The Effects of Magnesium on Hydroxyapatite Formation In Vitro from CaHPO₄ and Ca₄(PO₄)₂O at 37.4°C

Abstract.

The effects of magnesium ion on the formation of calcium-deficient hydroxyapatite [Ca₉HPO₄(PO₄)₅OH, CDHAp] from CaHPO₄ and Ca₄(PO₄)₂O dissolution were investigated using two magnesium sources: Mg₃(PO₄)₂ (chemical system 1) or MgCl₂· H₂O (chemical system 2) solutions. Because chloroapatite does not form from aqueous solutions, the use of two magnesium sources facilitated the determination of magnesium's role during synthetic hydroxyapatite formation in vitro and possible related effects during biomineralization. Isothermal calorimetry determined the progress of reactions. Two peaks are observed as heat is evolved during the formation of CDHAp in water at 37.4°C. The nucleation and growth of CDHAp are the corresponding mechanisms. Although the time for complete reaction and total heat-of-reaction ΔH_r remain constant, the height of the first peak is reduced as the concentration of magnesium ion approaches 4 mM in either chemical system. Magnesium does not substitute into CDHAp even though there are calcium vacancies available. Subsequent increases cause the remaining heat peak to broaden and the time required for complete reaction to approach 24 hours as the initial MgCl₂ concentration reaches 100 mM. Supersaturation limits chemical system 1 to Mg₃(PO₄)₂ concentrations below 10 mM. A MgCl₂ concentration of 3.16 M precludes CDHAp from forming for over 3 months; rather newberyite, MgHPO₄· 3H₂O, precipitates. The morphology and surface area of the CDHAp formed in 100 mM MgCl₂ solution are comparable to those of CDHAp formed in water. The surface areas are approximately 80 m²/g. Magnesium concentrations below 4 mM only inhibit nucleation whereas those above 4 mM inhibit growth as well. Magnesium phosphate complexes are more inhibitory than magnesium chloride complexes. Increasing the liquid-to-solids ratio or agitation significantly increases the induction period before reaction initiates. Increasing the liquid-to-solids ratio increases the time span for growth whereas increasing agitation decreases the time span for growth. The large inhibitory effect of agitation suggests quiescent systems are more suitable for determining the kinetics of HAp formation. A magnesium inorganic chemical activity (α_Mg=γ_Mg[Mg²⁺]) many times greater than that in biological fluids is required before inhibition of hydroxyapatite formation is realized.