What is the primary mechanism by which fluoride reduces caries incidence?

• A. Enhances remineralization of subsurface enamel and inhibits demineralization; also inhibits bacterial metabolism to some extent.
• B. Increases enamel hardness without affecting remineralization.
• C. Acts solely as an antibacterial agent with no remineralization effect.
• D. Creates a barrier on the enamel surface.

Answer: (A)

Fluoride reduces caries mainly by promoting remineralization of the enamel and making it harder to demineralize, with a secondary bonus of dampening bacterial acid production. When acid attacks occur, fluoride helps redeposit calcium and phosphate into the subsurface lesion, forming fluorapatite or fluoridated hydroxyapatite, which is more resistant to acid and less soluble than normal enamel. This shifts the balance toward mineral gain in the affected area, especially beneath the surface, aiding repair of early lesions. At the same time, fluoride lowers the overall solubility of enamel crystals, enhancing resistance to future acid attacks. While fluoride can also inhibit some bacterial metabolism, reducing acid production by plaque bacteria, this antimicrobial effect is secondary to its remineralization and acid resistance actions. It does not merely create a surface barrier or harden enamel without remineralization.