Magnesium-substituted tricalcium phosphate

Carrodeguas RG, De Aza AH, TurriUas X, Pena P, De Aza S. New approach to the beta-alpha polymorphic transformation in magnesium-substituted tricalcium phosphate and its practical impUcations. J Am Ceram Soc. 2008 91(4) 1281-6. 

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). 

Two aspects of the crystal chemistry of natural and synthetic apatites need to be recognized. First, the HA in bone is nonstoichiometric, has a Ca/P ratio of less than 1.67, and contains carbonate ions, sodium, magnesium, fluorine, and chlorine (Posner, 1985a). Second, most synthetic hydroxyapatites actually contain substitutions for the phosphate and/or hydroxyl groups and vary tom die ideal stoichiometry and Ca/P ratios. Oxyhydroxyapatite [Ca,o(P04) 0], a-tricalcium phosphate (a-TCP), )3-tricalcium phosphate O-TCP) or )8-Whitlockite [Caj(P04)J, tetracalcium phosphate (Ca4P209), and octocalcium phosphate [Cag(HP04)2(P04)4 5H20] have all been detected via x-ray diffraction (XRD), Fourier transform into spectroscopy (FITR), and chemical analyses (Kohn and Ducheyne, 1992 Ducheyne et al., 1986, 1990 Koch et al., 1990). These compounds are not apatites per se since the crystal structure differs from that of actual apatite. 

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