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 × 106 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/1. 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 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.

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.

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.

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.

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.

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.

Characterization of compression-molded UHMWPE, PMMA and PMMA/MMA treated UHMWPE: density measurement, FTIR-ATR, and DSC

Considered one of the weak links in the total hip replacement (THR), efforts to enhance the interfacial strength between bone cement and ultra-high molecular weight polyethylene (UHMWPE) acetabular cup had been conducted in this laboratory. Following the successful demonstration of high interfacial strengths for our new acetabular component design, the nature of physical, chemical, and thermal property of the compression-molded specimens, including UHMWPE, PMMA/MMA treated UHMWPE, and PMMA has been investigated in this study. Density results from a density gradient column showed that the molding processes and conditions were adequate for complete sintering of UHMWPE and PMMA powders. FTIR-ATR results gave a direct evidence that PMMA did exist in the PMMA/MMA treated UHMWPE matrix. It also revealed a clear diffusion-related behavior across the interface. Under the high temperature and pressure, the UHMWPE powders undergo drastic changes of their morphology and crystalline structures. These changes were examined by differential scanning calorimeter (DSC) which showed a large difference in terms of % crystallinity. The percent of PMMA deposited in the treated UHMWPE was 17.8%, 18.8%, and 24.3% from the analyses of density, FTIR-ATR, and DSC, respectively. Finally, an evidence of diffusive behavior at the interface exhibited diffusion of PMMA occurring across the interfaces between the treated UHMWPE and UHMWPE or PMMA.

Material removal during the sliding of hydroxyapatite against UHMWPE

Hydroxyapatite has been rubbed against ultra-high-molecular-weight-polyethylene (UHMWPE) under calcium-containg aqueous solutions. Further, hardness tests were carried out in air and in calcium-containing solutions whose pH ranged from pH 5 to pH 9. Hardness was found to vary with pH with a peak at around pH 7, i.e. - a chemomechanical effect was observed. Wear tests consisted in sliding hydroxyapatite samples against a UHMWPE disk for eight hours when lubricated by the same solutions as those used for the hardness tests. Volume loss, pH and calcium concentration were measured for up to 8 hours of sliding. Linking wear tests results with hardness results and supersaturation levels, it was concluded that two wear mechanisms occurred. A chemical mechanism depending on supersaturation occurred at the early stages of sliding. The wear rate was essentially independent of hardness during this stage. After a few hours, depending on the supersaturation of the lubricant, the chemical mechanism turned into a chemomechanical mechanism dependant on hardness.

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.

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 general axisymmetric contact mechanics model for layered surfaces, with particular reference to artificial hip joint replacements

A general axisymmetric contact mechanics model for layered surfaces is considered in this study, with particular reference to artificial hip joint replacements. The indenting surface, which represents the femoral head, was modelled as an elastic solid with or without coating, while the other contacting surface, which represents the acetabular cup, was modelled as a two-layered solid. It is shown that this model is applicable to current total hip joint prostheses employing ultra-high molecular weight polyethylene (UHMWPE) acetabular cups against metallic, metallic with coating or ceramic femoral heads as well as metal-on-metal combinations. The effect of cement is also investigated for these prostheses using this model. The use of a metallic bearing surface bonded to a UHMWPE substrate for acetabular cups is particularly examined for metal-on-metal hip joint replacements. Both the contact radius and the contact pressure distribution are predicted for examples of these total hip joint replacements, under typical conditions. Application of contact mechanics to the design of artificial hip joint replacements employing various material combinations is discussed.

[Life-cycle assessment of single-use versus reusable surgical drapes (cellulose/polyethylene-mixed cotton system)]

Surgical drapes made of cotton are under increasing competition with various disposable products and reusable draping systems (e.g., made of synthetic fabrics like polyester). When making a choice to use one of these medical devices in practical surgery, major aspects like handling, hygienic safety and costs, but also environmental effects have to be taken into account. In this study a mixed system for patient drapes (reusable cotton drapes combined with a reduced set of impermeable single-use drapes made of cellulose/polyethylene) was compared to a system that is only based on single-use drapes with regard to ecology [life-cycle assessment (LCA)]. The medical literature was reviewed to assess important medical aspects of the use of patient drapes, resulting in the statement that there are no conclusive arguments to support a clear hygienic superiority of one of these alternatives. Based on the conditions assumed and stated, the results of the LCA indicate that the mixed draping system is associated with two times more total energy consumption. In addition, more water is needed and more CO2 emissions are produced. However, draping with the single-use product results in more clinical waste. Regarding water pollution no system proved superior. It is difficult to compare and weigh various environmental aspects like the polluting cultivation of cotton in distant countries (reusable drapes) and the higher figure of transportation necessary to deliver the single-use product within Germany. It is an important disadvantage of the mixed system that it combines the ecological burden of both cotton drapes and the single-use alternative.

The association between rates of wear in retrieved acetabular components and the radius of the femoral head

Current evidence suggests that loosening of the acetabular socket is related to the volume of ultra-high molecular weight polyethylene wear debris generated at the articulating surfaces, through a process of bone resorption. Therefore it is important that the rate of volumetric wear is minimized in an attempt to extend the useful life of the procedure. Laboratory evidence indicates that a reduction in sliding distance would be beneficial in achieving this target and may be attained by a reduction in femoral head radius. To investigate the relationship between femoral head size and the rates of both volumetric wear and penetration, 200 acetabular components were retrieved at the time of revision surgery. The joints had femoral heads ranging in size from 11.1 to 19.8 mm radius. For those sockets that were loose at revision surgery, a significant correlation was observed, between the rate of volumetric wear and the radius of the femoral head. For this cohort an increase in radius of 1 mm resulted in an increased rate of volumetric wear of 5.1 (SE 1.4) mm3/yr. However, the explained variance in the regression was low and exemplifies the multifactorial nature of the wear process. In particular, it is anticipated that the activity of the patient will have a significant effect on the rate at which the debris is produced. No significant correlation was observed between the rate of linear wear and femoral head radius. These results would indicate a benefit in using head sizes of a smaller radius, which generate debris at a reduced rate, and therefore require more time to accumulate large volumes of wear products.

In vitro simulation of contact fatigue damage found in ultra-high molecular weight polyethylene components of knee prostheses

An in vitro simulation of fatigue loading of ultra-high molecular weight polyethylene (UHMWPE) knee components was carried out on a knee simulator and on a rolling and sliding wear tester. Tibial components for the knee simulator were gamma-sterilized, implantable components taken from manufacturing inventory. The rolling/sliding UHMWPE discs were machined from bar stock and either gamma sterilized in air and accelerated aged, or left as non-sterilized (controls). Cracking and delamination of samples that had been gamma sterilized in air and aged were observed in both types of tests. Contact fatigue damage was visible in as few as 150,000 cycles using the knee simulator at loads of 122 N (275 1b). The rolling/sliding samples showed signs of damage in as few as 130,000 cycles with an estimated stress of 15 MPa and 25 per cent sliding. However, cracking and delamination were not generated in the never-sterilized or recently sterilized controls. UHMWPE that has been gamma sterilized in air and aged is shown to be susceptible to contact fatigue damage. These results are important to the interpretation of in vitro total knee replacement simulations used to assess the performance of tibial bearings.

Sliding wear studies of selected nitride coatings and their potential for long-term use in orthopaedic applications

In the area of orthopaedic implants, particularly total hip joint replacements, a metal-plastic combination is still the most popular choice consisting of a femoral head fabricated from 316L stainless steel, Ti alloy or Co-Cr alloy in contact with an ultra-high molecular weight polyethylene (UHMWPD) acetabular cup. It is recently considered that wear of the UHMWPE cup is of major concern. Generation of the wear debris can have adverse effects on the body, both localized and systemic. It is envisaged that wear of the prosthetic components, particularly those fabricated from UHMWPE can be reduced through the use of surface coatings. The aim of this investigation was to deposit a selection of refractory element nitride-based coatings (TiN, TiA1N, ZrN) onto 316L stainless steel substrates, using physical vapour deposition (PVD) technology and to study their sliding wear behaviour in contact with both UHMWPE and 316L stainless steel pins, using a pin-on-plate testing rig. Tests were conducted in Ringers solution and Ringers solution plus bone cement particles. The volume of material removed from the pins served as an indication of their wear behaviour. Wear mechanisms were identified using scanning electron microscopy. The results of these findings and the potential for these coatings to be used in orthopaedic applications are discussed.

Metal release from total hip articulations in vitro: substantial from CoCr/CoCr, negligible from CoCr/PE and alumina/PE

We used a hip joint simulator to compare the metal release from CoCr/CoCr, CoCr/PE, and alumina/PE total hip articulations. The metal release was quantified by analyzing the Co, Cr, and Ni contents of the bovine serum lubricant used with atomic absorption spectroscopy. CoCr/CoCr articulations released substantial amounts of metal, whereas CoCr/PE was equal to the control, alumina/PE, in that metal release was negligible. The metal release was in accordance with the known clinical wear rates of CoCr/CoCr articulations. The largest dimensional changes occurred in polyethylene cups, the penetrations of CoCr heads to the polyethylene cups being twice that of the alumina head, which is consistent with clinical experience. The research on the wear behavior of different materials, aiming to find a prosthesis with negligible wear, needs to be continued. Due to the substantial metal release, the CoCr/CoCr articulation is hardly the final solution of the wear problem in total hip arthroplasty.

Osteolysis associated with cemented total knee arthroplasty

Osteolysis has not been mentioned as a complication or cause of failure of cemented total knee arthroplasties in long-term follow-up studies. We are aware of a single case report of osteolysis after cemented total knee arthroplasty. We report the case of an 87-year-old woman with massive osteolysis beneath a cemented tibial component.

An analysis of hylamer and polyethylene bearings from retrieved acetabular components

Hylamer and conventional polyethylene acetabular liners of the same design, revised for a variety of reasons, were examined and compared to assess the performance of Hylamer as a bearing material. Clinical damage modes, linear wear rates, oxidation levels, and mechanical properties were measured. In both series, many liners were retrieved for dislocation. Wear/osteolysis was the most common reason for retrieval in the Hylamer series, while none of the conventional polyethylene liners were retrieved for this reason. Nearly all liners exhibited abrasion, burnishing, scratching, and creep. The Hylamer liners had more cracking, delamination, and pitting. The Hylamer liners had an average linear wear rate of 0.32 mm/year, while the conventional polyethylene liners had an average wear rate of 0.20 mm/year. Due to sample size, no statistical difference in wear rate was noted between the two groups. In general, both the Hylamer and conventional polyethylene showed oxidation peaks subsurface, resulting from their exposure to gamma radiation in air. Liners with elevated oxidation had decreased ultimate tensile strength, elongation, and toughness. For given oxidation levels, the corresponding mechanical properties of Hylamer appeared lower than those of conventional polyethylene. The ultimate tensile strength values ranged from 14 to 33 MPa for Hylamer and 19 to 32 MPa for conventional polyethylene. Elongation ranges were 19% to 350% (Hylamer) and 80% to 375% (conventional). The Hylamer retrievals in this study gave initial indications of performance; Hylamer appeared to behave similarly, but not superiorly, to conventional polyethylene, in the early functional period with respect to clinical wear and clinical performance. Both Hylamer and conventional polyethylene liners were degraded by gamma sterilization in air, with Hylamer liners demonstrating greater property changes.

Role of polyethylene oxidation and consolidation defects in cup performance

Quality factors such as particle consolidation and oxidation have been claimed to impair the performance of ultrahigh molecular weight polyethylene implants, but no definite data estimate their real effect. To assess the influence of these quality determinants in wear, wear rate, and time in service at a single, well proven design, the percentage area of polyethylene sections occupied by nonconsolidated polyethylene particles, the presence of a white band, and the amount of polyethylene oxidation (through density curves) were evaluated in 92 retrieved Charnley acetabular components with available clinical data. The average percentage area of nonconsolidated polyethylene particles in cup sections was 3.1%. There were 11 cups showing a subsurface white band. The authors observed an average density in the deep polyethylene of 0.9420 g/cc after an average in vivo use of 9.8 years (range, 0.08-20.3 years). Banded cups showed significantly higher subsurface densities. When studying the relationship among clinical and material factors with performance variables, wear measurements (obtained through radiographic methods and direct measurements of polyethylene thickness in the collected implants) correlated with age at implantation, activity, and time in service. Weight was a determinant of the wear rate. None of the studied polyethylene quality factors showed a definite association with wear performance or time to failure in this series. Implant survivorship was not significantly impaired in the 22.225-mm Charnley low friction replacement by a subsurface white band or by a higher area occupied by nonconsolidated particles. Fewer nonconsolidated particles were not associated with longer survivorship. This design proved tolerance to polyethylene quality variations, in the signaled ranges, without a significant effect in the system performance.

Validation of a small punch testing technique to characterize the mechanical behaviour of ultra-high-molecular-weight polyethylene

The small punch or miniaturized disc bend test has been used successfully to characterize the ductility and fracture resistance of metals and ceramics with specimens measuring 0.5 mm in thickness. This study was performed to demonstrate the feasibility of performing small punch tests on implant grade ultra-high-molecular-weight polyethylene (UHMWPE). Large-deformation finite element simulations were developed and validated to explore the hypothesis that the macroscopic constitutive behaviour of UHMWPE may be inferred from a miniature specimen testing technique which can be used to characterize the ductility and work to failure for UHMWPE. The load-displacement curve was insensitive to cyclic preconditioning of the test specimen and only mildly sensitive to the loading rate. Furthermore, the initial slope of the small punch load-displacement curve was used to determine the elastic modulus of the UHMWPE with the help of the inverse finite element method. The ultimate goal of this research is to develop the capability to perform local measurements of material tensile and static fracture properties in as-manufactured, as-sterilized and as-retrieved UHMWPE components.

Bioactive heparin surfaces from derivatization of polyacrylamide-grafted LLDPE

Primary amine groups were introduced into polyacrylamide-LLDPE films, using the Hofmann degradation synthesis. The Hofmann degradation was studied at room temperature using sodium hypochlorite and sodium hydroxide at different concentrations. Diazotized heparin was covalently bound to the grafted LLDPE film via the primary amine groups. Surfaces were analysed with ESCA, ATR-IR, chloride titration and Toluidine Blue. Evaluation of the biological activity of the heparinized surfaces was made by measuring the capacity for binding antithrombin (AT) and inhibition of the activated coagulation factor XII (FXIIa). The heparinized surfaces were able to bind up to 3 pmol cm-2 of AT in solution with ionic strengths of I = 0.15 and I = 0.40. No activation of the adsorbed FXII was detected.

Evaluation of the wear of the pivot bearing in the Gyro C1E3 pump

To estimate the lifetime of the pivot bearing system of the sealless centrifugal Gyro C1E3 pump, pivot bearing wear phenomena of the C1E3 were studied. The pivot bearing system consisted of a male and female pivot made of ceramics and ultrahigh molecular weight polyethylene (UHMWPE), respectively. First, many pumping tests were performed with the C1E3 under various pumping conditions, and the effects of impeller position and fluid on wear were analyzed. Through these preliminary tests, it was found that the wear progress of the pivot bearing consisted of initial wear and stationary wear. Most of this initial wear is caused by the plastic deformation of the polyethylene female pivot. It also was observed that bovine blood was almost comparable to water in its effect on the stationary wear rate at the same rotational speed. Based on these results, a long-term pumping test was performed with the C1E3, and initial and stationary wear rates were determined. At the same time, the maximal loosening distance (LDmax) (permissible total wear) of the C1E3 was determined experimentally from hemolytic and hydraulic performance perspectives. By using experimentally determined parameters the lifetime of the pivot bearing system of the C1E3 was typically 10 years for right ventricular assist, 8 years for left ventricular assist, and 5 years for cardiopulmonary bypass.

The influence of scratches to metallic counterfaces on the wear of ultra-high molecular weight polyethylene

A number of studies of explanted metallic femoral heads have shown scratches or damage caused by bone cement, bone or metallic particles. This damage has been cited as a cause of increased wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. In this laboratory study, small scratches 2 mu m deep were made on smooth stainless steel surfaces at a spacing of 10 mm. These individual scratches were found to increase the wear rate of UHMWPE by a factor of 30 in unidirectional sliding and a factor of 70 in reciprocating motion. It is of particular concern that a single small scratch, which is not detected by the average surface roughness measurement Ra can cause such a dramatic increase in the wear of ultra-high molecular weight polyethylene.

Energy-shunting external hip protector attenuates the peak femoral impact force below the theoretical fracture threshold: an in vitro biomechanical study under falling conditions of the elderly

The first objective of this study was to design a hip protector that would effectively attenuate and shunt away from the greater trochanter the impact energies created in typical falls of the elderly. As the shock absorption material, the protector included the 12 mm-thick Plastazote, which was found to be the most efficient energy-absorbing material in our previous in vitro biomechanical tests. With an anatomically designed semiflexible outer shield of the protector (high density polyethylene), the impact surface was increased and the impact energy shunted away from the greater trochanter. In the second phase of the study, we determined the force attenuation capacity of this device in realistic (in vitro) falling conditions of the elderly. With the impact force of 6940 N used (a typical hip impact force measured in in vitro falling tests), the trochanteric soft tissue (25 mm-thick polyethylene foam) attenuated the peak femoral impact force to 5590 N and the tested protector to 1040 N. In the second series of this experiment, the peak femoral impact force was set to be so high (13,130 N) that the protector, if effective, should prevent the hip fracture in almost all cases. The trochanteric soft tissue attenuated this peak impact force to 10,400 N and the tested protector to 1810 N. Thus, the force received by the proximal femur still remained clearly below 4170 N, the average force required to fracture in vitro the proximal femur of the elderly in a fall loading configuration. In conclusion, our test results suggest that an anatomically designed energy-shunting and energy-absorbing hip protector can provide an effective impact force attenuation in typical falling conditions of the elderly. However, the efficacy of the protector in the prevention of hip fractures can only be evaluated in randomized clinical trials.

Metal wear and tissue response in failed titanium alloy total hip replacements

Thirteen total hip replacements with titanium alloy femoral components required revision for loosening at an average of two years after implantation. At revision the soft tissues around the implant were darkly stained and a proliferative membrane had invaded the cement-bone interface. The femoral components showed polishing of parts of their shot-blasted surfaces. Histology showed a fibroblastic reaction with abundant titanium lying free and within histiocytes, and a scanty foreign-body giant-cell reaction. Surface analysis of the removed femoral components and chemical analysis of the excised tissues is described. Tissue reaction in response to the metal-wear debris may have contributed to the early failure of these implants.

Comparison of the mechanical and histologic properties of Achilles tendons in New Zealand white rabbits secondarily repaired with Marlex mesh

Over the years, there have been numberous reports on the treatment of tendo Achillis ruptures. Most of these dissertations have dealt with the treatment of acute ruptures versus the more technically challenging, neglected, ancient rupture. This paper will focus on the neglected rupture, with specific attention directed towards a new technique of surgical repair, using Marlex mesh. Through the exhaustive testing of the tendo Achillis of New Zealand white rabbits at intervals of 1, 3, and 6 months postimplantation, and subsequent implantation in a 13-month follow-up in an adult, it has been demonstrated that this biosynthetic tendon complex very closely approximates the physical properties of the normal tendo Achillis. Furthermore, through the use of histologic examination, it has been shown that this material actually forms a frame or bridgework for ingrowth of normal, orderly, collagen bundles, closely resembling those found in the original tendinous structure.

Evaluation of long sampling tubes for remote monitoring by mass spectrometry

The long sampling tubes required for remote mass spectrometry alter the sampling system's performance characterized by sample flow, residence time, and 10 to 90% response time. We searched for an easy-to-handle tube with (1) a length of 30 m, (2) sample flow less than 50 ml.min-1, and (3) residence and response times approaching those predicted by our mathematical model. We tested tubes of various geometries and various commercially available materials by using them as inlet catheters for a quadrupole mass spectrometer (Centronic 200 MGA, Centronic Ltd, Craydon, UK). We measured their responses at 0 to 10% (on transients) and 10 to 0% (off transients) step changes in gas concentration for nitrogen, argon, nitrous oxide, oxygen, and carbon dioxide and 0 to 3% and 3 to 0% for halothane, enflurane, and isoflurane. With 5 polyethylene tubes, halothane response times were up to 38 times longer than predicted. One 30-m polyethylene tube combined a 158-ms response time for nitrogen and argon with a 2,205-ms response time for halothane. Teflon, polyvinyl chloride, and stainless steel also proved to be unsuitable because of unacceptable signal distortion: the carbon dioxide response time for a 30-m Teflon tube was 2,600 ms. A glass tube showed the least signal distortion but was hard to handle. Our requirements were fulfilled by a 29.77-m tube made from nylon with a 1.00-mm inside diameter to which a 0.23-m length of nylon with a 0.25-mm inside diameter was added at the patient end. It offers (1) sample flow equals 46 ml.min-1, (2) residence time equals 11.1 seconds, and (3) response times approaching our theoretical predictions, that is, 159, 164, 180, 159, 188, 302, 298, and 300 ms (means of on and off transients) for nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, halothane, enflurane, and isoflurane, respectively. This tube allows the accurate monitoring of breathing frequencies up to 25 and 50 breaths/min for volatile agents and gases, respectively.

Wear of ultra-high-molecular-weight polyethylene components of 90 retrieved knee prostheses

Wear of the ultra-high-molecular-weight polyethylene (UHMWPE) components in total knee arthroplasties is a potential long-term problem. Ninety total knees of various designs with implant times up to 10 years were retrieved. The wear noted in the majority of components was much greater than that noted in wear studies of acetabular components in total hip prostheses. Abrasion from cement or bone and delamination wear were particularly pronounced in the knee. Delamination, consisting of complete breakup of material in flakes and particles, appeared to be initiated by intergranular material defects and propagated by the excessive subsurface stresses beneath the contact zone. Material that was free of defects did not show delamination wear even after long time periods in a highly stressed, low-conformity design. Wear particles of UHMWPE can result in adverse tissue reaction with cellulitis, giant cell reaction, and necrotic tissue, and these effects could be cumulative with time. There is some evidence that particles can lead to bone resorption, including at the implant-bone interface, which could accelerate loosening. There is cause for concern as to the long-term effects of UHMWPE in total knee arthroplasty. This suggests the need for improved processing methods or more wear-resistant materials.

An evaluation of ultrastrong polyethylene fiber as an ophthalmic suture

An ultrastrong polyethylene fiber was evaluated as an ophthalmic suture. Properties of this fiber and of nylon, polypropylene, and polyester sutures were measured by standard techniques for fiber testing and for testing knot characteristics of sutures. Their behavior in cataract and keratoplasty surgery was assessed qualitatively. The ultrastrong polyethylene fiber has great tensile strength, high flexibility, and is very inelastic. Its strength and knot security provide safe incision closure and it has good biocompatibility. Ultrastrong polyethylene fiber is potentially superior to nylon, polypropylene, and polyester in the most important characteristics of a non-absorbable monofilament polymer ophthalmic microsuture.

Fluid-sorption phenomena in sterilized polyethylene acetabular prostheses

The weight changes due to fluid-sorption were measured in 62 radiation-sterilized acetabular sockets and 10 unsterilized discs. The materials included two types of ultra-high molecular weight (UHMW) polyethylene (RCH 1000; Hi-Fax 1900) and a carbon-fibre-reinforced polyethylene (CFPE). The fluid absorption curve was consistently biphasic. In the first 30 d soak-period (Phase 1), the initial rate of fluid absorption averaged 153 micrograms/d for conventional UHMW polyethylene and 278 micrograms/d for carbon-fibre-reinforced polyethylene. In Phase 2, beyond 30 d and up to 400 d, fluid absorption reduced to linear rates of 27 micrograms/d for UHMW polyethylene and 43 micrograms/d for CFPE. The latter soak-weight-gain values corresponded to only 0.00016%/d and 0.00034%/d respectively. There was little difference in absorption rates between sterilized and unsterilized samples. However soak rates were generally higher in water compared to serum.

A comparison of RCH 1000 and Hi-Fax 1900 ultra-high molecular weight polyethylenes

Hoechst RCH 1000 and Hercules Hi-Fax 1900 ultra-high molecular weight (UHMW) polyethylenes have been compared, for the first time, in tests, including wear, fatigue and creep, relevant to artificial human joints. In none of the tests was the behaviour of the Hercules material inferior to that of RCH 1000, and in the wear and creep tests it was superior.

Proplast and Plastipore

Sixteen Proplast and 52 Plastipore prostheses, removed at revision surgery, have been examined histologically and the findings illustrated. The macroscopic appearance of the prostheses was preserved. Fibrous tissue capsule formation was common. It has been expected that the prostheses will be invaded by fibrous tissue and thereby stabilized. Fibrous tissue ingrowth was consistently seen in Proplast but not Plastipore prostheses. Multinucleated foreign body giant cells were present in large numbers in both types. There was histological evidence of breakdown of the prostheses.

Histologic reactions to polyglactin-910, polyethylene and nylon microsuture

Three froms of 10-0 microsuture-monofilament polyglactin-910, polyethylene and nylon-were tested for histologic reaction. Polyglactin had the least long-lived reaction.

[Properties of polyfluoroethylene-copolymers for the denture base lining material (author's transl)]

Silicone rubber is conventionally used as a lining soft material for denture base. However it still has some properties to be improved, such as poor adhesive strenth with acrylic resin, discoloration, degradation and growth of candida. We attempted to apply chemically-stable polyfluoroethylene-copolymers for this purpose. The terpolymers consisting of difluoroethylene, tetrafluoroethylene and chlorotrifluoroethylene (or hexafluoropropylene) were prepared and tested for tensile strenth, adhesive strength to acrylic resin, water sorption, contact angle, shore hardness, and toxicity by hemolysis test, cell culture test and extraction test. One of the selected terpolymer showed less water sorption (0.1 mg/cm2), stronger tensile strength (160 kg/cm2) and adhesive strength (110 kg/cm2) than the silicone rubber lining material. The toxicity test results suggested that the terpolymer will pose a low potential acute toxicity in vivo. All the test results seem to suggest that the polyfluoroethylene elastomer described here is a promising soft liner material for denture base.