The prevalence of molar-incisor hypomineralization: a systematic review and meta-analysis

Molar-Incisor Hypomineralization (MIH) is a qualitative defect of enamel of unknown etiology, affecting one or more permanent molars and may include incisors. This condition is a clinical challenge and its prevalence is still uncertain given the recent increase in research. Thus, we aimed to comprehensively estimate the overall prevalence of MIH and associated characteristics. This systematic review is reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA). We searched articles using PubMed, MEDLINE, CENTRAL, Web of Science, SciELO, LILACS and TRIP databases, until July 2021. Heterogeneity and publication bias were computed via I2 test statistics and Egger’s significance test, respectively. Random-effects meta-analysis of prevalence were processed. We used the Strength of Recommendation Taxonomy [SORT] to grading the strength of evidence. Overall, 116 observational studies were included, with one study with moderate methodological quality and the remaining of high methodological quality. Subgroup analysis confirmed an influence of not using the 2003 MIH case definition (p = 0.0066). The pooled prevalence of MIH was 13.5% (95% CI 12.0–15.1, I2 = 98.0%). Affected incisors were seen in 36.6% (95% CI 30.0–43.7, I2 = 92.5%) of the cases. Lastly, the prevalence of hypomineralization of the second primary molars was observed in 3.6% of the MIH cases (95% CI 1.9–6.8, I2 = 96.3%). America was the continent with highest prevalence (15.3, 95% CI 12.8–18.3, p < 0.001, I2 = 96.3%) and Asia had the lowest prevalence (10.7, 95% CI 8.5–13.5, p < 0.001, I2 = 98.7%), however no continental differences were found. Sample size and year of publication were slight contributing factors to the heterogeneity in the analysis. Overall, these results were classified with a SORT A recommendation.

Molar-Incisor Hypomineralization (MIH) is designated as a qualitative defect of unknown etiology in the enamel development 1,2 . Since 2003, the European Academy of Pediatric Dentistry (EAPD) has proposed its first nomenclature to define a pathology of unknown etiology that affects one or more permanent molars and may include permanent incisors 1 .
As a potential oral public health concern, the prevalence of MIH became imperative to determine as a measure of interest in oral health programs. The prevalence of MIH was reported to range 2.8 to 40.2%, yet this inconsistency leads to a challenging interpretation and is mainly caused by the lack of standardization among clinicians/ researchers 3 . As a result, the EAPD introduced a diagnostic and classification system for MIH, with the purpose of improving epidemiological assessments 3,4 .
Two systematic reviews have estimated the prevalence of MIH between 13.1% and 14.2, with significant variances amid regions 5,6 . Moreover, Schwendicke et al. 5 estimated MIH prevalence on country scale via the Global Burden of Disease database, which may explain the variances between those regions. Additionally, both searches were conducted until mid 2017, and ever since, a number of new epidemiological studies have been published. However, other relevant information remains to be elucidated, namely the prevalence of moderate/ severe cases, prevalence of molars and incisors affected and the prevalence of hypomineralization of the second primary molars (HSPM). For these reasons, conducting a new systematic review on the topic would be convenient and relevant globally.
In this sense, and given the increase research on the prevalence of MIH, we aimed to comprehensively investigate the global prevalence of MIH, as well as its associated characteristics. www.nature.com/scientificreports/ male ratio (FMR), geographic location (latitude and longitude) and year of publication. The regression approach also allowed to quantify the percentage of heterogeneity that could be explained by that variable. All tests were two-tailed with alpha set at 0.05 except for the homogeneity test whose significance level cutoff was 0.10 due to the low power of the χ 2 test with a limited number of studies. Estimates were described with 95% confidence interval (CI).
Additional analyses. We employed the Strength of Recommendation Taxonomy (SORT) to appraise the strength and quality of the evidence 12 .

Results
Study selection. The online search retrieved strategy 2290 possibly relevant publications. After duplicates removal, 357 papers were judged against the eligibility criteria, and 1576 were excluded after titles and/or abstracts review. Among 138 articles assessed for full paper review eligibility, 22 articles were excluded with the respective reasons for exclusion detailed in the Supplementary S2. As a result, a final number of 116 observational studies were included for qualitative synthesis (Fig. 1). Inter-examiner reliability at the full-text screening was considered very substantial (kappa score = 0.915, 95% CI 0.895-0.925).
Studies characteristics. The characteristics of the included studies are presented in Table 1. We identified 116 different cohorts  from fifty different countries, across five continents. The year of publication of the included studies ranged between 2003 and 2021. Overall, a total of 135,181 participants were included in this review, being 52,876 girls and 52,872 boys, even though 18 manuscripts did not report on sex distribution. Thirty-four papers did not report the prevalence of MIH according to sex. Seven studies reported data on HSPM. Most studies recorded the MIH-related hypomineralization according to the diagnostic criteria of the EAPD case definition 1 , and others indices were also used such as the modified Developmental Defects of Enamel (mDDE) index 129 , the Kemoli 88 , Mathu-Muju and Wright 130 criteria, and the diagnostic criteria of Cho et al. 17,92 . Three cohorts had their data reported in more than one article (Petrou et al. 78 and Petrou et al. 109 ; Balmer et al. 13 and Balmer et al. 14 ; Negre-Barber et al. 110 and Negre-Barber et al. 111 ); thus, these papers were grouped under a single name study as follows: Petrou et al. 78,109 ; Balmer et al. 13,14 ; and, Negre-Barber et al. 110,111 . Also,   Supplementary S7). Detailed information on the definition of severity in each study was collectively presented in Table 3. Regarding the number of affected molars, estimates point to 24.3% of cases with one molar (95% CI 18.9-30.7, I 2 = 94.2%), 26.7% of cases with two molars (95% CI 23.9-29.7, I 2 = 65.0%), 18.1% of cases with three molars (95% CI 13.8-23.3, I 2 = 90.0%) and 26.8% of cases with four molars (95% CI 21.1-34.7, I 2 = 94.0%) (Supplementary S8-S11). The cases with affected incisors were estimated at 38.7% (95% CI 32.1-45.8, I 2 = 93.2%) (Supplementary S12), while cases with both molars and incisors were estimated at 42.1% (95% CI 34.9-50.0, I 2 = 95.5%) (Supplementary S13). Lastly, the prevalence of HSPM cases was estimated at 3.6% (95% CI 1.9-6.8, I 2 = 96.3%) (Supplementary S14). All the latter results had high heterogeneity.

Sex and geographic location (secondary outcomes).
We further analyzed whether the prevalence results were influenced by study sample size, female/male ratio, geographic location (latitude and longitude) and year of publication (Table 4).
Overall, MIH was influenced by the study sample size explaining 7.7% of the accounted heterogeneity, respectively. The year of publication (estimate = − 0.09, p = 0.023) demonstrated a slight influence on the prevalence of MIH cases with one molar affected (explained 11.6% of heterogeneity).
We then explored whether the prevalence between males and females would differ regarding MIH. Meta-analysis confirmed the latter result from meta-regression that MIH is not sex-related and females and males present a non-significant difference on the prevalence of MIH (0.986, 95% CI 0.940-1.035, I 2 = 32.6%, p = 0.564) (Fig. 2).
We further explored the prevalence of MIH per continent (Table 5). Among the five continents analyzed, America was the continent with highest prevalence (15.3, 95% CI 12.8-18.3, p < 0.001, I 2 = 96.3%) and Asia had the lowest prevalence (10.7, 95% CI 78.5-13.5, p < 0.001, I 2 = 98.7%). The differences between continents (test for subgroup differences) were not significant (p = 0.1643).  Mild (demarcated opacities affected less than one-third of the tooth surface, without post-eruptive enamel breakdown), moderate (demarcated opacities that affected at least one-third but less than two-thirds of the surface, without post-eruptive enamel breakdown; atypical caries lesions could affect less than two-thirds of the surface), and severe (demarcated opacities that affected more than two-thirds of the tooth surface, or the presence of post-eruptive enamel breakdown, atypical caries lesions larger than two-thirds of the surface, or large restorations with unusual shape, extended to smooth surfaces, or extraction of the tooth because of MIH) Mild (demarcated enamel opacities without enamel breakdown, occasional sensitivity to external stimuli but not brushing and only mild aesthetic concerns on discoloration of the incisors), and severe (demarcated enamel opacities with breakdowns, caries, persistent/spontaneous hypersensitivity affecting function and finally strong aesthetic concerns that may have socio-psychological impact) 25   www.nature.com/scientificreports/ Additional analyses. No publication bias was detected in the overall analysis (Table 2), except for the prevalence of cases with one molar affected (p = 0.004).
Using the SORT recommendation, we concluded the estimates obtained are classified as SORT A, that means, the results provide high level of confidence.

Discussion
Summary of main findings. The results of the present systematic review estimated a pooled prevalence of MIH at 13.5%. The moderate to severe cases of MIH were estimated at 36.3% of all cases. Having three molars affected with MIH is the least probable situation and affected incisors were seen in 36.6% of the cases. The prevalence of HSPM in MIH cases was estimated at 3.6%. The sample size was a significant source of heterogeneity for the overall MIH prevalence and the year of publication for the prevalence of one molar affected. Sex, year of publication and geographic location were not deemed influential factors in almost all the results. Continents showed no different prevalence on MIH, with the American continent displaying the highest prevalence and the Asian continent the lowest.
Quality of the evidence and potential biases in the review process. Overall, these results were categorized with a SORT A recommendation, which means that all studies found coherent conclusions regarding the prevalence of MIH and that these results are consistent and good-quality patient-oriented evidence. Furthermore, this is the first systematic review providing pooled estimates on molars and incisors affected with MIH and HSPM cases.
As previously presented, two previous systematic reviews have focused on the prevalence of MIH. Overall, our results provided similar prevalence to the one reported by Schwendicke et al. 5 (13.1%) and slightly above from Zhao et al. 6 (14.2%). However, comparing with the latter, the present systematic review expanded the number of countries (49), confirmed the downgrading of alternative case definition of MIH to the overall pooled estimate (while the previous reviews combined classifications), and present new prevalence estimates concerning clinical characteristics of MIH (molas and incisors affected, severity and HSPM).
Regarding the comparison between sexes, our result fully align with those by Schwendicke et al. 5 (OR 0.92; 0.81-1.04) and Zhao et al. 6 (regression estimate = 0.005, p-value = 0.938), which means that both girls and boys present similar distribution of MIH lesions.
When analyzing the prevalence among continents, the comparison with literature is not reasonable as we only accounted for the EAPD classification, and this explains why Oceania had no studies available (despite two publication by Mahoney et al. 118,119 ). Also, in Zhao et al. 6 , Africa was the continent with lowest prevalence, yet in our review Asia had the lowest prevalence. The American continent includes for the first time studies from the United States of America and Mexico which may explain a decrease in MIH prevalence from the two previous studies, however remains as the continent (super-region) with highest prevalence.
Regarding the methodological aspects, by comparing the EAPD with alternative diagnostic methods as a subgroup analysis we confirmed the downgrading potential of alternative methods to the overall estimates. Thus, this step methodological assortment into the analyses despite the substantial heterogeneity from the meta-analytical estimations. Also, our analyses on the severity, teeth affected and HSPM were severely reduced because this sort of data is still scarce. Future studies shall provide extensive information on these characteristics to confirm these results. Also, we were unable to explore hypothetical MIH-related factors (both medical, sociodemographic and environmental) once again because of the lack of relevant information, and this should be taken into account in future epidemiological studies. All in all, readers must bear in mind that although the overall prevalence seems to be constant over the time, new prevalence data has been pooled that contribute to understand the clinical characteristics of this enamel defect entity.
Strengths and potential limitations. This systematic review was conducted following PRISMA a strict guideline for data reporting, a comprehensive literature search and a meticulous predefined protocol. Furthermore, prior to any analysis, we compared the EAPD case definition with other classifications than the EAPD, and we confirmed substantial differences with a downgrading in prevalence when alternative methods were applied. We have attempted to explore ways to mitigate heterogeneity, and all studies used to compute estimates (and that employed the EAPD case definition) were of high methodological quality. Another advantage of this study is that www.nature.com/scientificreports/ we have expanded the search for potential sources of heterogeneity with the addition of geographic measures and the further assessment into the new prevalence estimates. Also, the number of included participants has increase, which is logical given the increase in studies included, yet this is a point to keep in mind. Nevertheless, there are a number of limitations important discussing. Almost half of the studies had not fulfilled the criterion of representativeness and this is a point where future studies shall be careful. These results should be prudently analyzed because of the elevated heterogeneity observed in some of the reported estimates, though from our analyses the heterogeneity mostly derives from the variability between regions already discussed in a previous study assessing meta-analysis of prevalence 131 , rather than the sources of heterogeneity considered as proven through meta-regression.
Also, a number of studies have not employed the EAPD case definition for MIH and after the subgroup analysis aforementioned they were not accounted for the analyses. It is essential that there is a standardization of the classification used, which is a topic already widely discussed in the literature 132 . Ergo, and given the results of the present systematic review, several challenges may emerge. First, a global partnership between all geographic representative associations shall be attained, to ultimately ensure a standardization of MIH reporting and, certainly, will encourage new and updated epidemiological and clinical data. Second, this suggested consensus will clarify the terminologies and guidelines towards a global alliance that will benefit all people affected by MIH. All in all, these may contribute to overcoming the lack of epidemiological data and a still methodologically unsettled reporting approach.
Only a percentage of the overall included studies reported data on the teeth affected with MIH, the severity of cases or HSPM cases. Several classifications for the severity of MIH have been proposed 133,134 , and some date before the EAPD 2003, such as Leppäniemi 135 or the Wetzel & Reckel scale 117,136 . Moreover, the MIH Treatment Need Index (MIH-TNI) was recently presented, which is 137 part of the Wuerzburg MIH concept. Nevertheless, the lack of a homogeneous definition may have contributed to the heterogeneity of results, making it urgent to establish a consensual severity classification.
Hence, future studies should focus on data on these prevalence characteristics to deepen our knowledge regarding the specifics of MIH. These information are of the utmost relevance for clinicians and may aid the development and implementation of future oral health programs.

Conclusion
The estimated prevalence of MIH was estimated at 13.5%. Moderate to severe cases of MIH were estimated at 36.3%. Affected incisors were seen in 36.6% of the cases. The prevalence of hypomineralization of the second primary molars in MIH cases was estimated at 3.6%. Overall, these results were categorized with a SORT A recommendation.

Data availability
Data is provided in the materials of the paper. www.nature.com/scientificreports/