The tribological performance and biological activity of the wear debris produced has been compared for highly cross-linked polyethylene, ceramic-on-ceramic, metal-on-metal, and modified metal bearings in a series of in vitro studies from a single laboratory. The functional lifetime demand of young and active patients is 10-fold greater than the estimated functional lifetime of traditional polyethylene. There is considerable interest in using larger diameter heads in these high demand patients. Highly cross-linked polyethylene show a four-fold reduction in functional biological activity. Ceramic-on-ceramic bearings have the lowest wear rates and least reactive wear debris. The functional biological activity is 20-fold lower than with highly cross-linked polyethylene. Hence, ceramic-on-ceramic bearings address the tribological lifetime demand of highly active patients. Metal-on-metal bearings have substantially lower wear rates than highly cross-linked polyethylene and wear decreases with head diameter. Bedding in wear is also lower with reduced radial clearance. Differential hardness ceramic-on-metal bearings and the application of ceramic-like coatings reduce metal wear and ion levels.

The wear, wear debris and functional biological activity of non-crosslinked and moderately crosslinked ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups have been compared when articulating against smooth and intentionally scratched femoral heads. Volumetric wear rates were determined in a hip joint simulator and the debris was isolated from the lubricant and characterized by the percentage number and volumetric concentration as a function of particle size. The volumetric concentration was integrated with the biological activity function determined from in vitro cell culture studies to predict an index of specific biological activity (SBA). The product of specific biological activity and volumetric wear rate was used to determine the index of functional biological activity (FBA). On smooth femoral heads the crosslinked UHMWPE had a 30 per cent lower wear rate, but it had a greater percentage volume of smaller, more biologically active particles, which resulted in a similar index of FBA compared with the non-crosslinked material. On the scratched femoral heads the volumetric wear rate was three times higher for the moderately crosslinked UHMWPE and two times higher for the non-crosslinked UHMWPE compared with the smooth femoral heads. This resulted in a higher wear rate for the moderately crosslinked material on the scratched femoral heads. All the differences in wear rate were statistically significant. There were only small differences in particle volume concentration distributions, and this resulted in similar indices of FBA which were approximately twice the values of those found on the smooth femoral heads. Both materials showed lower wear and FBA than for previously studied aged and oxidized UHMWPE gamma irradiated in air. However, this study did not reveal any advantage in terms of predicted FBA for moderately crosslinked UHMWPE compared with non-crosslinked UHMWPE.

Cross-linked polyethylene currently is being introduced in knee prostheses. The wear rates, wear debris, and biologic reactivity of non cross-linked, moderately cross-linked, and highly cross-linked polyethylene have been compared in multidirectional wear tests and knee simulators. Multidirectional pin-on-plate wear studies of noncross-linked, moderately cross-linked (5 Mrad), and highly cross-linked (10 Mrad) polyethylene showed a 75% reduction in wear with the highly cross-linked material under kinematics found in the hip, but only a 33% reduction under wear in kinematics representative of the knee. In knee simulator studies, with the fixed-bearing press-fit, condylar Sigma cruciate-retaining knee under high kinematic input conditions, the wear of 5 Mrad moderately cross-linked polyethylene was 13 ± 4 mm3 per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 ± 6 mm3/1 million cycles). For the low-contact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 ± 1 mm3 per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 ± 2 mm3/1 million cycles). The wear debris isolated from the fixed-bearing knees showed the moderately cross-linked material had a larger percentage volume of particles smaller than 1 μm in size, compared with GVF GUR 1020 polyethylene. Direct cell culture studies of wear debris generated in sterile wear simulators using multidirectional motion showed a increase (p < 0.05) in tumor necrosis factor-alpha levels and reactivity for GUR 1050 cross-linked polyethylene debris compared with an equivalent volume of noncross-linked GUR 1050 polyethylene. The use of cross-linked polyethylene in the knee reduces the volumetric wear rate. However, the clinical significance of reduced fracture toughness, elevated wear in abrasive conditions, and the elevated tumor necrosis factor-alpha release from smaller more reactive particles warrant further investigation.

Erythromycin-resistant staphylococci can be divided into two phenotypic classes based on their pattern of cross-resistance to other macrolides, lincosamides and type B streptogramins. Strains inducibly or constitutively resistant to all MIS antibiotics possess erythromycin ribosomal methylase (erm) genes, whereas strains inducibly resistant to only 14 and 15-membered ring macrolides and type B streptogramins harbour msrA. which encodes an ATP-dependent efflux pump. Dot-blot hybridization was used to study the distribution of ermA, ermB, ermC and msrA in five epidemiologically distinct groups of staphylococci. The most widely-distributed resistance determinant was ermC, which was detected in 112 (50·6%) of 221 isolates, alone in 106 isolates and in combination with a second erythromycin resistance determinant in six strains. MsrA was detected in 73 (33%) of isolates, alone in 65 and in combination with a methylase gene in eight strains. This determinant was responsible for erythromycin resistance in over one-third (36·4%) of clinical isolates of coagulase-negative staphylococci. ErmA and ermB were present in only a minority of isolates (5·9 and 7·2% of strains, respectively). The resistance determinants present in ten strains did not hybridize to any of the four probes although, in all cases, their resistance phenotype was consistent with the possession of a methylase gene. Interestingly, ermB was found exclusively in animal isolates of Staphylococcus intermedius, Staphylococcus xylosus and Staphylococcus hyicus, but not in coagulase-negative staphylococci of human origin. This determinant has previously only been found in a small number of epidemiologically related strains of Staphylococcus aureus.