Synthesis and Characterisation of Strontium and Magnesium Co-Substituted Biphasic Calcium Phosphates

Magnesium and strontium both play important roles in the growth of bone and so are
desirable ions for substitution into hydroxyapatite (HA) intended for use as bioinstructive bone
substitutes. A range of compositions were prepared by a solid state method based on the nominal
composition of HA (Ca10(PO4)6(OH)2), with various levels of strontium and/or magnesium
substitution: strontium-substituted HA (Ca8Sr2(PO4)6(OH)2), magnesium-substituted HA
(Ca9.8Mg0.2(PO4)6(OH)2 and Ca9Mg(PO4)6(OH)2), and strontium and magnesium co-substituted HA
(Ca7.8Sr2Mg0.2(PO4)6(OH)2 and Ca7Sr2Mg(PO4)6(OH)2). Materials were characterised by powder
X-ray diffraction, Fourier-transform infrared spectroscopy and Raman spectroscopy. These analyses
indicated that the co-substituted materials were composed of mixtures of strontium-substituted
hydroxyapatite and magnesium and strontium co-substituted β-tricalcium phosphate. In the
magnesium-substituted materials, increased magnesium content was related to increased proportion
of β-tricalcium phosphate phase, both with and without strontium co-substitution. The unsubstituted
and strontium mono-substituted materials, however, were pure apatite phase, suggesting that
magnesium was the destabilising factor in the phase compositions of the magnesium
mono-substituted and magnesium and strontium co-substituted materials