## 4 Discussion  
### Summary of Evidence  
In the present systematic review, articles that evaluated RME, SARPE, and/or MARPE by CT in the treatment of maxillary deficiency were included. The CT images were analyzed using Hounsfield units (HU), which is the most current method to determine bone density and has been used by dentists to quantify the structure and quality of maxillary and mandibular bone [30; 31]. The five articles included in the meta-analysis measured bone density in different areas of the MPS [15; 23; 25; 27; 28]. Despite variations in the articles' way of measuring, the presented measures were grouped into the anterior, middle, and posterior regions. In the anterior region, it was possible to include five articles, of which three were about RME [23; 25; 28], one was about SARPE [15], and one was about MARPE [27]. In the middle and posterior regions, it was possible to include four studies, two of which were about RME [23; 28], one was about SARPE [15], and one was about MARPE [27]. The variation in the way that the measurement is performed and the fact that they are different procedures with different effects on bone density after expansion may be contributing factors to the high values found for heterogeneity, even after our sensitivity analyses. Figure 4: Comparison of the average difference between ERMAC and MARPE [15; 27]. Figure 3: Comparison of the average difference for ERM [23; 25; 28]. The posterior region showed a significant decrease in suture density, regardless of the procedure, and minimal heterogeneity in cases in which only RME studies were used [23; 25; 28] and moderate heterogeneity in cases in which SARPE and MARPE were used [15; 27]. These data suggest that, for the posterior region, a longer retention time is needed so that the bone density after expansion is close to the initial value. The results of three studies [16; 24; 26] included in the review--and not in the meta-analysis-- corroborate that (a) 6 months after RME [24] and (b) 6 to 7 months after SARPE [16; 26], the bone density of the posterior region had not yet reached the baseline level. In the anterior and middle regions, from the meta-analysis results, there is no evidence to state that the density of the palatine suture is different 6 months after RME. A study included in the review and not in the meta-analysis found a similar result: 6 months after expansion with RME, there no significant changes reported [13]. Similarly, a study based on the tomographic evaluation of 17 children aged 5 to 10 years showed a completely ossified suture after 8 to 9 months of retention [32]. Although only one SARPE study was included in the meta-analysis [15], three articles were included in the review and all showed decreased suture density [15; 16; 26]; among these, the greatest differences in density were found in the anterior and middle regions. Only one study that evaluated MPS density after MARPE was included in the review and meta-analysis [27]; its results showed a greater decrease in bone density in the median region and in the midline. These differences between the results for SARPE and MARPE may be related to the injuries caused by the use of the chisel in the anterior region of the palate during the SARPE procedure [27] and also the mini-implants' location in cases of MARPE, which is usually in the middle portion of the palate, where consequently there will be a greater concentration of force during the active phase [27; 33], negatively influencing bone repair. According to the data presented in the meta-analysis, RME tended to have less of a difference in bone density than SARPE and MARPE. The literature reports that bone repair has been associated with the initial age of the patient when performing an expansion [34]; taking into account that SARPE and MARPE are treatment alternatives for adult patients with no growth potential, the age of the patients can then be a possible response for this difference.