# Upper Airway Changes in Diverse Orthodontic Looms: A Systematic Review and Meta-Analysis
## 1 Introduction  
The human upper airway could be defined as the airway space extending from the nose's nares and the mouth's stroma 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 nasopharyngeal, 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 thecraniofacial 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.  
Nasal obstruction and its inseparable companion, \"mouth breathing,\" have been assumed to affect dentofacial growth in the current literature. Although some researchers have found no association between the adequacy of the airway and dentofacial morphology [1]. It seems to be a consensus that the oropharyngeal (OP) and nasopharyngeal structures play roles in the development of the dentofacial complex [2].  
The etiology of malocclusions is believed to be multifactorial, and it could be considered erroneous to associate malocclusions with only breathing mode. Because the airway is assumed to play a role in dentofacial development, several studies have attempted to correlate patients with normal naso-respiratory functions with different malocclusions and airway dimensions.  
Despite this, the relationship between upper airway dimensions and skeletal malocclusion remains controversial. Some studies have shown no effect on the airway after orthodontic treatment [3], whereas others have reported a change in the airway [4]. The controversy regarding these outcomes might be related to the heterogeneity in the included articles, types of malocclusions, and imaging methods.  
In addition, there has been no meta-analysis published in the literature that has compared changes in the airway dimensions resulting from different types of orthodontic treatment by using cone-beam computed tomography (CBCT), a 3D imaging technique, and only one single meta-analysis that included a small number of studies [5]. Previous systematic reviews [5; 6; 7; 8] have investigated this subject based on data from only two-dimensional images.  
Therefore, the present meta-analysis focused only on studies that used computed tomography (CT) for airway evaluation, particularly CBCT. Through a meta-analysis, this study aims to assemble scientific evidence related to the effects of orthodontic treatment modalities on airway space.