Mandibular morphology and pharyngeal airway space: A cephalometric study

INTRODUCTION

Respiratory function plays a significant role in the development of the face and occlusion.^[1] It has been hypothesized that chronic nasal obstruction causes hyper divergent facial growth. Patients having “long face syndrome” are characterized by a vertically long lower face height, narrow alae, lip incompetency, a narrow maxillary arch, and a greater than normal mandibular plane angle.^[2] Heredity, muscle tonicity, repeated adenoidal infection and inflammation and other environmental factors also may influence facial growth.^[3,4]

There are significant relationships between the pharyngeal dimensions and craniofacial abnormalities. Craniofacial abnormalities such as maxillary retrusion, mandibular retrognathism, short mandibular body, and downward and backward rotation of the mandible in hyperdivergent patients may lead to narrowing of the pharyngeal airway space (PAS).^[5,6] Literature supports the notion that mandibular deficiency is frequently associated with a narrower PAS. It is believed that a retrognathic mandible and decreased space between the cranial column and the mandibular corpus might lead to a posterior postured tongue and soft palate, increasing the chances of impaired respiratory function and possibly causing nocturnal breathing problems.

Variations in PAS have also been described with some sleep disorders such as obstructive sleep apnea. Advancement and setback operations are standard procedures for the correction of mandibular retrognathism and prognathism, respectively. Surgery for the mandibular deformity alters skeletal and soft tissue components, including the PAS.^[7,8]

Changes in head posture significantly affect the size of the PAS. A significant relation (r = 0.807) between head posture and the PAS has been demonstrated.^[9,10]

Evaluation of the PAS thus has a very important role in diagnosis and treatment planning in patients with obstructive sleep apnea and dentofacial deformity. Cephalometric analysis of the airway does permit precise measurements in a sagittal plane and has the advantages of convenience, low cost and minimal exposure to radiation.

Therefore, the aims of the present study were to:

1. Examine the relationship between PAS in patients with different mandibular sizes (mandibular retrognathism and mandibular prognathism), when compared to normal subjects and to test the hypothesis whether mandibular retrognathism is associated with reduced PAS

2. Study the influence of mandibular morphology on pharyngeal length (PL).

MATERIALS AND METHODS

Ninety-two subjects were selected from the total patients registered at the Department of Orthodontics and Dentofacial Orthopedics, Manipal from January 2013 to July 2014. All the subjects selected were Indian females and were 15-30 years of age. The methodology was explained to the patients and only after they consented, they were included in the study.

Subjects with a history of previous orthodontic treatment, functional jaw orthopedic treatment, any surgery involving the jaws, or surgery for adenoids; breathing disorders (such as snoring and obstructive sleep apnea); cleft lip and palate; and any systemic disease affecting normal growth were excluded from the study. Based on the degree of sagittal mandibular development in relation to the anterior cranial base, all female subjects were divided into three groups: Mandibular retrognathism (n = 31; defined by angle SNB <76°), Normal mandible (n = 31; 76°≤ SNB ≤82°) and Mandibular prognathism (n = 30; SNB >82°).

Lateral cephalograms (Planmeca company, model Proline 2002cc, Finland, 66 kV, 10 mA, 1 s exposure) were taken with the orbital-auricular plane parallel to the floor and the teeth in centric occlusion or centric relation at the end of the expiratory phase. All subjects received the same instructions for radiographic positioning and were told to place the tongue in a relaxed position and to breathe through their nose after swallowing.

To overcome the influence of head posture on PAS and to standardize the head posture, a simple inclinometer with the help of spirit levels was prepared. An inclinometer is an instrument for measuring angles of slope, elevation or depression of an object with respect to gravity. In orthodontics, it can be used to record accurate registration of the head position of the patient. Ideally, such a device is constructed with contactless, precision potentiometers which can measure changes in single axis angles. In this study, inclinometer was prepared with the help of spirit level or bubble level, which is a plastic tube incompletely filled with colored spirit or alcohol leaving the bubble in the tube. This bubble positions in the center of the tube when placed on a horizontal surface. In the study, one spirit level was attached to the left arm of a pair of eyeglasses, parallel to the sagittal plane out of the subject’s visual range without touching his/her temple plane to determine the changes in pitch. A second spirit level was placed in the center perpendicular to the sagittal plane to measure changes in roll at the bridge of the eyeglasses. When the spirit level was adjusted parallel to the floor, the bubble would remain in the center [Figure 1]. The entire frame of the eyeglasses was made of plastic with no metal screws so it did not cast any shadow or artifact on the lateral cephalogram that could interfere with the analysis [Figures 2a, b, and 3] After tracing the cephalograms on lead acetate sheet, pharyngeal airway was measured and compared.

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Figure 1

Inclinometer prepared with the spirit levels

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Figure 2

(a and b) Patient positioned in the cephalostat with inclinometer

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Figure 3

Lateral cephalogram taken with the help of inclinometer

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The reference points and lines in the cephalometric analysis selected have been shown in Figure 4. Table 1 shows various variables used in the analysis.

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Figure 4

Various linear and angular measurements

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RESULTS

A comparison of various craniofacial morphologic variables obtained in the three groups is shown in Table 2. A statistically significant difference was noted in the mandibular body length (Go-Gn) between all the three groups [Table 3].

The values of McNamara upper and lower airway space (PAS-UP, PAS-TP) were comparable in all the three groups being maximum in the prognathic mandible and minimum in the retrognathic mandible group, but with no statistically significant difference.

Statistically significant difference was noted in uvula angulation between retrognathic and prognathic mandible (P = 0.007). This variable is large when the mandible is small. No significant difference was found among the three groups while comparing uvula length (UL) and uvula thickness (UT).

Craniocervical angulation (OPT/NSL) showed statistically significant difference between the three groups. A decreased craniocervical angulation correlated with a large mandible (P < 0.001).

In the study, the mean PL for retrognathic patients was significantly higher than prognathic patients (P = 0.032). The PL significantly correlated with lower anterior facial height (LAFH) in the entire study population [Figure 5, Table 4]. In prognathic patients LAFH and PL had significant moderate positive correlation (P = 0.026). However, no significant correlation was seen in normal and retrognathic patients between LAFH and PL [Table 5].

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Figure 5

Correlation between pharyngeal length and lower anterior facial height

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DISCUSSION

The pharynx is a tube-shaped structure that plays an important role in respiration and deglutition. The dimensions of the pharynx continue to grow rapidly until 13 years^[11] of age and then quite slow until adulthood.^[12] King^[13,14] had reported no significant change in the depth of nasopharynx after 12 years of age. In the present study, the age range of the subjects was 15-30 years to ensure that the pharyngeal structures had reached adult proportions. Our study sample consisted of only female subjects because they seek (in age group >15 years) orthodontic treatment more often than males.^[15] This also helped to avoid influence of gender on pharyngeal dimensions.^[16]

Head posture has been suggested to influence the dimensions of the PAS.^[8] Thus, in order to eliminate the effects of head posture on the dimension of the PAS, lateral cephalograms were taken with patients wearing a simple inclinometer.

Lateral cephalometric films are considered reliable to record airway dimensions^[17] as Cameron et al.^[18,19] found a significant positive relationship between nasopharyngeal airway size on cephalometric films and its true volumetric size as determined from cone beam computed tomography scan in adolescents.

Subjects were divided into three groups based on the SNB angle. However, this does not by pass the influence of cranial base inclination on mandibular position (SNB), so mandibular body length (Go-Gn) was measured for all the three groups which showed a statistically significant difference [Table 3]. The results of our study showed that the mandibular position (SNB) had no effect on the dimensions of the pharyngeal airway passage. No statistically significant difference (P = 0.47 and 0.094 respectively) in the (PAS-UP, PAS-TP) between the three groups was observed. Pearson’s correlation between PAS-UP and PAS-TP to other cephalometric variables is shown in Table 6a, 6b, 7a, 7b.

Past research on PAS has shown that a long soft palate was associated with smaller oropharyngeal depth and was more common among subjects who snored and had obstructive sleep apnea.^[19] According to Jena et al.^[20] and Muto et al.^[21] the increased length of the soft palate among subjects with mandibular retrognathism could be the result of the backward position of the tongue, which compressed the soft palate and resulted in decreased thickness and increased length of the soft palate. The soft palate was thicker among subjects with a prognathic mandible followed by retrognathic and normal mandible. On the contrary, Abu Allhaija and Al-K Abu hateeb^[22-24] reported a thinner soft palate among Class I subjects compared with Class II and Class III subjects. Therefore, the UL was correlated in subjects with different (prognathic and retrognathic) mandibular morphology. But no significant correlation was noted in this study.

Uvula angulation showed a statistically significant difference (P = 0.007) between mandibular retrognathism and prognathism. Maximum inclination of the soft palate was found in patients with mandibular retrognathism, literature^[20,21] explains that the mandibular retrognathism is associated with posterior location of the soft palate and narrowing of the oropharyngeal airway due to the backward position of the tongue and its contact with the soft palate.

Craniocervical angulation (OPT/NSL) was found be significantly different (P < 0.001) among the three groups with maximum value in retrognathic mandible group followed by normal and prognathic mandible group respectively. Thus subjects with retrognathic mandible had an extended head posture (increased craniocervical angulation) to compensate for the airway space. Moderate cranial extension was thought to decrease the resistance to airflow in the upper airway thickness.^[25] Woodside and Linder-Aronson^[26] also postulated that mouth breathers tip their head backward in an attempt to increase their airway.

Pharyngeal length showed statistically significant (P = 0.032) variation between mandibular retrognathism and prognathism patients. The mean PL for retrognathic mandible patients was significantly higher (P = 0.032) than prognathic mandible patients. These results correlate with the findings of Pae et al.^[27,28] who suggested that longer pharynx is more suspectible to collapse (the cross-sectional area of the pharynx being constant). Thus, patients with a retrognathic mandible are more predisposed to developing obstructive sleep apnea. On the contrary, Kerr reported that Class II subjects had short PL compared with Class III subjects.^[29]

The PL and LAFH in the whole study population showed a significant (P = 0.220) positive correlation. This finding correlates with Kerr^[29] and confirms the logical premise that subjects with long faces generally have longer nasopharynges. This can serve as an important diagnostic factor for obstructive sleep apnea.

A few limitations of this study are lack of sample size predetermination and the results based on two dimensional measurements on a lateral cephalogram. However future studies can focus on a bigger sample and 3 dimensional analysis of PAS for better results.

CONCLUSIONS

Hypothesis that mandibular retrognathism is associated with reduced PAS is rejected. There is no significant difference between PAS between patients with mandibular retrognathism, normal mandible and mandibular prognathism. Mandibular retrognathism patients show a significantly higher uvula angulation than patients with mandibular prognathism. Craniocervical angulation showed maximum value in retrognathic mandible group followed by normal and prognathic mandible group respectively. Mean PL for retrognathic mandible patients was significantly higher than prognathic mandible patients.