Early twice-daily toothbrushing and adolescent dental caries: a causal inference analysis using the Longitudinal Study of Australian Children

BACKGROUND: Dental caries is a highly prevalent chronic childhood condition, yet longitudinal evidence linking early oral hygiene behaviours to long-term outcomes remains limited. This study estimated whether twice-daily toothbrushing at age 2-3 years was associated with lower incident dental caries through adolescence, under a target trial emulation framework. METHODS: Data were from the Longitudinal Study of Australian Children (LSAC) B-cohort (N = 5,107). Children with caries at or before Wave 2 (age 2-3) were excluded, yielding an incident sample of 2,970. Associations were estimated using multivariable logistic regression, propensity score matching, inverse probability of treatment weighting (IPTW), targeted maximum likelihood estimation (TMLE), and g-computation. The outcome was carer-reported dental caries. Confounders were identified via directed acyclic graph (DAG) analysis. Effect modification by area-level socioeconomic position (SEP) was assessed through a pre-specified equity analysis. RESULTS: Among 2,941 children in the analytic sample, 1,559 (53.0%) developed carer-reported caries by age 14-15. Caries incidence was lower among children who brushed twice daily: 660 of 1,359 (48.6%) versus 899 of 1,582 (56.8%) in the less frequent group. The primary adjusted odds ratio (aOR) was 0.73 (95% CI: 0.63-0.85, p < 0.001). A dose-response gradient was observed (p-trend < 0.001). Effect estimates were similar across analytical approaches (OR range: 0.72-0.75). In non-disadvantaged areas, protective associations were observed (aOR = 0.65, 95% CI: 0.54-0.79). In disadvantaged areas, the association was weaker and not statistically significant (aOR = 0.91, 95% CI: 0.71-1.17; interaction p = 0.035). An exploratory falsification analysis using dental pain yielded a null result (aOR = 1.06, 95% CI: 0.91-1.23, p = 0.47), but this outcome was interpreted cautiously because dental pain may lie downstream of caries. CONCLUSIONS: Twice-daily toothbrushing by age 2-3 years was associated with a lower risk of incident adolescent caries. Estimates from multiple causal inference approaches were consistent with an absolute risk reduction of approximately 8% points (NNT ≈ 13), under assumptions of no unmeasured confounding, adequate positivity, and correct model specification. Protective associations appeared concentrated in non-disadvantaged areas, suggesting that oral hygiene promotion should be combined with broader preventive strategies addressing structural determinants of oral health.