Magnesium and Its Alloys as Implant Materials

Corrosion, Mechanical and Biological Performances

Mirco Peron, Filippo Berto, Jan Torgersen

• Publisher: CRC Press
• Year: 2020
• Topic: Technology & Engineering
• eBook ISBN:
  9781000048759
• Subtopic: Rheumatology, Orthopedics & Prosthetics
• Index: Technology & Engineering

Despite their tremendous potential, Mg and its alloys are not yet used in biomedical applications. This book aims to provide scientific insights into the challenges of the materials, and give an overview of the research regarding their mechanical properties, corrosion behaviour and biological performances. The authors intend to put the reader into the position to accurate discern the proper Mg-based material for his/her applications and to choose the proper improvement strategy to his/her cause. To this aim, the manuscript is structured as follow: in Section 2, the main challenges hampering the use of magnesium in biomedical applications and the common improvement strategies are listed. In Section 3, the most investigated Mg alloys are reported in separate sub-sections, detailing their mechanical properties, corrosion behaviour and biotoxicity. High-pure and ultra-high-pure Mg, Al-based Mg alloys, Zn-based Mg alloys, Ca-based alloys and RE-based Mg alloys have been considered. In Section 4, the alloys' performances with respect to the challenges is summarized providing the reader with useful information and suggestions on the potentially most suited choice. Finally, in Section 5, an outlook portraying the authors' opinion of the future development of the field will be provided. This book will allow biomedical engineers, surface scientists, material scientists, implant manufacturers and companies working on implant approval an overview of the state-of-the-art technologies adopted so far to overcome the drawbacks of Mg for biomedical applications. Particular emphasis is put on explaining the link between mechanical, corrosion and biocompatible properties of Mg and its alloys as well as their pros and cons. In doing so, the authors intend to put the reader into the position to accurate discern the proper Mg-based material for his/her applications and to choose the proper improvement strategy to his/her cause.

1 Introduction

1.1 Introduction

Biomedical implants have played a fundamental role in improving people’s health worldwide. They are used in applications such as orthopedics, cardiovascular stent and neural prosthetics, where there is abundant need to replace or repair fractured or diseased parts of the human body [1–4]. Among these, orthopedic surgery is characterized by the highest annual growth rate [3]. According to Long and Rach [5], almost 90% of the population over 40 years is affected by degenerative joint diseases. Total hip replacements are predicted to represent half of the estimated total number of operations in 2030 [6]. The surgical implantation of artificial biomaterials of specific size and shape is, in fact, an effective solution in restoring the load-bearing capacity of damaged bone tissue. Although outstanding mechanical and structural properties characterize bones, fractures can happen because of [7–9]

• stresses arising from daily activities
• sudden injuries
• bone infections and tumors, resulting in pathological fractures

Depending on their applications, implant devices can be classified into permanent or temporary. The former is required in applications such as joint replacements where a long-term existence in the human body is required. Appropriate materials are metals [10], polymers [11,12], ceramics [13,14] and composite materials [7,15], and a synthesis of their application has been recently published by the authors [16]. Polymers, such as PEEK (polyetheretherketone), are used as spinal cages, where the stress requirements are not high enough to require the use of metallic materials. In addition, van Dijk et al. [17] reported that polymeric cages significantly reduce the stress shielding phenomenon encountered with the application of titanium cages. Ceramic materials, such as Zirconia (ZrO₂), are used in dental applications due to their high strength (in vitro studies reported a flexural strength of 900–1200 MPa [18]), radiopacity and better esthetic [19,20]. Composite materials, such as carbon fiber-reinforced PEEK, have been studied for orthopedic applications such as hip-joint due to the possibility of tailoring their mechanical properties to those of human bones [21]. However, their long-term durability properties are still insufficient and thus metals are preferred [22]. Metallic materials are, in fact, generally used in load-bearing applications, where their high strength and fracture toughness render them preferable to ceramics (due to their higher fracture toughness), polymers (due to their high strength) and composite materials. However, metallic implants have two main problems. First, their elastic modulus highly differs from that of bone: for both stainless steel and cobalt–chrome alloys it is ten times higher, while for Ti-6Al-4V it is five times (Table 1.1).

Table 1.1

Comparison of the mechanical properties of natural bone with various implant materials [10,15,23–27]

Properties	Natural bone	Stainless steel	Ti alloy	Co–Cr alloy	Magnesium
Density (g/cm^3)	1.7–2.0	7.9–8.1	4.4–4.5	8.3–9.2	1.74–2.0
Elastic modulus (MPa)	3–20	189–205	110–117	230	41–45
Tensile strength (MPa)	80–150	480–620	930–1140	900–1540	170–270
Compressive yield strength (MPa)	130–180	170–310	758–1117	450–1000	65–100
Elongation at failure (%)	1–7	30–40	8–15	30–45	6–20
Fracture toughness (MPa m^1^/^2)	3–6	50–200	55–115	100	15–40

This leads to the occurrence of the stress shielding phenomenon, which is a phenomenological consequence of stress distribution changes that leads to the progressive bone desorption, phenomenon well described in ref. [16] and extensively reported in literature [28–37]. The second problem stems from their nonbiodegradability that leads to long-term complications, such as local inflammations due to the potential release of cytotoxic ions as a consequence of corrosion and/or wear processes [38–42]. Owing to these causes, the maximum service period of the permanent implant is around 12–15 years [43].

In contrast, temporary devices require biomaterials to stay inside the human body only for a restricted period, that is, as long as a bone heals (3–4 months [44,45]). Biodegradable materials have thus emerged to a greater extent in the fields of engineering scaffolds and bone fixators such as bone plates, screws, pins and stents, where materials that ideally degrade in the same manner and speed as natural bone heals are widely claimed [46–48]. Both natural and synthetic polymers have been studied extensively as biodegradable materials. Natural polymers such as polysaccharides and collagen have all produced favorable outcomes in tissue engineering applications [49–55], while synthetic polymers such as polyglycolic acid, poly-l-lactic acid, poly-dl-lactic acid and poly-capro lactone have been used as biodegradable sutures, drug delivery systems, fixation devices and low-load-bearing applications [56–63]. However, because of their low mechanical strength when compared to metals, polymers have been mostly used in soft tissue reconstruction and low-load-bearing applications. Moreover, they might absorb liquids and swell, leach undesirable products such as monomers, fillers and antioxidants. Furthermore, the sterilization process might affect their properties [7]. The combination of high strength and biocompatibility can be found in biodegradable metal alloys. Several of them, such as iron-based metals, Zn-based metals and tungsten, have been studied [64], but most of the scientific efforts focus on magnesium and its alloys [64–66]. Among metallic engineering materials, magnesium possesses, in fact, one of the best biocompatibility with human physiology and the best mechanical compatibility with human bones [67]. The density of magnesium and its alloys (1.74–2 g/cm³) is, in fact, very similar to that of cortical bone (1.7–2 g/cm³). In addition, the similarity of the elastic modulus of magnesium and its alloys with that of natural bones (Table 1.1) potentially reduces the possibilities of stress shielding in hard tissue applications [16,68]. Moreover, magnesium is the fourth most abundant element in the human body: the human body usually contains 35 g of magnesium per 70 kg of body weight, and it is recommended that an adult receives 240–420 mg daily [27]. It is a cofactor for many enzymes (it is involved in more than 300 enzymatic reactions), and it has roles in protein and nucleic acid synthesis, mitochondrial activity and integrity and in many other cellular functions [69–72]. Finally, Mg²⁺ ions, resulting from the degradation process (see Chapter 2), are reported to aid the healing and growth of tissue. Any excess of these ions is harmlessly excreted in the urine [24,73]. However, despite its many advantages, magnesium has some disadvantages. First, the strength of cast pure magnesium is too low compared to that of human bones [74]. This issue has been overcome by either alloying magnesium or reducing its grain size through thermo-mechanical processes (extrusion, rolling and so on) or severe plastic deformation techniques (ECAP, MAF and so on). Another disadvantage of magnesium and its alloys is their high corrosion rate in the body that may lead to a loss of mechanical integrity before tissues have sufficient time to heal. Moreover, hydrogen as a corrosion product together with the generation of respective hydrogen pockets can influence the healing process or, if the pockets are large, they may cause the death of patients through blocking of the blood stream [75,76]. Finally, the simultaneous action of the corrosive human body fluid and the mechanical loading can cause further complications of sudden fracture of implants due to corrosion-assisted cracking, such as stress corrosion cracking and corrosion fatigue [77,78].

Despite their high potential, magnesium and its alloys are not yet being utilized in biomedical applications, as the aforementioned challenges could yet not be overcome. In this work, we aim to provide scientific insights into these challenges and give a thorough state-of-the-art overview of the research conducted thus far, particularly focusing on the mechanical properties, corrosion behavior and biological performances of these materials. The majority of the alloys are initially introduced for industrial applications and then evaluated for biomedical applications. Here, the main alloys are reviewed with regard to their suitability for biomedical applications. Hence, their mechanical properties (tensile and compressive yield and ultimate strength, tensile elongation to failure), corrosion properties in terms of electrochemical propertie...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of Abbreviations
• Authors
• Chapter 1. Introduction
• Chapter 2. Challenges and Common Strategies
• Chapter 3. Synopsis of Properties of Biocompatible Mg and Its Alloys
• Chapter 4. Tackling the Challenges
• Chapter 5. Outlook
• Appendix A: Corrosion
• Appendix B: In Vitro Biocompatibility Assessment
• Index