Effect of cross-linking ultrahigh molecular weight polyethylene: Surface molecular orientation and wear characteristics (original) (raw)
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Wear behaviour of cross-linked polyethylene assessed in vitro under severe conditions
Biomaterials, 2005
The polyethylene (PE) for hip implants presents serious clinical problems; the production of debris may induce adverse tissue reactions that may lead to extensive bone loss around the implant and consequently osteolysis and implant loosening. Several attempts have been made to improve the wear properties of ultra-high molecular weight polyethylene (UHMWPE). More recently the attention of various researchers has been focused on cross-linked polyethylene (XLPE), due to its improved wear resistance with respect to conventional UHMWPE. This study was aimed at comparing the wear performances of clinically available acetabular liners (Zimmer Inc.) made of electron beam XLPE and conventional UHMWPE. To evaluate the influence of the material properties on wear, conventional UHMWPE and XLPE acetabular cups were tested against deliberately scratched CoCrMo femoral heads (R a =0.12-0.14 mm) in a hip joint wear simulator run for 3 million cycles with bovine calf serum as lubricant. Gravimetric measurements revealed significant differences between the wear behaviours of the two sets of acetabular cups: XLPE exhibited a wear rate about 40 times lower than conventional UHMWPE. Raman spectroscopy coupled to partial least-squares analysis was used to evaluate the possible crystallinity changes induced by mechanical stress (and thus the material wear resistance): only the UHMWPE cup which showed the highest weight loss displayed significant crystallinity changes. These results were correlated to the thickness of the plasticity-induced damage layer. The wear debris produced during the tests were isolated according to a validated protocol and imaged by scanning electron microscopy. The wear particles produced by XLPE were smaller than those produced by UHMWPE; the latter were observed as fibrillar and agglomerated particles. The mean equivalent circle diameter was 0.71 and 0.26 mm for UHMWPE and XLPE, respectively.
Polymer, 2005
Radiation-induced crosslinking has been shown to have a beneficial effect on the wear resistance of ultra high molecular weight polyethylene (UHMWPE). Since we postulate that crosslinking takes place through reactions involving terminal double bonds, unsaturated additives were added to UHMWPE in this study to enhance crosslinking. UHMWPE specimens soaked in 1,7-octadiene, methylacetylene and ethylene, respectively, were irradiated with electron beam to different doses in single or multiple passages. FTIR spectroscopy was used for the chemical characterisation of the crosslinked polymer. Tensile tests were performed with all samples in order to monitor changes in the mechanical properties.
Ultra high molecular weight polyethylene: Mechanics, morphology, and clinical behavior
Journal of the Mechanical Behavior of Biomedical Materials, 2009
Ultra high molecular weight polyethylene (UHMWPE) is a semicrystalline polymer that has been used for over four decades as a bearing surface in total joint replacements. The mechanical properties and wear properties of UHMWPE are of interest with respect to the in vivo performance of UHMWPE joint replacement components. The mechanical properties of the polymer are dependent on both its crystalline and amorphous phases. Altering either phase (i.e., changing overall crystallinity, crystalline morphology, or crosslinking the amorphous phase) can affect the mechanical behavior of the material. There is also evidence that the morphology of UHMWPE, and, hence, its mechanical properties evolve with loading. UHMWPE has also been shown to be susceptible to oxidative degradation following gamma radiation sterilization with subsequent loss of mechanical properties. Contemporary UHMWPE sterilization methods have been developed to reduce or eliminate oxidative degradation. Also, crosslinking of UHMWPE has been pursued to improve the wear resistance of UHMWPE joint components. The 1 st generation of highly crosslinked UHMWPEs have resulted in clinically reduced wear; however, the mechanical properties of these materials, such as ductility and fracture toughness, are reduced when compared to the virgin material. Therefore, a 2 nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1 st generation while also seeking to provide oxidative stability and improved mechanical properties.
Journal of Applied Polymer Science, 2013
The aim of this study was to explore the impact of the sequential irradiation and annealing process on the microstructure, thermooxidation behaviour and mechanical properties of GUR 1050 ultra-high molecular weight polyethylene (UHMWPE) with respect to the postirradiation annealed material. For this purpose, the effects of a variety of irradiation and annealing conditions on microstructure and mechanical properties were investigated. Differential scanning calorimetry was performed to characterize melting temperature, crystalline content and crystal thickness, whereas transmission electron microscopy provided additional insights into crystal morphology. Thermogravimetric experiments in air served to assess thermooxidation resistance and changes associated to radiationinduced crosslinking. Fatigue properties were studied from three different approaches, namely short-term cyclic stress-strain tests, long-term fatigue experiments and crack propagation behaviour. Likewise, three experimental techniques (uniaxial tensile test, impact experiments, and load to fracture of compact tension specimens) allowed evaluation of the fracture resistance. The present findings confirm sequentially crosslinked UHMWPE exhibited improved thermooxidation resistance and thermal stability compared to post-irradiation annealed UHMWPE. Also, the mechanical behaviour, including the fatigue and fracture resistance, of these materials was generally comparable regardless of the annealing strategy. Therefore, the sequential irradiation and annealing process might provide higher oxidation resistance, but not a significant improvement in mechanical properties compared to the single radiation dose and subsequent annealing procedure.
Journal of Materials Science-materials in Medicine, 2001
This paper investigates the benefits of combining roll-drawing and acetylene-enhanced crosslinking to alter the mechanical properties of the ultra high molecular weight polyethylene (UHMWPE) used in total hip and knee replacements, with the aim of improving its resistance to wear. UHMWPE was processed via crosslinking, roll-drawing and a combination of crosslinking and roll-drawing and subjected to gel content analysis, tensile
Journal of Arthroplasty, 1995
Several studies have indicated that degradation of ultrahigh-molecularweight polyethylene following gamma irradiation in air adversely affects the mechanical properties of the material; however, it is not known how this subsequently affects its wear rate. Wear studies have therefore been performed on three groups of ultrahigh-molecular-weight polyethylene: unirradiated material, recently irradiated material (aged for 2 months), and aged irradiated material (aged for 5 years). The aging took place in sterile packaging on the shelf. The wear studies were carried out on a tri-pin-on-disk wear tester, with a pin from each type of material being studied in each test. In each test the wear rate of the nonirradiated material was slightly lower than the 2-month-aged, irradiated material. The 5-year-aged, irradiated material had the highest wear rate, and this was significantly greater than that of the unirradiated material (P < .05). low wear rates in vivo have been quoted, 3 in general volumetric wear rates in vivo are much higher, with average values for acetabular cups estimated to be in the range 40 to 90 mm3/y. 4 It is recognized that in the body, femoral counterfaces that are initially very smooth can become damaged by bone-cement particles, bone particles, metallic debris from stems, and hydroxyapatite partides. These roughened counterfaces can then cause large increases in the wear rate of UHMWPE. 2 The effect of deterioration of the counterface on increased wear of UHMWPE has been well documented. In contrast, the effect of degradation of UI-IMWPE on wear has received less attention. Clinical wear takes place over much longer periods compared with "accelerated" laboratory tests, and there is increasing concern about the degradation and aging of UHMWPE following gamma irradiation, 5 with oxidative degradation affecting both density and
Surface and Coatings Technology, 2014
In this work the influence of two different irradiation techniques on the degree of crystallinity and nanomechanical properties of a medical grade UHMWPE is compared. One technique, widely used in the production of components for total joint replacement, is comprised by γ-irradiation followed by a thermal treatment above the melting temperature of UHMWPE and thus modifies the material's bulk. The other one, an alternative modification technique that affects only the near surface layers of UHMWPE, is swift heavy ion (SHI) irradiation. The effect of two types of ion beams (nitrogen and lithium) with different energies (33 and 47 MeV) and fluences (10 11 to 10 13 ions/cm 2) is investigated. Changes in degree of crystallinity are investigated by DSC and Raman spectroscopy while the nanomechanical propertieselastic modulus and hardnessare evaluated by nanoindentation tests. The γ-irradiated and remelted sample exhibits lower degree of crystallinity than the pristine material due to the hindered recrystallization process of the crosslinked chains. Concomitantly, this sample shows a reduction in hardness and elastic modulus of the bulk. On the other hand, SHI-irradiated samples display a large increase in degree of crystallinity and surface mechanical parameters with respect to pristine UHMWPE. The modification is confined to the ion target depth. The layer affected by the ion beam shows constant mechanical properties that appeared to be slightly influenced by the fluence in the studied range (around the optimum). Despite the changes induced by both techniques are completely different, they are able to enhance the wear performance of UHMWPE due to the beneficial change in elastic to plastic properties. Among SHI-irradiated samples, the N-ion (33 MeV and 1 × 10 12 ions/cm 2) exhibits the better combination of nanomechanical properties.
Materials, 2017
Ultra-high molecular weight polyethylene (UHMWPE) is the most common bearing material in total joint arthroplasty due to its unique combination of superior mechanical properties and wear resistance over other polymers. A great deal of research in recent decades has focused on further improving its performances, in order to provide durable implants in young and active patients. From "historical", gamma-air sterilized polyethylenes, to the so-called first and second generation of highly crosslinked materials, a variety of different formulations have progressively appeared in the market. This paper reviews the structure-properties relationship of these materials, with a particular emphasis on the in vitro and in vivo wear performances, through an analysis of the existing literature.
Irradiation of chemically crosslinked ultrahigh molecular weight polyethylene
Journal of Polymer Science Part B: Polymer Physics, 1996
Acetabular cups for artificial hip joints were prepared by compression molding of ultrahigh molecular weight polyethylene in the presence of peroxide. Peroxide crosslinking led to a decrease in the degree of crystallinity, peak melting temperature, and recrystallization temperature, as well as decreased crystal perfection and size. Peroxide crosslinked cups were sterilized with gamma rays at room temperature in air atmosphere to a n average dose of 3.4 Mrad. Irradiation produced further crosslinking in amorphous regions plus extensive chain scission of taut tie molecules and led to increased crystallinity and crystal perfection.
Biomaterials, 2006
Ultra high molecular weight polyethylene (PE) has been used for more than forty years as the bearing surface in total joint replacements. In recent years, there have been numerous advances in processing conditions that have improved the wear resistance of this material. In particular, crosslinking has been shown to dramatically improve the wear behavior of this orthopedic polymer in simulator studies. This benefit to wear resistance, however, is accompanied by a decrease in mechanical properties such as ultimate tensile strength, ductility, toughness and fatigue resistance. This degradation to mechanical properties may have serious implications for devices with high stress concentrations or large cyclic contact stresses. Tailoring microstructure for improved structural performance is essential for implant design. In this work we examined the role of crystallinity and crosslinking on the microstructure and mechanical properties of PE. Crystallinity was increased with a high pressure process and crosslinking was obtained with gamma irradiation. Crystallinity was beneficial to fatigue crack propagation resistance and when coupled with crosslinking a polymer with both wear and fatigue resistance was obtained.