**2. Metal alloys**

The use of metal implants in orthopaedics dates as far back as 1775 with the use of brass wire for fracture osteosynthesis [15]. All metal alloys consist of metals constituted from metallic and non-metallic elements which form a highly organised repeating microstructure. The solvent metal determines the name of the alloy and is considered the base or primary metal. Non-metallic alloying components or solutes, such as oxygen, carbon or nitrogen are added to the base metal, which alters its properties. In their pure form metals tend to have one of three crystalline arrangements. This can be body-centred cubic, face-centred cubic and hexagonal close packed. In the FCC arrangement each atom is in contact with 8 other atoms. Whereas in the BCC and HCP arrangement each atom is in contact with 12 other atoms. During alloy formation, when a molten mixture undergoes solidification, the alloying components substitute for atoms of the base metal in the crystalline arrangement. The size of the non-metallic alloying elements relative to the size of the metallic elements determine the alloy crystal arrangement. If the non-metallic and metallic atoms are of similar size a substitutional alloy is formed, whereby non-metallic elements substitute for metallic elements in the crystalline arrangement, such as brass. If the non-metallic atoms are smaller than the metallic atoms on the other hand then an interstitial alloy is formed, whereby non-metallic atoms occupy spaces in the crystalline structure, such as stainless steel.

The above metallic alloy crystals, group together to form clusters of crystals termed grains. Grains are imperfectly aligned with each other creating gaps between adjacent grains, termed grain boundaries. Microstructure defects such as grain boundaries as well as dislocations and vacancies can all act to increase the propensity for alloy failure. Macrostructure defects, such as scratches and voids can also be a focus for stress and precipitating failure. Several processing methods have been introduced to address these defects including cold and hot working, powder metallurgy techniques to reduce grain size, precipitation hardening and thermomechanical processing.

Metals are susceptible to chemical wear, which is typically the result of corrosion from reactions with the surrounding aqueous physiologic environment [16]. Corrosion is the undesirable dissolution of metal in a solution. This occurs through the formation of an anode and cathode, resulting in metal cation ejection. Typically, passivity through the formation of a thin oxide film on certain metals serves as a kinetic protective barrier. In arthroplasty implant modularity, which results in relative motion between two materials can disrupt the passive oxide layer, resulting in dissolution of metal particles, which is termed fretting corrosion. This has resulted in modular components falling out of favour in primary THRs. Other types of mechanically assisted corrosion such as pitting and crevice corrosion, result in the formation of localised defects and the formation of a stress riser. While implant longevity is a concern, the local and systemic effects of corrosion must also be considered. Therefore, knowledge of the relative corrosion resistance of various metal alloys is essential in TJA, to ensure that they are employed correctly.

Metal alloys with relevance to TJA fall into three main groups 1) stainless steel 2) alloys based on the Cobalt-Chrome (Co-Cr) system and 3) Titanium and its alloys, which will be discussed in this section.

### **2.1 Stainless steel**

Stainless steel was first used in orthopaedic implants in 1926 [17]. Later, Charnley and the Exeter group employed stainless steel for their femoral stems (**Figure 2**). An iron-based alloy, the stainless steel used in orthopaedic implants is austenitic American Iron and Steel Institute (AISI) 316 L. The number 300 indicates that it is a member of the 300 series of austenitic steel. Austenite steel denotes an FCC crystalline arrangement, with a solid solution of carbon in a nonmagnetic form of iron, which is stable at high temperatures. The FFC structure increases susceptibility to plastic deformation. The alloy includes 3% molybdenum, which increases resistance to pitting and 16% nickel which stabilises the austenitic structure, improving ductility and reducing the alloy's yield stress. The letter 'L' refers to the low carbon content of <0.03% which improves corrosion resistance by reducing sensitisation, a process which results in carbide formation in grain boundaries.

Furthermore, the addition of chromium to stainless steel results in the formation of a thin chromium oxide layer (Cr2O3), a process termed passivation which shields the alloy from corrosion. Despite these properties, stainless steel remains susceptible to stress and crevice corrosion. Stress corrosion occurs as a result of exposure to chloride- rich environments whereas crevice corrosion results from the disruption of the passive oxide layer which occurs with undulating deformation.
