Dentin carbonate apatite

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

The second mode of crystal formation that occurs in dentin is via matrix vesicles. These are phospholipid delimited packages of specialized enzymes, macromolecular complexes and ions, that induce the precipitation of amorphous calcium phosphate. At some point the latter crystallizes into carbonated apatite crystals, that have no preferred orientation [62], These appear smaller and denser than the crystals that form in the collagen framework. 

Indeed, in the light of current mineralogical theory (McConnell, 1973a) pertaining to carbonate apatites, there is no need to search for either a precursor or any other solid phase in bone — either crystalline or noncrystalline. The chemical composition is indicated for the principal constituents of bovine (dry, fat-free) bone, compared with dental enamel and dentin (Table 3.1.7). When converted to oxides, these values for bone become ... 

Comparison of Enamel and Dentine to DahHite. Infrared spectrophotometry and X-ray diffraction studies of natural apatites before and after thermal treatment, led Herman and Dallemagne (1963) to conclude that among the apatites they studied, francolite and quercyite are comparable and the results were explained by considering that CO3 groups are present within the apatitic lattice by substitution for P04" groups. Dahllite and Curasao carbonate-apatite contain calcite, but no evidence was found for the presence of calcite in francolite and quercyite. Enamel, dentine, and bone were found to be comparable to dahllite (Fig. 19.11). Spectra of... 

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. 

Scientifically speaking, biological apatites, such as apatite crystals of human dentin, enamel, and bone must be classified as carbonate apatites. These substances also demonstrate different levels of solubility because of their various degrees of crystallinity. Therefore, enarnel carbonate apatite is... 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

One further difference between the tissues should be noted briefly—that of turnover—which holds implications for the nature of the isotopic signal recorded and its interpretation. Bone is constantly resorbed and reformed during life, i.e., it turns over , whereas enamel and dentine do not, although secondary dentine can be later accreted. Enamel and dentine form during a discrete period in the individual s life. This means that carbon isotope dietary signals in bone, for both collagen and apatite, reflect diet integrated over years, whereas those in enamel and dentine increments reflect diet at time of formation. 

Among the apatite substituents, carbonate has been the subject of many studies. The apatite present in dental enamel is not a pure HA, but rather a corbonate apatite with a carbonate content of 2—3 %. There is still a controversy about the location of the carbonate in enamel, dentine and bone. While most recent studies agree that carbonate appears to be substituting within the lattice rather than existing in an amorphous phase, there is still some disagreement as to the actual position of the carbonate. The presence and location of the carbonate in dental enamel may relate directly to the risk of carious attack. Carbonate has been shown to be leaked preferentially from early carious lesions81. ... 

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

The presence of carbonate in dental enamel and dentine is an important factor which contributes to their susceptibility to carious attack (Zapanta-LeGeros et al., 1964). Carbonate increases the solubility of the apatite. The presence of carbonate... 

Substitutions in the HA structure are possible. Substitutions for Ca, PO4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH" groups in HA are replaced by F" the anions are closer to the neighboring Ca " ions. This substitution helps to further stabilize the structure and is proposed as one of the reasons that fluoridation helps reduce tooth decay as shown by the study of the incorporation of F into HA and its effect on solubility. Biological apatites, which are the mineral phases of bone, enamel, and dentin, are usually referred to as HA. Actually, they differ from pme HA in stoichiometry, composition, and crystallinity, as well as in other physical and mechanical properties, as shown in Table 35.7. Biological apatites are usually Ca deficient and are always carbonate substituted (COs) " for (P04). For... 

Dentine, p = 2.0-2.3 glan , like bone, contains about 72% apatite, 18% collagen some carbonate and small quantities of phospholipids, F, Na, Mg, and so on. The substitution of F for OH in dental apatite decreases the acid solubility and improves hardness and resistance to decay. For this reason, fluoride ions are sometimes added to toothpastes and drinking water supplies (Chapter 12.14) (Table 11.5). 

Slosarczyk et al. have used a wet method to obtain carbonated HAp powders [56, 57]. Calcium oxide (CaO), calcium nitrate, calcium tetrahydrate [Ca(N03)2-4H20] or calcium acetate [Ca(CH3COO)2-H20] were used as the calcium source. As the phosphorous source, phosphoric acid (H3PO4) or di-ammonium phosphate [(NH4)2HP04] were used. The molar ratio of Ca P was 1.67. Ammonium bicarbonate (NH4HCO3) or sodium bicarbonate (NaHCOs) were used as reactants to introduce groups. Biological apatites in natural bone, dentin, and enamel... 

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