Caries Management - Science and Clinical Practice. Группа авторов

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СКАЧАТЬ prevented by low concentrations of fluoride in the environment of the tooth. A similar effect is thought to be responsible for the reduced solubility of fluorhydroxyapatites in fluoride-free acid.³³ Here, partial dissolution of the solid produces enough fluoride ions to convert the crystal surfaces to fluorapatite.³²

      Fig. 2.7 Variation of solubility of hydroxyapatite and fluorapatite with pH.

      Fig. 2.8 The crystal lattice of hydroxyapatite (see Fig. 2.6) contains channels formed by stacked triangles of Ca²⁺ ions (orange), with successive triangles being rotated by 60°. In hydroxyapatite (A), the OH⁻ ions (large white circles representing the oxygen + small black circle representing the hydrogen) are too large to fit within the Ca triangles. In fluorapatite (B), the F⁻ ions (green) fit within the triangles, because they are smaller than the OH⁻ ion, and a more compact structure results. In fluorhydroxyapatites (C), F⁻ ions occupy some of the OH⁻ sites and can form hydrogen bonds with adjacent OH⁻ ions (dashed lines); this helps to stabilize the structure.

      Fig. 2.9 Effects of fluoride (F) on hydroxyapatite (HA) dissolution. HA and hydroxyapatites modified with fluoride (FHA, SFA) were added to acetate buffer, pH 5.0, containing 0, 0.1, or 5mg/L of F. The dissolution process was followed by measuring the release of Ca with time. FHA1 and FHA2 contained, respectively, 44 and 602mg/g F in the crystal lattice, whereas SFA1 and SFA2 were samples of HA powder that had been treated with F solution and contained respectively 60 and 706 mg/g F, which was concentrated at the crystal surfaces. The results show, first, that fluoride incorporated in the crystal structure or adsorbed to the crystal surfaces reduces dissolution rate, since the rates of FHA and SFA in fluoride-free buffer were lower than that of HA; and second, that the presence of fluoride in solution reduced dissolution rate, even that of HA, which contained no fluoride at all. (Data from ref.²⁹.)

      The lower solubility of fluorapatite also affects crystal growth. If fluoride ions are available in a solution supersaturated with respect to hydroxyapatite, crystal growth is accelerated. The solid that forms will not be hydroxyapatite but fluorapatite or a fluorhydroxyapatite, depending on how much fluoride is available, so the product of remineralization in the presence of fluoride tends to be less soluble than the mineral which had been lost.⁷

      If teeth are exposed to high concentrations of fluoride, for example, by treatment with fluoride varnish, a form of calcium fluoride (CaF₂), probably combined with phosphate, may be precipitated on the tooth surfaces.³¹ The Ca²⁺ ions required for precipitation of the CaF₂-like material are derived from the tooth mineral, so more of the precipitate is formed at lower pH. Although its formation removes Ca²⁺ ions from the tooth surface, CaF₂-like material could have a beneficial effect: since it is relatively soluble, it can act as a fluoride reservoir, maintaining raised concentrations of Ca²⁺ and F⁻ ions in the tooth environment.

      On the basis of this evidence, the prevailing current opinion is that strategies for caries prevention using fluoride should be aimed at maintaining low but sufficient concentrations of fluoride ions in the environment of the tooth, rather than at increasing the fluoride concentration in the tooth mineral.³¹ This is achieved by topical methods of fluoride administration such as toothpastes, mouth rinses, varnishes, and also by water fluoridation which, even though originally intended to reduce solubility of tooth mineral, has a significant topical effect³⁴ (see Chapter 12).

      The Cariogenic Challenge

      Dental plaque is an example of a biofilm, a film of micro-organisms adhering to a solid surface. Biofilms exist in a wide variety of types adapted to different habitats. Life in a biofilm requires physiological adaptations on the part of the constituent microorganisms and also provides several advantages, for instance, protection against antimicrobial agents.³⁵ The structure of biofilms varies widely, but in the case of dental plaque the constituent bacteria are closely packed together, occupying ca. 75% of the volume.³⁶ The remaining volume is made up of a matrix comprising proteins, carbohydrate polymers, and other substances (Fig. 2.10). Many matrix components, such as extracellular polysaccharides, are largely of bacterial origin, but others, including several proteins, originate from saliva and gingival crevicular fluid.

      Because of the dense structure of dental plaque, movement of nutrients and metabolic end products between the oral cavity and plaque, and within plaque, is mediated by diffusion, which is a relatively slow process (Fig. 2.11a). One consequence is that availability of nutrients or antibacterial substances will not be uniform but will vary with depth. For instance, when a nutrient is ingested, a gradient will be set up within the plaque (Fig. 2.11b), with the concentration falling toward the interior, and bacterial metabolism will steepen this gradient because utilization near the outer surface will make less nutrient available for inward diffusion (Fig. 2.12). This phenomenon has important effects on plaque ecology. For instance, most plaque bacteria are anaerobic (surviving only in the absence of oxygen), such as Veillonella or facultative (preferring to live in absence of oxygen), such as streptococci and Actinomyces, probably because oxygen is consumed by aerobic bacteria such as Neisseria at the plaque surface, and none reaches the inner plaque.³ A similar process probably limits exposure of plaque bacteria to antimicrobial agents, because such agents will be immobilized by strong interaction with bacteria in the outer plaque, and the concentration reaching the inner plaque may be too low to be effective. Metabolic end products clear slowly from plaque because their movement is similarly regulated by diffusion. These phenomena are central to the process of dental caries because they control the different phases of the cariogenic challenge.

      Fig. 2.10 Diagram to represent some aspects of plaque structure. Tooth surfaces exposed to saliva are covered by an acquired pellicle consisting of adsorbed salivary proteins. Initial bacterial colonists attach to the pellicle, and the plaque increases in bulk and complexity by attachment of further bacteria to the initial colonists. Attachment is mediated by specific receptors on bacterial surfaces (indicated by geometrical lock-and-key shapes). However, plaque cohesion is further enhanced by nonspecific interactions (dotted lines), e.g., calcium bridging, and by interactions between polymers forming the plaque matrix: proteins and extracellular polysaccharides (EPS) such as glucans.

      Fig. 2.11a, b Diffusion.

      a A single molecule (small red circle) in solution makes frequent small ‘jumps’ (single arrows). Even though each СКАЧАТЬ