Mechanics considerations for microporous titanium as an orthopedic implant material

This article investigates mechanics issues related to potential use of a recently developed porous titanium (Ti) material for load-bearing implants. This material may have advantages over solid Ti of enhancing the bone-implant interface strength by promoting bone and soft tissue ingrowth and of reducing the bone-implant modulus mismatch, which can lead to stress shielding. Experimental data from ultrasound experiments and uniaxial compression testing on microporous Ti are presented. Analytic models to predict its elastic modulus and Poisson's ratio are discussed, including "structural" approaches (Gibson and Ashby's cellular solids) and a "composite material" approach (Mori-Tanaka). Finally, two-dimensional finite element models based on optical micrographs of the material are presented. Simulations were performed for different conditions and levels of approximation. Results demonstrate that simple analytic models provide good estimates of the elastic properties of the porous Ti and that the moduli can be significantly reduced to decrease the mismatch between solid Ti and bone. The finite element simulations show that bone ingrowth will dramatically reduce stress concentrations around the pores.