The metal-on-polyethylene bearing surface was used in the version of the total hip replacement designed by Sir John Charnley in the 1960’s. The femoral component (ball) of this type of hip replacement is made of metal and glides against the acetabular (socket) surface, which is made of the most common type of plastic, polyethylene. The polyethylene portion is often simply a “liner” that is placed within a metal socket, which is tightly fitted into the acetabulum.
The first form of polyethylene used was ultra-high molecular weight polyethylene (UHMWPE). This was found to wear over time and release microscopic particles that became lodged in the lining (synovium) of the joint, resulting in inflammation of the hip joint. Over many years, the reaction between the hip joint lining and these particles would lead to a weakening and break down of bone around the joint (osteolysis). This bone destruction eventually lead to failure of the hip replacement. In the years since the introduction of polyethylene into hip replacements, its structure has been optimized to decrease the extent of wear over time.
Metal-on-polyethylene has a high success rate in older individuals as it usually lasts and functions well for 15-20 years, which is often the remainder of the individual’s life. Longevity of the prosthesis is especially important in younger individuals, which lead researchers to search for other bearing surfaces and better forms of polyethylene. In the 1990’s, a highly cross-linked polyethylene was developed, which was found to decrease particle formation and therefore cause less damage to the prosthesis overtime, lowering the rate of prosthetic loosening. Other bearing surfaces, such as ceramic-on-ceramic and metal-on-metal, have been shown to cause less wear and particulate release over time. However, they are not free from their own set of unique complications.
At this time, it is not clear which type of bearing surface is the best choice overall. The choice of bearing surface largely depends on the age and activity level of the patient, as well as the expertise of the surgeon performing the hip replacement surgery.