RME is a conventional treatment for the patients with maxillary transverse deficiency, but during the process of treatment, some undesired effects may also occur, among which alveolar bone absorption is very common. This systematic review and meta-analysis were to explore the changes of alveolar bone thickness and vertical height after RME. As far as we know, Giudice et al.  conducted a systematic review of the relevant contents but did not carry out a quantitative analysis. And there was also a comparison of buccal alveolar bone loss between mini-implant assisted rapid palatal expansion and conventional rapid palatal expansion, but without statistical analysis of effect of RME separately.  So, this is the first meta-analysis to evaluate the change in alveolar bone after RME. The results showed that RME could reduce the alveolar buccal bone thickness and marginal bone level of the retention teeth.
In this meta-analysis, after comprehensive studies retrieval and quality evaluation, a total of 8 articles are selected, 6 of which are statistically analyzed, and the remaining two items are only descriptive analysis due to the differences in methodology and data statistics. All the included literature was of medium or above quality.
According to the statistical results of this study, RME can lead to a decrease of buccal alveolar bone thickness and vertical bone level of maxillary first molars (Fig. 2,4). In the assessment of the buccal alveolar bone thickness of maxillary first molars, one study used hass appliance,  and the other two used hyrax appliances. [15, 17] However, previous studies  have shown that the effects of hass appliance and hyrax appliance on the buccal cortical thickness of molars were the same, and the results were not significantly affected after the sensitivity analysis.
As for the vertical bone level of the edge, two studies measured the left and right molars respectively, [15, 19] while the other two articles combined measurement of bilateral molars. [7, 18] Therefore, according to the Cochrane Handbook, we combined the left and right side data of the two items [15, 19] respectively, and then the sensitivity analysis was carried out. The results showed that there was no noticeable impact. The consistent results seemed to indicate the robustness of the meta-analysis results.
In the statistical study of palatal alveolar bone thickness of maxillary first molar, due to the high heterogeneity, no further discussion was made. Because of the massive difference and lack of relevant studies, the accuracy of the results needs more experimental research.
In each study, the confounding factors such as the activation scheme and appliance retention time may have an impact on the research results, which were slightly different. However, in the meta-analysis of buccal alveolar bone thickness and marginal vertical bone level of maxillary first molars, their influence may not affect the results because of the low heterogeneity between studies.
Finally, although the extensive search was performed, only six studies were selected for the meta-analysis. Because of the small number of included studies, the statistical capacity was insufficient, and the funnel plots of publication bias assessment were not carried out. Thus, the results of this systematic review should be considered with caution. Further high-quality original research is needed to draw more stable conclusions.
In clinical treatment, in order to increase the orthodontic force and reduce the occurrence of periodontal adverse reactions in the process of maxillary expansion, the method of increasing anchorage such as micro implant is often adopted in the process of clinical treatment. In addition, buccal cortical osteotomy and palatal suture osteotomy can also be used to reduce the resistance of bone in the process of maxillary expansion, so as to increase the success rate of maxillary expansion and reduce the damage of periodontal supporting tissue.
However, when using RME in clinical practice, it is better to evaluate the alveolar bone level of patients to determine whether it is sufficient for RME treatment. In addition, it is suggested that patients with rapid maxillary expansion should be followed up to regularly monitor the changes of alveolar bone, so as to minimize the side effects of rapid maxillary expansion.