Fluoroapatite crystals

Figure 3-22 SEM image after etching (2.5% HF, 10 sec) showing needlelike fluoroapatite crystals in a glass-ceramic.

The crystal stmcture of the calcium fluoroapatite has two different crystallographic sites for the Ca " ion. The Ca(I) site has a threefold axis of symmetry and is coordinated to six oxygen ions at the vertices of a distorted trigonal prism. The Ca(Il) ions are located at the corners of equilateral... 

The charge formation on the surface of tri-ionic crystals (apatite) is still more complex. Samani et al.56) postulate Ca2+ and HPO4- as PDI of fluoroapatite pH controls the concentration of these ions in the solution and the hydrolysis rate of appropriate surface species. In this way H+ and OH- indirectly become PDI. DobiaS et al.57) came to the same conclusion, but they also include F- as PDI. Somasundaran58 takes H+, OH-, and phosphate ions for major PDI Ca2+ and F- have a major influence on the potential of fluoroapatite. 

The presence of even one fluoride ion in the crystal slows the transformation to amorphous calcium monohydrogen phosphate. Thus, in the presence of fluoride (e.g., after using fluoridated toothpastes), fluoroapatite forms at the tooth surface and reduces the rate of caries development. The increased fluoride concentration at the tooth surface also inhibits lactate production. These observations explain why cleaning the teeth with fluoridated toothpaste prevent caries. Cleaning the teeth exposes the apatite at the enamel surface. In the absence of fluoride, there is no protection because the biofilm forms within a few... 

Fig. 16.6 Hydroxyapatite and fluoroapatite formation and dissolution, (a) Hydroxyapatite is transformed to fluoroapatite by isomorphous replacement. Fluoride ions diffuse into a hydroxyapatite crystal where they replace the hydroxide ions, (b) Fluoroapatite cannot dissolve as easily as hydroxyapatite. Right to left shows the solid-state rearrangement of hydroxyapatite to calcium monohydrogen phosphate, free calcium ions, and monohydrogen phosphate. The latter becomes mostly dihydrogen phosphate above pH 6.2. Arrows between (b) and (a) indicate enhanced apatite formation or slower changes to the amorphous solid if fluoride is present. Left to right shows the precipitation of calcium monohydrogen phosphate and its change to hydroxyapatite if an acid solution is made alkaline.

Phosphates play an enormously important role in biological systems. The genetic substances deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are phosphate esters (see Figure 9.11). Bones and teeth are constructed from collagen (fibrous protein) and single crystals of hydroxyapatite, Ca5(0H)(P04)j. Tooth decay involves acid attack on the phosphate, but the addition of fluoride ion to water supplies facilitates the formation of fluoroapatite, which is more resistant to decay. 

Prymak, O., Sokolova, V., Peitsch, T., Epple, M. he crystallization of fluoroapatite dumbbells fixrm supersaturated aqueous solution. Cryst. Growth Des. 6, 498 (2006)... 

Importance of Calcium 333 Sources of Calcium 333 Calcium Absorption 333 Serum Calcium 333 Ossification and Bone Metabolism 334 The Apatite Crystal 336 Fluoroapatite Bone Apatite... 

In their publication, LeGeros and LeGeros (1993) oudine the different apatites, ranging from natural apatite (minerals) to biological (human dentin, enamel, and bone) and synthetic (chemically synthesized) apatite. This publication clearly shows that apatite is a group of crystalline compounds. The most important of these compounds is calcium hydroxyapatite. All the related crystal structures, such as fluoroapatite, chloroapatite, and carbonate apatite are derived from it. 

Fluoroapatite (F-apatite), chloroapatite (Cl-apatite) and carbonate apatite (CO -apatite) are derived from hydroxyapatite. In F-apatite and Cl-apatite, the F and Cl ions assume the position of the OH ions. When F and Cl ions are inserted in the Ca triangle, their position in relation to the OH in hydroxyapatite changes (Fig. 1-23). As a result, the lattice parameters change compared with those of hydroxyapatite (Young and Elliot, 1966). When F ions are inserted in place of OH ions, the -axis is reduced and the c-axis remains constant (F = apatite a = 9.382 A, c = 6.880 A). The insertion of Cl ions enlarges the unit cell (Cl apatite a = 9.515 A, c = 6.858 A). The crystal structure of fluoroapatite is shown in Appendix 19. 

Another significant difference between needlelike apatite in glass-ceramics and other apatites is found in the fluoride content. The crystal structure of fluoroapatite compared with other apatites is shown in Section 1.3.2. The crystal structure of fluoroapatite is shown in Appendix Fig. 19. 

In a plastic (organic) polymen covalent, disulfide (covalent), H-bonds and dispersion forces in ceramics, mostly ionic and network covalent 25.43 Fluoroapatite is less soluble than hydroxyapatite, particularly in acidic solutions. Dental fillings must also be insoluble. 25.45 The molecule is long, flat, and rigid, so it should form a liquid crystal. 

Fiuoride. Fluoride is the most effective agent available for strengthening tooth resistance to acid demineralization. The mechanism by which fluorine increases caries resistance of the teeth is not fully understood. However, it appears that crystals of fluoroapatite can replace some of the calcium phosphate crystals of hydroxyapatite that are normally deposited during tooth formation, and that it may also replace some of the carbonate normally found in the tooth. Apparently these fluoride substances are more resistant to mouth acids. Fluorine may also inactivate oral bacterial enzymes which create acids from carbohydrates. 

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