Fluorinated hydroxyapatite

Lack of carbonated or fluorinated hydroxyapatite. The HAP formation by the reaction in Eq. 13.13 still does not produce a composition exactly the same as that of the bone. Bone contains carbonated and fluorinated apatite, or dahUite, and it is difficult to mimic such a composition by chemical reactions. 

There are a certain number of options to control and reduce dental caries, the biggest problem in tooth care. The use of fluoride salts is one of the most effective methods to prevent or slow down demineralization that causes tooth decay [16,17]. The action of fluoride can be explained by its antimicrobial action, its interaction with enamel to form a fluorinated hydroxyapatite compound (hydroxyfluorapa-tite or fluorapatite Ca5(P04)3F) by substitution of an hydroxyl ion in hydroxyapatite Ca5(P04)3(0H), which is more resistant to add than enamel on its own, and its repairing effect by formation of calcium and phosphate, which ranineralize the tiny lesions in which caries begin. 

Once fluoride ions react with bone, they are not easily dissolved out or exchanged by other elements. If bone is buried for long periods of time, the relative amount of fluorine in the bone gradually increases as a function of time the "fluoridation" process continues until the maximum amount of fluorine (necessary to convert all the hydroxyapatite to fluorapatite) is reached. The total concentration of fluor in carbonated fluorapatite can reach levels as high as above 3%. There is ample room, therefore, for an increase in the relative amount of fluorine in buried bone. Determining the relative amount of fluorine in buried bone may thus serve as a tool for dating bone. 

F. Freund, R.M. Knobel, Distribution of fluorine in hydroxyapatite studied by infrared spectroscopy, J. Chem. Soc. Dalton 11 (1977) 1136-1140. 

G. C. Maiti, F. Freund, Influence of fluorine substitution on proton conductivity of hydroxyapatite, J. Chem. Soc. Dalton Trans. 4 (1981) 949-955. 

H.W. Kim, J.C. Knowles, L.H. Li, H.E. Kim, Mechanical performance and osteoblastlike cell responses of fluorine-susbtituted hydroxyapatite and zirconia dense composite, J. Biomed. Mater. Res. 72 (2005) 258-268. 

The incorporation of fluoride in place of hydroxyl groups is chemically straightforward [59,60] and, as we have seen, results in a substance of greater resistance to acid attack. This is partly due to the greater electronegativity of fluorine, which means that the electrostatic attraction between Ca + and F is greater than that between Ca + and OH. As a result, the fluoridated apatite lattice is more stable than hydroxyapatite [61-63]. It is also more crystalline [64]. 

The potential of other nuclei for the study of surfaces is yet to be explored. Gottlieb and Luz (388) measured 2H spectra of a number of perdeuterated molecules adsorbed on active alumina and interpreted the results in terms of quadrupolar tensors. Yesinowski and Mobley (369) have shown that 19F MAS NMR can provide useful information about fluorinated surfaces of calcium hydroxyapatite, Cas(0H)(P04)3. In particular, l9F, 2H, and H MAS NMR may become powerful techniques for the study of interface systems in general. 

In humid environments, hydroxyapatite (Ca5(P04)30H), the main component of the inorganic bone and tooth matrix, is transformed into the more stable fluor-apatite (Ca5(P04)3F). In an idealized sample, fluorine uptake from the environment leads to a U-shaped concentration profile, which slowly develops into the bulk from the outer surface and from the marrow cavity inwards according to Fick s second law... 

The F content in recent bone or dentine apatite is normally less than 0.1 wt.%. For ancient specimen, F is known to diffuse during burial into bone material. Its enrichment is generally a part of many complex diagenetic changes of bone and tooth, which remains after their deposit. Fluorine can react with the bone and dentine mineral phase to form calcium fluoride compounds. It usually substitutes for hydroxyl ions in hydroxyapatite, leading to the less soluble fluorapatite compound (Ca10(PO4)6(F)2, FAP). 

Fluorine is widely distributed in Nature, representing about 0.065% of the earth s crust, making it the 13th most abundant element. It is more abundant than chlorine and much more abundant than common metals such as zinc and copper. Fluorine occurs in many minerals in which fluoride ion replaces hydroxide. The conversion of hydroxyapatite to flu-oroapatite strengthens tooth enamel. However, this would result in an increased brittleness in bones. An untested theory is that the widespread use of fluoride in drinking water, which had remarkable benefits in preventing dental caries in the 1960s, may be the cause of the rise in osteoporosis in the elderly population. 

Fluorine (F) and its metabolites are of importance in protecting teeth from caries. Fluorine is included in calcium hydroxyapatite, and it promotes the precipitation of calcium phosphate Ca(P03)2 and accelerates the remineralization. The necessary concentration of Fluorine added to drinking water to prevent caries is approximately 1 mg/L. Application of higher Fluorine concentrations (above 8 mg/L) leads to fluorosis. This is a disease that is characterized by a disturbance in the function of the thyroid gland. A long-term application of fluorine leads to intensive mineralization (possible precipitation of calcium sulfate), deformation of bones with possible accretion, and calcification of the connections. 

More recently - P- F REDOR has been used to examine the connectivity of the different atoms via the dipolar coupling (Pan 1995). The method was used to compare the effect of fluoride treatment of hydroxyapatite with fluoroapatite itself. The results suggest that a layer of fluoroapatite less than 1 unit cell thick is formed on the hydroxyapatite and illustrate the potential of this approach for more detailed modelling of the fluorine distribution in these materials. 

The most abundant natural sources of fluorine are the minerals fluorspar and cryolith (NajAlFs). Fluoroapatite (Ca5(P04)3F = 3Ca3(P04)2 CaF2 ) is, with hydroxyapatite (Ca5(P04)30H), a major component of tooth enamel, giving it its extreme mechanical strength and life-long durability. 

To counteract the high corrosion rate of Mg alloy implants electrochemically deposited coatings of fluorine-doped hydroxyapatite (FHAp) and brushite (DCPD) were developed by Bakhsheshi-Rad et al. (2014). As shown in Table 5.2 potentiodynamic polarisation measurements of uncoated and coated Mg-Ca alloys subjected to electrochemical corrosion in Kokubo s SBF (Table 7.8) confirmed the corrosion-resistant nature of the coatings. The anodic polarisation curve of the uncoated specimen shows a breakdown immediately after the... 

R., and Medraj, M. (2014) In-vitro corrosion inhibition mechanism of fluorine-doped hydroxyapatite and brushite coated Mg-Ca alloys for biomedical applications. Ceram. Int, 40 (6), 7971-7982. 

Elliott (1964) has stated that the frequency of the O—H band at 3570 cm in hydroxyapatite is lowered if fluorine partially substitutes for the hydroxyl ions. The shift depends on the amount of substitution, and is rather small a 50% substitution causes a lowering of only about 20cm However, he believed that the determination of the frequency of the O—H band to measure the amount of replacement of hydroxyl by fluoride ions in hydroxyapatite could be as sensitive as X-ray diffraction techniques and is not subject to the same limitation, namely, that the apatite must be well crystallized. 

Tooth enamel is composed of hydroxyapatite, whose simplest formula is Ca5(P04)30H, and whose corresponding ATjp = 6.8 X 10 . As discussed in the Chemistry and Life box on page 730, fluoride in fluorinated water or in toothpaste reacts with hydroxyapatite to form fluoroapatite, Ca5(P04)3F, whose = 1.0 X 10 . (a) Write the expression for the solubihty-constant for hydroxyapatite and for fluoroapatite. (b) Calculate the molar solubihty of each of these compounds. 

Two aspects of the crystal chemistry of natural and synthetic apatites need to be recognized. First, the HA in bone is nonstoichiometric, has a Ca/P ratio of less than 1.67, and contains carbonate ions, sodium, magnesium, fluorine, and chlorine (Posner, 1985a). Second, most synthetic hydroxyapatites actually contain substitutions for the phosphate and/or hydroxyl groups and vary tom die ideal stoichiometry and Ca/P ratios. Oxyhydroxyapatite [Ca,o(P04) 0], a-tricalcium phosphate (a-TCP), )3-tricalcium phosphate O-TCP) or )8-Whitlockite [Caj(P04)J, tetracalcium phosphate (Ca4P209), and octocalcium phosphate [Cag(HP04)2(P04)4 5H20] have all been detected via x-ray diffraction (XRD), Fourier transform into spectroscopy (FITR), and chemical analyses (Kohn and Ducheyne, 1992 Ducheyne et al., 1986, 1990 Koch et al., 1990). These compounds are not apatites per se since the crystal structure differs from that of actual apatite. 

Hydroxyapatite has the chemical composition Caio(P04)6(OH)2. In the body, its composition slightly differs from this formula (with the resulting material frequently called being dahlhte), for some calcium ions are replaced by other ions, and fluorine ions replace some of the (OH) ions. Because of their small size, it is rather difficult to determine the exact shape of the hydroxyapatite crystals in bone, which may also be different in different bones. Typically, they are platelet-shaped, with a thickness of only 5 nm and an edge length between 20 nm and 100 nm. These platelets are situated between the tropocollagen molecules (see figure 9.16). 

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