Caries Management - Science and Clinical Practice. Группа авторов

the more acidic plaque environment drives ecological selection of acidogenic, aciduric bacteria and this in turn increases the acidity of the plaque environment. Ultimately the ecological shift favors conditions acidic enough for caries to be initiated.

      Proponents of the specific plaque hypothesis recognize the powerful ecological effect of dietary sugar in determining the composition of plaque microflora, but would argue that only the increases in abundance of S. mutans are etiologically significant. However, while there is little doubt that S. mutans is a major agent of caries initiation,²² it is very likely that other acidogenic/aciduric bacteria play important roles in both initiation and progression of lesions.^14,15

      Fig. 2.4 Selected examples of population shifts in the microflora of plaque formed in situ, as a result of exposure to sucrose. “Sucrose” plaques were rinsed with 10% sucrose 6 times a day for 7 days while control plaques were exposed to equivalent numbers of saline rinses. Sucrose exposure results in reduced numbers of the nonacidogenic species, Streptococcus sanguinis, but increases in numbers of acidogenic species and of lactate-utilizing Veillonella. (Data from ref.¹⁸.)

       NOTE

      Caries is probably not a “classical” infectious disease, that is, one caused by a specific bacterium not normally found in the body. Instead, it is probably due to over-growth of acid-producing, acid-resistant members of the normal oral flora, driven by excessive consumption of sugars. However, some species, especially Streptococcus mutans, do have a prominent, well-documented role in caries etiology.

      Chemistry of Dental Minerals

      Solubility, Dissolution, and Crystal Growth

      The processes underlying the phenomena of demineralization and remineralization in caries are crystal dissolution and precipitation. In caries, the latter process is usually manifested as re-growth of partly-dissolved crystals, although precipitation of new crystals can occur. Dissolution and crystal growth are both surface-related processes. At the surface of a solid immersed in an aqueous solution (e.g., enamel crystals bathed in saliva), ions are constantly detaching from the surface and entering the solution and other ions are following the reverse path to become incorporated into the solid (A and B in Fig. 2.5). When the rates of these processes are equal, the solid is in equilibrium with the solution and no net dissolution or crystal growth will occur. In this situation, the solution is said to be “saturated” with respect to that particular solid. The concentration of dissolved solid in a saturated solution is a measure of the solid's solubility. When the solution contains less than the equilibrium concentration of dissolved solid it is said to be undersaturated and when the concentration of dissolved solid in solution is greater than at equilibrium, the solution is supersaturated. When in contact with an undersaturated solution, the rate of ions leaving the solid will tend to exceed that of ions leaving the solution. This means that the solid will tend to dissolve and that crystal growth is not possible. In a supersaturated solution, crystal growth will tend to occur, as more ions leave the solution and are added to the surface of the solid, but the opposite process of dissolution cannot take place. In this description, the use of “will tend to” rather than “will” is deliberate. The reason is that in the complex environment of the mouth, both dissolution and crystal growth can be heavily influenced by another surface-related process: adsorption to the crystal surface of ions or molecules that inhibit movement of ions between the solid and the solution (C in Fig. 2.5). Inhibitory substances, including macromolecules such as peptides and proteins (e.g., statherin) and low-molecular-weight substances such as the pyrophosphate ion, abound in biological fluids,²³ including saliva^1,2 and the interstitial fluid of plaque.

      Fig. 2.5 Crystal growth (A) and dissolution (B) as surface processes of exchange of ions/molecules between a solid surface and the bathing solution. On the right (C) is shown adsorption of a macromolecule, which blocks transfer of ions between the solid surface and the solution.

      In the foregoing an empirical definition of solubility is given. This is adequate for understanding the basis for demineralization and remineralization in caries, but it is important to understand that there exists a more sophisticated approach, based on fundamental principles of physical chemistry.²⁴ This approach is more generalized and allows predictions about dissolution and crystal growth in complex systems to be made. For example, it is possible to define quantitatively the state of a given solution with respect to dissolution and crystal growth of all possible solids by calculating for each one the degree of saturation (DS). This has a value of 1 in saturated solutions, >1 in supersaturated and <1 in undersaturated solutions. Furthermore, the greater the difference between the DS and the value of 1, the greater the potential chemical driving force for the respective process. However, it is difficult to exploit this approach fully because of uncertainties in defining the solubilities of the impure minerals found in dental tissues.

       NOTE

      Dental minerals are impure forms of a calcium phosphate—hydroxyapatite. They become rapidly more soluble as the pH of the aqueous environment falls. Hence, teeth lose mineral in response to pH falls due to acid production in plaque and can gain mineral when the pH rises again. Fluoride reduces solubility and dissolution of tooth mineral and promotes hydroxyapatite crystal growth, so exerts powerful preventive effects on the caries process.

      Minerals of Dental Tissues

      Dental hard tissues are composite materials in which crystals of mineral are intimately associated with an organic matrix. The mineral is a form of hydroxyapatite, a type of calcium phosphate which in its pure form has the formula Ca₅(PO₄)₃OH, and is the least soluble nonfluoridated calcium phosphate at neutral pH.²⁴ Hydroxyapatite belongs to a family of minerals (apatites) which share a similar crystal structure that is remarkable for its capacity for accepting substitutions of one ion for another.^25,26

      The composition of hydroxyapatite in dental hard tissues is altered by incorporation in the crystal structure of several “impurity” ions, especially magnesium, sodium, and carbonate (Fig. 2.6; Table 2.1), which originate from the tissue fluids during tooth formation. Impurity ions differ—in charge, size or both—from the Ca²⁺, , or OH⁻ ions which they replace (Fig. 2.6). These misfits disturb crystal structure and this in turn increases solubility. The exception to this rule is the fluoride ion, which both improves crystallinity and reduces solubility (see below). Enamel mineral contains fewer impurities than the mineral of dentin or cementum, the crystals are larger and more perfectly formed. Accordingly, enamel is only slightly more soluble than pure hydroxyapatite, while dentin is significantly more soluble,²⁷ although a reliable solubility has yet to be established.

      Fig. 2.6 In crystalline substances, the constituent ions or atoms are arranged in a regular, repeating array, which can be thought of as being made up of numerous