Properties and application spectrum of cast porous implants made of Ti-6Al-7Nb in coated and uncoated conditions

摘要

This thesis presents the development, the characterization and the application of cast Ti-6Al-7Nb porous implants with respect to repair defects in cancellous bone. In the framework of this project, it was aimed at fabricating a porous implant with high mechanical strength to resist the applied load, a relatively low Young's modulus to prevent stress shielding phenomena, controlled cell dimensions to stimulate both mineral and collagen osseointegration, high porosity to guarantee interconnection between different pores, corrosion resistance under stress conditions and good biocompatibility. To identify the mechanical stresses and deformation behaviour that a porous implant should fulfil in order to substitute cancellous bone, bovine and ovine cancellous bone samples were compression tested. Ovine and bovine cancellous bone samples were used as models to understand their mechanical behaviours and the relationship between morphology and mechanical properties of the bone. In addition, a 45ppi open-cell AlSi7Mg foam was studied to understand the deformation mechanisms occurring during the compression test of metallic foams and the results were compared with those of the cancellous bones and of the Ti-6Al-7Nb porous implants. During the fabrication of the cast Ti-6Al-7Nb implants, defects associated to the casting process such as α-case and heterogeneous microstructure were found to reduce the mechanical properties of the samples. Therefore, an aging treatment was development in order to homogenize their microstructure. Brittle α-case was removed by using a pickling process in a HNO3+HF solution supported by ultrasonic vibration. Samples in heat-treated and acid-milled condition show an improvement of their ductility and strength. In this project, the surface chemistry of the implants was modified by using calcium titanate reaction layer with the aim to promote osseointegration. CaTiO3 coating was formed on the surface by immersing the implants in a salt-bath containing Ca(NiO3)2+NaNO3 and KNO3 for 2 hr at different temperatures (350°C, 410°C, 450°C und 510°C). The samples coated at 350°C show the best results after the fretting and cyclic bending test in corrosive environment and therefore this reaction layer was used on the implants. To evaluate the biological fixation and to analyse the biomechanical behaviour of coated and uncoated implants, in-vivo test were carried out by inserting the implants in "defects" created in the cancellous bone area of the distal femoral metaphyses of rats and sheep. After a recovering time of 30 days in rats and 6 months in sheep, the implants were extracted from the bone by using push-out test. Biomechanical results reveal high osseointegration in the vicinity and inside the porous implants which indicate high biocompatibility of both coated and uncoated implants.