Ultrahigh Molecular Weight Polyethylene Specimens Research Articles

A novel method to eliminate the influence of absorbed lipids on the characterization of ultra-high molecular weight polyethylene using Fourier-transform infrared spectroscopy.

Fourier-transform infrared spectroscopy (FTIR) is one of the standard methods to analyze ultra-high molecular weight polyethylene (UHMWPE) in orthopedic implants. For retrieved components, lipid extraction using an organic solvent prior to the measurement is necessary to eliminate the influence of lipids absorbed in vivo. However, its influence on the measurement has not been substantially investigated. To investigate the influence of lipid extraction on the FTIR analysis of UHMWPE and to develop a novel method to obtain reliable results without inconvenient lipid extraction. FTIR analysis was repeatedly performed on UHMWPE specimens from retrieved components before and after lipid extraction under various conditions. A method to calculate the extent of influence of the absorbed lipids from the FTIR spectra was developed using a peak separation technique. An elevated temperature was necessary for lipid extraction; however, it had the potential to influence the results if the conditions were not properly controlled. The results obtained using the peak separation technique coincided with those obtained after lipid extraction. The use of the peak separation technique enables the efficient acquisition of reliable results without the need for lipid extraction.

Advanced shear tester for evaluation of asphalt concrete under constant normal stiffness conditions

This paper presents motivation and details on the development of an advanced shear tester (AST) device capable of investigating the response of 150-mm cylindrical specimens under constant normal stiffness (CNS) conditions. Based on the laboratory experience and field observations from the soil and rock engineering, CNS conditions are particularly desirable when the normal stress changes considerably during the shearing process. Such situations occur in asphalt pavement structures especially under certain loading configurations. The CNS conditions are complementary to the constant normal load conditions that are applied in the current shear-mode devices incorporated in testing of asphalt concrete (AC) and pavement interface properties. This paper demonstrates the complex state of stress in the upper AC layer of a typical pavement under moving truck wheel and shows the need for the CNS device. In addition to outlining unique AST design features, this paper presents also the verification effort conducted on the solid Ultra-High Molecular Weight Polyethylene specimens. The AST results matched very well the reference data that were obtained under the elastic regime in the stress-controlled mode from the unconfined uniaxial compression and indirect tension tests. Finally, this paper demonstrates the example results collected on the solid AC specimens prepared with two different nominal maximum aggregate sizes. The analysis confirmed the ability of the AST device to capture both practical and fundamental phenomena occurring during the shearing process in the AC. It is believed that AST-type devices that are rather simple in design yet allow to induce complex state of stress in the AC specimens should be more recognised in the future research efforts.

Surface Modification of Ultra-High Molecular Weight Polyethylene with Crosslinked Hyaluronic Acid and its Antiwear Performance

The surface of high-pressure crystallized ultra-high molecular weight polyethylene (UHMWPE) was modified for application as an artificial cartilage material. A UHMWPE surface pretreated by a series of processes, including treatment with O2-plasma and ethylenediamine solution, was coated with hyaluronic acid (HA). After that, adipic acid dihydrazide (AAD) was added to partially crosslink the HA coating in order to enhance its durability. The modified samples were verified by water contact angle measurement and Fourier transform infrared spectrometry. Both HA layers, original and crosslinked, were also quantitatively evaluated by carbohydrate chemistry assay according to the absorbance of the incident light. The tribological performance of the samples was evaluated by a pin-on-disk test rig lubricated by normal saline under an average pressure of 18 MPa and at a sliding speed of 0.03 m/s for 45 h. The wear resistance of the HA-coated UHMWPE specimens promoted by the crosslink process was superior to that of the original HA-coated sample, and that resistance was maintained after immersion in saline solution for one month.

Wear behavior of γ-irradiated ultra high-molecular weight polyethylene in a hip joint simulator

In this work, the wear behavior of cross-linked ultra-high-molecular weight polyethylene (UHMWPE) is discussed. The UHMWPE specimens are molded through injection molding techniques by varying the parameters of melting temperature (MT). The cross linking of UHMWPE was carried out by iridium 192 isotopes, where the specimen received 25 kGy energy. The wear tests were conducted on a hip joint simulator that was designed and fabricated in the laboratory. The contact loads are varied from 50 N to 100 N. The study revealed that MT has influenced the hardness and wear properties such as coefficient of friction and wear rate, respectively. γ-Irradiated UHMWPE has better wear and friction resistances than unirradiated UHMWPE. The worn out surfaces were examined with the help of scanning electron microscope, and it revealed the presence of wear mechanisms such as ironing, scratching, ploughing, plastic deformation, and fatigue wear.

The Wear Behavior of Cross-Linked UHMWPE under Dry and Bovine Calf Serum–Lubricated Conditions

The present study was performed to investigate the effects of gamma radiation on the wear behavior of unirradiated and irradiated ultra-high-molecular-weight polyethylene (UHMWPE) against Ti-6Al-4V under dry and lubricated conditions at different applied loads. The UHMWPE specimens were exposed directly to nominal doses of 0, 25, 40, 50, and 100 kGy. Scanning electron microscope (SEM) analysis of the worn surface of UHMWPE and Ti-6Al-4V was performed to understand the mechanism of wear involved between the contact surfaces during wear testing. From the wear test results, there were significant differences between the wear of unirradiated UHMWPE and UHMWPE irradiated at 25, 40, and 50 kGy under dry and lubricated conditions. SEM results for the worn surface of unirradiated and irradiated UHMWPE samples showed that dry and lubricated conditions affected the wear behavior of irradiated UHMWPE. Examination of the Ti-6Al-4V alloy counterface tested under dry conditions revealed that the maximum transfer layer thickness was as high as 22.79 μm. In addition, the UHMWPE irradiated at 100 kGy resulted in a more continuous and adherent transfer film on the Ti-6Al-4V surface compared to the unirradiated sample under dry conditions. Under serum-lubricated conditions, the transfer film layer on the Ti-6Al-4V alloy surface was thinner compared to under dry conditions.

Raman characterisation of conventional and cross‐linked polyethylene in acetabular cups run on a hip joint simulator

AbstractFive sets of differently sterilised conventional ultra‐high molecular weight polyethylene (UHMWPE) and cross‐linked polyethylene (XLPE) acetabular cups were run for 5 million cycles on a hip joint simulator in order to evaluate their wear behaviour in relation to material properties (PE grade, conventional or cross‐linked) and sterilisation method (ethylene oxide (EtO) treatment or γ‐irradiation). Gravimetric measurements revealed that conventional UHMWPE wore significantly more than XLPE. The differences in wear behaviour could be partly related to the orthorhombic contents obtained by Raman spectroscopy in the unworn areas of the cups: XLPE cups showed a significantly higher crystallinity degree than the UHMWPE specimens. Raman analysis showed that wear testing did not significantly modify the orthorhombic content of any of the tested acetabular cups. However, the set of cups that showed the highest weight loss, i.e. γ‐sterilised PE GUR1020, appeared the most homogeneously polished upon wear testing; from a molecular point of view, only this set of cups showed a significant increase of the I1130/I1060 intensity ratio, suggesting the occurrence of chain orientation. On the other hand, XLPE cups, despite the lowest weight loss undergone, showed a decrease in the amorphous content upon wear testing as well as a limited orthorhombic → monoclinic transformation, which did not appear detrimental. Copyright © 2011 John Wiley & Sons, Ltd.

The influence of injection molding on tribological characteristics of ultra-high molecular weight polyethylene under dry sliding

The objective of this study is to investigate the effects of various injection molding process parameters on the tribological properties of ultra-high molecular weight polyethylene (UHMWPE). The tribological properties, such as the friction coefficient and wear volume loss, were obtained using the Schwingum Reibung Verschleiss (SRV, oscillation friction wear) ball-on-plane wear tester. In addition, the mechanical properties of UHMWPE were investigated as well. The variable parameters of the injection molding process were melt temperature, mold temperature and injection velocity. Experimental results show that different wear contact loads and varied injection molding conditions influence the friction coefficient and wear volume loss of the UHMWPE specimens. As the sliding contact loads increased, the friction coefficient also increased. Moreover, the lowest wear volume loss mostly occurred in highest injection molding conditions. The morphologies of the worn surfaces and the specimen cross sections were examined with an optical microscope and a scanning electron microscope, respectively. Plastic deformations, grooves and wavelike formations are the main wear mechanism on the surface in the UHMWPE wear tests. Experimental results also showed that the tensile strength and surface hardness are affected by injection molding conditions and sliding contact loads.

The effect of pre-irradiation vacuum storage on the oxidation and wear of radiation sterilized UHMWPE

The adverse biological response to ultra-high molecular weight polyethylene (UHMWPE) wear debris in total joint replacement warrants the need for the material's improved wear resistance. The reaction of oxygen with radiation-induced free radicals within UHMWPE leads to oxidative degradation of the material. This work examines the effect of storing UHMWPE in vacuum at an elevated temperature (70 °C) prior to gamma irradiation in an attempt to remove pre-existing free oxygen from within the material and hence reduce oxidation that occurs during the irradiation process. UHMWPE specimens both with and without pre-irradiation vacuum were gamma irradiated to either 4 or 10 Mrad and then artificially aged to simulate the material's long-term oxidative behavior. Oxidation, degree of crosslinking, and wear resistance were measured. The benefit of pre-irradiation vacuum in terms of reduced oxidation, higher degree of crosslinking, and improved wear resistance measured after aging was observed for both 4 and 10 Mrad irradiation doses.

Effect of pre-heat treatment on the static and dynamic thermo-mechanical properties of ultra-high molecular weight polyethylene

In the present work, a dynamic thermo-mechanical analysis technique has been used to investigate the effect of pre-heat treatments on the visco-elastic properties of Ultra-High Molecular Weight Polyethylene (UHMWPE) as a function of frequency and temperature. These properties include storage modulus ( E′) and loss modulus ( E″). The UHMWPE specimens were pre-heated at 50, 80 and 100 °C for periods of 2 and 4 h, respectively. The effect of these pre-heat treatments on the quasi-static mechanical properties (Young's modulus, yield strength, fracture strength, ductility and micro-hardness) has also been investigated. Finally, the degree of crystallinity of the UHMWPE has been measured as a function of pre-heat treatment time and temperature. The results show a strong dependence of most of these properties on the degree of crystallinity which is a function of pre-heat treatment temperature and duration.

Defect initiation at subsurface grain boundary as a precursor of delamination in ultrahigh molecular weight polyethylene

In order to examine the initiation mechanism of delamination in ultrahigh molecular weight polyethylene (UHMWPE) knee components, a bi-directional sliding fatigue test was performed for three types of UHMWPE specimens: nonirradiated, gamma-irradiated (25 kGy) and gamma-irradiated (25 kGy) with 0.1% vitamin E added. Sliding surfaces of post-tested UHMWPE specimens were observed using an optical microscope and a scanning electron microscope. Also, surface roughness was measured at the sliding surfaces of UHMWPE specimens. Delamination was observed only in gamma-irradiated specimens. A networked structure of surface asperity that resembled grain boundary was observed prior to delamination in gamma-irradiated specimens. Surface roughness in the gamma-irradiated specimens, higher than in any other specimen, showed a rapid increase prior to delamination. Detailed observation using an optical microscope and a scanning electron microscope showed microscopic crack initiation along subsurface grain boundaries in gamma-irradiated specimens. These results suggest that subsurface crack initiation is a precursor of delamination and is accelerated by oxidative degradation due to gamma irradiation. Of the three types of specimens, UHMWPE with vitamin E added showed the lowest surface roughness values at all measuring points. The addition of vitamin E is effective in improving wear resistance and fatigue performance of UHMWPE.

Multiaxial fatigue behavior of conventional and highly crosslinked UHMWPE during cyclic small punch testing.

Previous observations of reduced uniaxial elongation, fracture resistance, and crack propagation resistance of highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have contributed to concern that the technology may not be appropriate for systems undergoing cyclic fatigue loading. Using a "total life" approach, we examined the influence of radiation crosslinking on the fatigue response of UHMWPE under cyclic loading via the small punch test. Our goal in this study was to evaluate the suitability of the small punch test for conducting miniature-specimen, cyclic loading, and fatigue experiments of conventional and highly crosslinked UHMWPE. We subjected four types of conventional and highly crosslinked UHMWPE to cyclic loading at 200 N/s and at body temperature in a small punch test apparatus. After failure, the fracture surfaces were characterized with the use of field emission scanning electron microscopy to evaluate the fatigue mechanisms. Cyclic small punch testing under load control was found to be an effective and repeatable method for relative assessment of the fatigue resistance of conventional and highly crosslinked UHMWPE specimens under multiaxial loading conditions. For each of the four conventional and highly crosslinked UHMWPE materials evaluated in this study, fatigue failures were consistently produced according to a power law relationship in the low cycle regimen, corresponding to failures below 10000 cycles. The fatigue failures were all found to be consistent with a single source of initiation and propagation to failure. Our long-term goal in this research is to develop miniature-specimen fatigue testing techniques for characterization of retrieved UHMWPE inserts.

Oxidation of Ultra-High Molecular Weight Polyethylene and Its Influence on Contact Fatigue and Pitting of Knee Bearings

Tibial hearings of total knee replacements are generally made from ultra-high molecular weight polyethylene (UHMWPE). Failure of those hearings has been found to result in many cases from subsurface crack initiation and propagation. Two types of subsurface-originated failure modes are investigated in this study: macroscopic contact fatigue and microscopic pitting. The cracks responsible for both modes of failure initiate in polyethylene material that has been embrittled by oxidation; this oxidation is an undesirable outcome of the gamma irradiation (in air) used to sterilize the tibial bearings. Accelerated aging was used in this work to achieve oxidation levels similar to those found in retrieved bearings. Tribotesting of the aged bearing materials was carried out under simulated service conditions using a rolling/sliding tester. Macroscopic fatigue cracks and microscopic surface pitting developed in UHMWPE specimens tested on the rolling/sliding tester, and the damage was similar to that found in retrieved tibial bearings. The fatigue cracks invariably initiated in the embrittled oxidized layer, and the number of cycles before initiation of the fatigue cracks was dependent on the oxidation level and on the contact stress. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Biodegradation of high-strength and high-modulus PE–starch composite films buried in several kinds of soils

The biodegradative behaviors of composite films of ultrahigh molecular weight polyethylene (UHMWPE) (6×106) and starch (ST) particles were investigated. The films were prepared by gelation–crystallization from dilute solutions of PE, in which ST particles were dispersed. The PE–ST compositions chosen were 3/1, 1/1, 1/3, and 1/5. The elongation was done up to 80 times in a hot oven at 135°C under nitrogen. Using the undrawn and drawn specimens, biodegradation of PE was examined by burying the blend materials for 16 months in three kinds of soils, namely, paddy soil, farmyard manure, and red clay. The weight decrease after the degradation was found to be dependent on ST content, draw ratio of original films, type of soils, and burying period. The weight loss was the highest for the specimen buried in paddy soil and lowest for that in red clay. Nevertheless, the changes in the tensile strength and Young's modulus after degradation were most prominent in the specimen buried in the red clay. This result is of interest since the weight decrease in this specimen was lowest. Drastic morphological changes were observed by scanning electron microscopy for the specimens buried in red clay. Significant chain scission of ultra UHMWPE could be observed. The specimen showed the largest decrease in storage modulus. No degradation, however, was observed for the ultradrawn UHMWPE specimen (1/0) buried in red clay. Thus it turned out that the degradation of PE in the composite materials buried in red clay is induced by the dispersed ST particles.

Effect of counterface on the tribology of UHMWPE in the presence of proteins

Artificial joints in orthopedics occupy a principal position owing to the increase in number of cases suffering from arthritis and associated diseases in addition to impairment caused by accidents. In this work, one of the most commonly used joint material, i.e. ultrahigh molecular weight polyethylene (UHMWPE), was tested against the duplex stainless steels instead of the conventional 316 L stainless steel. The UHMWPE was found to exhibit the lowest friction coefficient and wear rates when lubricated with water followed by globulin and glucose. The friction coefficient in the presence of egg albumen was higher along with high wear rates recorded. Post-test evaluation of surface roughness and wear scar/track analysis was performed to identify the wear mechanisms. Worn surfaces were analyzed using a differential scanning calorimeter for changes in crystallinity with sliding. The specimens tested under lubricated conditions with glucose, egg albumen and globulin indicated the presence of reaction products on the worn surface. Adhesive and corrosive wear mechanisms were the predominant modes of wear identified on the polymer samples. The wear tracks indicated that the proteins did react with the counterface material forming a thin deposit on them. Low temperature nitriding of the duplex stainless steel counterfaces were performed and the UHMWPE specimens were tested under similar conditions against the nitrided surfaces. Low temperature nitriding of the counterface did result in improved tribological behavior of UHMWPE and the corrosive effects were minimal.

Plasma immersion ion implantation of UHMWPE

Ultrahigh molecular weight polyethylene (UHMWPE) has been the material of choice in orthopaedic joint prostheses for over three decades owing to its proven low friction and excellent wear resistance when coupled with smooth Co-Cr-Mo alloy or austenitic stainless steel surfaces in vivo [1]. However, it is gradually recognized that the principal constraint on the longevity of total joint replacements is the body’s reaction to debris, which is mostly UHMWPE [2]. Therefore, how to improve the wear resistance of UHMWPE and thus reduce the volume of wear debris, which promotes implant loosening, is a timely task from both a scientific and technological point of view. Nitrogen ion implantation of UHMWPE has been proved to improve its tribological properties in terms of low friction and extremely low wear rate [3], however, it is difficult in practice to uniformly treat such three-dimensional objects as hip joint sockets without using sophisticated manipulating devices. Recently, an innovative hybrid technology, plasma immersion ion implantation (PI3), has been developed, which can, to a large extent, address the problems with ion implantation of components with complex shapes [4]. However, there appears to have been little, if any, work reported on the surface modification of UHMWPE using the innovative, hybrid PI3 technology with a view to improve its mechanical and tribological properties [5]. In the present investigation, an attempt has been made to explore the feasibility of surface modification of UHMWPE using the novel PI3 technology. The material used in the study was medical grade UHMWPE (GUR-412) in the form of 12.5 diameter, extruded and annealed rod which was obtained from Poly Hi Solidur Ltd (GB). The density of the homogeneous material and the average molecular weight are 0.932 g cm−3 and 3.6× 106 g mol−1, respectively. Specimens were treated in a Mark 1 PI3 unit at the Technical University of Clauthal with the pulse source being (PI3 treatment) or not being (plasma treatment) applied. Specimens were immersed in a low pressure (0.17 Pa) radio frequency plasma discharge, which was generated by a 300 W r.f. power at 13.56 MHz. Table I lists the treatment conditions for plasma treatment (PT) and PI3 treatments. By selecting appropriate pulse length and pulse frequency, the treatment temperature was controlled at below 90 ◦C. Although it has been claimed that ion implantation can decrease the surface roughness of polymeric materials [6], such an improvement was not observed in the present study. Indeed, surface relief appeared on PI3-H specimens largely because of the ion etching effect resulting from nitrogen ion bombardment. Slight surface discoloration was observed for all surface treated specimens, increasing in the order of PT, PI3-L and PI3-H. In view of the fact that the surface modified layers of UHMWPE material usually are very thin and the substrate very soft, the mechanical properties of the untreated and treated UHMWPE specimens were measured using a NanoTest 500. A standard procedure developed by Oliver and Pharr was adopted to process the load-depth data [7]. Fig. 1 depicts characteristic load versus depth hysteresis curves, showing the loadingunloading response of the PI3 treated as well as untreated material. It can be clearly seen that, to reach the same penetration depth (250 nm), a higher load was need for the treated surface relative to the untreated surface. It also demonstrated that PI3 treated material exhibited a higher elastic recovery and less creep during the dwelling time (10 s) than the untreated material.

Fatigue Strength of Polyethylene After Sterilization by Gamma Irradiation or Ethylene Oxide

The oxidation level of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in either air or Ar gas was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. The fatigue strength of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in air was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At the specimen surface, oxidation was highest for ultrahigh molecular weight polyethylene gamma irradiated in air, lower for ultrahigh molecular weight polyethylene gamma irradiated in Ar gas, and absent in unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At a depth of 3.5 mm below the specimen surface, oxidation levels were equivalent for ultrahigh molecular weight polyethylene gamma irradiated in either air or Ar gas whereas unsterilized and ethylene oxide sterilized specimens were again unoxidized. Thus, even in an inert atmosphere, oxidative degradation of gamma irradiated ultrahigh molecular weight polyethylene occurs. The 10 million cycle fatigue strength was similar for unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene whereas the fatigue strength of gamma irradiated in air ultrahigh molecular weight polyethylene was lower. Results of this study show that ethylene oxide gas does not degrade ultrahigh molecular weight polyethylene whereas gamma radiation in air causes changes in the polymer that adversely affect its mechanical properties. Ethylene oxide gas is a viable alternative to gamma radiation in air that avoids oxidation and fatigue strength degradation known to accompany irradiation of ultrahigh molecular weight polyethylene polymer bearing surfaces in total joint implants.

An observation on subsurface defects of ultra high molecular weight polyethylene due to rolling contact

Ultra high molecular weight polyethylene (UHMWPE) has been used in artificial joints for a few decades, and wear of UHMWPE has been one of the main problems. Though many other materials have been tested over the years, the best clinical results are still achieved with UHMWPE. This makes the study of UHMWPE, especially in relation to artificial joints, very important. Frequently, more severe wear can be observed in artificial knee joints than in artificial hip joints especially when the flaking-like wear occurs. This flaking-like wear can lead to significant destruction of the artificial knee joint. Macroscopically, artificial knee joints have combinational movements of rolling and sliding in order to simulate the motion of the normal knee joint. The components of motion are separated to make study easier. Fatigue tests of UHMWPE under the rolling contact condition were performed in this study. Three ceramic spheres were rolled over the UHMWPE specimen using 37 degrees C distilled water as a lubricant. The UHMWPE specimen was observed by the scanning acoustic tomography, microscopy, and SEM. Some subsurface defects could be observed by SAT even before experiments. Although the apparent wear is not observed on the surface, there was an increase in the number of observable subsurface cracks in the UHMWPE specimen. This shows that cracks occur under the surface after a 10(7) rolling contact loading, which is very close to the cyclic loading and unloading with very little friction compared to the sliding contact.