Shilesh C. Jani

Relationship between gravimetric wear and particle generation in hip simulators: conventional compared with cross-linked polyethylene.

Summary Hip-simulator studies have shown reduced gravimetric wear rates for inert-gas gamma-irradiated ultra-high molecular weight polyethylene when compared with conventional ethylene-oxide-sterilized ultra-high molecular weight polyethylene. Analysis shows a greater number of particles generated from inert-gas gamma-irradiated ultra-high molecular weight polyethylene. This study was undertaken to examine particle-generation rates of polyethylene with different levels of cross-linking and to correlate them with gravimetric wear data. Particle-generation rates did not correlate with gravimetric wear rates. Particle analysis should be performed to predict the in vivo behavior of bearing surface materials. Cross-linked ultra-high molecular weight polyethylene subjected to 10 Mrad (100,000 Gy) of gamma irradiation generated significantly fewer particles than ethylene-oxide-sterilized ultra-high molecular weight polyethylene; it also demonstrated a 96% reduction in the volume of particles.

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.

Evaluation of sequentially crosslinked ultra-high molecular weight polyethylene

This study was undertaken to investigate the effect of crosslinking ultra-high molecular weight polyethylene (UHMWPE) in a sequential manner to the final desired dose and to compare the results to single-dose crosslinking. To verify these results, an explanted, commercially available, sequentially crosslinked component was characterized. Finally, additional tensile testing was conducted to determine if tensile-sample thickness has a significant effect on the mechanical properties of UHMWPE. Based upon this well-controlled study with the same starting material, there is no apparent benefit of sequential crosslinking over crosslinking by single dose in any of the mechanical, thermophysical, physical, or oxidative properties evaluated in this study. In contrast, the soak temperature of the postirradiation heat treatment was more influential and exhibited statistically significant effects on the stability, structure, and properties of the resultant material. Compared to virgin material, crosslinking always resulted in decreases in tensile strength, elongation, and impact strength. These results were confirmed by characterization of a retrieved, sequentially crosslinked (X3) cup. All of the metrics derived for the retrieved cup were virtually identical to the sequential- and single-dose-crosslinked materials produced in this study. Examination of the effect of tensile-sample thickness demonstrated that there are significant effects on the resultant properties. In particular, the ultimate tensile strength of UHMWPE can be elevated by conducting tensile tests with thin specimens.