Discussion

In the present study, the posttreatment effects of FMR on dental caries in permanent teeth were apparent in Japanese university students aged between 18 and 25 years. Previous studies have also reported the preventive effects of FMR on dental caries in the permanent teeth of Japanese adults aged 20 years.¹⁶ The trend of increasing market share of fluoride toothpaste in Japan could have influenced the prevalence of dental caries in the permanent teeth of the subjects because age was found to be a statistically significant predictor of dental caries in the present study.¹⁷

Kishi et al.¹⁸ reported that 20-year-old Japanese adults who had participated in a daily 0.05% sodium FMR between 4 and 11 years of age and in a weekly 0.2% sodium FMR between 12 and 14 years of age presented with significantly lower caries than those who did not participate in such programmes (56%-71% difference in the mean DMFS). The former had fewer decayed tooth surfaces than the latter; the decay in the free smooth tooth surfaces especially was less than in the pits and fissures. These results in 20-year-old adults indicate that participation in FMR for 11 years between the ages of 4 and 14 years conferred posttreatment benefits.

In a study to evaluate the long-term caries-preventive effects of FMR, 637 subjects aged 20 to 39 years were divided into 4 groups: subjects who had participated in the programme from nursery to junior high school, those who had participated only in elementary school, those who had no experience of the programme, and others whose experience of the programme was unclear. Multiple regression analysis between the mean DMFT and the groups showed that those who participated in the programme from nursery to junior high school and those who participated only during elementary school had a negative association with the mean DMFT.¹⁶

In the present study, however, it was difficult to assess the preventive effect of preschool FMR because the programme timelines amongst the subjects were complicated. Whilst the World Health Organisation has emphasised that fluoride
mouth rinse is to be avoided in children younger than 6 years due to the risk of dental fluorosis resulting from daily ingestion of a certain amount of fluoride,6 the guidelines from the Ministry of Health, Labor, and Welfare of Japan recommend
the use of a fluoride mouth rinse from the age of 4 years.¹⁰ The present study failed to ascertain that the caries-preventive effects of FMR started at preschool were better than those of FMR started at elementary school; the benefits of the FMR started at preschool seemed to have disappeared. The association between age and dental caries in terms of the DMFS score was found to be statistically significant in this study, which is inevitable because dental caries is an irreversible and progressive disease. Ohara et al.¹² reported that the introduction of FMR had a positive effect on the prevention of dental caries based on the DMFS rates of the first mandibular molars of 12-year-old children. Greater differences in caries rates may be attributed to the age at the initial fluoride mouth rinse; 85% of the mandibular first molars would benefit from the fluoride immediately after eruption if the mouth rinse was started at 5 years, whilst only 25% of the molars would benefit if the rinse was started at 6 years. Besides, because fluorine is not available from general intake routes, such as fluoridated water, tablets, and foods, in Japan, dental fluorosis is not expected to be a problem.^7,19-20 Multiple regression analysis based on data from 1737 sixth graders in elementary schools comprising 67,672 schoolchildren obtained from the Dental Health Database of Niigata Prefecture, Japan, indicated  that FMR in preschool facilities, kindergartens, and elementary schools were statistically significant variables.²¹ Therefore, to prevent dental caries in the pit and fissure surfaces, especially of the first molars, it is recommended to start the FMR for children younger than 6 years.

Caries reduction by FMR at least during elementary school on different tooth surfaces indicates a more detailed impact of programme participation on the dental needs of adults. In this study, the free smooth surfaces had the highest percentage reduction in DMFS. Dirks22 pointed out that the repair of cavities in these surfaces seems relatively simple, but the durability of these restorations is much lower than those of fissure fillings. Moreover, there is a great danger to the integrity of the tooth and surrounding tissues. The percentage reduction in DMFS on proximal surfaces was intermediate, and their repair is more time-consuming than that of other types of fillings.22 The percentage reduction in DMFS was the lowest in the pit and fissure surfaces; dental caries on these surfaces tend to develop soon after the tooth erupts and progress rapidly. If dental caries in the pit and fissure surfaces remain incipient because of the programme, a preventive fissure sealant is applied instead of an operative treatment with filling materials.²³ The percentage reduction in the DMFS in the pit and fissure surfaces was the lowest, a 27.9-point difference compared to that for the free smooth surface. Comparing the mean values, the difference in reduction was 0.30 for surfaces and 41.1% for the mean DMFS. The benefit of fluoride was the lowest for the pit and fissure surfaces.

This study has a few limitations. First, it is observational and not experimental. Neither the subjects nor the preschool facilities, kindergartens, and elementary and junior high schools were randomly assigned to receive the programme. Second, although data on the FMR and individual use of fluoride-containing toothpastes were obtained, exposure to other sources of fluoride, such as fluoride in drinking water—which is uncommon in Japan—and topical application of fluoride in dental clinics, was unknown. Third, the programme timelines were complex; some children might have had only limited exposure to the programme because of the mobility of their families. This could likely contribute to the weakening of the preventive effect of the programme. In this study, it was impossible to obtain data regarding the socioeconomic inequalities amongst the subjects. However, all subjects belonged to the same university; thus, their educational and socioeconomic backgrounds during childhood were considered comparable. Finally, selection bias could limit the scope of our findings.

Indeed, participation in FMR at least during elementary school is not likely the most important preventive measure for the reduction of dental caries in adults, as FMR may only relate to a caries-preventive effect at the time. The subjects’ continuous fluoride exposure for their entire lives, such as using fluoride toothpaste with appropriate tooth-brushing as individual health practice, possibly through water fluoridation as a public health measure if it becomes feasible, and the other methods of delivering fluoride for the prevention of dental caries is inevitable.

In this study, FMR showed a significant benefit in caries prevention despite the decline in the prevalence of caries and increase in the use of fluoridated toothpaste. Moreover, although the effectiveness of decay prevention varied according to the tooth surface, the overall reduction in caries render the FMR beneficial. Multivariate analysis indicated that participation in FMR at least during elementary school, age and sex are statistically significant predictors for the reduction in the prevalence of dental caries in the permanent teeth.

Participation in an FMR at least during elementary school is a predictor for reduction in the prevalence of dental caries in permanent teeth in adults after investigating the posttreatment effects of FMR retrospectively and after adjustment for some confounding factors.

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