## 4 Discussions  
In this study, we aimed to compare the stresses between the radius and two carpal bones (the scaphoid and the lunate) in two wrist positions: extension and radial extension (position in DTM) and between an adult and elder model. Our finding shows that the von Mises, shear stresses and contact stresses were higher in the extension model compared with the radial-extension model and were higher for the radius-scaphoid interface in the adult model compared with the elder model. In the radius-scaphoid interface, the stress differences between the two wrist positions were smaller in the elder model (11.5% to \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Adult Model**} & \multicolumn{3}{c}{**Elder Model**} \\ \hline  & \multicolumn{1}{c}{**Extension**} & \multicolumn{1}{c}{\begin{tabular}{c} **Radial-** \\ **Extension** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **\% Of** \\ **Extension** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **\% Of** \\ **Extension** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Extension** \\ \end{tabular} } & \multicolumn{1}{c}{\
\begin{tabular}{c} **Radial-** \\ **Extension** \\ \end{tabular} } & \multicolumn{1}{c}{
\begin{tabular}{c} **\% Of** \\ **Extension** \\ \end{tabular} } \\ \hline \multicolumn{5}{c}{**Radius-Scaphoid interface**} \\ Maximal von Mises stress [MPa] & 19.9 & 8.2 & 41.1 & 16.7 & 3.8 & 22.5 \\ Maximal shear stress [MPa] & 11.2 & 4.7 & 41.5 & 9.4 & 2.1 & 21.7 \\ Maximal contact stress [MPa] & 29.7 & 10.4 & 35.1 & 29.1 & 5.9 & 20.2 \\ Average contact stress [MPa] & 8.4 & 2.8 & 33.6 & 8.5 & 1.0 & 11.5 \\ Contact area [mm\({}^{2}\)] & 6.4 & 6.5 & 100.9 & 7.3 & 3.7 & 50.2 \\ \hline \multicolumn{5}{c}{**Radius-Lunate interface**} \\ Maximal von Mises stress [MPa] & 9.5 & 6.0 & 62.7 & 10.9 & 7.5 & 68.8 \\ Maximal shear stress [MPa] & 5.4 & 3.4 & 63.0 & 6.1 & 4.2 & 68.8 \\ Maximal contact stress [MPa] & 15.5 & 10.7 & 68.7 & 18.6 & 10.7 & 57.5 \\ Average contact stress [MPa] & 5.3 & 3.1 & 57.2 & 5.1 & 4.1 & 81.0 \\ Contact area [mm\({}^{2}\)] & 3.4 & 7.6 & 222.2 & 3.9 & 5.0 & 127.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Maximal von Mises stresses, maximal shear stresses, maximal and average contact stresses [MPa] and contact area [mm2] in extension and radial-extension models in the adult and elder simulations. The percent of variable in the radial-extension model in relation to the extension model is calculated and presented as _”% of Extension”_. Figure 4: (a) von Mises stresses [MPa] and (b) min principal stress on the Radius cartilage [MPa] for the _adult model_ (left) and _elder model_ (right): (upper) _extension_, (bottom) _radial extension_. 22.5%) compared with the adult model (33.6-41.5%). During radial extension, the contact area at the radius-lunate interface was increased, more so in the adult model (222.2%) compared with the elder model (127.9%), while the contact area at the radius-scaphoid was not affected by the position of the wrist in the adult model (100.9%) but decreased in the elder model (50.2%) during radial extension. The altered stress distribution at the radiocarbon interface during radial extension compared with extension most likely originates from the differences in loading dynamics. As depicted in Table 1, approximately 24% of the force applied by the FCU during extension is divided between the other muscles during radial extension. Consequently, kinematics of the carpal bones is altered, as previously reported by [29], which showed that during flexion-extension, the scaphoid contributes mostly in the radiocarbon, whereas during DTM, it contributes almost equally in the radiocarbon and midcarpal joints. The lunate shows slightly different kinematics, as it contributes almost equally in both joints during flexion-extension motion, but is more prominent in the midcarpal joint during DTM [29].