Ultrastructural features of the bone response to a plasma-sprayed hydroxyapatite coating in sheep.

The intentions of this study were to characterize the macroscopic, microscopic, and structural aspects of a plasma-sprayed implant and to thoroughly investigate bone tissue response after its implantation in sheep. Therefore, we used scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, and energy-dispersive X-ray analyses. Assessment of the biomaterial prior to implantation showed a coating with irregular outlines and varying thickness, mainly consisting of hydroxyapatite (HA) covering a rough metallic implant core. Six months after insertion of the HA-coated Ti-6A1-4V implant, neither mechanical failure of the coating-substrate interface nor a significant loss of coating thickness was evident. However, an occasional lack of HA coating and phagocytosis of HA particles were noted. More generally, the implant was surrounded by well-mineralized bone investing the smallest cavities of the plasma-sprayed layer. Newly formed microcrystals with size, shape, and structure similar to those of bone apatite crystals were growing directly at the coating surface. These results suggest that the bone-bonding behavior of the considered grooved implant should provide satisfactory osseointegration and be suitable for fixed prostheses.