Our study found an association between S-FMR and children's oral health and the subsidy policy and children's oral health. However, no interaction effect existed between S-FMR and the subsidy policy. Our results are consistent with previous studies reporting improved oral health measures with S-FMR (8,19) and improved oral health measures with a decrease in the co-payment rate (20,21).
No association was found between S-FMR and the SCR of deciduous tooth extractions. S-FMR may have a preventive effect on tooth extractions caused by dental caries, but may not impact tooth extractions related to permanent teeth eruption. However, due to limitations in the available data, we could not definitively determine the cause of tooth extractions, which suggests that the effect of S-FMR on tooth extractions might be underestimated. Also, no association was found between S-FMR and the SCR of dental sealants. This could be because dental sealants are primarily applied to caries-free teeth immediately after the eruption. In areas with poor oral health status, even though S-FMR is introduced, many children may require treatment for decayed or previously treated teeth, making them ineligible for dental sealants. Or, many children receiving S-FMR may be less likely to be offered dental sealants because they have a low prevalence risk of dental caries and less frequent dental visits for treatment. The negative association between S-FMR and DMFT (decayed, missing, filled teeth) in 12-year-olds was found even after adjusting for the influence of the subsidy policy. This result reinforces that S-FMR is an effective universal approach to maintaining oral health in children (8).
A positive association between the subsidy policy and the SCR of deciduous tooth extractions was found. This finding was consistent with the previous studies that free out-of-pocket medical expenses encourage hospital visits (13,22). However, no association was found between the subsidy policy and the SCR of dental sealants. Promoting dental visits by the subsidy policy may increase the number of dental visits for treatment or by individuals who have decayed or treated teeth (not eligible for dental sealants). Additionally, no association was found between the subsidy policy and DMFT in 12-year-olds. This suggests that the effect of the subsidy policy on DMFT may be limited.
The interaction between S-FMR and the subsidy policy on all dependent variables was not found, possibly because of the different indicators of oral health with which each was associated: S-FMR was not associated with the SCR of deciduous tooth extractions but was negatively associated with DMFT. The subsidy policy was positively associated with the SCR of deciduous tooth extractions but not with DMFT. Our hypothesis that the subsidy policy was negatively associated with DMFT via a decrease in the SCR of deciduous tooth extractions was unsupported, although there was a very weak correlation (r= 0.19) between the SCR of deciduous tooth extractions and DMFT. Thus, although both interventions were expected to reduce economic barriers to access to care, there may be no interaction between the two. Researchers and governments must continue informing residents that S-FMR is an equitable and beneficial approach to maintaining oral health (23).
One implication is offered from this study. Many health interventions have a limited impact on vulnerable populations, such as those with low SES or poor health status, which leads to "inverse care laws" (24,25). While the medical-dental expense subsidy policy may have effects that apply to all populations, factors such as geographic access to clinics are also relevant to actual dental visits (26–28). Thus, the health effects of the subsidy policy among vulnerable populations may be limited. Whereas, S-FMR could help vulnerable populations because it targets entire institutions, such as schools, and may reduce DMFT, one of the indicators of oral health at the community level.
The strength of this study is that the validity of the results is ensured as the data targeted the entire population of Japan. The study had some limitations. First, causal relationships are unknown because this ecological study used cross-sectional data. An ecological study can lead to the ecological fallacy that associations observed between variables at the aggregated level do not necessarily represent associations at the individual level (29). However, this can be avoided when previous studies have shown causal relationships at the individual level (30), such as the association between subsidy policies and oral health (31), and S-FMR and oral health (8). Therefore, the potential for ecological fallacy in this study is limited.
Second, using SCR variables to indicate children's oral health in the community may not have been appropriate because the NDB-open data are only from patients who visited dentists. While the NDB database is highly representative because it includes data on all medical procedures, it does not include data on residents who do not use medical facilities. Therefore, reimbursement data is unlikely to accurately reflect local chronic disease prevalence rates for which residents have little awareness of the need for treatment. In fact, it has been found that medical visits are lower in areas with poor access to medical facilities (10,32,33). Additionally, in prefectures where universal approaches, such as S-FMR, have been successful, children may have fewer medical requirements for dental sealants. Therefore, the present study addressed this issue by examining the association between S-FMR and the subsidy policy, including DMFT, a dependent variable, other than NDB-open data.
Third, using DMFT as an indicator of children's oral health may not have been appropriate in identifying associations between the subsidy policy for children's oral health in the community because the subsidy policy may have decreased the number of D (untreated decayed teeth) and increased the number of F (filled teeth) among DMFT. Therefore, we conducted a sensitivity analysis using D, an indicator of untreated decayed teeth, as a dependent variable instead of DMFT. The results were similar to those in the main analysis (Supplementary Table 2). The subsidy policy does not appear to be associated with dental visits for prevention, at least in childhood.