# Evaluating the Degradation of WE43 for Implant Applications: Optical and Mechanical Insights
## 2 State of the Art  
### Classification of Magnesium and Its Alloys  
Current metallic implants predominantly consist of a titanium alloy or a CoCr-alloy, which has at least three times the Young's modulus and five to ten times the tensile strength compared to human bone (Figure 1) [13]. Due to the higher mechanical properties, stress shielding occurs when these materials are used as load-bearing implant materials. Efforts are being made to avoid this process. This can be achieved, for example, by choosing a different material. As a result, magnesium alloys are increasingly becoming the focus of research questions. Using magnesium alloys, the stress shielding of the bone is reduced and better healing of the bone is achieved [4].  
The group of magnesium alloys also includes the alloy WE43 [14; 15]. WE43 is approved for medical use and, in terms of mechanical properties and regulatory approval as a medical implant, is comparable to the magnesium screws commercially distributed by Syntellix [12]. In addition, this alloy has a similar density to bone, as shown in Table 1. Regarding tensile strength and Young's modulus, the values are above those of human bone and partially above those of pure magnesium [13].  
In addition to the mechanical properties, other characteristics like biocompatibility or manufacturability are also important for an implant material. During the degradation of magnesium alloys in aqueous solutions, including physiological fluids such as body replacement fluid, magnesium dissolves. As a result of the degradation, magnesium hydroxide (Mg(OH)2) and hydrogen gas (H2) are formed (Equation (1)). These degradation products must be manageable by the body. Therefore, it is important to select a degradation rate that aligns with the body's capacity to process the resultant degradation products [17; 18].  
\[Mg+2H_{2}O\rightleftharpoons Mg(OH)_{2}+H_{2} \tag{1}\]  
In addition to the time required for the removal of degradation products, the loss of mechanical strength due to the degradation process also plays a crucial role in determining the recommended degradation rate [19]. The degradation rate in millimetres per year (mm/y) should be less than 0.5 mm/y and higher than 0.2 mm/y to match the healing rate of the bone and correspond to the body's capacity to manage the resulting degradation products [20]. The degradation rate can be calculated using Equation (2) [21]. Early degradation can lead to failure of the implant. This, in turn, means that complete healing cannot be guaranteed. This is because the bone has not yet had sufficient time to regain strength and stability and to withstand the loads that occur [22]. The relevance of the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Density** & **Tensile Strength** & **Young’s Modulus** \\  & **in g/cm3** & **in MPa** & **in GPa** \\ \hline Average human bone & 1.80–2.10 & 110–130 & 10–30 \\ Pure magnesium & 1.74–2.00 & 90–190 & 41–45 \\ WE43 & 1.83–1.84 & \(\sim\)220 & \(\sim\)44 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Individual mechanical properties of human bone, pure magnesium, and magnesium alloy WE43 [14; 16].  
Figure 1: Schematic overview of the tensile strength via the modulus of elasticity of various materials compared to human bone (modified after [4]).  
required mechanical strength of the bioresorbable implant can be seen by comparing the degradation of the implant and the healing process of the bone in Figure 2. The implanted material should necessarily provide the required load-bearing capacity during the healing process, as it takes over the support function of the bone [23].  
\[P_{W}=\frac{3.65\times\Delta W_{r}}{\rho} \tag{2}\]  
\(P_{W}\) = degradation rate in mm/y (gravimetric); \(\Delta W_{r}\) = mass loss per area and time unit in mg/cm\({}^{2}\)/d; \(\rho\) = density of the sample in g/cm\({}^{3}\)[21].