International Journal ofEnvironmental Researchand Public Health
ArticleWider Dental Care Coverage Associated with Lower
Oral Health Inequalities: A Comparison Study
between Japan and England
Kanade Ito 1, *, Noriko Cable 2 , Tatsuo Yamamoto 3 , Kayo Suzuki 4 , Katsunori Kondo 5,6 ,Ken Osaka 7 , Georgios Tsakos 2 , Richard G. Watt 2 and Jun Aida 7
1

2
3
4
5
6
7

*

Department of Oral Care for Systemic Health Support, Health Sciences and Biomedical Engineering,
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University,
Bunkyo 113-8510, Japan
Department of Epidemiology and Public Health, University College London, London WC1E 7HB, UK;n.cable@ucl.ac.uk (N.C.); g.tsakos@ucl.ac.uk (G.T.); r.watt@ucl.ac.uk (R.G.W.)
Department of Disaster Medicine and Dental Sociology, Graduate School of Dentistry,
Kanagawa Dental University, Yokosuka 238-8580, Japan; yamamoto.tatsuo@kdu.ac.jp
Department of Policy Studies, Aichi Gakuin University, Nisshin 470-0195, Japan; ksuzuki@psis.agu.ac.jp
Department of Social Preventive Medical Sciences, Center for Preventive Medical Sciences, Chiba University,
Chiba 260-0856, Japan; kkondo@chiba-u.jp
Department of Gerontological Evaluation, Center for Gerontology and Social Science, National Center for
Geriatrics and Gerontology, Obu 474-8511, Japan
Department of International and Community Oral Health, Tohoku University, Sendai 980-8575, Japan;
osaka@m.tohoku.ac.jp (K.O.); j-aida@umin.ac.jp (J.A.)
Correspondence: ito.ocsh@tmd.ac.jp; Tel.: +82-3-5803-4969

Received: 30 June 2020; Accepted: 24 July 2020; Published: 31 July 2020


Abstract: Countries with different oral health care systems may have different levels of oral healthrelated inequalities. We compared the socioeconomic inequalities in oral health among older adults
in Japan and England. We used the data for adults aged 65 years or over from Japan (N = 79,707)
and England (N = 5115) and estimated absolute inequality (the Slope Index of Inequality, SII) andrelative inequality (the Relative Index of Inequality, RII) for edentulism (the condition of having nonatural teeth) by educational attainment and income. All analyses were adjusted for sex and age.
Overall, 14% of the Japanese subjects and 21% of the English were edentulous. In both Japan andEngland, lower income and educational attainment were significantly associated with a higher riskof being edentulous. Education-based SII in Japan and England were 9.9% and 26.7%, respectively,
and RII were 2.5 and 4.8, respectively. Income-based SII in Japan and England were 9.2% and 14.4%,
respectively, and RII were 2.1 and 1.9, respectively. Social inequalities in edentulous individuals exist
in both these high-income countries, but Japan, with wider coverage for dental care, had lower levels
of inequality than England.
Keywords: international comparison; edentulism; oral health inequality; universal health coverage;
slope index of inequality; relative index of inequality

1. Introduction
Health care systems are one of the structural social determinants of health inequalities [1]. There aresubstantial differences in the health care system between countries in terms of the coverage for healthcare across populations, for example, the target person, the scope of treatment available, the target
disease and the copayment [2–4]. Universal health coverage has been promoted in the Millennium

Int. J. Environ. Res. Public Health 2020, 17, 5539; doi:10.3390/ijerph17155539

www.mdpi.com/journal/ijerph

Int. J. Environ. Res. Public Health 2020, 17, 5539

2 of 10Summit at the United Nations and World Health Organisation (WHO), and expanding oral care inuniversal health coverage is desired [5,6].
There are differences in oral health care coverage across countries. Japan and England havea similar universal health care system. However, the dental health care system was the difference
between the two countries [7,8]. Japan has a public universal health care insurance system that includes
most of dental treatments. As a result, Japan is one of the countries providing the most accessible dental
care in the world from an international perspective. Japanese people attend dental care 3.2 times per
year (reported in 2011), which is the most frequent dental attendance pattern among the Organisation
for Economic Co-operation and Development (OECD) countries [9]. The percentage of Japanesehousehold expenditure on dental services is 0.4% [10]. In England, some dental care is also coveredby the public health care system, the National Health Service (NHS) [8]. In the United Kingdom(UK), dental care in the past year was 0.8 times per person (reported in 2011) [9], and the percentage
of household expenditure on dental services was 12.0% [11]. These large differences in health carecharacteristics and coverage could affect the level of inequalities in oral health at the population level.
In both Japan and England, there are inequalities in access to dental care [12–17]. Recently, universal
health coverage is promoted in the world, and it is considered to decrease oral health inequalities [1].
However, few studies compared the degree of oral health inequalities between the countries withdifferent coverages, and to the best of our knowledge, no international comparative study on oral
health inequalities has included Japan, the country with one of the highest oral health care coverage.
The global burden of disease study reported the prevalent nature and large burden of severe
tooth loss [18]. Edentulism (the condition of having no natural teeth) is the final form of severe toothloss. Accumulation of the experiences of dental caries and periodontal disease through life-course
cause edentulism [19,20]. Therefore, it is prevalent among the older population [21], and it can reflect
oral health inequalities determined by a wider range of dental health care across the life-course.
We hypothesised that inequalities in edentulism would be smaller in Japan with wider dental
care coverage than in England. The objective of this study is to compare the association between
socioeconomic status and edentulism among older adults in Japan and England.
2. Materials and Methods
2.1. Participants
We used cross-sectional data from the Japan Gerontological Evaluation Study (JAGES) [22] andthe English Longitudinal Study of Ageing (ELSA) [23]. The JAGES and ELSA are both ongoingprospective cohort studies investigating social and behavioural factors associated with a functional
decline or cognitive impairment among older individuals. The target population of the JAGES
was 169,215 community-dwelling people aged 65 years and older selected from 31 municipalitiesin 12 prefectures in Japan from August 2010 to January 2012 (Wave 3). A total of 112,123 people
participated in the JAGES (response rate = 66.3%). The ELSA data were collected between June 2010
and May 2011 (Wave 5) from a total of 10,274 community-dwelling people aged 50 years and older
living in England. Those certified for long-term care in JAGES and those under the age of 65 years in
ELSA were excluded. Then, both JAGES and ELSA data obtained from older adults aged over 65 years
and older with valid responses (no missing data) in the variables included in this study were used forour analyses (Figure 1).

and sex for both studies (JAGES = 2.5 (95% CI, 2.3; 2.6) and ELSA = 4.8 (95% CI, 3.7; 6.2)).
Overall, absolute oral health inequalities were smaller in Japan than in England. Education
appears to play a crucial role in determining oral health inequalities compared to income, particularly
in England.
Int. J. Environ. Res. Public Health 2020, 17, 5539

3 of 10Figure 1. Flowchart of the selection of participants for the study (JAGES, Japan Gerontological
Evaluation Study; ELSA, English Longitudinal Study of Ageing). * This response rate was taken fromcohort profile of ELSA [23].

Figure 1. Flowchart of the selection of participants for the study (JAGES, Japan Gerontological
Evaluation Study; ELSA, English Longitudinal Study of Ageing). * This response rate was taken from
Ethicalfor thecohort approval
profile of ELSA
[23].study was obtained from the Ethics Committee at Nihon Fukushi

2.2. Ethical Considerations

University, Japan (Approval number: 10–05). ELSA was approved by the South Central Berkshire
Research Ethics Committee through an application to the National Research Ethics Service [24].
2.3. Dependent Variable
The dependent variable for the present analysis was edentulism (= “Has no teeth”). The JAGES
self-report questionnaire on the current dental status of the participants used the following question:“How many natural teeth do you presently have?” There were four choices: (1) I have 20 or morenatural teeth, (2) I have 10 to 19 natural teeth, (3) I have 1 to 9 natural teeth, or (4) I have no natural
teeth. We categorised the answers as “dentate” (1–3) and answer “edentulous” (4). In the ELSA, thequestion was as follows: “In relation to your dental health, which of the following applies to you?”
There were four choices: (1) Has no natural teeth and wears dentures, (2) Has both natural teeth anddenture(s), (3) Has only natural teeth, or (4) Has neither natural teeth nor dentures. We categorised theanswers as “dentate” (2 and 3) and “edentulous” (1 and 4).
2.4. Explanatory Variables
We used equivalised annual income and educational attainment as indicators of participants’socioeconomic status. Income levels were classified as follows: lowest, low, middle and high (JAGES:≤12,500, 12,501–19,445, 19,446–30,619, ≥30,620 (USD, 1 USD = 100 JPY), ELSA: ≤11,911, 11,912–<16,459,
16,459–<23,810, >=23,810 (USD, 1 USD = 0.8 GBP), respectively). For educational attainment, we
acknowledge that possible country differences in the education system between Japan and England. We
consulted with the ELSA data management team and harmonised the education level between the twocountries (ELSA 3 and 4 Label = JAGES 1 and 2 (15 years or younger), ELSA 5, 6 and 7 Label = JAGES 3(16–18 years) and ELSA 8 Label = JAGES 4 (19 or older)). The samples were categorised according to
the age at which they completed their formal education as low (15 years or younger), middle (16–18
years) and high (19 years or older) for both the JAGES and ELSA. Age (65–69, 70–74, 75–79, 80–84 and≥85 years) and sex (men and women) were used as covariates.

Int. J. Environ. Res. Public Health 2020, 17, 5539

4 of 102.5. Data Analyses
Firstly, we described the demographic characteristics of the participants. For the participants’ age,
we used the median and first quartile–third quartile because our data were not distributed normally(Kolmogorov–Smirnov test, p < 0.001). Secondly, to show the sex- and age-adjusted association ofsocioeconomic status on edentulism, sex- and age-adjusted prevalence ratios were calculated fromPoisson regression analyses. Finally, we estimated the Slope Index of Inequality (SII) and the RelativeIndex of Inequality (RII) to evaluate absolute and relative inequalities [25] in edentulism. SII referents
the absolute difference in a health indicator between those with a higher and those with a lower
level of socioeconomic status. RII is a relative measure of inequality that indicates the ratio of healthstatus between the higher and lower levels of socioeconomic status groups. Both indicators were
calculated using regression-based methods [26]. To determine the explanatory factors in the association
between socioeconomic status (income and educational attainment) and edentulism, we built themodels for each country as follows. Model 1 tested the association between a socioeconomic status
variable (income or educational attainment) and edentulism (univariate model). Model 2 examined
the association between a socioeconomic status variable (incomes or educational attainment) andedentulism after adjusting for age and sex (age and sex adjusted model). All analyses were done by
Stata MP version 14 [26].
3. Results
We used the data obtained from 79,707 individuals in the JAGES and 5115 individuals in the ELSA
Wave that had no missing responses. There were 49.6% men in the JAGES and 45.6% men in the ELSA.
The median (first quartile–third quartile) age was 73.0 (69.0–77.0) years for JAGES and 73.0 (69.0–79.0)
years for ELSA. The prevalence of edentulism was 13.8% for JAGES and 20.6% for ELSA.
Table 1 shows the prevalence of sociodemographic attributes according to the dental status fromJAGES and ELSA. Women had a higher proportion of edentulism than men, especially in ELSA. Social
gradients in dental status were observed in both samples, and participants with higher education or
income had lower percentages of edentulism.
Table 1. Descriptive distribution of edentulous participants in Japan and England.
Total

JAGES (N = 79,707)
N

%

Total

ELSA (N = 5115)
N

%

Age
65–69 year old
70–74 year old
75–79 year old
80–84 year old
85+ year old

24,567
23,911
17,152
9506
4571

1331
2322
2904
2586
1881

5.4%
9.7%
16.9%
27.2%
41.2%

1537
1411
1029
640
498

172
244
239
190
209

11.2%
17.3%
23.2%
29.7%
42.0%

Men
Women

39,56840,139

5491
5533

13.9%
13.8%

2334
2781

417
637

17.9%
22.9%

LowestLow
MiddleHigh
Educational attainment
Low
MiddleHigh

21,076
19,455
18,916
20,260

4513
2386
2058
2067

21.4%
12.3%
10.9%
10.2%

1278
1280
1279
1278

356
322
247
129

27.9%
25.2%
19.3%
10.1%

36,08528,60315,019

1934
714
297

21.0%
10.5%
8.2%

2815
1645
655

779
233
42

27.7%
14.2%
6.4%

Sex

IncomeJAGES, Japan Gerontological Evaluation Study; ELSA, English Longitudinal Study of Ageing.

Men
Women

39568
40139

5491
5533

13.9%
13.8%

2334
2781

417
637

17.9%
22.9%

IncomeLowest
21076
4513
21.4%
1278
356
27.9%
Low
19455
2386
12.3%
1280
322
25.2%
Int. J. Environ. Res. Public Health 2020, 17, 5539
5 of 10Middle
18916
2058
10.9%
1279
247
19.3%
High
20260
2067
10.2%
1278
129
10.1%
Figures
2 and 3 show
the association between edentulism and socioeconomic status in Japan
Educational
attainment
and England. In both Japanand lower
attainment were
Low and England,
36085 lower
1934 income
21.0%
2815educational
779
27.7%
significantly and independently
both higher
risk of 1645
edentulism
Middle associated28603 with714
10.5%
233 (excluding
14.2% the middle
income of Japan, p < 0.001).
Income andeducational
High
15019
297 attainment
8.2% gradient655 were
42 appeared
6.4%to be greater
in England
than
in
Japan
in
edentulism.
JAGES, Japan Gerontological Evaluation Study; ELSA, English Longitudinal Study of Ageing.
2.50

***

***

2.21

2.13

Prevalence Ratio

2.00
1.71

***

***

1.81

1.50

***

1.22
1.001.00

JAGES

1.04

ELSA

1.00

0.50

0.00
High

MiddleLow

LowestFigure 2. The association between edentulism and equivalised annual income in Japan and England
Figure 2. The association between edentulism and equivalised annual income in Japan and England
estimated by Poisson regression analyses (adjusted for sex and age; JAGES N = 79,707, ELSA N = 5115).
estimated
Poissonregression
Int. J. Environ.
Res.byPublicHealth2020, 17, xanalyses (adjusted for sex and age; JAGES N = 79,707, ELSA N = 5115).6 of 10*** p < 0.001.
*** p < 0.001.

4.00

***

3.64

3.50

Prevalence Ratio

3.00
2.50

***

2.04
2.00

***

1.78

ELSA

***

1.50

JAGES

1.22
1.00 1.00

1.00
0.50
0.00
High

MiddleLow

Figure 3. The
The association
association between
between edentulism
edentulism andand educational
educational attainment
attainment in Japan and England
estimated5115).
estimated by Poisson
Poisson regression
regressionanalyses
analyses(adjusted(adjustedfor
forsexsexandandage;age;JAGES
JAGESN
N== 79,707, ELSA N ==5115).
*** pp << 0.001.

Table
shows
the and
SII and
RII socioeconomicof edentulisminequalities
in Japan andEngland. Theinabsolutesocioeconomic
Table2 2.
Absolute
relativeof edentulousness
JAGES and
ELSA.
difference in edentulism between groups with the lowest and highest income (SII for income) was(N = 79,707)
ELSA(univariate(N = 5115) models).
11.8% (95% CI, 11.0; 12.0) for JAGESJAGES
and 18.5%(95% CI, 15.0; 21.9) for ELSAAge and sex
Age and sex
After adjusting for age and sex,
significant
absolute income differencesin edentulism
forboth JAGES
Univariate
model
Univariate
model
adjusted model

IncomeEducational
attainment

SII (%) (95%CI)
RII (95% CI)
SII (%) (95%CI)

11.77 (11.04;
12.50)
2.47 (2.34; 2.61)
15.00 (14.18;
15.83)

9.24 (8.58; 9.90)
2.07 (1.96; 2.18)
9.93 (9.19;
10.68)

18.46 (15.02;
21.90)
2.40 (2.03; 2.83)
31.93 (28.22;
35.64)

adjusted model
14.35 (10.97;
17.73)
1.92 (1.63; 2.27)
26.65 (23.11;
30.20)

Int. J. Environ. Res. Public Health 2020, 17, 5539

6 of 10and ELSA (JAGES = 9.2% (95% CI, 8.6; 9.9) and ELSA = 14.4% (95% CI, 11.0; 17.7)) remained. The relative
income differences in edentulism (RII for income) were also significant after adjustment for age andsex for both JAGES and ELSA (JAGES = 2.1 (95% CI, 2.0; 2.2) and ELSA = 1.9 (95% CI, 1.6; 2.3)).
Table 2. Absolute and relative socioeconomic inequalities of edentulousness in JAGES and ELSA.
JAGES (N = 79,707)
Univariate
Model
IncomeSII (%) (95% CI)
RII (95% CI)

Educational
attainment

SII (%) (95% CI)
RII (95% CI)

11.77 (11.04;
12.50)
2.47 (2.34; 2.61)
15.00 (14.18;
15.83)
3.36 (3.13; 3.61)

Age and Sex Adjusted
Model
9.24 (8.58; 9.90)
2.07 (1.96; 2.18)
9.93 (9.19; 10.68)
2.45 (2.29; 2.63)

ELSA (N = 5115)
Univariate
Model
18.46 (15.02;
21.90)
2.40 (2.03; 2.83)
31.93 (28.22;
35.64)
5.88 (4.55; 7.61)

Age and Sex Adjusted
Model
14.35 (10.97; 17.73)
1.92 (1.63; 2.27)
26.65 (23.11; 30.20)
4.79 (3.70; 6.19)

SII, Slope Index of Inequality; RII, Relative Index of Inequality.

For education-based inequalities, the absolute difference in edentulism between groups with thelowest and highest educational attainment (SII for education) was 15.0% (95% CI, 14.2; 15.8) for JAGES
and 31.9% (95% CI, 28.2; 35.6) for ELSA. Even after adjusting for age and sex, significant absoluteeducational attainment differences in edentulism for both JAGES and ELSA (JAGES = 9.9% (95% CI,
9.2; 10.7) and ELSA = 26.7% (95% CI, 23.1; 30.2)) were observed. The relative educational attainment
differences in edentulism (RII for education) were also significant after adjustment for age and sex for
both studies (JAGES = 2.5 (95% CI, 2.3; 2.6) and ELSA = 4.8 (95% CI, 3.7; 6.2)).
Overall, absolute oral health inequalities were smaller in Japan than in England. Education
appears to play a crucial role in determining oral health inequalities compared to income, particularly
in England.
4. Discussion
The present study reports both absolute and relative socioeconomic (income and education)
inequalities in edentulism in Japan and England. In general, oral health inequalities were smaller inJapan, a country with one of the highest levels of public coverage for dental care, compared to England.
The prevalence of edentulism was higher in England than in Japan. Socioeconomic inequalities,
especially educational inequalities, were relatively larger in England than in Japan, and the differences
between the two countries were most obvious.
For oral health inequalities, our results were consistent with previous studies. Japanese national
health data showed that lower educational attainment and lower equivalent household expenditure
were associated with fewer remaining teeth [27], while lower income was associated with poor dental
status in Japanese older people [28]. Similarly, socioeconomic inequalities, number of remaining
teeth [29], edentulism [30], dental caries [31,32] and periodontal disease [33,34] were reported in the UK.
Using large cohort data with harmonised measures, our findings of oral health inequalities determinedby income and education offer extended support to previous studies.
Differences in accessibility to dental care could explain the present results of smaller oral healthinequalities in Japan than in England. Inequalities in access to dental care are also reported in bothcountries [16,35]. However, in the UK, out-of-pocket expenses for dental care were higher, and thefrequency of dental visits was lower than in Japan [9–11]. In England, previous studies showed that
almost 20% of the participants tried to get an NHS dental appointment, but failed to get one withina reasonable time [36,37]. People unable to book an NHS dental appointment had to visit a costly
private dentist instead. On the other hand, the Japanese insurance system provides a free accesssystem to dental care, allowing the Japanese to select any medical institution where they can receivehigh-quality medical and dental services at a low cost [38]. Moreover, Japan has a life-course oral
health care system [7]. This system is provided as part of general health services, and the programmeis based on health legislation. For preschool children, a national programme that includes physical,

Int. J. Environ. Res. Public Health 2020, 17, 5539

7 of 10

medical and dental examinations is conducted for all 18-month-olds and 3-year-olds. After the dental
examination, dental hygienists provide maternal and child oral health education. For school children,
every public primary, junior and senior high school has a school dentist. The school dentist performs
dental examination for the school children at least once a year. Moreover, all schools incorporate oral
health education into their curriculum. These systems are believed to be associated with low oral
health inequality in Japan.
The United Nations and WHO promote universal health coverage [5,39]. Our findings suggest
the possibility that more comprehensive dental care coverage reduces oral health inequalities. A wider
inclusion of dental care in universal health coverage is required to improve oral health and reduce oral
health inequalities. Notably, even in Japan, there are still significant levels of oral health inequalities,
suggesting room for improvement, such as a wide range of oral health promotion in the context ofsocial determinants of health [6,40].
Our study had several limitations and strengths. First, our study used a cross-sectional design, andhence, we could not establish a causal relationship between socioeconomic indicators and edentulism.
Second, the measurements were obtained from a self-administered questionnaire. However, clinicalassessment of remaining teeth is more accurate than self-administered questionnaires. Still, the validityof the self-reported number of remaining teeth was verified [31]. Finally, potential biases could be
likely due to fewer cases. However, the strength of this study was that it used large cohort data fromeach country with harmonised measures, offering the level of oral health inequalities in both absoluteand relative terms that were comparable.
5. Conclusions
Both absolute and relative inequalities in edentulism were found in Japan and England. Most of
the inequality measurements suggested that oral health inequalities were smaller in Japan, a country
with one of the highest levels of public coverage for dental care in the world.
Author Contributions: Conceptualization, K.I. and J.A.; methodology, K.K. and N.C.; software, T.Y.; validation,
G.T. and R.G.W.; formal analysis, J.A.; investigation, K.O.; resources, K.S. and K.O.; data curation, T.Y. and J.A.;
writing—original draft preparation, K.I.; writing—review and editing, N.C., K.S., T.Y., K.K. and J.A.; visualization,
K.I.; supervision, K.K., K.O., G.T. and R.G.W.; project administration, J.A.; funding acquisition, K.I., K.S., K.K. and
J.A. All authors have read and agreed to the published version of the manuscript.
Funding: This research was founded by JSPS (Japan Society for the Promotion of Science) KAKENHI Grant
Number (JP15H01972, JP20H00557, JP18KK0057, JP19H03860, JP15K21266, 20H00557), Health Labour Sciences
Research Grant (H28-Choju-Ippan-002, H30-Junkanki-Ippan-004, 19FA2001, 19FA1012), Japan Agency for
Medical Research and Development (AMED) (JP17dk0110017, JP18dk0110027, JP18ls0110002, JP18le0110009,
JP19dk0110034, JP19dk0110037), Open Innovation Platform with Enterprises, Research Institute and Academia(OPERA, JPMJOP1831) from the Japan Science and Technology (JST), a grant from Innovative Research Program on
Suicide Countermeasures (1-4), a grant from Sasakawa Sports Foundation, a grant from Japan Health Promotion
and Fitness Foundation, a grant from Chiba Foundation for Health Promotion and Disease Prevention, the 8020
Research Grant for fiscal 2019 from the 8020 Promotion Foundation (adopted number: 19-2-06), a grant from Niimi
University (1915010), grants from Meiji Yasuda Life Foundation of Health and Welfare and the Research Fundingfor Longevity Sciences from National Center for Geriatrics and Gerontology (29-42, 30-22), and also FUTOKU
Research Grant (H25), FUTOKU Foundation.
Acknowledgments: This research used data from JAGES (the Japan Gerontological Evaluation Study) and ELSA
(the English Longitudinal Study of Ageing). The authors are grateful to the ELSA data management team for theirassistance in harmonizing Japan and England education level and making the two comparable.
Conflicts of Interest: The authors declare no conflict of interest.

Int. J. Environ. Res. Public Health 2020, 17, 5539

8 of 10Abbreviations
CI
ELSA
OECD
JAGES
NHS
RII
UK
SII
WHO

Confidence Interval
English Longitudinal Study of Ageing
Organisation for Economic Co-operation and Development
Japan Gerontological Evaluation Study
National Health Service
Relative Index of Inequality
United Kingdom
Slope Index of Inequality
World Health Organisation

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