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The objective of this study was to systematically review prevalence of dental caries at different water fluoride levels and emphasize fluoride concentration of drinking water and prevalence of dental caries.
A comprehensive study was conducted using PubMed database. Inclusion criteria were predefined and some articles fulfilled these criteria. Study validity was assessed by some checklists. Surveys were conducted to determine prevalence of dental caries among individuals.
The heterogeneity in the group of children with deciduous teeth in terms of the amount of fluoride in drinking water and social class was significant, and the results of the studies in all the subgroups could not be pooled. However, the heterogeneity of group 2 for subjects with permanent teeth in terms of the fluoride level in drinking water and social class was not significant, and the results of the studies in each subgroup could be pooled together.
The meta-regression showed that tooth type and social class had a significant association with the difference in the prevalence of dental caries. Therefore, these variables were the sources of heterogeneity, and the studies must be grouped and subgrouped based on these variables.
Many studies have been conducted to explain the effects of water fluoridation in adults.
Broffitt et al. assessed caries incidence and risk factors for young adolescents at approximate ages of 9 and 13 years, and the results indicated that incidence of dental caries was low except for the occlusal surfaces of first molars.
The rationale for this review is that although two relevant systematic reviews on water fluoridation and dental carries have been conducted so far by McDonagh et al. in 2000 (reviewing the safety and efficacy of fluoridation of drinking water)
Search strategy
The MEDLINE database was used in this research. The time scope covered the starting date of the database to December 2014. Further searches were carried out through bibliographies of the included studies and also previous systematic reviews, especially the studies by McDonagh et al., hand-searched Index Medicus (from 1945 to 1963) and Excerpta Medica (from 1955 to 1973).
The full electronic search strategy for PubMed database included the following:
Dental caries (Mesh), 37,309 records Dental caries, 49,043 records #1 or #2, 49,043 records Water (title/abstract), 530,698 records Drinking water (Mesh), 2575 records Water supply (Mesh), 28,670 records #4 or #5 or #6, 537451 records Fluorin* (title/abstract), 6 records Fluorin* (title/abstract), 17,703 records Flurid* (title/abstract), 157 records Fluorid* (title/abstract), 37,624 records Fluorine (Mesh), 6996 records Fluorides (Mesh), 31,459 records Fluoridation (Mesh), 5437 records #8 or #9 or #10 or #11 or #12 or #13 or #14, 69,990 records #3 and #7 and #15, 2367 records.
Inclusion criteria
The inclusion criteria considered in this study were related to dental caries and fluoride levels in drinking water. The following criteria were considered for studies: (1) being an original study, (2) studying humans, (3) being related to fluoride in drinking water supplies and dental caries, (4) at least one group of individuals being included in the study, and (5) reporting measurable outcomes in a group accompanied by the amount of fluoride in its drinking water supply.
Data extraction and assessment of study validity
The inclusion criteria were examined by at least two reviewers. The reviewers independently extracted data from the studies and assessed their validity. Furthermore, the third reviewer checked the results. The validity was assessed based on the STROBE checklist adapted for cross-sectional study designs and the checklist of Centre for Evidence-based Social Service adapted for before–after study designs.
Outcome measures
The number of teeth becoming decayed, missing, and filled (DMFT) was used to classify dental caries. Dental caries was defined in this review as any level more than 0 on DMFT. In addition, a meta-analysis was conducted for obtaining summary measure with 95% confidence interval (CI) using Stata/SE 11.1 (Stata Corp LP, College Station, TX 77845, USA). Random effect models were employed for reporting the results.
Heterogeneity
The inconsistency was examined using the χ
2test at a significance level of 10%. In addition, heterogeneity was quantified across studies using I
2statistic.
Analysis
Measures of effect were plotted (95% CIs) if the data were in the right format. The heterogeneity was assessed based on visual examination of plots and χ
2statistic. Meta-regression was carried out for significant heterogeneity.
In addition, random effects models were used for combining the results. Furthermore, meta-regression was used for identifying the impact of study characteristics on the outcome for explaining any heterogeneity between studies. Data analyses were conducted with Stata/SE 11.1.
Thirteen articles met the inclusion criteria and were included in the study. Three articles that presented dental caries prevalence graphically were excluded from the study. Finally, 10 out of 13 articles, which contained all age groups, were selected. Hence, 10 articles including 56 study areas were reviewed. All the included articles were cross-sectional in design and were of evidence level B (low quality). A diagram showing the stages of a systematic review is presented in
Process of inclusion of studies for review and analysis.
Data categorization and analysis of subgroups were carried out to decrease the impact of confounding factors such as consumption of supplements or materials containing fluoride that can affect the relation between the fluoride levels in drinking water and dental caries.
According to dentists and epidemiologists, age is considered as the most significant variable in dental caries. Hence, categorization was firstly based on age. The second and third significant variables were considered the fluoride level in drinking water and the social level. Other factors such as exposure time to fluoride in drinking water and any exposure to fluoride in supplements, diet, and air might also influence the prevalence of dental caries, but they were not considered because of the lack of studies, not mentioning in most of the studies, and their insignificant role in the social level.
The categorization of variables included tooth type (deciduous and permanent), fluoride in drinking water (<0.7 and >0.7), and social class (high, moderate and low).
Finally, two groups were identified. First, studies were categorized based on tooth type and fluoride level in drinking water. Second, the individuals were categorized based on social level. According to
As it is illustrated in
Considering the following forest plot, there was no heterogeneity in group 2 for the children with permanent teeth, the fluoride level of <0.7 ppm, and high social class (subgroup 1). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 70.6% (95% CI, 66.2%–75.0%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of <0.7 ppm in drinking water and high social class (subgroup 1).
Regarding the forest plot below, there was no heterogeneity in group 2 for the children with permanent teeth, and the fluoride level of <0.7 ppm, and moderate social class (subgroup 2). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 76.6% (95% CI, 73.9%–79.3%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of <0.7 ppm in drinking water and moderate social class (subgroup 2).
Considering the forest plot below, there was no heterogeneity in group 2 for children with permanent teeth and the fluoride level of <0.7 ppm, and low social class (subgroup 3). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 78.9% (95% CI, 77.3%–80.6%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of <0.7 ppm in drinking water and low social class (subgroup 3).
Regarding the forest plot below, there was no heterogeneity in group 2 for children with permanent teeth and the fluoride level of >0.7 ppm, and high social class (subgroup 4). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 65.1% (95% CI, 57.7%–72.4%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of >0.7 ppm in drinking water and high social class (subgroup 4).
Regarding the forest plot below, there was no heterogeneity in group 2 for the children with permanent teeth, the fluoride level of >0.7 ppm, and moderate social class (subgroup 5). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 69.1% (95% CI, 64.4%–73.9%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of >0.7 ppm in drinking water and moderate social class (subgroup 5).
Considering the forest plot below, there is no heterogeneity in group 2 for the children with permanent teeth, the fluoride level of >0.7 ppm, and low social class (subgroup 6). Therefore, the pooled estimate of dental caries prevalence could be reported. The prevalence of dental caries in this subgroup was 69.2% (95% CI, 61.4%–77.0%)
Forest plot of dental caries prevalence for subjects with permanent teeth, a fluoride level of >0.7 ppm in drinking water and low social class (subgroup 6).
Meta-regression showed that tooth type and social class had a significant association with the difference in the prevalence of dental caries. These variables were the sources of heterogeneity. As a result, studies must be grouped and subgrouped based on these variables and the analyses must be carried out in the groups
Fluoride can be ingested from water supplies, beverages and nutriments, and its intake among individuals depends on food and use of fluoride supplements. In the current study, the variables included tooth type, fluoride in drinking water and social class. In addition, the number of included studies in this review considerably decreased because of considering all the detected confounders.
Analysis of the subgroups and examining the heterogeneity showed that in children in group 1, the heterogeneity among the studies in subgroups was statistically significant (P < 0.001). Therefore, the results of studies in the subgroups could not be pooled. However, in group 2, this heterogeneity was not significant, and the results of the studies in each subgroup could be pooled. In the same level of fluoride, the prevalence of dental caries in subjects with low social class was more than that in subjects with moderate social class. The prevalence of dental caries in subjects with low or moderate social class was more than that in subjects with high social class.
As it was mentioned earlier, two systematic reviews have been conducted in relation to fluoride concentration of drinking water and prevalence of dental caries. In the systematic review conducted in 2000, McDonagh et al. reviewed the safety and efficacy of fluoridation of drinking water through a search in 25 electronic databases and relevant websites. Another review was carried out by the Australian National Health and Medical Research in Australia in 2007.
The authors suggest an in-depth investigation into the geographical distribution of dental caries prevalence in different regions by considering all the confounding factors for further research in order to provide a wide epidemiological outlook on the problems explained in the current study. Since previous investigations have not focused on extracting a model for the geographical distribution of dental carries in relation to water fluoridation, this model is useful to adopt appropriate policies to decrease dental problems systematically.
As a result, regarding the heterogeneity, the results of studies in group 1 could not be pooled, but the results of studies in group 2 could be pooled together.
Acknowledgments
The authors appreciate the financial support of Tehran University of Medical Sciences. Furthermore, authors herewith acknowledge the Department of Community Oral Health at Tehran University of Medical Sciences.
Financial support and sponsorship
This study was supported by Tehran University of Medical Sciences.
Conflicts of interest
The authors of this manuscript declare that they have no conflicts of interest, real or perceived, and financial or nonfinancial, in this article.