Do current wear particle isolation procedures underestimate the number of particles generated by prosthetic bearing components?

Hip simulator serum samples containing ultra-high molecular weight polyethylene (UHMWPE) wear debris were digested in acid, and replicate digests were filtered through either a 0.2 or a 0.05 μm pore size membrane. The recovered particles were characterized using Fourier transform-infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Debris deposited on both the 0.2 and 0.05 μm membranes were identified as UHMWPE by FT-IR and were predominantly submicron and round, with occasional elongated fibrils. The mean and median diameters of the particles on the 0.05 μm membranes were significantly lower than those of the particles on the 0.2 μm membranes. Over half of the particles on the 0.2 μm membranes had diameters which were below the specified pore size, whereas only a small percentage (2.8%) of particles on the 0.05 μm membranes were smaller than the specified pore size. More than twice as many particles were recovered on the 0.05 μm membranes than the 0.2 μm membranes. These findings indicate that a substantial number of wear particles passed freely through the pores of the 0.2 μm membranes, which resulted in an underestimation of particle number and an overestimation of particle size. Because the cellular response to wear debris has been found to be dependent upon particle number and size, among other factors, the introduction of a new orthopaedic bearing material should be supported by an accurate description of wear particle parameters. To ensure an accurate description of particle characteristics, it is recommended that filter membranes with very fine pore sizes (at most 0.05 μm) be used to isolate UHMWPE wear debris from joint simulator serum and periprosthetic tissue.