Ceramic-on-ceramic bearings have become of great interest due to the substantial improvements in the manufacturing techniques and material properties and due to polyethylene wear debris-induced osteolysis and the issues with metal wear debris and ion release by metal-on-metal bearings. Edge loading conditions due to translational malpositioning (microseparation conditions) have been shown to replicate clinically relevant wear mechanisms and increase the wear of ceramic-on-ceramic bearings; thus, it was necessary to test new bearing materials and designs under these adverse conditions. The aim of this study was to assess the effect of increasing head size on the wear of BIOLOX(®) delta ceramic-on-ceramic bearings under edge loading conditions due to rotational (steep cup inclination angle) and translational (microseparation) malpositioning. In this study, six 36-mm ceramic-on-ceramic bearings (BIOLOX delta, CeramTec, Germany) were tested under standard and edge loading conditions using the Leeds II hip simulator and compared to the 28-mm bearings tested and published previously under identical conditions. The mean wear rate under standard gait conditions was below 0.1 mm(3)/million cycles for both the 28-mm and the 36-mm ceramic-on-ceramic bearings, and increasing the inclination angle did not affect the wear rates. The introduction of microseparation to the gait cycle increased the wear rate of ceramic-on-ceramic bearing and resulted in stripe wear on the femoral heads. Under microseparation conditions, the wear rate of size 36-mm bearings (0.22 mm(3)/million cycles) was significantly higher (p = 0.004) than that for size 28-mm bearings (0.13 mm(3)/million cycles). This was due to the larger contact area for the larger bearings and deprived lubrication under edge loading conditions. The wear rate of BIOLOX delta ceramic-on-ceramic bearings under microseparation conditions was still very low (<0.25 mm(3)/million cycles) compared to earlier generation ceramic-on-ceramic bearings (BIOLOX forte, 1.84 mm(3)/million cycles) and other bearing materials such as metal-on-metal bearings (2-8 mm(3)/million cycles).

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U ltra-high-molecular-weight polyethylene (UHMWPE) components for total joint replacement generate wear particles which cause adverse biological tissue reactions leading to osteolysis and loosening. Sterilisation of UHMWPE components by gamma irradiation in air causes chain scissions which initiate a long-term oxidative process that degrades the chemical and mechanical properties of the polyethylene. Using a tri-pin-on-disc tribometer we studied the effect of ageing for ten years after gamma irradiation in air on the volumetric wear, particle size distribution and the number of particles produced by UHMWPE when sliding against a stainless-steel counterface.The aged and irradiated material produced six times more volumetric wear and 34 times more wear particles per unit load per unit sliding distance than non-sterilised UHMWPE. Our findings indicate that oxidative degradation of polyethylene after gamma irradiation in air with ageing produces more wear. Osteolysis and subsequent loosening as a result of the generation of wear particles are now recognised as a major cause of long-term failure in total joint arthroplasty. Livermore, Duane and Murray 1 have shown a positive correlation between the amount of resorption of the proximal part of the femur and wear, and tests in vitro 2-4 have demonstrated that the macrophage response to particulate wear debris is dependent on the number and size of particles. These studies have shown that cells are more reactive to small phagocytosible particles and irregularly shaped particles than to larger and more regularly shaped particles. 3,4 It is therefore important to reduce the wear volume of ultrahigh-molecular-weight polyethylene (UHMWPE) and the number of wear particles to improve the long-term clinical performance. The volume, size, morphology and concentration of the wear particles produced are likely to depend on tribological factors such as the properties of the material and the loads and movements experienced at the contact surfaces. The smooth hard counterface (typically Ra = 0.01 to 0.05 m) slides against a concave surface producing UHMWPE wear particles by adhesive and fatigue wear. Many particles of various sizes are produced in each cycle. The wear rates and volumes increase dramatically as the femoral counterface becomes roughened due to third-body damage; the counterface asperities remove particles by abrasive actions or by low cycle fatigue.5 A second wear process that may produce larger polyethylene particles 5 is called 'macroscopic polymer asperity wear' and is associated with cyclic deformation and strain accumulation of much larger asperities (up to 10 m high) on the polymer surfaces. Evidence from explanted acetabular cups 6-8 has shown that fatigue failure of the polymer surface just below the asperity (=5 m deep) can produce long fibres or cigar-shaped particles of up to 100 m in length. Most polyethylene wear particles are submicron in size. 9,10Until recently, sterilisation of UHMWPE was by gamma irradiation in the presence of air causing the generat...

This study isolated and characterized UHMWPE particles from 3 explant groups: early Charnley hip failures (ECE; < 10 years), late Charnley hip failures (LCE; > 10 years) and early knee failures (EKE; < 10 years). Debris isolated from the 3 groups had percentage particle number and percentage volumetric concentration distributions that were not significantly different. The greatest number of particles were found in the 0.1-0.5 m size range and 19 -20.6% of the volumetric concentration was below 1 m in size in all groups. However, there were significant differences in the total volumetric concentration of debris isolated per g of tissue. LCE had significantly higher volumes of debris than ECE and EKE, there was no significant difference in the volume of debris from the EKE and ECE. The mean aspect ratio and mean irregularity ratio of the LCE group were also significantly higher than the ECE and EKE, suggesting that different wear mechanisms were occurring in the late Charnley group compared to the early Charnley and knee groups. These results also suggest that early knees, with normal surface wear, may have similar wear mechanisms to early Charnley hips and indicate that similar volumes of biologically active micrometer and sub-micrometer UHM-WPE particles were produced. This may have important implications in the longer-term outcome of total knee arthroplasties, because it indicates a similar potential for osteolysis induced by wear debris.