ARTICLE IN PRESS
international dental journal 0 0 0 ( 2 0 2 2 ) 1 − 8

Scientific Research Report

Co-Payments and Inequality in Gingival Bleeding and
Dental Visits
Noriko Nakazawa a, Taro Kusama a,b, Kenji Takeuchi a,b, Sakura Kiuchi a,c,
Tatsuo Yamamoto d, Katsunori Kondo e,f, Ken Osaka a, Jun Aida g*a
Department of International and Community Oral Health, Graduate School of Dentistry, Tohoku University, Sendai,
Miyagi, Japanb
Division for Regional Community Development, Liaison Center for Innovative Dentistry, Graduate School of Dentistry,
Tohoku University, Sendai, Miyagi, Japanc
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Miyagi, Japand
Department of Dental Sociology, Kanagawa Dental University, Yokosuka, Kanagawa, Japane
Center for Preventive Medical Sciences, Chiba University, Chiba, JapanfCenter for Gerontology and Social Science, National Center for Geriatrics and Gerontology, Obu, Aichi, Japang
Department of Oral Health Promotion, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental
University, Tokyo, JapanA R T I C L E

I N F OA B S T R A C T

Article history:

Objectives: Japan’s universal health insurance covers a wide range of dental treatments, andReceived 14 September 2022

the co-payment rates differ by age. We investigated whether the inequality in gingival

Received in revised form

bleeding and dental visits was smaller amongst those with lower co-payment rates.

11 November 2022

Methods: This cross-sectional study used data from the 2019 Japan Gerontological Evaluation

Accepted 17 November 2022

Study. The participants were functionally independent adults aged 65 years or older. The

Available online xxx

dependent variables were current gingival bleeding as a symptom of periodontal diseases anddental nonattendance for treatment in the past year. The independent variables were ridit

Key words:

scores of equivalent income and educational status. For covariates, we used age, sex, and the

Health inequality

number of remaining teeth. To evaluate the inequalities, we used the slope index of inequality

Social inequality(SII) and the relative index of inequality (RII). We also conducted stratified analyses by co-pay-

Oral healthment rates (30%, 20%, and 10%) to clarify the difference in inequalities by co-payment rate.

Universal health coverage

Results: A total of 15,389 participants were included in the analysis; their mean age was 71.8

Periodontal disease(SD = 4.1) and 51.8% were women. There were significant absolute and relative inequalities ingingival bleeding and dental visits by equivalent income and education. With regards to educational status, inequalities were lower with a decrease in the co-payment rate. In particular,
relative inequality by education in gingival bleeding was the largest amongst the 30% co-payment group (RII, 1.918; 95% confidence interval [CI], 1.386 to 2.656). For gingival bleeding, theabsolute and relative inequality by equivalent income were not significant amongst the 10%
co-payment group (SII,

0.003; 95% CI, 0.003 to 0.028; RII, 1.006; 95% CI = 0.676 to 1.498).

Conclusions: A low co-payment rate was associated with smaller inequalities in gingival
bleeding and dental visits by equivalent income and educational status.
Ó 2022 The Authors. Published by Elsevier Inc. on behalf of FDI World Dental Federation.
This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Introduction* Corresponding author. Department of Oral Health Promotion,
Graduate School of Medical and Dental Sciences, Tokyo Medical
and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo, 113The importance of oral health inequalities was highlighted in
8549 Japan.
the resolution on oral health that was adopted in the 74th
E-mail address: aida.ohp@tmd.ac.jp (J. Aida).
World Health Assembly in 2021.1 Previous studies have
Noriko Nakazawa: http://orcid.org/0000-0001-7743-8383
reported inequality in periodontal disease associated with
Sakura Kiuchi: http://orcid.org/0000-0002-8298-8643
socioeconomic status such as low education status and low
Katsunori Kondo: http://orcid.org/0000-0003-0076-816X
income.2−6 Periodontal disease is one of the main causes of
Jun Aida: http://orcid.org/0000-0002-8405-9872
https://doi.org/10.1016/j.identj.2022.11.009
0020-6539/Ó 2022 The Authors. Published by Elsevier Inc. on behalf of FDI World Dental Federation. This is an open access article under
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

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nakazawa et al.

tooth loss7−10 related to all-cause mortality.11−13 The inequality
in periodontal disease can lead to inequality in tooth loss andconsequently to general health inequalities. Additionally, there
were studies that reported inequality of dental attendance anddental service use caused by socioeconomic status.14,15
Universal health coverage (UHC) is an important measure to
improve access to health care.16 UHC is crucial for achieving
health and well-being in the Sustainable Development Goals,
which many countries have been working towards.17 Oral
health is an important area that should be covered in UHC.16,18
The prevalence of oral disease worldwide is very high, and it isa critical public health issue.19 However, oral health has been
neglected as part of UHC.16,20 In a meeting in 2019, the United
Nations highlighted the need to strengthen oral health in UHC.21
Japan has achieved UHC to a great extent; as a previous
study reports, almost everyone was insured in 1961.22 Japanese health care insurance plans are roughly divided into 2
types: employee and national health insurance plans.22 Bothplans cover the same types of medical services, including
dentistry. This universal health insurance system covers
most basic dental treatments.23 The co-payment for medical
and dental care decreases with age, with the rates being 30%
until 69 years, 20% for those between 70 and 74 years, and
10% for those 75 years or older.24
Japan has the most accessible dental treatment when considering their universal health insurance system. In a study
examining the per capita co-payment rate of dental treatment and the frequency of dental visits amongst countries of
the Organisation for Economic Co-operation and Development, the co-payment rate for dental treatment is low inJapan and the frequency of dental visits per capita is high.20
Co-payment rates are important because they affect accessto dental care. A previous study reported that amongst older
adults, the proportion of people who attended dental visitsvaried depending on the co-payment rate.25 However, they
did not mention whether the difference in the co-payment
rate reduced inequalities in dental visits. Additionally,another study investigated inequalities of denture use byincome or education status, and researchers found inequalities of denture use by co-payment rate; the inequality
appeared higher when the co-payment rate was higher.26
These results may be attributed to a decrease in the inequality
in dental visits due to the decrease in co-payments; however,
the authors did not report the association with dental visits.
Therefore, this study aimed to evaluate whether theinequality in gingival bleeding as one of the symptoms of
periodontal disease and dental attendance would be smallerwith a decrease in co-payment rate. We hypothesised thatthe inequalities in gingival bleeding and dental attendancewere smaller amongst those with lower co-payment rates.

factors and targeting functionally independent older adults
aged 65 years or older in Japan.27 The questionnaires were
distributed by mail to residents living in 63 municipalities
between November 2019 and January 2020. The questionnaire
consists of a core questionnaire and 8 versions (A to H) of supplemental questionnaires. Version D included dental-related
questions, and our study targeted those who answered this
supplemental questionnaire. The eligibility criteria of theanalysed population are as follows: (1) those who have atleast 1 tooth, (2) those who did not receive public assistance,(3) those who reported being functionally independent, and(4) those aged 69 to 79 years old, to align with the co-payment
rate−age bands, given that the co-payment rate changes in 5year increments in Japan’s universal health insurance system. No third-party data were used.

Methods

Covariates

Study settings and participants

For covariates, we used age, sex, and the number of remaining teeth. The analysis was conducted by each co-paymentgroup. Each co-payment group included a different age range.
Therefore, we used the age of people who are applicable toeach group as a continuous variable. Additionally, in thisstudy, the target population comprised people who had at

This cross-sectional study was based on self-reported questionnaires. Data from the 2019 survey of the Japan Gerontological Evaluation Study (JAGES) were used. The JAGES is an
ongoing large cohort study investigating health-related

Dependent variables
We used the following 2 dependent variables: gingival bleeding (as an indicator of periodontal disease) and dental nonattendance. Gingival bleeding is one of the major symptoms of
periodontal disease, and it was determined using the following yes/no question: “I got blood from my gums recently.”28,29
Dental visits were assessed using the question, “When did
you last visit the dental clinic for ‘treatment’ (including
adjustment of dentures)?” and the possible responses were
“during the past 6 months,” “6 months to 1 year ago,” “1 to3 years ago,” “more than 3 years ago,” and “I’ve never seen adentist.” Those who had not visited a dental clinic in the past
year were classified as not having dental attendance, andthose who had visited were classified as having dental
attendance.

Independent variables (socioeconomic indicators)
As socioeconomic indicators, we used equivalent householdincome and educational status. Equivalent household income
was calculated by dividing annual household income by thesquare root of the family members. Equivalent householdincome (in Japanese yen [JPY], where 1 USD  109 JPY) was
categorised as follows: (1) less than 1.00 million, (2) 1.00 to
1.99 million, (3) 2.00 to 2.99 million, (4) 3.00 to 3.99 million,and (5) 4 million or more. Educational status was categorisedas 9 years or fewer, 10 to 12 years, and 13 years or more. Nine
years of education is equivalent to junior high school;
12 years, to higher school; and 13 or more years, to a university or vocational school. Each education and income category was converted to a modified ridit score, which was
calculated by the midpoint of the range of the cumulative distribution of the population of participants in each category.

ARTICLE IN PRESS
co-payment rates and oral health inequality

least 1 tooth. Therefore, we used the number of remaining
teeth (1−4, 5−9, 10−19, and ≥20) as categorical variables.

Statistical analysis
To evaluate the absolute and relative inequality in gingival
bleeding and dental visits, we employed the slope index ofinequality (SII) and the relative index of inequality (RII).30 The
SII and RII were used to estimate the absolute differences andrelative ratio, respectively, of the prevalence of self-reported
gingival bleeding and the dental nonattendance for treatment
by socioeconomic inequality. If the SII is closer to 0 and the RII
is closer to 1, there is less inequality in the absolute and relative scales, respectively. In Japan, whilst the geographic
inequality of dental services has been decreasing, as of 2010,
21.4% of municipalities with a population of fewer than 5000
did not have dental clinics.31 Therefore, the ease of access todental clinics may vary by region. Consequently, we usedgeneralised estimating equations, considering municipalities
as clusters, as well as fitted models with binomial distribution
and the identity link function for SII and the logarithmic linkfunction for RII. Before the analyses, we conducted multiple
imputations to reduce selection bias. The missing values
were imputed by using multiple imputation by chained equations (MICE). We constructed 50 multiple imputed data sets
and combined the estimates using Rubin’s rule.32,33 For multiple imputation, we used all variables (age, sex, the number ofteeth, equivalent income, education status, gingival bleeding,and dental attendance for treatment).
First, we conducted an analysis including the interaction
term for overall participants to check the association between
the co-payment rate and socioeconomic indicators. Because
we could not obtain information from the government
directly about the actual co-payment rate for individuals in
the Japanese national health insurance system, the co-payment rate is determined by age.24 The data used in our study
included adults aged 65 or older, and the co-payment rate is
30% for those aged 65 to 69 years, 20% for those aged 70 to
74 years, and 10% for those aged 75 years and older.24 Therefore, we used the population aged 75 to 79 years as the 10%
co-payment group to align the age bands. Each age was used
as a new 3-category variable indicating the co-payment rate(65−69 = 30%, 70−74 = 20%, and 75−79 = 10% co-paymentrate). Second, the analyses were stratified by the co-payment
rate. We created 3 models for each analysis. Model 1 was theunivariate analysis, model 2 was adjusted for age and sex,
and model 3 was adjusted for age, sex, and the number ofremaining teeth. In model 2 and model 3, age was used as acontinuous variable. We used Stata MP version 16.1 to perform all analyses.

Ethical issues
The JAGES survey was approved by the Ethics Committee on
Research of Human Subjects at Nihon Fukushi University(No. 10−05). The additional 2019 survey used in this study
was approved by the National Center for Geriatrics and Gerontology (No. 1274−2), Chiba University (No. 3442), and theJapan Agency for Gerontological Evaluation Study Organization (No. 2019−01).

3

Results
The Figure presents the flowchart of the participants includedin this study. In the 2019 wave, 31,747 individuals answered
the questionnaires, including questions related to gingival
bleeding and dental visits (response rate, 69.1%). Subsequently, we excluded the individuals who disagreed with theuse of data for research purposes (n = 7394) or did not fulfill
the eligibility criteria (n = 8964) (the details of the eligibility
criteria are shown in the Figure). Finally, the analysis included
15,389 participants; their mean age was 71.8 years (SD = 4.1),
and 51.8% were women. The number of participants in the
30%, 20%, and 10% co-payment categories were 5025, 5800,
and 4564, respectively. Missing values for 2574 participants
were imputed by MICE (see Supplemental Table 1). Table 1
shows the prevalence of gingival bleeding and dental nonattendance for treatment stratified by each co-payment group.
In each co-payment group, the prevalence of gingival bleeding
was higher in both the lower equivalent income categories
and educational status. The number of people reporting nodental visits for treatment in the past year was high in thelower-socioeconomic-status category for each co-payment
rate. Before the analyses, we examined the interaction
between the co-payment rate and each socioeconomic status
for dependent variables. Although there were tendencies forthe effect of the independent variables to differ by the co-payment rate, there was no significant interaction between theco-payment rate and socioeconomic status (P ≥ .05), except
for income and co-payment in absolute inequalities of gingival bleeding (P < .05; see Supplemental Table 2).

Gingival bleeding
Table 2 shows the inequalities in the prevalence of gingival
bleeding in both absolute and relative scales. The result ofmodel 3, which was adjusted for age, sex, and the number ofremaining teeth, indicated significant absolute and relative
inequalities by equivalent income and education in gingival
bleeding in each co-payment group. Absolute inequalities (SII)
by equivalent income were as follows: 30% co-payment group,
0.054 (95% confidence interval [CI], 0.020−0.087) and 20% copayment group, 0.041 (95% CI, 0.013−0.069). Relative inequalities (RII) by equivalent income were as follows: 30% co-payment group, 1.583 (95% CI, 1.166−2.148) and 20% co-paymentgroup, 1.588 (95% CI, 1.159−2.174). Moreover, the SIIs by educational status were as follows: 30% co-payment group, 0.073(95% CI, 0.036−0.109); 20% co-payment group, 0.039 (95% CI,
0.010−0.068); and 10% co-payment group, 0.031 (95% CI, 0.002−0.059). The RIIs by educational status were as follows: 30% copayment group, 1.918 (95% CI, 1.386−2.656); 20% co-paymentgroup, 1.606 (95% CI, 1.164−2.216); and 10% co-payment group,
1.587 (95% CI, 1.074−2.345). However, amongst the 10% co-payment group, there were no significant inequalities of gingival
bleeding by equivalent income. The inequalities by educational
status were larger than those by equivalent income.

Dental nonattendance for treatment in the past year
Table 3 presents the results of inequalities of dental visits bysocioeconomic indicators. The absolute and relativeARTICLE IN PRESS
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nakazawa et al.

Fig – Flowchart of participants in this study.

inequalities of dental visits by equivalent income and education were observed in each co-payment group. Regarding
absolute inequality by equivalent income, smaller inequalities were observed in the lower co-payment group. The SIIs
by equivalent income were as follows: 30% co-paymentgroup, 0.093 (95% CI, 0.041−0.145); 20% co-payment group,
0.084 (95% CI, 0.037−0.130); and 10% co-payment group, 0.071(95% CI, 0.020−0.123). The trends of RII by income were notsimilar to the trends of the absolute inequalities that showedsmaller inequalities in the lower co-payment groups. The RIIs
by equivalent income were as follows: 30% co-paymentgroup, 1.287 (95% CI, 1.128−1.469); 20% co-payment group,
1.270 (95% CI, 1.108−1.455); and 10% co-payment group, 1.304(95% CI, 1.089−1.562). Whilst the same trend was notobserved for the SII and RII by equivalent income, theinequalities by education were smaller for lower co-payment
rate in both the absolute and relative scales. The SIIs by educational status were estimated as follows: 30% co-paymentgroup, 0.108 (95% CI, 0.052−0.164); 20% co-payment group,
0.086 (95% CI, 0.038−0.135); and 10% co-payment group, 0.049(95% CI, 0.002 to 0.100). In the 10% co-payment group, the SII
by education was not significant. The RIIs by educational status were as follows: 30% co-payment group, 1.339 (95% CI,
1.165−1.540); 20% co-payment group, 1.282 (95% CI, 1.112−1.477); and 10% co-payment group, 1.204 (95% CI, 1.011−1.434).

Discussion
To the best of our knowledge, this is the first study investigating that the inequalities of self-reported gingival bleeding oraccess to dental by equivalent income or educational status
are different for each co-payment rate. We also investigated
the interactions between the co-payment rate and socioeconomic status; however, we did not find significant interactions, except for between the 10% co-payment rate andincome in terms of the absolute inequality of gingival bleeding. Inequalities observed in gingival bleeding were smaller inthe lower co-payment group, and in the 10% co-paymentgroup, the inequalities of gingival bleeding by equivalent
income were not significant. The inequalities in dental visitswere also lower in those with a lower co-payment group,
except for the relative inequality by equivalent income. Additionally, the absolute inequality by education was not significant in the 10% co-payment group.
Our findings that smaller inequalities of oral health anddental attendance were observed in lower co-paymentgroups are consistent with previous studies.26,34 A study
conducted in Japan showed that the inequality of denture
use by income was smaller in the group with a lower copayment.26 Similarly, our results showed that amongst the
low co-payment group, inequalities in gingival bleeding anddental visits by equivalent income were lower. However,ARTICLE IN PRESS
5

co-payment rates and oral health inequality

Table 1 – Rate of gingival bleeding and no dental visits for treatment amongst participants in each co-payment category(N = 15,389).
65−69 years (n = 5025)

Sex
Equivalent
income (JPY)

Education status

Number of teeth

Men
Women
<100
≥100 to <200≥200 to <300
≥300 to <400
≥400
≤9
10−12
≥13
1−4
5−9
10−19
≥20

70−74 years (n = 5800)

75−79 years (n = 4564)

No.

Gingival
bleeding(%)

No dental
visit fortreatment (%)

No.

Gingival
bleeding(%)

No dental
visit fortreatment (%)

No.

Gingival
bleeding(%)

No dental
visit fortreatment (%)

2440
2585
393
1492
1319
1044
777
610
2415
2000
169
346
1028
3482

12.7
9.8
13.5
13.4
10.1
9.8
10.0
14.6
12.2
9.1
6.7
13.4
14.6
10.0

42.7
35.1
48.2
40.4
37.3
37.7
34.9
47.1
38.9
36.1
58.8
44.9
39.6
37.0

2813
2987
545
2096
1353
1035
770
1071
2696
2033
285
468
1292
3755

9.9
9.0
12.3
10.3
9.1
8.1
7.5
13.2
8.7
8.5
8.7
12.5
13.4
7.7

35.7
32.0
40.2
35.2
32.8
31.7
30.0
38.8
33.8
31.2
48.6
39.6
32.9
32.3

2166
2398
602
1754
1063
656
490
1304
1997
1263
316
514
1076
2658

9.2
6.4
7.3
8.0
7.7
7.7
7.4
9.0
7.6
6.7
6.8
9.8
10.4
6.4

29.9
27.6
33.0
30.4
26.8
25.3
26.0
31.8
28.1
26.5
40.6
35.6
25.8
27.1

only the relative inequality in dental visits by equivalent
income was not small amongst the lower co-paymentgroup. It has been reported that absolute and relative
inequalities do not always evaluate the same aspects ofinequalities.35 In our study, the proportion of higherincome groups who reported dental nonattendance in thepast year was also smaller, along with the co-payment rate.
This may be why the relative inequality in dental visits byequivalent income was not smaller in the lower co-payment group. Previous studies have observed dental healthinequalities by education level.15,36 The inequalities by education level reflect effects from childhood because one’s
education level is typically decided in a relatively early life
stage.15,37 Our study extends this finding by showing that
low co-payment rates may decrease the inequalities in gingival bleeding and dental visits by educational status.
The reduction in inequalities in gingival bleeding withthe low co-payment rate by equivalent income may be
explained by the reduction in inequalities in dental visits.
A previous study reported that the reduction in co-payment will increase the number of dental visits and improveself-rated oral health.38 The reduction in the co-payment
rate would have made it simpler for people to obtain dental
care, thereby improving access and leading to more people
receiving dental care; this includes periodontal disease
treatment, which relieves periodontal symptoms like gingival bleeding. Moreover, there were inequalities in gingival
bleeding and dental visits by education, which may be
explained as follows. Educational attainment determines
income, occupation, lifestyle, and health behaviour. Therefore, the inequalities by education may have accumulated
throughout the life course. However, because the inequalities were small amongst the lower co-payment groups,
especially in the 10% co-payment group, significant absolute inequality was not observed. This may suggest thatthe small co-payment may make dental visits moreaccessible.
The present study has several strengths. First, the data
used in this study were obtained from Japan, where themajority of the population benefits from UHC. Our results
suggested the possibility that the decline in the co-payment
rate could reduce socioeconomic inequality even under UHC.
Second, we used 2 different indicators of inequalities (SII and
RII), enabling us to assess the different aspects of inequalities.
Third, the sample size was relatively large (N = 15,389), covering 63 municipalities nationwide. This study also has several
limitations. First, our study design was cross-sectional andcould not infer the temporality between socioeconomic
inequality and gingival bleeding or dental visits. However,gingival bleeding or dental attendance is less likely to affectthe individual socioeconomic status. The second is that theeffect of age, for example, in inflammatory dysregulation,39
could not be completely excluded. Future studies should be
conducted amongst same-age people with different co-payment rates. The third is that the information on the co-payment rate was not obtained from the local goverment. In
Japan, the co-payment rate is generally determined by age
amongst older adults; however, for some high-income individuals, the co-payment rate is high regardless of age. Due toa lack of data from the local government, we could not consider these cases in our analysis. The fourth limitation is
information bias. We employed self-reported gingivitis anddental visits, leading to nondifferential misclassification andbiased estimates. Because the gingival bleeding question was
binary, nondifferential misclassification may have biased theestimates towards null. However, future studies employing
measurements with less misclassification are required for
dental visits.

Conclusions
The present study revealed the socioeconomic inequalities
in the prevalence of gingival bleeding and dental visitsamongst functionally independent older adults residing inJapan, where UHC strategies have been implemented.
However, these inequalities were attenuated by lower copayment rates. The reduction in co-payments is6

Table 2 – The absolute and relative inequalities of gingival bleeding in each co-payment by socioeconomic indicators.
Slope index of inequality
Socioeconomic
indicators

Relative index of inequality

No.

Model 1Model 2Model 3Model 1Model 2Model 3

IncomeCo-payment

30%
20%
10%

5025
5800
4564

0.052 (0.018 to 0.086)**
0.048 (0.019 to 0.076)**
0.002 ( 0.029 to 0.032)

0.056 (0.022 to 0.090)**0.050 (0.022 to 0.078)***0.003 ( 0.027 to 0.034)

0.054 (0.020 to 0.087)**
0.041 (0.013 to 0.069)**
0.003 ( 0.033 to 0.028)

1.611 (1.189 to 2.182)**1.680 (1.229 to 2.298)**
1.021 (0.687 to 1.518)

1.666 (1.230 to 2.257)**
1.734 (1.266 to 2.375)***1.069 (0.717 to 1.592)

1.583 (1.166 to 2.148)**1.588 (1.159 to 2.174)**
1.006 (0.676 to 1.498)

Education

Co-payment

30%
20%
10%

5025
5800
4564

0.076 (0.039 to 0.112)***0.046 (0.016 to 0.075)**
0.031 (0.002 to 0.061)*

0.079 (0.042 to 0.115)***0.049 (0.019 to 0.078)**
0.037 (0.008 to 0.065)*

0.073 (0.036 to 0.109)***0.039 (0.010 to 0.068)**
0.031 (0.002 to 0.059)*

1.946 (1.411 to 2.684)***
1.707 (1.246 to 2.339)***1.523 (1.030 to 2.251)*

2.041 (1.480 to 2.815)***1.801 (1.311 to 2.475)***1.665 (1.128 to 2.458)*

1.918 (1.386 to 2.656)***1.606 (1.164 to 2.216)**
1.587 (1.074 to 2.345)*

Model 1: crude; model 2: adjusted for sex and age; model 3: adjusted for sex, age, and the number of teeth.

Slope index of inequality
Socioeconomic indicators
IncomeEducation

Co-payment 30%
20%
10%Co-payment 30%
20%
10%

No.

Model 1Model 2Model 3Model 1Model 2Model 3

5025
5800
4564
5025
5800
4564

0.097 (0.045 to 0.148)***0.088 (0.042 to 0.134)***0.082 (0.030 to 0.133)**
0.113 (0.057 to 0.168)***0.087 (0.038 to 0.135)***0.058 (0.007 to 0.109)*

0.105 (0.054 to 0.157)***
0.094 (0.048 to 0.140)***0.085 (0.034 to 0.136)**
0.124 (0.069 to 0.179)***0.099 (0.051 to 0.148)***0.063 (0.012 to 0.114)*

0.093 (0.041 to 0.145)***0.084 (0.037 to 0.130)***0.071 (0.020 to 0.123)**0.108 (0.052 to 0.164)***0.086 (0.038 to 0.135)***0.049 ( 0.002 to 0.100)

1.287 (1.128 to 1.468)***1.291 (1.128 to 1.477)***1.325 (1.109 to 1.585)**
1.344 (1.169 to 1.545)***1.288 (1.120 to 1.481)***1.220 (1.026 to 1.450)*

1.329 (1.165 to 1.516)***1.319 (1.153 to 1.510)***1.340 (1.120 to 1.603)**
1.397 (1.216 to 1.606)***1.330 (1.156 to 1.530)***1.252 (1.052 to 1.490)*

1.287 (1.128 to 1.469)***1.270 (1.108 to 1.455)***1.304 (1.089 to 1.562)**
1.339 (1.165 to 1.540)***1.282 (1.112 to 1.477)***1.204 (1.011 to 1.434)*

Model 1: crude; model 2: adjusted for sex and age; model 3: adjusted for sex, age, and the number of teeth.
* P < .05;** P < .01;*** P < .001.

Relative index of inequality

nakazawa et al.

Table 3 – The absolute and relative inequalities of dental visits for treatment in each co-payment by socioeconomic indicators.

ARTICLE IN PRESS* P < .05;** P < .01;*** P < .001.

ARTICLE IN PRESS
co-payment rates and oral health inequality

considered an effective policy to eliminate oral healthinequalities even in a nation where UHC has already been
implemented.

Author contributions
N.N. and J.A. conceived of the study and wrote the first draft.
The analysis, data gathering, debate, and writing of the second and succeeding revisions were done by all writers. The
paper has been evaluated and reviewed by all authors, whohave also given their consent for submission.

Funding
This study used data from the Japan Gerontological Evaluation Study (JAGES). This study was supported by Japan Society
for the Promotion of Science (JSPS) KAKENHI Grant Number(JP15H01972), Health Labour Sciences Research Grant (H28Choju-Ippan-002, 22FA2001), Japan Agency for Medical
Research and Development (AMED) (JP18dk0110027,
JP18ls0110002, JP18le0110009, JP20dk0110034, JP21lk0310073,
JP21dk0110037, JP22lk0310087), Open Innovation Platform
with Enterprises, Research Institute and Academia (OPERA,JPMJOP1831) and from the Japan Science and Technology, a
grant from Innovative Research Program on Suicide Countermeasures (1−4), a grant from Sasakawa Sports Foundation, a
grant from Japan Health Promotion & Fitness Foundation, a
grant from Chiba Foundation for Health Promotion & Disease
Prevention, the 8020 Research Grant for fiscal 2019 from the8020 Promotion Foundation (adopted number: 19−2−06), andgrants from Meiji Yasuda Life Foundation of Health and Welfare and Research Funding for Longevity Sciences from the
National Center for Geriatrics and Gerontology (29−42, 30−22,
20−19, 21−20). The study’s funding sources were not involvedin the study’s planning, data collection, analysis, or interpretation or in the writing of the paper.

Conflict of interest
None disclosed.

Acknowledgements
This study was conducted using data from the Japan Gerontological Evaluation Study (JAGES) This work was supported by
JST SPRING, Grant Number JPMJSP211.

Supplementary materials
Supplementary material associated with this article can befound in the online version at doi:10.1016/j.identj.2022.11.009.

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