# Upper Airway Changes in Diverse Orthodontic Looms: A Systematic Review and Meta-Analysis
## 4 Discussion  
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; hence we undertook this systematic review. Twenty-seven studies were found to be appropriate for inclusion in this review, and the methodological quality of most of the studies was acceptable which permitted quantitative synthesis. The effects on the airway were assessed with orthopaedic protraction, rapid maxillary expansion, myofunctional appliances, fixed orthodontic therapy with extraction, and orthognathic surgery for correction of skeletal Class II and Class III malocclusion. Randomised controlled trials provide the highest level of evidence. However, well-conducted observational studies also contribute meaningful evidence to the existing scientific knowledge, especially when randomised clinical trials are unethical or unfeasible [16]. Because there were not enough randomised studies that evaluated the airway after different modalities of orthodontic treatment, prospective, and retrospective observational studies were also included in this review. Airway dimensions are assessed by various means including lateral cephalosporgrams, 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 and this review included studies evaluating the 3-D volume of the airway after the use of various orthodontic treatment approaches. Rapid maxillary expansion, introduced in the 19 th 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]. This review identified eight studies that assessed the impact of rapid palatal expansion on the airway where all of them were found to have significant improvements except for the study by [PERSON] et al. [4] and [PERSON] et al. [21]. The combined analysis showed a significant improvement in the upper airway, which is similar to the findings reported by [PERSON] et al. [22] in their 2017 systematic review. The mean age of the patient was higher in the studies by [PERSON] et al. [15] and [PERSON] et al. [23] and those authors used implant-supported maxillary expansion in contrast to other studies in which tooth-supported expanders were used in adolescents. For managing skeletal class III malocclusion, retrognathic maxilla expansion in combination with protraction is indicated for growth modulation. This modality has recently  
Figure 17: Funnel plot for maxillary expansion between experiment and control after removing the studies lies outside the funnel. 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 circummaximillary sutures and subsequent protraction initiates the cellular response resulting in the forward and downward movement of the maxillary complex [25]. This review included five observational studies that assessed the effect of maxillary expansion and protraction on the airway. All of the studies were retrospective except the study by [PERSON] et al. [26] which had a prospective study design. [PERSON] et al. [26] and [PERSON] et al.