The orthodontic extraction of second premolars: The influence on airway volume

Introduction

The extraction of second premolars and associated changes in the volume of the airway have not been previously explored. This retrospective study aimed to compare the volumetric changes of the airway preorthodontic and postorthodontic treatment in relevant extraction and control samples and to identify variables that may influence the outcome.

Methods

Cone-beam computed radiography scans of 54 patients with second premolar extraction and 59 nonextraction patients treated in a private orthodontic practice were matched for crowding. The average age for both samples was 15 years. The images were individually landmarked and measured by applying volumetric, linear, and angular parameters. The results were analyzed using repeated measures, such as variance analysis, correlation testing, and regression statistical analyses.

Results

There was a statistically significant increase in the airway volume for both groups ( P <0.05). The difference in increase between the groups was not statistically significant. Seven variables demonstrated a collectively significant effect on changes to airway volume ( F [7,112] = 38.48; P <0.001; r ² = 0.701), with 70% of the variation predicted by the variables. Multiple regression analyses indicated that changes to the area of minimum constriction (B = 32.45; t = 11.95; P <0.001) and changes to airway length (B = 94.75; t = 7.79; P <0.001) had a statistically significant effect on airway volume.

Conclusions

The volume of the airway increased in both the extraction and nonextraction samples. The biggest contributors to the increase were an increase in airway length and an increase in the area of minimum constriction, which likely occurred as a result of natural growth.

Highlights

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Airway volume changes after orthodontic treatment were assessed in 2 patient groups.
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Second premolar extraction treatment was compared with nonextraction treatment.
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Airway volume increased in both groups from the beginning to the end of treatment.
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The difference in increase between the groups was not statistically significant.
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Increases in airway length and the area of minimum constriction were influential.

Dental extractions are commonly prescribed by orthodontic therapy to facilitate treatment objectives. However, extraction-based orthodontic treatment has been a longstanding topic of controversy as it was initially contended that extractions were against the determined plan of a higher power. Tweed and Begg were among the first to identify that not all orthodontic patients could be managed without extractions, ^, and as the specialty grew, the controversy continued with claims that extraction-based therapy could flatten faces, compromise dentofacial esthetics, and cause temporomandibular disorder. ^, A more recent controversy has emerged and is associated with the claim that extractions may influence the airway and be implicated in the etiology of obstructive sleep apnea (OSA). ^,

The lateral cephalogram has been an integral aid to diagnosis and treatment planning in orthodontics, and traditionally, the most common method for orthodontists and dentists to assess the airway was via cephalometric radiography. Wang et al and Bhatia et al investigated the airway after the extraction of 4 first premolars for the management of bimaxillary protrusion and found a reduction in airway space. The potential limitation of this method of assessment is that it is a 2-dimensional (2D) representation of a 3-dimensional (3D) structure. Since its introduction in the late 1990s, cone-beam computed tomography (CBCT) has become a commonly used method for the 3D assessment of the craniofacial region. ^, The use of CBCT involves the capturing of a series of images while the scanning component rotates around an individual’s head and constructs a 3D volume. Recent developments in CBCT capturing methods have enabled less radiation exposure to patients than 2D radiographs. Furthermore, a recent study determined that CBCT was an accurate method for measuring pharyngeal volume, pharyngeal minimal cross-sectional area, anterior nasal volume, and nasal minimal cross-sectional area. This is of relevance to patients experiencing OSA as changes in total airway volume and the area of minimum constriction have been shown to be key features in the difference between patients with and without the condition. ^,

A recent systematic review found that pharyngeal airway volume was not affected by premolar extractions. The studies by Valiathan et al and Shannon were among the 7 that were included in the meta-analysis. Both investigations assessed the airway using CBCT scans after the extraction of first premolars in a growing patient population and found that the airway stayed the same or increased in total volume.

Pliska et al investigated volumetric changes of the airway in nongrowing participants who received either extraction or nonextraction orthodontic treatment and found that overall, there were no clinically significant changes in volume or area of minimum constriction between the 2 groups. Those undergoing extractions had at least 2 premolar teeth removed. However, it was not clear which teeth were extracted. Chen et al, in an adult population, found a decrease in the mean cross-sectional airway area within a sample group who underwent the extraction of the 4 first premolars. However, the total volume was not investigated.

A recent study has provided data regarding the influence of the orthodontic extraction of second premolars or second primary molars in which second premolars were agenic on the volume of the oral cavity proper (OCP). However, information regarding the influence of the extraction of second premolars only on the volume of the airway appears to be lacking. ^, Knowledge in this regard can support orthodontic and other clinical treatment considerations in addition to providing baseline data for further relevant research, particularly as total airway volume changes and the area of minimum constriction have been shown to be key features in the difference between patients with and without OSA. ^, It is also of relevance as recent surveys have indicated that there may be a trend toward a preference for extraction of second premolars compared with first premolars. ^, Therefore, this study aimed to investigate the volumetric changes of the airway in a sample of patients who underwent the extraction of the second premolars as part of their orthodontic treatment compared with a nonextraction control group and to subsequently evaluate influencing parameters. The null hypothesis was that the airway volume would not decrease because of the extraction protocol.

Material and methods

Ethical approval was provided by the University of Adelaide Human Research Ethics Committee.

Following a pilot study of 12 CBCT pairs, a sample of 52 patients was considered necessary to determine a significant difference in average airway volume of 450 mm ³ between pretreatment and posttreatment periods.

Using predetermined inclusion and exclusion criteria, a systematic search strategy was applied to identify patients treated by an experienced orthodontist in a private practice based in Adelaide, Australia between 2015 and 2021. The sample of patients corresponded to those evaluated in a recent investigation.

The inclusion criteria for the selection of the experimental and control groups were (1) a single-phase fixed appliance orthodontic treatment program without a history of previous orthodontic treatment and (2) complete patient records, including pretreatment and posttreatment CBCT images with appropriate extensions.

The exclusion criteria included (1) patients undergoing orthognathic surgery in conjunction with their orthodontic treatment and (2) patients presenting with craniofacial syndromes.

In addition, patients in the experimental group had undergone extraction of both second premolars in the maxilla or all second premolars as part of their orthodontic treatment were identified. Alternatively, patients had developmentally absent second premolars, for which the extraction of deciduous second molars was required along with the contralateral deciduous second molar or second premolar.

The experimental group was categorized according to the level of crowding, which was determined from the pretreatment records, into spacing or mild crowding (<3 mm) or moderate or severe crowding (>3 mm). Patients in the control group were matched according to the level of crowding.

The CBCT images of all selected patients were acquired via a CS9300 scanner (Carestream Health, Rochester, NY) [varying KVp and mA values; Voxel size 300 µm] Settings included a scan time of 12-20 seconds and a scanning area of 0.3 mm × 0.3 mm × 0.3 mm. The images were taken according to a standardized protocol requiring patients to breathe in and place their tongues on the palate during image acquisition. The data sets had ca 442 images saved in digital imaging and communications file format. The images were cleaned of noise to enable easy identification of landmarks and the computation of measurements.

After patient deidentification and randomization of the datasets, landmark determination was performed by 1 author (M.M.). ^, The landmarks within the scans were chosen for their familiarity, reliability, and clarity, and their ability to provide the most accurate calculations ( Table I ). Table II outlines the boundaries of the airway.

Table I

Landmarks used

Landmarks
A point
Anterior nasal spine
B point
Gonion left and right
Hamulus notch left and right
Lingula
Mandibular left and right central incisor
Mandibular left and right first permanent molar
Menton
Nasion
Posterior nasal spine
Sella
Maxillary left and right central incisor
Maxillary left and right first permanent molar

Table II

Boundaries of the airway

Boundary	Parameters
Anterior	The base of the tongue, soft palate, and anterior pharyngeal wall as defined by the difference in soft-tissue pixels
Lateral	The pharyngeal wall defined by the difference in soft-tissue pixels
Superior	Defined by bridging the line extending from the palatal plane (ANS-PNS) to the posterior wall of the pharynx
Inferior	The line from the most superior anterior point on the body of the third cervical vertebra to the posterior wall of the pharynx
Posterior	The pharyngeal wall defined by the difference in soft-tissue pixels

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

After landmark identification, a customized software facility was used to compute volumetric and linear measurements. The CBCT images were positioned using menton, anterior nasal spine, and posterior nasal spine landmarks as standardized reference points. Linear and volumetric calculations were based on those described in a recent study investigating the influence of the extraction of second premolars on the OCP.

Statistical analysis

Data were documented in an Excel spreadsheet (Microsoft, Redmond, Wash). Statistical analysis was performed using SPSS software (version 27; IBM, Aramonk, NY). The Shapiro-Wilks test was applied to determine normality. The underlying assumptions for the normality tests were evaluated for each analysis. No changes were deemed necessary for the normalization of the data. The normality assumptions were corroborated by the homogeneity of variance and Levene’s test, which identified that the populations were of equal variance, confirming an equal spread of the data and indicating qualification for analysis of variance as a primary statistical test.

Descriptive statistics were provided in means and percentages. Continuous variables were compared using t tests, whereas categorical variables were compared using the chi-square test to assess the similarity between groups at baseline.

Repeated measures analysis of variances tests were carried out to determine the differences across groups (experimental vs control) and over time. These were appropriate given that a failure to consider within-subject variance typically overestimates the sensitivity of the tests. The significance was set at P ≤0.05.

Correlation testing was conducted to evaluate linear relationships that could explain potential influences on the change in airway volume. This was followed by a more sophisticated statistical analysis using regression and multiple regression analyses to identify confounders, which are commonly identified as any variable that alters the coefficient by 10%. In addition, a multiple regression was computed to evaluate the collective impact as well as the individual contributions of the predictor variable influences on the changes to airway volume.

Results

A total of 113 patients satisfied the inclusion and exclusion criteria (54 in the experimental group and 59 in the control group). Table III indicates that age and the level of initial crowding did not differ significantly between the groups.

Table III

Descriptive statistics (n = 113)

Variable	Extraction (n = 54)	Nonextraction (n = 59)	P value
Sex
Male	18 (33)	24 (41)	0.420
Female	36 (67)	35 (59)
Age, y
Age at T0	15.00 ± 3.51	15.00 ± 3.32	0.743
<16	43 (80)	48 (81)	0.817
≥16	11 (20)	11 (19)
Skeletal classification
Class I	21 (39)	42 (71)	0.001
Class II	33 (61)	17 (29)
Initial Crowding
Spacing/mild (≤3 mm)	25 (46)	25 (42)	0.675
Moderate-severe (>3 mm)	29 (54)	34 (58)
MMA
<25	26 (48)	38 (64)	0.102
25-31	25 (46)	18 (31)
>31	3 (6)	3 (5)
Extraction
Maxillary second premolars	20 (37)	NA	NA
4 Second premolars	34 (63)	NA
Mean treatment length (mo)	23.5 (6.4)	21.5 (7.0)	0.110