INTRODUCTION

The past two decades have witnessed a renewed interest in the interaction between form and function in the craniofacial region. Two physiologic factors have received particular attention with regard to their possible relation to craniofacial development: adequacy of the pharyngeal airway and the postural relations of the head and the cervical column.

A normal airway is considered one of the important factors for the balanced growth of the 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. If this happens during growth it alters the forces affecting the facial skeleton. The mandible may not contact the maxilla during swallowing, permitting unrestrained vertical alveolar development and posterior tooth eruption.¹ Harvold² reported that the lower border of the mandible becomes steeper and the gonial angle increases in mouth-breathing animals. The lowering of the mandible was followed by a downward displacement of the maxilla. Thus, a change in breathing pattern led to a variety of skeletal and dental deformities in subjects that do not ordinarily develop malocclusions.

As there is close association between pharynx and craniofacial structures, a mutual interaction has long been assumed, and studies on subjects have been performed.^3–5 Various predisposing factors account for obstruction of the pharyngeal airways, such as allergies, infections, and environmental irritants,⁶ but there are also the anatomic features that predispose a person to narrower airway passages. Skeletal features such as retrusion of the maxilla and mandible, vertical maxillary excess, and vertical growth pattern of the mandible may lead to narrowing of the airway.^7,8 Furthermore, another study showed that hyperdivergent growth of the facial cranium or excessively vertical growth of the maxilla can result in extended head posture.⁹

Similarly, a positive correlation has been reported between nasal respiratory resistance and craniocervical posture. Increased craniocervical angulation has been reported in children with airway obstruction due to adenoids.¹⁰ Changes have been noted in pharyngeal airway space and head posture after tonsillectomy or adenoidectomy.¹¹

Cephalometry provides a lateral radiographic view of the head and neck in a standard plane with specific emphasis on bone and soft tissue landmarks. Because narrowing of the airway may be related to skeletal and pharyngeal abnormalities, it has been proposed that cephalometry may help to identify patients in whom the structural anomalies contribute to airway obstruction. Therefore, our objective in this study was to observe any difference in the nasopharyngeal and oropharyngeal airways of subjects with different vertical growth patterns and to determine any correlation with their craniocervical posture.

MATERIAL AND METHODS

The present study was conducted on 60 subjects in the Department of Orthodontics and Dental Anatomy, Dr. Z A Dental College, Aligarh Muslim University, Aligarh, India. Subjects ranging in age from 16 to 25 years were included because only small changes in the size of the nasopharynx and associated structures are expected after age 16.¹² In addition, subjects had a full complement of teeth (with the exception of third molars) and had plans to undergo orthodontic treatment. Further screening of subjects for inclusion was done after a detailed case history and clinical examination. Written informed consent was obtained from each participant or his or her parents, and ethical clearance was obtained from the institutional ethics committee.

Subjects who had any history of congenital defect, orthodontic treatment, surgery in the head and neck region, joint disorder, cervical spine disorder, neuromuscular disorder, or nasal obstruction were excluded from the study group. Pretreatment lateral cephalograms were taken with the subject in a natural head position^13,14 by the same operator (Rotograph plus cephalostat, Villa System Medical, Milan, Italy) with the film distance to the x-ray tube fixed at 5 feet. The study sample was divided into three groups based on mandibular growth patterns. The SN-MP¹⁵ angle was used to divide the sample into hypodivergent, normodivergent, and hyperdivergent growth patterns with values of <26°, 26°–38°, and >38° respectively.

The reference points and lines used in analysis were plotted, all lateral cephalometric tracings were scanned (Perfection V700 Photo scanner; Epson, Long Beach, Calif, USA), and output images were directly converted into a portable document format (PDF) in 1:1 ratio to the input. The nasopharyngeal and oropharyngeal areas were measured and analyzed using software (PDF XChange Viewer, Version 2.5 202, Tracker Software Products, Chemainus, BC, Canada) designed for Microsoft Windows, (version 2007, Redmond, WA, USA). Figures 1 and 2 show the reference points and lines used in analysis, nasopharyngeal and oropharyngeal areas, and craniocervical angle, respectively.

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RESULTS

To ascertain reliability, cephalometric films of 12 randomly selected subjects were retraced and remeasured at 3-week intervals. A paired sample t-test was used to determine measurement accuracy. No statistically significant difference was found between the first and second measurements (P > .05).

Pharyngeal Airway Measurements

Nasopharyngeal area

The ANOVA revealed significant differences in the mean nasopharyngeal area among the three groups (P < .05) (Table 1). The nasopharyngeal area was markedly reduced in the hyperdivergent group compared with the hypodivergent group, and this difference was statistically significant (P < .05). However, the mean nasopharyngeal areas of the hypodivergent versus the normodivergent groups and the hyperdivergent versus the normodivergent groups did not show any significant difference (P > 0.05) (Table 2).

Table 1. Measurements (Mean ± SD) for the Three Groups by Growth Pattern^a

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Table 2. Significance of the Mean Difference in Nasopharyngeal Area (mm²) Between Groups According to the Tukey Test

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Oropharyngeal area

The ANOVA revealed significant differences in the mean oropharyngeal area among the three groups (P < .001) (Table 1). Furthermore, a Tukey test (Table 3) revealed that the mean oropharyngeal areas of the hyperdivergent and normodivergent groups were significantly lower compared with that of the hypodivergent group (P < .001 and P < .05, respectively). In pairwise comparison, the normodivergent group was found to have a larger oropharyngeal area than the hyperdivergent group, but this difference was not statistically significant (P  =  .395).

Table 3. Significance of the Mean Difference in Oropharyngeal Area (mm²) Between Groups According to the Tukey Test

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Craniocervical Angles

Comparing the mean craniocervical angles (NSL/OPT and NSL/CVT) of the three groups, ANOVA revealed significant differences (P < .001) (Tables 1). A Tukey test (Table 4) revealed that the mean NSL/OPT of the hyperdivergent group was significantly higher than those of the hypodivergent group and the normodivergent group (P < .05). Similarly, the mean NSL/CVT of the hyperdivergent group was significantly higher than those of the hypodivergent group and the normodivergent group (P < .05). The mean NSL/CVT of the hypodivergent group was also higher than that of the normodivergent group (Table 5).

Table 4. Significance of the Mean Difference in NSL/OPT Between Groups According to the Tukey Test

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Table 5. Significance of the Mean Difference in NSL/CVT Between Groups According to the Tukey Test

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DISCUSSION

For the present study, pretreatment lateral head cephalograms of the subjects were taken in a natural head position to evaluate mandibular growth patterns, pharyngeal airway dimensions, and craniocervical posture. Controversy exists concerning the accuracy of this method as a radiograph depicts a two-dimensional view of a three-dimensional structure. We chose lateral cephalograms for this study as cephalometric analysis of the airway permits precise measurements in a sagittal plane at anatomically well-defined homologous locations. According to Riley and Powell¹⁶ pharyngeal airway space measured by cephalograms was highly correlated (r  =  0.92) with measurements using a three-dimensional computed tomography scan with considerably high accuracy in predictability. Cephalometry also offers considerable advantages over other techniques, including low cost; convenience; minimal exposure to radiation; and the ability to simultaneously analyze head position, craniofacial morphology, and pharyngeal airway.

In 1963, Bergland¹⁷ proposed the bony outline for measuring nasopharyngeal area. However, the Bergland triangle did not fit the outline of the nasopharynx. Later, Handelman and Osborne¹⁸ proposed the trapezoid form by joining palatal line, sphenoidal line, anterior atlas line, and pterygomaxillary line. This method reflects the limits of bony nasopharynx more accurately. Anatomically, the oropharynx is bounded anteriorly by the circumvallate papillae and the oropharyngeal isthmus, superiorly by the hard and soft palate, posteriorly and laterally by the respective pharyngeal walls, and inferiorly by the vallecula (base of epiglottis).¹⁹ The correlations between the different growth patterns of the mandible and the measurement of airway adequacy (nasopharyngeal area, oropharyngeal area) revealed an interesting pattern of associations.

After analyzing the data, we found that the nasopharyngeal area of patients with hyperdivergent growth pattern was markedly different from a normodivergent and hypodivergent growth pattern. However, no significant difference was found between the hypodivergent and the normodivergent group or between the hyperdivergent and the normodivergent group. Similarly, subjects with vertical growth patterns had significantly smaller oropharyngeal area than subjects with normodivergent and hypodivergent growth patterns. This was in agreement with studies done by Ackerman and Klapper²⁰ and Linder-Aronson and Backstrom²¹ who suggested that the nasopharyngeal space was lower in patients with a hyperdivergent growth pattern.

Kawashima et al.²² conducted a study on 45 healthy preschool children and reported a narrower pharyngeal space in patients with pronounced vertical features compared with control patients. Sosa et al.²³ suggested that patients with a larger pharyngeal area and a larger bony nasopharynx tended to have a more anteriorly positioned maxilla and mandible. Joseph et al.⁸ also suggested maxillary anteroposterior positioning as a potential mechanism to justify the decreased sagittal dimension of the nasopharynx in subjects with a hyperdivergent growth pattern. In the present study, narrowing of the nasopharyngeal area may be attributed to retrusion of maxilla, which is common in patients with a hyperdivergent growth pattern.²⁴

Ceylan and Oktay²⁵ studied the pharyngeal area in subjects with different types of sagittal malocclusions. They reported that the larger the ANB angle, the smaller the oropharyngeal area, and that this may be attributable to the tongue and mandible being in a different location in Class II malocclusion than in other skeletal configurations. In our study, a lower oropharyngeal area in the hyperdivergent group may be attributed to a downward and backward rotation of the mandible. With a downward and backward rotation of the mandible, the tongue base might be positioned more posteriorly and inferiorly, thus decreasing the oropharyngeal airway.

The present study also shows an interesting pattern of association between the vertical growth pattern of the mandible, craniocervical posture, and pharyngeal airway space. The craniocervical angles (NSL/OPT and NSL/CVT) of the hyperdivergent group were significantly higher than those of the normodivergent and hypodivergent groups. The observed correlations were in agreement with those described by previous authors⁹ who noted that an extension of the cranium relative to the vertebral column is associated with a large mandibular inclination and increased anterior facial height. Gresham and Smithells²⁶ also reported vertical development of the face in subjects with poor neck posture. This correlation supports the contention that there is a growth-coordinating mechanism that relates mandibular development to craniocervical angulations.

The association between head posture, craniofacial morphology, and pharyngeal airway can be explained by the soft tissue stretching hypothesis.²⁷ According to this hypothesis, change in airway adequacy causes neuromuscular feedback that results in changes in craniocervical angulations. In cases of long-term hyperextension of the head posture, these soft tissues stretch and create a dorsal and caudal force against the teeth and skeleton; such a force can restrict the forward growth of the maxilla and the mandible and redirect it more caudally, as seen in patients with a hyperdivergent growth pattern. The direction of the chains of events can probably be reversed, and any link in this sequence could be the site of a primary affliction that triggers a chain reaction. Though some studies have reported that a vertical growth pattern may cause a narrowing of the airway,^8,28 others have claimed that it is an impairment in the nasal airway that causes a hyperdivergent skeletal pattern in a subject.²⁹

Recently, interest has been focused on pharyngeal dimensions because of a potential relationship between the size and structure of the upper airway and sleep-induced breathing disturbances.³⁰ A narrow pharyngeal airway space is one of the predisposing factors for mouth breathing and obstructive sleep apnea, a condition of partial or complete upper airway obstruction that leads to increased resistance to airflow and potential cessation of breathing for 10 seconds or more.³¹ Anatomic abnormalities of the pharynx are thought to play a role in the pathogenesis of obstructive sleep apnea.³²

By the age of 4 years, the craniofacial skeleton has attained 60% of adult size, and by the age of 12 years, it is 90% of adult size.^33,34 Thus, early recognition and correction of clinically significant nasal obstruction caused by a hyperdivergent skeletal pattern or by adenotonsillar hypertrophy in young children is necessary to prevent further abnormal craniofacial development and irreversible damage to the pharyngeal reflexes, which can lead to the lifelong consequences of sleep-related disorders.

This study showed that the nasopharyngeal and oropharyngeal airways were smaller in subjects with a vertical growth pattern than in subjects with a normodivergent or hypodivergent growth pattern in obvious pharyngeal pathology free patients. A larger craniocervical angulation was seen as a compensating mechanism to maintain the airway in these patients. However, long-term, longitudinal studies with a larger sample should be conducted to confirm the results of the present study and to explore the comprehensive nature of the mechanism at work.

CONCLUSIONS

• Patients with a hyperdivergent growth pattern had smaller nasopharyngeal and oropharyngeal airways when compared patients with other growth patterns. Narrowing of the airway may be due to skeletal features common to these patients, that is, retrusion of the maxilla and the mandible.

• Reduced pharyngeal airways were typically seen in connection with a large craniocervical angulation.

• The difference in the pharyngeal airway dimensions and craniocervical posture between subjects with different mandibular growth patterns confirmed the presence of a comprehensive pattern of correlations among the three.