# Upper Airway Changes in Diverse Orthodontic Looms: A Systematic Review and Meta-Analysis
## 4 Discussion
[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 [PERSON] 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 [PERSON] 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 cephalospor. 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. In this review, we identified 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 [PERSON] et al. [29] and [PERSON] 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]. In this review, none of the included studies reported a significant volumetric change in the upper airway, and the results of the pooled analysis were inconclusive. The hyoid bone was displaced backward and downward with a large incisor retraction [32] and the evidence varies on its effect on the airway [33; 34]. Only the study by [PERSON] et al. [12] reported a change in the position of the hyoid bone in which the displacement was not statistically significant. 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]. We identified three studies measuring the volumetric changes in the airway after functional appliances in growing Class II patients with mandibular retrusion. Among all the studies, airway volume immediately after the treatment was compared with the pretreatment values, we conclude the effects on long-term changes could not explore the effect on long-term changes. The studies varied in the type of appliances used: [PERSON] et al. [39] used Herbst appliance, [PERSON] et al. [40] studied on Forsus, and [PERSON] et al. [41] compared removable twin block and Forsus with the control group. All studies reported a significant improvement in the upper airway except for the Forsus group in the study by [PERSON] et al. [41] in which insignificant changes were noticed. The pooled analysis revealed an insignificant increase in the airway volume which is in contrast with the result of the meta-analysis by [PERSON] et al. [42], who found a significant increase in the airway dimension at the OP region as measured with a cephalogram. In the systematic review by [PERSON] et al.