X-ray diffraction analyses of commercial and non-commercial ‘hydroxyapatite (HA)’ preparations as well as some biological apatites (enamel, dentin, bone) demonstrated a variability in crystallinity and composition before and after sintering. Coatings obtained by plasma-spraying calcium hydroxyapatite (HA) ceramic showed composition/crystallinity differences between the inner and outer layers of the coating; the coating composition consisting usually of: HA, amorphous calcium phosphate (ACP), α-TCP, β-TCP, tetracalcium phosphate (TTCP) and sometimes calcium oxide (CaO). The comparative extent of dissolution (EOD) in acid buffer of the non-sintered Ca-P materials was: ACP > DCPD > OCP > DCP > ‘AP’. For sintered materials, comparative EOD was: ACP*> TTCP > α-TCP > ß-TCP > HA. (‘AP’, calcium-deficient apatite; ACP, precipitated; ACP*, plasma-sprayed).
Results from this study underscored the need for the appropriate characterization of calcium phosphate materials using a combination of analytical techniques: x-ray diffraction, infrared and chemical. Since the dissolution of calcium phosphate materials depend on their composition and crystallinity, these results suggest that these materials used as bone grafts or implant coatings could affect their reactivity and stability in vivo.