wear In materials science, wear is the erosion of material from a solid surface by the action of another surface. It is related to surface interactions and more specifically the removal of material from a surface as a result of mechanical action.The need for mechanical action, in the form of contact due to relative motion, is an important distinction between mechanical wear and other processes with similar outcomes (source) . There ar many forms of wear : adhesive wear , fatique wear , fretting and erosive wear In artificial hip joints the major form is the abbrasive wear . Abrasive wear occurs when a harder or rougher surface slides across a softer surface. ASTM (American Society for Testing and Materials) define it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.
The first total hip designs (where both the femoral head and the acetabulum receive an implant) were of the metal-on-metal type. Relative rough surfaces caused increased wear and increased friction. Not well defined clearance between the two components inhibited good lubrication (when two indentical materials articulate, good lubrication is mandatory) and somethime too thin acetabular component deformed and "gripped"the head leading to early loosening. However from thos early designs it is known that there is probably no increase in the occurance of lymphomia and carcinoma. However it should be remembered that these early implants were only used in the relatievley older and less mobile patient. The new design resurfacing hip are especially indicated in the younger active population.
When -at that time- modern materials like Teflon and Polyethylene came available attempt were made to use those as articulation partmer of metal. And within the limitations of the material it became very succesfull. Polyethylene being the softer partner of the two was the one that would be wearing. If wear and diminishing of the size of the component would be the only problem, than a revision, exchanging an acetabular component for a new one . would be a simple solution. However it became apparant that the body may react to the generated wear particles. The human body recognizes these particle as "foreign" and may try to remove it from the body. The biological reponse would release a number of biochemical substances, that may not onloy attack the wear particle but also the bone around the implants. This may lead to early loosening and consequent implant failure.
Wear of implants can be reduced by making the metal part of the implant as smooth as possible, or by exchanging the metal head for a ceramic head or by coating the surface of the metal with a ceramic. Better characteristics of the materials on the acetabular side reduces wear also: cross linking polyethylene or using solid ceramic are measures to reduce wear. (click here for the various articulation options in total hip design)