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Increased longevity of total hip arthroplasty (THA) and its use in younger active patients has led to a greater incidence of clinical and radiographic apparent ultra high molecular weight polyethylene (UHMWPE) wear. In generic terms, wear is defined as the progressive loss of substance from the operating surface of a body occurring as a result of relative motion of the surface.1 With respect to THA, this wear is best quantitated through radiographic means as linear wear. Linear wear is the straight line distance the femoral head travels into the UHMWPE cup or insert and is expressed as millimeters per year. Several radiographic reviews of long-term performance of THAs have reported average rates of linear wear between 0.07 and 0.19 mm/year.2-9
Measured linear wear is the sum of true linear wear and creep.4 The magnitude of the contribution of creep, ie, time-dependent strain caused by stress to linear wear, is not known precisely. Rose et al10 attempted to determine the contribution of creep to linear wear. They collected and weighed all of the wear particles generated by individual prostheses in a hip stimulator and found the weight of the generated wear particles accounted for only l%-30% of the dimensional changes of the UHMWPE. The remainder of the dimensional changes were ascribed to creep.10 Thus, it is clear that creep accounts for a significant portion of measured linear wear.
MECHANISMS OF WEAR
The mechanisms of wear in THA are abrasion, adhesion, and fatigue.11 Abrasive wear is the removal of a material from the operating surface of a body by hard asperities on the countersurface (two-body wear) or by hard particles between the surfaces (threebody wear). Asperities are microscopic incongruities on the surface of the prosthesis.
In two-body wear, these asperities can remove material from the softer UHMWPE inserts. Three-body wear is most frequently caused by particles of polymethylmethacrylate that interpose themselves between the surfaces. They can grind off large quantities of UHMWPE from the acetabular components. Many authors believe three-body wear causes the accelerated wear occasionally observed in THA.4,12-14
Adhesive wear is defined as the transfer of material from one surface to another due to a process of solid-phase welding. This occurs when two asperities contact each other. Large pressures are generated over this small contact...