Wearing of polymer implants

Although replacement surgery is increasingly used for hip joints, these still suffer from limited lifetimes in the body due, at least in part, to the wear of the implanted polymer. 

In the context of widespread research aimed at understanding the wear processes in order to improve the materials and so prolong the effectiveness of the implant, researchers have examined a number of PE hip cups worn on a cobalt chrome femoral head before and after implantation in the body (Olley et al, 1999). In early investigations the use of permanganic etching prior to light and electron microscopy was shown to allow sensitive discrimination between the quality of 

More recently it has been possible to examine used hip cups which have been removed from the body. Electron microscopy of hip cup sections after permanganic etching reveals a layer of polymer, at least 10 pm thick extending inwards from the wear surface, which showed incipient cracking and appeared to have been embrittled by exposure to the body environment (Jordan et al, 2000). 

These results demonstrate that the embrittlement of the PE implants accompanies a microhardening of a surface layer and an increase in crystallinity. The two pieces of evidence are complementary and imply a reduction in the crack-blunting ability of the material, i.e. a diminution of the number of interlamellar tie molecules which connect adjacent lamellar stacks. In consequence the elastic properties of the material diminish and cause the material to microharden during wear. The increase in microhardness at the wear surface is partly because the amorphous component decreases in quantity and partly because its chemical nature changes as it undergoes simultaneous microhardening and loss of elasticity. 

During the past few years the microhardness technique has frequently been applied to the characterization of super-hard-surfaced polymers obtained by ion implantation and to plasma-deposited hard amorphous carbon films (Balta Calleja Fakirov, 1997). These products represent an entirely new class of materials that are lightweight and have the flexibility of polymers combined with a surface microhardness and wear resistance greater than those of metallic alloys (Lee et al., 1996). 

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Wear is defined as the loss of material from a surface as the result of relative motion. In this chapter, the wear processes in polymer implants are discussed. Polymers are used in a wide variety of implants in the human body such as joint replacement implants, pacemakers, catheters and heart valves. Wear of polymer implants is almost exclusive to joint replacement implants, such as those used to replace the hip or knee. These implants involve the articulation of a metal or ceramic against a polymer. Typically these implants operate with a mixed or boundary lubrication regime and, therefore, there is contact between the bearing surfaces that can lead to the generation of wear debris. The chapter is divided into sections that cover implants, wear processes, polymers used in implants, the effect of wear debris on the body and, finally, likely future trends. 

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