Abstract
The aim of this study is to evaluate the efficacy of uvulopalatopharyngoplasty and the corresponding postoperative airflow. Eleven patients diagnosed with obstructive sleep apnea syndrome who complained of snoring and apnea were enrolled in this study. Computational fluid dynamics (CFD) was implemented. CFD could be accomplished in nine cases. Airflow analysis was not possible in cases with a high respiratory event index (REI) score. Before surgery, stenosis was identified in the oropharynx and epiglottic area. And the airflow velocity and pressure were found to have significantly decreased in the oropharynx postoperatively, while in the epiglottic area, those data had increased postoperatively in some cases. The velocity and pressure of the oropharynx are related to REI score. From the CFD analysis, airflow analysis is important for evaluating the apnea state. It is suggested that the postoperative function can now be predicted preoperatively.
Keywords: Three dimensional computational fluid dynamics, Sleep apnea syndrome, Uvulopalatopharyngoplasty
Introdcution
Obstructive sleep apnea syndrome (OSAS) accounts for many of the causes of sleep apnea syndrome exhibiting apnea and hypopnea conditions resulting from obstruction of the upper airway. OSAS is also associated with higher cardiovascular and cerebrovascular risks, excessive daytime sleepiness, and traffic accidents [1].
In these diseases, it is important to find an anatomic stenosis site for diagnosis and treatment. As search methods for stenosis, analysis by macroscopic observation, endoscopic observation, cephalometric analysis [2–7], CT [8], and MRI [9] have been reported. However, these tests are for observing the resting state, and do not show the actual function and the airflow. It is important to analyze the state of airflow and also the wall pressure before and after treatment in the active state. Computational fluid dynamics (CFD) as a potential solution to this problem came to be applied. In recent years, with advances in computer technology, CFD can now be performed using personal computers. In the head and neck region, there have been many studies on nasal airflow analysis, and we have also reported airflow analysis of nasal septal perforations [10]. Case reports of CFD airflow analysis in oropharyngeal disease after adenotonsillectomy in children have been reported by Mihaescu et al. [11] and Luo et al. [12]. There have also been some case reports for adults [13, 14]. Additionally, one report evaluated airflow for an adult who underwent modified uvulopalatopharyngoplasty (UPPP) [15].
Previously, many surgical approaches had been tried for OSAS, but based on the ICSD-3 guidelines [16], CPAP has now become the gold standard. Other treatment options include weight loss, positional therapy, oral appliance therapy, and surgical interventions.
UPPP is not a mainstay nowadays, with reported success rates of 44% by Braga et al. [17] up to 80% by Friberg et al. [18], the latter involving modified UPPP.
Through the use of CFD, we examined the changes in airflow and pressure with CT to evaluate postoperative function. Our primary purpose of this study is to evaluate the efficacy of UPPP and to report on the postoperative airflow.
Materials and methods
Patients
Eleven patients with obstructive sleep apnea syndrome (OSAS) who complained of snoring and apnea were enrolled in this study.
All patients were evaluated with a portable device for SAS LS-330G (or LS300) (Fukuda Denshi Co., Ltd., Tokyo, Japan). The patients with respiratory event index (REI) > 40 with a portable device were diagnosed with OSAS. And patients with a REI < 40 underwent polysomnography (PSG), and they were diagnosed OSAS by PSG.
Additionally, all patients with tonsillar hypertrophy over grade II with Friedman’s palatine tonsil grading [19] underwent UPPP. Both before and 3 months after surgery, all patients underwent a CT scan and a portable apnea study.
The surgery was performed according to the method described by Fujita et al. [20]. The definition of surgical response and success was a reduction from the preoperative REI of at least 50% (response) and less than 20 events per hour (success) [21].
Computational fluid dynamics
Model generation
Three-dimensional (3D) models of the upper airway model with the nasal cavity and paranasal sinus were reconstructed using preoperative and postoperative CT images of 1.5-mm slice thickness. CT was taken while the patient was awake and in a supine position. The data were transferred through DICOM data, and the nasal geometry was determined using MIMICS 22 (Materialise Japan, Tokyo, Japan). The geometry was meshed using ICEM-CFD (ANSYS, Inc., Canonsburg, PA, USA), and the numerical simulation was performed using ANSYS CFX V15.0 (ANSYS, Inc., Canonsburg, PA, USA). For the ICEM-CFD simulation, the flow was assumed to be incompressible, in a quasi-state, and at a temperature of 25 °C. To account for the possible existence of turbulence, a κ-ε (kappa-epsilon) model was used. This model comprised about one million tetrahedral grids.
As for boundary conditions, all nasal walls were assumed to be a non-slip, rigid model. A velocity of 2 m/s airflow was applied to the outlet surface, and zero gauge pressure was applied to the pressure inlet as atmospheric pressure (Fig. 1). When the flow velocity is set to 2 m/s, the outlet cross-sectional area is about 2.5 cm2, which is 30 L per minute. This is the same value as in our previous reports [10], and this setting is simulating the resting breathing.
Fig. 1.
3D model a 3D model with boundary conditions of inlet and outlet b mesh model with pharyngeal volume (PV)
Pharyngeal volume (PV) was calculated by mesh model from choana to the base of the epiglottis level (Fig. 1b) [10].
CFD analysis
Pre- and postoperative models were compared for the streamline velocity (V, m/s), pressure (P, Pa) at the most narrow area in the oropharynx and around the epiglottis area.
Statistical analysis
The evaluation of mean differences was performed using a Wilcoxon signed rank test with matched samples for continuous variables. Regression analysis was used for the correlation among REI, BMI, PV, age, Velocity and Pressure by an analysis of variance. All analyses were performed using SPSS Statistics 25 (IBM, Armonk, NY, USA). P < 0.05 was considered significant. The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and institutional guidelines on human experimentation (Ethics Committee of Saitama Medical Center) and with the Helsinki Declaration of 1975, as revised in 2008.
Results
All patients underwent UPPP without any complications. Average REI was 56.2 (range, 12.5–95.1) in the preoperative stage, but was reduced to 16.9 (range, 1.2–67.0) postoperatively (Table 1). This difference was significant.
Table 1.
Background factor, PV and REI
| Case | Age (years) | BMI | REI | PV mm2 | ||
|---|---|---|---|---|---|---|
| B | A | B | A | |||
| 1 | 43 | 20.63 | 12.5 | 1.2 | 24098 | 27732 |
| 2 | 59 | 25.9 | 53.4 | 11.7 | 25505 | 23422 |
| 3 | 41 | 27 | 45.8 | 6.3 | 14659 | 26113 |
| 4 | 44 | 26.29 | 21.3 | 7.2 | 13747 | 17612 |
| 5 | 40 | 35.25 | 39.0 | 10.0 | 20982 | 23885 |
| 6 | 50 | 30.9 | 85.6 | 24.5 | 12335 | 14613 |
| 7 | 33 | 32.7 | 95.1 | 20.6 | 7943 | 8842 |
| 8 | 36 | 46.3 | 81.6 | 9.0 | 14659 | 15328 |
| 9 | 37 | 32.17 | 51.8 | 6.2 | 14293 | 15315 |
| 10 | 30 | 38 | 72.6 | 67.0 | 27737 | 49695 |
| 11 | 52 | 30.04 | 59.0 | 22.2 | 15192 | 28386 |
| Average | 42.3 | 31.4 | 56.2 | 16.9 | 17377.3 | 22813.0 |
| SD | 6.9 | 26.2 | 18.2 | 6230.7 | 10918.0 | |
| p-value | 0.001** | 0.003** | ||||
B before surgery, A after surgery, BMI body mass index, REI respiratory event index, PV pharyngeal volume
**P < 0.01
Only Case 10 showed a small reduction in REI (72.6–67.0) and continued to exhibit symptoms. For all other patients, symptoms such as snoring and apnea had disappeared.
Using the success criteria outlined by Sher et al.[21], 10 of 11 patients showed improvement in terms of symptoms and had an over 50% REI reduction, so the response rate was 91.0%. Additionally, 7 of 10 patients had REI values under 20, which corresponded to a success rate of 63.6%.
In terms of PV, there was a wider postoperative area, and this change was significant (Table 1). Only Case 2 had a smaller postoperative PV, but REI was still reduced in this case.
In Case 10, an increase in PV was found postoperatively but REI was still high. This was the only case for which UPPP was not effective.
Age was found not to be related to REI. A comparison between BMI and REI, a large BMI corresponded to a large REI, and this relationship was significant (Table 2).
Table 2.
Comparison between Age, BMI, PV, and REI
| p value | |
|---|---|
| Age and REI | 0.488 |
| PV and REI | 0.241 |
| BMI and PV | 0.713 |
| BMI and REI | ||||
|---|---|---|---|---|
| r | r2 | p value | Constant | Coefficient |
| 0.641 | 0.411 | 0.033* | − 20.2 | 2.43 |
*P < 0.05
There was no correlation between PV and preoperative REI (Table 2).
CFD analysis
Pre- and postoperative CFD could not be performed for two out of 11 patients. Case 10 still suffered from severe apnea after surgery, and Case 11 showed good recovery of apnea, but hypertrophy of the soft palate remained and there was an obstruction at the uvula area as shown by CT scan.
A preoperative CFD study was not possible for four out of nine patients, but such a study was possible postoperatively. We examined an integrated set of pre- and postoperative data and found that the average REI of eight studies who could not undergo CFD was 66.9 (± 22.9), and that of 14 studies who could undergo CFD was 19.2 (± 16.2). The former was significantly higher than the latter (p < 0.001) (Table 3). Additionally, five patients who underwent CFD before and after surgery were evaluated. In the oropharynx area, V and P were reduced after surgery with significant (p = 0.043), while in the epiglottis area, V and P were increased without significant (Table 4).
Table 3.
Availability of CFD study concerning REI
| Pre-op | Post-op | |
|---|---|---|
| Possible | 5 | 9 |
| Impossible | 6 | 2 |
| Average of REI ± SD | |
|---|---|
| Possible(14) | 19.2 ± 16.2 |
| Impossible(8) | 66.9 ± 22.9 |
p < 0.001
Table 4.
CFD data of pre and postoperative model
| V | P | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| B | A | B | A | |||||||
| Case | O | E | O | E | O | E | O | E | ||
| 1 | 4 | 9 | 1 | 60 | − 9 | − 25 | − 2 | − 1700 | ||
| 2 | 45 | 30 | 7 | 19 | − 1300 | − 850 | − 65 | − 470 | ||
| 3 | 65 | 23 | 16 | 9 | − 1900 | − 850 | − 140 | − 78 | ||
| 4 | 9 | 10 | 4 | 6 | − 50 | − 69 | − 18 | − 58 | ||
| 5 | 9 | 6 | 3 | 3 | − 55 | − 40 | − 9 | − 11 | ||
| 6 | 20 | 16 | − 200 | − 200 | ||||||
| 7 | 10 | 44 | − 240 | − 1050 | ||||||
| 8 | 17 | 12 | − 150 | − 130 | ||||||
| 9 | 3 | 6 | − 7 | − 22 | ||||||
| Average | 26.4 | 15.6 | 9.0 | 19.5 | − 662.8 | − 366.8 | − 92.4 | − 413.2 | ||
| SD | 27.1 | 10.4 | 7.1 | 19.5 | 881.6 | 441.4 | 91.9 | 585.6 | ||
| Average | 5case | 6.2 | 19.4 | − 46.8 | − 463.4 | |||||
| SD | 5.9 | 23.5 | 57.6 | 715.3 | ||||||
| p-value | 0.043* | 0.5 | 0.043* | 0.5 | ||||||
B before, A after, O oropharynx, E epiglottis, V velocity (m/s), P pressure (Pa)
*P < 0.05
The results of Case 1 were showed in Fig. 2a, b. Pre-operatively, a high-flow area was found in the oropharynx, while postoperatively, a high-flow area was found in the epiglottis area. The results of Case 5 were showed in Fig. 2c, d. In this case, the high-flow areas in the oropharynx and epiglottis disappeared after surgery.
Fig. 2.
Streamline of airflow a Case 1 pre-op, b Case 1 post-op, c Case 5 pre-op, d Case 5 post-op
All measurable integrated pre- and postoperative CFD data at the oropharynx area were compared with REI. REI was strongly influenced by velocity at the pharynx (Fig. 3, Table 5, r2 = 0.612, p = 0.001). Moreover pressure was significantly related to REI (Table 5).
Fig. 3.
Correlation between AHI and velocity of oropharynx
Table 5.
Comparison of CFD data with REI
| V | P | REI | |
|---|---|---|---|
| 1 | 4 | − 9 | 12.5 |
| 2 | 45 | − 1300 | 53.4 |
| 3 | 65 | − 1900 | 45.8 |
| 4 | 9 | − 50 | 21.3 |
| 5 | 9 | − 55 | 39.0 |
| 6 | 1 | − 2 | 1.2 |
| 7 | 7 | − 65 | 11.7 |
| 8 | 16 | − 140 | 6.3 |
| 9 | 4 | − 18 | 7.2 |
| 10 | 3 | − 9 | 10.0 |
| 11 | 20 | − 200 | 24.5 |
| 12 | 10 | − 240 | 20.6 |
| 13 | 17 | − 150 | 9.0 |
| 14 | 3 | − 7 | 6.2 |
| P value | r | r2 | |
|---|---|---|---|
| V and REI | 0.001** | 0.782 | 0.612 |
| P and REI | 0.001** | 0.779 | 0.607 |
V velocity (m/s), P pressure (Pa), REI respiratory event index
**P < 0.01
Discussion
We found that BMI was highly related to REI, which was consistent with the findings of a report on Japanese patients [22].
As for the effects of surgery, the response rate of our data was 91.0% and the success rate was 63.6%. Our procedure was nearly identical to that of classic UPPP. In our study, all patients had tonsil hypertrophy greater than grade 3. Tonsil hypertrophy patients could be a good indication of UPPP. Our data were better than those of Braga et al. but not than those of Friedman et al. Our procedure is appropriate, but we need further study to arrive at a better prognosis.
Although PV becomes wider postoperatively, it is not related to REI. There are many factors that have been indicated regarding airway space by cephalometry [3–5]. Chen et al. [23] and Nishimura et al. [24] reported that there was no correlation between PV and apnea–hypopnea index (AHI). This indicates that morphometric study alone does not work as a prognostic factor.
There have been some reports of CFD of OSAS for adults [25, 26], and Woolton et al. reported a CFD study involving children [27]. These authors evaluated the negative pressure or pressure drop that induces obstruction, but those reports were only on the pre-treatment state. Pre- and postoperative evaluations were reported by Powell et al. [28] before and after maxillo-mandibular advancement (MMA) and CPAP. In terms of UPPP, Yamamoto et al. [13] and Nomura et al. [14] reported small case studies for adults, and they showed the postoperative improvement of airflow. A case of adenotonsillectomy for children was also reported [11, 12], and the authors reported the efficacy of CFD.
There were some unmeasurable patients in both pre- and postoperative states in our study. In both states, patients with high REI scores could not undergo CFD study because of the complete obstruction of the pharynx. In other words, the CFD study failure group had high REI scores. The inability of CFD means a high degree of airway resistance and is thought to be an important factor in assessing the airway tract. Fortunately, almost all of the recovered patients could undergo postoperative study due to their reduced REI scores. The only exception was Case 11. This patient’s REI dropped from 59.0 to 22.2 but CFD could not be performed because uvula hypertrophy remained. In UPPP, the uvula is reduced by an appropriate amount, but, in this case, obstruction remained because of tonsil hypertrophy. We must therefore more precisely check the depth of the uvula during surgery to more accurately simulate the postoperative structure. Zhu et al. reported 11 cases of H-UPPP with an average pre-operative AHI score of 58.34, which was similar to our score of 56.2. Although Zhu et al. analyzed all 11 patients, 6 of our 11 cases could not undergo CFD. In our study, CTs were taken in the supine position, while their position in the Frankfurt plane is parallel to the floor, which simulates a standing position. In this case, the uvula is positioned caudally rather than posteriorly, which could explain why they were able to analyze all patients. While this positioning is efficient for aspiration, we believe that our position more accurately simulates real conditions. In addition, the fact that CFD cannot be measured could be also an index for evaluation.
We conducted a comparison of five cases for which pre- and postoperative CFD could be performed. In the oropharynx area, V and P were significantly reduced after surgery, indicating that the surgical procedure had alleviated stenosis in this area. There have been several previous reports on stenosis. Our previous two-case report [14] and a case report from Yamamoto et al. [13] and Xhu et al. showed similar results regarding the oropharynx area.
In contrast, Zhu et al. showed that three constricted areas are the upper surface of the soft palate, the area minimum and the superior border of the epiglottis, and that all areas were recovered postoperatively. In our study, V and P at the epiglottic area showed slightly induced reversals. We are not sure why this occurred, but we suspect that some mechanisms of functional and morphometrical change are dependent on the surgical technique. We could, however, speculate that the flow rate of the epiglottis is not related to OSAS status. Suga et al. [29] reported on a CFD study after Oral Appliance (OA) treatment. They found that AHI is related to the pressure of the epiglottis and also that pressure was decreased in this area. This could be explained by differences in the mechanism involved. Yajima et al. [30] reported on a CFD analysis of the cases after mandibular setback surgery in which the oropharynx was the stenosis area. This phenomenon was reversed of our study. The setback case of orthognathic surgery could create an apnea situation, and it would be worthwhile to preoperatively evaluate CFD to more accurately predict postoperative function.
In terms of the parametric factor for CFD, ΔP is recommended by some authors [12, 15, 27]. ΔP is calculated by a difference of two points. Zhu et al. [15] recognized that the velocity and pressure are worth measuring as ΔP in their study. In our study, V and P were identified as prognostic factors. In our previous nasal study [10], pressure gradient (nearly equal to ΔP) was more sensitive than pressure. In our study, obstructions were found in two sites, so we did not use ΔP analysis. In a future study, we could use ΔP analysis in segmenting the pharynx as the obstruction site. The summary of CFD reports for OSAS is listed in Table 6.
Table 6.
CFD reports for OSAS
| Treatment | Pre-op cases | Post-op cases | Factors | |
|---|---|---|---|---|
| Mihaescu et al. 2008 [11] | ped ad | 1 | 1 | P |
| Powell et al. 2011[28] | 2MMA 2CPAP | 4 | 4 | V |
| Tan et al. 2013 [25] | 10 | V P WSS | ||
| Luo et al. 2014 [12] | pedad | 10 | 10 | ΔP |
| Wootton et al. 2014 [27] | ped | 15 | R ΔP | |
| Yamamoto et al. 2017 [13] | UPPP | 1 | 1 | V P |
| Nomura et al. 2017 [14] | UPPP | 2 | 2 | V P WSS |
| Yajima et al. 2017 [30] | setback | 11 | 11 | ΔP |
| Liu et al. 2018 [26] | 1 | P | ||
| Zhu et al. 2019 [15] | H-UPPP | 11 | 11 | V P ΔP |
| Suga et al. 2019 [29] | OA | 15 | 15 | V P |
| Our 2021 | UPPP | 5 | 9 | V P WSS |
From the airflow analysis by CT, CFD could predict the REI score and could show the new findings from static analyses such as XP or CT. These results suggest that postoperative function can now be predicted preoperatively.
Conclusion
As for the effects of surgery, the response rate was 91.0% and the success rate was 63.6%. Our UPPP procedure was appropriate.
CFD could be implemented in nine cases. Airflow analysis was not possible in cases with high REI scores. Before surgery, stenosis was found in two places, the oropharynx and the upper glottic area, and the airflow rate was observed to decrease in the oropharynx, but, in the upper glottic area, there were cases that showed a postoperative increase. The correlation of REI and the V and P of the oropharynx was suggested.
This suggests that the postoperative function can now be predicted preoperatively. Flow analysis is as important as CT and cephalometry.
Authors' contributions
Dr. Nomura treated all cases and have full responsibility in this paper. Dr. Horikoshi, Dr. Kitano and Dr. Yamada co-operated the cases and checked all literature. Dr.Kondo helped the computer analysis. Dr. Kikuchi is the chief of the Department of Otolaryngology, Saitama Medical Center and has a responsibility of all patients’ outcome and paper publication.
Declarations
Conflict of interest
The Authors declare that they do not have any conflict of inrerest.
Availability of data and material (data transparency)
The authors confirm that the data supporting the findings of this study are available within the article.
Ethics approval
The study was approved by the Ethics Committee of Saitama Medical Center (No. 1276-III).
Consent to participate
Written patient consent was obtained.
Consent for publication
Written patient consent was obtained.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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