**1. Introduction**

Magnesium is one of the most common elements in nature. It constitutes 2.7% of the earth's crust and can be found in the form of minerals, such as dolomite, magnesite or kainite [1]. The mechanical properties of pure magnesium are poor, therefore alloying additives are introduced to improve them. Magnesium alloys, in which the major additive is aluminum – typically 6–10% – are the most widely used industrial alloys of magnesium. Zinc or manganese [2, 3] are added to improve corrosion resistance of magnesium alloys. Initially, magnesium alloys were produced mainly for military purposes. Due to the high specific strength and vibration damping capacity, magnesium alloys are mainly used in the automotive industry [3, 4].

Currently, research is carried out mainly on new groups of magnesium alloys, such as Mg-Ca, Mg-Zn and Mg-Zn-Ca, which have not been produced on an industrial scale so far. Studies are carried out on the use of these alloy groups as materials for implants, especially orthopedic ones. Injuries of the osteoarticular system, as well as diseases of the musculoskeletal system, including the continuous increase

in the incidence of bone cancer, are the main and the most common threat to the health of modern society. 2,710,000 cases of orthopedic fractures were noted in Poland in 2010. Due to the aging of the population, it is predicted that, in 2025, their number will reach 3,239,564, and 10 years later – over 4 million. In 2017, 85,488 joint arthroplasties (partial or total) were performed in Poland, including 56,688 of the hip and 27,653 of the knee. Surgical joining of broken bones through their correct connection and immobilization is performed with the use of bone plates, wires, clamps and/or screws. The use of these elements results in bone union and obtaining the correct bone structure, which, in turn, allows the patient to move, and thus return to basic life activity [5–8].

In medical practice, both long-term implants (e.g. joint prostheses) and short-term implants (e.g. plates, bone screws), used to stabilize broken bones, are produced from titanium alloys, cobalt alloys or stainless steel. Implants made of those are classified as neutral, i.e. neutral to the body, as long as protective layers (usually oxide) remain on their surface. Unfortunately, after some time, these layers become corroded and damaged, and implant components – which are usually biologically incompatible (toxic to the body) – pass into the human body, and thus are a threat to health and life. Resorbable biomaterials are an alternative to the metal alloys used so far for short-term orthopedic implants. The resorbable biomaterials used in medical practice so far include oxide glasses (composed of Na, K, Mg, Ca, Si and P oxides), ceramics based on calcium phosphates, e.g. hydroxyapatite [Ca10(PO4)6(OH)2] and polymers, such as polylactide, polyglycolide or copolymers of these materials. Unfortunately, their use in orthopedic implantology is limited. They are mainly used as fillers for bone defects, elements of dentures or coatings for medical implants [9**–**11]. This is due to poor mechanical properties of resorbable biomaterials. Mechanical strength of the resorbable materials used in medicine is 30–100 MPa [10, 12]. Consequently, mechanical properties of resorbable polymers and ceramics are a barrier to their use as biomaterial for implants, such as shortterm orthopedic implants. Accordingly, resorbable metallic biomaterials that can be used for orthopedic implants are necessary. Resorbable metallic biomaterials are an alternative to the metal alloys previously used for short-term orthopedic implants. Magnesium alloys are appropriate materials for resorbable metallic biomaterials.

This chapter presents the role of magnesium in the human body and its use in medicine. It presents the concept and potential applications of magnesium alloys in medicine, as well as classification of magnesium alloys as potential biomaterials due to the structure (amorphous, crystalline) and alloying elements (rare earth elements, noble metals etc.). The chapter also describes mechanisms and degradation behavior (in vitro) of magnesium alloys due to their structure. The impact of alloy additives (rare earth elements, noble metals) and protective coatings on the degradation process of magnesium alloys for biomedical applications in in vitro conditions has also been assessed.
