INTRODUCTION

Socioeconomic inequality in oral health is well documented within high-income countries, where oral disease is disproportionately prevalent in disadvantaged members of society [1, 2]. Country comparisons of oral health outcomes can provide insight into sociopolitical and health system factors that shape inequality [3-5]. For example, evidence suggests that liberal democracies with market-dominated economies and health systems accentuate differences in oral health between the rich and poor [4]. Canada and the United States, in particular, have consistently demonstrated low public health expenditure, social spending, and increases in income inequality over time (Table 1) [10-12]. One notable difference between the two countries is the availability of Canada's national system of universal health insurance, which covers physician and hospital care, yet excludes oral health care. The Canadian and American approaches to oral health care are actually quite similar, with most care financed by employer and individually-sponsored insurance and out-of-pocket payments, and limited contributions from government [6]. In addition, most care in both countries is delivered in the private sector by dentists on a fee-for-service basis. Nevertheless, as liberal democracies, Canada still provides more support to its citizens than the United States in terms of unemployment insurance, social assistance for the poor, tax credits, and other universal benefits [6]. Thus, despite similar demography and macroeconomic environments, the Canadian social safety is generally considered more extensive in terms of both population coverage and the level of benefits provided [7]. Given the potential role played by political and social institutions in mediating oral health inequality, it would be reasonable to speculate that the extent of such differences may impact the distribution of oral health-related outcomes in both countries. However, little comparative information on the magnitude of, and changes in, oral health inequality is available for Canada and the United States over time.

TABLE 1. Comparative framework to analyze oral health inequality over time in Canada and the United States Sociopolitical contexts Canada United States 1970s 2000s 1970s 2000s Total healthcare expenditurea 6.2 10.73 6.2 16.38 Public healthcare expenditurea 4.4 7.5 2.3 7.9 Public social spendinga 13.3 (1980) 15.7 12.8 (1980) 14.2 Income inequalityb 0.30 0.32 0.31 0.38 Oral healthcare system features Canada United States 1970s 2000s (%) 1970s 2000s (%) Total oral health expenditurec e 6 e 4.2 Public insuranced e 6.2 e 9 Private insuranced e 60 e 46.4 Out of pocket paymentsd e 40 e 40.1 Population coverage (%) Public insurance e 5.5 e 5 Private insurance e 62.6 e 60 No coverage e 32 e 35 a Expressed as % of GDP. b Expressed as Gini coefficient. c Expressed as % of total healthcare expenditure. d Expressed as % of oral healthcare expenditure. e Information not available. Source: Adapted from OECD 2020 Health spending (indicator) [6], OECD 2020 Social spending (indicator) [7], OECD 2020 Income inequality (indicator) [8], and Garbin Neumann and Quiñonez 2014 [9].

Elani et al. reported a declining prevalence of untreated decay and edentulism in both Canada and the United States from the 1970s until the first decade of the new millennium, along with a flattening of socioeconomic gradients for filled teeth outcomes, with more low-income individuals arguably consuming more restorative services in both countries over time [8]. While there was persistent inequality, improvements for untreated decay were higher in Canada and, for edentulism in the United States [8]. Farmer et al. supported these findings, reporting steeper income gradients in the United States than Canada, with adverse outcomes concentrated among the poor, which were attributed to the effects of income, gender, and age [2]. These are the only two studies using nationally representative data to compare the magnitude of, and changes in, oral health inequality in Canada and the United States, yet they have shortcomings. Elani et al. only measured the association between socioeconomic status on oral health, but not the extent to which differences in socioeconomic position might impact the distribution of oral health in the respective populations [9]. Farmer et al. used more robust measures to address the limitations of Elani et al.'s analysis, but only estimated the extent to which oral health outcomes were concentrated in certain segments of the population, and not changes in the magnitude of inequality over time.

Measuring and monitoring inequality in oral health is considered important, yet research on trends over time remains limited [13]. While it is known that the poor are worse-off than the rich, there is almost no information on changes in the magnitude of the gap between the best and worst-off members of society in Canada and the United States, particularly for clinical indicators. This study aims to quantify the extent to which differences in income impact the distribution of clinical oral health indicators, along with the percentage changes in inequality in Canada and the United States from the 1970s until the first decade of the new millennium.

METHODS Data sources

Data from four nationally representative surveys was used to obtain information on clinical oral health, demographic and socioeconomic status. For Canada, we used the Nutrition Canada National Survey 1970–1972 (NCNS) and the Canadian Health Measures Survey 2007–2009 (CHMS). The NCNS was conducted between October 1970 and September 1972 and collected data from 19,590 individuals aged 0–100 years, including Indigenous populations. The CHMS was conducted between March 2007 and 2009 and collected information from 5586 Canadians aged 6–79 years, excluding indigenous populations, institutionalized populations, and the Canadian Armed Forces. The NCNS and CHMS had unweighted response rates of 46.0% and 51.7%, respectively. Both surveys followed a stratified multistage sampling technique, collecting data over two phases, which included household interviews followed by clinical examination [14, 15].

For comparison with the NCNS and CHMS, we used the US Health and Nutrition Examination Survey 1971–1974 (HANES) and National Health and Nutrition Examination Survey 2007–2008 (NHANES). Both HANES and NHANES used stratified multi-stage probability samples to collect information from noninstitutionalized Americans aged 0–74 and 0–80 years, respectively. The unweighted response rates for the surveys were 74.0% and 75.4%, respectively. Demographic and socioeconomic data were collected via household interviews, while oral health information was collected via clinical examination [16].

Oral health outcomes

We focused on three clinical oral health outcomes; (i) ≥1 untreated decayed teeth, which included pit and fissure, occlusal, proximal, overt, and grossly decayed teeth that had never been restored, to represent untreated decay levels in each population; (ii) ≥1 filled teeth comprising all permanent amalgam, composite resin, and glass ionomer surface restorations along with previously filled teeth presenting with secondary decay and fractured/defective restorations; and (iii) edentulism, as an indicator of unmet treatment need, utilization of services, and history of dental disease. Individual tooth counts with the assessment of each tooth surface was carried out in three of the four surveys to estimate both prevalence and severity of oral disease, while in NHANES only a basic screening examination was conducted to assess the prevalence of oral conditions. In order to maintain comparability, all the oral health outcomes were dichotomized and analyzed as binary variables.

Income

To measure inequality in oral health, we used total annual income as a socioeconomic indicator, as it was consistently reported in an ordinal form across all four surveys. The NCNS and HANES reported total annual family income, while the CHMS and NHANES reported total annual household income. The income variable was further ranked into quintiles, from highest to lowest.

Indices of inequality

Two complex regression-based measures of inequality, the slope index of inequality (SII) and relative index of inequality (RII) were used to estimate absolute and relative inequality, respectively. The SII and RII not only reflect the socioeconomic dimension to inequality, they also incorporate the experiences of every socioeconomic group and are sensitive to changes in the distribution of socioeconomic groups in a population [17]. The SII and RII were estimated by the regression of the midpoint value of the health outcome for each socioeconomic group along with a cumulative distribution, represented by a ridit score. The ridit scores were calculated by ranking weighted proportions of the income variable from the highest to lowest income groups, and assigning each category scores ranging from 0 to 1, based on the midpoint of the cumulative distribution of individuals within each group [17]. The ridit scores were then incorporated in linear regression models, generating the regression coefficient, which represents the estimate of inequality. A positive value for the SII and an RII of greater than 1 is indicative of “pro-rich” inequality, meaning the outcome is disproportionately distributed among higher-income groups; while a negative value of the SII and an RII of less than 1 is indicative of “pro-poor” inequality, meaning the outcome is disproportionately distributed among lower-income groups [17].

Analysis

Data analysis using survey command was conducted in STATA version 15.0. Individuals aged ≥18 years, with complete data in all variables were included in the analysis. A very small percentage of participants ranging from 3% to 5% were excluded from the analysis due to missing data. Age-standardized distributions of oral health outcomes across income groups were estimated for each country at both time points. Direct age-standardization, using the US 2000 Census was performed to account for changes in distributions across time and country. The magnitude and direction of sex-adjusted oral health inequality was estimated along with percentage change in inequality over time. Finally, an unpaired t-test was conducted to determine the statistical significance of changes in the magnitude of inequality over time.

RESULTS Survey sample characteristics

The characteristics of the sample population are presented in Table 2. The gender and age distribution were similar in both countries in the 1970s and 2000s. The age-standardized prevalence of oral health outcomes by income category is presented in Figure 1. While income gradients persisted, the overall prevalence of untreated decay and edentulism decreased over time in both Canada (untreated decay: 64%–20.5%; edentulism: 16.4%–4.4%) and the United States (untreated decay: 45.7%–21.2%; edentulism: 15.5%–5.7%). For filled teeth, the overall prevalence increased over time in Canada (74.5%–89.7%), but remained stable in the United States (82%–82.6%). It also appears that increases in filled teeth among low and middle-income groups in Canada were greater than in the United States. Finally, while the income gradient for filled teeth remained in both countries, it was more delineated in the United States in the 2000s.

TABLE 2. Sample characteristics. Weighted proportions expressed as % and 95% CI Canada Canada United States United States 1970–1972 2007–2009 1971–1974 2007–2008 n = 11,142 n = 3715 n = 13,145 n = 4988 Age 18–39 49.0 (46.7, 51.4) 40.1 (36.1, 44.2) 48.9 (47.2, 50.6) 39.8 (36.8, 42.9) 40–59 32.7 (30.2, 35.3) 39.8 (37.3, 42.4) 34.8 (33.3, 36.3) 37.7 (35.3, 40.1) ≥ 60 18.3 (16.2, 20.5) 20.1 (16.7, 23.9) 16.3 (15.2, 17.4) 22.4 (20.2, 24.7) Sex Female 54.1 (52.2, 55.9) 50.6 (47.8, 53.6) 52.5 (51.5, 53.5) 51.1 (49.7, 52.6) Male 45.9 (44.1, 47.8) 49.4 (46.4, 52.4) 47.5 (46.5, 48.5) 48.9 (47.4, 50.3) Income Lowest 21.5 (18.6, 24.8) 24.7 (20.0, 30.1) 16.1 (14.5, 17.6) 22.5 (18.5, 26.4) Lower middle 18.7 (16.4, 21.4) 18.8 (16.5, 21.2) 14.4 (13.3, 15.5) 20.5 (17.8, 23.2) Middle 32.8 (29.9, 35.7) 16.6 (14.5, 18.8) 23.5 (22.1, 25.0) 15.7 (13.3, 18.0) Higher middle 18.3 (15.0, 22.1) 12.5 (10.5, 14.8) 24.1 (23.0, 25.2) 20.3 (17.1, 23.5) Highest 8.7 (7.0, 10.8) 27.5 (22.3, 33.5) 21.9 (19.8, 23.9) 21.0 (16.7, 25.4) Age-standardized prevalence of oral health outcomes by income category. Weighted proportions and 95% CI. Decayed and filled teeth outcomes based on dentate population. Edentulism based on the whole population [Color figure can be viewed at wileyonlinelibrary.com] Income-related inequality in oral health outcomes

As seen in Table 3, among dentate adults, there was significant absolute income-related inequality (SII) in the prevalence of untreated decay at both time points; however, this decreased by approximately 31% in Canada and remained unchanged in the United States. Relative income-related inequality (RII) for untreated decay increased significantly over time in both countries. The increase in relative inequality in Canada (91%) was half of that in the United States (189%). For filled teeth, both the SII and RII declined significantly over time in both countries. The reduction in the SII in Canada (79%) was almost double that in the United States (38%). For filled teeth, the RII decreased over time by 63% and 16% in Canada and the United States, respectively. For edentulism, the SII decreased by 57.1% in Canada and 50.9% in the United States, while the RII rose by 200% in Canada and 78% in the United States.

TABLE 3. Sex adjusted absolute and relative oral health inequality and changes over time in Canada and United States SII Direction of change RII Direction of change 1 ≥ untreated decaya Canada 1970–1972 29.2***(20.2, 38.2) 1.54***(1.34, 1.78) 2007–2009 20.2***(13.3, 27.2) ↓ 2.95**(1.60, 5.50) ↑ % change 30.8NS 91.5* United States 1971–1974 26.2***(21.5, 30.9) 1.67***(1.52, 1.83) 2007–2008 29.4***(22.9, 35.9) ↓ 4.83*** (3.14, 7.44) ↑ % change 12.2NS 189*** 1 ≥ filled teetha Canada 1970–1972 −46.5***(−54.4, −38.5) 0.55***(0.49, 0.62) 2007–2009 −10.0**(−15.6, −4.4) ↓ 0.9**(0.85, 0.95) ↓ % change 78.5*** 63.6*** United States 1971–1974 −32.2***(−36.1, −28.3) 0.68***(0.65, 0.71) 2007–2008 −19.7***(−23.9, −15.5) ↓ 0.79*** (0.75, 0.83) ↓ % change 38.8*** 16.1*** Edentulismb Canada 1970–1972 30.3***(24.0, 36.7) 4.29***(3.06, 6.02) 2007–2009 13.0***(7.4, 18.7) ↓ 12.9***(4.62, 35.7) ↑ % change 57.1*** 200.6* United States 1971–1974 21.6***(18.6, 24.7) 5.11***(4.01, 6.5) 2007–2008 10.6***(7.1, 14.2) ↓ 9.1*** (3.76, 22.08) ↑ % change 50.9*** 78.4*** Abbreviation: NS, not significant. *** p < 0.001, ** p < 0.01, *p < 0.05. a Decayed and filled teeth estimate based on dentate population. b Estimates for edentulism based on the whole population. DISCUSSION

Absolute inequality in the prevalence of untreated decay and edentulism decreased over time in Canada. In the United States, absolute inequality decreased for edentulism only, and remained unchanged for untreated decay. However, relative inequality for untreated decay and edentulism increased over time in both countries. For untreated decay, the increase in relative inequality in Canada was half of that in the United States; for edentulism, relative inequality more than doubled in Canada compared to the United States. For filled teeth, both absolute and relative inequality declined over time in both countries, with improvements among lower and middle-income groups appearing more pronounced in Canada than in the United States. Overall, apart from edentulism, the magnitude of oral health-related inequality in untreated decay and filled teeth appears to be worse in the United States than in Canada.

In their pioneering study, Sanders et al. demonstrated that high population coverage for social benefits contributes significantly toward mitigating oral health inequality, and that a high reliance on private dental insurance is ineffective in achieving equity in population oral health [4]. These findings help to explain our own. For instance, a higher level of welfare benefits in Canada covering larger portions of the population [10-12] may have contributed to lower oral health-related inequality than in the United States, despite a high reliance on private dental insurance in both countries. Sanders et al. and other authors also suggest that the population's oral health and inequality therein might be impacted by the unequal distribution of income (or income inequality) in a country [3, 4]. Canada has had lower income inequality than the United States (Table 1); thus, despite higher levels of social spending in the United States over the past 35 years, low population coverage in regard to this spending and higher income inequality relative to Canada [10-12] may explain why oral health-related inequality appears to be worse in the United States than Canada.

On the other hand, both absolute and relative inequality for filled teeth declined over time, albeit to a greater extent in Canada than in the United States. The narrowing of inequality for this outcome is indicative of an increasing uptake of dental services among lower-income individuals. Both countries have predominantly privatized oral health care systems, suggesting there would be similar barriers in accessing d