The microstructure of ultra-high molecular weight polyethylene used in total joint replacements

The microstructure of ultra-high molecular weight polyethylene (UHMWPE) has been studied using a range of techniques. Both the unprocessed base powder and ram-extruded polymer have been examined using optical microscopy, scanning and transmission electron microscopy and small-angle light scattering. By examining the microstructure of samples compression moulded at a range of temperatures, techniques have been developed to assess the degree of consolidation of the processed polymer. The raw polymer is a powder with a particle size in the range 50-250 microm. These particles are themselves agglomerates of much finer particles typically 0.5-1 microm in size. It has been suggested that these sub-micron particles may be the origin of the sub-micron wear debris found in tissues around total joint replacements. However, examination of the ram-extruded polymer, from which implants are machined, shows a different structure from the powder, with no evidence of retention of the 0.5-1 microm structure seen in the powder in the processed material. It thus appears that the similarity in size between the sub-micron wear debris particles and the fine structure seen in the unprocessed UHMWPE resin is coincidental. Processed UHMWPE does show a 'memory' of the grain boundaries between powder particles and the degree of consolidation can be assessed by observing the distinctiveness of these boundaries.