Oxyapatite in hydroxyapatite coatings

Plasma sprayed hydroxyapatite coatings are used as bioactive surfaces for i ncreasing the fixation of bone to the dental implants or orthopaedic prosth eses. The variability in the performance of these coatings is partly attrib uted to the chemical phases that form at high temperatures in the plasma or fast cooling rates upon deposition. Hydroxyapatite can be accompanied by a n amorphous phase constituent, tricalcium phosphate, tetracalcium phosphate , calcium oxide or the rarely mentioned oxyapatite. The high temperatures i n the plasma produces a hydroxyl-depleted layer on the outside of the trave rsing hydroxyapatite particle. It is this dehydroxylated area which may for m oxyapatite upon deposition. Higher cooling rates produce an amorphous pha se, but a lower cooling rate will lead to the formation of oxyapatite. Exam ination of the coating at various depths with X-ray diffraction reveals a h igher oxyapatite content in the underlying layers. The surface in contact w ith water vapour in air can be modified by the inclusion of hydroxyl ions t o form oxyhydroxyapatite or hydroxyapatite. Th is presence and varying amou nts of oxyapatite with coating thickness could influence the dissolution an d mechanical performance of the coating for dental and orthopaedic prosthes es. (C) 1998 Kluwer Academic Publishers.