INTRODUCTION

Significant relationships between the pharyngeal structures and both dentofacial and craniofacial structures have been reported.1–7 Skeletal features such as retrusion of the maxilla and mandible and vertical maxillary excess in hyperdivergent patients may lead to narrower anteroposterior dimensions of the airway.8 On the other hand, the oropharyngeal airway has been claimed to affect the growth of craniofacial structures. To breathe through the mouth, one must maintain an oral airway, and, to accomplish this, the mandible and the tongue are displaced downward and backward and the head is tipped back. These postural changes suggest the possible effect on the relationship of teeth as well as the direction of jaw growth, which may become more downward and backward.9

Recently, interest has been focused on uvulo-glosso-pharyngeal dimensions because of a potential relationship between size and structure of upper airway and sleep-induced breathing disturbances.1011 It has been shown that Obstructive Sleep Apnea patients have aberrated skeletal and soft tissue patterns that reduce airway space.1012 The face and anterior cranial base tend to be retruded,1314 the cranial base angle reduced,15 the mandible short or retrognathic (or both),1617 and the lower face height and maxillomandibular planes angle increased.1314 Moreover, the hyoid bone is usually located inferior in relation to the mandibular plane, the tongue and soft palate are enlarged,10151819 and the posterior airway space is reduced.1720

The hyoid bone and its musculature occupy an important role in the maintenance of the pharyngeal airway.21 Several studies have shown that changes in hyoid bone position tend to be related to changes in mandibular position.22–29 Battagel et al29 reported a more posterior position of the hyoid bone in Class II subjects with a narrower upper airways and Adamidis and Spyropoulos30 showed that hyoid bone in Class III subjects lay more anteriorly. The effect of the change in anteroposterior position of the mandible on hyoid bone position and the pharyngeal airway space is well documented.529–32 Surgical mandibular advancement resulted in anterior positioning of the hyoid bone and widening of the minimal pharyngeal airway space,31 whereas surgical mandibular setback was associated with reduction of the saggital dimensions of the oropharyngeal airway space.32 Moreover, Battagel et al29 suggested that mandibular advancement with protrusion splint was associated with a proportionate increase in oropharyngeal dimensions.

On the basis of this close relationship between uvulo-glosso-pharyngeal structures and the facial skeleton, a difference in size and position of soft and hard tissue structures of upper airway in different anteroposterior skeletal patterns was hypothesized. This study, using lateral cephalograms, was carried out to determine whether the uvulo-glosso-pharyngeal dimensions of subjects with normal nasal breathing could be affected by the different anteroposterior jaw relationships.

MATERIALS AND METHODS

Lateral cephalometric radiographs were selected from the files of orthodontic patients at the orthodontic department, Jordan University of Science and Technology Dental Teaching Centre. Records for 1520 orthodontic patients were screened and 90 subjects (45 females and 45 males, ages 14–17 years) were included with the following criteria: no history of orthodontic treatment; breathing comfortably through the nose; normal vertical occlusal relationship (Max/Man planes angle 25.5° ± 5).

Cephalometric radiographs were taken with a Siemens Orthophos-5 machine (Siemens AG, Munich, Federal Republic of Germany) using a standardized technique and a fixed anode-midsaggital plane distance. The subjects were asked not to swallow, not to move their heads and tongues, and to contact their teeth lightly while the radiographs were being exposed. The magnification of the radiographic machine, which was not corrected, was 11.3.

Lateral skull radiographs were traced on acetate paper and 10 hard and soft tissue cephalometric points were registered yielding 12 linear measurements (Figure 1). The measurements were performed manually using a ruler to the nearest 0.1 mm.

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The films were divided into three groups according to the ANB angle:

• group 1, skeletal Class I (ANB angle 1–5)

• group 2, skeletal Class II (ANB angle >5)

• group 3, skeletal Class III (ANB angle <1).

In addition, each group was divided into two subgroups according to sex.

RESULTS

The results of the statistical analysis are shown in Tables 1 through 3. In Class I, sex differences were detected in the vertical position of the hyoid bone in relation to the mandibular plane and in relation to a line connecting retrognathion to C3 (P < .05 and P < .001, respectively), and in the anterior-posterior position of hyoid bone relative to retrognathion (P < .05). In Class III subjects, sex differences were found in the vertical airway length (VAL) (P < .01), the vertical position of hyoid bone in relation to mandibular plane and in relation to a line connecting retrognathion to C3 (P < .01 and P < .001, respectively), and the anterior-posterior position of hyoid bone relative to retrognathion (P < .05). No sex differences were found in Class II subjects.

TABLE 1.  Means, Standard Deviations and P Values for Sex Differences in Each of the Three Skeletal Groups^a

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DISCUSSION

The nasopharyngeal dimensions continue to grow rapidly until 13 years of age35 and then slows until adulthood.3637 In this study, the age range was 14–17 years to ensure that the oropharyngeal structures had reached adult size.

The anteroposterior skeletal relationship was determined using Jordanian norms38 of the ANB angle (ANB = 3 ± 2°). ANB angle is considered the most commonly used cephalometric measurement for evaluation of anteroposterior jaw relationship.3940 The validity of this measurement has been investigated by several researchers. Jacobson41 showed that ANB angle does not provide adequate assessment of jaw relationship because rotational growth of the jaws and the anteroposterior position of nasion influence the ANB angle. Furthermore, Hussels and Nanda42 reported that the vertical lengths from nasion to point B and from point A to point B are usually affected. On the other hand, Oktay39 and Ishikawa et al43 reported that ANB angle is one of the most reliable and accurate measurements of the anteroposterior jaw relationship.

Mouth breathing results in a number of postural changes, such as downward and forward tongue position and head extension.24242644 Because the mode of breathing affects mandibular position, inclination of the mandibular plane, and head posture, only subjects with a normal breathing pattern were included in this study. Because head posture has been suggested to influence the dimensions of the oropharyngeal airway,45–47 cephalometric radiographs of the subjects were recorded with the head in a natural position.4748

Long face subjects have backward growth direction expressed in an increase in face height and gonial angle. It has been found that adenoid obstruction is most common among long face subjects.4950 Opdebeeck et al24 reported smaller nasopharyngeal cavities in long face subjects compared with short face ones. In our study, only subjects with normal vertical skeletal relationship were included to eliminate any effect on nasopharyngeal airway caused by changes in the vertical plane.

Sex differences were not detected in tongue and soft palate dimensions. Pharyngeal dimensions were not affected by sex except in Class III subjects where VAL was longer in males. These findings are in agreement with those reported in the literature,46736 which suggest that sex differences in the pharyngeal dimensions are not present. However, vertical and anteroposterior position of the hyoid bone showed sex differences in Class I and III subjects only, which may reflect a sex difference in neck thickness.

Hyoid bone position is of great clinical interest because it plays an important role in maintaining the upper airway dimensions.51 In this study, the hyoid bone position was different in the different skeletal patterns. In Class II subjects, hyoid bone was located in an upward and backward position, whereas it was located in downward and forward position in Class III subjects. These findings were in agreement with those reported by others.52–54 Yamaoka et al52 found that tongue root was situated more posterior in Angle's Class II compared with Angle's Class III females.

Our study revealed that anteroposterior pharyngeal airway dimensions were not affected by the changes of the ANB angle. This was in agreement with earlier studies suggesting that the anteroposterior dimension of the upper airway is usually maintained by adaptation of both tongue and hyoid bone.2325 Because the hyoid bone is located more posterior in Class II skeletal pattern, the genioglossus, the main protruder of the tongue, generates upper airway dilating forces to maintain upper airway patency.54

However, Kerr5 reported that Class II subjects had a larger nasopharyngeal area than Class I subjects with malocclusion. On the other hand, the VAL was affected by the change in the anteroposterior jaw relationship and Class II subjects had short VAL compared with Class III subjects. Pae et al55 reported that as hyoid bone position moves inferiorly, the pharyngeal length become longer because the hyoid bone and epiglottis are in a close anatomical relationship. The hyoid bone position detected in Class II and Class III in this study explains the difference in upper airway length in Class II and Class III subjects.

Our findings regarding the relationship between uvulo-glosso-pharyngeal structures and anteroposterior skeletal pattern are inconsistent with those that reported no relationship between pharyngeal structures and the ANB angle.475657 However, the variables used to measure pharyngeal airway in the previous studies differed from those used in this investigation, which makes the comparison more difficult.

Although significant correlation was found in this study between anteroposterior skeletal relationship and airway dimensions, it was a low correlation, which was in agreement with that reported by Kerr.5 The position of the hyoid bone and width of inferior pharyngeal space were correlated with the change in ANB angle. As the ANB increases, the inferior pharyngeal space is reduced and the hyoid bone moves upward and backward.

CONCLUSIONS

• Sex differences in the pharyngeal dimensions were found in Class I and Class III skeletal patterns.

• The position of hyoid bone and the width of inferior pharyngeal space had a significant but weak correlation with the change in ANB angle.