## 4 Discussion  
[PERSON] et al. analyzed the mechanical properties of a porous biomaterial consisting of a titanium alloy Ti6 Al4V. The test specimens were produced by additive electron beam melting (EBM) technology and tested for mechanical properties using a static pressure test. Cylindrical samples with a diameter of 10 mm were produced in three variants: honeycomb structure, orthogonal structure, and layer structure. Pore sizes ranged from 500 \(\upmu\)m to 600 \(\upmu\)m. The results showed that the porous structure of the material significantly reduced the stress shielding effect. The compressive strength of these materials was in the range of 163 MPa to 286 MPa and [PERSON]'s modulus of elasticity was in the range of 14.5 GPa to 38.5 GPa, which is similar to cortical bone values [27]. [PERSON] et al. in their study dealt with tetrahedron and octahedron topologies made of Ti6 Al4V alloy. The samples were produced by SLM technology and had the shape of a block, the width varied from 9 mm to 14 mm and the height from 15 mm to 22 mm. The pore size for the tetrahedron was determined to 500 \(\upmu\)m and for the octahedron to 770 \(\upmu\)m. For each topology, four different percentual porosities were chosen (50%, 60%, 70%, and 75%) to ensure that the entire porosity range was covered. By mechanical testing, it was determined that the stiffness of the material was 1.2 GPa to 4.6 GPa, the maximum compressive strength was in the range of 31 MPa to 228 MPa, and [PERSON]'s  
Figure 18: Microstructure of sample: (a) 1 mm (b) 100 \(\upmu\)m (c) 500 \(\upmu\)m.  
modulus of elasticity decreased significantly with an increase in porosity [28]. This trend of decreasing Young's modulus with increasing porosity percentage for porous structures made of Ti6 Al4V alloy has also been reported in publications by other authors [12; 29; 30; 31; 32; 33]. Another important factor in the formation of porous structures is the size of the individual pores. The change of these dimensions in porous structures has a fundamental effect on the permeability of cells and nutrients, the flow of blood through the pores of the material as well as the influence of growth factors. For this reason, the optimal pore size varies considerably depending on the application [34]. [PERSON] et al. conducted a study in which they implanted titanium prostheses into the rabbit femoral bone. They assumed that the optimal pore size for bone growth is in the range of 100 \(\upmu\)m to 400 \(\upmu\)m. They formed an implant structure made of titanium material, the pores of which reached a size of 50 \(\upmu\)m to 125 \(\upmu\)m It was found that 12 weeks after implantation the growth of bone structures was not affected by the pore size or the thickness of the implant structure itself. The average value of bone growth ranged from 64% to 78% under non-load-bearing conditions [35]. [PERSON] et al. used a 4 mm diameter disk on which a coating of commercially pure titanium with a thickness of 1 mm was applied in their study. These implant structures were subsequently implanted into the rabbit tibial bone. Four weeks after implantation, they found that bone growth occurred in micropores smaller than 10 \(\upmu\)m [36]. On the other hand, [PERSON] et al. used SLM technology to create porous plates and porous cylinders from commercially pure titanium powder. Titanium plates were implanted into the rabbit tibial bone and titanium cylinders were implanted into the rabbit femoral bone. The porosity was 65%, with pore sizes of 300 \(\upmu\)m, 600 \(\upmu\)m, and 900 \(\upmu\)m. Individual observations were made after 2, 4, and 8 weeks. Histological examination revealed that the implant structure with a pore size of 600 \(\upmu\)m showed a higher rate of bone overgrowth after 2 weeks. After 4 to 8 weeks, this condition equalized, and all porous structures achieved the same rate of bone growth.