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A coating technique was developed which produced a thin, homogeneous and very adhered layer of hydroxyapatite with controlled composition, density and crystallinity, without grit-blasting pre-treatment of the pure titanium or titanium alloy. The pulsed layer deposition (PLD) layer followed perfectly the geometry and topography of the surface, so that, within the project, plane, cylindrical and small animal hip prosthesis have been successfully coated. The coatings have been produced by applying a new PLD method based upon an excimer laser beam focussed on a hydroxylapatite (HA) target. The ablated material forms a plume rendering film growth on a substrate located in front of it. Adhesion strengths of the coatings without grit blasting have been greater than 58 MPa, and in all cases it was bonding adhesive which failed. This result especially interesting from the industrial point of view, since the main drawback of the conventional plasma spray (PS) method is the lack of adhesion and subsequent spallation of the coating, resulting in major medical complications, such as implant loosening and severe inflammation.
In-vitro testing was performed for cytotoxicity and biocompatibility demonstration, as well as screening of the coating with best potential for in-vivo performance. Cylindrical implants have been tested in rabbit tibia and dog jaw for comparison of the bone apposition at the PLD, PS coated and the bare titanium surface. The results in rabbits gave several inflammatory cases for the plasma sprayed coated cylinders, while none occurred for the PLD ones. The superior capability of the thin PLD coating in promoting osteointegration in the trabecular bone of rabbit tibia has been fully demonstrated, since the bone contact after 6 months was similar for the bare titanium and the plasma sprayed one, but 43% more on the 2 µm thick layer produced by the laser technique. This result revolutionizes the field because, until now, the state of the art technology dictated PS coatings of at least 50 µm, that had to be partially resorbed to promote bone bioactivity. It has been demonstrated that a 2 µm thick, well adhered and homogeneous coating can perform better and after 6 months it has not resorbed, so that the thickness may well be reduced further.
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