A simulation model for the degradation of magnesium-based bone implants.

The development of degradable bone implants, in particular made of metal materials, is an emerging field. The advantage of degradable implants is that they do not have to be removed later. In order to be able to develop and scale appropriate implants for different applications, it is necessary to know the change in mechanical properties of the implant during the degradation process in general and at different locations. One area of bone implants are bone substitute materials. They are deployed when there is a defect in the bone which cannot be filled autonomously by the body. In this study, a numerical degradation model of magnesium-based bone substitute materials is developed using the finite element method. Computational models are being developed to reduce experimental animal research in future. Magnesium is a naturally occurring material which is needed to build enzymes in the body. Additionally, magnesium has a Young's modulus close to native bone, wherefore it is attractive for medical applications with bone contact. The simulation model is based on the assumption that the degradation is a diffusion-controlled process driven by the dissolution of magnesium. The model is adapted to a 3D open-pored structure made of the magnesium alloy LAE442. Previous studies showed that implants made of LAE442 lose stiffness without a volume reduction. To simulate the change in mechanical properties, a concentration-dependent Young's modulus is assumed. With this model the formation of the degradation layer is computable as well as the change in mechanical properties, as measured by the effective Young's modulus of the structure. The movement of the interface between the not-degraded and degraded material is modelled using the level set method.