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Original Article
5 (
1
); 22-28
doi:
10.4103/2321-1407.148021

Mandibular morphology and pharyngeal airway space: A cephalometric study

Department of Orthodontics, Manipal College of Dental Sciences, Manipal University, Manipal, Karnataka, India
Department of Orthodontics, Jaipur Dental College, Jaipur, Rajasthan, India
Case Western Reserve University, Cleveland, Ohio, USA
Department of Orthodontics, Manipal College of Dental Sciences, Manipal University, Manipal, Karnataka, India
Address for correspondence: Dr. Siddharth Mehta, Assistant Professor, Department of Orthodontics and Dentofacial Orthopedics, Manipal College of Dental Sciences, Manipal University, Manipal - 576 104, Karnataka, India. E-mail: sidmehta100@gmail.com
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 3.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.
How to cite this article: Mehta S, Lodha S, Valiathan A, Urala A. Mandibular morphology and pharyngeal airway space: A cephalometric study. APOS Trends Orthod 2015;5:22-8. Source of Support: Nil. Conflict of Interest: None declared.

Abstract

Introduction

Mandibular retrognathism is considered to be the most important risk factor for upper airway obstruction.

Aim

This cross-sectional study intended to examine the relationship between craniofacial morphology and the pharyngeal airway space (PAS) in patients with mandibular retrognathism and mandibular prognathism, when compared to normal subjects. The study also analyzed the influence of mandibular morphology on pharyngeal length (PL).

Materials and Methods

The PAS was assessed in 92 females (age 15-30 years) further divided into three groups - Group 1- normal mandible (76°≤ SNB ≤82°; n = 31); Group 2-mandibular retrognathism (SNB <76°; n = 31); Group 3-Mandibular prognathism (SNB >82°; n = 30). All subjects were examined by lateral cephalometry with head position standardized using an inclinometer. Craniocervical angulation, uvula length, thickness and angulation were compared among different groups.

Results

The results showed no statistically significant difference in the pharyngeal airway between the three groups. Measurements of PL showed statistically significant higher values for retrognathic mandible group than normal and prognathic mandible group.

Conclusion

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.

Keywords

Head posture
mandibular prognathism
mandibular retrognathism
obstructive sleep apnea
pharyngeal airway

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.

Figure 1: Inclinometer prepared with the spirit levels
Figure 2: (a and b) Patient positioned in the cephalostat with inclinometer
Figure 3: Lateral cephalogram taken with the help of inclinometer

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.

Figure 4: Various linear and angular measurements
Table 1: Various variables used in the cephalometric analysis
Variables Description
OPT/NSL Defined as the craniocervical angulation at the uppermost part of the cervical spine. It is defined as the inside angle between the intersection of OPT line (line joining Cv2tg and Cv2ip points) and the NSL line (extension of SN plane)
Where Cv2tg point is tangent point of OPT line on the odontoid process of the second cervical vertebra and Cv2ip point is the most inferior posterior point on the corpus of the second cervical vertebra
PAS-UP Defined as the minimal sagittal linear distance between the uvula and the posterior pharyngeal airway space
PAS-TP Defined as the minimal sagittal linear distance between the back of the tongue and the posterior pharyngeal airway space
PL Pharyngeal length measured as the linear measurement between the PNS to the base of epiglottis
UL Uvula length measured as the linear distance from point PNS to the tip of soft palate outline
UT Uvula thickness as the linear measurement of the maximum cross sectional dimension of soft palate outline
UA Uvula angulation measured as the inside angle between the intersection of palatal plane (ANS-PNS) to the line joining PNS to the tip of soft palate outline
LAFH Lower anterior facial height measured as the linear distance between the cepahlometric points ANS to Me
Go-Gn Mandibular body length measured as the distance between cephalometric points Go-Gn

PAS – Pharyngeal airway space; LAFH – Lower anterior facial height; PL – Pharyngeal length; UA – Upper airway; Go-Gn – Gonion to Gnathion; PNS – Posterior nasal spine; ANS – Anterior nasal spine; UL – Uvula length; UT – Uvula thickness

Statistical analysis

All statistical analyses were performed using the SPSS software package (SPSS for Windows Xp, version 13.0, SPSS Inc, Chicago). Descriptive data that included arithmetic mean, standard deviation and range values were calculated for each variable as well as for each group and were used for analysis. A P < 0.05 was set to be statistically significant. ANOVA followed by Post-hoc Tukeys test was used to compare the difference among the groups for all the variables under study [Table 2]. Pearson correlation coefficient was done to assess the correlation between different variables.

Table 2: Comparison of various cephalometric variables using ANOVA followed by Post-hoc Tukeys test
Variable Group n Mean SD P Post-hoc test
UL (mm) Normal mandible 31 34.10 3.28 0.331
Retrognathic 31 34.68 3.75    
Prognathic 30 33.40 2.91    
UT (mm) Normal mandible 31 8.97 1.38 0.813
Retrognathic 31 9.03 1.62    
Prognathic 30 9.20 1.32    
UA (®) Normal mandible 31 132.24 6.20 0.007 2>3
Retrognathic 31 133.71 7.11    
Prognathic 30 128.07 7.69    
OPT/NSL (®) Normal mandible 31 98.32 6.85 <0.001 2>1>3
Retrognathic 31 100.65 7.77    
Prognathic 30 92.03 9.02    
PAS-UP (mm) Normal mandible 31 9.84 3.30 0.47
Retrognathic 31 9.13 2.94    
Prognathic 30 10.03 2.80    
PAS-TP (mm) Normal mandible 31 10.00 3.41 0.094
Retrognathic 31 9.95 3.03    
Prognathic 30 11.53 3.08    
LAFH (mm) Normal mandible 31 66.45 5.52 0.053
Retrognathic 31 69.00 4.07    
Prognathic 30 66.07 5.49    
PL (mm) Normal mandible 31 61.61 5.16 0.032 2>3
Retrognathic 31 64.29 5.83    
Prognathic 30 60.30 6.76    

SD – Standard deviation; PL – Pharyngeal length; LAFH – Lower anterior facial height; PAS – Pharyngeal airway space; UL – Uvula length; UT – Uvula thickness; UA – Upper airway; Go-Gn – Gonion to Gnathion

All cephalometric tracings were done by a single investigator (SM). Tracings for 10% of the study subjects were repeated at random by the same investigator after 1 week for assessment of intra-examiner reliability. Intra-examiner reliability was calculated using intra-class correlation coefficient. The values ranged from 0.94 to 0.98 showing acceptable agreement between the tracings.

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].

Table 3: Comparison of SNB and Go-Gn values among 3 groups
SNB value as well as Go-Gn (mandibular body) was taken
Variable Group n Mean SD P Post-hoc test
SNB Normal mandible 31 78.77 1.84 <0.001 3>1>2
Retrognathic 31 72.77 1.97    
Prognathic 30 83.75 1.17    
Go-Gn Normal mandible 31 78.13 5.05 <0.001 3>1>2
Retrognathic 31 76.23 4.41    
Prognathic 30 81.40 3.60    

SD – Standard deviation; Go-Gn – Gonion to Gnathion

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].

Figure 5: Correlation between pharyngeal length and lower anterior facial height
Table 4: Correlation in the whole sample
LAFH PL
Pearson correlation 0.220*
Significant (two-tailed) 0.036
n 92

*Correlation is significant at the 0.05 level (two-tailed). Significant positive weak correlation between LAFH versus PL. LAFH – Lower anterior facial height; PL – Pharyngeal length

Table 5: Correlation in the individual groups
Groups LAFH PL
Normal mandible Pearson correlation −0.162
Significant (two-tailed) 0.384
n 31
Retrognathic Pearson correlation 0.195
Significant (two-tailed) 0.293
n 31
Prognathic Pearson correlation 0.406*
Significant (two-tailed) 0.026
n 30

*Correlation is significant at the 0.01 level (two-tailed). No significant correlation seen in normal and retrognathic patients between LAFH versus PL. In prognathic patients LAFH versus PL had significant moderate positive correlation. LAFH – Lower anterior facial height; PL – Pharyngeal length

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.

Table 6a: Correlation between PASUP and other variables
PASUP SNA SNB SNP UL UT UA OPTNSL PASTP LAFH PL
Pearson correlation 0.056 0.092 0.096 −0.208* −0.086 −0.071 0.308** 0.663** 0.039 −0.101
Significant (two-tailed) 0.595 0.386 0.363 0.046 0.413 0.503 0.003 0.000 0.716 0.338
n 92 92 92 92 92 92 92 92 92 92

*Correlation is significant at the 0.05 level (two-tailed); **Correlation is significant at the 0.01 level (two-tailed). LAFH – Lower anterior facial height; PL – Pharyngeal length; UL – Uvula length; UT – Uvula thickness; UA – Upper airway

Table 6b: Correlation between PASUP and other variables in individual groups
Groups PASUP SNA SNB SNP UL UT UA OPTNSL PASTP LAFH PL
Normal mandible Pearson correlation −0.004 −0.017 0.036 −0.331 −0.045 0.149 0.576** 0.760** 0.046 −0.213
Significant two-tailed 0.982 0.927 0.846 0.069 0.809 0.425 0.001 0.000 0.805 0.249
n 31 31 31 31 31 31 31 31 31 31
Retrognathic Pearson correlation −0.010 −0.245 −0.247 −0.180 −0.057 0.026 0.332 0.681** 0.223 −0.172
Significant two-tailed 0.956 0.185 0.180 0.332 0.762 0.891 0.068 0.000 0.229 0.356
n 31 31 31 31 31 31 31 31 31 31
Prognathic Pearson correlation −0.152 0.124 0.120 −0.027 −0.197 −0.287 0.299 0.526** −0.009 0.158
Significant two-tailed 0.423 0.513 0.528 0.887 0.296 0.124 0.108 0.003 0.962 0.404
n 30 30 30 30 30 30 30 30 30 30

*Correlation is significant at the 0.05 level (two-tailed); **Correlation is significant at the 0.01 level (two-tailed). LAFH – Lower anterior facial height; PL – Pharyngeal length; UL – Uvula length; UT – Uvula thickness; UA – Upper airway

Table 7a: Correlation between PASTP and other variables in the whole study sample
PASTP SNA SNB SNP UL UT UA OPTNSL PASUP LAFH PL
Pearson correlation 0.045 0.182 0.185 −0.011 −0.083 0.046 0.203 0.663** −0.016 0.061
Significant two-tailed 0.672 0.083 0.077 0.916 0.433 0.661 0.052 0.000 0.880 0.561
n 92 92 92 92 92 92 92 92 92 92

LAFH – Lower anterior facial height; PL – Pharyngeal length; UL – Uvula length; UT – Uvula thickness; UA – Upper airway

Table 7b: Correlation between PASTP and other variables in individual groups
Groups PASTP SNA SNB SNP UL UT UA OPTNSL PASUP LAFH PL
Normal mandible Pearson correlation −0.070 0.005 0.090 0.003 −0.071 0.298 0.473** 0.760** −0.168 −0.099
Significant two-tailed 0.707 0.977 0.631 0.987 0.704 0.103 0.007 0.000 0.365 0.598
n 31 31 31 31 31 31 31 31 31 31
Retrognathic Pearson correlation −0.121 −0.019 −0.019 0.062 0.088 0.137 0.247 0.681** 0.162 0.031
Significant two-tailed 0.518 0.921 0.921 0.742 0.636 0.463 0.181 0.000 0.385 0.869
n 31 31 31 31 31 31 31 31 31 31
Prognathic Pearson correlation −0.193 0.029 −0.035 −0.005 −0.374* −0.032 0.305 0.526** 0.132 0.383*
Significant two-tailed 0.308 0.880 0.853 0.977 0.042 0.866 0.102 0.003 0.486 0.037
n 30 30 30 30 30 30 30 30 30 30

LAFH – Lower anterior facial height; PL – Pharyngeal length; UL – Uvula length; UT – Uvula thickness; UA – Upper airway

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.

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