Caries Management - Science and Clinical Practice. Группа авторов
Чтение книги онлайн.
Читать онлайн книгу Caries Management - Science and Clinical Practice - Группа авторов страница 29
Название: Caries Management - Science and Clinical Practice
Автор: Группа авторов
Издательство: Ingram
Жанр: Медицина
isbn: 9783131693815
isbn:
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+,
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 basic units (unit cells). The diagram above shows the arrangement of calcium, phosphate, and hydroxyl groups in hydroxyapatite, as if looking down the long axis of a crystal. The outline of one unit cell, which has the formula Ca10(PO4)6(OH)2, is marked by a bold dashed line. The dotted triangles in middle mark groups of three Ca2+ ions which are rotated 60° in successive layers of the structure, forming a “channel” in which lie the OH− ions (cf. Fig. 2.8). The remaining calcium ions form two columns of single ions. For clarity, the ions are not shown to scale. In reality, the oxygen atoms would be much larger and would fill most of the space within the structure. (Modified from Brown WE, et al: Ann Rev Mater Sci 1976;6:213–235.) The table compares lattice ions and impurity ions that can take their place, to show the disparities in charge and size (except for fluoride).
The solubility of a particular form of calcium phosphate, as defined by the concentration of dissolved solid in a saturated solution, is not constant but varies according to the solution composition. The dominant factor in calcium phosphate chemistry is pH: the solubility of all calcium phosphates increases as the pH falls below 7. The solubility of hydroxyapatite increases particularly rapidly with pH, by a factor of ca. 10 per pH unit fall (Fig. 2.7).
Fluoride and Calcium Phosphate Chemistry
Fluoride has profound effects on solubility of hydroxyapatite and dental minerals. It is readily incorporated into the apatite structure because F− ions can replace OH− ions. Fluorapatite, in which all OH− is replaced by F−, is less soluble than hydroxyapatite at pH7 or below (Fig. 2.7). In hydroxyapatite crystals, OH− ions are located within channels formed by triangular groups of Ca2+ ions, but lie between the triangles25,26 (A in Fig. 2.8). In fluorapatite, the slightly smaller F- ion fits within the triangles (B in Fig. 2.8) and this is associated with a denser, more stable crystal structure. Partial substitution of F− for OH− produces fluorhydroxyapatites, which are thought to be stabilized by hydrogen-bonding between adjacent F− and OH− ions24 (C in Fig. 2.8).
Important though they are, the effects of fluoride on crystal structure are probably much less significant than the reactions which take place between crystal surfaces of apatites and fluoride dissolved in the bathing liquid. If fluorhydroxyapatite, or even pure hydroxyapatite—which of course contains no fluoride ions—is placed in an acidic solution containing low concentrations of fluoride, the rate of dissolution is lower than in one that is fluoride-free28,29 (Fig. 2.9). This phenomenon is due to replacement of OH− ions by F− ions at the crystal surfaces, probably not much deeper than one unit cell thickness.30 The F− ions stabilize the surrounding Ca2+ ions.30 Regions of the crystal surface in which F− has replaced OH− are in effect converted into fluorapatite, so are much less soluble than nonfluoridated regions.28,31,32 When exposed to acidic conditions, nonfluoridated regions of the crystal surfaces dissolve at the rate normal for hydroxyapatite, while the fluoridated regions will dissolve more slowly or not at all. The overall rate of dissolution is therefore slower than in the absence of fluoride, and the higher the concentration of fluoride in the ambient solution the greater the effect, because a greater proportion of the crystals surfaces is converted to fluorapatite. Thus, dissolution of tooth mineral СКАЧАТЬ