Clinical effect on the alveolar bone of maxillary molar after rapid maxillary expansion: a systematic review and meta-analysis

Background This study assessed the effects of rapid maxillary expansion (RME) on alveolar bone cortical thickness and vertical bone level of maxillary rst molar. methods Eight studies were selected for the systematic review, and six studies were statistically selected in meta-analysis. The thickness of the distal buccal alveolar bone was signicantly reduced of both left (MD 0.53;95% CI:0.15–0.90) and right (MD 0.61;95% CI:0.28–0.94) sides of the maxillary rst molar after RME. The same was true for the left (MD 0.63;95% CI:0.28–0.98) and right (MD 0.63;95% CI:0.36–0.91) sides of the mesial buccal side. And the vertical distance between the cusp tip and the buccal alveolar crest increased signicantly after RME (SMD − 0.92;95% CI: -1.20–0.64). However, the study of palatal cortical thickness of maxillary rst molar needs more clinical trials because of its high heterogeneity (left: I 2 = 92%; right: I 2 = 86%).


Results
Eight studies were selected for the systematic review, and six studies were statistically selected in metaanalysis. The thickness of the distal buccal alveolar bone was signi cantly reduced of both left (MD 0.53;95% CI:0.15-0.90) and right (MD 0.61;95% CI:0.28-0.94) sides of the maxillary rst molar after RME. The same was true for the left (MD 0.63;95% CI:0.28-0.98) and right (MD 0.63;95% CI:0.36-0.91) sides of the mesial buccal side. And the vertical distance between the cusp tip and the buccal alveolar crest increased signi cantly after RME (SMD − 0.92;95% CI: -1.20-0.64). However, the study of palatal cortical thickness of maxillary rst molar needs more clinical trials because of its high heterogeneity (left: I 2 = 92%; right: I 2 = 86%).

Conclusions
According to current studies, RME can reduce the buccal cortical thickness of maxillary rst molars and increase vertical bone loss. More research is needed to determine the stability of the results. However, it is advisable to evaluate the alveolar bone before treatment. Background Maxillary transverse de ciency is a kind of common malocclusion in clinical orthodontic. The patients often show the symptoms of dental crowding, posterior crossbite and so on. [1] For patients with maxillary transverse de ciency, maxillary expansion is a standard treatment. This technology was rst proposed by Angell in the 1860s. [2] It expands the middle palatal suture by applying lateral force against the teeth and the alveolar bone around the teeth. [3] However, the rapid maxillary expansion device is loaded on maxillary rst molars and premolars. During RME, massive orthodontic forces are transmitted to the alveolar bone through the teeth. [4] So, some undesirable effects may occur in the related teeth and their supporting tissues, including marginal bone loss, tipping of maxillary teeth, bending of the alveolar bone, reduction of buccal bone thickness, and periodontal damages. [5][6][7][8][9][10] For the sake of de ning the change of alveolar bone after RME, computed tomography was used in studying basal bone changes after RME for the rst time. [11] It could show all anatomical structures and assess the buccal and palatal bone thickness through the superposition of multiple planes. [12] Besides, compared with computed tomography, cone-beam computed tomography (CBCT) has higher resolution and lower radiation dose, which is widely used in dentistry. [13] At present, there are many reports about the changes in the buccal and palatal cortex after rapid maxillary expansion. Still, there are some differences in the methods of measurement and research results. [6,7,14−19] Up to now, there is only one systematic review which has limited statistics on this issue, but no relevant meta-analysis has been published. The purpose of this study is to assess existing literature, performing an updated systematic review, meta-analysis to evaluate and compare the effect on the cortex of molar alveolar bone after RME.

Protocol and registration
This systematic review protocol was registered under the PROSPERO register with the number CRD42021228114 (www.crd.york.ac.uk/prospero).

Search strategy
An extensive electronic search was conducted through databases including PubMed, Web of Science, Scopus, and Cochrane Central Register of Controlled Trials (CENTRAL) by the speci c search strategy-((maxillary expansion OR rapid maxillary expansion OR RME OR palatal expansion OR rapid palatal expansion OR RPE) AND (alveolar bone OR periodontal)). On this basis, we personalized the retrieval strategy of each database. Besides, we also searched the grey literature in databases. The search strategy of each database was reported in Table 1. Also, the related journals were searched manually.
There was no language restriction during the literature search Two researchers independently completed the literature search. Full articles selected from the abstract were independently evaluated by the two researchers. Finally, the two researchers discussed their results to reach the same agreement.
Criteria for included studies Studies were included by following selection criteria-Participants, intervention, comparison, outcome, study design (PICOS) format.
(2) Intervention (I): RME protocol with hass or hyrax application. Moreover, the studies were also excluded as follows: (1) Double publications with the same data; (2) Studies involving subjects with craniofacial anomalies, systemic diseases, previous surgery or another orthodontic intervention; (3) Literature with incomplete data description.; (4) Review articles, case reports, descriptive studies or abstracts.

Risk of bias assessment
We adopted the assessment system created and modi ed by Saltaji [20] and Yi [21] for risk of bias assessment, which assessed the risk of bias based on four aspects as following: study design, study measurements, statistical analysis and baseline information ( Table 2). [22] Statistical analysis Review Manager (RevMan5.3; Nordic Cochrane Centre, Cochrane Collaboration, Copenhagen, Denmark) was used for quantitative analysis. The mean differences (MD) with their correspondent 95% con dence intervals (95% CI) were chosen as the treatment effects for the analysis of buccal and palatal cortex thickness and the standardized mean differences (SMD) with their 95% CI were chosen for the analysis of buccal marginal bone level. For studies with high heterogeneity(I 2 > 50%), we adopted the random effect model. Otherwise, the xed-effect model was used. The study with P < 0.05 was found to be statistically signi cant. To explore the stability of the research results, a sensitivity analysis was carried out by omitting relevant studies.

Risk of bias assessment
The results of the risk of bias assessment are shown in Table 4. In the eight included studies, two studies took a low risk of bias, and the other six studies took a medium risk of bias. Among the six articles for quantitative analysis, one took the low risk, and four took the medium risk.

Data analysis
Six studies reported the buccal and palatal cortex thickness or buccal marginal bone level of molar alveolar bone after RME. The feasible data was statistically analyzed to judge the in uence of RME for alveolar bone. For the measurement of buccal marginal bone level, the left and right data in two studies [15,19] were combined by Formula from the Cochrane Handbook. [24] The results of the meta-analysis are listed in Fig. 2 The remaining two studies were not selected for the meta-analysis [6,23] because of the lack of comparability of data. Garib et al. [6] found that RME reduced the thickness of the buccal alveolar bone and increased the thickness of palatal alveolar bone of posterior maxillary teeth. Brunetto et al. [23] found that reduction of alveolar bone height was detected after RME.

Sensitivity analysis
As for the measurement of buccal cortex thickness, Ballanti et al. [16] used hass appliance, while the other two studies in meta-analysis applied hyrax appliance. In another group of the measurement of buccal marginal bone level, the left and right data in two studies [15,19] were combined. Therefore, we did a sensitivity analysis by omitting the studies separately. The results are shown in Table 5, which made no change.
Discussion RME is a conventional treatment for the patients with maxillary transverse de ciency, 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. [25] 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. [26] So, this is the rst 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 rst molars (Fig. 2,4). In the assessment of the buccal alveolar bone thickness of maxillary rst molars, one study used hass appliance, [16] and the other two used hyrax appliances. [15,17] However, previous studies [6] 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 signi cantly 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 rst 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 rst molars, their in uence 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 metaanalysis. Because of the small number of included studies, the statistical capacity was insu cient, 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. [27] 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. [28] However, when using RME in clinical practice, it is better to evaluate the alveolar bone level of patients to determine whether it is su cient 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.

Conclusion
According to limited evidence, RME can lead to a decrease of buccal cortical thickness and vertical bone level of alveolar bone in maxillary rst molar. However, due to the lack of included studies, these statements are not inclusive. So, these results should be evaluated with caution. More high-quality clinical studies are needed to determine the relevant conclusions further.