Fluoride provides protection against caries at three levels:
• it increases the resistance of enamel to demineralisation
• it enhances the reservoir of ions for remineralisation
• it interferes with bacterial cell metabolism in plaque
It is now accepted that constant exposure to low levels of fluoride in drinking water and toothpaste will have an effect on enamel throughout life. Note that fluoride in toothpaste may be even more beneficial than drinking fluoridated water. However, the effect of fluoride is reduced as the pH of the saliva falls and this has important implications when interpreting the inter-relationship of the risk factors.
The fluoride ion will not only prevent initial lesions developing, but will also stabilise established lesions. Fluoride can -
• contribute to remineralisation of incipient enamel caries
• partly remineralise carious dentine and thus slow down or arrest the caries process in the cavitated coronal lesion
• remineralise root surface lesions to the extent that restoration may not be necessary
Topical fluoride is more effective in inhibiting smooth surface caries. It is less effective in fissure or interproximal caries because of the difficulty of removing stubborn or mature plaque. Daily application of topical fluoride to demineralised root surfaces over a period of 2-4 months will lead to significant hardening of the exposed dentine indicating that a remineralising balance has been established. The surfaces of such remineralised lesions can become glass-like in texture, as a result of this hypermineralisation.