Research Progress on Biodegradable Porous Metallic Bone Implants

The clinical efficacy of traditional inert metallic bone implants is limited by stress shielding caused by mechanical mismatch and by the need for a second operation to remove the implant. Biodegradable metals have become a focus of research in bone repair materials because they degrade and are absorbed in vivo, their mechanical properties can be tuned, and their biocompatibility is favorable. The introduction of a porous structure further modulates the apparent elastic modulus of the material to match the host bone tissue and provides a suitable microenvironment for cell adhesion, proliferation, and osseointegration. This review summarizes the characteristics and the current status of research on the three major biodegradable metal groups, namely, the Fe-, Mg-, and Zn-based systems. It compares the advantages and limitations of fabricating porous metallic scaffolds by electrodeposition, sintering, space-holder casting, and additive manufacturing. A mechanistic framework is proposed that links alloy composition, pore architecture, degradation kinetics, and the time-dependent evolution of mechanical properties, in which the weakening caused by matrix dissolution competes with the strengthening caused by the deposition of corrosion and mineralization products. The mechanical compatibility window defined by matching the mechanical degradation rate of the scaffold to the rate of bone regeneration is then analyzed, and a dimensionless ratio of the two rate constants is proposed as a criterion for this matching. Finally, future directions in this field are discussed, with the aim of providing a reference for the development of high-performance biodegradable porous metallic bone implants.