Fluoridated apatites

Most uses of fluoride ions in biomaterials involve apatite compounds either as initial compounds or as a terminal location for fluoride ions. A description of fluoridated apatites will first be given, including structural and characterisation aspects, thermodynamics, solubility and reactivity. 

Several techniques have been used to characterise fluoridated apatites, especially XRD, fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (NMR). 

The use of H NMR to investigate the structure of fluoridated apatite appears to be of crucial importance, particularly for determining the active surface sites, suggested to be calcium hydroxyl (Ca-OH) and phosphorus (POH) groups [51],... 

Fig. 8. Evolution of synthetic and biological apatites in the presence of fluoride ions. The increase of pH and/or phosphate concentration in solution favours the formation of fluoridated apatite, whereas the increase of fluoride and/or calcium concentration favours CaF2 formation. At physiologic pH and mineral ions concentrations (in all body fluids), the formation of fluoridated apatite is favoured.

Synthetic fluor-containing apatites are prepared and investigated for biomedical applications and serve also as models to understand the formation of biological fluorapatites and some of their properties. The synthesis of fluoridated apatites has been accomplished in various ways from simple ion exchange in solution to more elaborate techniques involving sol-gel routes or thermal processes. Two main classes of synthesis routes are presented in this chapter high-temperature routes and low-temperature solution routes. 

Several high-temperature methods leading to fluoridated apatites can be found in the literature they involve solid-gas reaction, pyrolysis or crystal growth processes. 

Other fluoridation agents can be involved. After 2 h at 900°C, fully fluoridated apatite (a = 2) is formed [115]. Applying this process to HA-dense ceramics leads to similar results. 

Fluoridated apatite crystals can grow using the dual membrane system involving on the one hand a calcium acetate solution and on the other hand a phosphate solution at physiological temperature with a pH of 6.5. lijima et al. showed that the combination of fluoride ions, added to the phosphate solution, and amelogenin (a major protein in the enamel extracellular matrix), present in the reaction space between the two membranes, controlled the transformation of octacalcium phosphate (OCP) into fine rod-like fluoridated apatite crystals with habit, size... 

Other coating processes involving fluoridated apatite have been investigated to improve the long-term adhesion and promote osteointegration of cementless titanium-based metal implants pulsed laser deposition, electron beam deposition and ion beam sputter deposition techniques, and sol-gel methods, for example. They lead to fluor-containing calcium phosphates (apatites in most cases) with different compositions and crystallinity states. 

Despite many advantages fluoridation may also have some drawbacks. One of the most important is increased formation of dental calculi. As fluoridation considerably reduces the solubility of apatites, the supersaturation of saliva with respect to apatite is appreciably increased and may favour the spontaneous precipitation of fluoridated apatites and the formation of dental calculi. This phenomenon would occur especially in the case of fluoridated toothpaste, and most of them... 

The data concerning the effect of fluoride-containing apatites on cell adhesion, proliferation and expression seem rather disparate, probably because of the variation of other surface characteristics and the use of different cell strains. No real negative effect of fluoride-containing substrates has been reported so far and the osteoblast cells seem to behave similarly [184,185] or better [183,186,187] on fluoridated apatites than on HA surfaces. Some reports mention weaker attachment and proliferation, compensated for by improved collagen matrix production [188]. The shape of cells appeared different on fluoridated apatites than on HA. Other authors have found an improvement in cell attachment [183] which has been attributed to the change in surface charge of FA compared to HA. 

A.T. Kreinbrink, C.D. Sazavsky, J.W. Pyrz, D.G.A. Nelson, R.S. Honkonen, Fast magic angle spinning F NMR of inorganic fluorides and fluoridated apatitic surfaces, J. Magn. Reson. 88 (1990) 267-276. 

M. Braun, C. Jana, NMR spectoscopy of fluoridated apatites, Chem. Phys. Lett. 

E.C. Moreno, M. Kresak, R.T. Zahradnik, Physicochemical aspects of fluoride-apatite systems relevant to the study of dental caries. Caries Res. 11(suppl. 1)(1977) 142-171. 

M. Okazaki, Y. Miake, H. Tohda, T. Yanagisawa, J. Takahashi, Fluoridated apatite synthesized using a multi-step fluoride supply system. Biomaterials 20 (1999) 1303-1307. 

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]. 

Rocks which contain calcium chloride or fluoride (apatites) are decomposed according to the equations... 

Although several borate halides exist in Nature, few synthetic examples have been described. Because their stoichiometries mimic that of the fluoride apatite, Ca5(P04)3p, the borates AE5(B03)3X (A = Ca, Sr X = F, Cl, Br) have generated some interest. The structures of these materials are substantially different from that of apatite, and they vary depending on the identity of the A and X atoms. Interestingly, in the compound Ca5(B03)3p, one of the Ca atoms and the F atom may be replaced by a Ln = lanthanide atom and an O atom, respectively, to give materials of the type Ca4Ln(B03)0. ... 

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