Evaluation of the upper airway dimensions following Herbst appliance treatment in adolescents: A retrospective study

Study participants

Lateral cephalograms of consecutive patients who were treated with Herbst appliance at the Department of Paediatric Dentistry and Orthodontics of the University of Hong Kong between 2000 and 2012 were screened, and all 44 subjects (23 boys, mean age = 13.3 ± 1.1 years, and 21 girls mean age = 12.6 ± 1.1 years) who fulfilled the following criteria were selected as study participants [Table 1]

1. Chinese patients

2. Age at the baseline: 10–14 years for girls and 12–16 years for boys (ranges represent the average age of the pubertal growth peak ± 2 years)^[19]

3. Duration of the therapy: 10–14 months

4. A conventional Herbst appliance without a headgear used during the therapy

5. Quality cephalograms recorded at the beginning and end of Herbst appliance therapy in the natural head posture, displaying recognizable upper airway structures, and not during deglutition

6. No craniofacial syndromes or clefts.

Longitudinal cephalometric data were obtained from 34 Chinese adolescents (15 boys and 19 girls). At the age of approximately 12 years, they participated in an oral health survey [Table 1], during which they were reviewed during various periods (3–6 months, 5 years, and 15 years) to observe the reproducibility of the natural head posture.^[20,21] None of these adolescents underwent any orthodontic treatment during the review period, and none of them exhibited craniofacial syndromes or clefts. The ANB angle and Wits appraisal displayed skeletal Class I pattern in 23 adolescents, and skeletal Class II pattern in 11 adolescents.

The present study was a retrospective study. All the cephalometric data were anonymized and de-identified before analysis, and an ethics approval has been granted by the Institutional Review Board of the University of Hong Kong/Hospital Authority Hong Kong West Cluster (IRB Reference Number: UW 12-405).

Cephalometric analysis

All cephalograms were scanned (Epson Expression 1649XL, Seiko Epson Corp., Japan) and the analyzed using CASSOS software (Soft Enable Technology Limited, Hong Kong, China). The upper airway variables included eight linear variables and one angular variable: Three variables for the soft palate, four for upper airway depth, and two for the position of the hyoid bone.^[18] Craniofacial variables consisted of three angular and two linear variables [Table 2 and Figure 1]. All linear variables were corrected according to the actual magnification.

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Figure 1:

Landmarks and measurements of the upper airway.

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Data analysis

An examiner (MG) measured all variables. Cephalograms from 15 randomly selected participants (two radiographs per participant) were measured on separate occasions at a 2-week interval. The method error was calculated by Dahlberg’s formula,^[22], $ME=\sqrt{{\displaystyle \sum d^2/2n}}$ where ∑d is the difference between the pairs of replicate measurements, and n is the number of replicate measurements. The method errors for the linear and angular measurements were not significant using a paired t-test and did not exceed 1 mm and 1°, respectively.

All variables were presented as the mean ± standard deviation. A paired t-test was used to compare the changes between pre- and post-Herbst appliance therapy. A two- sample t-test was used to compare the changes between boys and girls. The Pearson correlation analysis was conducted to analyze the relationship between changes in the upper airway and craniofacial variables. The levels of statistical significance were set at P < 0.05, P < 0.01, and P < 0.001. All statistical analyses were performed using Statistical Package for the Social Sciences (SPSS) software (IBM SPSS Statistics, Version 20, IBM Corp.).

Because the growth data used in the present study comprised longitudinal data of 38 adolescents measured at various review intervals and dissimilar review durations, the quantitative calculation was difficult. Hence, the visual inspection method was used to compare the changes in the upper airway variables with growth curves generated by the locally weighted scatterplot smoothing (LOESS) method. The LOESS method is a flexible nonparametric regression technique that does not require a priori specification of a function to fit a particular model to data.^[23] In SPSS, “% of points to fit” was set at 85% for the LOESS option and “Triweight” for the kernel option, similar to a previous study.^[24] A line representing the mean change in pre- and post-Herbst appliance therapy was drawn on the same scatterplot. The slope of this line was compared with that of the growth curve. A steeper slope indicated a greater change in an upper airway variable following Herbst appliance therapy compared with that in the same variable because of natural growth, whereas a flatter slope indicated a smaller change.

Effect of Herbst appliance therapy on the nasopharynx

The effect of FA therapy in the nasopharyngeal airway above the palatal plane is difficult to determine because the size of this region is greatly influenced by adenoid involution during the pre- and early adolescence period,^[28] therefore, the present study used pharyngeal level crossing the palatal plane to represent the nasopharynx and found that the average change in nasopharyngeal depth following Herbst appliance therapy in both boys and girls was smaller than that in nasopharyngeal depth after natural growth. This effect was observed potentially because of the headgear effect of the Herbst appliance.^[29] The headgear effect refers to the influence of inhibition of the maxillary growth as well as the distalization and intrusion of the upper molars produced by the Herbst appliance with the posterior-upward force on the maxillary complex, which is similar with the effect of a high- pull headgear.^[29]

Several studies have reported that headgear treatment can reduce pharyngeal depth in adolescents.^[7,30] Another study demonstrated that headgear treatment is likely to be associate with an increase in oropharyngeal depth.^[31] However, no studies have reported the headgear effect of a FA on the upper airway. The present study suggested that FA therapy potentially restricts the growth of the nasopharynx.

Effect of Herbst appliance therapy on the oropharynx and soft palate

In the present study, the oropharynx was divided into two regions: Upper (retropalatal) and lower (retroglossal) regions. Because the soft palate is the anterior wall of the retropalatal oropharynx, its size and angulation are likely to be associated with the narrowing of this region,^[32] and the FA treatment may change its dimensions.

Jena et al.^[8] reported that the length and inclination of the soft palate decreased following Twin block appliance therapy, whereas the thickness of the soft palate and retropalatal oropharynx increased accordingly. The present study demonstrated a mild improvement in the inclination of the soft palate in both boys and girls, whereas changes in retropalatal oropharyngeal depth and the size of the soft palate following Herbst appliance therapy were the same as those observed after natural growth.

The possible explanation of these different results is that the headgear effect is likely to influence the upper region of the oropharynx, but only for a fixed FA, such as a Herbst appliance, because the delivered force is more consistent than removable FAs, such as a Twin Block appliance, or an Activator. Further research is required to compare these two types of appliances.

When the tongue moves forward in response to FA therapy, the depth of the lower region of the oropharynx is likely to increase.^[33] The present study demonstrated that the retroglossal pharynx increased following the Herbst appliance therapy, but only in girls.

A similar result was reported by Iwasaki et al.^[34] Using cone- beam computed tomography, the researchers observed that following Herbst appliance therapy, the depth of the retroglossal airway increased more in Herbst appliance treated group than the changes in the Class I control group, whereas there was no difference in the retropalatal airway depth between two groups. Because their study comprised a combination of boys and girls as participants, no sex-related differences were reported.

Effect of Herbst appliance on the hypopharynx

Iwasaki et al.^[34] reported that a Herbst appliance can improve hypopharyngeal depth, but again no sex-related differences were reported. The present study demonstrated that only boys displayed a greater increase in hypopharyngeal depth compared with the increased that occurred from natural growth.

Therefore, the present study illustrated that Herbst appliance therapy can improve the upper airway dimension distinctly in boys and girls; for girls, the site was the retroglossal oropharynx; and for boys, the site was the hypopharynx.

Boys displayed improvement in a lower position than girls did probably because of the different increase in the facial height. In the present study, boys demonstrated a greater increase in the anterior and posterior facial heights than girls did although did not reach the level of significance. Furthermore, the Pearson correlation analysis revealed a positive association between the change in hypopharyngeal depth and the increase in the anterior and posterior facial heights.

Because the participants were Chinese, these results may not be consistent among other races. Mislik et al.^[35] reported that no sex-related differences in oropharyngeal depth were observed in Caucasian children aged >9 years, whereas Gu et al.^[18] reported sex-related differences were observed in 12-year-old Chinese children. Furthermore, Hanggi et al.^[14] reported that the changes in the oropharyngeal depth following activator- headgear and fixed appliance therapy of Caucasian boys and girls were similar. Therefore, the growth potential is not similar in Chinese and Caucasian children, yielding potentially dissimilar results after FA therapy in these two races.

Effect of Herbst appliance therapy on the position of the hyoid bone

Although the present study revealed that the hyoid bone moved forward and downward significantly (except the vertical movement in girls) following Herbst appliance, none of these changes were caused by the treatment instead by natural growth. Ulusoy et al.^[17] observed that the change in the hyoid bone position following activator appliance therapy occurred not in the treatment period but in the retention period, whether the participants of the present study displaying the same result needs further investigation that including the retention period.

Implication and limitations

Although the present study demonstrated that the Herbst appliance potentially restricted growth in the nasopharynx, the absolute value of the nasopharyngeal depth increased, with the improved inclination of the soft palate. Therefore, the influence of the headgear effect on the upper airway cannot be overemphasized. The main effects of Herbst appliance therapy were the enlargement of retroglossal oropharyngeal depth in girls and hypopharyngeal depth in boys. Hence, Herbst appliance therapy may be beneficial to children with potential SDB and Class II malocclusion. Schutz et al.^[6] reported the Herbst appliance could improve nocturnal breathing in growing children.

Because of ethical limitations, we could not obtain pure longitudinal cephalometric data of Class II untreated Chinese adolescents. The participants from our growth data comprised adolescents with both skeletal Classes I and II patterns. Their summarized growth pattern is likely to overestimate from that of adolescents with pure Class II pattern,^[36] therefore, the effects of Herbst appliance treatment might be underestimated in the present study. Furthermore, the visual inspection method was merely qualitative and not quantitative. Therefore, the results of the present study must be interpreted with caution and further confirmed with a multicenter trial with a larger sample size and proper control subjects.