**3.6 Histological analysis**

Fig.17 showed the tissue response to the Mg-4.0Zn-0.2Ca alloy pins implantation at 1, 2 and 3 months. It could be clearly seen that some lymphocytes were identified in histological tissue in 1 month after operation, but there was no visible evidence of multinucleated giant cells. After 2 months implantation, there was an active bone formation, which was evident by large number of new disorganized trabeculae. After 3 months implantation, new bone tissue was formed around the magnesium implant. In comparison with the histological microstructure obtained at the cortical bone near the implantation site, as shown in Fig.17, no difference could be found in the histological microstructure between the new bone and the cortical bone.

Magnesium alloys have attracted much attention as potential biodegradable bone implant materials due to their biodegradability in the bioenvironmental as well as their excellent mechanical properties such as high strength and an elastic modulus close to that of bone. In this paper, the in-vitro cytotoxicity and in-vivo biocompatibility of new kind of Mg-4.0Zn-0.2Ca alloy was studied. The cytotoxicity test indicated that the Mg-4.0Zn-0.2Ca alloy had no cytotoxicity. Rabbit implantation indicated that the Mg-4.0Zn-0.2Ca alloy did not cause any inflammation reaction. One month after operation, all magnesium implants were fixed tightly. There was no gap between the bone and the residual implant. Optical images from Fig.15 and SEM microstructure from Fig.16 showed clearly that there was a degradation layer formed on the surface of the magnesium implants. Histological images showed that new bone tissue was in contact with the magnesium implant through this degradation layer.

Research on Mg-Zn-Ca Alloy as Degradable Biomaterial 203

In this paper, we developed ternary Mg-Zn-Ca alloys as biodegradable materials. The

The mechanical properties of the as-cast Mg-Zn-Ca alloys can be tailored by the Zn and Ca content. The tensile strength can be increased form 105MPa to 225Mpa, and the elongation

The in-vitro degradation of Mg- Zn- Ca alloys revealed that Zn and Ca not only elevated the corrosion potential of the magnesium alloys, but also influence their corrosion current. A protective layer of Mg(OH)2 and other Mg/Ca phosphates was formed on the surface of Mg- Zn- Ca alloys when immersed in SBF solution ,which declined the degradation rate. In vitro cytotoxicity assessments indicated that Mg-4.0 Zn-0.2Ca alloy did not induce

Implanted in rabbits, the alloy did not induce inflammation reactions or affect the new bone formation. We could draw a conclusion that the Mg-4.0Zn-0.2Ca alloy had good

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toxicity in L-929 cells and are suitable for biomedical applications.

**4. Conclusion** 

biocompatibility.

**5. References** 

1093.

6(2010) 1813.

following conclusions can be drawn.

can be increased from 4.2% to17 %.

Biomaterials 26 (2005) 3557.

Line scanning in Fig.17 proved that large amount of Ca and P was found around the magnesium implant. In the present study, the histological analyses revealed that this magnesium-containing calcium phosphate degradation layer could promote or accelerate the new bone formation.

Fig. 17. Tissue response of the Mg-Zn-Ca alloy implantation at 1, 2 and 3 months (a) Mg-4.0Zn-0.2Ca 1 month, (b) Mg-4.0Zn-0.2Ca 2 month, (c) Mg-4.0Zn-0.2Ca 3 month, (d) cortical bone.

In-vivo degradation was a very complex process, so it was difficult to accurately assess the degradation rate of an implant material. F. Witte et al [35] found that the corrosion of a magnesium rod in the medullar cavity was not homogeneous in all cross-sections by used micro-computed tomography. In our study, the cross-section area of the residual implant was calculated to describe the degradation rate of the Mg implant. Due to the inhomogeneous corrosion of a magnesium rod in the medullary cavity, the calculated degradation rate based on the images was similar to the real in-vivo degradation rate of magnesium implants.

In-vivo degradation, compare with other metal implants, is an ultimate merit of magnesium alloy. After implantation in the rabbit, Mg-4.0Zn-0.2Ca alloy would be reacted with body fluid on the surface and get dissolved in the surrounding body fluid. At first, the released Mg2+, Zn2+ and Ca2+ could be absorbed by the surrounding tissues and excreted through the gastrointestinal route and the kidney. However, with the increasing time of implantation, more Mg2+, Zn2+ and Ca2+ ions are dissolved into the solution, the local pH near the surface of the Mg implants could be >10[36]. As a result, an insoluble magnesium-containing calcium phosphate would be precipitated from the body fluid on the surface of the magnesium implant and tightly attached to the matrix, which retarded degradation. In addition, the corrosion layer on the Mg-4.0Zn-0.2Ca alloy contained Mg, Ca and P, which could promote osteoinductivity and osteoconductivity, predicting good biocompatibility of magnesium. Therefore, it is proposed that the Mg2+, Zn2+ and Ca2+ released during degradation should be safe.
