Fluoride in the drinking water and dental caries experience by tooth surface susceptibility among adults

Background: Dental caries is the most prevalent non-communicable health condition globally. The surface-based susceptibility hierarchy indicates that surfaces in the same group have similar susceptibility to caries, where the most susceptible group consists of occlusal surfaces of first molars and buccal surfaces of lower first molars, and the least susceptible surfaces are smooth and proximal surfaces of first premolars, canines and incisors. Therefore, fluoride in the drinking water could impact one group more than the other group. The present study examined the association between fluoride levels in the drinking water and dental caries experience in adults in the context of varying tooth surface susceptibility. Methods: Data from the cross-sectional National Lithuanian Oral Health Survey conducted in 2017-2019 included a stratified random sample of 1398 35-74-year-olds (52% response rate). Dental caries experience in dentine was measured at a surface level. The surfaces were grouped according to their caries susceptibility (group 1 being the most and group 4 the least susceptible), and dental caries experience was calculated separately for each susceptibility group, creating four outcomes. Information about explanatory variable, fluoride levels in the drinking water, was provided by the water suppliers. The questionnaire inquired about potential determinants: sociodemographic characteristics and oral health-related behaviors. Chi-square, Mann-Whitney U and Kruskal Wallis tests were used for descriptive statistics, and linear regression analyses to examine the association between fluoride levels and four outcomes. Results: The proportions of median decayed, missing, filled surfaces decreased following the surface-based susceptibility hierarchy (group 1-33%, group 2-28%, group 3-24%, group 4-15%). When adjusted for potential determinants, higher-level fluoride (>= 0.7 ppm vs < 0.7 ppm) in the drinking water associated with lower dental caries experience in all surface-based susceptibility hierarchy groups; Group 1: beta = - 0.23 (95 %CI - 0.44; - 0.001), Group 2: beta = - 0.44 (95 %CI - 0.82; - 0.07), Group 3: beta = - 1.14 (95 %CI - 1.88; - 0.41) and Group 4: beta = - 6.28 (95 %CI - 9.29; - 3.30). Conclusions: The higher-level fluoride in the drinking water associated with lower dental caries experience in adults and this was observed in all surface-based susceptibility groups. However, there is a need to validate the surface-based susceptibility hierarchy in longitudinal adult studies.