Hydroxyapatite phosphate, dissolution

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

Mechanism of Action Inhibits formation, growth, and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces ... 

Russell, R. G. G., Miihlbauer, R. C., Bisaz, S., Williams, D. A., Fleisch, H. The influence of pyrophosphate, condensed phosphates, phosphonates and other phosphate compounds on the dissolution of hydroxyapatite in vitro and on bone resorption induced by parathyroid hormone in tissue culture and in thyroparathyroidectomized rats. Calc. Tiss. Res. 6, 183... 

Christoffersen M. R., Dohrup J., and Christoffersen J. (1998) Kinetics of growth and dissolution of calcium hydroxyapatite in suspensions with variable calcium to phosphate ratio. J. Cryst. Growth 186, 283-290. 

Fig. 16.6 Hydroxyapatite and fluoroapatite formation and dissolution, (a) Hydroxyapatite is transformed to fluoroapatite by isomorphous replacement. Fluoride ions diffuse into a hydroxyapatite crystal where they replace the hydroxide ions, (b) Fluoroapatite cannot dissolve as easily as hydroxyapatite. Right to left shows the solid-state rearrangement of hydroxyapatite to calcium monohydrogen phosphate, free calcium ions, and monohydrogen phosphate. The latter becomes mostly dihydrogen phosphate above pH 6.2. Arrows between (b) and (a) indicate enhanced apatite formation or slower changes to the amorphous solid if fluoride is present. Left to right shows the precipitation of calcium monohydrogen phosphate and its change to hydroxyapatite if an acid solution is made alkaline.

Apatites must undergo a solid-state transition to amorphous calcium phosphate before they can dissolve and the spontaneous replacement of hydroxide with fluoride ions slows the rate at which this transition occurs (Fig. 16.6b). Conversely, as an acid environment becomes more alkaline, fluoride ions promote the precipitation and crystallization of amorphous calcium monohydrogen phosphate/calcium fluoride into fluoro- and difluoro-apatites faster than amorphous calcium phosphate would crystallize into hydroxyapatite. Thus, fluoride ions have two effects on enamel that protect from caries they slow enamel dissolution in lactic acid and promote its re-precipitation and crystallization when the lactic acid is neutralized. 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

Apatite dissolution phosphate rock and synthetic hydroxyapatite (HAP). The... 

If natural water phosphate levels were controlled by equilibrium of the water with the thermodynamically stable calcium phosphate solid— calcium hydroxyapatite, Ca50H(P04)3—phosphate levels would be so low that we would not be concerned with phosphate as a nutrient for photosynthetic aquatic organisms. However, levels of phosphate exist in receiving waters that are far in excess of those predicted by equilibrium with hydroxyapatite because, not only is the rate of formation and dissolution of this thermodynamically predicted solid slow, but also calcium phosphate solids of higher solubility form and then transform very slowly into hydroxyapatite. 

Bone is the body s calcium reservoir. PTH stimulates bone resorption leading to the dissolution of hydroxyapatite and release of calcium and phosphate into the blood. This action of PTH appears to be the major mechanism for the rapid elevation of blood calcium levels. PTH also maintains blood calcium levels by promoting calcium reabsorption from the renal tubules. 

Table 9.21 shows that the corrosion resistance of stainless steel increases upon coating with hydroxyapatite. The presence of calcium phosphate in solution, due to dissolution of hydroxyapatite, seems to be the cause for these changes. The same table indicates that calcium phosphate is detrimental to the corrosion resistance of titanium, both in terms of film breakdown potential and corrosion rate under passive conditions. 

These techniques are bas not only on the principle that lead-containing phosphates with the apatite structure are highly insoluble, but also that rapid reactions occur with apatite and lead ions at the sohd/aqueous solution interface [12, 13, 15, 20, 29, 48, 53, 56]. Removal of lead from aqueous solutions using synthetic hydroxyapatite gives aqueous lead concentrations below the maximum contamination level after Ih [12, 53]. Other workers [9] observed the formation of calcium-lead apatite solid-solutions after 3 mins contact between synthetic hydroxyapatite and aqueous solutions containing lead, and no lead was detected in the aqueous solution after 24 h contact. However, the efficiency of lead removal depends on the characteristics of the phosphate rock employed [15]. It has been shown that the composition and crystallinity of the phosphate influence the speed of the surface reactions [4, 44]. More highly crystalline solids have lower solubilities and dissolution rates, making the apatite less reactive [4]. The presence of fluoride in the hydroxyapatite structure decreases its solubility and dissolution rate, while the presence of carbonate decreases structural stability, and increases solubility and the dissolution rate [4, 35]. 

Several authors [11-16,18,19,41,42] have suggested that the precipitation of the lead-bearing apatites pyromorphite and hydroxypyromorphite results from prior dissolution of hydroxyapatite, which is much more soluble than the lead phases. Continuous dissolution of hydroxyapatite was observed as the result of the formation of less soluble species [12, 15, 29, 53]. Some workers have studied the effect of different phosphate amendments (synthetic... 

Based on observed tissue response, synthetic bone-graft substitutes can be classified into inert (e.g., alumina, zirconia), bioactive (e.g., hydroxyapatite, bioactive glass), and resorbable substitutes (e.g., tricalcium phosphate, calcium sulfate). Of these, resorbable bone-graft substitutes are preferred for bone defect filling because they can be replaced by new natural bone after implantation, p-tricalcium phosphate (Ca3(PO )2, p-TCP) is one of the most widely used bone substitute material, due to its faster dissolution characteristics. Preparation of magnesium-substituted tricalcium phosphate ((Ca, Mg)3(PO )2, p-TCMP) has been reported by precipitation or hydrolysis method in solution. These results indicate that the presence of Mg stabilizes the p-TCP structure (LeGeros et al., 2004). The incorporation of Mg also increases the transition temperature from p-TCP to a-TCP and decreases the solubility of p-TCP (Elliott, 1994 Ando, 1958). 

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