Evaluation of vitamin E-diffused highly crosslinked polyethylene wear and porous titanium-coated shell stability: a seven-year randomized control trial using radiostereometric analysis.

AIMS: Vitamin E-diffused, highly crosslinked polyethylene (VEPE) and porous titanium-coated (PTC) shells were introduced in total hip arthroplasty (THA) to reduce the risk of aseptic loosening. The purpose of this study was: 1) to compare the wear properties of VEPE to moderately crosslinked polyethylene; 2) to assess the stability of PTC shells; and 3) to report their clinical outcomes at seven years. PATIENTS AND METHODS: A total of 89 patients were enrolled into a prospective study. All patients received a PTC shell and were randomized to receive a VEPE liner (n = 44) or a moderately crosslinked polyethylene (ModXLPE) liner (n = 45). Radiostereometric analysis (RSA) was used to measure polyethylene wear and component migration. Differences in wear were assessed while adjusting for body mass index, activity level, acetabular inclination, anteversion, and head size. Plain radiographs were assessed for radiolucency and patient-reported outcome measures (PROMs) were administered at each follow-up. RESULTS: In total, 73 patients (82%) completed the seven-year visit. Mean seven-year linear proximal penetration was -0.07 mm (sd 0.16) and 0.00 mm (sd 0.22) for the VEPE and ModXLPE cohorts, respectively (p = 0.116). PROMs (p = 0.310 to 0.807) and radiolucency incidence (p = 0.330) were not different between the polyethylene cohorts. The mean proximal shell migration rate was 0.04 mm per year (sd 0.09). At seven years, patients with radiolucency (34%) demonstrated greater migration (mean difference: 0.6 mm (sd 0.2); p < 0.001). PROMs were lower for patients with radiolucency and greater proximal migration (p = 0.009 to p = 0.045). No implants were revised for aseptic loosening. CONCLUSION: This is the first randomized controlled trial to report seven-year RSA results for VEPE. All wear rates were below the previously reported osteolysis threshold (0.1 mm per year). PTC shells demonstrated acceptable primary stability through seven years, as indicated by low migration and lack of aseptic loosening. However, patients with acetabular radiolucency were associated with higher shell migration and lower PROM scores. Cite this article: Bone Joint J 2019;101-B:760-767.

A Fully Functional Drug-Eluting Joint Implant.

Despite advances in orthopedic materials, the development of drug-eluting bone and joint implants that can sustain the delivery of the drug and maintain the necessary mechanical strength in order to withstand loading has remained elusive. Here, we demonstrate that modifying the eccentricity of drug clusters and the percolation threshold in ultrahigh molecular weight polyethylene (UHMWPE) results in maximized drug elution and in the retention of mechanical strength. The optimized UHMWPE eluted antibiotic at a higher concentration for longer than the clinical gold standard antibiotic-eluting bone cement while retaining the mechanical and wear properties of clinically used UHMWPE joint prostheses. Treatment of lapine knees infected with Staphylococcus aureus with the antibiotic-eluting UHMWPE led to complete bacterial eradication and to the absence of detectable systemic effects. We argue that the antibiotic-eluting UHMWPE joint implant is a promising candidate for clinical trials.

Alpha-tocopherol-doped irradiated UHMWPE for high fatigue resistance and low wear.

Longevity of total joints has been compromised by wear and fatigue of ultrahigh molecular weight polyethylene (UHMWPE) components. Crosslinking reduces UHMWPE wear, but combined with postirradiation melting, also reduces its fatigue strength, therefore limiting its use in high-stress applications. We hypothesized that a lipophilic antioxidant (alpha-tocopherol, alpha-T) can protect UHMWPE against oxidation eliminating the need for postirradiation melting of crosslinked UHMWPE and improve its fatigue strength. To test these hypotheses, 65- and 100-kGy irradiated, alpha-T-doped and subsequently gamma-sterilized UHMWPE were used. (I) alpha-T-doped irradiated UHMWPEs showed significantly lower oxidation levels (0.48+/-0.25 and 0.44+/-0.06) compared to 100-kGy irradiated UHMWPE (3.74+/-0.16) after 5 weeks of accelerated aging at 80 degrees C in air. (II) Wear rate of alpha-T-doped irradiated UHMWPE (1.9+/-0.5, and 0.9+/-0.1mg/million cycles (MC) for 65- and 100-kGy irradiated UHMWPE, respectively) were comparable to that of 100-kGy irradiated/melted UHMWPE (1.1+/-0.7mg/million cycles). (III) The stress intensity factor at crack inception ( DeltaKi) of 100-kGy irradiated UHMWPE increased significantly upon doping with alpha-T from 0.74 to 0.87MPam(1/2) ( p<0.01 ). The DeltaKi for the 100-kGy irradiated and melted UHMWPE, currently in clinical use, was 0.55MPam(1/2). Doping with alpha-T eliminated the need for postirradiation melting to protect irradiated UHMWPE against long-term oxidation. The fatigue strength was improved by 58% for alpha-T-doped 100-kGy irradiated UHMWPE compared to irradiated and melted UHMWPE. The increase in oxidative stability of alpha-T-doped UHMWPE is attributed to the ability of alpha-T to react with peroxy free radicals on lipid chains and arrest the oxidation reactions. The improved fatigue strength is attributed to the increase in plasticity of UHMWPE due to the lipophilic nature of alpha-T.

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.

Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept.

The design of femoral and acetabular components of metal-on-polyethylene total hip arthroplasty implants has been dominated by the limitations of the wear properties of ultra-high molecular weight polyethylene (UHMWPE). As a result, the commonest femoral head diameters used range from 22 to 32 mm, the latter producing maximal volumetric wear. Cross-linking has been shown to improve significantly the wear resistance of acetabular components when tested in vitro against conventional femoral head sizes (22-32 mm). We expanded the study of the wear behavior of 1 type of electron-beam cross-linked UHMWPE with femoral head diameters ranging from 22 to 46 mm. The simulated gait studies showed that wear was independent of head size for the range of femoral head sizes studied. Even for the 46-mm femoral head, wear was reduced significantly using criteria of gravimetric and geometric measurements and morphologic appearance of the machining marks out to 11 million cycles of simulated gait.

A critical assessment of proximal macrotexturing on cemented femoral components.

We analyzed the cement-metal interface of 3 different types of femoral components that had proximal macrotexturing after in vitro insertion and after fatigue testing designed to produce debonding and micromotion. These components were compared with clinical retrieval specimens. The cement did not flow into the macrotexturing; rather, hollow, brittle volcanoes or calderas were formed. These fragile protrusions of cement become worn down or abraded by debonded components. This abrasion of cement may contribute to the early and aggressive osteolysis seen in some of these early failures with proximal macrotextured components. The formation of these volcanos and calderas can be aborted by placing bone-cement onto the macrotexturing before stem insertion. This simple technique allows the macrotexturing to be filled with cement.

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.

Identification and quantification of irradiation in UHMWPE through trans-vinylene yield.

trans-Vinylene unsaturations generated in ultrahigh-molecular-weight polyethylene (UHMWPE) after radiation treatment scales linearly with the absorbed dose level, and can serve as an internal dosimeter for determining the radiation dose level for sterilizing or crosslinking UHMWPE. We measured the trans-vinylene concentration using infrared spectroscopy and generated calibration curves for ultrahigh-molecular-weight polyethylene (ram-extruded GUR 1050) after cold e-beam, cold gamma, and warm e-beam irradiations. The trans-vinylene content increased linearly with the absorbed dose level in all cases except with cold gamma irradiation at dose levels higher than approximately 70 kGy. At that dose level the trans-vinylene content of the warm irradiated samples was higher than those of the cold irradiated samples. Following postirradiation melting, the trans-vinylene content of the gamma irradiated samples increased. Such trans-vinylene quantification can be an internal dosimeter for crosslinked UHMWPE when radiation is used for improving its wear resistance or sterilizing the device made thereof. It can also be utilized to assess the spatial uniformity of the absorbed radiation dose level.

Interlaboratory reproducibility of standard accelerated aging methods for oxidation of UHMWPE.

During accelerating aging, experimental uncertainty may arise due to variability in the oxidation process, or due to limitations in the technique that is ultimately used to measure oxidation. The purpose of the present interlaboratory study was to quantify the repeatability and reproducibility of standard accelerated aging methods for ultra-high molecular weight polyethylene (UHMWPE). Sections (200 microm thick) were microtomed from the center of an extruded rod of GUR 4150 HP, gamma irradiated in air or nitrogen, and circulated to 12 institutions in the United States and Europe for characterization of oxidation before and after accelerated aging. Specimens were aged for 3 weeks at 80 degrees C in an air circulating oven or for 2 weeks at 70 degrees C in an oxygen bomb (maintained at 503 kPa (5 atm.) of O2) in accordance with the two standard protocols described in ASTM F 2003-00. FTIR spectra were collected from each specimen within 24 h of the start and finish of accelerated aging, and oxidation indices were calculated by normalizing the peak area of the carbonyl region by the reference peak areas at 1370 or 2022 cm(-1). The mean relative interlaboratory uncertainty of the oxidation data was 78.5% after oven aging and 129.1% after bomb aging. The oxidation index measurement technique was not found to be a significant factor in the reproducibility. Comparable relative intrainstitutional uncertainty was observed after oven aging and bomb aging. For both aging methods, institutions successfully discriminated between air-irradiated and control specimens. However, the large interinstitutional variation suggests that absolute performance standards for the oxidation index of UHMWPE after accelerated aging may not be practical at the present time.

A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award.

Increasing cross-linking has been shown in vitro and in vivo to improve markedly the wear resistance of ultra-high-molecular-weight polyethylene (UHMWPE). The reduction in the mechanical properties of polyethylene under certain methods used to produce cross-linking has been a concern, however. These reductions are known to result from the processes used to increase the cross-link density and could affect the device performance in vivo. We present a novel method of increasing the cross-link density of UHMWPE in which UHMWPE is irradiated in air at an elevated temperature with a high-dose-rate electron beam and subsequently is melt-annealed. This treatment improves markedly the wear resistance of the polymer as tested in a hip simulator, while maintaining the mechanical properties of the material within national and international standards. This method leads to the absence of detectable free radicals in the polymer and, as a result, excellent resistance to oxidation of the polymer.

Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty.

Despite the recognized success and worldwide acceptance of total joint arthroplasty, wear is a major obstacle limiting the longevity of implanted UHMWPE components. Efforts to solve the wear problem in UHMWPE have spurred numerous detailed studies into the structure, morphology, and mechanical properties of the polymer at every stage of its production from original resin into stock material and final fabricated form. Scientific developments in this field are occurring at an accelerating rate, and periodic review of UHMWPE technology is therefore increasingly necessary. The present article provides a four-part comprehensive review of technological advancements in the processing, manufacture, sterilization, and crosslinking of UHMWPE for total joint replacements. The first part of this article describes the recently updated nomenclature of UHMWPE, including the process of resin production and conversion to stock material. The second part outlines the methods of manufacturing UHMWPE into joint replacement components and provides overviews of alternate forms of UHMWPE, namely carbon-fiber reinforced UHMWPE (Poly II) and UHMWPE recrystallized under high temperature and pressure (Hylamer). The third part summarizes the sterilization and degradation of UHMWPE. Newly developed methods for accelerating the oxidation of UHMWPE after sterilization (for preconditioning of test specimens), as well as methods for quantifying the oxidation of UHMWPE, are also discussed. Finally, the fourth part reviews the development and properties of crosslinked UHMWPE, a promising alternate biomaterial for total joint replacements.

Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE).

Crosslinking has been shown to improve the wear resistance of ultra-high molecular weight polyethylene in both in vitro and clinical in vivo studies. The molecular mechanisms and material properties that are responsible for this marked improvement in wear resistance are still not well understood. In fact, following crosslinking a number of mechanical properties of UHMWPE are decreased including toughness, modulus, ultimate tensile strength, yield strength, and hardness. In general, these changes would be expected to constitute a precursor for lower wear resistance, presenting a paradox in that wear resistance increases with crosslinking. In order to understand better and to analyze this paradoxical behaviour of crosslinked UHMWPE, we investigated the wear behavior of (i) radiation-crosslinked GUR 1050 resin, (ii) peroxide-crosslinked GUR 1050 resin and (iii) peroxide-crosslinked Himont 1900 resin using a bi-directional pin-on-disk (POD) machine. Wear behavior was analyzed as a function of crystallinity, ultimate tensile strength (UTS), yield strength (YS), and molecular weight between crosslinks (Mc). The crosslink density increased with increasing radiation dose level and initial peroxide content. The UTS, YS, and crystallinity decreased with increasing crosslink density. While these variations followed the same trend, the absolute changes as a function of crosslink density were different for the three types of crosslinked UHMWPE studied. There was no unified correlation for the wear behavior of the three types of crosslinked UHMWPE with the crystallinity, UTS and YS. However, the POD wear rate showed the identical linear dependence on Mc with all three types of crosslinked UHMWPEs studied. Therefore, we have strong evidence to propose that Mc or crosslink density is a fundamental material property that governs the lubricated adhesive and abrasive wear mechanisms of crosslinked UHMWPEs, overriding the possible effects of other material properties such as UTS, YS and crystallinity on the wear behavior.