## 5 Discussion
When planning mandibular advancement, the correct timing is vital to achieve the maximum skeletal effects. The randomized clinical trials reviewed by [PERSON] et al. [17] in 2006 all agree that the amount of additional mandibular growth recorded after the use of functional appliances appears to be significantly higher when applied at the peak growth stage. A measure of the efficacy of functional appliances can be obtained by dividing the total mandibular length gained by the total number of months of treatment performed (efficacy coefficient). For the Herbst appliance it has been reported as 0.28 mm/month, the Twin Block 0.23 mm/month, the Bionator 0.17 mm/month, the Activator 0.12 mm/month, and the Frankel 0.09 mm/month. As expected, the most effective device turns out to be the only fixed, and therefore compliance-independent, one. Among the removable appliances, the Twin Block has the highest efficacy coefficient, and is very commonly used due to its high acceptability and ability to produce results rapidly over time [6]. In our sample the average treatment time with F22\({}^{\copyright}\) Young was 10 months. Dividing this by the total mandibular elongation (+3.016 mm) we acquire a value of 0.3 mm/month. This is even higher than that of the Herbst appliance, which, coupled with the high acceptability of removable appliances, makes the F22\({}^{\copyright}\). Young a promising option, at least in the short term. Indeed, the device was highly accepted by all patients treated: nobody reported any particular pain, discomfort or tooth sensitivity for all treatment time [18]. The maintenance of the therapeutic position was ensured by the use of class II elastics directly applied to specific cuts performed on the aligners. The mandibular advancement can also lead to an improvement of sleep quality, with a contemporary tongue reposition, increasing the airway space and facilitating the superior respiratory system [19; 20]. Recent studies have been conducted with the aim of investigating and quantitatively comparing the skeletal and dental effects obtained after a period of treatment with functional orthodontics on laterolateral radiographs. For example, [PERSON] et al. [13] compared changes in skeletal, dentoalveolar and soft tissue levels measured via cephalometry on radiographs taken before and after treatment with Twin Block, and confirmed an increase in the overall mandible length (Co-Gn) of 3.21 mm over a period of 12 months of treatment. Our results are just as good, if not better, revealing an increase of 3 mm over 10 months of observation. As for the SNB angle, [PERSON] et al. [13] found an improvement of 1.56\({}^{\circ}\) from the start of treatment, alongside a 1.82\({}^{\circ}\) reduction in the ANB. Similarly, [PERSON] and [PERSON] [21] found a 1.8\({}^{\circ}\) reduction in the ANB angle in a group of patients treated via Twin Block. Our data are in line with those findings. However, in order to interpret the results obtained in a meaningful fashion, it is important to be aware of the limitations of this study, one of which being the influence of natural growth, which may have differed to a greater or lesser extent in the patients analysed. Nonetheless, the changes seen in our group of patients, treated around the growth spurt, would have not normally be observed with natural growth. In fact, a longitudinal study comparing untreated class II (study group) and class I (control group) malocclusion patients during the stages of growth CSV3-CSV4 showed that the Co-Gn tends to vary significantly, less in the former than in the latter (a difference of about 2 mm). This means that in Class II subjects, the initial deficit remains constant during growth, without self-correcting, and the mandibular size discrepancy with respect to class I subjects therefore increases [22]. It is, however, important to note that we did not have a control or comparison class II group to compare the outcomes of our device with no treatment or those associated with another device, respectively.