserve the airway capacity after sagittal compression. (Am J Orthod Dentofacial Orthop 2010;138:292-9)
T
he relationship between respiration and craniofacial morphology—airway anatomy, nasal flow, and orthognathic surgery—has been the subject of many studies.1-5 Mandibular setback surgery can reduce the volume of the pharyngeal airway because of the positional changes of the tongue.2,6 Pharyngeal airway constriction after surgery might lead to sleep apnea syndrome.6-8 The results from a 12-year postoperative observation suggested continuous narrowing of the airway.2 a
Chairman, Department of Orthodontics, Kooalldam Dental Hospital, Incheon, Korea. Postgraduate student, Department of Industrial Engineering, Seoul National University, Seoul, Korea. c Assistant professor, Department of Orthodontics, Section of Dentistry, Seoul National University Bundang Hospital, Seoul, Korea. d Professor, Department of Oral & Maxillofacial Surgery, School of Dentistry and Dental Research Institute, Seoul National University, Seoul, Korea. e Professor, Department of Orthodontics, School of Dentistry and Dental Research Institute, Seoul National University, Seoul, Korea. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: Jong-Wan Kim, Department of Orthodontics, Seoul National University Bundang Hospital, 300 Gumi-dong, Bundang-gu, Sungnam-si, Gyunggi-do, 110-749, South Korea; e-mail, [email protected]. Submitted, March 2009; revised and accepted, October 2009. 0889-5406/$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2009.10.036 b
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Therefore, the positional changes in the tongue after mandibular setback surgery might cause postural adaptation of the tongue to preserve the airway,9 which can influence surgical stability.7 Another study reported that the hyoid bone finally returned to its original position after an immediate backward, downward movement by surgery, and the position of the hyoid bone might be related to the stability of the surgical results.10 On the other hand, Takagi et al11 reported that the size of the oropharyngeal airway did not change after mandibular setback surgery, even though there was downward movement of the hyoid bone. They suggested that physiologic mechanisms to maintain the pharyngeal airway can occur after the posterior location of the tongue through surgery. A long-term study on the changes in the airway after the mandibular setback surgery showed a gradual physiologic readaptation of the pharyngeal airway morphology to its original dimensions,12 even though the lower facial morphology and pharyngeal airway morphology had changed after surgery.13 Because most evaluations of the pharyngeal airway were carried out with cephalometric radiographs, there were some limitations, such as the lack of soft-tissue detail and the difficulty of 3-dimensional (3D) analysis.5,14
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Fig 1. Airway acquisition of the projected images from the CT data with V-works.
Computerized tomography (CT) can provide accurate anatomic information of the soft and hard tissues.15-17 Montgomery et al14 reported that the nasopharynx volume could be calculated by CT rather than with a silicone rubber cast. CT can allow an accurate invivo assessment of the 3D relationship of the facial structures. In this study, we evaluated the volumetric, planar, and linear changes in the pharyngeal airway after mandibular setback surgery on CT and lateral cephalometry, and compared the results from the volumetric and linear data. MATERIAL AND METHODS
Fig 2. Landmarks, reference planes, and measurements for the volumetric analysis on the CT: 1, FH plane; 2, PT point; 3, PNS; 4, C3 (the inferior and anterior point of the third cervical vertebra); 5, plane passing through the right and left PT points and PNS; 6, plane parallel to FH passing through PNS; 7, plane parallel to FH passing through C3 point; 8, nasopharyx, the airway space between planes 5 and 6; 9, oropharynx, the airway space between planes 6 and 7.
Twelve patients (5 men, 7 women; mean age, 25.5 years) who underwent mandibular setback surgery at Seoul National University Dental Hospital in Korea were enrolled in this study. All subjects had a bilateral sagittal split ramus osteotomy for the correction of mandibular prognathism by 1 surgeon (M.J.K.). The facial CT scan and lateral cephalometric radiograph of each subject in the supine position were taken preoperatively within 1 month (T0) and postoperatively about 6 months (T1). The CT scans were made by using a Somatom Plus 4 (Siemens, Erlangen, Germany) with 1.5-mm section intervals, 1.0-mm slice thickness, and 512 3 512
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Fig 3. Procedure for pharyngeal volumetric analysis with the CT images: A, 3D thresholding CT image of the airway and oral cavity; B, nasopharynx, oropharynx, and oral cavity on the sagittal CT image; C, pharynx and oral cavity range before analysis; D, the treated oropharynx image for volumetric analysis.
matrix. The head and neck were supported and fixed with a hard pillow while taking the CT. The resulting 2-dimensional image data were stored in Digital Imaging and Communications in Medicine (DICOM) format. Each subject was scanned from the cranial vault to the chin. Lateral cephalometric radiographs were taken (CX90SP, Ashahi, Tokyo, Japan). The data were analyzed with V-Works software (version 4.0, CyberMed, Seoul, Korea) for CT (Fig 1) and V-Ceph software (version 4.5, CyberMed) for cephalometry. The pharynx is located behind the nasal and oral cavities and the larynx, extending from the cranial base to the level of the sixth cervical vertebra and the lower border of the cricoid cartilage. It can be divided into 3 parts: nasopharynx, oropharynx, and laryngopharynx.18 The nasopharynx is located behind the nasal cavity and above the soft palate. Anteriorly, it is connected to the nasal cavity. Posteriorly, it continues downward to the oropharynx. The oropharynx is the opening into the oral cavity by an isthmus and extends from the second to the fourth cervical vertebrae. The laryngopharynx
joins the oropharynx at the level of the pharyngoepiglottic fold and hyoid, and then continues to the level of the sixth cervical vertebra. For volumetric analysis, the pharynx, nasopharynx, and oropharynx were defined as follows. For the volumetric analysis of the airway on CT, the volume of the pharynx, which consisted of the nasopharynx and the oropharynx, was assessed. The landmarks and planes were defined as the Frankfort horizontal (FH) plane, the most posterior point of the pterygomaxillary fissure (PT point),19 posterior nasal spine (PNS), and the third cervical vertebra (C3)20 (Fig 2). The nasopharyx was defined as the airway space between a plane parallel to FH passing through PNS and a plane passing through the right and left PT points, and the PNS. The oropharynx was defined as the airway space between a plane parallel to FH passing through PNS and a plane parallel to FH passing through C3. The pharynx was defined as the sum of the nasopharynx and theoropharynx. V-works software was used to reconstruct the airway space from the CT slice images. The airway volume in the area of interest was calculated
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Fig 4. Landmarks, planes, and measurements for pharyngeal linear analysis on the cephalometric radiographs. A, Landmarks and reference planes for linear analysis: 1, FH plane; 2, porion (Por); 3, posterior reference line (PRL) (FH-perpendicular plane passing through Por); 4, PNS; 5, most posterior point of soft palate (PSP); 6, PSP plane (FH-parallel plane passing through PSP); 7, PSP0 (intersecting point of the posterior pharyngeal wall [PPW] and PSP plane); 8, PSP00 (intersecting point of PRL and the PSP plane); 9, intersecting point of the mandibular inferior border and posterior area of the tongue (PTO); 10, PTO plane (FH-parallel plane passing through PTO); 11, PTO0 (intersecting point of the PPW and PTO plane); 12, PTO00 (intersecting point of the PRL and PTO plane). B, Measurements for linear analysis: 1, soft palate length (distance PNS-PSP); 2, middle airway space (distance PSP-PSP0 ); 3, middle pharyngeal depth (distance PSP-PSP00 ); 4, middle soft-tissue thickness (distance PSP0 -PSP00 ); 5, inferior airway space (distance PTO-PTO0 ); 6, inferior pharyngeal depth (distance PTO-PTO00 ); 7, inferior soft-tissue thickness (distance PTO0 -PTO00 ).
from the number of voxels ranging from the minimum value (–1024 HU) to –600 HU to exclude the soft and hard tissues (Fig 3, A).21-23 The airway selected by the threshold value was divided into the planes used to the define the airway spaces (Fig 3, B and C). The volume of each part was measured by exporting the divided spaces for volumetric analysis to the surface on demand model (Fig 3, D). Linear analysis of the pharyngeal airway morphology on the lateral cephalometric radiographs was based on the method of Pae et al24 and Saitoh.13 The landmarks and reference planes for linear analysis are shown in Figure 4, A. The following 7 measurements were defined for linear analysis on the cephalometric radiograph: soft palate length, middle pharyngeal depth, middle airway space, middle soft-tissue thickness, inferior pharyngeal depth, inferior airway space, and inferior soft-tissue thickness (Fig 4, B). The position of the hyoid bone was analyzed by CT to determine the effects of mandibular setback
surgery. The landmarks, planes, and measurements for the linear analysis of the hyoid bone on CT are shown in Figure 5. The axial section area of the airway was analyzed by CT to evaluate the changes. For area analysis of the axial section on CT, the landmarks and planes are shown in Figure 6. The measurements were defined as the area on the PNS (area of the airway on the PNS plane), area on C2 (area of the airway on the C2 plane), and area on C3 (area of the airway on the C3 plane). Statistical analysis
The airway volume was calculated twice 3 weeks apart to evaluate the reliability of the measuring method. The surgical changes in the pharyngeal airway and hyoid bone position were evaluated by comparing the volumetric and linear measurements and the axial section area of the airway at T0 and T1 by using a paired t test. P \0.05 was considered significant.
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Fig 5. Landmarks, planes, and measurements for the linear analysis of the hyoid bone position on CT: 1, sella; 2, sella line (SL) (FH-perpendicular plane passing through sella); 3, FH plane; 4, H point (the most anterior and superior point on the hyoid bone body); 5, GH point (midpoint between both ends of the greater horn of the hyoid bone); 6, H-X (distance H-SL); 7, H-Y (distance H-FH plane); 8, GH-X (distance GH-SL); 9, GH-Y (distance GH-FH plane). H-X and GH-X were defined as positive when anterior to the SL and as negative when posterior to the SL.
RESULTS
The reliability of the method for measuring the airway volume was high, with an error variance from 0.22% to 1.04% of the total mean value (Table I). The errors in the estimation ranged from 78.15 to 157.40 mm3. The results of the volumetric analysis from the CT data showed no significant difference between the T0 and T1 images of the nasopharynx, oropharynx, and pharynx (P .0.05; Table II). The volume of the pharynx, including the nasopharynx and oropharynx, changed from 17.63 3 103 mm3 at T0 to 16.09 3 103 mm3 at T1. There was little change in the nasopharynx volume: 6.70 3 103 mm3 and 6.69 3 103 mm3 at T0 and T1, respectively. The oropharynx volume decreased from 10.92 3 103 mm3 to 9.40 3 103 mm3. Linear analysis of the cephalometric radiographs showed decreases in all measurements after surgery except for soft-palate length (Table III). Some measurements decreased significantly from T0 to T1, such as middle pharyngeal depth (P \0.01) and inferior pharyngeal depth
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Fig 6. Landmarks and planes for the axial section of the airway: 1, FH plane; 2, PNS; 3, C2 (inferior and anterior point of the second cervical vertebra); 4, C3 (inferior and anterior point of the third cervical vertebra); 5, PNS plane (FH-parallel plane passing through PNS); 6, C2 plane (FH-parallel plane passing through C2); 6, C3 plane (FH-parallel plane passing through C3).
Table I. Reliability of the methods for the volumetric measurements of airway morphology in CT Measurement
Difference, mean 6 SD (mm3)
SE (mm3)
C (%)
Nasopharynx Oropharynx Pharynx
166.15 6 121.09 171.05 6 145.52 88.65 6 67.41
144.32 157.40 78.15
1.04 0.75 0.22
Difference, Absolute difference between double measurements; SE, standard error; C, error variance as a percentage of the total mean (SE 3 100/[mean 1 1 mean 2]).5
(P \0.05). Although the airway space (middle and inferior) and soft-tissue thickness (middle and inferior) were reduced, they showed no significant change (P .0.05). The position of the hyoid bone in the CT images changed posteriorly (Table IV). The horizontal position of the hyoid bone body and the horizontal position of the greater horn of the hyoid bone decreased significantly (P \0.05), but there were no significant changes in their vertical positions (P .0.05). Although the axial section areas of the airway (areas on PNS, C2, and C3) were reduced, there was no significant difference between T0 and T1 (P .0.05) (Table V).
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Table II.
Volumetric changes in the airway on the CT T0
T1
Measurement
Mean 6 SD (1 5 1000 mm3)
Mean 6 SD (1 5 1000 mm3)
Sig
Nasopharynx Oropharynx Pharynx
6.70 6 1.86 10.92 6 3.15 17.63 6 3.86
6.69 6 2.17 9.40 6 3.09 16.09 6 4.54
NS NS NS
Table III. Linear measurements of airway morphology in the cephalometric radiographs
Measurement Soft-palate length Middle pharyngeal depth Middle airway space Middle soft-tissue thickness Inferior pharyngeal depth Inferior airway space Inferior soft-tissue thickness
T1
Mean 6 SD (mm) Mean 6 SD (mm) Sig 35.55 6 3.51 26.36 6 5.07 15.15 6 3.79 12.59 6 7.13 24.64 6 4.70 14.15 6 3.51 11.77 6 7.20
36.45 6 4.41 23.95 6 4.38 13.25 6 3.49 11.91 6 7.53 23.05 6 4.53 13.50 6 3.59 10.77 6 7.06
Position of the hyoid bone in CT
Measurement
Sig, Significance of paired t test; NS, not significant (P .0.05).
T0
Table IV.
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HB-X HB-Y GH-X GH-Y
T0
T1
Mean 6 SD (mm)
Mean 6 SD (mm)
Sig
23.00 6 8.62 94.50 6 11.07 –14.77 6 5.55 81.86 6 8.99
19.27 6 9.31 95.36 6 10.55 –18.73 6 7.47 82.64 6 9.17
* NS * NS
Sig, Significance of paired t test; NS, not significant (P .0.05); *P \0.05. H-X, Horizontal position of the hyoid bone body (distance H-SL); H-Y, vertical position of the hyoid bone body (distance H-FH plane); GH-X, horizontal position of the greater horn of hyoid bone (distance GHSL); GH-Y, vertical position of the greater horn of the hyoid bone (distance GH-FH plance). H-X and GH-X were defined as positive when anterior to the SL and as negative when posterior to the SL.
NS †
NS NS * NS NS
Axial section area, horizontal to the FH plane at the level of PNS, C2, and C3 in the CT
Table V.
T0
T1
Measurement
Mean 6 SD (mm)
Mean 6 SD (mm)
Sig
Sig, Significance of paired t test; NS, not significant (P .0.05); *P \0.05; †P \0.01.
Area on PNS Area on C2 Area on C3
339.58 6 68.27 232.95 6 65.71 231.58 6 54.53
316.14 6 110.33 206.31 6 69.78 189.92 6 76.18
NS NS NS
DISCUSSION
Sig, Significance of paired t test; NS, not significant (P .0.05).
The reliability of the method used to measure the airway volume was high. The errors in the estimation ranged from 78.15 to 157.40 mm3, and the error variances were from 0.22% to 1.04% of the total mean value. A comparison of these values with the airway volume showed low error in the airway calculation. CT can provide reliable data for analyzing the airway. Mandibular setback surgery can affect cranial posture, and a changed cranial posture might affect the hyoid bone position. However, Bibby and Preston20 and Bibby25 reported that the hyoid triangle was maintained regardless of cranial posture, and the position of the hyoid bone was independent of cranial posture. Therefore, a change in hyoid bone position can only indicate the effects of posterior movement of the mandible, not a change in cranial posture. Some studies suggested that there is constriction of the airway after mandibular setback surgery.6,7 In this study, the results from linear analysis on cephalometry showed posterior movement of the oral structures: soft palate, tongue, and hyoid bone (Table III). Although middle and inferior airway spaces, and middle and inferior soft-tissue thicknesses on the cephalometric analysis did not change significantly from T0 to T1, the measurements decreased after the mandibular setback surgery.
The significant decreases in middle and inferior pharyngeal depths meant that the soft palate and tongue were positioned posteriorly by mandibular repositioning. In the linear analysis on CT, the hyoid bone was moved posteriorly by approximately 4 mm (Table IV). These results suggest that mandibular setback can affect the positions of the soft palate, tongue, and hyoid bone. The hyoid bone can affect the pharynx after mandibular setback surgery via muscle attachments.2 However, middle and inferior airway spaces and middle and inferior soft-tissue thicknesses showed no significant differences because the amounts of reduced middle and inferior pharyngeal depths were dispersed to the airway space and soft-tissue thickness. The results from volumetric analysis on CT showed that the airway volume did not change significantly after mandibular setback surgery, even though there was a small decrease in each measurement with the exception of the nasopharynx (Table II). This suggests that the airway might transform to preserve the airway capacity in the changed environment after mandibular setback surgery. The airway volume could be conserved with some deformation capability.26,27 It appears that the absolute volume of the airway might be not be
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Fig 7. Axial sections of the airway in the CT images: A, axial section image at T0 showed that the airway was a circle; B, at T1, the airway was constricted sagittally with lateral expansion.
reduced significantly by mandibular setback surgery according to CT. This suggests that the airway might be deformed both anteroposteriorly and laterally to preserve its volume. Although volumetric analysis in the CT showed no significant difference between T0 and T1 (P .0.05), the oropharynx had a tendency to decrease after mandibular setback (Table II). A slight decrease in the oropharynx suggests that mandibular setback surgery might affect the lower part, such as the oropharynx. Linear analysis showed changes in the lower part, such as decreased pharyngeal depth and distal movement of the hyoid bone. The axial section areas decreased after mandibular setback surgery (Table V). However, there was no significant difference in axial section area of the airway between T0 and T1. Distal movement of the posterior structures after mandibular setback surgery appears to be inevitable. However, the axial section views at T0 and T1 showed that the airway was constricted in the sagittal direction and expanded laterally (Fig 7). These results suggest the possibility of other physiologic deformations of the airway to preserve the airway volume under sagittal compression of the airway. There were some limitations in this study, such as the small sample size and accurate measurements of the images. Cone-beam CT can provide a short taking time and reduce the breathing effects on airway volume. In addition, the calibrating apparatus, such as airway phantoms, could make the airway volume more accurate and more valid.28 Other methods, such as magnetic resonance imaging, can be effective in analyzing the changes in soft tissues.29
CONCLUSIONS
The structures around the mandible inevitably moved posteriorly after mandibular setback surgery according to linear analysis. However, according to volumetric analysis, the airway was not reduced significantly by mandibular setback surgery. It is possible that physiologic deformation occurs to preserve the airway capacity after sagittal compression. REFERENCES 1. Ceylan I, Oktay H. A study on the pharyngeal size in different skeletal patterns. Am J Orthod Dentofacial Orthop 1995;108: 69-75. 2. Eggensperger N, Smolka W, Iizuka T. Long-term changes of hyoid bone position and pharyngeal airway size following mandibular setback by sagittal split ramus osteotomy. J Craniomaxillofac Surg 2005;33:111-7. 3. Gross AM, Kellum GD, Morris T, Franz D, Michas C, Foster ML, et al. Rhinometry and open-mouth posture in young children. Am J Orthod Dentofacial Orthop 1993;103:526-9. 4. Hellsing E. Changes in the pharyngeal airway in relation to extension of the head. Eur J Orthod 1989;11:359-65. 5. Kluemper GT, Vig PS, Vig KW. Nasorespiratory characteristics and craniofacial morphology. Eur J Orthod 1995;17:491-5. 6. Hochban W, Schu¨rmann R, Brandenburg U, Conradt R. Mandibular setback for surgical correction of mandibular hyperplasia— does it provoke sleep-related breathing disorders? Int J Oral Maxillofac Surg 1996;25:333-8. 7. Tselnik M, Pogrel MA. Assessment of the pharyngeal airway space after mandibular setback surgery. J Oral Maxillofac Surg 2000;58:282-5. 8. Riley RW, Powell NB, Guilleminault C, Ware W. Obstructive sleep apnea syndrome following surgery for mandibular prognathism. J Oral Maxillofac Surg 1987;45:450-2. 9. Chen F, Terada K, Hua Y, Saito I. Effects of bimaxillary surgery and mandibular setback surgery on pharyngeal airway
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