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Materials selection of femoral head and acetabular cup components

The choice of design for the femoral head and acetabular cup components can be broken down into two main categories: hard-on-hard and hard-on-soft. Hard-on-hard describes implants in which both components are made from either metal or ceramic, and hard-on-soft describes those in which the femoral head is made of a metal or ceramic and the acetabular cup is a polymer.

Hard-on-Hard Implants

Metal-on-metal implants were first developed in the 1960s but have since been greatly improved. Cobalt chrome is a popular choice, although some studies have found that metal-on-metal implants can cause elevated levels of the metal ions in urine and the bloodstream. This indicates that wear produces particles that enter the body, and which may have an adverse effect. This is particularly a problem for people with poor kidney function.

Another possibility is implants in which both the femoral head and the acetabular components are made from a ceramic, such as alumina or zirconia. The main issue to note with these is that the ball and the cup must be manufactured as a pair. They must exactly fit one another, otherwise chipping will occur, and ceramic particles will be present in the joint.

Hard-on-Soft Implants

Early implants had a metal femoral head and an acetabular component made from ultra high molecular weight polyethylene (UHMWPE), and this is still one of the most popular styles of implant. UHMWPE has densely packed linear polyethylene chains, which gives increased crystallinity and improved mechanical properties, although it leads to a decrease in ductility and fracture toughness. The main problem with this combination of materials is wear of the acetabular cup, which can lead to the formation of small particles of the polymer and inflammation. A further operation may be also be required at a later date to replace the worn component. Studies have shown that increasing the cross-linking in the polyethylene significantly reduces wear, leading to more durable acetabular components, thus increasing the lifetime of an implant

Alternatively, ceramics such as alumina or zirconia can be used to manufacture the femoral head. These can be polished to give a very smooth surface and have a much lower wear rate than metal on polyethylene. These improved wear properties are dependent on a small, uniform grain size in the ceramic, so its microstructure must be carefully controlled during the manufacturing process.

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