Effect of surgical mandibular advancement on pharyngeal airway dimensions: a three-dimensional computed tomography study

Abstract

The aim of this study was to quantify the changes in pharyngeal airway space (PAS) in patients with a skeletal class II malocclusion managed by bilateral sagittal split ramus osteotomy for mandibular advancement, using three-dimensional (3D) registration. The sample comprised 16 patients (mean age 21.69 ± 2.80 years). Preoperative (T0) and postoperative (T1) computed tomography scans were recorded. Linear, cross-sectional area (CSA), and volumetric parameters of the velopharynx, oropharynx, and hypopharynx were evaluated. Parameters were compared with paired samples t -tests. Highly significant changes in dimension were measured in both sagittal and transverse planes ( P < 0.001). CSA measurements increased significantly between T0 and T1 ( P < 0.001). A significant increase in PAS volume was found at T1 compared with T0 ( P < 0.001). The changes in PAS were quantified using 3D reconstruction. Along the sagittal and transverse planes, the greatest increase was seen in the oropharynx (12.16% and 11.50%, respectively), followed by hypopharynx (11.00% and 9.07%) and velopharynx (8.97% and 6.73%). CSA increased by 41.69%, 34.56%, and 28.81% in the oropharynx, hypopharynx, and velopharynx, respectively. The volumetric increase was greatest in the oropharynx (49.79%) and least in the velopharynx (38.92%). These established quantifications may act as a useful guide for clinicians in the field of dental sleep medicine.

As clinicians, our areas of focus are confined to dental, skeletal, and soft tissue relationships. More often than not, we tend to overlook the functional spaces during the pre-treatment evaluation phase, especially pharyngeal spaces. The inter-relationship between airway dimensions, craniofacial morphology, and orthognathic surgery has become an area of study in recent years.

Research has established that various types of dysgnathia may predispose a patient to changes in airway space. A class II skeletal malocclusion due to mandibular deficiency is considered a risk factor for sleep apnoea due to underlying oropharyngeal airway deficiencies. Class II patients have narrower pharyngeal dimensions, hence constrictions may occur at one or more levels: behind the soft palate, posterior to the tongue base, or in the hypopharyngeal region.

Analysis of the airway focuses on linear measurements, area, and volume, since volume information alone might not necessarily represent or identify the locations of the relevant constrictions. Researchers have mainly evaluated the pharyngeal airway space (PAS) on a two-dimensional (2D) basis via specific landmarks on cephalograms, measuring a variety of linear and area parameters. However, the more contemporary three-dimensional (3D) technology of computed tomography (CT) has expanded diagnostic capacities significantly. This provides 3D reconstruction and permits visualization of areas of interest in various planes. 3D CT also allows independent visualization of the internal anatomical structures, such as the airway, by sculpting. It has made cross-sectional and volumetric investigations of the PAS possible for analysis of the complex airway anatomy.

Reports in the literature on 3D PAS changes in skeletal class II patients undergoing surgical mandibular advancement are sparse. In order to bridge the gap in knowledge, this prospective study was performed to quantify the changes in PAS in adult patients with skeletal class II malocclusion undergoing combined orthodontic–surgical treatment for mandibular advancement by bilateral sagittal split ramus osteotomy (BSSRO), using 3D registration.

Materials and methods

Sixteen patients (10 male, six female; age range 18–26 years) who underwent mandibular advancement surgery were enrolled in this study. BSSRO for the correction of mandibular retrognathism was performed by the same surgeon, using rigid fixation. All study subjects required a mandibular advancement not exceeding 7 mm. All subjects underwent a period of fixed appliance orthodontic treatment before and after surgery.

The research proposal was approved by the institutional research ethics committee, and informed consent was obtained from all subjects. Inclusion criteria were skeletal class II malocclusion due to mandibular retrognathism and class II molar and canine relationship. Exclusion criteria were cleft, systemic disorders, temporomandibular joint disorders, craniofacial syndrome, asymmetries, breathing problems, or previous craniofacial surgery.

All CT examinations were performed using a 36-detector CT scanner (Brilliance; Philips Healthcare, Japan) with the patient in the supine position and the head and neck in a neutral position; the Frankfort horizontal plane was perpendicular to the floor. The imaging protocol used was 120 kV, 150 mA, and a 400 ms rotation time, with a slice thickness less than 0.5 mm and increments of 0.4 mm, using a detector collimation of 64 mm × 0.5 mm. The subjects were instructed to maintain maximum intercuspation with the tongue touching the palate, to avoid swallowing, and to breathe smoothly during the scanning period. Scans were carried out in maximum intercuspation because centric occlusion minimizes the variability in mandibular and soft tissue measurements often associated with the rest position. Axial sections were obtained starting at the infra-orbital margin and progressing to the intervertebral space between the fourth and fifth cervical vertebrae. All sections were perpendicular to the airway lumen to allow accurate assessment of the linear and volumetric measurements of the PAS. The CT datasets were stored in DICOM format (Digital Imaging and Communications in Medicine). The scans of each subject were taken preoperatively within 1 week of surgery (T0) and postoperatively after about 5 months (T1).

3D models were reconstructed from axial images and analyzed by a single operator. The lower pharyngeal portion was divided into the velopharynx (Va), oropharynx (Or), and hypopharynx (Hy), as proposed by Claudino et al. The superior boundary of the velopharynx was the palatal plane (anterior nasal spine (ANS) to posterior nasal spine (PNS)) extending to the posterior wall of the pharynx, and the inferior boundary was the plane parallel to the palatal plane that passes from the uvula. The superior limit of the oropharynx was the lower limit of the velopharynx, and its lower limit was a plane parallel to the palatal plane intersecting the upper point of the epiglottis. The upper limit of the hypopharynx was the lower limit of the oropharynx, and its lower limit was a plane parallel to the palatal plane intersecting the lower and most anterior point of the fourth cervical vertebra. Landmarks used and measurements made in this study are shown in Tables 1 and 2 .

Table 1
Landmarks used in the study.
Name Definition
Palatal plane Plane extending from ANS to PNS
Uvula (U) The most inferior point of the soft palate
Epiglottis (E) Upper point of the epiglottis
C4ai Antero-inferior point of the fourth cervical vertebra

ANS, anterior nasal spine; PNS, posterior nasal spine.

Table 2
Measurements made in the study.
Name Definition a
Distance – sagittal dimensions
Velopharyngeal (Va) Va – Sag The largest sagittal dimension along a plane parallel to the palatal plane intersecting U
Oropharyngeal (Or) Or – Sag The largest sagittal dimension along a plane parallel to the palatal plane intersecting the upper point of E
Hypopharyngeal (Hy) Hy – Sag The largest sagittal dimension along a plane parallel to the palatal plane intersecting C4ai
Distance – transverse dimensions
Velopharyngeal (Va) Va – Trans The largest transverse dimension along a plane parallel to the palatal plane intersecting U
Oropharyngeal (Or) Or – Trans The largest transverse dimension along a plane parallel to the palatal plane intersecting the upper point of E
Hypopharyngeal (Hy) Hy – Trans The largest transverse dimension along a plane parallel to the palatal plane intersecting C4ai
Cross-sectional area (CSA)
Velopharyngeal (Va) Va – CSA Along a plane parallel to the palatal plane intersecting U
Oropharyngeal (Or) Or – CSA Along a plane parallel to the palatal plane intersecting E
Hypopharyngeal (Hy) Hy – CSA Along a plane parallel to the palatal plane intersecting C4ai
Volume (Vol)
Velopharyngeal (Va) Va – Vol The upper limit was the palatal plane; the lower limit was a plane parallel to the palatal plane intersecting U
Oropharyngeal (Or) Or – Vol The upper limit was the lower limit of the velopharynx; the lower limit was a plane parallel to the palatal plane intersecting the upper point of E
Hypopharyngeal (Hy) Hy – Vol The upper limit was the lower limit of the oropharynx; the lower limit was a plane parallel to the palatal plane intersecting C4ai

a For definitions of U, E, and C4ai, see Table 1 .

The preoperative and postoperative measurements were recorded by a single investigator. The cross-sectional area (CSA) was calculated automatically using the software Extended Brilliance Workspace version 4.5.4 (Philips Healthcare) at determined levels on the axial slices. For volume evaluation, sculpting was done to isolate the desired airway section by removing the undesired structures, together with any artefacts or background scatter. CSA was calculated in square millimetres and volume in cubic millimetres.

Statistical analysis

The data collected were compiled in a Microsoft Excel Worksheet. Data were analyzed using SPSS version 17.0 software (SPSS Inc., Chicago, IL, USA). To assess intra-investigator error, seven randomly selected scans were measured again 2 weeks after the first evaluations. An intra-investigator reliability test to measure method error was conducted using Cronbach’s alpha value. A Cronbach’s alpha value closer to 1 indicates reliability between measurements made at separate time points, and a value closer to 0 indicates unreliability. Means and standard deviations were calculated. The preoperative (T0) and postoperative (T1) measurements were compared with paired samples t -tests. P < 0.05 was considered statistically significant.

Results

In the present study, no difference in values was found by patient age or sex ( P > 0.05). The data were equally distributed. Cronbach’s alpha ranged from 0.80 to 0.931, demonstrating high reliability between measurements of the same anatomical structure taken 2 weeks apart.

The mean age of the sample was 21.69 ± 2.80 years. The mean mandibular advancement was 6.5 ± 0.97 mm. The results of the paired samples t -tests for the comparison of preoperative and postoperative measurements are shown in Tables 3–6 . The linear, CSA, and volumetric measurements made at T0 and T1 are presented in Figs 1–3 .

Table 3
Measurements of the pharyngeal airway space along the sagittal plane (in millimetres, mm).
Parameter Mean SD Variance P -Value Percentage increase Mean increase Increase per mm advancement
Va – Sag 0.00041 8.97% 0.87 0.13
T0 9.7 1.14 1.29
T1 10.57 1.16 1.34
Or – Sag 0.00019 12.61% 1.53 0.24
T0 12.13 1.89 3.58
T1 13.6 1.88 3.55
Hy – Sag 0.00032 11.00% 1.28 0.20
T0 11.64 1.55 2.42
T1 12.92 1.44 2.08
SD, standard deviation; T0, preoperative; T1, postoperative.

Table 4
Measurements of the pharyngeal airway space along the transverse plane (in millimetres, mm).
Parameter Mean SD Variance P -Value Percentage increase Mean increase Increase per mm advancement
Va – Trans 0.00021 6.73% 0.95 0.15
T0 14.11 2.12 4.49
T1 15.06 2.44 5.95
Or – Trans 0.00093 11.50% 2.15 0.33
T0 18.69 2.11 4.45
T1 20.84 1.29 3.66
Hy – Trans 0.00045 9.07% 1.59 0.24
T0 17.53 1.86 3.47
T1 19.12 1.96 3.84
SD, standard deviation; T0, preoperative; T1, postoperative.

Table 5
Cross-sectional area (CSA) measurements of the pharyngeal airway space (in square millimetres, mm 2 ).
Parameter Mean SD Variance P -Value Percentage increase Mean increase Increase per mm advancement
Va – CSA 0.00037 28.81% 49.82 7.66
T0 172.9 7.97 63.5
T1 222.72 11.01 121.28
Or – CSA 0.00076 41.69% 116.89 17.98
T0 280.39 10.98 120.49
T1 397.28 9.53 90.75
Hy – CSA 0.00027 34.56% 93.23 14.34
T0 269.76 11.74 137.79
T1 362.99 13.12 172.1
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Jan 16, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Effect of surgical mandibular advancement on pharyngeal airway dimensions: a three-dimensional computed tomography study
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