Changes in Oral Health Indicators Due to Implementation of Health Insurance Coverage for First Molar Dental Sealant for Children and Adolescents in Korea

Background: The effectiveness of a dental sealant reimbursement policy for children and adolescents was verified by comparing changes in first molar oral health indicators before and after policy implementation. Methods: Korea National Health and Nutrition Examination Survey data were analyzed: the fourth period (2007–2009) provided data before, and sixth period (2013–2015) data after policy implementation. The proportion of individuals with first-molar sealant, decay-missing-filled first molar permanent teeth, and single crowns in the group aged 11-20 years were calculated. Data were analyzed using chi-square for complex samples and the complex samples general linear model. Results: Compared with non-beneficiaries, among policy beneficiaries, sealant ownership increased by 7.7%, and the number of permanent teeth with sealant per capita was about double; the proportion of individuals with decay-missing-filled permanent teeth decreased 9.1%, and the average decay-missing-filled permanent teeth index per person decreased by about a quarter; the rate of single-crown holders decreased by 2.7%, and the average single-crown index decreased by 0.03 per person. The number of sealants increased with age and household income. The caries rate was higher in females and older respondents. Conclusions: A system is needed to ensure sealant is supplied to vulnerable groups; a follow-up system is required for long-term sealant maintenance.

regarding oral health around the world, the global average decay-missing-filled permanent teeth index was reduced to less than [3]. In addition, OECD member countries also compare their decay-missing-filled permanent teeth index among 12-year-olds to understand their oral health status [4].
To reduce the prevalence of dental caries in children and adolescents in Korea, the government implemented free-sealant projects through public health centers from 2002 to 2010 [5]. These publicly based oral health projects played an important role in reducing the decay-missing-filled permanent teeth index among 12-year-olds from 3.25 in 2003 to 2.17 in 2006 [6]. Subsequently, as the effectiveness of sealant was supported, this treatment was included as a health insurance benefit for the first molar for children aged 6 to 14 years from December 2009 [7]. In October 2012, the second molar was additionally covered, and in July 2013, the age of coverage was extended to under 18 years [7]. Previous studies [8,9] have shown that sealant treatment is increasing with the expansion of sealant health insurance. However, it has been confirmed that differences in the use of sealant treatment according to socioeconomic factors still exist [8,9]. This may be attributed to the fact that government-led sealant projects are rapidly curtailed or abolished after the reimbursement [10], resulting in a decrease in the supply of sealant to vulnerable groups.
On the other hand, the permanent caries prevalence rate in 12-year-olds decreased slightly by 0.9% over 3 years, but the decrease observed from 7.8% in 2015 (about 5 years after sealant reimbursement) [11] to 6.9% in 2018 (about 8 years after the reimbursement) [12] was not significant.
Given these phenomena, it is necessary to evaluate the effect of sealant treatment after the implementation of the reimbursement policy for children and adolescents on the change in their oral health indicators. Sealant is also one of the first preventive services in Korea to be included in a reimbursement policy. An assessment of its usefulness is required using various indicators, given that dental prevention services are provided as health insurance benefits. Therefore, this study aimed to identify the effectiveness of the sealant reimbursement policy for children and adolescents by comparing the differences in first-molar oral health indicators between non-beneficiaries and the eligible individuals who did not receive benefits during the extended sealant coverage, as well as differences according to sociodemographic characteristics.

Study participants
In this study, we obtained approval to analyze the raw data from the Korea National Health and Nutrition Examination Survey (KNHANES), which can be accessed through its website [13]. The KNHANES is a national survey that identifies the current state and related trends of the health and nutritional status of the population, establishes target indicators and evaluation data for the Health Plan, and calculates suitable health indicators for comparison with other countries [13]. In order to improve the representativeness of the sample and the accuracy of the estimations, the sample area was extracted using the multi-stage stratified colony probability extraction method, which is a complex-sample design method [14].
For the analysis of data in the present study, the fourth period (2007-2009) of the KNHANES was selected as the dataset before the sealant reimbursement policy, and the sixth period (2013-2015) as the dataset after the reimbursement policy was implemented.
Participants' age was selected to be 11-20 years in order to compare oral health indicators after 5-6 years after policy implementation in 2010, based on an age of 6-14 of those who received reimbursement benefits.
The target tooth was selected as the first molar, which is covered by the sealant reimbursement policy. An indicator was created by converting the code of the "tooth condition" in the "oral examination" section of the KNHANES examination. The dependent variables were first molar with or without sealant, permanent caries, and single crown.

Statistical analysis
Statistical analysis was conducted using SPSS statistical package. The proportion of individuals with first-molar sealant, decay-missing-filled permanent teeth, and singlecrown among individuals aged 11-20 years were calculated, and a complex-sample chisquare test was conducted to identify significant differences according to the factors under investigation. In addition, the average number of instances of sealant, decaymissing-filled permanent teeth, and single crown for the first molar in participants aged 11-20 years were calculated. In order to compare the difference between non-insured and insured households regarding sealant treatment, and to compare and analyze the differences according to the related factors, a descriptive statistical analysis considering complex samples for each independent variable was conducted using the complex samples general linear model.  (Table 1). was higher in women, younger age groups, for those whose father had higher education, and those with higher household income. Age, household income, and parental status were significant factors (p < 0.000). Mother's education level was less different among the non-beneficiaries. In the beneficiary, the higher the mother's education level, the higher the sealant retention, and the association was significant (p < 0.05).  DMFT index) was higher in women, in the older age group, those living in the city, and those whose father's education was lower. In the non-beneficiaries, the association was significant for gender and age group, and in the beneficiaries, for father's education.  Table 4 shows the application of sealant reimbursement among Korean children and adolescents, and the differences in single-crown treatment according to sociodemographic characteristics.
Compared with the non-beneficiaries, the SC rate fell by 2.7% (8.7% vs 6.0%). The SC index per person decreased by 0.03, with 0.11 for the non-beneficiaries and 0.08 for the beneficiaries, but the difference was not significant. Single-crown holdings were higher in females, those in the higher age group, and those with a higher level of father's education. Prior studies have shown that high partial copayments will be particularly burdensome for low-income individuals [18]. The relationship between socioeconomic factors and sealant has weakened after the reimbursement policy, but still existed [6]. In this study, there was a significant difference in sealant retention according to parental education and household income. Similar studies have reported that household income is associated with a higher sealant experience. [19].
Meanwhile, some developed countries provide free preventive dental services for children and adolescents. In the French dental system, children and adults are eligible for free preventive dental services every 3 years from 3 to 24 years of age [20]. In addition, Sweden regards children's dental care as part of the nation's universal welfare, and services for dental health for children and adolescents up to 19 years of age are provided free of charge [21].
In 2012, Korea piloted the family dentist system for vulnerable children under 18 years of age living in Seoul for the first time [22]. Since then, this system has been introduced and implemented in some areas. The family dentist system in Korea is a scheme that provides dental medical services such as oral examination, preventive care, and treatment in connection with the public health center and local dental clinic for low-income children [23]. This system has shown positive effects in terms of oral health awareness and behavior, and it is also very positive that students who have low access to dental healthcare and who do not receive dental services can be beneficiaries of the system [24].
Since 2020, the family dentist system for children has been piloted as a government-led project. In addition, since October 2017, deductibles have been reduced from 30-10% because of sealant reimbursement [25]. Nevertheless, in order to provide dental preventive services to vulnerable groups who do not benefit from the reimbursement system, schemes such as the family dentist system for children should be promoted, and measures to supplement the limitations and minimize problems between the systems should be implemented.
The proportion individuals with decay-missing-filled permanent teeth decreased by about quarter-fold in the beneficiaries compared with the non-beneficiaries, 2.09 in the nonbeneficiaries and 1.57 in the beneficiaries. In addition, the single-crown retention rate decreased 2.7% (8.7% for the non-beneficiaries to 6.0% for the beneficiaries). The number of single crowns per person decreased by 0.03 (from 0.11 to 0.08), but this was not statistically significant. In other words, both the occurrence of caries and fixed dentures decreased, but these were not significant changes when compared to values before the reimbursement policy. In addition, according to the outpatient ranking of multi-frequency diseases in Korea, dental caries in 2010-2018 ranked 6-7th, which indicates no significant change over 8 years [26]. It is supposed that dental caries are decreasing in children and adolescents, but the caries that have advanced to adulthood and need treatment have not been reduced.
As a result, there seems to be a need to determine how long sealant treatment in children and adolescents can last into adulthood. Some previous studies have shown that the retention and lifespan of sealants act as a beneficial factor in the prevention of tooth decay [27,28]. That is, after sealant treatment, if left out partially, the risk of caries seems inevitable. Choi et al. insisted on the factors that can improve retention of sealant treatment and asserted the importance of follow-up management to institutionalize and manage return visits after treatment [29]. Although sealant is less costly and more effective as a preventive policy than the expensive post-treatment of caries, the caries prevention effect of sealant will not last into adulthood without considering the loss and maintenance of sealant. In the future, a follow-up system that can ensure sealant retention will need to be implemented.
This study was meaningful as it confirmed the change in oral health indicators and the difference of sociodemographic factors after the sealant reimbursement policy was implemented. However, there were some limitations. First, we could not track the age of the participants limited to cross-sectional studies; moreover, there were restrictions in identifying causal relationships, although associations between oral health indicators and sociodemographic characteristics were identified. In the future, cohort studies are needed to track the experience with sealant treatment to confirm changes in oral health indices and promote sealant maintenance.

Conclusions
Sealant treatment for the first molar in children and adolescents aged 11-20 years was significantly higher in reimbursement policy beneficiaries compared with nonbeneficiaries, and caries was significantly lower. In addition, differences in sealant and caries indicators according to sociodemographic characteristics were also identified. Therefore, a system to prevent and treat dental caries based on universal health coverage should be established, and a follow-up system is needed to monitor the retention of sealant.

Consent for publication
"Not applicable" Availability of data and materials