Theoretical insights into bone grafting silicon-stabilized alpha-tricalcium phosphate

Silicon-stabilized tricalcium phosphate (Si-TCP) is an excellent bone graft substitute being totally resorbed by the body and replaced by natural bone. Experimental studies show that coatings and bulk ceramics based on this material have superior bioactivity not existing in traditional hydroxyapatite materials. However, the mechanisms through which Si and other dopants affect the properties are not known. We have performed ab initio density functional calculations to investigate the effect of Si dopants on these materials. The results show that Si2O7 species can be formed with weak binding in bulk alpha-TCP with an oxygen vacancy for charge compensation, and that 2SiO4 substitution for a pair of PO4 groups with an excess Ca2+ for charge compensation also leads to a stable structure. With an increase of Si concentration, the former is less bound and the latter becomes more stable, and is a good candidate for the form of Si in Si-stabilized alpha-TCP. The stability of the Si-substituted TCP seems to be determined by the P-P distance of the pair of PO4 groups to be replaced before substitution. The Si-doping leads to a pronounced change in the Ca-O bond lengths, and has little effect on the P-O bonds.