Low in vitro third-body wear on total hip prostheses induced by calcium sulphate used for local antibiotic therapy.

In case of implant associated infection, implant preservation is associated with high failure rates. Therefore, a removal or exchange of the implant is most often mandatory for treatment success. Alternatively, under certain conditions, local antibiotic delivery can be applied - preserving the implant, using for example calcium sulphate as a resorbable carrier. In this work, third-body wear on total hip prostheses caused by calcium sulphate particles was tested in a hip simulator. Inlays made of ultra-high-molecular-weight polyethylene (UHMWPE) and cross-linked polyethylene (XLPE) against 28 mm CoCrMo heads and 36 mm alumina pairings were tested in triplicate, both with and without calcium sulphate particles in the test liquid. Neither the alumina articulations nor the CoCrMo heads were affected by the calcium sulphate particles since calcium sulphate is a relatively soft material. The polyethylene inlays showed 39-89 % higher wear during exposure compared to references, but wear returned to normal when no more particles were added. Thus, calcium sulphate might be used as antibiotic carrier even in the presence of total hip prostheses without fearing excessive third-body wear.

Calf serum constituent fractions influence polyethylene wear and microbial growth in knee simulator testing.

Calf serum lubricants consisting of various polypeptide constituent fractions are routinely used in knee wear simulators as part of the standardized test protocol. Three calf sera (bovine, new-born and alpha) were diluted as per the recommendation of ISO 14243-3 and used in displacement-controlled knee wear simulators to investigate their effects on polyethylene wear. Biochemical analyses included measuring total polypeptide degradation, electrophoretic profiles and low-molecular weight polypeptide concentrations to elucidate their involvement in the wear process. The effects of the various calf sera constituent fractions on microbial growth were also explored. The polyethylene wear rates and the results from the biochemical analyses for the three calf serum lubricants were all found to be statistically significantly different from each other. The lubricant derived from the alpha-calf serum was closest in constituent fractions to human synovial fluid. It also showed the lowest polyethylene wear rate (14.38 +/- 0.85 mm3/million cycles) and the lowest amount of polypeptide degradation (7.77 +/- 3.87%). Furthermore, the alpha-calf serum lubricant was associated with the least amount of change in the electrophoretic profile, the least change in low-molecular weight polypeptide concentration, and the lowest microbial growth in the presence of sodium azide (a microbial inhibitor conventionally used in implant wear testing). Replacing sodium azide with a broad spectrum antibiotic-antimycotic eradicated the microbial growth. Some speculation was entertained regarding the effect of alpha-calf serum on colloid-mediated boundary lubrication. Based on the results, it was recommended that ISO 14243-3 be modified to include guidelines on calf serum constituent fractions that would favour using alpha-calf serum in order to improve the fidelity of the simulation in knee implant wear testing.

Fractionation and characterization of particles simulating wear of total joint replacement (TJR) following ASTM standards.

Reactions of bone cells to orthopedic wear debris produced by the articulating motion of total joint replacements (TJRs) are largely responsible for the long-term failure of such replacements. Metal and polyethylene (PE) wear particles isolated from fluids from total joint simulators, as well as particles that are fabricated by other methods, are widely used to study such in vitro cellular response. Prior investigations have revealed that cellular response to wear debris depends on the size, shape, and dose of the particles. Hence, to have a better understanding of the wear-mediated osteolytic process it is important that these particles are well characterized and clinically relevant, both qualitatively, and quantitatively. In this study we have fractionated both ultra-high molecular weight polyethylene (UHMWPE) and Ti particles, into micron (1.0-10.0 µm), submicron (0.2-1.0 µm), and nanoparticle (0.01-0.2 µm) fractions, and characterized them based on the following size-shape descriptors as put forth in ASTM F1877: i) equivalent circle diameter (ECD), ii) aspect ratio (AR), iii) elongation (E), iv) roundness (R), and v) form factor (FF). The mean (± SD) ECDs (in µm) for micron, submicron, and nanoparticles of UHMWPE were 1.652 ± 0.553, 0.270 ± 0.180, and 0.061 ± 0.035, respectively, and for Ti were 1.894 ± 0.667, 0.278 ± 0.180, and 0.055 ± 0.029, respectively. The values for other descriptors were similar (no statistically significant difference). The nanofraction particles were found to be more sphere-like (higher R and FF values, and lower E and AR values) as compared to larger particles. Future experiments will involve use of these well characterized particles for in vitro studies.

Wear comparison between a highly cross-linked polyethylene and conventional polyethylene against a zirconia femoral head: minimum 5-year follow-up.

Highly cross-linked polyethylene (HXLPE) was developed to reduce wear of articular bearing surface in total hip arthroplasty patients. Several studies have shown reduced wear of HXLPE compared with conventional polyethylene; however, these studies had used HXLPE in combination with a Co-Cr metal head. The purpose of this study was to compare the 5-year in vivo wear of HXLPE with that of conventional PE using a zirconia femoral head. Forty-five hips with a Trilogy HXLPE (Zimmer, Warsaw, Ind) were matched and compared with a control group of 20 conventional Trilogy PE hips. The 2-dimensional linear wear rate was significantly less in the HXLPE group between 1 and 5 years postoperation (P < .001). The results show that HXLPE reduces short-term polyethylene wear against not only a Co-Cr head but also a zirconia head.

Effect of head surface roughness and sterilization on wear of UHMWPE acetabular cups.

The impact of femoral head surface roughness on wear of gamma-irradiation sterilized (3 MRad in nitrogen, crosslinked) and nonsterilized (not crosslinked) UHMWPE acetabular cups has been evaluated. Gravimetric wear testing was performed in a hip joint simulator for 2 x 10(6) cycles. CoCrMo heads were used with different surface roughness (R(a) = 15 nm and R(a) = 400 nm). The surface roughness after wear test was unchanged for the roughened heads, whereas the initially smooth heads showed a few scratches. The roughened heads increased the wear of the acetabular cups 2-fold. The gamma-irradiated cups tested against rough heads underwent the highest wear. The absorption of water was highest for the gamma-irradiated cups (0.0204% compared to 0.0031% after 85 days). Raman spectroscopy showed small but significant crystallinity changes in the wear zone, where the gamma-irradiated cups with the most extensive abrasion increased in crystallinity, whereas the nonsterilized cups underwent a crystallinity decrease.

The creep and wear of highly cross-linked polyethylene: a three-year randomised, controlled trial using radiostereometric analysis.

The creep and wear behaviour of highly cross-linked polyethylene and standard polyethylene liners were examined in a prospective, double-blind randomised, controlled trial using radiostereometric analysis. We randomised 54 patients to receive hip replacements with either highly cross-linked polyethylene or standard liners and determined the three-dimensional penetration of the liners over three years. After three years the mean total penetration was 0.35 mm (SD 0.14) for the highly cross-linked polyethylene group and 0.45 mm (SD 0.19) for the standard group. The difference was statistically significant (p = 0.0184). From the pattern of penetration it was possible to discriminate creep from wear. Most (95%) of the creep occurred within six months of implantation and nearly all within the first year. There was no difference in the mean degree of creep between the two types of polyethylene (highly cross-linked polyethylene 0.26 mm, SD 0.17; standard 0.27 mm, SD 0.2; p = 0.83). There was, however, a significant difference (p = 0.012) in the mean wear rate (highly cross-linked polyethylene 0.03 mm/yr, SD 0.06; standard 0.07 mm/yr, SD 0.05). Creep and wear occurred in significantly different directions (p = 0.01); creep was predominantly proximal whereas wear was anterior, proximal and medial. We conclude that penetration in the first six months is creep-dominated, but after one year virtually all penetration is due to wear. Highly cross-linked polyethylene has a 60% lower rate of wear than standard polyethylene and therefore will probably perform better in the long term.

Wear particle analysis of highly crosslinked polyethylene isolated from a failed total hip arthroplasty.

Polyethylene wear particles are one of the most important factors affecting the results of total hip arthroplasty (THA). To reduce wear generation and to achieve better long-term results of THA, highly crosslinked polyethylene (HXPE) has recently been introduced and come into wide use. Thus far, however, there have been no reports on in vivo analysis of HXPE wear particles. We isolated HXPE wear particles from periprosthetic tissue of a failed THA and analyzed using scanning electron microscope. The number of particles was 5.33 x 10(7) g(-1). Particle size (equivalent circle diameter) was 0.66 +/- 0.40 microm (mean +/- standard error). Aspect ratio and roundness were 1.37 +/- 0.26 and 1.44 +/- 0.67, respectively. All the particles were round shaped, and "fibrils" or "shreds" were not detected. Thus far, this was the first report on in vivo wear particle analysis of HXPE. HXPE generated less, smaller, and rounder particles, compared with the corresponding reported values for particles generated from conventional polyethylene. These characteristics might affect macrophage response, osteolysis, and long-term results of THA with HXPE.

Compliance calibration for fatigue crack propagation testing of ultra high molecular weight polyethylene.

Ultra High Molecular Weight Polyethylene (UHMWPE) total joint replacement components under certain conditions are at risk of fatigue fracture. Thus, the fatigue crack inception/propagation resistance of UHMWPE is of interest. During fatigue crack propagation tests of UHMWPE, crack growth is often followed visually; however, this approach can be time consuming and requires that the specimen be accessible during testing. The objective of this study was to demonstrate the applicability of the compliance method for fatigue crack propagation tests of UHMWPE. We hypothesized that the standard calibration coefficients developed for metals may not be appropriate for UHMWPE and that different UHMWPE materials would require different compliance calibration coefficients. Three UHMWPE materials: sterilized (30 kGy); highly crosslinked and annealed (100 kGy, 130 degrees C); and highly crosslinked and remelted (100 kGy, 150 degrees C) were examined under ambient conditions. The results support the applicability of the compliance method for determination of crack length during fatigue crack propagation testing of UHMWPE. As hypothesized, the standard calibration coefficients were found to be inaccurate for UHMWPE. New UHMWPE-specific calibration coefficients were determined which predicted the crack growth behavior accurately. Also, as hypothesized, the compliance calibration coefficients for the three materials were significantly different. This is the first reported study to demonstrate the applicability of a compliance method to measure crack length in UHMWPE.

Fluid composition impacts standardized testing protocols in ultrahigh molecular weight polyethylene knee wear testing.

Wear of total knee replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Weight gain due to fluid absorption during testing is corrected using a load soak station. In this study, three sets of ultrahigh molecular weight polyethylene tibial plateau were tested against highly polished titanium condyles. Test 1 was performed in two different institutions on the same simulator according to the standard ISO 14243-1, using two testing lubricants. Test 2 and test 3 repeated both previous test sections. The wear and load soak rates changed significantly with the lubricant. The wear rate decreased from 16.9 to 7.9 mg weight loss per million cycles when switching from fluid A to fluid B. The weight gain of the load soak specimen submersed in fluid A was 6.1 mg after 5 x 10(6) cycles, compared with 31.6 mg for the implant in fluid B after the same time period. Both lubricants were mixed in accordance with ISO 14243 (Implants for surgery - wear of total knee-joint prostheses), suggesting that calf serum should be diluted to 25 +/- 2 per cent with deionized water and a protein mass concentration of not less than 17 g/l. The main differences were the type and amount of additives that chemically stabilize the lubricant throughout the test. The results suggest that wear rates can only be compared if exactly the same testing conditions are applied. An agreement on detailed lubricant specifications is desirable.

Wear, debris, and biologic activity of cross-linked polyethylene in the knee: benefits and potential concerns.

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 mm per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 +/- 6 mm/1 million cycles). For the low-contact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 +/- 1 mm per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 +/- 2 mm/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 mum 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.

Wear of retrieved UHMWPE hip liners.

After the gamma-irradiation sterilization, the most widely used orthopaedic grade polymer bearing liner material for the total joint replacement, ultra-high molecular weight polyethylene (UHMWPE), degrades through the progressive in vivo oxidation. The oxidative degradation makes UHMWPE brittle and leads to reduction of its mechanical properties. In this study, the effect of the in vivo post-irradiation ageing time on the wear of UHMWPE was investigated. Twelve retrieved polyethylene hip liners implanted for 3-16 years and then stored in the air for 1.5-8 years were used. Two types of the pin-on-disk wear testing were conducted. The uni-directional repeat pass rotating and the linear reciprocating wear testing were done with stainless steel disks against stationary polyethylene pins under 4MPa at 1Hz with bovine serum lubrication. Wear of the retrieved polyethylene hip liners does not have significant correlation with the in vivo or total ageing time. The linear reciprocal sliding motion generated a more pronounced wear than the uni-directional repeat pass sliding motion. This indicates that the kinematic motion significantly affects the wear of aged UHMWPE, having a brittle, white band region.

Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC).

Exothermic heat flow rates (Q=microW=microJ/s), as a function of elapsed time, were measured by isothermal heat-conduction microcalorimetry (IHCMC) in order to study the aggregate rate of physico-chemical change in specimens of unsterilized and sterilized ultra-high-molecular-weight polyethylene (UHMWPE). Standard protocols for performing the IHCMC tests were developed and are described. Use of the standard protocols yielded the desired results-data that were not significantly different among either replicate sets of unsterilized specimens or as a function of which calorimeter test well was used. Heat flow rates measured in air at 20 degrees C, 25 degrees C, 35 degrees C, and 45 degrees C yielded estimates of activation energies of 47, 11, and 41 kJ/mol for unsterilized, gamma-radiation sterilized, and ethylene oxide gas (EtO) sterilized polymer, respectively. These results support the ideas that (a). initial exothermic degradation takes place much more easily in the radiation-sterilized material, due to direct oxidation of readily available free radicals, and (b). the much slower degradation process in EtO-sterilized UHMWPE is not appreciably different than in unsterilized polymer. Comparison with other activation energy data suggests that the rate-limiting process in EtO- or un-sterilized polymer is oxygen diffusion into the polymer. For shelf storage in air, for periods up to 8 months, the mean exothermic heat flow in air, at 25 degrees C (Q(m)) [determined from the Q values averaged over the time period between 15 and 20 h after test start], from UHMWPE gamma-radiation sterilized in air was significantly higher than for unsterilized material (2.91+/-0.11 vs. 0.73+/-0.11 microW). The higher rate can be attributed to oxidation of radiation-induced free radicals in the polymer near its surface. For the gamma-irradiated polymer, the decline in Q(m) with shelf storage time suggests that, eventually, degradation might become oxygen diffusion limited in this case also. However, in vivo, surface wear of an UHMWPE articular component may continue to expose unoxidized free radicals, keeping the exothermic reaction rate high and, possibly, continuing to produce an oxidized UHMWPE surface prone to wear.

Comparison of wear, wear debris and functional biological activity of moderately crosslinked and non-crosslinked polyethylenes in hip prostheses.

The wear, wear debris and functional biological activity of non-crosslinked and moderately crosslinked ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups have been com pared 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.

Micro-wear patterns on UHMWPE tibial inserts in total knee joint simulation.

The objective of this study was to examine both simulator and retrieved total knee replacement polyethylene inserts to confirm, using scanning electron microscopy, whether similar micro-wear patterns to those seen on retrieved inserts were reproduced on simulator specimens. The simulator specimens consisted of samples subjected to sliding and rolling movement (Experiment 1) and to sliding movement only (Experiment 2). Samples from Experiment 1 demonstrated longitudinal patterns in the middle of the wear track and transverse patterns in the anterior and posterior ends, whereas in Experiment 2, only transverse patterns were observed. In the retrieved specimens, both longitudinal and transverse patterns were observed. The results showed that the simulator study reproduced similar patterns of micro-damage on polyethylene, and that the longitudinal micro-wear pattern was related to the rolling movement that is distinctive in knee kinematics.

Surface micromechanics of ultrahigh molecular weight polyethylene: Microindentation testing, crosslinking, and material behavior.

The wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) is critical to the success of total joint replacements. Recent attempts to modify the wear behavior of UHMWPE by processing, in particular, crosslinking UHMWPE have shown promise to increase wear resistance, but concerns persist regarding other mechanical properties. It is also unclear what specific surface mechanical properties govern the wear resistance seen in these materials. The goal of this study was to demonstrate a custom-built surface mechanical test system and method that measures the micromechanical response of microtomed UHMWPE surfaces to depth-sensing microindentation tests. The surface structure of these UHMWPE materials was also examined using scanning electron microscopy and atomic force microscopy. A custom designed microindentation test system assessed the microindentation behavior of three UHMWPE resins: 1. Hylamertrade mark, 2. GUR-1020 CMS, and 3. Marathontrade mark-a lightly crosslinked material. The effects of material and indentation depth were studied. Microindentation tests were performed with indentation depths ranging from 2 to 45 microm. Four different measurements of surface micromechanical behavior were obtained including the surface modulus, microhardness, hysteresis energy (irreversible work done to the sample per unit cycle) and its associated energy dissipation factor, and loading slope. Statistically significant differences in each of these parameters were found for each material. Generally, Hylamer had the largest values for these parameters, followed by the GUR resin and then the Marathon. Surface modulus was independent of depth of testing and found to be 651 MPa for Marathon, 738 MPa for GUR, and 1015 MPa for Hylamer (Modulus for bulk UHMWPE is 540 MPa for Hylamer, 620 for GUR, and 1380 for Hylamer). The microhardness varied between 67 and 162 MPa depending on material and depth of testing. Surface structural characterization shows that the microtoming process for surface preparation generated distinct surface features that varied between materials. Intermittent drawn ribbons of polymer with oriented crystals were observed in both scanning electron microscopy and atomic force microscopy. The surface density and size of these features were characteristic of the materials with the Hylamer having the fewest, but largest ribbons, followed by GUR and then Marathon.

Accelerated aging studies of UHMWPE. I. Effect of resin, processing, and radiation environment on resistance to mechanical degradation.

The resin and processing route have been identified as potential variables influencing the mechanical behavior, and hence the clinical performance, of ultra-high molecular weight polyethylene (UHMWPE) orthopedic components. Researchers have reported that components fabricated from 1900 resin may oxidize to a lesser extent than components fabricated from GUR resin during shelf aging after gamma sterilization in air. Conflicting reports on the oxidation resistance for 1900 raise the question of whether resin or manufacturing method, or an interaction between resin and manufacturing method, influences the mechanical behavior of UHMWPE. We conducted a series of accelerated aging studies (no aging, aging in oxygen or in nitrogen) to systematically examine the influence of resin (GUR or 1900), manufacturing method (bulk compression molding or extrusion), and sterilization method (none, in air, or in nitrogen) on the mechanical behavior of UHMWPE. The small punch testing technique was used to evaluate the mechanical behavior of the materials, and Fourier transform infrared spectroscopy was used to characterize the oxidation in selected samples. Our study showed that the sterilization environment, aging condition, and specimen location (surface or subsurface) significantly affected the mechanical behavior of UHMWPE. Each of the three polyethylenes evaluated seem to degrade according to a similar pathway after artificial aging in oxygen and gamma irradiation in air. The initial ability of the materials to exhibit post-yield strain hardening was significantly compromised by degradation. In general, there were only minor differences in the aging behavior of molded and extruded GUR 1050, whereas the molded 1900 material seemed to degrade slightly faster than either of the 1050 materials.

Accelerated aging studies of UHMWPE. II. Virgin UHMWPE is not immune to oxidative degradation.

In Part I of this series, we showed that aging at elevated oxygen pressure is more successful at increasing the depth to which degradation occurs although it, too, generally causes greater degradation at the surface than at the subsurface. Therefore we hypothesized that thermal degradation alone, in the absence of free radicals, could be sufficient to artificially age UHMWPE in a manner analogous to natural aging. In the present study, virgin and air-irradiated UHMWPE (extruded GUR 1050 and compression-molded 1900) were aged up to 4 weeks at elevated oxygen pressure, and the mechanical behavior at the surface and subsurface was examined. All the materials were substantially degraded following 4 weeks of aging, but the spatial variations in the nonirradiated materials more closely mimicked the previously observed subsurface peak of degradation seen in naturally aged UHMWPE following irradiation in air. This aged material could provide a more realistic model for subsurface mechanical degradation, making it suitable for further mechanical testing in venues such as wear simulation.

Comparison of the properties of annealed crosslinked (Crossfire) and conventional polyethylene as hip bearing materials.

In 1998, orthopaedic manufacturers started to introduce highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) for total hip replacement bearings. Today's highly-crosslinked UHMWPEs materials are irradiation processed with a total dose ranging between 50 and 105 kGy, depending upon the manufacturer. Each manufacturer has adopted a different route for producing their highly crosslinked UHMWPE that includes a combination of three important processing steps: an irradiation step, an intra or post-irradiation thermal processing step, and a sterilization step. This paper reviews the choices available to an implant designer when developing a highly-crosslinked UHMWPE as an orthopaedic bearing material. We suggest that the application of annealing rather than re-melting in the thermal processing step allows the retention of important mechanical properties in the finished material. This approach will be illustrated with test data on Crossfire (Stryker Howmedica Osteonics, Mahwah, NJ), an annealed, highly-crosslinked UHMWPE developed specifically for total hip replacements. We compare the physical, mechanical, and wear properties of Crossfire with that of conventional (N2-Vac) UHMWPE and with materials produced using published melt irradiation technology. At the short term (2 and 3 years) the results demonstrated through clinical follow-up, clinical results for Crossfire, are encouraging. Longer follow-up is necessary to confirm the benefits to patients from reduction of debris released from the articulation.

Comparative wear and wear debris under three different counterface conditions of crosslinked and non-crosslinked ultra high molecular weight polyethylene.

The wear debris generated from ultra high molecular weight polyethylene (UHMWPE) have been recognised as one of the major causes of failure in total hip replacements (THR). It is essential to reduce the wear debris generated from UHMWPE acetabular cups in order to minimise this problem. Debris in the submicron size range is believed to have greater osteolytic potential. It is now known that crosslinked UHMWPE acetabular cups have reduced volumetric wear rates but little is known about the influence of crosslinking on the size and morphology of the wear debris. In this study, the wear of grade GUR 1020 crosslinked (vacuum gamma irradiated), GUR 1120 crosslinked (acetylene enhanced irradiated) and non cross linked (ethylene oxide sterilised) GUR 1020 UHMWPE was compared in multidirectional pin-on-plate wear tests under three different counterface conditions (smooth, isotropically rough and scratched counterfaces). Multidirectional motion was chosen because this motion was closer to the relative motion in the natural hip. From this study, better wear resistance of crosslinked UHMWPE compared with non-crosslinked UHMWPE was demonstrated for the smooth counterface conditions. However, in the rough and scratched counterface conditions, the vacuum gamma irradiated crosslinked material produced significantly higher wear rates than the non-crosslinked material. The analysis of the wear debris showed that the majority of the volume of the acetylene enhanced crosslinked UHMWPE wear debris was in the most biologically active size range (0.1 to 0.5 microm). In contrast, the non-crosslinked material and the vacuum gamma irradiated crosslinked material had a greater proportion of the volume of the debris in the larger size ranges which are less biologically active. This has important implications for its osteolytic potential.

A depth profile of oxidation and gel fraction in gamma-irradiated silane crosslinked and ultra high molecular weight polyethylenes.

The depth profile of oxidation index and gel fraction has been measured for two silane crosslinked poly(ethylene) (SXLPE) acetabular cups (one gamma irradiated in air, and one non-irradiated, both with a shelf-life of 13 years) and for two UHMWPE components (one gamma irradiated in air and one non-irradiated, with shelf-lives of 13 and 7 years, respectively). Only the irradiated UHMWPE exhibited any variation in these properties with depth. The oxidation profile (maximum 1 mm below surface) has been explained to result from reduced levels of diffused oxygen with depth, giving rise to a balance of alkyl and peroxyl radicals (and hence maximum carbonyl production) just below the surface. The gel fraction profile (maximum 4 mm below surface) is also attributed to the lower levels of diffused oxygen with depth, causing crosslinking to dominate in the bulk and chain scission to dominate at the surface. The resistance to oxidative degradation in the non-irradiated SXLPE was attributed to the use of antioxidants in the polymer processing.