**1. Introduction**

162 Recent Advances in Arthroplasty

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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,

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 165

The coefficient of friction μ is a dimensionless number, defined as the ratio F/N between the friction force F and the normal force N acting to press the two bodies together. The kinetic coefficient of friction μk is the coefficient of friction under conditions of macroscopic relative motion between the two bodies, while the static coefficient of friction μs is the coefficient of friction corresponding to the maximum friction force that must be overcome to initiate macroscopic motion between the two bodies. F is approximately linearly proportional to N

is sometimes called "Amontons Law". The value of μ can be expected to depend significantly on the precise composition, topography and history of the surfaces in contact, the environment to which they are exposed, and the precise details of the loading conditions. Although tables of coefficients of friction have been published, they should not be regarded as anything more than general indications of relative values under the specific conditions of measurement. The coefficient of friction usually lies in the range from 0 to 1,

In order to initiate sliding, the bonds between the contact points need to be broken, thus explaining the higher force required to initiate movement with respect to maintaining it. The friction begins by cyclic elastic or plastic deformation of contact spots of the real contact area. Then it is transformed into elastic or plastic deformation energy within interlocking surface asperities and/or leads to crack initiation and propagation. This deformation may be responsible for the generation of particles. This (progressive) loss of particulate debris from the surface of a solid body due to mechanical action has been defined as wear. The adhesion of the surface atoms and molecules of body and counterbody contributes to friction. The probability of adhesion depends on mechanical properties and the tendency of atoms and molecules to react chemically. Both the deformation and adhesion contributions to friction can be distinctly lowered by means of surface modifications or coatings as well as by lubrication. At least 90% of the introduced energy is dissipated into heat, leading to an increase of the temperature within the contact zone. Depending on local normal forces and the relative velocity, the average as well as the flash temperatures may rise. Although the average temperature is primarily governed by the normal force, the flash temperature depends mostly on the relative velocity and lasts for only a few nanoseconds or milliseconds. The remaining 10% is dissipated by storing mechanical energy within lattice defects generated by cyclic elastic and plastic deformation, phase transformation or chemical reaction of body, counterbody, interfacial medium, and environment. Friction also

results in the transfer of force from the articulating areas to the fixed interfaces.

across the interface, and the roughness of the mating surfaces.

Friction can be reduced by lubrication. The principal idea behind lubrication is to interpose a material between two contacting solids to minimize interaction between them. For example, wetting of the surfaces reduces adhesion. The extent of fluid film formation plays an important role in the wear process of artificial joints in vivo. The effectiveness of a lubricant film can be defined by the specific film thickness which is dependent on the viscosity of the lubricant, the relative velocity between body and counterbody, the pressure

F = μN (1)

over quite large ranges of N. The equation

**2.2 Lubrication** 

although there is no fundamental reason why it need do so.

carbon fiber–reinforced UHMWPE with a potentially improved wear resistance was clinically introduced. In the 1980s, a high-pressure recrystallized form of UHMWPE was clinically introduced for its improved creep resistance. In the late 1990s, many researchers confirmed that crosslinking of UHMWPE can substantially improve the wear performance of the polyethylene in hip joint simulators. Based on these in vitro analyses, a number of radiation cross-linked materials have been clinically introduced in the late 1990s.
