Hip prostheses can be differentiated according to the materials used to make them. Currently, these materials are divided into three types:
- Metal: certain metals such as stainless steel, cobalt-chrome or titanium are used to make the femoral stem.
- Polymers: Polyethylene, which is a very hard plastic and the most commonly used in the world, is the main material used in this category. It is an inert and perfectly biocompatible substance that was introduced in orthopedics in the 1960s as a component of the cemented acetabulum prosthesis. Today this material is still used for some patients but the disadvantage is the risk of plastic wear over time and therefore a shorter prosthesis lifespan. However, this risk remains mitigated because some patients have been able to keep this type of prosthesis for 30 years and others for only a few years.
- Ceramics: Ceramics are a good alternative because of their strength. A titanium cup can be used with a ceramic insert.
The movable area between the femoral head and the cup forms what we call friction torque. It is the most fragile part of the prosthesis, particularly in terms of wear. There are four possible pairings:
- Ceramic-polyethylene
- Ceramic-ceramic
- Metal-polyethylene
- Metal-metal
Each friction pair has advantages and disadvantages, and it is up to the orthopedic surgeon to choose the most appropriate one based on several criteria such as the patient’s age, physical activity, bone specificity, etc.
It should be noted that, in general, the use of metal on metal implants is not recommended. Some of the companies that manufactured them decided in 2010/2011 to stop marketing them, and in the interests of patients, to recall those not being used. The problem stems from the friction between the different elements of the implant that can detach small metal particles that then enter the bloodstream. In the hip joint, these small particles can cause allergic reactions leading to localized pain and lesions.