Synthetic and biological hydroxyapatites: crystal structure questions.

Experimental and theoretical studies have demonstrated that biological and synthetic hydroxyapatites display crystal structure similarities and differences that affect greatly the bioactivity of the synthetic materials. Recent developments in structure research of bioapatites report experimental values of the hydroxyl concentration in biological apatites in the range of 20-0% of the amount in synthetic hydroxyapatite. Theoretical calculations find the hexagonal space groups energetically unfavorable for hydroxyapatite crystallization. Different configurations are proposed for the carbonate substitution in B-type carbonate apatites.