Objective To evaluate the caries prevention effect of different fluoride application frequencies in 3-year-old children with active caries.

Methods A total of 337 3-year-old children with dental caries from 12 kindergartens in Changning District of Shanghai were divided into three groups： group 1 received fluoride application once every three months， group 2 received fluoride application once every six months， and the control group received no fluoride application. At baseline， parents completed a questionnaire survey， and oral examinations were conducted at baseline， 7 months， and 16 months after the fluoride intervention to assess the incidence of new dental caries， new dmft， and new dmfs of children. Multivariate logistic regression analysis was used to analyze the relationship between fluoride application and new dental caries， with new dental caries as the dependent variable and local fluoride application， child's gender， age in months， parents' highest education level， frequency of sugary intake （desserts， candies， sweet beverages）， age of toothbrushing initiation， frequency of daily toothbrushing， use of fluoride toothpaste， parental assistance in child’s toothbrushing， and daily use of dental floss as the independent variables.

Results There were no significant differences in baseline general conditions and oral health behaviors among group 1 （121 cases）， group 2 （123 cases） and the control group （93 cases） （all P>0.05）. After 16 months' intervention， three children were lost to follow-up due to transferring schools. The new caries rates of group 1 （120 cases）， group 2 （121 cases） and the control group （93 cases） were 5.83%， 24.79% and 52.69%， respectively. The incidence of new caries was significantly lower in group 1 compared to group 2 and the control group （P<0.001）， and it was also significantly lower in group 2 compared to the control group （P<0.001）. The newly-increased caries of group 1， group 2 and the non-fluoride control group were all 0.12 0.52， 0.36 0.68 and 1.16 1.39， respectively. The mean increment of new caries was lower in group 1 compared to group 2 （P=0.040） and the non-fluoride control group （P<0.001）， and it was lower in group 2 compared to the non-fluoride control group （P<0.001）. After adjusting for factors such as gender， parents' education level and child's oral health behaviors by multivariate logistic regression， the results showed that fluoride application once every three months could significantly reduce the risk of dental caries （OR=0.04， P<0.001）.

Conclusion Local use of fluoride has a certain effect on the prevention and treatment of dental caries in children with active caries. Fluoride application once every three months effectively reduces the incidence of new caries and slows down the progression of caries lesions. It is suggested that local application of fluoride should be implemented once every three months for children with active caries.