Wear ranking of hard on hard bearings for prosthetic hip joints

With the increasingly ageing population across the developed world the numbers of people requiring surgical procedures to replace damaged, worn and arthritic joints is hugely increasing. In fact these operations are now the most common surgical procedure carried out in hospitals with 250,000 total hip replacement (THR) procedures carried out in the EU annually.

The standard design of the THR comprises of a metal or sometimes ceramic head running against an ultra high molecular weight polyethylene (UHMWPE) acetabular cup (Fig. 1). THR can be successful for in excess of 20 years however up to 10% of prostheses fail prematurely. The 75% early failure results from the process of aseptic loosening. This is classified as mechanical failure of the joint through loosening of one or more of the prosthetic components. General loosening of the prosthesis has been widely attributed to generation of wear debris resulting from the relative movement of the femoral head and acetabular cup. The debris generated at the tribological interface accumulates in the surrounding tissue. The debris initiates the body's immune system into producing macrophages which aggressively attacks the debris and also the immediate surrounding bone tissue. This causes bone resorption and consequent loosening (Fig. 2). For standard THR systems the primary wear debris generated form the UHMWPE component and takes the form of micro- and submicro-scale polymeric particles [1].