Trajectory of inequalities in children’s and adolescence's caries in Japan

Objectives: Geographical inequalities in oral health, particularly in children’s and adolescence’s caries have been reported in Japan. However, their longitudinal trajectories have not been evaluated. We aimed to examine the trajectory of geographical inequalities in relation to caries prevalence in childhood and adolescence between all 47 prefectures in Japan.
Methods: This ecological study used national survey of caries prevalence age at 1.5-year-old in 2011, 3-year-old in 2013, and 5-year-old in 2015 as childhood, and caries prevalence of age at 12 to 17-year-old as adolescence between 2010 and 2017 among all 47 prefectures in Japan. We used caries prevalence as dependent variable and mean prefectural income per capita as independent variable between each periods. We calculated inequalities in 1.5 to 5-year-old children’s and 12 to 17 year-old adolescent’s caries prevalence using the Slope Index of Inequality (SII) and Relative Index of Inequality (RII), which are measures of absolute- and relative-health inequality, respectively.
Results: Between 1.5 to 5-year-old, the median of caries prevalence increased 2.2% to 39.2%, and the SII increased from 1.37 (95% confidence intervals [CI]: 0.89, 1.86) to 17.30 (95% CI: 11.52, 23.06). In contrast, the RII decreased from 1.92 (95% CI: 1.45, 2.86) to 1.62 (95% CI: 1.34, 2.05). Between 12 to 17-year-old, the median of caries prevalence increased 48.7% to 59.3%, and the SII and RII decreased from 15.03 (95%CI: 7.93, 22.14) to 8.17 (95% CI: 2.21, 14.12) and from 1.37 (95% CI: 1.17, 1.68) to 1.15 (95% CI: 1.04, 1.30), respectively.
Conclusions: Inequality measurements of longitudinal trajectory of caries prevalence were varied by age and measurement type (absolute and relative). Because they are affected by longitudinal increment of caries prevalence, interpretations should be carefully done in public health sectors.