Apatite, biological Calcium hydroxyapatite

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. 

A variation of calcium phosphate is the major component of bones and teeth in all vertebrates including humans. These calcium phosphates are usually referred to collectively as biological apatites, which are nonstoichiometric compounds based on pure apatites, Ca5(P04)3X, where X can be fluorine (F), chlorine (Cl), or hydroxyl (OH). (These are called fluoro-, chloro-, and hydroxyapatite, respectively.) In biological apatites the calcium cations can be replaced with varying amounts of strontium, magnesium, sodium, and potassium ions, and the phosphate anions can be replaced with hydrogen phosphates and carbonates. 

The first and primary protective effect of fluoride is due to its strong, spontaneous reaction with metal ions. Biologically, the most important of these ions is the calcium ion, large amounts of which interact with phosphate to form bones and teeth. Studies show that fluoride reduces apatite solubility in acids by an isomorphic replacement of hydroxide ions with fluoride ions to form fluoro-hydroxyapatite and difluoro-apatite (Fig. 16.6a). 

Mention has already been made of substitution within the crystal lattice. A foreign ion, provided that it is similar in size to the ion which it replaces, may exchange for a normal hydroxyapatite constituent. This process is called heteroionic exchange, whereas the exchange of like ion for like is called isoionic exchange. There are, thus, two ways in which ions other than calcium, phosphate or hydroxyl may become part of the structure of biological apatite. Firstly,... 

Difficulties arise, not so much in explaining why mineralization readily and regularly occurs in certain tissues, but rather why other tissues, which resemble them in many ways, do not normally mineralize. Thus it is relatively easy to explain how crystals of a very sparingly soluble substance such as hydroxyapatite can be formed in bone, on the basis of the concentrations of calcium and phosphate ions present in blood these are sufficiently high to permit small crystals of biological apatite to grow at the expense of ions in solution. It is more difficult to appreciate why, under apparently similar conditions, a tissue such as skin which, like bone, contains... 

It now remains to explain the apparent paradox that the ionic product for calcium and monohydrogen phosphate ions in serum exceeds the solubility product of both hydroxyapatite and biological apatite and yet crystals of these substances are not formed in the bloodstream. Of the several reasons for this, the most basic one can be demonstrated by a simple experiment. 

The other ceranfic widely used are phosphate salts of calcium, with the chosen phase usually being hydroxyapatite. This material is conventionally prepared by thermal methods at temperatures well in excess of 1000°C. As a result of their preparation at high temperatures, the salts are carbonate free and are made up of much larger and more perfect crystals than those found in biological apatite minerals including bone. The imperfect crystalline structure of bone mineral leads to the natural material being soluble and reactive with respect to body fluids. In contrast, the synthetic materials are much less reactive than those found in living tissue and problems with biocompatibility can arise. 

Heating bone tissue to a temperature of about 750 °C ensures a complete biological decontamination [3]. When the temperature increases above this threshold the start of a complex metamorphic transformation of the bone tissue occurs. The thermal decomposition of stoichiometric hydroxyapatite undergoes at temperatures over 800 °C, with the initial formation of oxy-hydroxyapatite and oxy-apatite, followed by the oxy-apatite decomposition into various forms of tricalcium phosphate and/or calcium oxide [4,48]. An endothermic phenomenon can be identified in the range 800-1000 C, assigned to the modification of the hydroxyapatite crystalline lattice parameters, which takes place shortly before the initiation of its decomposition in beta-tricalcium phosphate [(3-TCP, CajCPO ) ] [49]. 

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