Absence of an association between socioeconomic indicators and traumatic dental injury: a systematic review and meta-analysis

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1 Dental Traumatology 2015; 31: ; doi: /edt Absence of an association between socioeconomic indicators and traumatic dental injury: a systematic review and meta-analysis Patrıcia Corr^ea-Faria 1, Carolina C. Martins 1, Marcelo B onecker 2, Saul M. Paiva 1, Maria Letıcia Ramos-Jorge 3, Isabela A. Pordeus 1 1 Department of Pediatric Dentistry and Orthodontics, School of Dentistry, Federal University of Minas Gerais, Belo Horizonte, MG; 2 Department of Orthodontics and Pediatric Dentistry, School of Dentistry, Federal University of S~ao Paulo, S~ao Paulo, SP; 3 Department of Pediatric Dentistry, Federal University of Vales do Jequitinhonha and Mucuri, Diamantina, MG, Brazil Key words: tooth injuries; systematic review; tooth deciduous; socioeconomic factors Correspondence to: Patrıcia Corr^ea-Faria, Av. Antonio Carlos, 6627, Faculdade de Odontologia, UFMG - Campus Universitario , Belo Horizonte, MG, Brazil Tel.: patriciafaria.faria09@gmail.com Accepted 6 April, 2015 Abstract Objective: The aim of this study was to perform a systematic review and search for scientific evidence on the association between socioeconomic indicators and traumatic dental injury (TDI) in the primary dentition. Methodology: The PubMed, ISI, LILACS, Cochrane Library, and Embase databases were searched for articles addressing possible associations between socioeconomic indicators and TDI in the primary teeth in journals dating from the inception of the databases through to December Two independent reviewers performed data extraction and analyzed the quality of the studies. Meta-analysis was undertaken. Pooled estimates were calculated with a 95% confidence interval (CI) and odds ratios (OR). Results: Sixteen articles were included in the systematic review. Children from families with household income less than two times average salary (U$ 592) (OR: 0.77; 95% CI: ) or more than three times the average salary (U$ 888) (OR: 0.76; 95% CI: ) had a significantly lower chance of having TDI in the primary dentition. TDI was not associated with socioeconomic status (high vs low OR: 0.77; 95% CI: ; high vs medium OR: 1.03; 95% CI: ; medium vs low OR: 0.70; 95% CI: ), house ownership (owned vs rented OR: 1.28; 95% CI: ), mother s schooling (OR: 0.89; 95% CI: ), or father s schooling (OR: 1.01; 95% CI: ). Conclusion: The scientific evidence demonstrates that socioeconomic indicators are not associated with TDI in the primary dentition. The evidence of an association between a low income and TDI is weak. In general, studies had low risk of bias. Further prospective cohort studies are needed to confirm this association. The literature states that socioeconomic factors are important health determinants. Individuals from less privileged socioeconomic classes are more prone to diabetes, disabilities, a shorter life expectancy, and obesity (1 3) due to limited access to healthcare and preventive services as well as the adoption of unhealthy behaviors (4, 5). Socioeconomic indicators have also been associated with oral health status in children. Children from families with a low socioeconomic status tend to have a greater frequency of dental caries (6 8) and periodontal problems (6). The association between socioeconomic factors and traumatic dental injury (TDI) has also been investigated (9 15). According a literature review published in 2009, most studies investigating the influence of socioeconomic factors on TDI in the permanent dentition have found no such association (16). Moreover, there is no consensus in the literature regarding the influence of socioeconomic factors on TDI in the primary dentition. While some studies report that TDI is associated with the number of children in the family (17), family structure (18), mother s schooling (18, 19), household income (20), and type of school (11, 12, 21), other investigations have not confirmed these associations (9, 14, 15, 22 24). Such divergences cast doubts regarding evidence of the role of socioeconomic factors in the occurrence of TDI. Thus, a systematic review and meta-analysis is needed. The aim of this study was to perform a systematic review and meta-analysis of articles addressing the association between socioeconomic indicators (e.g., mother s schooling, household income, type of school) and TDI in the primary dentition. 255

2 256 Corr^ea-Faria et al. Material and methods Review methods The present systematic review was undertaken in accordance with the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (25) (protocol number: PROSPERO CRD ). Eligibility criteria Epidemiological studies (cross-sectional, case control, cohort, clinical trials) addressing possible associations between socioeconomic indicators (e.g., mother s schooling, household income, number of children, type of school) and TDI in the primary dentition were included in the present systematic review. Review articles, case reports, expert opinions, studies involving the permanent dentition or pooled data on both the primary and permanent dentitions, and studies addressing particular groups (e.g., patients with cerebral palsy) were excluded from the review. No restrictions were imposed regarding language or date of publication. Search strategy One of the researchers (P.C.F) searched electronic databases in December 2013 for relevant studies. Searched databases are shown in Table 1 with the, respectively, the strategies. Manual searches were also performed of the lists of references in the studies included. The Reference Manager Software Ò (Reference Manager, Thomson Table 1. Search strategy used for each database Database Search strategy Medline through PubMed ( ISI Web of Science ( Cochrane Library ( cochrane.org/index.htm) Latin American and Caribbean Health Sciences (LILACS) through Virtual Health Library ( Embase ( online-tools/embase) ((tooth injuries [MeSH terms] OR teeth injur* [Text word] OR tooth injur* [Text word] OR traumatic dental injur* [Text word] OR TDI [Text word]) AND (overjet [Text word] OR overjet, dental [MeSH] OR dental overjet [MeSH] OR lip coverage [Text word] OR socioeconomic factors [MeSH terms] OR social class [MeSH terms] OR socioeconomic status [MeSH terms] OR educational status [MeSH terms] OR income [MeSH terms] OR risk factors [MeSH terms]) AND (animals [MeSH terms] NOT humans [MeSH terms])) (tooth injury AND educational status; tooth injury AND risk factors) (tooth injury AND social class) Reuters, New York, NY, USA, version ) was used to organize the list of studies. Study selection Study selection was carried out independently and in duplicate by two researchers (P.C.F and C.C.M). The title and abstract were read for the initial selection of studies (Fig. 1). As a calibration exercise, an initial amount of 10% of retrieved studies was selected independently by the reviewers. In cases of disagreements, a consensus was reached after fully discussion of inclusion and exclusion criteria. After this, the independent selection followed for the remaining of studies. The disagreements were again reached by discussion. Studies considered relevant and those that met the inclusion criteria were submitted to full-text analysis. In cases of disagreement, decisions regarding eligibility were discussed between the researchers until consensus was reached. When the information in the title was insufficient and the abstract was unavailable, the full text was obtained for further analysis. Full texts were read and analyzed based on the eligibility criteria. Studies that did not meet these criteria were excluded. Authors were contacted and asked to send their full texts when the study was not found and when additional information on the study was required. Data extraction Data extraction was performed using a chart designed specifically for this purpose. Information was collected on the study design, setting (location where clinical examinations were conducted), sample size, age, index used for TDI, socioeconomic indicators analyzed, statistical tests, effect measures (odds ratio, relative risks), confidence interval, and P-values (Table 2). For cases in which the necessary data were not available in the article, the authors were contacted directly. Data were extracted for categorical variables considered risk factors for TDI and recorded based on the distribution and frequency of TDI among children exposed to risk factors vs the frequency among children not exposed to risk factors, as reported by the authors: high vs medium socioeconomic status (SES); medium vs low SES; high vs low SES; parent s schooling > eight years vs < eight years; home owned vs rented; high income (>2 or >3 times the average salary) vs low income (<2 or <3 times the average salary). Income was measured in terms of the Brazilian minimum monthly wage (BMW), a standard used for this type of assessment which was approximately US$ 296 during the data-gathering period. Thus, the value of U $ 296 was considered the average salary in this systematic review. Two times the average salary was U$ 592 and three times the average salary was U$ 888. SES was investigated using the Brazilian Economic Classification system (17), Social Vulnerability Index (SVI) (10, 19, 24), and criteria of the Brazilian Advertisers Association (ABA-ABIPEME) (11). The Brazilian Economic Classification system and ABA-A- BIPEME define economic status in categories, and it is

3 Socioeconomic indicators and traumatic dental injury 257 Identification 2566 of records identified through searches of PubMed (555), Web of Science (378), Cochrane (204), Lilacs (240) and Embase (1189) databases 1403 duplicate records removed Screening 1163 records screened 1125 excluded Fig. 1. Flowchart of article selection process. Eligibility Included 38 full-text articles assessed for eligibility 16 studies included in qualitative synthesis (15 cross-sectional and 1 cohort) 9 studies included in quantitative synthesis (meta-analysis) 22 full-text articles excluded: 4 involved permanent dentition 6 no investigation of factors associated with TDI 3 sample comprised only of children with TDI 1 abstract presented at event 8 no investigation of socioeconomic factors based on the ownership of items by the family such as bathroom, television, and the schooling level of head of the family (26, 27). The SVI includes 20 variables for quantifying access to housing, schooling, income, jobs, legal assistance, health, and nutrition. This index measures social access and determines to what extent the population of each region of the city is vulnerable to social exclusion (28). Assessment of study quality The quality of the studies was evaluated by two researchers using the Newcastle Ottawa Quality Assessment Scale (NOS) for cohort/case control studies (29). Cross-sectional studies were evaluated using the modified Newcastle Ottawa Scale (29). Risk of bias was evaluated for each question (Fig. 2). For each question-based entry, the judgment was as follows: Yes, for low risk of bias and it owned a point (*), and No, for high risk of bias and it did not own a point (30). A system of points was used for studies that fulfilled the methodological requirements and ranged from 1 (very poor) to 9 (high) points for cohort studies and from 1 (very poor) to 8 (high) points for cross-sectional studies. Disagreements were resolved by consensus. This scale evaluates case definition, representativeness of the sample, sample selection (e.g., community, hospital), eligibility criteria, adjustment for confounders, acquisition of data on the dependent variable, description of bias, non-response rate. Data synthesis The Comprehensive Meta-Analysis software program (Biostat, Englewood, NJ, USA), version 2, was used for the meta-analysis (31). Heterogeneity among the studies was evaluated using I 2 statistics (32). Metaanalysis was conducted when I 2 was below 50%, as values above 50% would signify moderate to high heterogeneity, which may preclude meta-analysis (31). A sensitivity test was conducted when heterogeneity was moderate to high to exclude studies that would increase heterogeneity. A random effect model was used when heterogeneity was high, and a fixed effect model was used when heterogeneity was low (33 35). For categorical data, risk measures, 95% confidence intervals (CIs), and P-values were described in forest plots and summary risk measures were calculated. Results Study selection Figure 1 displays a flowchart outlining the number of articles identified at each step of the literature search. The search of databases provided a total of 2566 articles. The screening process of the title identified 38 articles that met the inclusion criteria, among which 16 articles were included after reviewing the full text: 15 cross-sectional studies and one cohort study. No clinical trials or case control studies were found (Table 2). Main reasons for exclusion of studies were as follows:

4 258 Corr^ea-Faria et al. Table 2. Studies on factors associated with traumatic dental injury in primary dentition Authors, year, location of study Design Setting Sample size Age of subjects Socioeconomic variables Statistical analysis Outcomes Study quality Mu~noz et al., 2006; Chile (22) Viegas et al., 2006; Brazil (19) Granville-Garcia et al., 2010; Brazil (12) Granville-Garcia et el., 2006; Brazil (21) Oliveira et al., 2007; Brazil (9) Feldens et al., 2008; Brazil (18) De Vasconcelos Cunha Bonini et al., 2009; Brazil (36) Cross-sectional Schools years Type of school Chi-square test Type of school not associated with TDI (P = 0.16) Cross-sectional National Day of Children s Vaccination Cross-sectional Private and public schools and preschools Cross-sectional Private and public schools and preschools Cross-sectional National Children s Vaccination Day Cohort Municipal health center Cross-sectional National Children s Vaccination Day years Mother s schooling, Social Vulnerability Index Chi-square test Mother s schooling (P = 0.01) and Social Vulnerability Index (P = 0.008) associated with TDI years Type of school Chi-square test Type of school associated with dental trauma (P < 0.001) years Type of school Chi-square test and logistic regression Type of school associated months Mother s schooling, home ownership year Mother s schooling, income per capita, mother s occupational status, family structure, number of residents in home months Mother s schooling, father s schooling, income, mother s employment status, father s employment status, home ownership, change of address in previous year, overcrowding, family structure with TDI; children in private schools 1.27-fold more likely to suffer TDI (OR: 1.27; 95% CI: ) Chi-square test, simple and multiple regression Neither mother s schooling (P = 0.762) nor home ownership (OR: 0.82; 95% CI: ) associated with TDI Simple and multiple logistic regression Children from non-nuclear families (OR: 2.28; 95% CI: ) and those whose mother s had a higher level of schooling (OR: 2.61; 95% CI: ) at greater risk of TDI; income per capita (P = 0.723), mother s occupational status (P = 0.328), and number of residents in home (P > 0.05) not associated with TDI Chi-square test and simple logistic regression No socioeconomic indicators associated with TDI (P > 0.05). 4 (8) 4 (8) 4 (8) 5 (8) 5 (8) 8 (9) 7 (8)

5 Socioeconomic indicators and traumatic dental injury 259 Table 2. Continued Authors, year, location of study Design Setting Sample size Age of subjects Socioeconomic variables Statistical analysis Outcomes Study quality Jorge et al., 2009; Brazil (10) Ferreira et al., 2009; Brazil (20) Robson et al., 2009; Brazil (11) Wendt et al., 2010; Brazil (23) Viegas et al., 2010; Brazil (24) Dutra et al., 2010; Brazil (17) Cross-sectional National Children s Vaccination Day Cross-sectional National Children s Vaccination Day Cross-sectional Day care centers and preschools years Mother s schooling and Social Vulnerability Index Chi-square test, simple and multiple logistic regression Mother s schooling associated with TDI (P = 0.001); children from families with high social vulnerability had greater chance of exhibiting TDI than other children (P = 0.045) months Household income Chi-square test Prevalence of TDI increased significantly with family income (OR: 1.44; 95% CI: ) years Economic criteria of Brazilian Advertisers Association, socioeconomic status, type of school Cross-sectional Schools months Mother s schooling, household income Cross-sectional Day care centers and preschools Cross-sectional National Children s Vaccination Day months Social vulnerability index, parents schooling, number of residents in home, household income years Mother s schooling, household income, economic class, number of children in family Chi-square test, simple and multiple logistic regression Socioeconomic status not associated with TDI (P > 0.05); Children attending public schools had nearly twofold greater chance of having TDI than those attending private schools (OR: 1.66; 95% CI: ). Chi-square test No socioeconomic indicators associated with TDI (P > 0.05) Chi-square test Social vulnerability index (OR: 1.07; 95% CI: ), parents schooling (OR: 0.84; 95% CI: ), number of residents in home (OR: 0.76; 95% CI: ), and household income (OR: 0.85; 95% CI: ) not associated with TDI Univariate and multiple regression models Number of children associated with TDI (OR: 2.0; 95% CI: ); economic class (P > 0.49), income (P = 0.31), mother s schooling (P < 0.62) not associated with TDI 6 (8) 5 (8) 7 (8) 5 (8) 6 (8) 6 (8)

6 260 Corr^ea-Faria et al. Table 2. Continued Authors, year, location of study Design Setting Sample size Age of subjects Socioeconomic variables Statistical analysis Outcomes Study quality T umen et al., 2011; Turkey (13) Piovesan et al., 2012; Brazil (15) Goettems et al., 2012; Brazil (14) Cross-sectional Day care centers years Father s education, mother s education, household income Cross-sectional National Children s Vaccination Day Cross-sectional Private and public schools months Household income, mother s education, father s education months Type of school, mother s schooling, household income Chi-square test, Fisher s exact test, simple and multiple logistic regression Simple and multiple regression Chi-square test and Fisher s exact test Father s schooling (P = 0.763), mother s schooling (OR: 0.481; 95% CI: ), and household income (P = 0.142) not associated with TDI Household income (PR: 0.87; 95% CI: ), mother s schooling (OR: 0.90; 95% CI: ), and father s schooling (OR: 0.94; 95% CI: ) not associated with TDI Type of school (P = 0.21), mother s schooling (P = 0.62), and household income (P = 0.30) not associated with TDI 6 (8) 7 (8) 6 (8)

7 Socioeconomic indicators and traumatic dental injury 261 Case definition Representativeness of the sample Sample selection Elegibility criteria Adjustment for confounders Acquisition of data on the dependent variable Muñoz et al., 2006 (22) * * * * Viegas et al., 2006 (19) * * * * Granville-Garcia et al., 2010 (12) * * * * Granville-Garcia et el., 2006 (21) * * * * * Oliveira et al., 2007 (9) * * * * * De Vasconcelos Cunha Bonini et al., 2009 (32) Description of bias Non response rate * * * * * * * Jorge et al., 2009 (10) * * * * * * Ferreira et al., 2009 (20) * * * * * Robson et al., 2009 (11) * * * * * * * Wendt et al., 2010 (23) * * * * * Viegas et al., 2010 (24) * * * * * * Dutra et al., 2010 (17) * * * * * * Tümen et al., 2011 (13) * * * * * * Piovesan et al., 2012 (15) * * * * * * * Goettems et al., 2012 (14) * * * * * * Fig. 2. Assessment of study quality. Risk of bias was evaluated for each question. For each question-based entry, the judgment was as follows: Yes, for low risk of bias and it owned point (*), and No, for high risk of bias and it did not owned a point. studies reporting permanent dentition, absence of statistical data reporting socioeconomic indicator and TDI, impossibility to extract data, no investigation of socioeconomic factors, and duplicate study found in different databases. Methodological quality assessment Agreement between the reviewers on each item of the Newcastle Ottawa Scale was 100%. The characteristics of the studies are displayed in Table 2. The quality of the cross-sectional studies ranged from four to seven on the eight-point scale (Table 2, Fig. 2). In general, studies had low risk of bias. Only four item were judged as high risk of bias in some studies: failure to report eligibility criteria (exclusion criteria) (9, 12, 13, 15, 17, 19, 21 23), failure to adjust for confounding variables through logistic regression analysis (12, 14, 19, 20, 22 24), description of bias (9 14, 17, 19 24, 36), and failure to report missing data (9, 12, 19 22) (Fig. 2). The score of the cohort study was eight on the ninepoint scale (Table 2). In this study was judged as low risk of bias, except for the assessment of outcome. Socioeconomic indicators investigated The following socioeconomic indicators were investigated: parents schooling (9, 10, 13 15, 17 19, 23, 24, 36), parents employment status (18, 36), home ownership (9, 36), having changed address in the previous year (36), income (13 15, 18, 20, 23, 24, 36), family structure (18, 36), number of residents in the home (18, 24, 36), number of children in the family (17), type of school (11, 12, 14, 21, 22), and socioeconomic status (10, 11, 17, 19, 24). Table 2 shows a synthesis of independent studies regarding each socioeconomic indicator. Socioeconomic indicators were collected through interviews (9, 10, 12, 15, 18) or questionnaires answered by parents (11, 13, 14, 17, 19, 22 24, 36). Socioeconomic status (SES) SES was investigated in five studies (10, 11, 17, 19, 24). Two articles reported data for this category and therefore were included in the meta-analysis. It was not possible to group data from the remaining articles for meta-analysis (10, 19, 24). In the analysis of subgroups, no significant association was found between SES and TDI (high vs low: OR: 0.77; 95% CI: ; high vs medium: OR: 1.03; 95% CI: ; medium vs low: OR: 0.70; 95% CI: ) (Fig. 3). SES was categorized as high and low (10, 19, 24) or high, medium and low (11, 17). Schooling Parents schooling was analyzed based on years of study in 11 articles (6, 9, 10, 13 15, 17 19, 23, 24, 36). Three articles investigated father s schooling (13, 15, 36), nine articles investigated mother s schooling (9, 10, 13 15, 17 19, 36), and one study investigated parent s/caregiver s schooling with no gender distinction (24). Five articles were included in meta-analysis, which was performed considering mother s schooling (13 15, 17, 36) and father s schooling (13, 15, 36). No statistically significant association was found between TDI and schooling (mothers OR: 0.89; 95% CI: ; fathers OR: 1.01; 95% CI: ) (Fig. 4). Home ownership Home ownership was investigated in two articles (9, 36) and was dichotomized as owned or rented, the latter of which was considered exposure to the outcome. In the meta-analysis, no significant association was found between house ownership and TDI (OR: 1.28; 95% CI: ) (Fig. 5).

8 262 Corr^ea-Faria et al. Group by Subgroup within study Study name Statistics for each study Odds ratio and 95% CI Odds Lower Upper ratio limit limit P-value High/low Dutra et al., 2010 High/low High/low Robson et al., 2009 High/low High/low High/medium Dutra et al., 2010 High/medium High/medium Robson et al., 2009 High/medium High/medium Medium/low Dutra et al., 2010 Medium/low Medium/low Robson et al., 2009 Medium/low Medium/low Favors higher SES Favors lower SES Fig. 3. Forest plot of meta-analysis for two cross-sectional studies evaluating socioeconomic status (SES) and TDI in primary dentition analyzed by subgroups. Pooled summary effect measures [odds ratio (OR) and 95% confidence interval (CI)] indicate that socioeconomic status (SES) was not significantly associated with TDI. Heterogeneity I 2 : high/medium = 0.00%; medium/ low = 39.26%; high/low = 41.06%. Random effect model used. Group by Subgroup within study Study name Schooling for Statistics for each study Odds ratio and 95% CI Odds Lower Upper ratio limit limit P-value Father De Vasconcelos Cunha Bonini et al., 2009 Father Father Piovesan et al., 2012 Father Father Tümen et al.,2011 Father Father Mother De Vasconcelos Cunha Bonini et al., 2009 Mother Mother Dutra et al., 2010 Mother Mother Goettems et al., 2012 Mother Mother Piovesan et al., 2012 Mother Mother Tümen et al.,2011 Mother Mother Favors >8 years Favors <8 years Fig. 4. Forest plot of meta-analysis for five cross-sectional studies evaluating parents schooling in years and TDI in primary dentition analyzed by subgroups. Pooled summary effect measures [odds ratio (OR) and 95% confidence interval (CI)] indicate that neither mother s nor father s schooling was significantly associated with TDI. Heterogeneity I 2 : father s schooling = 0.00%; mother s schooling = 0.00%. Fixed effect model used. Study name Statistics for each study Odds ratio and 95% CI Odds Lower Upper ratio limit limit P-value De Vasconcelos Cunha Bonini et al., Oliveira et al., Favors own house Favors rented Fig. 5. Forest plot of meta-analysis for two cross-sectional studies evaluating home ownership and TDI in primary dentition. Pooled summary effect measures [odds ratio (OR) and 95% confidence interval (CI)] indicate that home ownership was not significantly associated with TDI. Heterogeneity I 2 = 0.00%. Fixed effect model used.

9 Socioeconomic indicators and traumatic dental injury 263 Income Income was investigated in eight studies (13 15, 17, 18, 20, 23, 24, 36). Only Brazilian studies in which household income was evaluated based on the average salary were included in the meta-analysis (15, 17, 20, 24, 36). In this systematic review, the value of U$ 296 was considered the average salary. After grouping the data (> two and two times average salary), a statistically significant association was found between household income and TDI. An income of more than two times average salary (OR: 0.77; 95% CI: ) and more than three times average salary (OR: 0.76; 95% CI: ) was a protective factor for TDI (Fig. 6). Discussion The present systematic review was conducted to investigate the association between socioeconomic indicators and TDI in an effort to clarify the strength of evidence on this association in the primary dentition (37 39). This systematic review involved the search of different electronic databases with restrictions regarding language or date of publication. After the search was selected 16 articles, and 15 of these had a cross-sectional design. This type of study does not allow the determination of a causal relationship among the variables (17), which compromises the quality of evidence. Moreover, the studies included in this review had flaws, such as the failure to describe the eligibility criteria and non-response rate (9, 12, 17, 21, 22). Despite these limitations, the articles analyzed had in general low risk of bias which can be strong points, such as an adequate sample size calculation (10, 11, 14, 15, 17, 20, 24, 36), systematic, representative sampling (9, 10, 15, 17, 19, 21, 24, 36), and the use of examiners who had undergone a training and calibration exercise (9, 11, 13 15, 17, 19, 21, 24, 36), demonstrating the reliability of the data reported. Study quality was assessed by Newcastle Ottawa Scale. The selection of this instrument was due to characteristics such as being easy to use, convenient, provide clarity and completeness for the evaluation of studies to be used in the meta-analysis (29). While funnel plot analysis could not be performed to investigate publication bias due to the limited number of studies (40), this aspect was analyzed qualitatively. Publication bias occurs when predominantly statistically significant outcomes are reported in literature (34). This was not the case with the present review, as the studies analyzed presented both significant (P < 0.05) and nonsignificant outcomes (P > 0.05) in the direction of better and worse socioeconomic indictors. The majority of studies were developed in Latin America. Similar results are found in systematic reviews performed to determine factors associated with TDI in the permanent dentition (16, 39). According to Aldrigui et al. (39), this is due to the greater investment in research on TDI in Latin America. This result detaches the need of further investigations on the influence of socioeconomic indicators, especially in countries outside South America. By one side, main reasons of TDI in primary dentition may be fall at home (41) and this may be a reason common to children from other countries. However, this systematic review reports socioeconomic indicators, and the inclusion of South American studies in this systematic review can weaken the generalization of the findings to the rest of the world. For example, South America concentrates developing countries. On the other side, North America is made of developed countries, very distinct socioeconomic realities. Among the socioeconomic indicators submitted to meta-analysis, only monthly household income was associated with TDI in the primary dentition. An income of more than two times average salary and more than three times average salary was a protective factor for TDI. Neither family income nor per capita income was associated with TDI in the majority of studies (13 15, Group by Subgroup within study Study name Minimum wage Statistics for each study Odds ratio and 95% CI Odds Lower Upper ratio limit limit P-value >2/<2 Ferreira et al., 2009 >2/< >2/<2 de Vasconcelos Cunha-Bonini et al, 2009 >2/< >2/<2 Piovesan et al., 2012 >2/< >2/< >3/<3 Ferreira et al., 2009 >3/< >3/<3 de Vasconcelos Cunha-Bonini et al., 2009 >3/< >3/<3 Viegas et al., 2010 >3/< >3/<3 Dutra et al., 2010 >3/< >3/< Favors higher income Favors lower income Fig. 6. Forest plot of meta-analysis for five cross-sectional studies evaluating income and TDI in primary dentition analyzed by subgroups. Pooled summary effect measures [odds ratio (OR) and 95% confidence interval (CI)] indicate that lower income (>2 vs <2 times the average salary and >3 vs>3 times the average salary) was significantly associated with TDI. Heterogeneity I 2 : >2/<2 = 0.000%; >3/<3 = 0.000%. Fixed effect model used. Average salary = U$ 296. Two times the average salary = U$ 592. Three times the average salary = U$ 888.

10 264 Corr^ea-Faria et al. 18, 23, 24, 36). Ferreira et al. (20) found a statistically significant association between income and TDI (OR: 1.44; 95% CI: ), as children from families with a higher income had a greater rate of the outcome. However, caution should be exercised in the interpretation of this finding. Monthly household income was evaluated using different cutoff points, which hinders the comparison of the results. For the meta-analysis, it was only possible to group studies that considered the Brazilian monthly minimum salary as the reference, which limits the extrapolation of the findings to other populations in which income is measured in a different fashion or other currency, demonstrating a certain degree of publication bias with regard to the geographic region in which the studies were conducted. Moreover, the odds ratio regarding the association between income and TDI was close to 1, which indicates a weak association. SES was not associated with TDI in the meta-analysis. The lack of a significant association was also found in the majority of studies (11, 17, 24). However, Viegas et al. (19) and Jorge et al. (10) found a statistically significant association between SES and TDI, as children from families with a high degree of social vulnerability had greater chance of exhibiting TDI (OR: 1.51; 95% CI: ) (10). This divergence may be explained by the use of the Social Vulnerability Index (10, 19, 24) and the Brazilian Economic Classification (17), which are specific indices for populations in Brazil. Parents schooling was not associated with TDI, which is in agreement with data reported in different studies (9, 13 15, 17, 24, 36). In contrast, mother s schooling has been associated with TDI among Brazilian children (10, 18, 19). However, divergent findings are reported in these studies. While Feldens et al. (18) found a greater prevalence rate of TDI among children whose mothers had a higher level of schooling, Jorge et al. (10) and Viegas et al. (6) found that TDI was more common among children whose mothers had a lower level of schooling. While parents schooling was evaluated based on years of study, different cutoff points were employed. The majority of studies used a cutoff point of eight years of study, whereas other studies categorized this variable based on the level of education (elementary, high school, and university) (36) or other measures in years (0 6 years, 7 12 years, and years) (10). These differences in the categorization of schooling as well as the joint evaluation of mother s and father s schooling (24) hinder the comparison of the results. Therefore, it is not possible to affirm whether mother s or father s schooling is associated with TDI. Familial characteristics, such as parents employment status (18, 36), a change of address in the previous year (36), family structure (18, 24, 36), number of children in the family (17), and number of residents in the home (18, 24, 36), were only evaluated qualitatively. These variables were addressed in a small number of studies, which resulted in a lack of scientific evidence to support such associations. Only type of school (12, 21) and family structure (18) were associated with TDI in at least one study. Moreover, the studies cited report divergent findings. In this systematic review were observed limitations such the use of different variables for the evaluation of socioeconomic indicators. This use of multiple variables reveals that there is no single variable that is suitable in isolation to assess a SES. SES is a multidimensional concept that includes resource-based components (e.g., income) and prestige components (e.g., schooling) (42). Thus, the significant association between variables such as parent s schooling and TDI does not mean that the socioeconomic conditions are directly related to this oral problem, as there is still no single measure can evaluate SES. The standardization of variables employed for the investigation of the association between socioeconomic indicators and TDI is needed for the acquisition of scientific evidence of such an association. Therefore, it is desirable to use statistical methods as latent class analysis (LCA). LCA as a method was used to combine several dimensions of SES, such aspects of an individual s social and economic position, into one indicator (43). Other suggestion was the use of the Human Development Index allows comparisons between different populations. This index is a combination of indicators of life expectancy, educational attainment, and income for the evaluation of social and economic development (44). While the Human Development Index does not allow the determination of the association between TDI and socioeconomic indicators on the individual level, the multilevel analysis of this index on the population level is important to the establishment of preventive measures directed at TDI. TDI results from the interaction of clinical, environmental, and behavioral factors (45). It is therefore necessary to understand the etiological factors associated with this outcome in order to establish effective preventive measures. Although socioeconomic characteristics are subject to rapid changes, it is possible to prevent TDI with the child supervision by parents/guardians (46) and develop effective educational campaigns in regard to causes and prevention (41). Conclusion In conclusion, the scientific evidence indicates no association between socioeconomic indicators and TDI. The association between a low income and the occurrence of TDI in the present systematic review is weak and doubtful. Prospective observational studies are needed to address this issue further. Acknowledgements This study was supported by the following Brazilian fostering agencies: National Research Commission (CNPq; Ministry of Science and Technology) and the State of Minas Gerais Research Foundation (FAPEMIG). Conflict of interest The authors declare that they have no conflict of interests.

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