The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection

During total hip arthroplasty, both the femoral and acetabular bearing surfaces are surgically replaced with metallic, polymeric, and/or ceramic components. Throughout the twentieth century, many different combinations of these materials have been proposed and examined as bearing surfaces for total hip arthroplasty. Metal-on-metal total hip replacements were first implanted by Wiles in the 1930s and later developed in the 1950s and 1960s by pioneering surgeons like McKee and Ring. In 1958, Charnley introduced a ‘‘low-friction arthroplasty’’ based on the principle of a metallic femoral component articulating against a polymeric acetabular component, and in 1970, Boutin developed the first ceramic-on-ceramic total hip replacement. Charnley’s hard-on-soft bearing concept eventually dominated the hard-on-hard bearing alternatives. Today the most widely accepted bearing couple (i.e., combination of bearing materials for the hip joint) consists of a femoral head made from cobalt chromium molybdenum (cobalt chrome or CoCr) alloy articulating against a polymeric component fabricated from ultrahigh molecular weight polyethylene (UHMWPE). The use of the CoCr/UHMWPE bearing couple has proved consistent results in total hip arthroplasties around the world for the past four decades. (Older 2002) In 1998, an estimated 700,000 UHMWPE hip components were implanted worldwide. Based on the report of major orthopaedic manufacturers, at most 200,000 metal-on-metal 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. Therefore, the overwhelming majority (over 90%) of total hip arthroplasties currently in service throughout the world includes an UHMWPE or a modified UHMWPE component and is based upon Charnley’s original concept of hard-on-soft bearing. Despite the worldwide acceptance of total hip arthroplasty, wear of the UHMWPE component is a major obstacle limiting the longevity of these reconstructions. It is well established that particulate debris generated from the hard-on-soft articulating surfaces initiates a cascade of adverse tissue response leading to osteolysis and in certain cases loosening of the components. Extending the longevity of total joint replacements using alternative bearing technologies with improved wear behavior has been the subject of ongoing research in the orthopaedic community. Since the 1970s, researchers have attempted to improve the tribological characteristics of UHMWPE by modifying the polymer’s structure, with the ultimate goal of improving the in vivo wear performance of hip bearings. In the 1970s,