INTRODUCTION

More attention has been paid to sleep pathology in recent years due to the high level of comorbidity and complexity of the disease. Obstructive sleep apnea-hypopnea syndrome (OSAHS) is a chronic disorder characterised by repetitive episodes of total or partial collapse of the upper airway during sleep, causing progressive asphyxia for more than 10 seconds (Spicuzza et al, 2015.).

Some risk factors have been described as possible triggers of OSAHS, such as age, male sex, obesity and craniofacial deformities; the latter two present a positive correlation in patients diagnosed with OSAHS (Neelapu et al., 2017; Albajalan et al., 2011).

Some investigations show a relation between airway size and morphological facial pattern; the position of the maxilla and the mandible in the anteroposterior direction influence this volume (Sprenger et al., 2017.) This anatomical variation may be further exposed where there is a vertical facial increment in conjunction with mandibular retraction, causing pharyngeal reduction of the airway and respiratory collapse (Neelapu et al., 2017).

Studies show that different morphological patterns are linked with an increase or diminution of the vertical facial position, and with sagittal alterations of the maxilla and mandible, generating alterations in the airway volume (Muto et al. 2008; Zheng et al. 2014). This morphology creates the conditions for alterations in the respiratory mechanism (Zheng et al. 2014) and increases the risk of associated pathologies (Gong et al. 2018). In this way, morphological craniomaxillofacial anomalies generate significant functional changes which involve oral dysfunction, labial incompetence, functional alterations and muscular imbalances (Sprenger et al. 2017).

In cases of diagnosis of severe respiratory alterations like OSAHS, different treatment modalities have been proposed like the continuous positive airway pressure (CPAP) mask. The main problem with these is the failure of patients to adhere to the treatment, since 17-54 % of patients do not use the CPAP for a prolonged period. Surgical alternatives have also been proposed. One viable option is orthognathic surgery (Ferraz et al. 20016) since it allows an increase in the anteroposterior and lateral relations of the maxilla, with consequent increase in airway volume (Veys et al., 2017; Zaoui et al., 2019).

The aim of this research was to analyse the airway volume in subjects with different skeletal conditions by 3D imaging.

MATERIAL AND METHOD

A cross-sectional study was carried out to identify the airway volume in adult subjects with different sagittal skeletal anomalies where orthognathic surgery is indicated; all the images obtained were used for pre-surgery facial diagnosis. The subjects enrolled in the study voluntarily, signing an informed consent document. The investigation safeguarded the integrity of the participants and complied with the Helsinki Declaration.

The subjects included were subjects of both sexes, aged over 18 years, who presented a Class II facial deformity (Steiner analysis with angle >2º) or a Class III deformity (Steiner analysis with angle <2º); subjects with previous facial surgery, background of facial trauma or conditions which signified alterations of the facial morphology were excluded.

Images were taken of all the subjects using the NewTom 3D Tomograph, Model VGi EVO (Verona, Italy), viewing field 24 x 19 cm. and exposure parameters 110 kV, 8 mA, 15 sec. The image was obtained by a technician who specialises in this kind of imaging; the patient was positioned immobile in a vertical position while the image was taken, with lips in repose and without forcing any position of the body.

The image was analysed using the NNT New Tom software (Imola, Italy) by a specialist in Maxillofacial Radiology who delimited the pharyngeal space in a rectangular area; the reference points used were:

- Anterior position: posterior nasal spine in the sagittal plane and choanae in the axial plane.

- Posterior position: posterior wall of the pharynx.

- Upper position: highest point of the nasopharynx.

- Low position: low hyoid bone, at the inferior edge of the vertebral body of C4.

Based on these variables, a statistical analysis was carried out to determine the relation between them. Spearman's test was used, with threshold of statistical significance p<0.05.

RESULTS

In this research, 115 subjects were included. According to Steiner's proposed definition, there were 71 skeletal Class II subjects (61.7 %) and 44 Class III subjects (38.3 %) (Figs. 1 and 2). The subjects included presented a mean age of 27.8 ± 11.6 years (range 18 to 63 years). 46 subjects (40 %) were male and 69 (60 %) were female. Table I identifies the airway volumes observed in the different measurement levels.

Using Spearman's test, the airway volume was significantly greater in Class III subjects (p=0.034). The subjects also presented significant differences by sex (p=0.0159), although this difference may be associated with the fact that the Class II group of subjects (smaller airway volume) contained almost twice as many female as male subjects.

When each measurement was examined independently, it was observed that at all levels the subjects with Class II characteristics presented smaller airway measurements than Class III subjects. In the minimum area reported in each class, Class III presented an area 10.4 mm2 larger than Class II; in the maximum area, Class III presented an area 46.6 mm2 larger than Class II; in the total volumetric measurement, Class III subjects presented a mean volume 4.1 mm3 larger than Class II subjects.

Fig. 1 Subjects with Class III skeletal deformity and observation in airway. 

Fig. 2 Subjects with Class II skeletal deformity and observation in airway. 

Table I Distribution of the 115 subjects included by type of facial deformity and airway volume. 

DISCUSSION

Facial morphology defines conditions of form and function which are essential for life and quality of life. In these analyses, 3D studies have evolved to explain morphology better and to define the best treatments.

Studies with 2D images have been used for many years and help to define certain standards (^{Olate et al, 2016}); however they still present limitations due to the three-dimensional form of the human body. In 2014, our group (Olate et al., 2014) indicated dimensional changes in the airway by 2D analysis, showing that subjects with Class II facial deformity presented a smaller airway area based exclusively on sagittal measurements. We also indicated that the nasopharynx presented limited differences, showing that the position of the mandible was the most important in this definition since a more retracted position of the hyoid bone was also related with a more retracted position of the mandible.

The morphology of the airway has been reviewed analysing respiratory pathologies like obstructive sleep apnea-hypopnea syndrome (OSAHS). This syndromic disease has a great impact on quality of life and the presence of comorbidities. ^{Jung et al. (2019}) indicated that patients with OSAHS present a high risk of cardiovascular morbidity; ^{Kim et al. (2019}) also indicated that patients with OSAHS are associated with sub-clinical systemic arteriosclerosis. Thus OSAHS and respiratory pathology are today a potentially destructive problem with important connotations for public health.

In children, a relation has been reported between skeletal alterations like mandibular divergence and the presence of OSAHS, although no cause-effect relation has been identified between craniofacial anomalies and respiratory obstructions (^{Galeotti et al., 2019}). Also in children, a correlation has been reported between reduction in cortical bone tissue and the presence of OSAHS, probably also determining an alteration in bone homeostasis (^{Eimar et al., 2019}).

Our investigation studied the airway conditions of patients with skeletal deformities. OSAHS was not found in any of these patients, however skeletal alterations have been defined as risk factors for the presence of OSAHS. ^{Sakakibara et al. (1999}) indicated that in obese patients the increase of soft tissues in the airway zone are an important element in the presence of OSAHS, while in non-obese patients skeletal alterations are more important in the presence of OSAHS. Similar conclusions were presented by ^{Yu et al. (2003}), who indicated that narrow nasopharynx and narrow oropharynx are important risk factors for patients with OSAHS.

Although the relation between airway reduction and OSAHS is real, attention has focused on more specific aspects of these relations. ^{Mouhanna-Fattal et al. (2019}) compared craniofacial relations in groups with and without OSAHS and they found that the airway volume was smaller in subjects with OSAHS but that this was not necessarily related with craniofacial morphology. ^{Cillo Jr. et al. (2012}) found no relation between any of the specific cephalometric variables used and the presence of OSAHS.

Our results showed that subjects with Class II skeletal deformity presented a smaller airway volume than subjects with Class III deformity; however, some subjects with Class III presented a similar airway volume to Class II subjects. Thus maxillomandibular morphology is not always associated with a reduction in airway volume, and it may be that other variables contribute synergistically to the respiratory pathology.

The airway volume cannot be the only value related with the presence of OSAHS (^{Susarla et al., 2010}). Furthermore, it is difficult to obtain ranges of normality and abnormality for the airway since variables like body mass index, height etc. should be included in determining what volume should be considered normal for the respiratory function and in this scene, surgical treatments like maxillomandibular advancement or body mass reduction (in obese patients) are confirmed as suitable therapies for augmenting the airway and reducing OSAHS (^{Cillo Jr. & Dattilo, 2019}).

Thus we conclude that airway volume is an important variable in these analyses. It is clear that subjects with Class II deformity have a smaller airway volume than Class III subjects, thus in the presence of OSAHS, one of the treatment objectives must consider modification and augmentation of the airway. Based on our research we may conclude that Class II subjects present a smaller airway volume than Class III subjects.