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Association between developmental defects of enamel and early childhood caries in children under 6 years old: A systematic review and meta-analysis

Early childhood caries (ECC) are an oral health problem worldwide in children under 6 years of age. This disease of rapid development has a multifactorial etiology, and one of the possible risk factors is developmental defects of enamel (DDE), such as hypoplasia and opacities. The aim of this systematic review was to evaluate the association between DDE and ECC in children under 6 years of age. An electronic search was conducted until March 2022 using Medline (PubMed), Scopus, Science-Direct, LILACS, Web of Science, Cochrane Library, EBSCO-Host, EMBASE, and Google Scholar and complemented with a manual search, with no restrictions on language or date of publication. Longitudinal studies of children under 6 years of age with primary dentition were included. A total of 1158 studies were found, of which 651 records were reviewed by title and abstract, and 24 articles were selected for full-text evaluation. Finally, nine studies that met the selection criteria were included in the qualitative synthesis. Study quality and certainty were assessed using the Newcastle-Ottawa scale and the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) tool. Three cohort studies of good quality were included in the meta-analysis. A risk associated with DDE (RR = 1.94; 95% CI: 1.52–2.49) and a risk associated with enamel hypoplasia (RR = 5.45; 95% CI: 1.84–16.14) were found. The results for diffuse opacity (RR = 1.21; 95% CI: 0.18–8.15) and demarcated opacity (RR = 1.26; 95% CI: 0.43–3.65) were not significant. GRADE analysis presented low and very low certainty of evidence. It was concluded that there is an association between DDE and ECC. However, the results should be interpreted with caution because of the limitations of the study. The protocol for this study has been registered in PROSPERO under identification number CRD42021238919.


Introduction
Early childhood caries (ECC) are defined as the presence of one or more cavitated or non-cavitated lesions or missing or filled teeth due to caries in the primary dentition of children younger than 6 years of age [1,2]. The two key parameters for ECC are the age of the child and the involvement of the primary dentition; one characteristic is that its progression is often fast and can ultimately result in the complete destruction of teeth [1,3,4,5,6]. Recent studies have shown that the worldwide prevalence of ECC ranges from 23% to 90%; in the case of Latin America, it exceeds 50% [1,7].
A variety of observational studies with good evidence have demonstrated the multifactorial etiology of dental caries in children, associating them with various factors such as biofilm, frequent sugar intake,

Methods
The present article followed the guidelines of writing according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) [25] ( Table 1). The protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) under registration number CRD42021238919 after a preliminary search was performed.
The question addressed for the systematic review was "What is the association between DDE and ECC in children under 6 years of age?" The PECO strategy was utilized: P: children under 6 years old E: the presence of DDE C: absence of DDE O: development of ECC

Eligibility criteria
Studies investigating the association between DDE and the prevalence of ECC in a representative sample of children were included.
For the article search, the included studies were systematic reviews and meta-analyses as well as cohort and case-control studies that included a control group. Both systematic reviews and meta-analyses were considered only for conducting a manual search of the reference lists.
The study designs that were excluded were cross-sectional studies, experimental studies, letters to the editor, literature reviews, and animal studies.
Cross-sectional studies were excluded because ECC is a disease that develops over time, and its progression is best evaluated via longitudinal studies [22,23].
The conditions of the participants were as follows: Inclusion criteria included children in preschool under 6 years of age, presenting with primary dentition and the absence of any systemic condition.
Exclusion criteria included children with any fixed or removable appliances, the presence of molar-incisor hypomineralization, and dental fluorosis.

Search strategy
An electronic search was conducted and updated through March 16, 2022, using the following databases: Medline (PubMed), Scopus, Science Direct, LILACS, Web of Science, Cochrane Library, EBSCO-Host, and EMBASE complemented with a search of Google Scholar as well as a manual search. Keywords were selected from the DeCs-health science descriptors, the Medical Subject Headings (MeSH) of the U.S. National Library of Medicine, and uncontrolled vocabulary. In addition, a manual search of the reference lists of the selected systematic reviews was performed. Articles were selected based on predefined eligibility criteria, with no restrictions on language or publication date and no definite time of follow-up, using the web application Rayyan QCRI; studies that were either published or approved for publication were considered. The references were organized using a reference manager software (Mendeley desktop, Elsevier). Systematic reviews and meta-analysis were not considered for inclusion among the final selection of studies for the qualitative and quantitative analyses, as only references of the primary studies included in those articles were considered.
The definitions considered for the development of this systematic review were the following: -ECC: caries development in children under 6 years old [26,27,28]. -DDE: quantitative or qualitative alterations in the enamel [12], considering: Data collection process 10 Describe method of data extraction from reports (e.g., piloted forms, independently, in duplicate) and any processes for obtaining and confirming data from investigators.

6
Data items 11 List and define all variables for which data were sought (e.g., PICOS, funding sources) and any assumptions and simplifications made.

6-7
Risk of bias in individual studies 12 Describe methods used for assessing risk of bias of individual studies (including specification of whether this was done at the study or outcome level), and how this information is to be used in any data synthesis.

Synthesis of results 14
Describe the methods of handling data and combining results of studies, if done, including measures of consistency (e.g., I 2 ) for each meta-analysis.

8
Risk of bias across studies 15 Specify any assessment of risk of bias that may affect the cumulative evidence (e.g., publication bias, selective reporting within studies).

7
Additional analyses 16 Describe methods of additional analyses (e.g., sensitivity or subgroup analyses, metaregression), if done, indicating which were pre-specified.

Study selection 17
Give numbers of studies screened, assessed for eligibility, and included in the review, with reasons for exclusions at each stage, ideally with a flow diagram.

8
Study characteristics 18 For each study, present characteristics for which data were extracted (e.g., study size, PICOS, follow-up period) and provide the citations.

8-9
Risk of bias within studies 19 Present data on risk of bias of each study and, if available, any outcome level assessment (see item 12).

8-9
Results of individual studies 20 For all outcomes considered (benefits or harms), present, for each study: (a) simple summary data for each intervention group (b) effect estimates and confidence intervals, ideally with a forest plot.

9-10
Synthesis of results 21 Present results of each meta-analysis done, including confidence intervals and measures of consistency.

9-10
Risk of bias across studies 22 Present results of any assessment of risk of bias across studies (see Item 15). 9 Additional analysis 23 Give results of additional analyses, if done (e.g., sensitivity or subgroup analyses, metaregression [see Item 16]).

Summary of evidence 24
Summarize the main findings including the strength of evidence for each main outcome; consider their relevance to key groups (e.g., healthcare providers, users, and policy makers).

11-15
Limitations 25 Discuss limitations at study and outcome level (e.g., risk of bias), and at review-level (e.g., incomplete retrieval of identified research, reporting bias).

Conclusions 26
Provide a general interpretation of the results in the context of other evidence, and implications for future research.

Funding 27
Describe sources of funding for the systematic review and other support (e.g., supply of data); role of funders for the systematic review. Enamel hypoplasia: quantitative deficiency of enamel, either localized or general lack of enamel surface [11,29,30]. Diffuse opacity: diffuse opacity of enamel of white color presented as a translucent alteration of the enamel to a variable degree [11,29,30]. Demarcated opacity: delimited opacities of the enamel of white, yellow, or brown color, involving an alteration in the translucency of the enamel to a variable degree [11,29,30].
The first stage of the study selection was performed by two reviewers (SCS and KHUK) to remove articles that were irrelevant based on the title and abstract, and in case of discrepancy, a third reviewer (MBBH) was involved to resolve the issue. The same reviewers then performed the second stage of study selection independently and in duplicate after reading the articles' full text, and any discrepancy was resolved by the same third reviewer. The final data extraction was performed by four reviewers (KHUK, JCBL, CRY, and ZRA) and verified by a final reviewer (GTR), and any disagreement was resolved by consensus.

Assessment of risk of bias and quality of evidence
Quality assessment was performed for each included study in duplicate and independently by two reviewers (GPS and JAM) using the Newcastle-Ottawa scale (NOS) [31]. Discrepancies were resolved by consensus with the participation of a third evaluator when necessary (KHUK). The NOS tool is a classification system that assigns a maximum of nine stars in three categories: selection of participants (four stars), comparability (two stars), and measurement of exposure in case-control studies and outcomes in cohort studies (three stars).
The following factors were considered in the comparability assessment: number of dental caries, type of DDE, sex, and age.
To determine the level of quality of each included study, results were converted to good, fair, and poor quality based on previous studies [32,33] as follows: -Good quality: 7-9 stars -Fair quality: 5-6 stars -Poor quality: 0-4 stars The certainty of the evidence was evaluated using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) tool [34]. The assessment was evaluated in duplicate by two reviewers (GPS and JCBL), and discrepancies were resolved with the participation of a third evaluator (KHUK).

Risk of caries assessment
Subgroup analysis was performed according to the type of DDE: enamel hypoplasia, diffuse opacity, and demarcated opacity. Based on the articles found, primary second molar hypomineralization was not considered.
The risk for ECC was assessed considering DDE in general, as well as enamel hypoplasia and diffuse and demarcated opacities independently. The adjusted odds ratio (aOR) and adjusted relative risk (aRR), P-values, and 95% confidence intervals (CIs) were obtained for all studies and, if not directly provided, were calculated manually and individually.
Data extraction was processed using the Review Manager software (RevMan 5.4) [35] of the Cochrane Collaboration. Relative risk (RR) was obtained for DDE, enamel hypoplasia, diffuse opacity, and demarcated opacity.
Heterogeneity was assessed using the analysis of x 2 and the I 2 index. A random-effects model was used for the meta-analysis when heterogeneity as determined by the I 2 index was above 50%, considering a confidence interval of 95%. For I 2 values <50%, a fixed-effects model was used. Measurements of relative risk (RR) and frequencies for the studies were used for the statistical analysis, with a significance level of 0.05.
however, no risk of bias was identified. A subgroup analysis was performed, and a meta-regression analysis was not necessary.

Results
After the search, a total of 1158 articles were found, including four systematic reviews. After the elimination of 507 duplicated articles, a total of 651 articles were analyzed by title and abstract. A total of 627 articles were excluded based on their title and abstract because they were not related to the topic, reported different outcomes, or were from different populations, resulting in 24 studies that were ultimately selected for full-text assessment. After reading the full text, nine articles were included for the qualitative synthesis, and three were included for the quantitative synthesis (meta-analysis), as presented in the flow chart ( Figure 1).
For the study selection, reviewer calibration was performed previously to obtain a suitable inter-rater reliability value (K ¼ 0.7). This calibration process consisted of unifying the criteria among reviewers by selecting studies from a predetermined set of 20 articles and then comparing the results with the selection of an expert (GTR).
The list of articles selected for full-text evaluation is included in Table 3, while articles excluded at the full-text stage and the exclusion reason as well as the studies included for the qualitative analysis are presented in Table 4. For all studies with missing data, the authors were contacted, and the article was dismissed if they failed to reply.
The language of the final articles included was predominantly English, and the studies were from various countries (Brazil, Australia, the United States, and China). All of the studies were published between 2006 and 2020. Of the nine items, 50% received funding. Among the included studies, the participants were male and female children under 6 years of age. A summary of the extracted data, including country, study design, sample size, age, the index used, results, and remarks, is presented in Table 5.

Level of evidence and risk of bias (quality)
Quality assessment was performed for each of the nine longitudinal studies (cohort and case-control). Previously, a calibration of the two evaluators was performed with an expert (KHUK) by unifying criteria and evaluating different articles using the NOS scale, thus achieving an appropriate value of kappa for both the case-control (K ¼ 0.9) and cohort (K ¼ 0.8) studies, with a final kappa value (K ¼ 0.729), which showed high concordance.
All studies included in the qualitative analysis were evaluated according to their quality using the NOS tool [31] for both case-control and cohort studies; four articles of good quality, three of fair quality, and two of poor quality were found ( Table 6). The decisions by item are listed in Table 7.
The certainty level was evaluating using the GRADE tool [34]. Analysis was performed for each outcome included in the meta-analysis, and the grade of recommendations was obtained (Table 8). A low certainty of evidence was determined for the association between DDE and ECC, and very low level of certainty was determined for the association of  enamel hypoplasia and ECC, diffuse opacities and ECC, and demarcated opacities and ECC.

Measurement of exposures and outcomes
All of the articles included in the narrative synthesis and metaanalysis were analyzed using the NOS for both the three case-control studies and the six cohort studies. After the evaluation of risk of bias, four studies of good quality, including one case-control and three cohort studies, were found. For the meta-analysis, only good-quality cohort studies were included.
The majority of studies reported that the examiners had undergone training and calibration to perform the clinical examination and reported results for the coefficient of kappa of 0.83-0.93.
Various indices for the evaluation of DDE and ECC were utilized in the nine articles included. For the evaluation of DDE, the DDE index of the     F ed eration Dentaire Internationale (FDI) [42] was the most widely used; it includes codes for hypomineralization (diffuse opacity and demarcated opacity) and hypoplasia (missing enamel and reduction in thickness). For ECC, as they affected primary dentition, most of the studies evaluated the lesions of dental caries by decayed, extracted, and filled (deft) for primary teeth, according to the World Health Organization's (WHO) parameters [43].

Risk of caries according to DDE classification
For the meta-analysis, only the high-quality cohort studies were considered, which included only three studies in total [8,9,36]. Meta-analysis of case-control studies was not performed because only one case-control study presented good quality [41], and a meta-analysis with a combination of data between case-control and cohort studies was considered inappropriate. Figure 2 shows the risk of ECC found for each category of DDE evaluated, including all DDE in general, enamel hypoplasia, diffuse opacities, and demarcated opacities. The forest plot in Figures 2a and 2b represent only those with significant RR greater than one associated with DDE (RR ¼ 1.94; 95% CI: 1.52-2.49) and enamel hypoplasia (RR ¼ 5.45; 95% CI: 1.84-16.14) . Figures 2c and 2d show a RR of greater than one, which is not significant, associated with diffuse opacity (RR ¼ 1.21; 95% CI: 0. 18-8.15) and demarcated opacity (RR ¼ 1.26; 95% CI: 0.43-3.65).
Heterogeneity of the study (I 2 ) and p-value (p < 0.01) were also calculated. The results showed a measure of consistency of heterogeneity of 0% (Z ¼ 5.26, p < 0:00001) for the presence of DDE, 97% (Z ¼ 3.06, p < 0:002) for enamel hypoplasia, 93% (Z ¼ 0.20, p ¼ 0:84) for diffuse opacity, and 76% for demarcated opacity (Z ¼ 0.43, p ¼ 0:67). This determined the use of a fixed-effect model for the presence of DDE in general and the use of a random-effect model for enamel hypoplasia, diffuse opacity, and demarcated opacity.

Discussion
The results of the present study showed that children under 6 years old affected by DDE had twice the risk of developing caries, compared to those with no DDE. The children with enamel hypoplasia presented as Quality assessment: Good quality (7-9 stars), fair quality (5-6 stars) and poor quality (0-4 stars) as classified on previous studies [32,33].  much as five times higher risk of developing ECC, while children with diffuse or demarcated opacities, do not present a risk of developing ECC. The prevalence of DDE ranges from 25% to 79% [14,15,16,17], and is highly variable depending on the sample size of the various studies. Moreover, according to Oliveira et al. [14], 78.9% of infants at 36 months of age already have at least one tooth affected. Within these defects, enamel hypoplasia is observed, presented as the absence of enamel or as a reduction of the thickness of enamel, as is hypomineralization, also known as opacities, which may appear as diffuse or demarcated [3,4,44]. These defects may be found in both permanent and primary dentition, and diverse studies have associated the presence of DDE in primary dentition with risk factors such as low birth weight [5,44], malnutrition [3], problems during pregnancy [4], premature delivery [5,18], infections during childhood [5], lack of breastfeeding [4], social factors [4,5], sex, and age [45].
DDE are alterations in the normal development of tooth enamel, which can affect its quantity or quality [11,12] and may make affected teeth more susceptible to the development of carious lesions. Several studies have associated the development of dental caries with DDE, considering them to be risk factors for caries [8,14,36]. Although there have been previous systematic reviews of this topic, they have included cross-sectional studies or studies in children with mixed or permanent dentition, with no quality evaluation of the evidence used [10,46]. Moreover, the role of the different types of DDE and their association with ECC remain unclear.
In children under 6 years of age, the development of ECC is a problem of oral health concern, and it is one of the most prevalent chronic diseases in children [47]. The presence of cavities in primary dentition is also considered a predictor of the development of caries in permanent dentition [48,49], it is highly important to address this health problem from a very early age. As an oral disease with a high number of cases in children-it has been reported in approximately 50% of preschoolers worldwide [47]-it is of utmost importance to recognize the various risk factors for its development. As DDE are considered to constitute a risk factor for the development of ECC in several studies, the following systematic review aimed to evaluate the association between DDE and ECC prevalence in children under 6 years of age.
The results of the meta-analysis showed that children who had teeth affected by DDE had nearly twice the risk of developing caries, compared to those with no DDE (RR ¼ 1.94; 95% CI: 1.52-2.49). Based on this, the present study identified DDE as a risk factor for dental caries in primary dentition, as reported in the systematic reviews conducted by Costa et al. [10] and Kirthiga et al. [2]. In Costa et al.'s [10] the risk values obtained were slightly higher (OR ¼ 3.32, CI: 2. 41-4.57). This may have been because both cross-sectional and longitudinal studies were included, and there was no categorization according to the quality of studies, unlike in this meta-analysis. In addition, Kirthiga et al.'s [2] review considered only two studies for their meta-analysis, which obtained a much higher value (OR ¼ 14.62, CI: 6.10-35.03) than that of the present study. Kirthiga et al. [2] considered the study of Oliveira et al. [14] as part of their meta-analysis, which presented very wide confidence intervals (OR ¼ 14.9, CI: 5.48-38.63) that may have affected their final results. In this meta-analysis, de Oliveira et al.'s [14] study, although included in the qualitative synthesis, was not considered for the quantitative analysis because it did not present good quality. In the Vargas-Ferreira et al.'s [46] systematic review, which evaluated the association between DDE and the development of dental caries in children and adolescents among 6-14 years old, they found an RR ¼ 2.22 (CI: 1.39-3.54), very similar to this study. Thus, regardless of age, DDE can be considered a risk factor for the development of dental caries.
Although the present results are consistent with previous systematic reviews regarding DDE as a risk factor for dental caries, the role of each DDE type in the prevalence of ECC remains unclear. Within the classification of DDE, there is a higher prevalence of diffuse opacities and hypoplasia [36,50,51], although the results differ slightly from what was found in a preliminary study conducted by the authors of the present study showing that in a population of children from 2 to 5 years old in the region of Pichanaki, Peru, there was a higher prevalence of demarcated opacities when analyzing the second primary molars.
In this meta-analysis, regarding the data of enamel hypoplasia, the risk of caries increased, as children with enamel hypoplasia presented as much as five times higher risk of developing ECC (RR ¼ 5.45; 95% CI: 1.84-16.14). It should be noted that only for Targino et al.'s [36] study was missing enamel considered as hypoplasia. However, when assessing the diffuse and demarcated opacities, the risk was not only less than that for hypoplasia, but neither opacity type presented significant results. Diffuse opacities were not considered a risk factor for ECC ( The results of this meta-analysis demonstrate that within the different types of DDE, enamel hypoplasia represents a true risk for the development of dental caries, as reported by Costa et al. [10]. This may occur because hypoplasia, most notably the missing enamel, can form undermined surfaces, such as cavities, pits, or grooves [11,12], which may allow for greater retention of dental plaque, as well as areas with reduced tooth enamel, which can lead to lower protection against cariogenic bacteria-generated acids, with the subsequent progression of carious lesions. Moreover, in the meta-analysis of the different types of DDE, the heterogeneity was considerable for enamel hypoplasia, diffuse opacity, and demarcated opacity. It is possible that, due to the nature of the studies, the different sample amounts, ages, and follow-up of participants may have been reflected in the heterogeneity. In addition, unlike Targino et al. [36] and Paixão-Gonçalves et al. [8], who used the FDI index for DDE, Hong et al. [9] used a different index and also examined hypoplasia in the second primary molars, which may have been reflected in the high heterogeneity of the hypoplasia analysis. Enamel hypoplasia is a type of defect that involves the enamel surface and is associated with reduced and localized thickness [30,52]; it is typically the most prevalent of all DDE and generates risk of developing carious lesions [9,39,53]. The prevalence of hypoplasia has been reported to be 28% [54] and is considered a significant predictor of dental caries in children under 6 years old [9,55]. Opacities involve an alteration in the translucency of the enamel to a varying degree; they can appear in white, cream, yellow, or brown colors and vary greatly in their extension, location, and distribution [30,52]. The prevalence of demarcated opacities is variable, having been reported as low as 5.0% [50], while diffuse opacities are commonly more prevalent (23.1%) [50]; similarly, these opacities have been associated with caries development [37]. Although the results of the present study only obtained significant values for hypoplasia, that opacities could also play an important role in the development of ECC should not be ruled out. Therefore, more studies are needed to address the association of the different types of DDE in relation to the development of dental caries.
DDE constitute a risk factor for the development of ECC, and enamel hypoplasia presents an even higher risk for the development of dental caries, as evidenced in this study. Thus, early management of these issues is of utmost importance to prevent the progression of carious lesions. The present study considered only cohort studies with the best quality for this meta-analysis in addition to stratifying the results into subgroups according to the type of DDE. The results may be valuable for decisionmaking regarding the prevention of dental caries in primary dentition, however, they should be interpreted with caution. Based on the certainty level, only low and very low levels of certainty were determined for all the outcomes studied, including the association of DDE and ECC, enamel hypoplasia and ECC, diffuse opacities and ECC, and demarcated opacities and ECC. The quality of evidence was low due to serious inconsistency and imprecision, due to the high heterogeneity of the studies included in each meta-analysis, few events reported, and wide confidence intervals. Moreover, the meta-analyses only included 2 to 3 studies, possibly affecting the certainty level.
Ismail et al. [22], who studied the progression of caries on the primary tooth surface level over a period of two years, showed that the rate of progression of moderate caries is 9.6 times higher than that of healthy surfaces. The initial and moderate carious surfaces progressed to extensive caries 6.1 and 20.6 times, respectively, relative to the healthy surfaces. This indicates that the progression of caries over time is necessary for evaluation, especially on non-healthy surfaces. Based on the progression of dental caries over time, cross-sectional studies were not considered in our analysis, as it is known that ECC is a multifactorial disease that takes time to develop. According to previous studies [22,23], this development can take 24 months, as can diagnosis; therefore, longitudinal studies would be more accurate for studying the development of dental caries. In addition, cross-sectional studies represent lower and weaker scientific health evidence, compared to case-control and cohort studies, according to the latest pyramid of scientific evidence proposed by Murad et al. [24]. Moreover, they are not considered appropriate for use in providing clinical recommendations, as stated by the SIGN guidelines, in comparison to longitudinal studies [56]. The previous systematic reviews that have been published [2,10] included cross-sectional studies, which may have reduced the level of evidence and the degree of clinical recommendation of the studies; therefore, the focus of the present systematic review and meta-analysis was to search for only case-control and cohort studies.
Regarding the application of the results found in the present study, children under 6 years of age need to be evaluated for the presence of any kind of DDE, as this condition could represent a risk of developing ECC. Populations with a high prevalence of these enamel defects should be monitored to avoid the progression of carious lesions from an early age. Although the results of this study showed a significant risk of ECC for both DDE in general and hypoplasia, the meta-analyses were performed with only three studies; therefore, the results should be interpreted carefully. It is possible that teeth with hypoplasia may be more vulnerable to the development of dental caries than those with opacities. However, it should not be ruled out that both hypoplasia and opacities could manifest a high risk for ECC. Considering that DDE may be a risk factor for ECC, it is important to consider that all children under 6 years of age with primary dentition who present any kind of DDE may require more extensive dental care to prevent the development and progression of ECC.
As few good-quality studies on the subject address the association between each type of DDE and ECC, it is necessary to perform goodquality longitudinal studies to elucidate the true role according to the type of DDE, differentiating between hypomineralization (diffuse and demarcated opacities) and hypoplasia (missing enamel and reduced thickness) in children under 6 years of age.

Limitations
This review had several limitations, including the limited number of studies on the subject, with the majority being cross-sectional studies and few being longitudinal studies. Likewise, regarding the small number of studies on DDE related to ECC, we received limited responses from the authors when we contacted them. In addition, among the nine articles included, the quality varied greatly, with only four studies showing good quality, three showing fair quality, and two showing low quality. These limitations have implications for future research.
There was no standardization among studies to measure the results of both dental caries and DDE. Some studies evaluated these variables in a general way, while others did so according to the different types, including enamel hypoplasia and opacities, which meant that not all of the included studies were comparable.
Regarding the certainty of evidence, the GRADE analysis showed a low and very low level of evidence. An additional limitation was that the meta-analysis was conducted using only three studies; thus, the findings should be considered with extreme caution because this limitation results in evidence of very low strength.

Conclusion
The association between DDE and ECC has been demonstrated based on the results of this systematic review and meta-analysis. In addition, within the types of DDE, enamel hypoplasia represented a risk for the development of dental caries in children under 6 years old. A low level of certainty was determined for the association between DDE and ECC, and very low levels of certainty were determined for enamel hypoplasia, diffuse opacities, and demarcated opacities in relation to ECC.
Children with DDE may, therefore, require more dental care to prevent the development and progression of ECC. However, the results of this systematic review and meta-analysis should be interpreted with caution because of the limitations of the study.