Trajectory of social inequalities in the treatment of dental caries among preschool children in Japan

Objectives: The longitudinal trend of dental health inequalities among preschool children has not been described. In this study, we aimed to measure the trajectory of both relative and absolute inequalities in caries treatment among preschool children in Japan. Methods: We used data from the Longitudinal Survey of Babies in the 21st Century (LSB21), which is an ongoing national representative longitudinal study. The target population was families residing in Japan with newborn baby/babies born between 10 January and 17 January or 10 July and 17 July 2001. Data regarding caries treatment history and socioeconomic status (SES) were collected using a self-reported questionnaire. SES was assessed on the basis of the parents’ educational attainment. In total, 35 260 children were followed from 2.5 to 5.5 years through annual surveys. To evaluate absolute and relative inequalities, we calculated the slope index of inequality (SII) and relative index of inequality (RII), respectively. Results: The rate of caries treatment at the age of 2.5 years was <10% for all SES groups; this increased to more than 30% at 5.5 years of age. Children with lower SES received more frequent caries treatment, and both absolute and relative social inequalities were statistically significant. SII showed a significant increase throughout the follow-up period, with values of 4.13% (95% confidence interval [CI], 3.16; 5.09) and 15.50% (95% CI, 13.68; 17.32) at 2.5 and 5.5 years of age, respectively. In contrast, RII decreased with an increase in the treatment rate for all groups, with values of 1.83 (95% CI, 1.59; 2.11) and 1.53 (95% CI, 1.46; 1.61) at 2.5 and 5.5 years of age, respectively. Conclusion: Our results suggest that the rate of caries treatment is higher for preschool children with lower SES in Japan, with significant widening of absolute inequalities along with the growth of the children.