Oxyapatite in hydroxyapatite coatings

Plasma sprayed hydroxyapatite coatings are used as bioactive surfaces for increasing the fixation of bone to the dental implants or orthopaedic prostheses. The variability in the performance of these coatings is partly attributed to the chemical phases that form at high temperatures in the plasma or fast cooling rates upon deposition. Hydroxyapatite can be accompanied by an amorphous phase constituent, tricalcium phosphate, tetracalcium phosphate, calcium oxide or the rarely mentioned oxyapatite. The high temperatures in the plasma produces a hydroxyl-depleted layer on the outside of the traversing hydroxyapatite particle. It is this dehydroxylated area which may form oxyapatite upon deposition. Higher cooling rates produce an amorphous phase, but a lower cooling rate will lead to the formation of oxyapatite. Examination of the coating at various depths with X-ray diffraction reveals a higher oxyapatite content in the underlying layers. The surface in contact with water vapour in air can be modified by the inclusion of hydroxyl ions to form oxyhydroxyapatite or hydroxyapatite. Th is presence and varying amounts of oxyapatite with coating thickness could influence the dissolution and mechanical performance of the coating for dental and orthopaedic prostheses. (C) 1998 Kluwer Academic Publishers.