Biodegradable magnesium alloys for orthopaedic applications

Abstract

There is increasing interest in the development of bone repair materials for biomedical applications. Magnesium (Mg)-based alloys have a natural ability to biodegrade because they corrode in aqueous media; they are thus promising materials for orthopaedic device applications in that the need for a secondary surgical operation to remove the implant can be eliminated. Notably, Mg has superior biocompatibility because Mg is found in the human body in abundance. Moreover, Mg alloys have a low elastic modulus, close to that of natural bone, which limits stress shielding. However, there are still some challenges for Mg-based fracture fixation. The degradation of Mg alloys in biological fluids can be too rapid, resulting in a loss of mechanical integrity before complete healing of the bone fracture. In order to achieve an appropriate combination of bio-corrosion and mechanical performance, the microstructure needs to be tailored properly by appropriate alloy design, as well as the use of strengthening processes and manufacturing techniques. This review covers the evolution, current strategies and future perspectives of Mg-based orthopaedic implants.

Keywords: biodegradability; magnesium alloys; mechanical behaviour; microstructure; orthopaedic application.

Figure 1. (A) Locking compression plate. (B) Radiograph of distal tibial fracture treated with a locking plate. Reprinted from Bastias et al. Copyright 2014 European Foot and Ankle Society. (C) Interlocking nail. (D) Radiograph of femoral fracture treated with locking nail. Reprinted from Hsu et al. Copyright 2019, with permission from Elsevier.

Figure 2. (A-C) Principles of locking compression plate (A), intramedullary nailing (B), and screw (C).

Figure 3. Optimal degradation behaviour of a magnesium-based implant in bone fracture healing. The blue and yellow lines indicate the mechanical integrity and biodegradation rate, respectively.

Figure 4. Schematic illustration of the corrosion of magnesium in an aqueous environment: (A) The dissolution of magnesium via the anodic reaction. The cathodic reaction increases the pH and produces H₂, while hydrolysis reduces the pH. Intermetallic particles act as cathodic sites and consume the electrons produced by the anodic reaction. (B) Chloride ions in the solution attack and dissolve the Mg(OH)₂ film.

Figure 5. The tensile strength and elongation of various magnesium alloys. (A) Reprinted from Lu. (B) Reprinted from Chen et al. Copyright 2014, with permission from Acta Materialia Inc.

Figure 6. (A) As-built WE43 scaffold with diamond lattice fabricated by selective laser melting. Reprinted from Li et al. Copyright 2017, with permission from Acta Materialia Inc. (B) Honeycomb-structured magnesium scaffold produced by laser perforation. Reprinted from Tan et al. Copyright IOP Publishing. Reproduced with permission. All rights reserved. Scale bars: 1 mm.

Figure 7. Micro-computed tomographic three-dimensional images of as-cast Mg-3Zn-0.3Ca alloy. (A) Before immersion test. (B) After immersion test. Scale bars: 2 mm. Reprinted from Lu et al. Copyright 2018, with permission from Acta Materialia Inc.

Figure 8. (A) Fluoroscopic image of cross-section of magnesium rod in a guinea pig femur. E: endosteal bone formation; I: implant residual; P: periosteal bone formation. (B) Three-dimensional reconstruction of remaining AZ91D in the femur of a guinea pig. (C) Three-dimensional reconstruction of remaining LAE442 in the femur of a guinea pig. Scale bars: 1.5 mm. A-C were reprinted from Witte et al.^, Copyright 2005 & 2006, with permission from Elsevier Ltd.