A three layer structure model of fluoridated enamel containing CaF2, Ca(OH)2 and Fap

Objectives The aim of this study was to develop a molecular-based structural model of human teeth after fluoridation with a commonly used amine fluoride, which is highly significant for understanding the effectiveness of topical fluoridation. Methods This multi method study used XPS, MAS-NMR and Raman-spectroscopy measurements in order to analyze powdered synthetic hydroxylapatite (HAp), powdered human enamel samples and human enamel pieces treated with amine fluoride (Elmex® fluid) in vitro. Results The results lead to a complete structural characterization of the fluoridation products. A three layer composition of calcium hydroxide, calcium fluoride and an apatite species was identified. Significance The top surface CaF2 layer acts as a fluoride reservoir and covers a layer of antimicrobial effective Ca(OH)2. Ca(OH)2 is a well-known therapeutic agent. However, up to now Ca(OH)2 has not been described as a reaction product after topical fluoridation. Below the Ca(OH)2 layer an acid resistant apatite species (FAp) was detected which reached directly into the bulk enamel HAp species. The three layer composition identified helps to understand the influence of fluoride application in the pathogenic mechanisms of tooth decay. Each component in this newly suggested structure model has a specific function, which explains how topical fluoridation of enamel reduces dental caries and influences its pathogenic mechanisms.