We present a new approach to designing three-dimensional, physically realisable porous femoral implants with spatially varying microstructures and effective material properties. We optimise over a simplified design domain to reduce shear stress at the bone-prosthetic interface with a constraint on the bone resorption measured using strain energy. This combination of objective and constraint aims to reduce implant failure and allows a detailed study of the implant designs obtained with a range of microstructure sets and parameters. The microstructure sets are either specified directly or constructed using shape interpolation between a finite number of microstructures optimised for multifunctional characteristics. We demonstrate that designs using varying microstructures outperform designs with a homogeneous microstructure for this femoral implant problem. Further, the choice of microstructure set has an impact on the objective values achieved and on the optimised implant designs. A proof-of-concept metal prototype fabricated via selective laser melting demonstrates the manufacturability of designs obtained with our approach.