Objective:

To determine how orthodontic treatment with first premolar teeth extracted and the associated arch dimensional changes in bimaxillary proclination patients affect the upper airway dimensions.

Materials and Methods:

Pre- and postorthodontic treatment cephalograms and dental casts of 40 bimaxillary proclination patients (ages ranged between 18 and 23 years) were used for this study. Patients were all treated with extraction of the four first premolars. Cephalometric radiographs were used to measure airway dimensions, and dental casts were used to measure the changes in the arch dimensions. A paired t-test was used to detect differences at P < .05.

Results:

The results showed statistically significant reductions in tongue length (P < .05), posterior adenoids thickness (AD2-H) (P < .05), upper and lower incisor inclination, and lower incisor to A-Pog line (P < .001). Considering the dental cast results, statistically significant reductions in upper arch length, lower arch length, and lower intermolar width were also found (P < .001). The only statistically significant increase was recorded for the upper intercanine width (P < .001).

Conclusions:

Extraction of the first premolars for the treatment of bimaxillary proclination does not affect upper airway dimensions despite the significant reduction in tongue length and arch dimensions.

Bimaxillary proclination can be defined as a condition where the upper and lower incisors are proclined and protrusive, which results in increased lip procumbency. Keating1 suggested that upper incisor inclination of 115° and lower incisor inclination of 99° or more, together with an interincisal angle of 125° or less, are features to diagnose bimaxillary proclination.

Bimaxillary proclination is more common in African-American2,3 and Asian populations4,5; nevertheless, it can be seen in almost every ethnic population.6 The etiology of bimaxillary proclination seems to be multifactorial and consists of genetic and environmental factors, such as mouth breathing, tongue and lip habits, and tongue volume.4 Although the overall tooth size of maxillary and mandibular dentition was found to be larger than that of the corresponding control sample,7 dentition in bimaxillary proclination patients usually adapts to the underlying skeletal and surrounding soft tissues,1 resulting in less crowding because the incisor proclination compensates for such potential crowding.8 

The major orthodontic treatment goal in bimaxillary proclination is to reduce the proclination of the maxillary and mandibular incisors with a resultant decrease in soft-tissue procumbency and convexity.6 A common treatment approach for such patients is to extract the four first premolars and then retract the anterior teeth.9 

Lateral cephalograms have been used extensively to study airway dimension. The reliability of lateral cephalograms in assessing airway dimension has been studied before, and findings suggested that cephalometric head film can provide valuable information about airway size.10 Also, the commonly used landmarks of the airway structures can be reliably identified, as reported by another study.11 

The effect of premolar extraction on airway dimension has been studied before,12 however, changes in the upper airway dimensions that may occur because of the extraction of first premolar teeth in a sample of patients with bimaxillary proclination, who may have different airway dimensions,13 has not been studied before.

Dental arch expansion is associated with airway dimensional changes,14,15 and because the effect of extraction treatment will be reflected on the arch dimensions, as reported in many studies,1621 and because most of the extraction spaces in patients with bimaxillary proclination would be used for incisor retroclination and correction of lip procumbency, one would think that altering incisor and soft-tissue position and arch dimension could affect tongue position and therefore the upper airway dimension.

The aims of this study were to investigate the effect of fixed orthodontic treatment with first premolar teeth extraction in patients with bimaxillary proclination on the upper airway dimensions and to study the relationship between upper and lower arch dimensional changes and the changes in upper airway dimension.

This retrospective study was carried out on the available pre- and postorthodontic treatment records of patients who had bimaxillary proclination and were treated at Dental Teaching Centre of Jordan University of Science and Technology. An ethical approval for the conduction of this study was obtained from the Institution of Research Board at Jordan University of Science and Technology. The study material consisted of 40 patients with bimaxillary proclination (27 women, 13 men) who had fixed orthodontic treatment with extraction of upper and lower first premolar teeth. Subjects were selected according to the following criteria:

  1. Pretreatment bimaxillary proclination (upper incisor to maxillary plane angle, UI/Max ≥ 115°), lower incisor to mandibular plane angle (LI/Mand ≥ 99°), and interincisal angle less than 125°.

  2. No medical history, that is, the selected patients did not suffer from any medical disease. Patients with medical history of pharyngeal pathology and/or nasal obstruction, snoring, obstructive sleep apnea, adenoidectomy, and tonsillectomy were excluded.

  3. A minimum age of 18 years at the start of treatment.

  4. Pre- and postorthodontic treatment lateral cephalograms and dental casts of adequate diagnostic quality.

  5. Patients had no crowding or spacing at the start of treatment.

  6. Pretreatment class I molar relationship.

  7. Treatment consisted of fixed orthodontic appliances only.

Patients' ages ranged between 18 and 23 years (average age was 19.21 ± 1.46 years). Pre- and posttreatment lateral cephalograms were manually traced using acetate tracing paper carefully attached to the radiographs. During tracing, the room was darkened and the viewing screen was blanked off, showing only the radiograph. Magnification of radiographs was corrected and calibrated according to the magnification factor and using the radiopaque ruler (calibration marker). Lateral cephalometric and dental cast measurements were all performed by the same investigator.

Thirty-one landmarks (21 landmarks for sagittal airway measurements and 10 for skeletal and dental measurements) were identified for each cephalogram, yielding 24 linear and 7 angular measurements (Figure 1). Definition of the different points and measurements used are shown in Table 1. All cephalometric measurements were performed manually using a ruler to the nearest 0.1 mm to measure the linear distance between the two points, making the measurement (Table 1 and Figure 1) and protractor to the nearest 0.5° to measure the angular measurements. Pre- and postorthodontic treatment dental casts (upper and lower) were also analyzed manually using a divider and an orthodontic ruler to the nearest 0.1 mm. Dental cast measurements included intercanine, interpremolar, and intermolar widths and arch length. The transverse distances of the canines were measured between their cusp tips. The interpremolar widths were measured between their buccal cusps. The intermolar widths were measured between the mesiobuccal cusps of the first molars. Arch length was measured between the mesiobuccal cusp of the first molar on one side to the mesiobuccal cusp of the first molar on the contralateral side, using a piece of wire that was adapted between the mesiobuccal cusp of the first molars to the tip of the buccal cusp of the premolars, canines, and incisor edges on both sides. The wire was then straightened and measured on a ruler.

Figure 1

Cephalometric measurements of the airway dimensions.

Figure 1

Cephalometric measurements of the airway dimensions.

Close modal
Table 1

Points and Measurements Used to Assess Upper Airway Dimension

Points and Measurements Used to Assess Upper Airway Dimension
Points and Measurements Used to Assess Upper Airway Dimension

Error of the Method

Ten lateral cephalograms and 10 pairs of dental casts were randomly selected and reanalyzed after a one-month interval. Dahlberg's formula22 , for double measurement determination was used to calculate the error of the method of measurements. Houston coefficient of reliability23 was calculated for all measured variables.

Statistical Analysis

Statistical analysis was performed using the Statistical Package for Social Science (SPSS) computer software (SPSS 17.0, SPSS Inc, Chicago, Ill).

Descriptive statistics (means and standard deviation) were calculated for all the measured variables. A paired t-test was conducted to detect the differences between pre- and posttreatment measured variables for the same individuals. Significance was predetermined at .05 levels.

Error Testing

Dahlberg's error ranged from 0.06 mm for middle airway space to 0.19 mm for arch length, indicating that there were no significant differences between any of the measured variables at two different time points. the Houston coefficient of reliability was calculated and was above 90% for all measured variables.

Treatment Changes

To determine the overall effect of orthodontic treatment of bimaxillary proclination, changes between pretreatment and posttreatment cephalometric and dental cast measurements were determined and are presented in Tables 2 through 4. Means and standard deviations of pretreatment and posttreatment measurements and the associated P values are listed.

Table 2

Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Dental and Skeletal Relationships

Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Dental and Skeletal Relationships
Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Dental and Skeletal Relationships
Table 3

Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Soft Tissues and Airways Dimensions

Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Soft Tissues and Airways Dimensions
Means and Standard Deviations of Pretreatment and Posttreatment Measurements for Soft Tissues and Airways Dimensions
Table 4

Means and Standard Deviations of Pretreatment and Posttreatment Dental Cast Measurements

Means and Standard Deviations of Pretreatment and Posttreatment Dental Cast Measurements
Means and Standard Deviations of Pretreatment and Posttreatment Dental Cast Measurements

Cephalometric Measurements

The upper and lower incisors were retroclined a mean of 10.26° (±5.76) and 9.70° (±4.83), respectively. Lower incisors were retracted relative to the A-Pog line by a mean of 3.37 mm (±2.17). The mean interincisal angle was increased 18.90° (±8.73) as a result of treatment. All of these changes were found to be statistically significant (P < .001). Skeletally, none of the measured variables were found to be statistically significant.

Soft Tissues and Airways Dimensions

Tongue length was reduced as a result of orthodontic treatment; the mean reduction was 1.75 mm (±4.10). This reduction in the tongue length was statistically significant (P < .05).

The mean reduction in the measured upper adenoid thickness (AD2-H) as a result of treatment was 1.01 mm (± 2.36). This was also statistically significant (P < .05).

Dental Cast Measurements

For the upper arch, statistically significant results were recorded for both arch length and intercanine width (P < .001 and P < .05, respectively). The upper arch length decreased (mean decrease of 13.77 ± 4.70 mm), whereas intercanine width increased (mean increase of 0.94 ± 2.23 mm). For the lower arch, highly statistically significant results were recorded for both arch length and intermolar width (P < .001). The mean reduction in the lower arch length was 16.52 mm (± 15.64) and in the intermolar width was 2.25 mm (± 2.24).

This retrospective study aimed to reveal the effect of premolar extraction as part of fixed orthodontic treatment and the associated arch dimensional changes on the upper airway dimensions in bimaxillary proclination patients.

Lateral cephalometric radiographs were used to measure airway dimensions and dental casts were used to measure changes in arch dimension. Using lateral cephalograms to assess the airway dimension is considered a reliable method.10 

It has been found that dental arch expansion was associated with airway dimensional changes,14,15 and because the effect of extraction treatment will be reflected on the arch dimensions, as reported in many studies,1621 dental arch dimensions were recorded and related to the changes in upper airway dimensions.

The effect of growth may play a role when evaluating dimensions of the pharyngeal airway. It has been found that a greater rate of changes in the soft-tissue measurements of the posterior pharyngeal wall occurred between 6 and 9 years and between 12 and 15 years.24,25 In this study, the age range was 18 to 23 years to ensure that the oropharyngeal structures had reached the adult size and that effect from growth would not affect the results.

After upper and lower first premolars extraction and retraction of the anterior segment, significant retroclination of upper incisor to maxillary planes angle, lower incisor to mandibular planes angle, and retraction of lower incisor to A-Pog was observed. These results were expected because the aim of treatment was to reduce the incisal proclination and the lip procumbency. These findings were also in agreement with other studies.26,27 Interincisal angle was increased as a result of upper and lower incisor retroclination and was almost in agreement with the increase (18.1°) recorded by Bills et al.6 

In this study, skeletal measurement (SNA, SNB, and ANB angles) did not change significantly as a result of first premolar extraction. This was in agreement with another study,27 which reported that these angles did not change significantly in both minimum and high anchorage extraction cases. On the contrary, Sharma28 reported a significant retraction in the skeletal and soft-tissue points A and B, with mean reduction of 2.367° and 1.95° in SNA and SNB angles, respectively. The possible cause of this disagreement may be related to the usage of palatal root torque in the upper and lingual root torque in the lower arch wires during the incisor retraction in Sharma's study28 to prevent labial movement of the roots. In contrast, in this study torque was intentionally not incorporated in the arch wire, and the incisors inclination was reduced significantly as mentioned previously. This indicates that the possible slight labial incisor root movement was responsible for moving A and B points anteriorly and compensating for the backward movement due to incisor retraction.

The only significant finding concerning the airway measurements was for the upper adenoid thickness (AD2-H), in which the mean reduction was 1.01 mm. Despite this reduction, the airway dimensions were not affected in any location. This was in agreement with another study, which reported that extraction of four premolars with retraction of incisors did not affect oropharyngeal airway volume.12 

On the other hand, Germec-Cakan et al.27 carried out a study to investigate the upper respiratory airway dimensions in nonextraction and extraction subjects treated with minimum or maximum anchorage. They reported that the superior airway space and the middle airway space were increased significantly in the minimum anchorage group. The mean increase in these dimensions was approximately 1.5 mm after treatment. On the other hand, the middle airway space and the inferior airway space were reduced significantly in maximum anchorage group. They explained this difference between the two groups by a mesial molar movement after resolution of anterior crowding, which was, on average, 3 mm in the minimum anchorage group. This mesial molar movement might have increased the posterior tongue space and so the superior and middle airway dimensions. The other possible cause they mentioned was the growth. In the maximum anchorage group, on the other hand, they suggested that after significant incisor retraction, the tongue space might have reduced and resulted in a significant reduction of the middle airway space and inferior airway space, which was approximately 3 mm.

The other significant finding in the present study was tongue length, which was reduced significantly (mean reduction  =  1.75 mm). This reduction is still in line with expectations because the restriction of the tongue after bimaxillary proclination treatment is considered to be the main cause of relapse and space reopening. Hyoid bone position was also not affected significantly in this group. This was in agreement with findings reported earlier.27 

In this study, a significant increase in the upper intercanine width was recorded after premolar extraction. This is possibly due to distal movement of the canines to the extraction spaces and so a wider part of the arch. This finding was in agreement with findings reported by others.1619,21 

Neither upper interpremolar nor intermolar width changed significantly in this study. However, intermolar width was reduced by a mean of 0.97 mm, which was in agreement with the 0.88 mm reduction reported by Isik et al.21 and the 0.53 mm reduction reported by Kim and Gianelly.20 This reduction may be due to the mesial movement of these teeth toward the narrower anterior part of the arch.17,29,30 

In the lower arch, the only significant change in arch width was in the lower intermolar, which was reduced significantly after treatment. This was in agreement with the 0.94 mm reduction reported by Kim and Gianelly20 and with the findings of Gardner and Chaconas.31 The cause behind this reduction may again be due to mesial movement of the molar teeth toward the narrower part of the arch.

Regarding arch length, the result of this study revealed a significant decrease in both upper and lower arch lengths due to extraction. This was expected because of the extraction of four units and was in agreement with the findings of another study.32 

  • Upper and lower first premolar extraction for the treatment of bimaxillary proclination does not affect the upper airway dimensions.

  • Reduction in arch dimensions as a result of extraction treatment does not affect the upper airway dimensions.

The study was supported by the Deanship of Research/ Jordan University of Science and Technology [grant number 100/2010].

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