Experimental Study Of Osteointegration On A Novel Sustained Zinc-releasing Hydroxyapatite Coating Material

The development of orthopaedic surgery in the recent30years can be attributed to the development of surgical technic and the advance of the implanted biomaterials. Implants has a great significance in the modern orthopaedic clinical treatment, it is the carrier of many different treatment strategies. The total joint replacement has been used for the standard operation in the joint reconstruction, on the other hand, the internal fixation has been widely used in the high-energy orthopaedic trauma area. For these modern clinical treatment technology, the metallic implants were widely adopted for its good strength and ductility. But the metallic material is a typical biologically inert material. It is very difficult for metal to take osteointegration with bone completely and tightly. For this reason a new technology which increaseed the biological response of metallic material through bioactive coatings was developed. The research on metallic implants surface modification using bioactive coatings and their cell and tissue response is one of the most attractive aspects of biomaterials. Through bioactive coatings the material will stimulate cell activity to promote bone growth initially and get long term stability between implants and bone finally. Hydroxyapatite(HA) has been widely used to improve the tissue bonding between implants and bone. However, the clinical application of hydroxyapatite coating shows it will be dissolved easily and has low osteogenic activity, which may result in slow osteointegration in the early stage. Zinc ions has been reported that it can promote the proliferation and differentiation of osteoblasts. In this study we want to use tricalcium phosphate(TCP) as carrier of zinc ions, then we can get the coatings of fluoride hydroxyapatite(FHA)incorporated ZnTCP powders by sol-gel method. And finally test the osteointegration of this novel coating.At first, we prepared nano-sized TCP powders with zinc ions through depositing of precursor amorphous calcium phosphate in the lower temperature. Then the coatings were prepared by a sol-gel technique, and nanosized Zn-containing tricalcium phosphate particles were added to the Ca(NO3)2-P2O5-HPF6sol system to incorporate Zn ions into fluoridated hydroxyapatite (FHA) coatings. We changed the quantity of Zn ions in the coatings by adding more Zn ions into TCP particles or adding more ZnTCP particles into the FHA coatings. The derived Zn-releasing fluoridated hydroxyapatite coatings demonstrated a slow and sustained Zn release behavior. And the result of electron microscope scan and Zn-releasing test showed that the coating with6.2wt%of zinc ions has best Zn-releasing ability.Sceond, the osteoblastlike cell cultures (MG63) were performed on the ZnTCP/FHA coatings. The cell adhesion and proliferation were evaluated by MTT assay. The surface of coatings were scaned by electron microscopy. The result showed there were more osteoblastlike cells on surface of the ZnTCP/FHA coatings. The proliferation of these cells were more quickly. And these osteoblastlike cell have higher activity than those on traditional FHA coatings. Then the activity of ALP of the cultured cells were measured by potassium ferricyanide method. The gene expression of Collagen I and Osteocalcin were evaluated through RT-PCR test. We found these cells on the ZnTCP/FHA coatings have higher ALP activity and the expression of Collagen I and Osteocalcin were on a higher level than those on Ti and FHA coatings. This result showed ZnTCP/FHA coatings had better biocompatibility and osteogenic activity compared with FHA coatings in vitro study.Finally, three different implants, which consisted of Ti,Ti with FHA coating and Ti with ZnTCP/FHA coating, were inserted into the tibia of16New Zealand white rabbits. For the bone fluorescently labeling, tetracycline was injected subcutaneously10and11weeks after the operation. These rabbits were euthanized12weeks after the operation. The tibia specimens were harvested. Biomechanical experiments were used to test the bonding strength between bone and implant. The results showed the ZnTCP/FHA coating group has a higher anti-pullout strength. With fluorescence microscopy, tetracycline labels were clearly observed as two thin yellow lines in both the FHA and ZnTCP/FHA groups, indicating the newly formed bone tissue. The formation of new bone were also found in HE stained slices of both the FHA and ZnTCP/FHA groups. But in ZnTCP/FHA coating group the connections between bone and implant were more closely, and the new bone were connected to the implant directly along the coating surface. The results showed the ZnTCP/FHA coatings have a more rapid osteogenesis and a more complete osseointegration in vivo study.Our study demonstrated the novel sustained zinc-releasing hydroxyapatite coating has a good biocompatibility and a more complete osseointegration both in vitro and in vivo study. It indicated that this coating has good clinical application prospects.