A porous substrate or implant for implantation into a human or animal body constructed from a structural material and having one or more regions which when implanted are subjected to a relatively lower mechanical loading. The region(s) are constructed with lesser mechanical strength by having a lesser amount of structural material in said region(s) relative to other regions. This is achieved by controlling pore volume fraction in the regions. A spacer is adapted to define an open-cell pore network by taking a model of the required porous structure, and creating the spacer to represent the required porous structure using three-dimensional modelling. Material to form the substrate about the spacer in infiltrated the scaffold structure formed.