## 4 Discussion  
Class II malocclusion is the most common orthodontic problem in the population [1]. As stated by [PERSON] et al. [11], it can be expressed in multiple forms, both skeletal and dental, and their differential diagnoses can help in the selection of the most appropriate treatment approach. Among these factors, mandibular retrognathism shows a prevailing frequency [2,3]. In these cases, the use of functional appliances capable of stimulating growth and anterior repositioning of the mandible is particularly indicated. The aim of this study was to analyze the skeletal and dentoalveolar effects induced by the treatment of Class II malocclusion with a removable functional appliance followed by fixed appliances. Over the decades, numerous types of functional devices--both fixed and removable--have been proposed and analyzed in the literature. According to information reported by recent systematic reviews of the literature [11, 12, 13], they have been proven effective in the treatment of Class II malocclusion, resulting in a correction of the latter through both skeletal and dentoalveolar effects. The skeletal correction of Class II malocclusion is the result of the combined action of effects on the maxilla and mandible. The PK appliance was designed with the aim of achieving stimulation of mandibular growth, both in its horizontal and vertical components, and control of vertical skeletal relationships. The latter is obtained through careful management of the occlusal plane and the facial height index; the goal, in fact, is to maintain the anterior facial height and to increase the posterior facial height. The patients belonging to the treated group underwent a therapy consisting of two phases: the first treatment phase was performed with the PK appliance for 15 months, with the aim of controlling the sagittal and vertical positions of the maxilla, promoting mandibular growth, and establishing a Class I molar relationship and a correct relationship between the incisors. Once a Class I molar relationship was reached, the second phase of treatment was performed with fixed appliances with the aim of aligning and leveling the arches to obtain a stable occlusion. This study has highlighted the ability of the PK appliance to act on the sagittal growth of the maxilla, resulting, at the end of treatment, in a statistically significant reduction (\(-2.2^{\circ}\), \(p=0.0000\)) of the value of the SNA angle. This result is in line with the results reported by [PERSON] and collaborators [13], who underlined the ability of functional removable devices to exert a restraining action on the growth of the maxilla. In contrast, the PK appliance did not demonstrate any modifying effects on the size and antero-posterior position of the mandible. Therefore, the statistically significant reduction in the ANB angle and in the Wits appraisal (\(-2.2^{\circ}\), \(p=0.0000\) and \(-3.4\) mm, \(p=0.0000\)) appears to be due to the action of the device mainly on the maxilla. As previously stated, one of the goals of the PK appliance is to control the vertical dimension. According to [PERSON] [14], anterior rotation of the mandible occurs when the posterior facial height shows greater development than the anterior facial height. Mandibular rotation appears to be closely related to changes in the occlusal plane during growth [15]. The posterior portion of the dentition, moreover, represents one of the main factors influencing the functional positioning of the mandible [16; 17]. Longitudinal growth studies conducted by [PERSON] and [PERSON] [18] revealed that changes in the occlusal plane can change the growth pattern of Class II malocclusions and, specifically, that the continued leveling of the posterior occlusal plane is accompanied by a simultaneous decrease in the angle of the mandibular plane.