The objectives of the project were achieved: the powders and the coated materials have shown their biocompatibility, i.e. the ability to perform in a specific application with an appropriate host response. From the technological point of view, it was demonstrated the feasibility of a ceramic component with an osteoconductive coating applied on the surface. Moreover, during the setup of the manufacturing process, interesting results were obtained in the development of a energy saving, environmental friendly binder system for Injection Molding.
The biocompatibility tests carried out in vitro with human osteoblasts, the cells responsible for bone formation and repair, gave a preliminary demonstration of the coating osteoconductivity based on cell phenotype expression and on collagen Type I synthesis.
The results of in vivo tests in NZW rabbits, carried out up to 12 months, demonstrates osteogenetic activity at the bone-implant interface. Macroscopic examination, X-ray and light-microscopy of the samples shows differences in bone formation between coated and uncoated specimens. The bone healing process is faster in HA coated implants than in uncoated ones. Histomorphometry shows statistically significant differences in bone formation and bone resorption: bone formation rate, number of osteoblasts and percentage of the implant surface covered by the newly formed bone are higher in coated than in uncoated samples, and the resorption index is lower in HA coated implants.
These satisfactory and promising results do not allow to draw conclusions about long-time biocompatibility of the coating, but data confirm that the HA coating may activate bone formation by osteoconduction. The coating seems also to accelerate the process of bone remodelling around the implant. In fact, the newly formed trabeculae, randomly arranged in the early stages of the osteogenetic process, show a gradual orientation, with their axes parallel to the implant. Contemporary the transformation of the early woven bone into the mature lamellar bone is enhanced. This process of secondary remodelling is aimed at restoring the mechanical equilibrium of the bone-implant complex.
The presence of macrophages multinucleated-giant cells in close contact with HA crystals could suggest a process of coating resorption or degradation. This last feature is rarely present at 1 year and cannot be shown up to 6 months after bone implantation. In our opinion, this is not a reliable evidence of the HA resorption due to osteoclastic activity. Moreover this observation is not in agreement with the previous data of other authors, who reported that the HA degradation started much more quickly at 1 or 2 months after implantation. We can only speculate that it would depend on the different physical and chemical characteristics of the coating tested.