## 4 Discussion  
In the present study, potentialities and disadvantages of different processing routes are compared in terms of: viability (cost and potential industrial application), reproducibility (reliability), limitations (porosity range, size, shape and distribution) and transversality of stablished or optimized protocols \begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties** & **Experimental** & **FEM** \\ \hline \(E\), GPa & 31 & 39 \\ \(\sigma_{y}\), MPa & 170 & 153 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Prediction of mechanical properties for the case of study corresponding to a porous compact fabricated by 40 vol.% NH4 HCO3 space holder. Figure 4: Example of Von Mises stress distribution at macroscopic yield. The case studied corresponding to a porous compact fabricated by a 40 vol.% NH4 HCO3 space holder. to other interested emerged and/or higher value added sectors. The framework used to make that comparison is based on the capability to achieve an optimum equilibrium between mechanical (\(E\) and \(\sigma_{y}\), according according to bone tissue to substitute) and biofunctional behaviour (allowing bone ingrowth and obtaining an inside porous roughness to improve osteointegration and infiltration with bioactive materials). Concerning to the microstructural results, in conventional PM, compaction pressure and sintering temperature have a significant effect in porosity (see Table 2). In general, temperature has the highest effect over porous morphological parameters as well as final porosity. For the temperature range studied, when temperature increases, an approximately 70% porosity reduction is observed. Porosity range that can be achieved through the implementation of this technique is from 13.6% to 44.8%, reaching the highest values by LS at 1000 \({}^{\circ}\)C sintering temperature. In this case, the porosity becomes more interconnected (44.1% at 0 MPa and 28.0% at 13 MPa). As it was expected, when temperature increases: (a) pore size is reduced, a 12% for 0 MPa compaction pressure level and a 29% for the highest pressure value; (b) pores are more rounded (shape factor, \(F_{f}\), close to one); (c) closed or isolated porosity is higher; (d) porous contiguity decreases until reaching zero value (isolated porosity); and (e) the distance between porous is 2.64 higher. Moreover, in general, compaction pressure effect is lower than the temperature influence. Regarding to the space-holder technique, the type of space holder used defines the porous morphology (irregular surface), fracture behaviour and plastic deformation of the specimens. These factors could affect, to some extent: (1) preserving the pore size (related to the space-holder volume); (2) the pore walls roughness; (3) increasing of interconnectivity between the pores; (4) the collapse of the porous structure of specimens (structural integrity of the samples during the process of space-holder evacuation); (5) the manufacturing cost of the specimens (time and resources); (6) the reproducibility of the evacuation space-holder process (it depends on the number and control of the process parameters) and (7) the risk of the residual space-holder content, from the point of biomedical view. In this context, considering the range of compaction pressure, it is observed how the use of space holders allows for achieving higher porosity levels and similar sizes to the original space holder distribution compared to the conventional PM technique. The total porosity could be controlled by varying space-holder content, although a small proportion of isolated micro-porosity has been observed (Figure 1 and Table 2). This micro-porosity is produced during the sintering of titanium powders. The role and the comparison of the studied space-holder size (NaCl and NH\({}_{4}\)HCO\({}_{3}\)) should be analysed by considering the particle size distribution of the starting powders (Table 1). By using 30 vol.% of space-holder content, \(D_{eq}\) of the obtained pores is around 2.6 greater employing NaCl.