The biomechanical problems of polyethylene as a bearing surface.

The metal backings of acetabular components can reduce the available polyethylene thickness, often to an alarming extent. This study indicated that the pitting and cracking of thin polyethylene surfaces have some similarities to tibial and patellar bearings and that creep-related deformation occurred more frequently in thin polyethylene components. Additionally, it appears that dimensional tolerances of the polyethylene inserts are difficult to maintain and may result in a nonuniform fit of both the femoral head into the component and the component into its own metal backing, which can lead to component separation. It is difficult to accurately measure the changes in material and mechanical properties of polyethylene over time. An additional ramification is that flaws, such as voids in the polyethylene, cannot be attributed to problems of bulk supply, fabrication, or postmanufacturing treatment. Orthopedic device manufacturers should keep samples of each lot of polyethylene used and provide components with serial numbers so that the source, composition, and properties of the original bulk material and material "as fabricated" can be documented. This would permit researchers studying revisions or postmortem samples to determine the changes in the polyethylene over time in vivo, thus improving the understanding of this crucial material. If manufacturers were to include dimensions and tolerances of the polyethylene inserts in their product literature, accurate measurements of wear of retrieved specimens may be possible.