The human upper airway could be defined as the airway space extending from the nose’s nares and the mouth’s stoma down to the cricoid cartilage in the larynx. The nasal and oral cavities merge in the area known as the pharynx, which is divided anatomically into three sections: the nasopharynx, oropharynx, and hypopharynx.

The shape and dimensions of the upper airway passages influence the volume of air passing through them. The close anatomical relationship between the upper airway and the craniofacial and dental structures dictates their influence on each other. The normal growth and development of craniofacial structures depend on a patent airway and nasal breathing.

Orthodontists have long been interested in airway analysis because of the airway’s potential importance to the normal development of the craniofacial region as well as its involvement in the diagnosis and treatment of mouth breathing and sleep disorders. Clinicians need to evaluate the upper airway objectively and identify the normal and abnormal anatomical boundaries and dimensions. Although two-dimensional lateral cephalograms have been used for decades to evaluate the airway’s shape, size, position, and relationship with other anatomical structures, they lack the information to illustrate the anatomically complex airway structure in three dimensions. Lateral cephalograms could be used as an initial screening method for airway evaluation, but three-dimensional cone-beam computed tomography (CBCT) images provide a more reliable and comprehensive tool for airway assessment and volumetric measurements. The findings of snoring and interrupted sleep together with diagnostic images obtained during the dental examination can provide indications of possible airway disorders and support the need for referral to a medical specialist referral.

2. Upper Airway Changes in Diverse Orthodontic Looms

Respiratory function has a considerable influence on the development of the craniofacial complex, and the upper airway assessment has been an important concern for the orthodontist while performing different modalities of the management of dental and skeletal malocclusion [2]. Controversies exist regarding the influence of orthodontic treatment on the upper airway.

Airway dimensions are assessed by various means including lateral cephalograms, and CBCT. Although the pharyngeal airway is a 3-D structure. Conventional lateral cephalography provides only the sagittal and vertical dimensions and thus has limited value for an accurate assessment. The transverse dimension of the airway has been found to be variable for a similar area of the nasopharyngeal airway, which raises concerns over the conclusions drawn from the lateral head film [17]. Thus, it would be more accurate to provide a 3-D analysis of the airway assessment.

Rapid maxillary expansion, introduced in the 19th century, is routinely performed for the correction of posterior crossbite and the creation of space to relieve crowding. This technique is now believed to be helpful for additional purposes, such as reduction of nocturnal enuresis [18], improvement of impaired nasal respiration [19], and relief from obstructive sleep apnoea [20].

For managing skeletal class III malocclusion, retrognathic maxilla expansion in combination with protraction is indicated for growth modulation. This modality has recently received increased attention as several studies have suggested improvement in the airway dimension facilitating the management of obstructive sleep apnoea [15,20,24]. Separation of the midpalatal suture decreases the resistance of the circummaxillary sutures and subsequent protraction initiates the cellular response resulting in the forward and downward movement of the maxillary complex [25]. Fu et al. [26] and Alrejaye et al. [3] enrolled cleft patients, whereas noncleft subjects were evaluated in the other studies. The pharyngeal anatomy of cleft patients was found to be different from the noncleft children and there was a varying effect of skeletal protraction among the cleft patients as compared with non-cleft patients [27]. Only the study by Fu et al. [26] showed a significant increase in the pharyngeal airway volume after expansion and protraction, however, this difference was insignificant when combined with the data from other studies. In contrast, the systematic review by Lee et al. [5] found a significant increase in the upper airway after rapid maxillary expansion and protraction with nonsignificant changes in the lower airway when assessed on two-dimensional lateral cephalogram.

The extraction of one or more teeth is frequently indicated in contemporary orthodontics for the management of various dentoskeletal problems. Premolars are most often extracted for crowding correction and retraction of anteriors which can result in a considerable number of changes in the hard and soft tissues of the dentofacial region [28]. Distal movement of the incisors could lead to the encroachment of space with posterior displacement of the tongue narrowing the upper airway. There are Five observational studies that assessed the upper airway after the extraction of premolars and the retraction of anteriors. All the studies were retrospective and compared airway changes after orthodontic treatment in patients with and without extraction of at least two premolars. Studies by Stefanovic et al. [29] and Valiathan et al. [30] included adolescent patients, whereas the other three studies included adult patients (>18 years). During the period of active craniofacial growth (i.e., 8 to 18 years of age), the length and volume of the airways increase. Thus, in adolescents’ treatment effect, if any, can be compensated by the growth of tissues surrounding the airway in adolescents [31].

A posteriorly positioned mandible is commonly associated with obstructive sleep apnoea [35] and its advancement is believed to facilitate an increase in the upper airway volume which mitigates the apnoea [36]. Functional appliances enhance the growth of the mandible by repositioning it anteriorly, however, recent evidence suggests that the advancement consists of dentoalveolar changes with only minimal skeletal changes [37,38].

A retrognathic maxilla and mandible can compromise the upper airway volume and are associated with obstructive sleep apnoea [35,44,45]. Orthognathic surgery involves the manipulation of the jawbones in which their position is changed from the surrounding craniofacial structures. This may cause morphological alteration of the airway resulting in further respiratory complications such as obstructive sleep apnea. However, there is conflicting evidence on the effects of this surgery on the airway [46,47].

Only the study by Li et al. [23] considered CBCT in a supine position with the FH plane perpendicular to the ground. The dimension of the upper airway is sensitive to the body position [52], and the volume at the supine position is important because obstructive sleep apnoea occurs only during sleep. Although CBCT provides a clear picture of the hard and soft tissues at a single point in time, it does not provide any information on the muscle tone or susceptibility of collapse. Hence, the use of CBCT alone is not valuable for the diagnosis of obstructive sleep apnoea [10].