**3. Alternate bearing surfaces**

During total hip arthroplasty, both the femoral and acetabular bearing surfaces are surgically replaced with metallic, polymeric, and/or ceramic components.(Fig.6) Throughout the twentieth century, many different combinations of these materials have been explored as candidate bearing surfaces for total hip arthroplasty. Today the most widely accepted bearing couple (i.e., combination of bearing materials for the hip joint) consists of a femoral head fabricated from cobalt chromium molybdenum (CoCr) alloy articulating against a polymeric component fabricated from ultrahigh molecular weight polyethylene (UHMWPE). The use of the CoCr/UHMWPE bearing couple has provided consistent results in total hip arthroplasties around the world for the past four decades. In 1998, an estimated 1.4 million UHMWPE components were implanted worldwide, with approximately half of these bearings being implanted in the hip. At most 200,000 metal-onmetal or ceramic-on-ceramic components have been implanted in patients worldwide between 1988 and 2000, corresponding to less than 10% of total hip replacements during the same time period. Despite the recognized success and worldwide acceptance of total hip arthroplasty, wear of the UHMWPE component is a major obstacle limiting the longevity of these reconstructions.

Fig. 6. Different current bearing surface options; A, metal-on-polyethylene, B, metal-onmetal, and C, ceramic-on-ceramic
