Karlis A. Gross

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. This presence and varying amounts of oxyapatite with coating thickness could influence the dissolution and mechanical performance of the coating for dental and orthopaedic prostheses.

Osteoclast resorption of thermal spray hydoxyapatite coatings is influenced by surface topography.

Coating characteristics such as composition, crystallite features and topography collectively impact the cell response. The influence from splats has not yet been assessed for hydroxyapatite (HAp) thermal spray coatings. The objective of this work is to (a) survey the topography on commercial implants, (b) ascertain topography formation from single splats, and (c) determine the osteoclast resorption pattern on a topographically refined coating compared to dentine. Coatings on dental implants, an orthopedic screw, a femoral stem and a knee implant were studied for reference. The effects of substrate pre-heat, roughness, spray distance and particle size on the coating roughness and topography were studied. Human-derived osteoclasts were placed on a coating with refined topography and compared to dentine, a polished coating and polished sintered HAp. A pre-heat of at least 200°C on titanium was required to form rounded splats. The greatest influence on coating roughness and topography arose from particle size. A 2-fold increase in the mean particle size from 30 to 72 μm produced a significant difference (P<0.001) in roughness from 4.8 and 9.7 μm. A model is shown to illustrate topography formation, nanostructure evolution on single splats, and the topography as seen in commercial implants. Osteoclasts showed a clear preference for activity on coatings with refined topography. A one-way ANOVA test revealed a significantly greater pit depth (P=0.022) for dentine (14 μm) compared to the as-sprayed and polished coating (5 μm). Coatings with topography display a similar number of resorption pits with dentine, but a 10-fold greater number than polished coatings, emphasizing the importance of flattened droplet topography on implant surfaces.