Potential errors in FTIR measurement of oxidation in ultrahigh molecular weight polyethylene implants

A simple, rapid and accurate method for the sample preparation and quantification of meso- and microplastics in food and food waste streams

Plastics are produced and used in large quantities worldwide (e.g. as food packaging). In line with this, plastic particles are found throughout the ecosphere and in various foods. As a result, plastics are also present in energy-rich waste biomass derived from the food industry, supermarkets, restaurants, etc. These waste streams are a valuable source for biogas production but can also be used to feed insects that in turn upcycle it into new high-value biomass. In both applications, the remaining residue can be used as fertilizer. Due to the present plastic particles, these applications could pose a continued threat to the environment, and both human and animal health. Therefore, the need of determining the (micro)plastic content to assess the potential danger is rising. In this research, a closed-vessel microwave-assisted acid digestion method was developed to accurately determine meso- and microplastic contents in food (waste) matrices by solubilising this food matrix. Polyvinyl chloride (PVC) food packaging foil was used to develop the method, using a full factorial design with three parameters (nitric acid concentration (c(HNO₃)), temperature (T), and time (t)). According to this model, the best practical conditions were c(HNO₃) = 0.50 mol/L, T = 170 °C, and t = 5.00 min. Subsequently, the method was tested on five other plastics, namely high- and low-density polyethylene (HDPE and LDPE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), mixed with a food matrix, resulting in a mean plastic recovery of 102.2 ± 4.1%. Additionally, the polymers were not oxidised during the microwave digestion. For PVC and PS hardly any degradation was found, while HDPE, LDPE, and PP showed slight chain degradation, although without recovery loss. In conclusion, the method is an accurate approach to quantify the total meso- and microplastic content in food (waste) matrices with minimal change in their intrinsic characteristics.

Influence of Accelerated Aging on the Wear Behavior of Cross-Linked Polyethylene Liners—A Hip Simulator Study

Sequential cross-linked and annealed ultra-high-molecular-weight polyethylene (SX-PE) is known as a low-wear articulating partner, especially for total hip endoprostheses. Aging of polymeric materials, irrespective of if induced by shelf or in vivo life, can degrade their tribological and mechanical properties. However, changes in wear behavior of aged SX-PE liners have not been not quantified so far. An accelerated aging procedure, to simulate shelf and in vivo aging, was performed on thin SX-PE liners after five million load cycles using a simulator (“worn-aged”) as well as on new SX-PE liners (“new-aged”). A subsequent hip simulator test was performed with both thin SX-PE liner sets in combination with large-diameter ceramic femoral head, representing a combination known as advantageous for treatment after revision because of dislocation. Oxidation indices were measured on the liners after each step of the procedure. SX-PE liners after accelerated aging show bedding-in phases during simulator test, which was a characteristic only known from clinical investigations. Hence, the wear rates of the “new-aged” ((1.71 ± 0.49) mg/million cycles) and of the “worn-aged” ((9.32 ± 0.09) mg/million cycles) SX-PE were increased in the first period compared to new unaged SX-PE liners. Subsequently, the wear rates decreased for “new-aged” and “worn-aged” inserts to (0.44 ± 0.48) mg/million cycles and (2.72 ± 0.05) mg/million cycles, respectively. In conclusion, the results show promising effects of accelerated aging on SX-PE liners in simulator testing and for potential long-term use in clinical applications.

Oxidation of Second Generation Sequentially Irradiated and Annealed Highly Cross-Linked X3™ Polyethylene Tibial Bearings

Since the first use of ultra-high-molecular-weight polyethylene as a bearing material, research and development efforts have sought to improve wear resistance, increase longevity and lessen the potential for debris mediated adverse tissue responses. A series of second generation sequentially cross-linked and annealed tibial bearings were analysed after several bearings sent for routine retrieval analysis showed oxidative degradation including subsurface whitening, cracking and gross material loss. Evaluation incorporated visual and white banding assessment, mechanical testing and spectroscopy analysis. Whilst visual observation and white banding assessment confirmed oxidative changes, a decrease in mechanical properties and increasing ketone oxidation index as a function of time in vivo suggest time dependent oxidative degradation. Clinically relevant degradation of the sequentially cross-linked and annealed tibial bearings was observed.

Oxidation and fusion defects synergistically accelerate polyethylene failure in knee replacement

We have previously reported upon a cohort of patients with premature failure of such material and postulated upon the impact of abnormally high concentrations of type 2 fusion defects whereby there is a lack of particle cohesion due to incomplete diffusion. In vivo oxidation has been purported to underscore the premature failure of polyethylene. The mechanism of such remains poorly delineated. New data has now been obtained by determining substrata oxidative profiles of 10 failed Kinemax Plus modular tibial insert analyses in conjunction with fusion defect detection. The full thickness of a series of cores was analysed using infra-red spectroscopy to identify higher levels of oxidation in loaded used material at both the articulating and non-articulating regions. A comparison was made to an unused control. Articulating, loaded, areas exhibited greater local concentrations of oxidised material and wider variation of such consistent with the higher presence of fusion defects. Subsurface analysis confirmed the presence of a major oxidative peak 2mm below the surface for all loaded areas irrespective of wear. Additionally we were able to identify a second major oxidative focus about halfway between the inferior (tibial baseplate) surface and the articulating area. We believe that the combination of high oxidation and fusion defects represents a second high stress zone consistent with the observation of tibial baseplate polyethylene dissociation and backside wear with resultant catastrophic material failure.

In vivo oxidation contributes to delamination but not pitting in polyethylene components for total knee arthroplasty

The aim of this study was to better understand how in vivo oxidation contributes to fatigue damage in total knee arthroplasty (TKA). A total of 119 tibial inserts were consecutively collected after revision surgery. Of the 119 polyethylene retrievals, 29 were gamma sterilized in air (historical), whereas the remaining 90 were gamma sterilized in nitrogen (conventional). Surface damage assessment and characterization of oxidation were performed on all the retrievals. Delamination was significantly more prevalent and extensive in the longer-term, highly oxidized, historical tibial inserts. Pitting damage, in contrast, seemed to be equally prevalent between both retrieval groups and was not correlated with in vivo oxidation. Our findings support our hypothesis that in vivo oxidation is a contributing factor to delamination, but not pitting, in TKA. Despite the lower oxidation displayed by conventional retrievals, this study provides strong evidence that delamination secondary to in vivo oxidation may occur during the second decade of implantation.

The natural history of polyethylene oxidation in total disc replacement

STUDY DESIGN

This study is an evaluation of wear and oxidation in retrieved total disc replacements (TDRs). Forty-eight CHARITE TDRs were retrieved from 41 patients after 7.8 years of average implantation. All implants were removed because of intractable back pain and/or facet degeneration. Three unimplanted implants served as controls.

OBJECTIVE

Our aim was to determine whether gamma-sterilized polyethylene components implanted in the spine oxidize in vivo, and if so, whether polyethylene oxidation has clinical relevance for the long-term performance of TDRs.

SUMMARY OF BACKGROUND DATA

The natural history of polyethylene oxidation following gamma sterilization and long-term implantation in the spine has not yet been investigated.

METHODS

Oxidation and oxidation potential were measured at the rim and dome of 47 components using Fourier transform infrared spectroscopy. The wear patterns of each retrieved polyethylene core were analyzed at the rim and dome.

RESULTS

Oxidation was significantly higher at the rim, as compared with the dome of the cores. Hydroperoxide index was also significantly higher at the rim, as compared with the dome. Dome penetration rate was negatively correlated to implantation time (P < 0.0001) but not correlated to oxidation or hydroperoxide index (P > 0.05). Implants with evidence of chronic rim loading had higher rim oxidation.

CONCLUSION

The data support our hypothesis that, for the historical packaging methods employed by the manufacturer, polyethylene oxidation and oxidation potential were significantly higher at the rim as opposed to the dome. The mechanism is governed by access to oxygen in vivo and may be accelerated under certain combined modes of repeated rim loading. Our findings have clinical significance in cases of chronic impingement, when the rim has to support repeated loading for the lifetime of the implant.

Gamma inert sterilization: a solution to polyethylene oxidation?

BACKGROUND

In the 1990s, oxidation was found to occur in ultra-high molecular weight polyethylene total joint replacement components following gamma irradiation and prolonged shelf aging in air. Orthopaedic manufacturers developed barrier packaging to reduce oxidation during and after radiation sterilization. The present study explores the hypothesis that polyethylene components sterilized in a low-oxygen environment undergo similar in vivo oxidative mechanisms as inserts sterilized in air. In addition, the potential influence of the different sterilization processes on the wear performance of the polyethylene components was examined.

METHODS

An analysis of oxidation, wear, and surface damage was performed for forty-eight acetabular liners and 123 tibial inserts. The mean implantation time was 12.3+/-3.7 years for thirty-one acetabular liners that had been gamma sterilized in air and 4.0+/-2.5 years for the seventeen acetabular liners that had been gamma sterilized in inert gas. The mean implantation time was 11.0+/-3.2 years for the twenty-six tibial inserts that had been sterilized in air and 2.8+/-2.2 years for the ninety-seven tibial inserts that had been gamma sterilized in inert gas. Oxidation and hydroperoxide levels were characterized in loaded and unloaded regions of the inserts.

RESULTS

Measurable oxidation and oxidation potential were observed in all cohorts. The oxidation and hydroperoxide levels were regional. Surfaces with access to body fluids were more heavily oxidized than protected bearing surfaces were. This variation appeared to be greater in historical (gamma-in-air-sterilized) components. Regarding wear performance, historical and conventional acetabular liners showed similar wear penetration rates, whereas a low incidence of delamination was confirmed for the conventional tibial inserts in the first decade of implantation.

CONCLUSIONS

The present study explores the impact of industry-wide changes in sterilization practices for polyethylene. We found lower oxidation and oxidation potential in the conventional acetabular liners and tibial inserts that had been gamma sterilized in inert gas as compared with the historical components that had been gamma sterilized in air. However, we also found strong evidence that conventional components undergo mechanisms of in vivo oxidation similar to those observed following gamma irradiation in air. In addition, gamma sterilization in inert gas did not provide polyethylene with a significant improvement in terms of wear resistance as compared with gamma sterilization in air, except for a lower incidence of delamination in the first decade of implantation for tibial inserts.

The performance of gamma- and EtO-sterilised UHWMPE acetabular cups tested under severe simulator conditions. Part 1: role of the third-body wear process

Due to its excellent combination of properties, ultra-high-molecular-weight-polyethylene has been used for the last 30 years in the replacement of damaged articulating cartilage for total joint replacement surgery. However, in some cases, wear, failure and delamination have been observed. Polyethylene performance may be affected by oxidation during consolidation of the resin, sterilisation of the finished specimens and post-irradiation storage. In order to evaluate the influence of the sterilisation method (gamma-irradiation and ethylene oxide(EtO)-treatment) and third-body particles on the ultra-high-molecular-weight-polyethylene wear behaviour, gamma- and EtO-sterilised ultra-high-molecular-weight-polyethylene acetabular cups were tested against CoCrMo femoral heads in a hip joint simulator run for 2.5 million cycles in bovine calf serum in the presence of third-body PMMA particles. Weight loss measurements revealed that the gamma-sterilised acetabular cups exhibited a significantly lower wear rate than those EtO-sterilised. Moreover, significant differences were found for each type of sterilisation between the gravimetric wear trends obtained until 2.5 million cycles in the presence and in the absence of PMMA particles.

Interaction of oxidation and crosslinking in gamma-irradiated ultrahigh molecular-weight polyethylene

The interaction between oxidation and crosslinking in gamma-irradiated ultrahigh molecular-weight polyethylene with and without artificial aging was studied. The effect of the atmosphere during irradiation (air vs. low oxygen) occurred primarily within about 0.5 mm of the surface, that is, the depth to which oxygen had diffused when the polyethylene specimen was machined and when it was irradiated. Irradiation in the presence of oxygen induced oxidation instead of crosslinking, so that the level of crosslinking achieved was lower than that which normally would occur at the same dose in the absence of oxygen. Subsequent artificial aging reduced the gel content (crosslinking) and had a maximal effect on the surface and subsurface regions for the gamma-air and gamma-low oxygen polyethylenes, respectively. Thus the storage environments and durations prior to irradiation and prior to artificial aging must be taken into account when attempting to duplicate the oxidation-crosslinking profiles that occur with actual implants in clinical use. In addition, the oxidation mechanisms initiated by the artificial aging method used in this study (i.e., heating in air to 80 degrees C) initiated somewhat different oxidative reactions from those that occur during prolonged shelf life at room temperature or in vivo. In particular, the formation of a peak of oxidation below the free surface of the polyethylene is due to the combined effects of the distribution of residual free radicals and the diffusion gradient of the oxygen. The interactive relationship between oxidation and crosslinking characterized in the present study provides a fundamental basis for understanding the wear behavior of gamma-sterilized components in past clinical use. It also provides guidelines for the development of polyethylenes with improved resistance to oxidation and wear, with particular relevance to estimation of the amount of crosslinking need- ed to potentially eliminate the clinical problem of osteolysis.

Interlaboratory validation of oxidation-index measurement methods for UHMWPE after long-term shelf aging

An international oxidation index standard would greatly benefit the orthopedic community by providing a universal scale for reporting oxidation data of ultra-high molecular weight polyethylene (UHMWPE). We investigated whether severe oxidation associated with long-term shelf aging affects the repeatability and reproducibility of area-based oxidation index measurement techniques based on normalization with the use of 1370- or 2022-cm(-1) infrared (IR) absorption reference peaks. Because an oxidation index is expected to be independent of sample thickness, subsurface oxidation was examined with the use of both 100- and 200-microm-thick sections from tibial components (compression-molded GUR 1120, gamma irradiated in air) that were shelf aged for up to 11.5 years. Eight institutions in the United States and Europe participated in the present study, which was administered in accordance with ASTM E691. On average, the 100-microm-thick samples were associated with significantly greater interlaboratory relative standard uncertainty (40.3%) when compared with the 200-microm samples (21.8%, p = 0.002). In contrast, the intralaboratory relative standard uncertainty was not significantly affected by the sample thickness (p = 0.21). The oxidation index method did not significantly influence either the interlaboratory or intralaboratory relative standard uncertainty (p = 0.32 or 0.75, respectively). Our interlaboratory data suggest that with the suitable choice of specimen thickness (e.g., 200 microm) and either of the two optimal oxidation index methods, interlaboratory reproducibility of the most heavily oxidized regions in long-term shelf-aged components can be quantified with a relative standard uncertainty of 21% or less. Therefore, both the 1370-cm(-1) and the 2022-cm(-1) reference peaks appear equally suitable for use in defining a standard method for calculating an oxidation index for UHMWPE.

Oxidation of orthopaedic UHMWPE

Retrieved EtO sterilised acetabular cups usually show much less degradation than gamma-ray sterilised cups. Some of our retrieved EtO sterilised cups did, however, reveal unexpected bulk oxidation. It was observed that this oxidation was always accompanied by whitening of the material. This whitening was found to be due to a break-up of the compression moulded material into its original particles. It was noticed that there was no oxidation in all parts, where the break-up and whitening appeared. The oxidation did, however, occur exclusively in the parts where there was a badly consolidated material. Upon examining shelf aged, unsterilised samples, it was found that the degradation was also present here. This shows that the observed phenomenon is not due to the service in vivo and that it must originate from the processing step. Just as for the retrieved samples, the shelf aged cups only showed oxidation in the bulk and not at the surface. It was concluded that the material used for the cups had been badly fused together during the compression moulding and that the machining had created a bad stress situation in the cups leading to a break-up of the particles. The mechanism that initiates the oxidation is not known, but it is believed that the distribution depends on how the internal stresses have acted to break up the structure. In the areas where the particles have been separated, there is probably a higher availability of oxygen than what is normally observed in UHMWPE.

Effects of the sterilisation method on the wear of UHMWPE acetabular cups tested in a hip joint simulator

Ultra-high molecular-weight-polyethylene is the most commonly used bearing material in total joint replacement. Wear of polyethylene is a Serious Clinical problem that limits the longevity of orthopaedic implants. Information on degradative changes in the material properties and on the methods used for the sterilisation of polyethylene may help in the selection process of orthopaedic implants with the best wear resistance. This study was performed to investigate the effects of the sterilisation method (gamma irradiation and ethylene oxide treatment) on the wear and on the changes in physical properties of polyethylene acetabular cups. At this purpose, gamma-sterilised and ethylene oxide (EtO)-sterilised acetabular cups were tested against CoCr femoral heads in a hip joint simulator run for 5 million cycles in bovine calf serum. The crystallinity of the cups was evaluated by micro-Raman spectroscopy as a function of the inner surface position. The partial least square calibration was used to correlate the Raman spectra with the crystallinity of the polymer measured by differential scanning calorimetry. The analysis performed on soak control acetabular cups demonstrated that the gamma-sterilised cups are significantly more crystalline than the EtO-sterilised ones. The mean crystallinity values obtained for the gamma-sterilised and EtO-sterilised soak control cups were 65.0% and 63.4%, respectively. Weight loss measurements revealed that the gamma-sterilised acetabular cups exhibited a lower wear rate than that by EtO-sterilised. Thc Raman results obtained on gamma-sterilised and EtO-sterilised acetabular cups showed that the changes in surface crystallinity were mainly caused by irradiation rather than by the mechanical friction during the in vitro tests.

Interlaboratory studies to determine optimal analytical methods for measuring the oxidation index of UHMWPE

Fourier transform infrared spectroscopy has emerged as the technique of choice for the quantification of oxidation in ultra-high molecular weight polyethylene used in orthopedic implants. We initiated interlaboratory studies to determine the method of normalization, hence quantification, that provided the highest level of reproducibility across multiple institutions. The goal of this research was to identify optimal normalization methods that minimize the experimental uncertainties associated with interlaboratory reproducibility and intralaboratory repeatability of oxidation index measurements. Test samples were prepared from GUR 4150 HP, gamma irradiated in air, and had a shelf age of two years. Samples were analyzed according to ten oxidation index test methods during two interlaboratory studies, which were conducted in accordance with ASTM E691. Variations in reproducibility and repeatability were evaluated using analysis of variance (ANOVA). The basis of the test methods (peak area-based vs. peak height-based), as well as the normalization method, were both found to be associated with significant differences in reproducibility (p = 0.0006 andp < 0.0001, respectively). Normalization techniques based on the 1370 and 2022cm(-1) peaks areas were found to be the most reproducible methods, and were associated with mean interlaboratory uncertainties of 16.5% and 24.2%, respectively. Repeatability of the test methods was not sensitive to the normalization technique; the mean intralaboratory repeatability for all of oxidation index measurements was found to be 10.2%. The results of this interlaboratory research will be a useful basis for the development of a new oxidation index standard for the orthopedics community.

Effect of sterilization method and other modifications on the wear resistance of acetabular cups made of ultra-high molecular weight polyethylene. A hip-simulator study

BACKGROUND

Wear of ultra-high molecular weight polyethylene acetabular cups in hip prostheses produces billions of submicrometer wear particles annually that can cause osteolysis and loosening of the components. Thus, substantial improvement of the wear resistance of ultra-high molecular weight polyethylene could extend the clinical life span of total hip prostheses. It has become apparent that the conditions under which ultra-high molecular weight polyethylene cups have been sterilized can markedly affect their long-term wear properties, and new sterilization methods and other modifications have been developed to minimize the negative effects.

METHODS

In the present study, a hip-joint simulator was used to assess whether it is preferable to sterilize ultra-high molecular weight polyethylene cups without gamma irradiation, to avoid radiation-induced oxidative degradation, or to sterilize with gamma irradiation while the cups are packaged in a suitable low-oxygen atmosphere to minimize oxidation while retaining the increased wear resistance conferred by the radiation-induced cross-linking. Ion-implanted cups and cups made of a highly crystalline polyethylene (Hylamer) also were investigated. Cups made of each material were subjected to wear-testing prior to and after artificial thermal aging to accelerate oxidative degradation.

RESULTS

The results of the present study demonstrated that the cross-linking induced by gamma irradiation improves the wear resistance of ultra-high molecular weight polyethylene, while oxidation reduces it. Without thermal aging, the two types of cups that were sterilized with gamma irradiation while in low-oxygen packaging exhibited about a 50 percent lower rate of wear than did either the nonsterilized cups or the nonirradiated cups sterilized with gas plasma. There was a comparable advantage in the rate of wear after fourteen days of thermal aging. However, after thirty days of aging, the cups sterilized with gamma irradiation in low-oxygen packaging wore several times faster than did the nonirradiated cups. Ion-implanting improved the wear resistance without thermal aging, but after extensive thermal aging the oxidation and wear were greater than those of the controls. Hylamer cups (that is, those that were sterilized with gas plasma) exhibited wear properties very close to those of the nonsterilized ultra-high molecular weight polyethylene cups (the controls) with or without aging.

CONCLUSIONS

Sterilizing an ultra-high molecular weight polyethylene acetabular cup without radiation (for example, with ethylene oxide or gas plasma) avoids immediate and long-term oxidative degradation of the implant but does not improve the inherent wear resistance of the polyethylene. Sterilizing with use of gamma irradiation with the implant packaged in a low-oxygen atmosphere avoids immediate oxidation and cross-links the polyethylene, thereby increasing its wear resistance, but long-term oxidation of the residual free radicals may markedly reduce the wear resistance. Ideally, cross-linking with gamma irradiation to reduce wear should be done in a manner that avoids both immediate and long-term oxidation.

Wear of gamma-crosslinked polyethylene acetabular cups against roughened femoral balls

Crosslinking of ultrahigh molecular weight polyethylene has been shown to markedly improve its wear resistance in clinical studies and laboratory tests using hip joint simulators. However, because most of the laboratory studies have been done under clean conditions using prosthesis-quality, highly polished counterfaces, there is concern regarding how well an intentionally crosslinked polyethylene acetabular cup will resist abrasion by a femoral ball that has been damaged by third-body abrasion in vivo. To investigate this, conventional and radiation crosslinked-remelted acetabular cups of ultra-high molecular weight polyethylene were tested in a hip joint simulator bearing against smooth femoral balls and against balls with moderate and severe roughening. Cups were tested with and without aging to accelerate any oxidative degradation. The crosslinked cups were produced by exposing extruded GUR 4150 bar stock of ultrahigh molecular weight polyethylene to 5 Mrad gamma radiation under a partial vacuum and then the bars were remelted to extinguish residual free radicals. Artificial aging at 70 degrees C under 5 atm oxygen for 14 days induced negligible oxidation in the crosslinked and remelted material. Against smooth balls, the wear of the crosslinked cups, with or without aging, averaged approximately 15% of that of the conventional cups. Against the moderately rough balls, the wear rate of the conventional cups was unchanged, whereas the wear rate increased slightly for the nonaged and aged crosslinked cups, but was still only 26% and 20% of that of the conventional cups, respectively. Against extremely rough balls, the mean wear rates increased markedly for each material such that during the final 1 million cycle interval, the average wear rates of the nonaged and the aged crosslinked cups were 72% and 47% of that of the conventional cups, respectively. That is, the crosslinked polyethylene showed substantially better wear resistance than conventional polyethylene across the range of ball roughnesses, with or without accelerated aging.