Pharyngeal airway evaluation after isolated mandibular setback surgery using cone-beam computed tomography

Introduction

In this study, we investigated volumetric and dimensional changes to the pharyngeal airway space after isolated mandibular setback surgery for patients with Class III skeletal dysplasia.

Methods

Records of 28 patients who had undergone combined orthodontic and mandibular setback surgery were obtained. The sample comprised 17 men and 11 women. Their mean age was 23.88 ± 6.57 years (range, 18-52 years). Cone-beam computed tomography scans were obtained at 3 time points: before surgery, average of 6 months after surgery, and average of 1 year after surgery. Oropharyngeal, hypopharyngeal, and total volumes were calculated. The lateral surface and anteroposterior dimensions at the minimal axial areas for oropharyngeal and hypopharyngeal volumes and mean mandibular setback were determined.

Results

The mean mandibular setback was 9.93 ± 5.26 mm. Repeated measures analysis of variance determined an overall significant decrease between the means for 6 months and up to 1 year after surgery for oropharyngeal and hypopharyngeal volumes, anteroposterior at oropharyngeal, lateral surface at oropharyngeal, and anteroposterior at hypopharyngeal. No strong correlation between mandibular setback surgery and pharyngeal airway volumes or dimensions was determined.

Conclusions

After mandibular setback surgery, pharyngeal airway volume, and transverse and anteroposterior dimensions were decreased. Patients undergoing mandibular setback surgery should be evaluated for obstructive sleep apnea and the proposed treatment plan modified according to the risk for potential airway compromise.

Highlights

  • Pharyngeal airway volume is decreased after isolated mandibular setback surgery.

  • Lateral and anteroposterior oropharyngeal dimensions also decrease significantly.

  • Anteroposterior hypopharyngeal dimension decreases.

  • Amounts of mandibular setback and pharyngeal airway change are not strongly correlated.

  • Amount of mandibular setback surgery could not predict pharyngeal airway changes.

In patients with severe skeletal Class III dysplasia, combined orthodontic-orthognathic surgical treatment provides an esthetic and functional solution. Isolated mandibular setback surgery is a treatment option for the correction of this dysplasia. An important aspect of this surgical correction is that it causes changes in the position of the hyoid bone and the base of tongue. The posterior shift of the tongue base creates an increase in contact length between the soft palate and the tongue base and can decrease the pharyngeal airway space. The resultant changes in hard and soft tissues after mandibular setback surgery have been shown to produce a shift in oropharyngeal characteristics to a morphology associated with sleep-disordered breathing, typical of obstructive sleep apnea (OSA).

OSA is characterized by repeated increases in resistance to airflow in the upper airway, causing obstruction. It is also characterized by the periodic partial or complete collapse of the upper airway that results in episodes of hypopnea (diminished airflow of at least 30%, lasting at least 10 seconds) or apnea (absent airflow). The collapse of soft tissues in the upper airway, including the retropalatal and retroglossal regions of the oropharynx, play a role in the etiology of OSA. Epidemiologic estimates of OSA prevalence are about 4% for men and 2% for women in the age group of 30 to 60 years in the United States when considering subjective daytime sleepiness. Approximately 1 in 5 adults has at least mild OSA, and 1 in 15 adults has OSA of moderate or worse severity.

Initial research performed to evaluate the effect of mandibular setback surgeries on the pharyngeal airway space have been evaluated with lateral cephalograms. The limitations of a lateral cephalogram are that it is a static, 2-dimensional image that does not adequately represent the 3-dimensional volumetric data. Recently, cone-beam computed tomography (CBCT) has been used to evaluate the airway changes 3 dimensionally. The majority of CBCT studies examining pharyngeal airway volume changes have patients undergoing a combination of maxillary advancement and mandibular setback surgery. Thus, there is limited evidence in the literature describing the effect of isolated mandibular setback surgeries on pharyngeal airway space using CBCT. Further research may elucidate whether a setback alone contributes to a negative impact on the airway and possibly exacerbate OSA.

The aims of this study were to evaluate volumetric and dimensional changes in the pharyngeal airway space for patients who have undergone isolated mandibular setback surgery with CBCT, and also to determine whether a relationship exists between mandibular setback surgery and pharyngeal airway volumes or dimensions.

Material and methods

For this study, the records of 28 patients who had undergone combined orthodontic and isolated mandibular setback surgery to correct Class III skeletal dysplasia were obtained. The sample included 17 men and 11 women. Their mean age was 23.88 ± 6.57 years, with a range of 18 to 52 years. The sample was retrieved from the Department of Orthodontics at Pusan National University Hospital, Busan, South Korea. The setback surgery consisted of sagittal split ramus osteotomy of the mandible with rigid fixation. CBCT scans were obtained at 3 time points: T1 (before surgery), T2 (an average of 6 months after surgery), and T3 (an average of 1 year after surgery). The inclusion criteria for this study were adults with Class III skeletal deformities who had undergone mandibular setback surgery and orthodontic treatment. The exclusion criteria were severe facial asymmetry or syndrome, and symptoms of temporomandibular disorders or respiratory disease.

The presurgical and postsurgical apnea-hyponea index values and the body mass index values of the subjects were not available for this study.

All patients underwent a CBCT examination (DCTpro; Vatech, Seoul, Korea) for assessment of upper airway volume and skeletal changes. The patients were seated in the upright position with maximum intercuspation. The Frankfort horizontal plane was parallel to the floor. Head orientation was the same for each CBCT image performed by the same experienced operator. The patients were asked not to swallow during the scan. The maxillofacial regions were scanned for 24 seconds using a CBCT machine with a voxel size of 0.3 mm, a field of view of 20 × 19 cm, a tube voltage of 90 kV(p), and a tube current of 4.0 mA. Images were imported into imaging software (version 11.5; Dolphin Imaging and Management Systems, Chatsworth, Calif) and used to view, analyze, and manipulate the CBCT scans.

The calculation of the anteroposterior surgical movement was measured by converting the CBCT scan from a 3-dimensional volume to a lateral cephalogram image. A reference plane was drawn through sella and nasion, and then 7° was subtracted. A perpendicular line was drawn through the corrected horizontal plane from nasion, and then the distance to B-point was measured and compared before and after surgery ( Fig 1 ).

Fig 1
Lateral cephalogram demonstrating measurement of surgical movement.

To isolate the pharyngeal airway and volumetric measurements, orientation of the CBCT scan included the horizontal reference plane that was defined bilaterally by porion with right orbitale. This condition was verified on the midsagittal plane. The transporionic plane was oriented vertically, defined bilaterally by porion and perpendicular to the horizontal reference plane. The midsagittal plane was oriented vertically, defined by nasion, and perpendicular to the other reference planes. The 2 defined volumes included the oropharyngeal (OP) and hypopharyngeal (HP) volumes. The superior border of OP was bounded by a line from the most superior anterior point of cervical vertebrae 1 to the posterior tip of the hard palate. The inferior border was defined by a line parallel to the superior border from the most inferior anterior point of cervical vertebrae 2 to the base of the tongue. This inferior border also formed the superior border of the HP, and the inferior border of the HP was a line parallel to the superior border from the most inferior anterior point of cervical vertebrae 4 to the anterior border. The anterior border comprised the posterior soft palate and the base of tongue. The posterior border was the posterior pharyngeal wall ( Fig 2 ).

Fig 2
Segmentation of OP and HP volumes.

After isolation, the volume of each segment and total volume (TV) were calculated. In addition, the minimum axial area was determined, and then the lateral surface length (TR) and the anterior posterior lengths (AP) were measured ( Fig 3 ).

Fig 3
Segmentation of TR and AP.

Statistical analysis

Data analysis was performed using SPSS software (version 23.0; IBM, Armonk, NY). Descriptive statistics calculated the means and standard deviations for mandibular setback and relapse, as well as the OP and HP volumes and TV, TR, and AP at each time point. The Shapiro-Wilk test demonstrated normal distribution of the data. Paired t tests were used to verify significant differences in the mean airway volumes, and the TR and AP measurements. The Pearson correlation was used to determine whether a relationship existed between the amount of mandibular setback and pharyngeal volumes and dimensions.

Multiple linear regressions were conducted at each time point interval (T2-T1, T3-T2, T3-T1) using all variables that evaluated the airway volumes or dimensions. The forward method was used to include or exclude variables in the adjusted model. The significance level for the statistical tests was set at 0.05.

For reliability testing, 10% of the variables were randomly selected and remeasured after 2 weeks. Cronbach alpha interitem correlation was the statistic used to determine reliability. As a general rule, intraclass correlations greater than or equal to 0.80 are considered adequate. A Cronbach value of 0.8 or greater was met for all measurements.

Results

From the cephalometric data, the mean mandibular setback was 9.93 ± 5.26 mm, and the mean mandibular relapse from T3 to T2 was 1.13 ± 3.11 mm.

From the volumetric data, means and standard deviations for OP and HP volumes, TV, and TR and AP are presented in Table I .

Table I
Descriptive statistics for oropharyngeal volume (OP), hypopharyngeal volume (HP), total volume (TV), anteroposterior at oropharyngeal (AP at OP), lateral surface at oropharyngeal (TR at OP), anteroposterior at hypopharyngeal (AP at HP), and lateral surface at hypopharyngeal (TR at HP)
Variable T1 T2 T3
OP (mm³) 16279.31 ± 4627.48 11868.04 ± 5600.73 12317.64 ± 5595.85
HP (mm³) 14199.36 ± 4246.32 11049.49 ± 4969.84 11943.75 ± 4154.69
TV (mm³) 30478.67 ± 8319.83 22917.53 ± 9824.03 24261.40 ± 8916.27
AP at OP (mm) 13.50 ± 3.34 9.77 ± 3.49 10.25 ± 3.72
TR at OP (mm) 29.06 ± 5.46 24.10 ± 6.12 24.74 ± 6.01
AP at HP (mm) 14.50 ± 3.34 11.35 ± 4.04 11.78 ± 4.30
TR at HP (mm) 28.60 ± 9.03 27.20 ± 6.59 27.85 ± 7.31
T1 , Before surgery; T2 , 6 months after surgery; T3 , 1 year after surgery. n = 28.

Thus the null hypothesis can be rejected and the alternative hypothesis accepted. There was a significant difference in the pharyngeal airway (OP and HP volumes and TV, TR at OP, and AP at HP) after mandibular setback surgery ( Fig 4 ).

Fig 4
Graph representing changes in mean volumes at T1, T2, and T3.

Paired t tests (after Bonferroni correction to control the family-wise error) identified the intervals during which there were significant changes for the OP and HP volumes and TV, in addition to the TR and AP. Generally, significant decreases were noted from T2 to T1 and from T3 to T1, but no significant changes were noted from T3 to T2. The only exception was for TR at HP (lateral surface length at hypopharyngeal), where there was no statistically significant change over time ( Table II ).

Table II
Paired t tests denoting significant changes between T1 and T2, and T1 and T3, but not T2 and T3, for all evaluated variables, except for TR at HP where no differences were noticed between all time points
Variable Time point Time point Mean difference SE Significance
OP (mm³) T1 T2 4411.26 806.63 <0.001
T3 12304.14 1057.24 <0.001
T2 T3 −449.60 545.71 0.417
HP (mm³) T1 T2 3149.87 763.16 <0.001
T3 2255.60 705.77 0.001
T2 T3 −894.26 540.17 0.109
TV (mm³) T1 T2 7561.13 1497.01 <0.001
T3 6217.27 1566.01 <0.001
T2 T3 −1343.86 974.16 0.179
AP at OP (mm) T1 T2 3.72 0.61 <0.001
T3 3.25 0.73 <0.001
T2 T3 −2.18 1.64 0.196
TR at OP (mm) T1 T2 4.96 1.14 <0.001
T3 4.32 0.82 0.001
T2 T3 −0.63 1.24 0.478
AP at HP (mm) T1 T2 3.14 0.83 0.001
T3 2.71 0.79 0.001
T2 T3 −0.42 0.66 0.524
TR at HP (mm) T1 T2 1.40 1.53 0.369
T3 0.75 1.31 0.555
T2 T3 −0.65 1.08 0.570
OP , Oropharyngeal volume; HP , hypopharyngeal volume; TV , total volume; AP at OP , anteroposterior at oropharyngeal; TR at OP , lateral surface at oropharyngeal; AP at HP , anteroposterior at hypopharyngeal; TR at HP , lateral surface at hypopharyngeal.

Calculations were done to determine the volume percentage change. All 3 volumes showed statistically significant percentage decreases from T2 to T1 and T3 to T1. However, T3 to T2 showed a nonsignificant percentage change ( Table III ).

Table III
Volume percent change over the 3 time points
Percent change T2−T1 T3−T2 T3−T1
Oropharyngeal volume −26.2% ± 2.4% 6.2% ± 2.5% −22.3% ± 3.3%
Hypopharyngeal volume −21.7% ± 2.4% 7.5% ± 3.1% −14.0 % ± 2.3%
Total volume −24.0% ± 2.2% 10.5% ± 2.7% −18.4% ± 2.7%

P <0.05, a statistically significant decrease.

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Dec 12, 2018 | Posted by in Orthodontics | Comments Off on Pharyngeal airway evaluation after isolated mandibular setback surgery using cone-beam computed tomography
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