| With more and more new kinds of medical metal implants applied in clinical work, many difficult problems are resolved. In the orthopaedics field, especially, medical metal implants are used widly in clinical application, such as implants for bone and repair materials for hard tissue repair. But metal implants are kinds of foreign bodies for living organism. There are very big differences between them on physical and chemical properties. In the orthopaedics clinical application, many serious problems are usualy due to these differences. To resolve these problems, the biocompatibility of medical metal implants is going to be improved in order to stable the connection between bone tissue and implants and ensure the long-term results of implants operation.In the orthopaedics clinical application, the most two important biocompatibilities of medical metal implants are biomechanics and physical chemistry interaction between bone tissue and implants. It is showed a serial of biological behaviour, such as the integratation in direction, the transmission and location of stress, and biological behaviour on surface and so on. In a word, mechanics and surface properties of implants are concerned about seriously by orthipedist. So there two approaches to improve the biocompatibilities of medical metal implants in this study. One is to improve the mechanics property, and the other is surface property. After the modification, we compare the new implant with normal one applied in clinic to validate the effects of the modification using an animal model.To improve the mechanics property, the intramedullary nails made of both Ti-24Nb-4Zr-7.9Sn alloy (weight percent) with elastic modulus ~33 GPa and the Ti-6Al-4V ELI alloy with modulus ~110 GPa were inserted into marrow cavity of rabbits to fix the fractured tibia. The early effect on healing of fracture tibia and new bone formation around the nails in the marrow cavity were investigated up to 4 weeks after implantation. To improve the surfce property, the micro-grooved surface is chosen. Our early investigation showed better cell proliferation and best cell differentiation on the micro-grooved surface at the cell scale (50μm), and osteoblastic cells on the micro-grooved surfaces also displayed a more similar morphology to osteoblastic cells in vivo. So the test surfaces are micro-grooved surface with the scale of 50μm and smooth surface.1. Early Effect of Ti-24Nb-4Zr-7.9Sn Intramedullary Nails on Fractured BoneObjective A multifunctional titanium alloy with ultra-low elastic modulus and high strength has been developed very recently for potential biomedical applications. In this study, the bone healing and stability of implants in a rabbit tibial fracture model were investigated using intramedullary nails made of either Ti-24Nb-4Zr-7.9Sn alloy or Ti-6Al-4V ELI.Methods A total of 15 New Zealand white rabbits of both sexes were used as experimental animals. After tibial fracture along transverse direction was introduced in both sides of tibiae closing to the middle part, the nail was inserted into the marrow cavity. Postsurgerily and weekly, all the rabbits were examined weekly by X-ray. At the 4th week, 5 sacrificed rabbits were selected randomly to perform biomechanical testing including nails were pulled out test and callus tensile test. Other 5 rabbits were sacrificed randomly for Micro-CT analysis. The BMD and BVF were detected. The rest of last 5 rabbits were used for histological observations of the callus formation around the distal end of the tibiae. Two-tailed t-test was the one used for statistical analysis using SPSS software version 12 for windows (SPSS, Chicago, IL, USA). Statistical significance was set at a probability p<0.05.Results The radiograph analyses showed that there were no signs of deformity in any of the fractured tibiae except the excluded four rabbits due to deep infection after implantations. The macroscopic observation also showed that the fracture lines of all rabbits almost disappeared by the generation of new callus around the fracture areas up to 4 weeks. The pull-out force of nails and tensile force of the newly formed callus located at the fracture site are presented in the Tab. 2. The mean pull-out forces of nails was 127.8N in experimental and 110.9N in control groups (p=0.009), showing significant statistical difference. However, the mean tensile force of the newly formed callus were almost identical being 172.3N and 170.7N (p=0.76) for both groups, showing no statistical difference. Quantitative data of bone regeneration obtained by the micro-CT analysis are given in Tab. 3. It shows that there was significant difference between the experimental and control groups. The mean BMD in experimental group is 197.3 mg/cm3 but 140.9 mg/cm3 in control group while the mean BVF is also higher in experimental group. Two and three dimensional micro-CT observations also show consistent results. After staining the cut slices by Masson trichrome, histological observation confirmed that the tissue around the implanted nails are new bone callus. The mean BVF in experimental group was 45.8% vs. 32.6% in control group, similar to the data obtained by BVF from micro-CT analysis.Conclusion These results suggested that, in the early stage of fixation, the nails with ultra-low elastic modulus improved the new bone formation in the marrow cavity. 2. Effect of surface micro-topography of titanium test piece with low elastic modulus on new bone formation in tibea of rabbitObjective From the results of test one, the advantage of Ti-24Nb-4Zr-7.9Sn could be see. To compare the effect of different alloies with different surface, Ti-24Nb-4Zr-7.9Sn and Ti-6Al-4V implamts with regulated micro-topography in 50μm and smooth surface were implanted into tibea of rabbit. After 4 weeks, new bone formations on surface were compared.Methods 5 rabbits were used. Materials Ti-24Nb-4Zr-7.9Sn and Ti-6Al-4V were made into square implants with 0.5cm in length and 1.0mm in high. Two kinds of surface were made on both materials, and then 4 kinds of implants were implanted into the middle part of tebea of rabbit. There were 4 sites for 4 kinds of implants. Both Ti-24Nb-4Zr-7.9Sn implamts were put into left tibea and Ti-6Al-4V in right. Both smooth impalnts were put into upper sites, and micro-topography in lower sites. Postoperative radiograghy was made to ensure the position of implants. At the 4th week, all animals were sacrificed for Micro-CT and histopathology analysis. BMD and BVF were detected in Micro-CT. After embeded in MMA, tissue was sliced for micro-X-ray, white light, polarized light and toluidine blue observation. Computer image analysis was made for slice using toluidine blue stain. Two-tailed t-test was the one used for statistical analysis using SPSS software version 12 for windows (SPSS, Chicago, IL, USA). Statistical significance was set at a probability p<0.05.Results The grayscale image of new bone formation could be seen clearly from Micro-CT. BMD and BVF analysis showed that the formation of new bone around micro-topography surface is better than smooth on and more new bone were seen around Ti-24Nb-4Zr-7.9Sn implants. Bone resorption was seen around the Ti-6Al-4V implants. From micro-X-ray, white light, polarized light and toluidine blue observation, the same results were seen. The images from histopathology analysis could ensure the grayscale image from Micro-CT. After toluidine blue stain, the BVF by computer image analysis was similar to Micro-CT.Conclusion Compared to smooth surface, micro-topography with the scale of 50μm surface have better effect on induction for bone formation. In the condition of same surface, Ti-24Nb-4Zr-7.9Sn implants with low elatic modulus improved the new bone formation on the surface of implants in the marrow cavity. In a word, both low elatic modulus Ti alloy and micro-topography with the scale of 50μm have satisfactory promotive effects on new bone formation around implants. |
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