## 4 Discussion  
Class II malocclusion, classified on the basis of the relationship of the lower molar to the upper molar, affects a large group of individuals. It has been estimated that it involves about 14.7% of the US population, with the prevalence decreasing from 22.6% in the 8-11 years age group to 15.6% between 12 and 17 years old, ending up at 13.4% between 18 and 50 years old [15]. The National Center for Health Statistics reported that 20.4% of 6- to 11-year-olds have a bilateral class II molar relationship [16], while among 12- to 17-year-olds the prevalence drops to 14.5% [17]. Several studies have shown that there is no significant difference in the distribution of malocclusions between males and females [18]. Racial, national, and ethnic factors have a certain influence on the incidence of malocclusions and on the type of growth: a higher frequency of the dolichofacial type has been found in Caucasian subjects and a higher prevalence of the brachyfacial type in Black subjects [19]; class II malocclusions were found to be more frequent in Whites, especially of northern European origin, while class III prevailed in Asian populations [20]. In a recent study, [PERSON] et al. demonstrated that children with skeletal class II malocclusion due to mandibular retrusion have reduced airway dimensions compared to children with class I malocclusion [21]. Similarly, [PERSON] reported that subjects with class II malocclusion show smaller nasopharyngeal dimensions than subjects with class I [22]. [PERSON] and [PERSON] evaluated the oropharyngeal volumes of patients with different dentofacial skeletal patterns. The oropharyngeal airway volumes of patients with class II malocclusion were small compared with those of class I and class III patients. They therefore reported that the position of the mandible relative to the skull base influences the volume of the oropharyngeal airways [23]. In recent years, in order to perform a precise evaluation of the values of the initial and final upper airway volumes, cone beam computed tomography (CBCT) has been extensively used, representing an appropriate method for airway analysis, besides being a fundamental tool in the diagnostic phase as well as in the clinical planning in dentistry [5]. [PERSON] evaluated differences in airway shape and volume between growing subjects with different facial patterns, using CBCT. A statistically significant association was found between lower airway volume and the antero-posterior relationship of the maxillae, and between airway volume and facial size [24]. All these authors therefore demonstrated a statistically significant relationship between the position of the mandible and the dimensions of the naso-oropharyngeal airways. If no upper nasopharyngeal airway disease related to adenoid or tonsil hypertrophy or chronic  
Figure 6: Box and Whiskers Plot graph representing total volume (VTOT) values at T0 (1) and at T1 (2). respiratory problems is found, early correction of a skeletal class II could eliminate the possibility of having respiratory disorders during sleep, such as snoring [25]. With an appropriate treatment plan, an improvement in the sagittal mandibular position can favor an increase in the dimensions of the naso-oropharyngeal airways as well as improve the facial profile and dento-alveolar relationships [26]. Furthermore, the hyoid bone plays a key role in keeping the airways open during sleep. As already described in the literature, in subjects affected by obstructive sleep apnea, the hyoid bone is found on average in a lower and more retruded position than in healthy subjects [27]. [PERSON] compared the position of the hyoid bone in subjects with different malocclusions and demonstrated that the position of the hyoid bone varies according to the malocclusion [28]. The hyoid bone is positioned more supero-ventrally in individuals with class II malocclusion than in those with class I occlusion [29].