WEAR MECHANISMS OF UHMWPE IN TOTAL JOINT REPLACEMENTS

The wear of ultra-high molecular weight polyethylene (UHMWPE) has beco me a major focus regarding the long-term clinical performance of total joint replacements. Clinical retrieval analysis has revealed the exis tence of two distinct wear mechanisms that are operational in both tot al hip and total knee replacements. While the majority of the wear deb ris produced from the acetabular component is less than a micron in si ze and often particulate or fibrous in shape, much larger thin flakeli ke wear debris is produced from the tibial component. Two theoretical wear models that are based on the scale of intimate asperity interacti ons are proposed to account for the observed differences in hip and kn ee wear. Cyclic plastic strain accumulation is identified as the commo n cause for wear debris generation in both hip and knee replacements. In the case of acetabular cup wear, the scale of plastic deformation i s limited to the sites of intimate micro-asperity contacts and the wea r rate is defined by a critical strain criterion. In the case of tibia l component wear, however, plastic deformation spreads into the subsur face to the site of macro-asperity contacts, and material can be remov ed by subsurface cracking and delamination. In both cases, the wear ra te is strongly affected by the ultimate tensile strength and breaking elongation of the UHMWPE material.