Experimental and numerica analyses concerning the use of low-density materials for hip prostheses

The most suitable materials or materials combination for hard tissue replacement prostheses have to possess properties such as biocompatibility, excellent resistance to degradation, strength to sustain the cyclic loading of the joint and a high wear resistance to minimize debris generation. In this frame, materials characterized by lower elastic modulus (E) have an additional benefit due to stiffness reduction, which can result in the decreasing of stress shielding. This is a keypoint because stress shielding can cause the well known “implant loosening”. As far as the latter problem is concerned, titanium alloys have a strong advantage if compared to more conventional stainless steel (E=210 GPa) and cobalt-based alloys (E=190MPa) due to their lower elastic modulus (110GPa). Nevertheless, titanium stiffness is still higher than human bones (E=15GPa) and, therefore, the implant loosening can be just reduced but not completely avoid. Thus, the introduction of low-density materials, i.e. metal foams, can give a string improvement to the solution of the aforementioned problem. In this work, experimental tests were performed by using simplified specimens to highlight the influence that the density of different materials has on the “loosening” phenomenon. Furthermore, numerical simulations were carried out in order to numerically evaluate the shear stress field obtained by changing materials in the designed tests. These stress components, in fact, are mainly responsible of the unsticking between prosthesis and bone. More in particular, in this phase two different commercial codes were utilized to provide a comparison among the results and to validate their effectiveness. The numerical campaign was finally completed analysing a traditional test according to ISO 7206 standard.