## 3 Results
### Mechanical Testing  
Figure 12 shows loading curves for implants tested under axial compression. The testing machine crosshead travel (mm) is plotted on a horizontal axis; the testing load (N) is plotted on the vertical axis. Shown also are the photographs of implant specimens after fracture. Table 2 shows maximum load values for specimens under axial compression. The results show that the BI001 specimen is almost three times stronger, compared to the other three specimens. This can be explained by a denser packing of a wireframe, resulting from the use of the UC001 unit cell with rib thickness of 0.4 mm and dimensions of L = 2.3 mm, W = 2.3 mm, H = 1.7 mm. Moreover, the BI001 specimen features three rows of contact surfaces in the upper part of the model, thus forming a larger area of contact with the loading plate and providing more uniform and less intense redistribution of load from the testing machine. At the same time, the BI004 specimen also features three rows of contact surfaces at the upper part of the model; however, the thinner and longer ribs of the UC004 unit cell (thickness = 0.3 mm, L = 2.0 mm, W = 2.0 mm, H = 4.1 mm) make the structure susceptible to premature local buckling and result in a significantly lower strength of the implant. The BI002 specimen has two rows of contact surfaces in the upper part of the model and features higher strength compared to that of the BI004 specimen, due to the higher thickness of the ribs of the UC002 unit cell (rib thickness = 0.7 mm, L = 1.6 mm, W = 1.6 mm, H = 2.3 mm). The lowest strength was demonstrated by the BI003 specimen, having only one row of contact surfaces. The increase in unit cell dimensions (from UC001 to UC004) results in lower maximum load the specimen can carry and in significantly lower physical permeability of an implant, which is necessary for successful integration of the implant into the patient organism. Reducing unit cell dimensions while keeping the thickness of the ribs the same or lower is currently impossible with the SLA printing process. Due to the low strength of the printed layers, they tend to be destroyed by a printer spreading blade when applying new layers of ceramic paste (see Figure 1). Smaller unit cell dimensions in combination with an increase in rib thickness result in a collapse of gaps between neighboring ribs, thus making the removal of excess feedstock from printed parts almost impossible (even in an ultrasound bath). Figure 11: SEM images of the BI004 implant after sintering: (**a**) scaffold; (**b**) enlarged view of a pore; (**c**) enlarged view of a rib; (**d**) sintered ceramics. Table 4 also shows the effective compressive strength of implants, calculated by division of the maximum load by the area of a cylindrical base plate of 4 mm in diameter. According to the available data, the ultimate compression strength in the longitudinal direction constitutes 193 MPa for a cortical bone and \(\sim\)50 MPa for a trabecular bone [51], which is comparable to the effective compressive strength of the BI001 implant (65.1 MPa) shown in Table 4.  
Figure 13 shows the distribution of vertical strains at the surface of the BI002 specimen during testing at the 185.7 N load, obtained by digital image correlation. We also measured strains between reference points A and B, located at the cylindrical base plate and in the lower part of the wireframe, respectively (see Figure 13). Before loading, the distance between points constituted 7.26 mm. At the load of 185.7 N, the distance between reference points A and B decreased by \(\Delta U_{ABexp}=-0.00356\) mm; thus, the resulting strain constituted \(\varepsilon_{ABexp}=-0.000492\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **BI001** & **BI002** & **BI003** & **BI004** \\ \hline Maximum load at a specimen, N & 817.5 & 279.9 & 93.0 & 221.6 \\ Effective compressive strength of the implant1, MPa & 65.1 & 22.3 & 7.4 & 17.6 \\ \hline \hline \end{tabular} \({}^{1}\) The cross-section area of the implant is taken to be the area of a circle of 4 mm in diameter.  
\end{table}