Hip replacements consist of several different components and all of them can fail by becoming loose or just simply wearing out. The most common part to wear out is the socket - here is a picture of a socket that was taken out by Mr. Miles because the plastic had worn out completely. It was replaced with another socket. The other parts of the hip can wear out, including the stem and the head, but this is much rarer. It is more common for the components to loosen from the bone that they are attached to. This can be quite destructive to the bone of both the pelvis and the femur. It may need complex techniques and special intruments to reconstruct the bones in order to resotre the function of the affected hip.