Effect of nutrient-based alloying elements on biodegradable magnesium alloys: Evolution, challenges, and strategies for orthopaedic applications

In recent years, magnesium (Mg) alloys have become increasingly popular in orthopaedic applications as biomaterials. Unlike traditional implants such as cobalt-chrome, stainless steel, and titanium alloys, Mg alloys offer notable advantages, including outstanding biodegradability and biocompatibility. This characteristic eliminates the need for a second surgery after the bone healing process, a distinct advantage for patients. Additionally, Mg alloys address the issue of stress shielding, a common problem with other materials. Despite facilitating the osteoconductive process, their rapid degradation in physiological conditions poses a challenge, compromising mechanical strength and hindering bone tissue recovery. This degradation leads to tissue alkalization and the formation of hydrogen bubbles, hindering the recovery rate of bone tissues and limiting the applications of Mg alloys. And the rapid degradation of magnesium alloys in physiological conditions accelerates corrosion and compromises mechanical integrity, affecting their load-bearing capacity. Enhancing structural integrity is essential to ensure sufficient strength during bone healing, aligning the degradation rate with the physiological process. To reduce the fast degradation rate, extensive research has been conducted in mechanical and corrosion-based studies, focusing on altering the biomedical performance of Mg alloys through alloying elements, processing routes, and other strategies. One approach involves mixing pure magnesium with nutrient materials and reinforcing it with hydroxyapatite. These modifications aim to match the corrosion rate with the healing rate of bone tissue. This paper explores the significance of biodegradable Mg alloys, providing a comprehensive review of their evolution and development. It emphasises enhancing the mechanical and corrosion properties of Mg alloys by adjusting the percentage of alloying elements, employing specific processing strategies, and incorporating reinforcements. The discussion particularly emphasizes the impact of nutrient elements, binary and ternary alloys, as well as hydroxyapatite composites of magnesium-based alloys in physiological conditions. Furthermore, the review highlights emerging technologies like Laser Powder Bed Fusion (LPBF), offering a general perspective on improving the mechanical and corrosion properties of Mg alloys for orthopaedic use.