A biomechanical evaluation of cast porous Ti-6Al-7Nb implants for orthopedic applications by using a comparative finite element analysis

Over the past decade, new developed and refined titanium casting technologies make it possible to produce dental and medical implants. Avoiding impurities and changes of the surface layer by α case formation are major challenges in titanium casting. In this context, the present results show the effect of four types of investments on the α case layer of Ti67-castings. In brief, Y2O3 based investment materials are a reliable and promising choice for the production of titanium castings compared to Invest-Ti-T, Al2O3 and ZrSiO4 based investment materials, although the processing of this material is comparatively difficult.
It can be concluded that an α case layer with a suitable thickness has a positive effect on titanium castings and can help to improve mechanical properties such as Young’s modulus, flexure stress, and fatigue resistance. In the light of these results, 15 µm was determined as a critical thickness value for an α case layer.
The main objective of this study was to evaluate the implant stability numerically after four weeks and six months implantation time for rat and sheep implants, respectively. The FEA was performed to simulate biomechanical push-out tests.
The simulated average maximum push-out forces are higher than the experimentally measured values. Both 3D model curves reach the maximum push-out force at larger displacements. The computationally predicted stiffness values for both 3D models exhibited a high agreement with the experimentally calculated stiffness. The second stage of 2D model curves were similar to that of measured curves. However, no clear non-linear behavior was observed for both 3D model curves. Meanwhile, 2D models showed a uniform and continuous sliding stage, while implants eluding occurred faster and sliding stages curves were narrow in 3D models. Even though different stress magnitudes were obtained for both groups of rat and sheep implants, the stress distribution at the bone interface was similar.
It can be concluded that 3D simulations were more sensitive than the 2D FE analyses. It should be noted that a detailed bone model is crucial and unit cell assumptions enhances the accuracy of FE analyses. Elongated tetrakaidecahedron unit cells improved the prediction of cancellous bone behavior and the highest accuracy of FE prediction was also achieved.
FEA results allow to make statements about the average behavior of the implant bone interface. High correlations were found between push-out strength as estimated from FE and the experimentally measured push-out strength.