The degradation prediction of Structural Mg-based implant for orthopaedic applications

Magnesium (Mg) alloys with a porous structure have been found to serve as high-potential orthopaedic biomaterials due to its super lightweight, closer elastic moduli to that of natural bones, which would minimize stress shielding and implant loosening. Therefore, a novel design of Mg implant having a coral-like open-cell porous interior and an outer solid casing was achieved. Different porosities of porous interiors, combined with different wall thicknesses of outer casing, were designed. By implementing a modified continuum damage mechanics (CDM)-based biodegradation model into finite element (FE) simulations, the mechanical properties and degradation rates of the implant were predicted. Suitable porous implant structure was then determined and fabricated with comprehensively considering the tissue regeneration and implantation strength. An in-vivo rabbit model was employed to evaluate the degradation behaviours of the implant at different time points. In addition, the CDM-based model was applied for the degradation prediction of the solid-type Mg implant, microstructural configurations of Mg-Zn-Mn (ZM) alloys that depicted from SEM were employed as the geometrical FE models, in which the Mg matrix, grain boundary and second phase were included. Predicted results revealed that the grain boundary had poor corrosion resistance while the second phase facilitated delaying corrosion expansion. Furthermore, in-vitro tests were carried out and consistent results were obtained, i.e., the grain refinement made the entire corrosion process more uniform and severe corrosion in local areas was avoided, and the intergranular second phase was beneficial to delay the corrosion process. This study suggested that designing implant structures from a biomimetic structure perspective is an effective way to meet the strength requirements and meanwhile benefit the ingrowth of bone tissue and expedite the healing process. Moreover, the application of FE simulation, rather than experimental testing, is believed to be an efficient method for the degradation evaluation of Mg-based implants.