In this article, we propose a methodology for the rational design of scaffold constructs in bone-tissue engineering. The construct under investigation is a sandwich structure with an Intramedullary rod (IM), a Biological Sponge (BS) and an External sleeve (ES). The IM rod provides axial resistance, BS facilitates the growth of new bone and ES provides stability to the construct by resisting torsion and bending. We demonstrate that only select combinations of stiffness between IM and ES facilitate the growth of new bone. Perren's interfragmentary strain theory is employed to clearly identify regions favoring bone growth from those favoring the formation of cartilage. Finally, design maps are constructed that clearly identify the combinations facilitating timely bone growth.