INTRODUCTION

It is accepted that maxillofacial growth and development in children is affected by both genetic and environmental factors. Familial aggregation and genetic background have been shown to contribute to prognathism1,2 and other facial and occlusal growth patterns.2–5 On the other hand, normal respiratory activity influences the growth of maxillofacial structures, favoring their harmonious growth and development.6,7 The presence of any obstacle in the respiratory system, especially in the nasal and pharyngeal regions, causes respiratory obstruction and forces the patient to breathe through the mouth.8 The close association between mouth breathing and maxillofacial morphology9–12 suggests that more attention should be paid to the effect of an obstructed airway on maxillofacial growth and development in children.

Some articles have assessed the nasal and pharyngeal airway from lateral cephalograms.13–16 Class II patients have a tendency for a narrower anteroposterior pharyngeal dimension, specifically in the nasopharynx at the level of the hard palate and in the oropharynx at the level of the tip of the soft palate and the mandible.13 The Class II division 1 malocclusion is associated with a narrower upper airway structure without retrognathia.17 However, there are few reports relating airway width and prognathism, especially in children. Therefore, an understanding of prognathism's etiology is crucial for its clinical correction.

Jacobson et al.18 suggested that prognathism was due to not only a longer mandible but also a mandible positioned more anteriorly relative to the maxilla. McNamara19 noted that in 8-year-old children, (1) the wider the lower pharyngeal airway, the more anterior the position of the tongue in the oral cavity, either as a result of habitual posture or due to an enlargement of the tonsils, and (2) prognathism can be associated with a forward tongue position and enlarged tonsils. In other words, habitual mandibular posturing associated with the enlargement of the tonsils might be associated with prognathism. Therefore, the aim of this study was to examine whether pharyngeal airway width was associated with anteroposterior mandibular position in prognathic children.

MATERIALS AND METHODS

Data Preparation

The system used in this study has been described in detail elsewhere,20 but a brief description follows. Each cephalogram was traced, and 9 skeletal points and 27 soft tissue points were systematically digitized using a mechanical three-dimensional digitizing system (Micro Scribe G2X, Immersion, San Jose, Calif; Figure 1). The 27 soft tissue points consisted of 17 points on the posterior pharyngeal wall, 9 points on the posterior outline of the soft palate, and 1 point at the intersection of the posterior border of the tongue and the inferior border of the mandible. The coordinates from each subject were transformed to a standardized plane using a custom-made program written in Microsoft Visual C++ (Microsoft, Redmond, Wash). In this standard plane, Sella was the origin, the FH plane was parallel to the x-axis, and all measurement points were rotated to match this reference plane. From the 27 soft tissue points, upper pharyngeal width was measured from the posterior outline of the soft palate to the closest point on the posterior pharyngeal wall, and the lower pharyngeal width was measured from the intersection of the posterior border of the tongue and the inferior border of the mandible to the closest point on the posterior pharyngeal wall, according to the method of McNamara (Figure 1),19 using a custom-made program written in Microsoft Visual C++. These custom-made programs were made by coauthor Emi Inada.

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RESULTS

The upper and lower pharyngeal width differences between the normal-occlusion and prognathic girls are shown in Table 1. The lower pharyngeal width of prognathic girls was significantly larger than that of girls with normal occlusion (P  =  .01), while there was no significant difference in upper pharyngeal width.

Table 1 Mean Upper and Lower Pharyngeal Airway Width (mm) of Normal-Occlusion and Prognathic Girls

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Correlations between the horizontal coordinates of each skeletal point and the two pharyngeal widths are shown in Table 2. A significant positive correlation was found between the horizontal coordinate of Ar and the lower pharyngeal airway width in both the normal-occlusion and prognathic girls.

Table 2 Correlations Between the Horizontal Coordinates of Each Skeletal Point and the Two Pharyngeal Widths in Normal-Occlusion and Prognathic Girls^a

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DISCUSSION

The association between mouth breathing and craniofacial morphology has been well studied.7,10,12,22–24 The designations “adenoid face” and “long face syndrome” have been used to characterize the typical appearance in patients with obstructed airways.25 Many previous studies have shown that adenoid or tonsillar obstruction of the airway might cause abnormal maxillofacial development.9,26,27 de Freitas et al.28 suggested that patients with Class I or Class II malocclusions and vertical growth patterns had significantly narrower upper pharyngeal airways than patients with Class I or Class II malocclusions with normal growth patterns. Vargervik et al.29 showed that prognathism can be induced by mouth breathing in animals. Hultcrantz et al.30 showed that 77% of patients with open bite and 50% to 65% of patients with buccal crossbites and prognathism were normalized 2 years after tonsillectomy.

The evaluation of upper and lower pharyngeal widths in this study followed the method of McNamara.19 He noted that the standard lower pharyngeal width was 10 mm to 12 mm and that it did not change appreciably with age. In addition, he suggested that (1) apparent airway obstruction, indicated by an opening of 5 mm or less in the upper pharyngeal measurement, could be used as an indicator of possible airway impairment, and (2) lower pharyngeal airway widths greater than 15 mm, due to habitual posture or the enlargement of tonsils, might cause anterior positioning of the tongue.19

Kawashima et al.31 reported a lower pharyngeal airway width in preschool children of 12.1 mm. Abu Allhaija and Al-Khateeb14 reported the average lower pharyngeal airway width of 15 skeletal class I girls from 14 to 17 years of age to be 12.9 ± 5.2 mm. The lower pharyngeal width of the normal-occlusion girls in this study is similar to these previous results. The lower pharyngeal widths of prognathic girls in this study were also similar to those of 15 older prognathic girls ranging from 14 to 17 years of age (14.9 ± 5.4 mm).14 Iwasaki et al.32 also reported that the lower pharyngeal widths of girls with Class III malocclusion were significantly larger than that of girls with Class I occlusion, while there was no significant difference in the upper pharyngeal width evaluated by cone-beam computed tomography.

The lower pharyngeal airway of prognathic girls was wider than that of girls with normal occlusion. Our results suggest that the upper pharyngeal airway does not influence prognathism, and an anterior position of the tongue or enlargement of the tonsils might be characteristics of prognathism in children about 8 years of age.

Ar, which is the point on the condylar posterior edge, could indicate whether the condyle is located more anteriorly and the lower pharyngeal airway is wider (Figure 2). However, only the horizontal coordinate of Ar in both groups was positively correlated with lower pharyngeal width. During childhood, tonsillar hypertrophy increases with age and reaches a peak at 5 to 6 years in both control and obstructive sleep apnea groups.33 Kawashima et al.34 suggested that the tongue base was more anteriorly positioned with tonsillar hypertrophy. This could maintain a normal airway cross-sectional area, even when the lateral dimension was reduced by enlarged palatine tonsils.

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Any change of neuromuscular activity, such as that of positioning the tongue anteriorly, might also displace the mandible anteriorly. Although Jacobson et al.18 stated two causes of prognathism (a mandible that is too long and a mandible that is too far forward), our study suggests that excessive forward positioning of the mandible might compensate for the presence of an obstacle such as an enlarged tonsil.

Finally, these results indicate that the more anterior the condyle position, the wider the lower pharyngeal airway in children about 8 years of age. A wider pharyngeal airway could be an important factor causing prognathism. Therefore, the lower pharyngeal airway should be examined more closely for malpositioning of the tongue or enlarged tonsils in prognathic children before treatment.

CONCLUSIONS

• Prognathic girls had a significantly wider lower pharyngeal airway compared with the airways in girls with normal occlusion.

• More anterior positioning of the mandible of 7- and 8-year-old children resulted in a wider lower pharyngeal airway.