How fluoride protects dental enamel from demineralization

Introduction: How fluoride (F-) protects dental enamel from caries is here conveyed to dental health-care providers by making simplifying approximations that accurately convey the essential principles, without obscuring them in a myriad of qualifications. Materials and Methods: We approximate that dental enamel is composed of calcium hydroxyapatite (HAP), a sparingly soluble ionic solid with the chemical formula Ca10(PO4)6(OH)2. Results: The electrostatic forces binding ionic solids together are described by Coulomb's law, which shows that attractions between opposite charges increase greatly as their separation decreases. Relatively large phosphate ions (PO43-) dominate the structure of HAP, which approximates a hexagonal close-packed structure. The smaller Ca2+ and OH- ions fit into the small spaces (interstices) between phosphates, slightly expanding the close-packed structure. F- ions are smaller than OH- ions, so substituting F- for OH- allows packing the same number of ions into a smaller volume, increasing their forces of attraction. Dental decay results from tipping the solubility equilibrium Ca10(PO4)6(OH)2 (s) & x21d4; 10Ca2+ (aq) + 6PO42- (aq) + 2OH- (aq) toward dissolution. HAP dissolves when the product of its ion concentrations, [Ca2+]10x[PO43-]6x[OH-]2, falls below the solubility product constant (Ksp) for HAP. Conclusion: Because of its more compact crystal structure, the Ksp for fluorapatite (FAP) is lower than the Ksp for HAP, so its ion product, [Ca2+]10x[PO43-]6x[F-]2, must fall further before demineralization can occur. Lowering the pH of the fluid surrounding enamel greatly reduces [PO43-] (lowering the ion products of HAP and FAP equally), but [OH-] falls much more rapidly than [F-], so FAP better resists acid attack.