Trends and multidimensional disparities in permanent teeth caries among primary and secondary school students in Henan Province, China, 2019–2025: a serial cross-sectional study

Dental caries remains one of the most prevalent chronic diseases among children and adolescents worldwide. Although China has implemented school-based oral health programs, long-term trends in caries prevalence and filling rates—particularly their dynamic relationship and age‑varying disparities—remain poorly understood. This study aimed to analyze seven‑year trends in permanent teeth caries and filling rates and examine the effects of gender, urban–rural residence, and age on caries risk among primary and secondary school students in Henan Province, China. This was a serial cross-sectional study comprising seven independent annual cross-sectional surveys conducted from 2019 to 2025 in Henan Province, China. A total of 1 804 129 students participated in annual oral health examinations using a stratified cluster sampling design. Caries prevalence, untreated caries rate (Decayed component only), and filling rate (Filled component only) were calculated. Temporal trends were analyzed using the Cochran‑Armitage trend test. Multivariable binary logistic regression was performed to examine main effects and two‑way and three‑way interactions (rural × age, female × age, female × rural, and female × rural × age) on caries risk, adjusting for survey year. Caries prevalence among secondary school students showed an initial increase from 29.44% in 2019 to 30.33% in 2020, plateaued during 2021–2022 (29.73–29.79%), and then declined steadily to 25.85% by 2025 ( χ² trend =1554.085, P < 0.001). Untreated caries followed a similar pattern ( χ² trend =1828.321, P < 0.001). However, the filling rate remained persistently low at 4.11–5.12% with no significant change over the seven‑year period ( χ² trend =2.006, P = 0.157). Multivariable analysis revealed that females ( OR = 1.432, 95% CI : 1.388–1.478) and older age groups ( OR for age ≥ 15 years: 6.213, 95% CI : 6.053–6.377) had higher caries risk, while rural residents ( OR = 0.815, 95% CI : 0.791–0.840) had lower caries risk. Significant interactions were observed: urban–rural disparities narrowed with age ( OR = 0.845 for age ≥ 15 years, 95% CI : 0.815–0.875), while gender disparities widened with age ( OR = 1.173 for age ≥ 15 years, 95% CI : 1.132–1.215 ). The three-way interaction (female × rural × age) was also significant ( OR = 1.070, 95% CI : 1.020–1.123, P = 0.005), indicating that the widening of gender disparities with age was marginally more pronounced in rural settings. Although permanent teeth caries prevalence among students in Henan Province declined after 2023, the filling rate remained critically low, indicating a substantial and potentially widening treatment gap. The novel findings that urban–rural disparities narrow while gender disparities widen with age suggest that intervention strategies should be age‑tailored, with particular attention to urban older students and adolescent females.