Residual factors associated with poor oral hygiene among Japanese kindergarten children: a cross-sectional study

Objectives To identify behavioural and household factors associated with poor oral hygiene among Japanese kindergarten children in a population with high health awareness, using the Debris Index-Simplified (DI-S) as a clinical proxy for early oral hygiene deterioration. Design Cross-sectional study. Setting Seven kindergartens in Sapporo city, Japan. Participants Of the 1229 kindergarten children invited, 871 provided parental consent (consent rate: 70.9%). Among them, 675 children aged 1–6 years who completed both the questionnaire and oral examination (completion rate: 54.9%). Most post-consent losses were due to logistical and staffing constraints. Children were stratified into ≤3 year and ≥4 year academic classes. Primary and secondary outcome measures The primary outcome was oral hygiene status based on the DI-S scores (categorised as good (DI-S=0) or poor (DI-S>0). The secondary outcome was the presence of dental caries, defined as decayed, missing and filled primary teeth: dmft≥1. Multivariable logistic regression was used to estimate associations between poor oral hygiene and behavioural and household factors. Results Among the 675 children, 168 children (24.9%) exhibited poor oral hygiene (DI-S>0) and 89 children (13.2%) had dental caries (dmft≥1). In multivariable analysis adjusted for demographic, dental and dietary variables, poor oral hygiene was significantly associated with being from a multiple-child household (OR 1.67, 95% CI 1.16 to 2.42) and irregular juice consumption (OR 1.60, 95% CI 1.07 to 2.38). Age-stratified analysis revealed that these associations persisted among those in the ≥4 year class, with additional significance for infrequent tooth brushing (<2 times/day; OR 1.81, 95%CI 1.00 to 3.26). Sensitivity analysis using dmft revealed that the significant associations differed: male sex, age and irregular snacking were significant, while household composition and juice consumption were not. Conclusions In a high health awareness Japanese preschool population, poor oral hygiene was independently associated with household composition and juice consumption, rather than traditional dental behaviours. These findings suggest the need to broaden preventive strategies beyond routine dental guidance to include family structure and dietary patterns, particularly in low-prevalence settings. Differential associations between DI-S and caries experience emphasise the utility of early clinical indices in oral health promotion.