## 3 Results
### Custom-Made Implant 1  
The biomechanical test of reference implant 1 showed that the implant withstood an axial load of 2.6 kN for 10,000,000 cycles without failure, which implies a fulfillment of the pre-defined acceptance criterion [26]. To identify the safety margin and the failure mechanism, the peak load was increased by 500 N after completion of each 1,000,000 cycles. One sample showed a fracture in the screw and stem in load step 4.6 kN, one sample showed a fracture of the proximal-anterior part of the box at 3.6 kN, and one sample completed the load step of 4.6 kN without any visible failure. It is worth mentioning that implant fracture is inevitable with increasing loads and that the observed failure is not necessarily clinically relevant. For this specific off-the-shelf implant, several studies and register data demonstrate that this implant provides reliable and durable reconstruction of the knee joint [27; 28; 29].  
The FEAs of reference implant 1 and custom-made implant 1 were performed with screw pre-tension values of 5 kN, 8 kN and 10 kN. Obtained values of tensile stress, van Mises stress and micromotion were compared between reference implant 1 and custom-made implant 1, in terms of absolute values and relative values (Table 2). Custom-made implant 1 showed a maximum tensile stress of 36% and a maximum van Mises stress of 74% and 76%, compared to reference implant 1, respectively. Custom-made implant 1 showed a maximum micromotion of 113% at a screw pre-tension of 5 kN compared to reference implant 1. With increased screw pre-tension to 8 kN and 10 kN, the maximum micromotion of custom-made implant 1 was 68% and 63% compared to reference implant 1, respectively.  
The maximum tensile stress of reference implant 1 occurred in the proximal-anterior area of the box (Figure 4a). The maximum tensile stress of custom-made implant 1 was observed in a more posterior area compared to reference implant 1. The observed crack in the second sample of reference implant 1 in the proximal-anterior area of the box could be explained by the high tensile stress in this area (Figures 4a and 5b).  
The FEA results of custom-made implant 1 were verified by a biomechanical test. This test showed that the implant withstood a cyclic loading of 2.6 kN for 10,000,000 cycles without any visible failure and fulfilled, thereby, the required performance criteria. The observed displacement was in an expectable range (Figure 5a). A stepwise increase in the peak load of 500 N after completion of every 1,000,000 cycles led to a fracture of the screw at a load level of 4.6 kN after 13,485,000 cycles (Figure 5c).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Max. Tensile Stress [MPa]**} & \multicolumn{2}{c}{**Max. Van Mises Stress [MPa]**} & \multicolumn{2}{c}{**Max. Micromotion [mm]**} \\ \hline
**Screw Tension** & **5 kN** & **8 kN** & **10 kN** & **5 kN** & **8 kN** & **10 kN** & **5 kN** & **8 kN** & **10 kN** \\ \hline Reference & 1381 & 1736 & 2020 & 1734 & 1736 & 1801 & 0.171 & 0.0232 & 0.0197 \\ implant & 100\% & 100\% & 100\% & 100\% & 100\% & 100\% & 100\% & 100\% & 100\% \\ Custom-made & 509 & 628 & 725 & 1283 & 1283 & 1 376 & 0.194 & 0.0159 & 0.0124 \\ implant & 36\% & 36\% & 36\% & 74\% & 74\% & 76\% & 113\% & 68\% & 63\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: FEA results of custom-made implant 1.  
Figure 4: Reference implant 1 and custom-made implant 1. (**a**) Maximum tensile stress (\(\sigma_{\max}\)) and tensile stress distribution of reference implant 1 as well as (**b**) custom-made implant 1.