Introduction
The objective of this research was to evaluate the effect of orthodontic extraction on the pharyngeal airway volume and Minimum cross-sectional area (MCA) in growing and adult patients.
Methods
Seven databases, unpublished gray literature, and the references list of the identified articles were electronically searched for relevant studies that met our eligibility criteria. Included studies assessed the effect of dental extraction or sagittal dental movements on pharyngeal airway dimensions. The quality of the included studies was assessed using the methodological index for nonrandomized studies. In addition, a meta-analysis was conducted using the RevMan 5 (Nordic Cochrane Centre, Cochrane Collaboration, Copenhagen, Denmark).
Results
In 7 studies, 268 treated patients with a mean age of 19.1 ± 7.6 years and 342 nonextraction control group subjects with a mean age of 19.3 ± 7.2 years were included. Compared with the control group, no statistically significant difference was found in total, nasopharyngeal, glossopharyngeal, oropharyngeal volume, or MCA ( P >0.05) in the extraction group except in oropharyngeal volume in which a statistically significant increase in the volume 0.41 cm 3 (95% CI, 0.05-0.80; P = 0.03) was detected. The clinical significance of this increase is questionable. Included studies showed a moderate to high risk of bias.
Conclusions
There is no strong evidence to support the concept that premolar extractions in bimaxillary protrusion or crowded growing and adult patients reduce either pharyngeal airway volume or MCA. Moreover, as the level of evidence was considered very low for all variables, the magnitude, and direction of the summaries have to be interpreted with caution. Future studies with better quality could significantly affect the direction and strength of the results. Trial registration number: PROSPERO CRD42018089924.
Highlights
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Pharyngeal airway volume is no affected by premolars extraction.
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Minimal cross-sectional area is no affected by premolars extraction.
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Cone-beam computed tomography is an imaging tool to evaluate the pharyngeal airway volume.
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The level of evidence was considered very low for all variables.
The pharyngeal airway is a complex 3-dimensional (3D) structure and shown to be affected by different craniofacial skeletal patterns. Accurate evaluation of the pharyngeal airway requires 3D measurement of the minimim cross-sectional area (MCA) and airway volume. The MCA is the anatomic location perpendicular to the direction of airflow as visualized in the axial plan and which the degree of its constriction dictates the resistance to airflow. From the available literature, it was found that assessment of the pharyngeal airway is highly dependent on the method of assessment. Cone-beam computed tomography (CBCT) has been recently used as a 3D imaging tool for the evaluation and accurate 3D visualization of both the MCA and the airway volume. Recent studies showed that CBCT is superior to the traditional 2-dimensional (2D) methods in assessing anteroposterior (AP) distances and corresponding cross-sectional areas of the pharyngeal airway and that significant differences were found when comparing 2D lateral cephalogram vs 3D computed tomography as the imaging tool. Although CBCT is more superior to 2D lateral cephalogram as an imaging tool of the pharyngeal airway, it has limitations such as cost, absence of a standard protocol for airway imaging, and the inherent nature of being a static image of a dynamic structure ,
Bimaxillary protrusion and severe tooth size arch length discrepancy both are conditions that often result in the extraction of premolars to alleviate crowding or retract anterior teeth and establish proper occlusion and esthetics. However, orthodontic interventions such as extraction and distalization, which alter the AP dimensions, have been blamed for potentially exacerbating the risk of obstructive sleep apnea (OSA). , This hypothesis is based on the assumption that premolar extraction and bimaxillary retraction decrease the arch length and oral cavity size. This leads to restriction of the tongue space and more posterior positioning of the tongue, potentially leading to constriction of the airway. Conversely, 3D imaging studies, which assessed pharyngeal airway cross-sectional area after premolar extraction, found a morphologic change in the pharyngeal airway but no difference in both the MCA and volume. Sharma et al evaluated the changes in pharyngeal airway size after premolar extraction in bimaxillary protrusive adults and concluded that the pharyngeal airway size diminished during treatment. This was a 2D study that did not take into account the volume of the airway but measured the AP dimensions on cephalograms. In 2015, Hu et al conducted a systematic review to evaluate the effect of teeth extraction for orthodontic treatment on the pharyngeal airway and stated that overall extraction could lead to narrowing of the pharyngeal airway space, especially the oropharyngeal airway. They mentioned that exceeding heterogeneity was present among the included studies. The formerly mentioned systematic review pooled the results of the studies using 2D and 3D imaging tools together, which could have affected its results.
To our knowledge, this is the first systematic review to extract and analyze data exclusively from 3D imaging tools to evaluate the effect of orthodontic treatment aimed at sagittal (AP) changes on the pharyngeal airway volume. We believe this systematic review will be highly informative to all orthodontists in general and clinicians who diagnose and manage OSA patients.
Objectives
This study aimed to perform a systematic review to investigate the effect(s) of orthodontic treatment modalities aimed at occlusal and sagittal changes on pharyngeal airway dimensions assessed through 3D CBCT imaging.
Material and methods
Protocol and registration
The protocol of this systematic review was registered on the National Institute for Health Research database ( https://www.crd.york.ac.uk/prospero/ ); trial registration number: PROSPERO CRD42018089924.
Eligibility criteria
The inclusion criteria were developed according to the Population, Intervention, Comparison, Outcomes, and Study criteria ( Table I ). The primary outcome was identified as pharyngeal airway volume. Only studies using 3D imaging (computed tomography, CBCT) as outcome assessment to evaluate the pharyngeal airway dimensions were included in this systematic review. Exclusion criteria were as follows: (1) research type: animal studies, case reports, editorials, or opinions; (2) research sample: patients with cleft palate, medically compromised patients, pharyngeal airway dysfunction, the syndrome of OSA; and (3) intervention: orthodontic treatment aimed at only the transverse or vertical dimension and not involving the sagittal dimension. Two-dimensional lateral cephalogram as a method of outcome assessment.
| PICOS | Description |
|---|---|
| Population | Adult and growing patients with angle Class I, II, and III malocclusion |
| Intervention | Orthodontic treatment aimed at sagittal dimension: extraction and distalization |
| Comparator (control group) | Nonextraction or no treatment |
| Outcome | One-year: volumetric airway changes (total airway volume, nasopharyngeal, oropharyngeal, and hypopharyngeal) Two-year: hyoid bone displacement and pharyngeal airway MCA |
| Study design | Retrospective, prospective cohort, randomized controlled, or quasirandomized |
| Question | What is the effect of the orthodontic treatment aimed at sagittal changes on the pharyngeal airway volume? |
Information sources, search strategy, and study selection
This systematic review and meta-analysis were conducted following as close as possible the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. An electronic search without language restrictions in Medline, PubMed, Cochrane Library Central, Scopus, Web of Science, LILACS, EBSCO, CINHAL published up to October 25, 2020. Gray unpublished literature was searched through clinicaltrials.gov , PROQUEST. An additional manual search of references in the included studies, sleep journals was also conducted. We used the search terms combination [“Malocclusion, Angle Class I, II, III” OR “adult” OR “growing” AND “orthodontic extraction” OR “Distalization” AND (“Pharynx” OR “Naso-pharynx” OR “Hypopharynx” OR “Oropharynx”)]. A list of full keywords and search strategies for each database can be found in Supplementary Table I .
Data item and collection
Two independent authors (N.O. and T.E.) conducted the study selection with discrepancies being resolved by discussion with a third author (C.F.M.). Title and abstract screening were performed by grading studies as yes, no, or maybe on the basis of the information provided by the title and abstract. The full text was located for all articles graded with yes or maybe, and studies when no abstract was available or the information available was inconclusive in reaching a decision. The inclusion and exclusion criteria were applied to the full-text articles and when questions remained, efforts were made to contact the authors of the study.
Two reviewers (N.O. and S.K.), who then crosschecked and reviewed the text in full to decide whether they were eligible, conducted an initial screening through titles and abstracts independently. Disagreements were resolved through discussion, when necessary, by seeking the opinion of a third reviewer (C.F.M.). The following data were extracted from the studies included in the final analysis: title, author, year of publication, study design, age and gender of research objects, sample size, orthodontic intervention, evaluation parameters, and other statistical data. Two investigators (N.O. and S.K.) extracted the data and crosschecked to confirm their accuracy. Any discrepancies were resolved through discussion and by seeking the opinion of a third investigator (C.F.M.).
Data extraction was performed with the primary outcome measure sought was dimensional changes in any region of the pharyngeal airway. The pharyngeal airway was defined as including the nasopharynx, oropharynx, and hypopharynx. Information related to the study samples, including sample size, age, gender, and patient selection criteria, were recorded. Details of the intervention and specifics of the timing of treatment were retrieved from all included studies. The modality and technique used to quantify the pharyngeal airway volume and the exact time points that these were recorded and extracted. Measurements were accepted from 2 specific time points only: immediately before treatment (T1) and immediately after treatment (T2).
Risk of bias (quality assessment) in individual studies
Prospective or retrospective studies reporting on the pharyngeal airway space change on orthodontic sagittal intervention were selected. The quality of included studies was assessed using the methodological index for nonrandomized studies, which is customized for quality evaluation of nonrandomized controlled studies and comprises 12 items, with each item scored from 0 to 2 with a total score of 24. On the basis of other researchers, 0-12 showed a high risk of bias, 13-18 showed a moderate risk of bias, and 19-24 showed a low risk of bias ( Supplementary Table II ). Two reviewers assessed the quality of included studies independently in accordance with the methodological index for nonrandomized studies, and a conclusion was reached after disagreements were resolved through discussion. The overall quality of evidence (confidence in effect estimates) for the primary and secondary outcomes was rated using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) approach.
Summary measures and approach to synthesis
Extracted data were statistically analyzed using RevMan software (version 5.3; Nordic Cochrane Centre, Cochrane Collaboration, Copenhagen, Denmark). All available data extracted from the included studies were continuous variables; mean and standard deviation (SD) with a 95% confidence interval (CI) was used to estimate the treatment effect. Cochrane test (I 2 statistic) was used to evaluate statistical heterogeneity. Low heterogeneity ( P >0.10, I 2 <50%) means a fixed-effects model may be employed to conduct the meta-analysis. A random-effects model was adopted in all meta-analyses. The statistical significance for the testing of hypotheses was set at P <0.05. If enough studies were available, Funnel plots would be used to detect publication bias.
Additional analyses
Sensitivity analyses and subgrouping analyses were prespecified to deal with studies at higher risk of bias, and other potential sources of heterogeneity such as differences in the sample age group, to isolate their influence on the overall outcome. Sensitivity and subgroup analyses were undertaken using the RevMan 5 software.
Results
Study selection and characteristics
A total of 3433 studies were retrieved following the previously defined search strategy. After duplicate removal and initial title and abstract screening ( Supplementary Tables III and IV ), there were 1765 studies included, and 1751 studies were excluded, leaving 14 studies for full-text review , , , After the full articles were retrieved and careful application of the eligibility criteria, 5 studies were excluded for having no control group, , including a surgical intervention and using 2D lateral cephalogram as a method of outcome assessment. , Finally, only 9 retrospective cohort studies met the eligibility criteria and were included in our systematic review , The Preferred Reporting Items for Systematic Reviews and Meta-Analyses flowchart ( Fig 1 ) shows the process of selecting studies for meta-analysis. The included studies were published from 2010 to 2020. A complete list of excluded studies with reasons can be found in Supplementary Table III . From these 9 studies, 7 were included in the quantitative synthesis, as 1 study did not adequately report outcome data in the control group, and the second study acquired the 3D CBCT volume at different time points. The results of essential features and methodological quality assessment of the included studies are listed in Table II , and descriptions of the outcome variables are listed in Table III .
| Author, y | Type of study | No. and gender | Age, y | Type of intervention | Comparison | Outcome measurement | Time points | Quality score | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Volume | MCA | HB | U1 | T0 | T1 | Score | Definition | ||||||
| Park et al, 2018 | R | 6M/27F EG:16 CG:17 |
22.18 ± 3.99 | Distalization extraction of premolars | Distalization nonextraction | Velopharyngeal Glossopharyngeal Total airway |
Velopharyngeal Glossopharyngeal Total airway |
✓ | Immediately before distalization | After distalization | 13 | Moderate risk of bias | |
| Pliska et al, 2016 | R | EG: 8M/18F CG: 17M/31F |
30.4 ±11.4 | Extraction of at least 2 premolars | Nonnxtraction | Nasopharyngeal Velopharyngeal Glossopharyngeal Total airway |
Velopharyngeal Glossopharyngeal Total airway |
Pretreatment | Posttreatment | 14 | Moderate risk of bias | ||
| Shannon, 2012 | R | EG: 11M/16F CG: 30M/31F |
13.5 | Extraction 4 premolars | Nonextraction | Oropharyngeal Velopharyngeal Glossopharyngeal Total airway |
Velopharyngeal Hypopharyngeal |
✓ | Pretreatment | Posttreatment | 12 | High risk of bias | |
| Stefanovic et al, 2013 | R | EG:15M/16F CG:15M/16F |
12.97 ± 1.15 | Extraction 4 premolars | Nonextraction | Nasopharyngeal Oropharyngeal |
Oropharyngeal | Pretreatment | Posttreatment | 13 | Moderate risk of bias | ||
| Valiathan et al, 2010 | R | EG: 10M/10F CG: 20 |
11.3-15.6 | Extraction | Nonextraction | Oropharyngeal | Oropharyngeal | Pretreatment | Posttreatment | 13 | Moderate risk of bias | ||
| Zhang et al, 2015 | R | EG: 5M/13F CG: 18 |
24.1 ± 3.8 | Extraction of 4 or 2 premolars | Untreated | Nasopharyngeal Velopharyngeal Glossopharyngeal Total airway |
Velopharyngeal Glossopharyngeal Total airway |
✓ | ✓ | Pretreatment | Posttreatment | 17 | Moderate risk of bias |
| Leslie and Harris, 2013 | R | EG: 23/41 NE: 32/48 |
Growing | 4 Premolar extraction | Nonextraction | Velopharyngeal Glossopharyngeal Oropharyngeal |
Oropharyngeal | Pretreatment | Posttreatment | 14 | Moderate risk of bias | ||
| Joy et al, 2020 | R | EG: 20M/21F NE: 22M/20F |
26 ± 7.1 | 6 premolars and 35 had 4 premolars | Nonextraction | Nasopharyngeal Velopharyngeal Glossopharyngeal |
Nasopharyngeal Velopharyngeal Glossopharyngeal |
✓ | Pretreatment | Posttreatment | 15 | Moderate risk of bias | |
| Chen et al, 2018 | R | EG: 10M/15F CG: 10M/15F |
EG and CG 12.2 ± 1.3 |
4 Premolars | Nonextraction | Nasopharyngeal Oropharngeal Hypopharyngeal |
Oropharyngeal Hypopharyngeal |
✓ | Pretreatment | Posttreatment | 12 | High risk of bias | |
| Variable | Superior and inferior boundaries |
|---|---|
| Nasopharyngeal volume | From the top of the airway to a line passing through the posterior nasal spine |
| Oropharyngeal volume | From the palatal plane (ANS-PNS) line extending to the posterior wall of the pharynx, the line passing from the most inferoanterior point on the body of the third cervical vertebra and the base of the epiglottis |
| Oropharyngreal MCA | The minimum cross-sectional area within the boundaries |
| Velopharyngeal volume | From the horizontal level of the palatal plane to the horizontal level of the end of the uvula |
| Velopharyngeal MCA | The minimum cross-sectional area within the boundaries |
| Glossopharyngeal | From the horizontal level of the end of the uvula to the horizontal level of the C3 |
| Glossopharyngeal MCA | The minimum cross-sectional area within the boundaries |
| Total pharyngeal volume | Total of nasopharyngeal and oropharyngeal volume from Sella to the level of C3 |
Risk of bias within studies
From the 9 included studies, 7 studies showed a moderate risk of bias , , , and were initially considered appropriate for quantitative synthesis, only 2 studies showed a high risk of bias. ,
Results of individual studies, meta-analysis, and additional analyses
Nasopharyngeal volume
The 4 included studies enrolling a total of 269 patients showed that there was no statistically significant decrease in the nasopharyngeal airway volume after premolars were extraction ( Fig 2 , A ). The mean decrease in volume was −0.09 cm 3 (95% CI, −0.27 to 0.10; P = 0.36) and heterogeneity test among those studies showed homogeneity: τ 2 = 0.00; χ 2 = 1.24, degrees of freedom (df) = 3 ( P = 0.74); I 2 = 0%. The test for subgroup differences showed no statistical significant difference ( P = 0.54).
