## 4 Discussion
Assuming that the posterior portion of the occlusal plane is the key to changing the position of the mandible, it is plausible to infer that the leveling of this plane may result in an increase in the height of the mandibular ramus and, consequently, a flattening of the mandibular plane. An excessive curve of Spee in the lower arch, in fact, can be leveled by freeing the posterior teeth from the resin of the appliance, inducing their extrusion. The analysis of the results obtained in the vertical plane from the present study confirmed a statistically significant increase in the angular value of SN-palatal plane (\(1.1^{\circ}\), \(p=0.020\)) at the end of treatment in the treated group compared with that in the control group. The increase in this angle is indicative of a downward inclination of the palatal plane. This result, therefore, is in line with the findings of [PERSON] and [PERSON] [7] and reflects one of the desired effects induced by the device. The latter, in fact, was designed to exert control of verticality through the presence of lateral thickness planes and three-lobed springs. Specifically, the latter are able to progressively activate the device in order to obtain a greater activation in the posterior region than in the anterior one. On the contrary, no effects of the device on the occlusal plane were found, since, in the treated group, there were no statistically significant changes in the values of SN-oclusal plane and SN-mandibular plane. As for dentonalveolar changes, the group treated with the PK appliance showed significant decreases in the values of the overjet and overbite, and a statistically significant improvement in the molar relation compared with the control group (respectively \(-2.9\) mm, \(p=0.0000\); \(-2.5^{\circ}\), \(p=0.0000\); \(3.6\) mm, \(p=0.0000\)). In contrast, there were no significant changes in the inclinations of the upper incisor with respect to the palatal plane and of the lower incisor with respect to the mandibular plane. The improvement in the values of the overjet and overbite, therefore, can be related to the results obtained at the skeletal level, specifically with the reduction in the SNA angle and the downward inclination of the palatal plane. The skeletal and dentonalveolar results listed above must be analyzed by taking into account the timing for the treatment with the PK appliance in patients with Class II malocclusion. The literature is unanimous in establishing as the ideal timing of treatment with functional appliances the period coinciding with the peak of pubertal growth [19; 20], corresponding to the CS3-CS4 stage of maturation of the cervical vertebrae [21]. The group undergoing treatment with the PK appliance, however, was mostly composed of prepubertal-stage subjects (mainly cervical stages CS1 and CS2) at the beginning of active therapy (92%). Moreover, at the end of the observation, most of the treated group was still in the prepubertal stage (stages CS1-2, 16%) or in the circumpubertal stage CS3 (52%), when the beginning of the pubertal growth peak was identified. By virtue of this observation, it is plausible to infer that the lack of skeletal effects induced by the device on the mandible, both on its size and its sagittal position, may be related to inappropriate treatment timing. This hypothesis is in line with the conclusions of the study conducted on the Twin-block device by [PERSON] and collaborators [22]. According to this study, in fact, early treatment with the Twin-block appliance can correct Class II malocclusion mainly through dentoalveolar modifications, such as the reduction of the overjet and the improvement of the molar relationship. These effects lead to an overall reduction in the severity of the malocclusion, while the skeletal effects of early treatment are mild and probably of low clinical significance. In addition, according to [PERSON] et al.