The Effect Of The Bone Formation On New Low Elastic Modulus β Ti Alloy Implant-bone Interface

Background:Because of their outstanding biocompatibility and mechanical function, excellent corrosion resistance to biological environments, Titanium (Ti) and its alloys have been widely utilized in medical practice, Nevertheless, there is still a problem that the difference of elastic modulus between implants and bone tissue exists. At present the Ti and its alloys used in clinic have higher elastic modulus than bones, and it will affect transferring the stress to the bone. The significant difference in elastic modulus between implants and bone tissue can lead to stress shielding effect, and thereby may cause bone absorption or poor osseointegration. To achieve the success it is very important to control the stress shielding effect and try the best to transfer the stress to the bone. Therefore, the study of Ti and its alloys with a low elastic modulus become more and more hotter. The implants with a low elastic modulus shares loads with the bone to facilitate bone growth. Now the clinically used Ti–6Al–4V has an elastic modulus of 110GPa, which is lower than 200GPa of stainless steel, but still higher than 10-30 GPa of natural bone. Ti–24Nb–4Zr–7.9Sn is a newβ-type titanium alloy developed recently for biomedical application. It shows higher strength(800-900MPa) than that of Ti-6Al-4V(600MPa),and relatively lower elastic modulus(33GPa).It may be a new substitute material used for biomedical application in the future.Objective:To investigate the influence of implants on new bone formation and the effects of stress distribution on the interface with two different elastic modulus. (Ti–24Nb–4Zr–7.9Sn with modulus of elasticity about 30GPa and Ti-6Al-4V with modulus of elasticity about 110GPa).Methods:Thirty adult New Zealand white rabbits were selected in this study. Two different kinds of modulus of elasticity titanium alloy implants were inserted into proximal tibia by Slaets'method. The left lateral tibia was low elastic modulus group (Ti–24Nb–4Zr–7.9Sn,modulus of elasticity about 30GPa ). The right one was high elastic modulus group (Ti-6Al-4V,modulus of elasticity about 110GPa)., After surgery, 9 animals were sacrificed respectively at 4 weeks, 8 weeks, 12 weeks , X-ray, histology, biomechanics and micro-CT were used to observe the results. The tubiform region were defined as the region of the interest 1 (ROI1) of which the center is the implant and the radius is 1mm. The Anisotropy (AS), bone mineral density (BMD), bone volume fraction (BVF) and structure model index (SMI) had been analyzed in ROI1 by Micro-CT. In the histology the orbicular region on the middle cross section of the implants were defined as the ROI2, of which the center is the implants and the radius is 0.5mm. The bone formation ratio (BFR) and bone contact ratio (BCR) had been analyzed in ROI2.Results:X-ray examination: X-ray films of 4, 8, 12 weeks after operation show no significant difference between the low elastic modulus group and the high elastic modulus group.Histological examination: In the ROI2, at the 4, 8, 12 weeks after the operation, the BCR of the low elastic modulus group have better performance than high elastic modulus group (P<0.05). At the 4 weeks after the operation, the BFR of the two groups shows no difference, and at the 8 and 12 weeks after the operation, BFR of the low elastic modulus group have better performance than high elastic group (P<0.05).Micro-CT: in the ROI1, at the 4 weeks after the operation, the BMD, BVF, SMI and AS show no obviously differences between the two groups. At 8 weeks and 12 weeks after the operation, the BMD,BVF, SMI and AS show the significant differences between the two groups and the low elastic modulus group is obviously better than the high elastic modulus group(P<0.05).Biomechanics test: At 4 weeks after the operation the maximum force (Fmax) of the low elastic modulus group is higher than the high elastic modulus group but there are no significant differences between two groups. At 8 weeks and 12 weeks after the operation, the Fmax of the low elastic modulus group is significant higher than the high elastic modulus group.Conclusion:In our study, the new low elastic modulusβTi alloy implants of Ti-24Nb-4Zr-7.9Sn, compared with Ti-6Al-4V used in clinic, permits more stress to be transmitted into bones ,results in less stress concentrated and less bone resorption. It is better for new bone formation in the interface and improving the bone contact radio, so it will be easy to achieve osseointegration, and help to enhance the biological stability. We believe that the new low elastic modulusβTi alloy will have a bright prospect of orthopedic applications.