The selection of the space-holder is one of the most crucial steps because it defines many of the properties of the resulting foam, including cell shape, cell size and macroporosity. The space-holder should be inert and represent the size and shape of the desired pores. The porosity may be adjusted anywhere between 50 and 85% without the filler material becoming a part of the resultant foam. It is also important to select a spacer that has limited or no solubility in titanium, as this incorporation will affect the mechanical properties of the resulting foam.