Solubility hydroxyapatite systems

Most foods and drinking waters contain enough fluoride to result in the incorporation of significant amounts of fluoride into this mineral whereby the solubility decreases. Therefore, the system hydroxyapatite-fluorapatite is primarily of importance for the prevention of dental caries. However, in this context its theoretical treatment is important for geochemists who may be confronted with so-called subregular solid solutions. 

Nevertheless, fluoride does lead to a reduction in the solubility of hydroxyapatite in aqueous solution, even in the absence of trace levels of fluoride in solution, and hence can be seen to have an effect in the solid state as well [57], Apatites are complex and diverse materials which have the general formula Caio(P04)eX2 (X = F, Cl, OH) and they represent a crystallographic system, in which there can be considerable replacement of species. Thus, with little or no change in the dimensions of the crystal lattice, there can be exchanges of OH for F, Ca + for Sr +, and PO4 for CO and all of these are known to occur in biological systems. Natural hydroxyapatite, for example, is often partially carbonate substituted [58]. 

Dietz, V. R., Rootare, H.M., and Carpenter, F.G. The surface composition of hydroxyapatite derived from solution behaviour of aqueous suspensions. J. Coll. Sci. 19, 87-101 (1962). Rootare, H.M., Dietz, V.R., and Carpenter, F.G. Solubility product phenomena in hydroxyapatite water systems. J. Coll. Sci. 17, 179-206 (1962). 

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the "c" axis. In its pure form, F, OH, or Cl occupy sites along this axis to form fluorapatite, hydroxyapatite, or chlorapatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are essentially never foxmd. Of the possible substituting ions, carbonate ion is by far the most important followed by Na, SO , and Mg " ". The most common form of natural apatite is francolite, a highly substituted form of carbonate fluorapatite deposited in marine systems. The substitution of CO3 ions into the mineral lattice has a substantial effect on apatite solubility (Jahnke, 1984). More studies are required, however, before the effects of all substituting ions are understood and an accurate assessment of the solubility of complex, natural apatites can be made. 

Bioceramics as Bone Substitutes The development of the so-called bioceramics is based on the knowledge that native bone is essentially composed of a more or less carbonated hydroxyapatite (HA) Caio(P04)(,(OH)2. With respect to the need for low solubility or of a controllable resorption, different compounds of the calcium phosphate system Ca(OH)2-H3P04-H20 have been applied for bone substitutes or bone fillings (Table 5.1). 

Table 12.1 shows the phases existing in the system CaO-PjOj-HjO. Out of these only monocalcium phosphate (anhydrous or monohydrate) is reasonably soluble in water, whereas the solubility of all other calcium phosphates is very limited. At any pH above 4.8 hydroxyapatite is the phase that is least soluble, and hence the one that is thermodynamically stable in this system. 

S Living systems utilize soluble ionic matter, such as Na+ and K (in combination with neurotransmitters) as a means of communication between the brain and the body and as nanomotors for movement and flexing muscles (Ca +). Insoluble ionic material, in the form of hydroxyapatite, Caio(P04)6(OH)2, creates our skeleton and our teeth by weaving in the protein collagen, which provides enormous strength and flexibility to these parts of the body. 

Approximately 10% of the human population (with regional differences indicating both genetic and environmental factors [33]) is affected by the formation of stones or calculi in the urinary tract. Urolithiasis is not only a painful condition, but also causes annual costs to the health system in the order of billions of dollars in the USA alone [34, 35]. Based on their composition, structure and location in the urinary tract, renal stones have been classified into 11 groups and their formation mechanisms have been discussed together with alterations in urinary parameters and metabolic risk factors for renal lithiasis [35]. Approximately 70% of these stones contain calcium oxalate monohydrate (COM) and dihydrate as major components, while other calculi are composed of ammonium magnesium phosphate (struvite), calcium phosphates (hydroxyapatite and brushite), uric acid and urates, cystine and xanthine. An accurate knowledge of the solubilities of these substances is necessary to understand the cause of renal or bladder calculi formation and find ways towards its prevention and treatment [36]. 

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