## 3 Results
\begin{table}
\begin{tabular}{|c|c|c|c|c|} \hline \multicolumn{4}{|c|}{**GPR1 Model (Training Data)**} \\ \hline \multirow{2}{*}{**Target Variable**} & \multicolumn{2}{c|}{**Medial**} & \multicolumn{2}{c|}{**Lateral**} \\ \cline{2-5}  & \(\mathbf{R^{2}}\) & **RMSE** & \(\mathbf{R^{2}}\) & **RMSE** \\ \hline Contact area [mm\({}^{2}\)] & 1.00 & 0.045 & 1.00 & 0.046 \\ \hline Contact pressure [MPa, peak] & 1.00 & 0.0020 & 1.00 & 0.018 \\ \hline Contact pressure [MPa, average] & 1.00 & 0.00077 & 1.00 & 0.00072 \\ \hline Tensile stress [MPa, peak] & 1.00 & 0.0030 & 0.99 & 0.12 \\ \hline Tensile stress [MPa, average] & 1.00 & 0.0015 & 1.00 & 0.00083 \\ \hline Tensile strain [-, peak] & 0.97 & 0.0022 & 0.92 & 0.0035 \\ \hline Tensile strain [-, average] & 0.98 & 0.00066 & 0.98 & 0.00076 \\ \hline Shear stress [MPa, peak] & 1.00 & 0.0015 & 1.00 & 0.040 \\ \hline Shear stress [MPa, average] & 1.00 & 0.00082 & 1.00 & 0.00067 \\ \hline Shear strain [-, peak] & 0.98 & 0.0029 & 0.89 & 0.0049 \\ \hline Shear strain [-, average] & 0.99 & 0.00064 & 0.99 & 0.00073 \\ \hline Pore pressure [MPa, peak] & 1.00 & 0.0043 & 1.00 & 0.055 \\ \hline Pore pressure [MPa, average] & 1.00 & 0.0021 & 1.00 & 0.0013 \\ \hline Degeneration [mm\({}^{3}\)] & 1.00 & 0.031 & 1.00 & 0.070 \\ \hline \multicolumn{4}{|c|}{**GPR1 Model (Validation Data)**} \\ \hline \multirow{2}{*}{**Target Variable**} & \multicolumn{2}{c|}{**Medial**} & \multicolumn{2}{c|}{**Lateral**} \\ \cline{2-5}  & \(\mathbf{R^{2}}\) & **RMSE** & \(\mathbf{R^{2}}\) & **RMSE** \\ \hline Contact area [mm\({}^{2}\)] & 0.98 & 2.67 & 0.80 & 16.46 \\ \hline Contact pressure [MPa, peak] & 0.95 & 0.19 & 0.88 & 0.30 \\ \hline Contact pressure [MPa, average] & 0.97 & 0.040 & 0.85 & 0.14 \\ \hline Tensile stress [MPa, peak] & 0.92 & 0.44 & 0.82 & 0.67 \\ \hline Tensile stress [MPa, average] & 0.94 & 0.14 & 0.83 & 0.33 \\ \hline Tensile strain [-, peak] & 0.86 & 0.0046 & 0.66 & 0.0068 \\ \hline Tensile strain [-, average] & 0.94 & 0.0012 & 0.88 & 0.0025 \\ \hline Shear stress [MPa, peak] & 0.92 & 0.17 & 0.83 & 0.27 \\ \hline Shear stress [MPa, average] & 0.95 & 0.058 & 0.85 & 0.13 \\ \hline Shear strain [-, peak] & 0.88 & 0.0053 & 0.68 & 0.0084 \\ \hline Shear strain [-, average] & 0.96 & 0.0015 & 0.89 & 0.0033 \\ \hline Pore pressure [MPa, peak] & 0.94 & 0.49 & 0.79 & 0.86 \\ \hline Pore pressure [MPa, average] & 0.96 & 0.19 & 0.86 & 0.36 \\ \hline Degeneration [mm\({}^{3}\)] & 1.00 & 1.74 & 1.00 & 2.61 \\ \hline \end{tabular}
\end{table} 
Table 2: Accuracy of the trained GPR1 models with training and validation data for different target variables. Statistics were performed with the Pearson correlation (\(\mathbf{R^{2}}\)) and the Root Mean Squared Error (RMSE) between outcomes from GPR1 model and FEA outputs; \(p<0.001\) in all cases. \begin{table}
\begin{tabular}{|c|c|c|} \hline \multicolumn{3}{|c|}{**GPR2 Model (Training Data)**} \\ \hline \multirow{2}{*}{**Target Variable**} & \(\mathbf{R^{2}}\) & **RMSE** \\ \hline \multirow{2}{*}{\(\mathrm{JS}_{\mathrm{MED-Xray}}\) [mm]} & 0.62 & 0.73 \\ \cline{2-3}  & 0.65 & 0.50 \\ \hline \multirow{2}{*}{\(\mathrm{W}_{\mathrm{Xray}}\) [mm]} & 0.96 & 1.35 \\ \hline \end{tabular}
\end{table} 
Table 3: Accuracy of the trained GPR2 models with training and validation data for different target variables (knee geometry). Statistics were performed with the Pearson correlation (\(\mathbf{R^{2}}\)) and the Root Mean Squared Error (RMSE) between outcomes from GPR2 model and manually measured knee joint dimensions; \(p<0.001\) in all cases. Figure 4: ROC (receiver operating characteristic) curves with AUC (area under the ROC curve) values for discrimination between KL01 and KL34 groups for atlas-based FEA and GPR-based predictions. The KL0 and KL1 groups were merged into the KL01 group. This was justified by the significant differences between KL0 and KL34 groups and KL1 and KL34 groups (Figures 3 and 4).