There are four main criteria to consider when selecting suitable implant materials.
- Biological Conditions
The human body is not an easy environment for a material to function in for prolonged periods. It must be able to operate for many years at a temperature of 37°C in a very moist environment. Also, bone growth can only occur in acidic conditions, as these are necessary for mineral deposition, so the implant must be capable of withstanding this for many years without corroding.
Implant materials must be designed to minimise the adverse reactions associated with introducing a foreign material to the body. The immune system will typically attack anything that has originated outside the body, leading to inflammation. Elevated levels of particular metals in the bloodstream can lead to various problems including cytotoxicity and carcinogenesis. It is therefore crucial to choose materials that will have a minimal negative impact on the body.
- Cytotoxicity - Having a toxic effect on cells, caused by increased levels of metal ions in the bloodstream.
- Carcinogenesis - The formation of cancerous cells, which can be caused by elevated levels of certain metal ions in the body.
- Materials Properties
The materials used in each component of the hip implant must have suitable properties to allow them to replace the natural tissue and continue to perform the same functions.
The mechanical property requirement for the femoral stem is primarily to support the loads that are applied, and therefore the modulus of the implant material is one of the main criteria. The femoral head and acetabular cup components are required to act as bearing surfaces, and therefore, the coefficient of friction and wear rates of these materials will be important.
As with most areas of materials science, cost is an important contributing factor to the selection of materials. Often, manufacturers must strike a suitable balance between a material’s performance and cost.
These issues are discussed in the following two sections, along with information about the materials that are generally chosen for these components.
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