There is some debate about what is an ideal bearing surface for the interface between the head and the liner. (see illustration below) Putting two hard surfaces (hard-on-hard) rubbing against each other with a fluid lubricant can potentially create a situation with almost no wear, like in a car motor. While this is conceptually ideal, there are significant issues with "hard-on-hard" bearing surfaces: metal on metal, ceramic on ceramic, or hybrid. They can function well, but any possible theoretic benefit likely does not outweigh the risk. The risk of these surfaces comes from the potential for fracture of the material, intolerance to impact or debris, and most commonly, generation of metal particles in the hip space. These particles can induce rapid corrosion, wear, and formation of inflammatory masses that can be challenging to manage and may require revision of the hip, loss of muscle attachments, hip instability, and infection.