Review question/objective

There is a lack of explicit theoretical basis and clarification for the association between social inequalities and population oral health outcomes in the dental literature. This scoping review aims to gauge and explore the existing evidence on theoretical explanations for pathways and mechanisms through which social inequalities affects population oral health. The objectives of this scoping review are:

1. To organize and present the existing evidence on the association between social inequalities and population oral health according to social theories.

2. To identify and categorize conceptual and measurement alternatives used in the existing evidence to measure social class or socio-economic inequalities according to either stratification or relational approach.

3. To highlight the existing gap in evidence for future research implications in this field.

Specifically, this review seeks to answer the following question:

* What is the nature and extent of social theories being used as a basis to explain the associations between social inequalities and population oral health in the existing literature?

Background

In the past few decades a vast amount of literature on the effects of social inequalities on a range of population health outcomes has emerged. This includes outcomes such as mortality, life expectancy, self-rated health, hypertension and suicide, among others.1-11 Studies assessing the effects of area level inequalities on population level health outcomes are termed as studies of 'social ecology'.1 Since the 1980s, Richard Wilkinson, in a series of publications, highlighted how income inequality (a marker of social inequality) is a key determinant of population health and is relevant to economic and public health policy.9,11-13 Several studies in the 1990s examined the relationship between inequality and population health outcomes and reported consistent findings regarding detrimental effects of income inequalities.5,14-17 Despite these findings, a systematic review of 98 aggregate studies examining income inequality and population health concluded that there is little support that income inequality is a major, generalizable determinant of population health differences within or between countries but that it may have a direct influence on some population health outcomes, like homicides, in some contexts.8 In the past two decades many studies have preferred a multi-level modelling over an ecological study design for social ecology studies due to its advantage of clarifying the independent effects of individual and aggregated level inequalities on population health outcomes.18 A recent meta-analysis of multi-level studies reported that income inequality was only associated with modest excess risk of premature mortality and poor self-rated health.7 While the evidence on the effect of social inequalities on population health outcomes remains conflicting, a recent study confirms positive associations between income inequality and mortality in a panel survey of 21 developed countries over 30 years.19

Studies of social inequalities and population oral health outcomes

Oral health is an integral part of general health. Oral diseases affect 3.9 billion people and untreated dental caries (tooth decay) is the most prevalent disease globally.20 Oral diseases significantly affect quality of life20 and the associated health care has significant costs. There are also links between oral health and chronic non communicable diseases,21-26 including an association between tooth loss and pre-mature mortality.27,28 Considering the pervasiveness of oral diseases it is important to study and assess the effects of social inequalities on population oral health outcomes.

Researchers have investigated relationships between inequalities and population oral health outcomes both ecologically29-31 and using a multi-level technique.32-35. Bernabe and Sheiham29 in their ecological study of the 50 richest nations (based on the Gross National Income [GNI]) reported that income inequality was significantly and inversely related to the number of filled teeth and decayed, missing, filled permanent teeth (DMFT) scores and restorative treatment but not to the number of missing and decayed teeth.

A study conducted in the United States reported state Gini coefficients (a widely used measure of inequality),36 to be associated with tooth loss.32 Celeste and Fritzell37 assessed the association between different levels of Gini, lagged and current, with dental caries and periodontal outcomes in Brazil. Their study reported that while lagged Gini was neither related to caries or periodontal diseases, current Gini was associated only with dental caries but not periodontal diseases. Several other studies of social ecology also reported negative effects of income inequality on population oral health outcomes.30,34,35,38-40

It should be recognized that studies mostly focused on income inequality as a measure of social inequalities, and thus other aspects of social inequality such as social class, gender, race etc. were largely ignored. One exception is the Brazilian multi-level study by Peres and Peres41 assessing the association between the contextual Human Development Index (HDI) and dental pain. This study found higher prevalence of dental pain in ethnically black girls with lower educational attainment. The study also reported that students from areas of low HDI had higher prevalence of dental pain compared to those from areas of higher HDI, regardless of individual characteristics.

Theoretical pathways explaining relationship between inequalities and population health outcomes

Apart from the strength of the association between social inequalities and population oral health outcomes, one of the key debates persistent in the field of social epidemiology has been around the theoretical pathways by which the association can be explained. Studies which attempt to compile these theoretical pathways have done it in many different ways. While Mel Bartley in her book "Health Inequality" described the theories that explain health inequalities and extrapolated these explanations to explain the social ecology studies1, Wagstaff and Doorslaer attempted to clarify various hypotheses which had been advanced to explain the negative effects of income inequalities on population health.42 Several other studies attempted to explain the existing pathways43 as well as propose new theoretical pathways.44 Overall, the leading social theories, which are proposed in the literature to explain this relationship, are:1

1. Material explanations: In areas where there is more inequality, a larger population lives under material disadvantage and exposure to potential physical hazards hence, there is greater social production of disease.1

2. Cultural and behavioral explanations: Amongst individuals in a lower social position, the existing perception of social disadvantage reinforces a 'habitus' for compromised self-care which leads to persistent ill behavior. People who are not wealthy may feel less valued and hence are not likely to improve their health through healthy behaviors.1

3. Psychosocial explanations: In more unequal societies, people at lower social positions perceive themselves at a low status which affects their psychological wellbeing, affecting the endocrine mechanisms and leading to production of disease. It is also proposed that such relative perceptions further affects the quality of social relationships and reduces social capital, which additionally affects these mechanisms.1

4. Neo-material explanations: Under the neo-material explanations the focus remains on the lack of material factors as a result of the inadequate public policies and public services for people, which leads to disease production.8

5. Life course explanations: Accumulation of different material conditions through the life course of an individual has also been proposed to be a reason for social production of disease.1 The life course explanation focuses on the key stages of life when the consequences of inequalities can lead to development of diseases in the future.

6. Political economy explanations: Some social epidemiologists argue that, in addition to the mechanisms linking existing social inequalities and population health outcomes, the socio-political processes, which lead to the production of social inequalities in the first place, are also of great importance in social production of disease. These socio-political processes primarily focus on power, politics, economics and rights as the key societal determinants of health. These socio-political theories propose that, to varying degrees, a society's political and economic institutions as well as priorities determine societal levels of disease.45 The leading theoretical explanations are:

* Neo-materialist explanations45,46 which are an extension of the either theories proposed by Marx regarding how power differences between social classes according to ownership of means of productions may lead to disease production47 or Weber's theories of how life chances linked to occupation and educational qualifications are distributed and affect the social production of disease.1

* Neo-liberal explanations which suggest that neo-liberalism leads to greater income inequalities, depletion of social cohesion and welfare systems, thus leading to the production of disease.3

Apart from the above discussed social theories, Wagstaff and Doorslaer42 also attempted to clarify various hypotheses which had been aiming to explain the negative effects of income inequalities on population health. According to their study the various hypotheses that explain this relationship were:

1. Absolute income hypothesis:48,49: individual absolute income explains the observed health effects of aggregate income inequality8

2. Relative income hypothesis (RIH):50 it is income relative to a social group average that is important.8

3. Deprivation hypothesis: a variant of the RIH and predicts that it is income relative to poverty that is important.8

4. Relative position hypothesis:51 it is an individual's position in the income distribution that matters.8

5. Income inequality hypothesis:51 the amount of income inequality in a community matters for health in addition to absolute income.8

Ultimately, they concluded that studies measuring inequality at an area level were largely insufficient for discriminating between competing hypotheses and only individual level studies have the potential to discriminate between most hypotheses.42 The focus of the current review is on the social theories which are proposed to explain the income inequality hypothesis amongst all mentioned above. Central to all discussed hypotheses and theories is the issue of quantification of social position and social inequality.1 Different measurements of social position and social inequalities reflect different processes and great caution is required while interpreting theoretical pathways relating social inequality to population health outcomes without giving due importance to the type of measurement.

While the importance of social theories and hypothesis has been discussed in the general health literature, studies on oral diseases have given less importance to theoretical explanations and mechanisms of production. This is despite oral diseases being commonly referred as social diseases due to both their causation as well as their social consequences.52 A recent review by Baker and Gibson pointed out that there is sparse discussion on either the basis of the theory or its application in social oral epidemiology.53 They further report that a search for "theory" in 682 articles indexed under "dental public health" or "oral epidemiology" in Web of Science database from 190 -2012 showed only three articles including the word "theory".

Methods

A systematic review is a type of literature review which adheres tightly to a set methodology that aims to limit bias, by seeking to identify, appraise and synthesize relevant studies to answer a particular research question.54 Considering the uncertainty regarding the overall evidence on the theoretical pathways in the literature on social inequalities and population oral health, a conventional systematic review is not an appropriate method to assess the collective evidence on this research topic. A scoping review is a review vehicle which accommodates this limitation as one of its primary objective is to determine the extent, range and nature of any research activity.55 Due to its relevance and suitability over a systematic review as a method55,56 to address the highlighted research gap, a scoping review is proposed to address this significant gap in literature. Initial searches of the Pubmed, EMBASE and Cochrane databases confirm that no systematic or scoping reviews with similar objectives have been published.

The stages of a scoping review are similar to that of a systematic review (systematic selection, collection and summarization of existing knowledge in a broad thematic area), one of the main variations is that rather than emphasis on the detailed appraisal of the identified evidence sources, the focus is to collate the existing evidence in a thematic or analytical framework using a narrative synthesis.55,56 Studies which have proposed a methodological framework for conducting a scoping review have recommended the following stages.55,57

STAGE I: Identifying the research question

STAGE II: Identifying relevant studies

STAGE III: Study selection

STAGE IV: Charting the data

STAGE V: Collating, summarizing and reporting the results

Inclusion criteria

Types of participants

This review will consider all studies regardless of specific age group, gender or geographical context.

Context

i. Studies assessing an association between the following measurements of social inequalities at an aggregate level: measurements of social inequality, income inequality, Gini Index, income share, Robinhood index, slope index of inequality, relative index of inequality and relative concentration index

ii. Social inequalities: occupational measurements, employer-employee relationship, ethnicity, gender, caste, power relations and the following clinical and self-reported outcomes:

* Self-reported outcomes: self-rated oral health, number of teeth, tooth loss, oral health related quality of life (OHRQoL) and dental pain

* Clinical outcomes: dental caries (DMFT), periodontal disease

Concept

To use social theories as explanations for pathways and mechanisms through which social inequalities affects population oral health.

Types of studies

The review will consider all types of studies which measure social inequality at an aggregated level (area level, country level or multi country level) and will exclude studies which measure either inequality at individual level or which account for social position at an individual or aggregated level.

Search strategy

The search strategy aims to find both published and unpublished studies. A three step search strategy will be utilized in this review. An initial limited search of MEDLINE will be undertaken followed by analysis of the text words contained in the title and abstract, and of the index terms used to describe the article. A second search using all identified keywords and index terms will then be undertaken across all included databases. Thirdly, the reference list of all identified reports and articles will be searched for additional studies. Only studies published in English will be considered for inclusion in this review. Studies published post 1900 will be considered for inclusion in this review. As the search for the scoping review may be quite iterative, the reviewers will revisit the search strategy when they become more familiar with the evidence base, as additional keywords sources and potentially useful search terms may be discovered.58

The databases to be searched include:

PubMed, Medline (Ovid), EMBASE, Web of Science, ERIC (Education Resources Information Center), Sociological Abstracts, Social Services Abstracts

The search for unpublished studies will include:

Reference lists, book chapters and contact with experts; Thesis (ProQuest) and Conference abstracts.

Data collection

A data charting form will be developed by collective discussion among all the reviewers and the variables will be reported in the following categories:

a. Use of theoretical explanations

b. Choice of variables to measure exposure (social inequality)

c. Evidence, explanation and justification for the choice of variables

The data charting form will be consistently updated in consultation with the reviewers and experts on the basis of emerging information from the studies. Existing resources on data charting from selected studies in systematic reviews of theories60 will also be referred in development of the form.59 Two investigators will independently perform data charting of the first five studies and will pilot the developed data charting form. They will meet to cross check the extracted formation and strategies will be formulated to address the discrepancies. Following the existing methods adopted for data charting of scoping reviews,60 the author will chart all the extracted information and a second reviewer will cross check the information independently. Any disagreements will be resolved either through discussion or by a third reviewer.

Data mapping

A "narrative review" or a "descriptive analysis" of the extracted contextual or process oriented information can be conducted in place of data synthesis for the purposes of a scoping review.55 A 'qualitative content analysis' is also an alternative for this step. The proposed a-priori approach to data mapping will be to attempt to categorize the included studies both by the type of social theory used and the extent to which social theory is drawn upon by the authors. The criterion to analyse this aspect in the selected studies will be to assess them under the following categories drawn and modified from previous systematic reviews conducted with objectives to evaluate theory and its use:59,61

1. Explicitly theory based: Study explicitly stated a theory and provided a direct test of one or more of the hypotheses deduced from a named theory in order to design the study.

2. Some conceptual basis: Study in which theory was judged to have been used in the study, but where the study did not provide a test of any of the hypotheses deduced from the theory in order to design the study. Studies included in this category were those where the authors stated that they had employed a theory within the study or where the study described a framework or approach that appeared to be theoretically based.

3. Theoretical construct used: Studies included in this category are those in which one or more constructs were examined within the study, but the use of constructs was not embedded within the framework of a theory. Where a construct was referred to within the context of a theory, but it was the only component of the theory that was measured and considered, this was considered to be use of the theory within the 'some conceptual basis' category.

4. Post hoc explanation: Study uses theory retrospectively to explain the results of the study or to stimulate further discussion.

5. Indirect use: Study does not name or disclose any theoretical basis but the discussion of results are directed towards one of the social theory.

6. No theory: When the study has no theoretical basis.

A sub-analysis will focus on the choice of measurement variables for social inequality. The selected studies will be categorized on the basis of the aggregate level of measurements and how they are quantified.

The reviewers also propose to conduct further mapping of the evidence in order to map out the available literature on the topic. This will be carried out in two ways. Firstly, systematic mapping will be used to aid identification of which research questions are answerable, and in what ways.62 The reviewers will examine the statistical and methodological techniques through which included studies have assessed the association between social inequality and population oral health outcomes. Descriptive mapping will then be used to provide a snapshot of the wider field in which the literature in the review is located.62 The existing resources on the key wording tool will be modified according to the objectives of the review and will be used to describe the field of available literature.62

Conflicts of interest

The reviewers have no potential conflicts of interest.

Acknowledgements

The authors like to acknowledge the assistance and guidance provided by The Joanna Briggs Institute, Adelaide, Australia by sharing the unpublished manuscript on guidelines for conducting a scoping review. Ankur Singh is supported by doctoral scholarships from the International Postgraduate Research Scholarship (IPRS) and Australian Postgraduate Awards (APA) by the Australian government.

References