## 4 Discussion
Although the differences between the obtained \(D_{eq}\) can lead to a decrease in a 15% using 40 vol.% of space holder. The increasing of interconnectivity (\(\lambda\)) for NH\({}_{4}\)HCO\({}_{3}\) implies balances between the differences due to an original space-holder size employed. Moreover, a round shaped porosity is formed (slightly shape boundary related to the sintering stage). Nevertheless, a small part of the porous, for NaCl as a space-holder, could be preserving the cubic geometry according to the original morphology, whereas the pores are shaped due to the employment of the NH\({}_{4}\)HCO\({}_{3}\) showing an elliptical and elongated morphology. Additionally, the porous titanium specimens produced by NH\({}_{4}\)HCO\({}_{3}\) manifest a better porous homogeneity distribution than the ones fabricated with NaCl; during the powder compaction, the NaCl is less fractured than the NH\({}_{4}\)HCO\({}_{3}\). In spite of that, the NaCl removal is more costly, less repeatable and less feasible to industrial implementation. Regardless of the nature of the space-holder, increasing their content has some important outcomes: pore shape factor is reduced (irregular morphology of porous boundary); the contiguity (\(C_{pore}\)) is kept constant for both cases; interconnected porosity for 30 vol.% of space holder represents the 75% of the total porosity, while 40 vol.% of space holder reaches 78 %. Both values of the interconnectedporosity and the suitable size of the pores promote the bone ingrowth. However, its influences on the mechanical behaviour should be considered (see below). At this point of this reported research, the results and discussion have been focused on the analysis of each fabrication method independently, where the influence of the processing parameters on the porosity has been evaluated. The following stage is now emphasised on a general comparison of both processing routes. A significant difference in morphology and porosity distribution among the specimens produced via conventional PM and a space-holder technique is observed (see in Figure 1). In conventional PM, the obtained pores present an irregular shape and a more homogenous distribution than in the space-holder process, in which the morphology of the pores reproduces the employed space-holder geometry. The \(D_{\alpha q}\) obtained by using a space holder are larger than those resulted from the conventional PM (Table 2). These results verify the potential and versatility of the space-holder method to control the shape, the size, the proportion and distribution of the porosity, in order to achieve biofunctional and biomechanical equilibrium of the implants. This notwithstanding, both industrial application viability and low cost of conventional PM routes are well known features. Within the context of porous implants in contact with bone tissue, some previous papers have reported that a suitable bone ingrowth can be achieved with a mean pore size of around 50 \(\upmu\)m [41; 42]. However, there are other studies where the optimal bone ingrowth happens for pore size range of 100-200 \(\upmu\)m [9; 43; 44]. Nevertheless, several authors' works show a better infiltration achievement with polymers or bioactive glasses if the pore size overcomes 200 \(\upmu\)m [45]. Accordingly, the conventional PM route manifests a drawback and thus only 6% of the pores (0 MPa and 1000 \({}^{\circ}\)C), which assures doubtful bone ingrowth, being almost a non-existent possibility to perform infiltration tests. On the other hand, the space-holder method allows for achieving successful results of optimal bone growth and better infiltration of the polymers or bio-glasses in the pores [45]. Considering the mechanical behaviour analysis results (Figure 2), to achieve a suitable stiffness range (20-25 GPa), it is necessary to obtain a total porosity between 40-55%.