Y. Okazaki,     National Institute of Advanced Industrial Science Technology, Japan

Abstract:

Metal material principles of orthopaedic implants are integrated in understanding the factors affecting the performance and durability. This chapter reviews aspects governing biocompatibility such as corrosion resistance and mechanical compatibility, and the related testing methods. The future perspective regarding materials and testing methodology are also considered.

Key words

orthopaedic implant

metallic materials

microstructure

corrosion resistance

fatigue property

mechanical compatibility

durability of device

testing method

2.1 Introduction

Many types of metallic biomaterial devices have widely been used to replace failed hard tissues; namely, bone screws, bone plates, compression hip screws (CHS), intramedullary fixations, short femoral nails, artificial hip joints, artificial knee joints, spinal instruments, and dental implants. Orthopaedic implants require biomechanical and biochemical compatibilities, as well as biological safety. Therefore, many types of metallic orthopaedic devices, manufactured from metallic materials with excellent mechanical properties and structural stability, are used worldwide in the orthopaedic field. In order to determine biomechanical and biochemical properties, as well as biological safety, many mechanical, chemical and biological tests are conducted during device developments. This chapter will provide the test methods for characterising the appropriate materials for metallic orthopaedic devices such as osteosynthesis and artificial joints. Recent topics on the mechanical, chemical, and biological test results of the new biocompatible materials will be introduced, with the developments of new orthopaedic devices.

2.2 Standardised implantable metals

Stainless steels, cobalt (Co)–chromium (Cr)–molybdenum (Mo) alloys, commercially pure titanium (CP Ti), and Ti alloys are widely used in orthopaedic surgery. Many types of these alloys have been standardised on the basis of international and national standards; for example, International Organisation for Standardisation (ISO), American Society for Testing and Materials (ASTM), and Japanese Industrial Standard (JIS). As well as the alloy specifications, main corrosion test methods and corrosion testing solutions for metals used for orthopaedic implants are also standardised as guides (Table 2.1).1 − 19 In the orthopaedic field, given that mechanical compatibility has an effect on the long-term clinical result, materials for these devices should have excellent mechanical strength and ductility. Figure 2.1 shows the correlations between minimum values of ultimate tensile strength (σUTS) and total elongation (T. E.) of these materials specified in ISO and JIS, namely, stainless steel, high-nitrogen stainless steel, Co–Cr–Mo alloys, and Ti materials (CP Ti and Ti alloys). The T. E. of various materials continuously decreases with increasing σUtS. The σutS of the cold-worked CP Ti grade 4 containing small amounts of oxygen (O) and iron (Fe) is close to those of the Ti alloys. The main applications of these alloys are shown in Table 2.2. Stainless steel (ISO 5832-1 and ISO 5832-9), CP Ti and Ti alloys are widely used in osteosynthesis devices, while C–28Cr–6Mo alloys are used in artificial joint bearing parts. Artificial hip joints, consisting of two types of stems and immobilising bones, with or without cement, are widely used in many countries. However, for the cement-type stem, the consumed amount of Ti alloys has tended to decrease recently. This is due to corrosion caused by micromotion between the stem and the cement mantle (formed by cement inserted into bone), particularly when less-stiff Ti alloy is used for the artificial hip stem.20 In contrast, the Co–28Cr–6Mo alloy and high-nitrogen (N) stainless steel, with high strength and stiffness, has become the most popular cement-type stem material. On the other hand, for the cementless stem, the use of Ti alloy with high biocompatibility has become popular. Figure 2.2 shows the relationship between σUTS and Vickers hardness with tensile test specimens (diameter: 1.5 or 3 mm, gauge length: 9 or 15 mm) taken from orthopaedic devices: bone plates, compression hip screws (CHS), intramedullary rods, short femoral nails, artificial hip stems, artificial knee femoral components, and tibial stems. The σUTS of forged Co–28Cr–6Mo alloys and stainless steels used for artificial hip stems is very high. Figure 2.3 shows typical optical micrographs and transmission electron microscopy (TEM) images of the hot-forged Co–28Cr–6Mo alloy and cold-worked high-nitrogen stainless steel (ISO 5832–9) used for the cement-type stem. In the TEM images of hot-forged Co–28Cr–6Mo alloys, much finer fcc γ (gamma) structures are observed. For high-carbon Co–28Cr–6Mo alloys, M23C6 carbide precipitates in the γ matrix.21 In cold-worked high-nitrogen stainless steel, CrNbN precipitation is observed in the fine austenitic (γ) phase in Fig. 2.3(d).22