For example, some have poor mechanical properties and display limited osteoinduction in the clinic [ 16 , 17 ]. Metallic materials are another alternative for use in the repair or replacement of diseased or damaged bone tissue. Metallic materials currently widely used in orthopedics include stainless steel and titanium alloys because they are mechanically strong and resistant to fracture [ 18 — 21 ].
Furthermore, the elastic moduli and tensile strength of metals and bone are significantly different, which can cause stress shielding and result in weakening of surrounding bone. These inert implants also often need to be removed via invasive secondary surgeries once the bone fracture has completely healed. To minimize trauma to the patients and decrease medical costs, biodegradable implants could be used to replace traditional metal implants and remove the need for secondary surgeries [ 22 — 26 ].
Magnesium Mg alloys have a reputation for being revolutionary biodegradable metal materials in orthopedic applications due to their good biocompatibility, biodegradability, and acceptable mechanical properties [ 27 — 30 ]. The fourth most plentiful cation in the human body, Mg is an element essential in many metabolic processes and is primarily stored in bone tissue.
Mg is taken into the body daily in substantial amounts, stimulates the growth of bone cells, and accelerates the healing of bone tissue. Moreover, Mg alloys have mechanical properties similar to those of bone. Mg alloys are lightweight with densities 1. Therefore, the stress shielding from the notable mechanical mismatch between natural bone and metal implants should be mitigated [ 35 — 37 ]. Therefore, Mg alloys are expected to become biocompatible, biodegradable, lightweight, and load-bearing orthopedic implants [ 22 , 38 — 40 ].
While research on Mg alloys as bone implants has led to significant progress over the past 20 years, rapid degradation of these materials inside the human body is still a major obstacle hampering their use in the clinic. As biodegradable materials, it is important that the rate of implant degradation matches the rate of healing of the bone tissue, which generally consists of an early inflammatory stage lasting from 3 to 7 days, a reparative stage that leads to a strong healing union lasting about months, and then a remodeling phase that can last months to years [ 41 — 43 ].
Therefore, it is necessary for the implant to remain stable for at least 12 weeks [ 22 ]. However, the currently available Mg alloys degrade too quickly to hold well during implantation.
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This fast degradation results in the formation of hydrogen gas cavities, rapid loss of mechanical integrity of the implants, and adverse host tissue reactions, such as local swelling and significant pain within the first week after surgery [ 44 — 46 ]. There have been a number of recent opportunities and challenges in the development of Mg alloys for use in bone repair. Therefore, it is necessary to summarize the findings of the researchers in this field. Compared to recently published reviews [ 27 , 47 — 53 ], this paper is more targeted and specifically discusses biodegradable Mg alloys to be used in bone repair.
We review the alloying design, surface modifications, and the in vitro and in vivo biological performance of Mg in bone repair. Novel insights that have been used to improve the compatibility and reliability of biomedical Mg alloys in the bone reconstruction field are also discussed. Adequate strength, ductility, fatigue resistance, and biocorrosion resistance are important characteristics for biodegradable implants to be used in orthopedic applications. Because adding alloying elements can improve mechanical properties and decrease the corrosion rate of Mg by modifying the structure and phase distribution, several Mg alloys have been designed to meet the requirements of bone repair implant materials [ 30 , 32 , 60 ].
Careful selection of alloying elements is the first step in designing Mg alloys. To strengthen Mg-based materials, adding elements such as Al, Zn, Ca, Ag, Ce, and Th can generate different microstructures and improve the mechanical properties of the resulting Mg alloy [ 71 — 74 ]. Biocompatibility also needs to be considered. Previous reports have shown that biological nutrients e. With the development of biodegradable Mg alloys, researchers have started trying to endow Mg alloys with new biomedical functions through alloying.
Ca, Sr, Ag, and Cu as biofunctional trace metallic elements have been confirmed to promote bone cell activation and stimulate new bone formation. In addition to promoting osteogenesis, these elements also inhibit bacterial infection after implantation, thereby effectively decreasing morbidity and mortality, by making the environment alkaline and releasing antimicrobial metallic ions [ 81 — 86 ].
Due to having a combination of good mechanical properties and corrosion resistance, some commercial Mg alloy systems have been selected as biodegradable Mg alloys at an early stage. It has been reported that AZ31 and AZ91 alloys release hydrogen upon degradation in physiological environments, leading to a significant increase in both pH and Mg ion concentration [ 90 ].
In Hank's solution, the AZ31 alloy degrades more slowly than the AZ91 alloy, but there is no significant difference in vivo [ 91 , 92 ]. Short-term in vivo studies of AZ31 and AZ91 alloys have also revealed that a biocompatible Ca phosphate protective film layer covers their surfaces and increases the formation of new bone mass around the implants [ 92 , 93 ]. WE series alloys have good biocorrosion resistance because they form a rare-earth RE oxide film in aqueous environments.
It has been reported that WE54 1. Witte et al. However, an increase in Al ion concentration in the brain is associated with the occurrence of Alzheimer's disease and severe hepatotoxicity has occurred after the administration of RE elements, such as Y, Ce, and Pr [ 6 ]. However, the extremely high rates of degradation of Mg-Zn-Zr alloys are alarming and restrict their future development.
Ca, acting as a grain-refining agent in Mg alloys, can stabilize grain size at levels up to 0. As a major component of human bone, Ca is essential for bone cell signaling and beneficial to bone healing. It has been reported that Mg-1Ca alloy does not induce cytotoxicity and osteoblasts and osteocytes are highly active around Mg-1Ca alloy pins implanted in rabbit femoral shafts, thus demonstrating good biocompatibility and bioactivity [ 84 ]. Strontium Sr and Ca belong to the same family and have similar physical and chemical properties and biological functions.
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Zinc Zn is one of the most abundant essential nutrients in the human body and is safe for use in biomedical applications [ ]. The rate of Mg corrosion can be reduced by increasing the mass fraction of Zn mixed with Mg, thus strengthening the mechanical properties of Mg through solid solution hardening [ ]. Cai et al. Mg-6Zn alloy has good biocompatibility in vitro based on hemolysis and MC3T3-E1 cell adhesion assays [ ].
Among these, Mg-Nd alloy has a much slower corrosion rate than the other alloys [ 74 ]. Mg-Y alloy was prepared using a zone solidification method and improved corrosion resistance and mechanical properties [ ]. Mg-Y-Zn alloy contains an interesting combination of preferred microstructural, mechanical, electrochemical, and biological properties, making it very promising for use as a biodegradable implant material [ ].
Alloying elements in Mg alloys may exist in the form of second-phase particles and precipitate in grains or grain boundaries, substantially enhancing mechanical properties through second-phase strengthening. Figure 1 presents the typical morphologies of second phases for Mg alloys and Table 1 presents the second phases of biodegradable Mg alloys. Compared to Mg matrix, second phases have higher potentials and may facilitate corrosion, leaching into the physiological environment accompanied with the degradation of the matrix. Kannan investigated the degradability of Mg 17 Al 12 phase in simulated body fluid SBF using electrochemical measurements and found that the degradation rate of Mg 17 Al 12 was lower than that of bare Mg.
Our previous study demonstrated that pitting corrosion occurs with crackings for Mg 17 Al 12 phase in Hank's solution and degrades much slower than AZ31 alloy and pure Mg [ ]. When assessing Mg alloy implants for use in bone repair, the stability of second phases and Mg matrix under different conditions may have significantly influenced degradation and biological responses to the implant in the body. Yang et al. The second phases had higher phase stability than Mg matrix, but the phase stability was quite different for different types of second phases and second-phase-4H 2 O systems [ 71 ].
In order to evaluate the effect of second phases on the biological safety of biodegradable Mg alloy implants, Mg 17 Al 12 second phase from Mg-Al-Zn alloys was investigated for in vitro biocompatibility and phagocytosis by macrophages. Mg 17 Al 12 second phase did not induce hemolysis and had excellent cytocompatibility. Mg 17 Al 12 particles are processed in endolysosomal compartments and lysosomes play a major role in digesting Mg 17 Al 12 particles [ ]. However, not all the alloying elements in Mg alloys form second-phase particles.
In the solution, the original crystal structure of magnesium remains unchanged, but a lattice distortion is produced and thus the motion of dislocations becomes impeded, which leads to the enhancement of strength of Mg.
Gao et al. They found enhanced hardness as the Y content increased at room temperature because of large differences in the atomic radii of Y and Mg and a relatively wide range of solubilities [ ]. Moreover, solid solution alloying also potentially affects degradation of Mg alloys by improving corrosion resistance by reducing internal galvanic corrosion between the second phase and Mg matrix.
Zhang et al. Therefore, solid solution might be a feasible alternative for generating a single-phase Mg alloy and can help improve the corrosion resistance of Mg alloys in orthopedic applications. During casting and refining, magnesium always introduces superfluous amounts of impurity elements. Impurity elements in Mg alloys usually include iron Fe , nickel Ni , and copper Cu [ 66 , ]. These elements can significantly accelerate Mg corrosion when their concentrations exceed the limits of tolerance [ — ]. Below the tolerance limits, no impurity particles are formed and, thus, no electrochemically active cathodic sites exist to accelerate corrosive attack, which keeps the corrosion rate very slow.
When levels are above the tolerance limits, Fe, Ni, and Cu in Mg alloys significantly increase the corrosion rate due to the low solubility of these elements and their distinctly more noble position in the electrochemical series [ 66 ]. Atrens et al. Recent studies have shown that adding silicon Si to the reactive impurity elements Fe, Ni, and Cu is detrimental to corrosion, as it plays a critical role in promoting the formation and growth of Fe-rich particles.
Lee et al.
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In addition to accelerating corrosion, excessive impurity elements are also harmful to biocompatibility. For example, Ni leaching into the body has toxic biological effects and high levels of Cu exert a toxic effect at cell surfaces [ ]. In order to reduce impurity during casting and refining, the crucible, stirrer, and mold containing no such elements are prudently utilized [ 31 ]. As the chemical properties of Mg alloys are very active, a large amount of nonmetallic inclusions is also produced during casting and refining which act as additional major impurities in Mg alloys [ ].