## 4 Discussion  
The purpose of this paper was to evaluate RPE effects in terms of BD of the MPS in growing patients (10-14 years). A low-dose CBCT protocol was used for the better identification of landmarks and to reduce the radiation exposure of the patients. The ROI in the MPS showed a lack of statistically significant differences from T0 (preoperative) to T1 (one year after the beginning of the therapy). According to [PERSON] et al., (2013) [20], who found similar results between T0 (preoperative) and six months after the beginning of RPE, they also found a significant reduction in density between T0 and the end of the active expansion phase (14 days after T0), which was caused by the orthopaedic forces of RPE that determined the lateral displacement of the two hemi-maxillae, as confirmed by [PERSON] et al., (2020) [21].  
The RPE was kept in place as a passive retainer to avoid transverse maxillary relapse and to maintain the two halves of the maxilla separately while mineralization of the MPS increases.  
As ageing increases, the degree of ossification of the MPS increases; the impact of the RPE was mainly on tipping the teeth after puberty. CBCT evaluation of the MPS is highly recommended before the use of the RPE [5], in order to avoid the failure of maxillary expansion in terms of a lack of increase in transverse width, and in terms of adverse effects such as periodontal attachment loss, increased mobility, uncontrolled tipping, root reabsorption, necrosis, and vestibular fenestration [22].  
Females showed significantly higher BD of the MPS than males, and the first null hypothesis was rejected.  
No difference between different vertical skeletal patterns (hypodivergent vs. normodivergent, hyperdivergent vs. normodivergent, hypodivergent vs. hyperdivergent) and between BD values before (T0) and after RPE (T1) was found, and thus the second and fourth null hypotheses were not rejected. Similar results were found in the literature by [PERSON] [17].  
Class III skeletal individuals showed a significantly higher BD values than the Class II group at T0, but not at T1. No differences between other sagittal skeletal pattern groups were found, so the third null hypothesis was partially rejected. These results were confirmed in the literature [17].  
The MPS showed homogeneous density values before treatment (T0) in each category (550.17 HU, 561.35 HU, 551.08 HU, 546.52 HU, 552.84 HU, 563.70 HU, 558.72 HU, and 555.04 HU).  
The RPE produced maxillary expansion, and MPS opening was successful in all patients recruited in this retrospective study. In the literature, it is reported that it was possible to open sutures with RPE when BD values of the MPS ranged from 563.3 to 741.7 HU, as confirmed by [PERSON] (2013) [20].  
The MPS also showed homogeneous density values after treatment (T1) in each group (541.92 HU, 550.25 HU, 545.23 HU, 541.16 HU, 543.72 HU, 553.85 HU, 552.29 HU, and 550.03 HU), and the density values of the MPS at T1 were similar to T0, suggesting that a similar rate of suture reorganization occurs one year after the use of RPE, caused by reorganization and bone deposition along the MPS.  
There are two limitations to this study. First, the suture margins' poor resolution was not well defined in the low-dose CBCT protocol. The second limitation was the implementation of the inclusion criteria according to age range (from 10 to 14 years), without distinguishing the patients according to the five stages of MPS maturation described by [PERSON] (2013). Further research should be designed to prove the utility of CBCT in the evaluation of the mean bone density of the MPS using Hounsfield units.