HXLPE acetabular liners were introduced to reduce wear-related complications in THA. However, post-irradiation thermal free radical stabilization can compromise mechanical properties, leave oxidation-prone residual free radicals, or both. Reports of mechanical failure of HXLPE acetabular liner rims raise concerns about thermal free radical stabilization and in vivo oxidization on implant properties. The purpose of this study is to explore the differences in the mechanical, physical and chemical properties of HXLPE acetabular liner rims after extended time in vivo between liners manufactured with different thermal free radical stabilization techniques. Remelted, single annealed and sequentially annealed retrieved HXLPE acetabular liners with in vivo times greater than 4.5 years were obtained from our implant retrieval laboratory. All retrieved liners underwent an identical sanitation and storage protocol. For mechanical testing, a total of 55 explants and 13 control liners were tested. Explant in vivo time ranged from 4.6 – 14 years and ex vivo time ranged from 0 – 11.6 years. Rim mechanical properties were tested by microindentation hardness testing using a Micromet II Vickers microhardness tester following ASTM standards. A subset of 16 explants with ex vivo time under one year along with five control liners were assessed for oxidation by FTIR, crystallinity by Raman spectroscopy, and evidence of microcracking by SEM. No significant difference in in vivo or ex vivo was found between thermal stabilization groups in either set of explants studied. In the mechanically tested explants, there was no significant correlation between in vivo time and Vickers hardness in any thermal stabilization group. A significant correlation was found between ex vivo time and hardness in remelted liners (r=.520, p = .011), but not in either annealed cohort. ANCOVA with ex vivo time as a covariate found a significant difference in hardness between the thermal free radical stabilization groups (p 0.1) was found in retrieved remelted (25%), single annealed (100%) and sequentially annealed (75%) liner rims. Crystallinity was increased in the subsurface region relative to control liners for both annealed, but not remelted, liner rims. Hardness was increased in oxidized rims for both annealed cohorts but not in the remelted cohort. Microcracking was only found along the surface of one unoxidized remelted liner rim. Mechanical properties were reduced at baseline and worsened after in vivo time for remelted HXLPE liner rims. Rim oxidation was detected in all groups. Oxidation was associated with increased crystallinity and hardness in annealed cohorts, but not remelted liners. Increased crystallinity and oxidation do not appear to be directly causing the worsened mechanical behavior of remelted HXLPE liner rims after extended in vivo time

Introduction. HXLPE acetabular liners were introduced to reduce wear-related complications in THA. However, post-irradiation thermal free radical stabilization can compromise mechanical properties, leave oxidation-prone residual free radicals, or both. Reports of mechanical failure of HXLPE acetabular liner rims raise concerns about thermal free radical stabilization and in vivo oxidization on implant properties. The purpose of this study is to explore the differences in the mechanical, physical and chemical properties of HXLPE acetabular liner rims after extended time in vivo between liners manufactured with different thermal free radical stabilization techniques. Material and methods. Remelted, single annealed and sequentially annealed retrieved HXLPE acetabular liners with in vivo times greater than 4.5 years were obtained from our implant retrieval laboratory. All retrieved liners underwent an identical sanitation and storage protocol. For mechanical testing, a total of 55 explants and 13 control liners were tested. Explant in vivo time ranged from 4.6 – 14.0 years and ex vivo time ranged from 0 – 11.6 years. Rim mechanical properties were tested by microindentation hardness testing using a Micromet II Vickers microhardness tester following ASTM standards. A subset of 16 explants with ex vivo time under one year along with five control liners were assessed for oxidation by FTIR, crystallinity by Raman spectroscopy, and evidence of microcracking by SEM. Results. No significant difference in in vivo or ex vivo was found between thermal stabilization groups in either set of explants studied. In the mechanically tested explants, there was no significant correlation between in vivo time and Vickers hardness in any thermal stabilization group. A significant correlation was found between ex vivo time and hardness in remelted liners (Δ=.520, p=.011), but not in either annealed cohort. ANCOVA with ex vivo time as a covariate found a significant difference in hardness between the thermal free radical stabilization groups (p<.0005, η. 2. = 0.322). Post hoc analysis revealed hardness was significantly lower in the retrieved remelted group compared to both the single annealed (p=.001) and sequentially annealed (p<.0005) cohorts. Hardness was significantly higher in the retrieved remelted liners compared to controls (p=.007), with no different in either annealed cohort (figure 1). Detectable subsurface oxidation (OI > 0.1) was found in retrieved remelted (25%), single annealed (100%) and sequentially annealed (75%) liner rims (figure 2). Crystallinity was increased in the subsurface region relative to control liners for both annealed, but not remelted, liner rims. Hardness was increased in oxidized rims for both annealed cohorts but not in the remelted cohort. Microcracking was only found along the surface of one unoxidized remelted liner rim. Conclusion. Mechanical properties were reduced at baseline and worsened after in vivo time for remelted HXLPE liner rims. Rim oxidation was detected in all groups. Oxidation was associated with increased crystallinity and hardness in annealed cohorts, but not remelted liners. Increased crystallinity and oxidation do not appear to be directly causing the worsened mechanical behavior of remelted HXLPE liner rims after extended in vivo time. For any tables or figures, please contact the authors directly

Introduction. HXLPE acetabular liners were introduced to reduce wear-related complications in THA. However, post-irradiation thermal free radical stabilization can compromise mechanical properties, leave oxidation-prone residual free radicals, or both. Reports of mechanical failure of HXLPE acetabular liner rims raise concerns about thermal free radical stabilization and in vivo oxidization on implant properties. The purpose of this study is to explore the differences in the mechanical, physical and chemical properties of HXLPE acetabular liner rims after extended time in vivo between liners manufactured with different thermal free radical stabilization techniques. Material and Methods. Remelted, single annealed and sequentially annealed retrieved HXLPE acetabular liners with in vivo times greater than 4.5 years were obtained from our implant retrieval laboratory. All retrieved liners underwent an identical sanitation and storage protocol. For mechanical testing, a total of 55 explants and 13 control liners were tested. Explant in vivo time ranged from 4.6 – 14.0 years and ex vivo time ranged from 0 – 11.6 years. Rim mechanical properties were tested by microindentation hardness testing using a Micromet II Vickers microhardness tester following ASTM standards. A subset of 16 explants with ex vivo time under one year along with five control liners were assessed for oxidation by FTIR, crystallinity by Raman spectroscopy, and evidence of microcracking by SEM. Results. No significant difference in in vivo or ex vivo time was found between thermal stabilization groups in either set of explants studied. In the mechanically tested explants, there was no significant correlation between in vivo time and Vickers hardness in any thermal stabilization group. A significant correlation was found between ex vivo time and hardness in remelted liners (r=.520, p=.011), but not in either annealed cohort. ANCOVA with ex vivo time as a covariate found a significant difference in hardness between the thermal free radical stabilization groups (p<.0005, η. 2. = 0.322). Post hoc analysis revealed hardness was significantly lower in the retrieved remelted group compared to both the single annealed (p=.001) and sequentially annealed (p<.0005) cohorts. Hardness was significantly higher in the retrieved remelted liners compared to controls (p=.007), with no different in either annealed cohort. Detectable subsurface oxidation (OI > 0.1) was found in retrieved remelted (25%), single annealed (100%) and sequentially annealed (75%) liner rims. Crystallinity was increased in the subsurface region relative to control liners for both annealed, but not remelted, liner rims. Hardness was increased in oxidized rims for both annealed cohorts but not in the remelted cohort. Microcracking was only found along the surface of one unoxidized remelted liner rim. Conclusion. Mechanical properties were reduced at baseline and worsened after in vivo time for remelted HXLPE liner rims. Rim oxidation was detected in all groups. Oxidation was associated with increased crystallinity and hardness in annealed cohorts, but not remelted liners. Increased crystallinity and oxidation do not appear to be directly causing the worsened mechanical behavior of remelted HXLPE liner rims after extended in vivo time. For any figures or tables, please contact authors directly

Introduction: Despite significant progress at the molecular level the etiology of aseptic loosening is still unclear. Fibrosis of the new capsule is an invariable finding at revision hip arthroplasty. Tissue fibrosis has been demonstrated in varies pathologic conditions due to elevated oxidative stress. The present retrospective study was designed to proof the hypothesis that peri-prosthetic fibrosis in aseptic loosening may be caused by elevated oxidative stress and represent an initial step in the pathomechanism of aseptic loosening.

Material and methods: Levels of malondialdehyde (MDA), oxidized (GSSG) and reduced (GSH) gluthatione were assayed as markers of oxidative stress in retrieved capsules of 28 loose hips (Group I) and 12 hips revised for high rate of wear (Group II). Collagen in the periprosthetic tissues was measured as hydroxiproline content and semiquantitatively by electrophoresis. In four representative cases electron microscopy was performed.

Results: MDA level as well as GSH/GSSG and GSH/ GSSG² ratios showed elevated oxidative stress in group I compared to group II and controls. SDS-PAGE electrophoresis showed higher molecular bands in 20 patients compared to controls. Hydroxiproline level in group II is significantly higher than in group I (p< 0.05). MDA, GSH and GSSG correlate significantly with hydroxiproline. A negative correlation between collagen content and osteolysis was established.

Discussion and conclusion: Higher oxidative stress plays role in aseptic loosening of hip arthroplasty. The present data support the hypothesis that the process is initiated by excessive fibrosis which consequently might lead to increase of intraarticular pressure and to extension of the joint space.

Summary. Fifteen irradiated, vitamin E-diffused UHMWPE retrievals with up to three years in vivo service showed no appreciable oxidation, nor change in material properties from a never-implanted liner, and showed a 94% decrease in free radical content. Introduction. Radiation cross-linking, used to improve wear resistance of ultra-high molecular weight polyethylene (UHMWPE) bearings used in total joint arthroplasty, generates residual free radicals which are the precursors to oxidative embrittlement. First generation materials adopted thermal treatments to eliminate or reduce free radical content, but came with compromises in reduced mechanical properties or insufficient stabilization. A second generation alternative method infuses an antioxidant, vitamin E, into irradiated UHMWPE to stabilise free radicals while maintaining fatigue strength. In vitro studies predict excellent oxidation and wear resistance in vitamin E-stabilised bearings, but the long-term in vivo oxidation behavior, influenced by lipid absorption and cyclic loading, remains largely unknown. Our aim was to investigate in vivo changes in UHMWPE surgically-retrieved explants that were radiation cross-linked and stabilised by vitamin E. Patients & Methods. Fifteen surgically-retrieved irradiated, vitamin E-diffused and inert-gamma sterilised bearings (E1™, Biomet, Inc., Warsaw IN) with in vivo durations ranging from 3 days to 36.6 months were analyzed at unloaded rim/eminence and the articular surface along with one never-implanted component. Total lifetime of components was summed as shelf storage prior to implantation, in vivo duration and ex vivo duration in air. Fourier Transform Infrared Spectroscopy (FTIR) was used to measure carbonyl index (CI; per ASTM F2102-01ε1) both before and after 16 hour hexane extraction to. Extracted thin films were also reacted with nitric oxide to quantify hydroperoxides, an intermediate oxidation product associated with oxidation potential. Cross-link density was calculated from gravimetric swelling analysis per ASTM F2214. Crystallinity measurements were performed regionally using differential scanning calorimetry (DSC). Free radical content was measured by electron spin resonance (Memphis, TN). Results. Irradiated and vitamin E-diffused retrievals showed scratching at the articular surface, but retained machining marks up to three years in vivo, indicative of no measurable wear. Retrievals showed no significant oxidation at the time of surgical removal with maximum post-hexane carbonyl indices in the barely detectable range (MCI=0.029–0.154), located at the surface of retrievals. Ex vivo oxidation was not observed after 18 months of aging in air at room temperature. There was no increase in hydroperoxides (never-implanted HI=0.62±0.04; retrieval HI= 0.62±0.04), nor change in cross-link density (never-implanted: 0.275±0.015 mol/dm. 3. ; retrieval: 0.295±0.016 mol/dm. 3. ) or crystallinity (never-implanted: 58.3±1.4%; retrievals: 60.0±3.5%). Lipid penetration increased with time, showing a higher rate of diffusion in loaded regions. Free radical content was observed to decay with increasing in vivo duration (R. 2. =0.44; p<0.05), and by one order of magnitude (94%) by 36.6 months. A stronger negative correlation (R. 2. =0.65) was observed between the total lifetime of the liner and free radical content. Discussion/Conclusion. The free-radical scavenging activity of the vitamin E appears to successfully prevent both in vivo and ex vivo oxidation for short durations. Without an increase in hydroperoxides, the oxidation cascade initiated by radiation-induced and lipid-derived free radicals appears to have been halted. Retrievals also gave no indication of wear in this timeframe, similar to improved wear resistance seen in first generation materials. Continued monitoring will be necessary at longer implant durations

Radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the bearing of choice in joint arthroplasty. The demands on the longevity of this polymer are likely to increase with the recently advancing deterioration of the performance of alternative metal-on-metal implants. Vitamin E-stabilized, cross-linked UHMWPEs are considered the next generation of improved UHMWPE bearing surfaces for improving the oxidation resistance of the polymer. It was recently discovered that in the absence of radiation-induced free radicals, lipids absorbed into UHMWPE from the synovial fluid can initiate oxidation and result in new free radical-mediated oxidation mechanisms. In the presence of radiation-induced free radicals, it is possible for the polymer to oxidize through both existing free radicals at the time of implantation and through newly formed free radicals in vivo. Thus, we showed that reducing the radiation-induced free radicals in vitamin E-stabilized UHMWPE would increase its oxidative stability and presumably lead to improved longevity. We describe mechanical annealing, low pressure annealing, and warm irradiation of irradiated vitamin E blends as novel methods to eliminate 99% of radiation-induced free radicals without sacrificing crystallinity. These are significant improvements in the processing of highly cross-linked UHMWPE for joint implants with improved longevity

Since its introduction in total hip replacements in the 1960's, Ultra High Molecular Weight Polyethylene (UHMWPE) has played a major role as a bearing component material for joint arthroplasty. Concerns were raised when issues of wear resistance became apparent, and therefore Highly Crosslinked Polyethylenes were introduced. Such materials undergo a thermal treatment to quench the free radicals and reduce progressive oxidation. However, said thermal treatment weakens the material mechanical properties and hence the use of antioxidants has been proposed and implemented in clinical use, mainly Vitamin-E. This can be added to the material before or after irradiation. If it is done before, part of the anti-oxydant is consumed during irradiation and so will not be available for its main purpose, and part reacts before irradiation with the free radicals thus reducing the crosslinking effect. If it is added after irradiation, high temperatures are required in order to diffuse it in the bulk material, and anyway the surface will be mainly rich in antioxidant. However, Vitamin-E tends to neutralize the free radicals on the oxidized lipid chain present in our body fluids and so in direct contact with the prosthetic components: such mechanism reduces the Vitamin-E quantity available for anti-oxidation purposes in the long run. A UHMWPE doped with Hindered Amine Light Stabilizer (HALS) has been developed and tested for applications in large joint replacements where highest resistance to wear and tough mechanical properties are simultaneously required, such as tibial inserts for knee joints or acetabular inserts for large diameter heads. Mechanical and biocompatibility tests were run in accordance with ASTM F 2565-06 and ISO 10993-1 with successful results and good reproducibility. In particular, electro spin resonance exhibited a very high level of free radicals in the three samples, which confirms the properties of this new material. Free radicals are the result of the activation of the HALS molecules during irradiation, creating nitroxide radicals that will destroy the residual alkyl radicals responsible for the oxidation before and after implantation. Biocompatibility tests proved absence of cytotoxicity, sensitization, irritation, genotoxicity or pyrogenic reactions. The possible future applications for this new material in the arthroplasty field will be discussed along with the expected advances and advantages

Highly cross linked polyethylenes fall into two classes depending on whether annealing or remelting are used in processing. Annealed polyethylenes contain free radicals. Remelted polyethylenes have reduced mechanical properties but no free radicals. Research has now produced a highly cross linked polyethylene (SXL) that combines the advantages of each class. GUR 1020 polyethylene was sequentially cross linked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (Mrad total). Free radical concentration was measured by electron spin resonance. Accelerated aging was carried out in an oxygen bomb under 5 atmospheres of oxygen at 70 degrees C for 14 days. Tensile properties were determined according to ASTM D638. Wear measurements to 5 million cycles were made on an MTS hip joint simulator at 1 Hz using the Paul load curve with maximum load of 2450 N with alpha fraction bovine calf serum. Free radical concentration was 14 x 10(14) spins/g for SXL compared to 1550 x 10(14)spins/g for GUR 1020 irradiated to 3 Mrad in nitrogen (gamma-N2). The maximum oxidation index was 0.09 for SXL, 0.09 for unirradiated UHMWPE, and 1.27 for gamma-N2 respectively. Mechanical properties exceeded the ASTM F648 specification and were unchanged by oxidative challenge. Wear rates were 1.35 cubic mm per million cycles for SXL and 46 cubic mm per million cycles for gamma-N2 respectively. Wear particle sizes were similar for the two materials. Sequential irradiation and annealing provides more complete cross linking of free radicals with a consequent reduction in free radical level. SXL has the same resistance to oxidative challenge as unirradiated polyethylene. Mechanical properties exceed the ASTM F648 values. Wear is reduced by 97% compared to that of gamma-N2. Sequential irradiation and annealing preserves the microstructure by avoidance of melting yet minimizes free radicals

Summary. Understanding of the role of the radical-generating ability of wear particles of the existing and new implant materials as well as application of efficient antioxidants is one of the necessary conditions for improvement of the results of joint replacements. Introduction. Functioning of joint prostheses is accompanied by a continuous formation of wear particles and their accumulation in surrounding tissues. The impact of microroughnesses of joint prosthesis friction units may bring about chemical bond breakage and free-radical generation on a newly-formed wear surface. Wear particles of orthopedic alloys are capable to produce free radicals, and Co-Cr-Mo alloy particles are especially active. Free radicals generated by wear particles can cause oxidation and reduced wear resistance of polyethylene. Oxidised polyethylene particles stimulate the activity and release of bone-resorbing cytokines by human monocytes/macrophages. The ability of free radicals to cause damage to surrounding tissues and implant components makes it necessary to estimate comprehensively the radical-generating activity of wear particles of different orthopedic materials and develop the ways of its inhibition. Methods. Artificial Co-Cr-Mo alloy wear particles were obtained using dry friction of a ball against a disk. The radical-generating ability of orthopedic alloy wear particles was estimated by oxygen consumption using the model reaction of cumene oxidation. The radical-generating ability of wear particles was determined at different moments after their formation and storage at room temperature and humidity. In the experiments, a pro-inflammatory action of wear particles during their continuous formation was also simulated. Fresh cobalt alloy wear particles were used for a consecutive triple oxidation of 2 ml of cumene at a particle concentration of 0.3 mg/ml. After the first 40 min oxidation, a suspension of particles in cumene was centrifuged, and the used particles were removed. Fresh particles were added to oxidised cumene, and the second and third oxidations were carried out in a similar way. The ability of some antioxidants to inhibit the radical-generating ability of cobalt alloy wear particles was also determined. Results. Fresh cobalt alloy wear particles demonstrated an expressed radical-generating ability which remained practically at the initial level after a one-week storage. The ability gradually reduced in the process of storage. After a one-month storage the particles’ radical-generating ability decreased 2.6 times. A six-month storage of cobalt alloy particles resulted in a tenfold reduction of the radical-generating ability as compared to that of fresh particles. The intensification of radical formation was studied during three consecutive oxidations of cumene by wear particles. It was established that each consecutive oxidation of cumene by fresh wear particles occurred with a growing radical-generation ability. That parameter of the newly-formed particles increased more than two- and threefold during a consecutive double and triple cumene oxidation, respectively. Synthetic antioxidant BHT and natural antioxidant alpha-tocopherol were used for inhibition of wear particles-initiated free-radical reactions. Introduction of the antioxidants inhibited cumene oxidation with an antioxidant dose-dependent duration of this effect. In a mixture of alloy and orthopedic polyethylene particles, alpha-tocopherol completely inhibited the radical-generating activity of alloy particles thus preventing the polymer's oxidative destruction. Conclusion. The use of commercially available particles of orthopedic alloys with an uncontrolled duration storage in experiments considerably reduce or do not reveal the negative effects conditioned by their radical-generating ability. A proper study of the effect of the radical- generating ability of wear particles on the properties of implant components and surrounding tissues is possible only with the use of fresh particles. Permanent generation of free radicals in the process of wear of joint prosthesis metal components creates conditions for self-potentiation of negative free radical reactions during joint replacement. This requires the necessity of a preclinical estimation of the radical-generating ability of orthopedic materials and application of efficient antioxidants during the post-implantation period

Introduction. Inradiation cross-linked and melted ultrahigh molecular weight polyethylene (UHMWPE) total joint implants, the oxidation potential is afforded to the material by by post-irradiation melting. The resulting cross-linked UHMWPE does not contain detectable free radicals at the time of implantation and was expected to be resistant against oxidation for the lifetime of the implants. Recently, analysis of long-term retrievals revealed detectable oxidation in irradiated and melted UHMWPEs, suggesting the presence of oxidation mechanisms initiated by mechanisms other than those involving the free radicals at the time of implantation. However, the effect of oxidation on these materials was not well studied. We determined the effects of in vitro oxidation on the wear and mechanical properties of irradiated and melted UHMWPEs. Materials and Methods. Medical grade slab compression molded UHMWPE (GUR1050) was irradiated using 10, 50, 75, 100, 120 or 150 kGy. The irradiated and melted UHMWPEs were accelerated aged at 70°C for 2, 3, 4, 6 and 8 weeks at 5 atm of oxygen. Oxidation profiles were determined by first microtoming 150 μm cross sections; these were then extracted by boiling hexane for 16 hours and vacuum dried for 24 hours. They were then analyzed on an infrared microscope as a function of depth away from the surface. An oxidation index was calculated per ASTM 2102 as the ratio of the area under the carbonyl peak at 1740 cm-1 to the area under the crystalline polyethylene 1895 cm-1 peak. The cross-link density was calculated as previously described (Oral 2010). The wear rate was determined using a custom-designed pin-on-disc wear tester against CoCr polished discs at 2 Hz and a rectangular path of 5 × 10 mm in undiluted bovine serum (Bragdon 2001). Tensile mechanical properties were determined using Type V dogbones according to ASTM D638. Results and Discussion. Oxidation increased as a function of aging duration for all UHMWPE samples. The cross-link density decreased non-linearly with increasing oxidation and the wear rate increased non-linearly. The dependence of wear on cross-link density was different for freshly irradiated, unoxidized samples in contrast to aged and oxidized samples (Figure 1). The elongation at break and the ultimate tensile strength decreased with increasing oxidation (Figure 2) and the modulus increased with increasing oxidation. There was an increase in the oxidation rates and oxidation levels of irradiated and melted UHMWPEs with increasing radiation dose (Figure 1), which suggested that regardless of the presence of residual free radicals, increased cross-linking made the material more prone to oxidation and oxidative degradation. The wear rate was not very sensitive to oxidation with an increase only observed at an oxidation index of 1 (Figure 3), suggesting a significant level of degradation and oxidative damage only at this level of oxidation. In contrast, the tensile strength and elongation-at-break were very sensitive to oxidation, showing severe degradation at low oxidation levels. Significance. This is the first study exploring the effects of simulated oxidation in irradiated and melted UHMWPEs without detectable free radicals known to cause oxidation. We have shown that when oxidation occurs, severe degradation may occur in irradiated and melted UHMWPEs

Introduction Contemporary highly crosslinked polyethylenes fall into two classes (annealed or remelted). Annealed polyethylenes contain free radicals. Remelted polyethylenes have reduced mechanical properties but no free radicals. SXL provides the advantages of both classes. Materials and Methods GUR 1020 polyethylene was sequentially crosslinked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (SXL). The following were measured: free radical concentration (electron spin resonance), oxidation resistance (5 atmospheres of oxygen at 70 degrees C for 14 days), and tensile properties (ASTM D638). Hip simulator wear was determined (MTS machine, 5 million cycles, 1 Hz, Paul load curve with maximum load of 2450 N, alpha fraction bovine calf serum). Results Free radical concentrations were 14 x 1014 and 1550 x 1014 spins/g for SXL and GUR 1020 irradiated to 3 Mrad in nitrogen (gamma-N2) respectively. Maximum oxidation index was 0.09 for SXL, 0.09 for unirradiated UHMWPE, and 1.27 for gamma-N2. SXL tensile properties exceeded ASTM F648 and were unchanged by oxidative challenge. Wear rates were 1.35 and 46 mm3 per million cycles for SXL and gamma-N2 respectively; wear particle sizes were similar. Discussion and Conclusions Sequential irradiation and annealing provides more complete crosslinking with reduction in free radical level. SXL has the same resistance to oxidative challenge as unirradiated polyethylene. Mechanical properties exceed the ASTM F648 values. Wear is reduced by 97% compared to that of gamma-N2. SXL is the basis for next generation highly crosslinked UHMWPE

Osteolysis secondary to ultra-high molecular weight polyethylene (UHMWPE) wear is a leading cause of late-term implant failure via aseptic loosening in patients treated with total hip arthroplasty (THA). Radiation crosslinking of UHMWPE has been shown to decrease wear. However, the resulting polymer (crosslinked-PE) has a high free radical content. Two different methods that have been used to reduce the remaining free radicals are mechanical annealing and chemical stabilization using Vitamin E, a free radical scavenger. The primary purpose of the current study was to evaluate and compare the wear properties of vitamin E-doped crosslinked-PE (VEPE) and one formulation of mechanically annealed crosslinked-PE using radiostereometric analysis (RSA) in patients five years after primary THA. We also sought to understand the association between polyethylene wear and patient-reported outcome measures (PROMs). Three-hundred and five patients from six international centers were enrolled. Seventy-six percent were treated with highly-crosslinked (95 kGy) VEPE liners, and the rest received moderately-crosslinked (50 kGy) (ModXL), mechanically annealed liners. Data was collected prospectively at one-, two-, and five-year intervals. At the 5-year follow-up, proximal femoral head penetration into the VEPE liners (median = 0.05mm (range, −0.03–1.20)) was significantly lower than the penetration into the ModXL liners (median = 0.15mm (range, −0.22–1.04)) (p<0.001). In the VEPE cohort the median proximal penetration did not increase from one- to five-year follow-up (p=0.209). In contrast, there was a significant increase in femoral head penetration for the ModXL group (p<0.001) during that same time. Multivariable regression showed that the only variable predictive of increased wear was ModXL liner type (B=0.12, p<0.001). There were no differences in PROMs between the liner groups, and there was no correlation between polyethylene wear and PROMs for the cohort as a whole. The current study is the largest analysis of polyethylene wear at five-year follow-up using the RSA technique. We observed similar bedding in through the two-year interval between the two liner types, however, there was significantly more wear in the ModXL cohort at five-years. Currently, the wear rates for both liner groups are below the osteolysis threshold and have not led to any implant failures via aseptic loosening. Continued follow-up will provide a better understanding of the association between wear rate and clinical outcomes

Nitric oxide is a free radical which in vivo is solely produced during the conversion of the amino acid arginine into citrulline by nitric oxide synthase enzymes. Recently, the importance of nitric oxide on inflammation and bone metabolism has been investigated. However, the knowledge regarding possible in vitro effects of arginine supplementation on chondrogenic differentiation is limited. ATDC5, a cell line which is derived from mouse teratocarcinoma cells and which is characterized as chondrogenic cell line, were proliferated in Dulbecco's Modified Eagle Medium (DMEM)/F12 and subsequently differentiated in proliferation medium supplemented with insulin, transferrin and sodium-selenite and where arginine was added in four different concentrations (0, 7.5, 15 and 30 mM). Samples were harvested after 7 or 10 days and were stored at −80 °C for subsequent RNA isolation for qPCR analysis. To determine chondrogenic differentiation, Alcian Blue staining was performed to stain the proteoglycan aggrecan, which is secreted by differentiated ATDC5 cells. All measurements were performed in triplo. Alcian Blue staining showed a qualitative increase of proteoglycan aggrecan secretion in differentiated ATDC5 cells after treatment with 7 and 15 mM arginine, with additional increased expression of ColII, ColX, Bmp4 and Bmp6. Treatment with 30 mM arginine inhibited chondrogenic differentiation and expression of aforementioned genes, however, Cox-2 and Vegfa gene expression were increased in these samples. Bmp7 was not significantly expressed in any experimental condition. The obtained results are suggestive for a dose-dependent effect of arginine supplementation on chondrogenic differentiation and associated gene expression, with 7.5 and 15 mM as most optimal concentrations and implications for apoptosis after incubation with 30 mM arginine. A future recommendation would be to investigate the effects of citrulline in a similar experiment, as this shows even more promising results to enhance the nitric oxide metabolism in sepsis and bone healing

Abstract. Background. Progressive muscle ischaemia results in reduced aerobic respiration and increased anaerobic respiration, as cells attempt to survive in a hypoxic environment. Acute compartment syndrome (ACS) is a progressive form of muscle ischaemia that is a surgical emergency resulting in the production of Lactic acid by cells through anaerobic respiration. Our previous research has shown that it is possible to measure H+ ions concentration (pH) as a measure of progressive muscle ischaemia (in vivo) and hypoxia (in vitro). Our aim was to correlate intramuscular pH readings and cell viability techniques with the intramuscular concentration of key metabolic biomarkers [adenosine triphosphate (ATP), Phosphocreatine (PCr), lactate and pyruvate], to assess overall cell health in a hypoxic tissue model. Methods. Nine euthanised Wistar rats were used in a non-circulatory model. A pH catheter was used to measure real-time pH levels from one of the exposed gluteus medius muscles, while muscle biopsies were taken from the contralateral gluteus medius at the start of the experiment and subsequently at every 0.1 of a pH unit decline. The metabolic biomarkers were extracted from the snap frozen muscle biopsies and analyzed with standard fluorimetric method. Another set of biopsies were stained with Hoechst 33342, Ethidium homodimer-1 and Calcein am and imaged with a Zeiss LSM880 confocal microscope. Results. Our study shows that the direct pH electrode readings decrease with time and took an average of 69 minutes to drop to a pH of 6.0. The concentrations of ATP, pyruvate and PCr declined over time, and the concentration of lactate increased over time. At pH 6.0, both ATP and PCr concentrations had decreased by 20% and pyruvate has decreased by 50%, whereas lactate had increased 6-fold. The majority of cells were still viable at a pH of 6.0, suggesting that skeletal muscle cells are remarkably robust to hypoxic insult, although this was a hypoxic model where reperfusion was not possible. Conclusions. Our research suggests that histologically, skeletal muscle cells are remarkably robust to hypoxic insult despite the reduction in the total adenine nucleotide pool, but this may not reflect the full extent of cell injury and quite possibly irreversible injury. The timely restoration of blood flow in theory should halt the hypoxic insult, but late reperfusion results in cellular dysfunction and cell death due to localised free radical formation. Further research investigating the effects of reperfusion in vivo are warranted, as this may identify an optimal time for using pharmacological agents to limit reperfusion injury, around the time of fasciotomy to treat acute compartment syndrome. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Introduction. In vivo, UHMWPE bearing surfaces are subject to wear and oxidation that can lead to bearing fatigue or fracture. A prior study in our laboratory of early antioxidant (AO) polyethylene retrievals, compared to gamma-sterilized and highly cross-linked (HXL) retrievals, showed them to be more effective at preventing in vivo oxidation. The current analysis expands that early study, addressing the effect of:. manufacturing-variables on as-manufactured UHMWPE;. in vivo time on these initial properties;. identifying important factors in selecting UHMWPE for the hip or knee. Methods. After our prior report, our IRB-approved retrieval laboratory received an additional 96 consecutive AO-retrievals (19 hips, 77 knees: in vivo time 0–6.7 years) of three currently-marketed AO-polyethylenes. These retrievals represented two different antioxidants (Vitamin E and Covernox) and two different delivery methods: blending-prior-to and diffusing-after irradiation cross-linking. Consecutive HXL acetabular and tibial inserts, received at retrieval, with in vivo time of 0–6.7 years (260 remelted, 170 annealed) were used for comparison with AO-retrievals. All retrievals were analyzed for oxidation and trans-vinylene index (TVI) using a Thermo-Scientific iN10 FTIR microscope. Mechanical properties were evaluated for 35 tibial inserts by uniaxial tensile testing using an INSTRON load frame. Cross-link density (n=289) was measured using a previously published gravimetric gel swell technique. Oxidation was reported as maximum ketone oxidation index (KOI) measured for each bearing. TVI was reported as the average of all scans for each material. Cross-link density and mechanical properties were evaluated as a function of both TVI and oxidation. Results. Minimal increase in oxidation was seen in these AO-retrievals, out to almost 7 years in vivo. In contrast, HXL-retrievals showed increasing KOI with time in vivo (annealed-HXL = 0.127/year, remelted-HXL = 0.036/year, p<0.001). HXL oxidation rate was higher in knees (0.091/year) than in hips (0.048/year), p<0.001. Cross-link density (XLD) correlated positively with TVI for both HXL (Pearson's correlation=0.591, p<0.001) and AO (Pearson's correlation=0.598, p<0.001) retrievals. AO-materials had higher TVI for the same or similar XLD than did HXL polyethylene. XLD correlated negatively with KOI for HXL retrievals (Pearson's correlation=−0.447, p<0.001). Mechanical properties varied by material across all materials evaluated, with tensile toughness correlating negatively with increasing TVI (Pearson Correlation=−0.795, p<0.001). Discussion. Irradiation cross-linking has been used effectively to improve wear resistance. Residual free radicals from irradiation are the target of AO-polyethylene, to prevent loss of UHMWPE XLD, resulting from in vivo oxidation of free radicals as seen in HXL retrievals, and toughness, resulting from oxidation or initial remelting. Despite different manufacturing variables, AO-polyethylene retrievals in this cohort had minimal oxidation and no change in XLD or toughness due to oxidation. However, toughness did vary with irradiation dose as did cross-link density. To achieve the same level of cross-linking as HXL-polyethylene required a higher irradiation dose in blended AO-polyethylene. AO-polyethylenes evaluated in this study had toughness that decreased with irradiation dose, but avoided loss of toughness due to remelting. Because AO-polyethylenes did not oxidize, they did not show the decrease of cross-link density, and potential loss of wear resistance, seen in HXL-polyethylene. For any figures or tables, please contact authors directly

One of serious issues in total hip arthroplasty (THA) is the osteolysis which results in aseptic loosening caused by the wear particles from a polyethylene (PE) acetabular cup. In addition, oxidation degradation of PE cup resulting in the fracture or the severe wear caused by the reduction of mechanical properties in vivo is also the issue. The oxidation degradation is considered to be induced by residual free radicals generated by gamma-ray irradiation for cross-linking to reduce wear or for sterilization. In this study, (1) wear property, (2) oxidation degradation of retrieved PE and highly cross-linked PE (CLPE) cups against alumina ceramic femoral heads, and (3) the correlation between those properties were evaluated. The radiographic wear of six conventional PE cups with the mean follow-up of 19.1–23.3 years and 60 CLPE cups with the mean follow-up of 3.1–9.1 years were measured by a non-radiostereometric analysis method (Vectorworks. ®. 10.5 software package). As a retrieval analysis, 26 retrieved acetabular cups were evaluated; 16 cups were ethylene oxide gas-sterilized conventional PE cups with clinical use for 16.0–24.9 years and 10 cups were gamma-ray-sterilized CLPE cups with clinical use for 0.9–6.7 years. The linear and the volumetric wear were measured using a three-dimensional (3D) coordinate measurement machine. The shapes of unworn and worn surfaces with 15- and 30-point intervals, respectively, were measured. Oxidation degradation of the surface, sub-surface and inner for both worn and unworn parts of the retrieved cups was measured using a Fourier-transform infrared (FT-IR) spectroscopy. Oxidation indices were calculated using the peak at 1740 cm. −1. and 1370 cm. −1. according to ASTM F2012. In the radiographic analysis, the linear wear rate of CLPE cups was significantly lower than that of conventional PE cups [Fig. 1]. In the retrieval analysis, the linear wear rate of CLPE cups (mean: 0.07 mm/year) showed a 51% reduction (p = 0.002) compared to conventional PE cups (mean: 0.14 mm/year) [Fig. 2]. The retrieval and the radiographic analysis for both conventional PE and CLPE cups showed similar results (p = 0.7 and 0.1, respectively). Maximum oxidation indices for CLPE cups were similar to those of conventional PE cups regardless of the difference of clinical duration [Fig. 3]. This result is different from in vivo wear, which increases as the clinical duration. For both conventional PE and CLPE cups, the oxidation indices of subsurface were higher than those for surface. The worn parts showed higher oxidation indices than those for unworn parts. From the results, even when the free radicals were so few or absent, the oxidation degradation would be induced in vivo. In conclusion, the wear resistance for CLPE cups was greater than that for conventional PE cups from both radiographic and retrieval analyses. The in vivo oxidation degradation might not be caused by only residual free radicals. It was found that oxidation degradation of PE cups when used with alumina ceramic femoral heads is not correlated to their wear properties

Remelted highly cross linked UHMWPEs have no detectable free radicals but the mechanical and fatigue properties are reduced because remelting changes the microstructure. Annealed highly cross linked UHMWPEs maintain the microstructure and mechanical properties but contain free radicals. A novel sequential irradiation and annealing process preserves the microstructure while providing enhanced oxidation resistance. 6_B_Material_e_Methods: GUR 1020 polyethylene was sequentially cross linked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (SXL). Density was measured according to ASTM D1505. Crystallinity and thermal properties were determined according to ASTM D3417. Crystallite size/lamellar structure was determined by small angle x-ray scattering. Accelerated aging was carried out in an oxygen bomb under 5 atmospheres of oxygen at 70 degrees C for 14 days. SXL density was 939.2 kg/cubic meter, identical to that for unirradiated UHMWPE and UHMWPE irradiated in nitrogen to 3 Mrad (gamma-N2). SXL crystallinity was 61.7%, compared to 61.3% and 59.2% for gamma-N2 and virgin UHMWPE, respectively. The long period spacing, crystal thickness and amorphous thickness were 38.2, 23.6 and 14.6 nm respectively for SXL and 38.9, 23.0 and 15.9 for gamma-N2. There was no statistical difference. Accelerated aging resulted in a white band for gamma-N2 with an oxidation index of 1.27. The response of SXL was the same as virgin UHMWPE e.g. crystallinity and density were unchanged with no white band formation and an oxidation index of 0.09. By avoiding remelting, sequential irradiation and annealing preserves polyethylene microstructure. The sequential process allows more efficient cross linking of free radicals resulting in an oxidation resistance equivalent to that of virgin UHMWPE

Irradiation cross-linking of UHMWPE has been shown to reduce wear while generating free radicals that oxidise in the presence of oxygen or oxidising species. Various methods have been used to minimise or eliminate the effect of these free radicals including below-melt annealing, remelting, Vitamin E infusion, or the use of other antioxidants. Each method has benefits and drawbacks with respect to wear properties, mechanical properties, and chemical properties. Accelerated aging techniques are used to evaluate the efficacy of new methods in stabilising free radicals in highly cross-linked UHMWPE. Various procedures have been described for aging standard gamma-air sterilised UHMWPE to produce oxidation levels that represent shelf-aged bearings. An important factor in evaluating and comparing these aging techniques is validating that they reproduce the profile of oxidation (depth and magnitude) seen both in gamma-air, shelfaged polyethylene and in clinical retrievals. Moreover, the resulting oxidation level in the aged UHMWPE should predict the fatigue and/or wear damage seen in retrieved gamma-air inserts and liners. The present study compared clinically relevant UHMWPE samples aged with ASTM 2003-00, (Method B: 70°C, 5 atm O2, 14 days) and a published lower temperature, lower oxygen-pressure environment (63° C, 3 atm O2, 28 days). Longer aging times (35 to 42 days) were also tested to examine oxidation rate and time to onset of mechanical degradation. Both published methods result in oxidation of gamma-air and gamma-barrier sterilised polyethylene, but have little effect on remelted or antioxidant stabilised crosslinked polyethylene. These aging protocols, however, did not bring standard polyethylene to the critical oxidation level necessary for the fatigue damage that is seen in retrieved inserts and liners. Oxidation of gamma-air and gamma-barrier sterilised UHMWPE increases exponentially with time on the shelf or in the two aging environments. Of note, longer aging times (35 to 42 days) that bring standard UHMWPE to sufficiently high oxidation levels for fatigue to occur also cause increased oxidation levels in remelted UHMWPE. Oxidation increases were the smallest in antioxidant UHMWPE, though still detectable. While this oxidation is not high enough in remelted material or antioxidant material to cause the fatigue damage seen in gamma-air sterilised UHMWPE, it does raise concerns about the published aging techniques and the long term stability of the new materials in vivo. Relying on artificial aging techniques that do not adequately challenge even gamma-air polyethylene may conceal unforeseen weaknesses of new materials. Using longer aging times for existing techniques or novel aging approaches may be necessary to effectively evaluate the long term stability of new bearing materials

Aims: In recent years, radiation crosslinking has become an important processing step in the manufacture of ultra-high molecular weight polyethylene (PE) components of joint replacement prostheses due to its associated high wear resistance. Gamma or electron beam radiation treatment is usually followed by a heating step, either complete melting or annealing of PE close to but below the melting temperature for a specific time duration. The heat treatment is performed to decrease free radical concentration within the crystalline lamellae in order to make PE more oxidation resistant. In this study, we hypothesized that high pressure processing of PE would be advantageous if it is performed only after irradiation and quenching of free radicals and that it would be detrimental if it preceded irradiation. We used accelerated oxidation, mechanical tests and wear tests to show. Methods: Ram-extruded rod stock of GUR 1050 PE (Ticona, Bayport, TX) was purchased from MediTECH Medical Polymers (Fort Wayne, IN) and machined into cylinders to snugly fit into a custom-built high-pressure cell. A Carver hydraulic press was used to apply a pressure of 500MPa to PE specimens preheated to various temperatures, slow cooled to room temperature followed by pressure release. The PE cylinders and untreated control PE were subjected to 50kGy gamma radiation, which is a dose sufficient for a high degree of crosslinking in PE. A Parr bomb reactor filled with oxygen gas and operating at 5atm pressure and 70_C temperature was used to oxidize PE for a period of 14 days, according to ASTM standard F2003–02, and later characterized using Fourier Transform Infrared Spectroscopy (FTIR). A second batch of PE was first irradiated, melted and then subjected to high pressure processing. ASTM standard tensile tests were conducted to determine whether there was any increase in mechanical properties. Scanning electron microscopy (SEM) and differential scanning calorimeter (DSC) were used to characterize the lamellar morphology. Results: The morphological characterization techniques, SEM and DSC, showed that high pressure processing increased the crystallinity as well as lamellar thickness regardless of whether the process was conducted prior to or after irradiation. FTIR showed that there was a monotonic increase in oxidation with lamellar thickness if the irradiation was carried out after high pressure processing. Several mechanical properties such as modulus and yield stress of PE increased with increase in crystallinity, which is desirable for applications where PE is subjected to high stresses. Conclusions: High-pressure processing benefits the mechanical properties of crosslinked PE when it is conducted after irradiation and melting. However, if it conducted prior to irradiation and is not followed by thermal treatment, it can lead to more trapped free radical and excessive oxidation. Therefore, it is important to employ this processing method after irradiation so that it improves the mechanical properties of crosslinked PE

Introduction. The optimum UHMWPE orthopaedic implant bearing surface must balance wear, oxidation and fatigue resistance. Antioxidant polyethylene addresses free radicals, resulting from irradiation used in cross-linking, that could oxidize and potentially lead to fatigue damage under cycles of in vivo use. Assessing the effectiveness of antioxidant (AO) polyethylene compared to conventional gamma-sterilized or remelted highly cross-linked (HXL) polyethylene is necessary to set realistic expectations of the service lifetime of AO polyethylene in the knee. This study evaluates what short-term antioxidant UHMWPE retrievals can reveal about: (1) oxidation-resistance, and (2) fatigue-resistance of these new materials. Methods. An IRB-approved retrieval laboratory received 25 AO polyethylene tibial insert retrievals from three manufacturers with in vivo time of 0–3 years. These were compared with 20 conventional gamma-inert sterilized and 30 HXL (65-kGray, remelted) tibial inserts of the same in vivo duration range. The retrievals were. (1) analyzed for oxidation and trans-vinylene index (TVI) using an FTIR microscope, and (2) inserts of sufficient size and thickness were evaluated for mechanical properties by uniaxial tensile testing using an INSTRON load frame. Oxidation was reported as maximum oxidation measured in the scan from the articular surface to the backside of each bearing. TVI was reported as the average of all scans for each material. Average ultimate tensile strength (UTS), ultimate elongation (UE), and toughness were the reported mechanical properties for each material. Results. Maximum oxidation values differed significantly across material types (p=0.018, Figure 1). No antioxidant retrieval exhibited a subsurface oxidation peak, in contrast to conventional gamma-sterilized (55%) and highly cross-linked (37%) retrievals that exhibited subsurface oxidation peaks over the same in vivo time (Figure 2). Trans-vinylene index (TVI) correlated positively with nominal irradiation dose (p<0.001). Mechanical properties varied by material, with tensile toughness correlating negatively with increasing TVI (p<0.001, Figure 3). Discussion. AO polyethylene was developed to address the problem of free radicals in polyethylene resulting from irradiation used in cross-linking or sterilization. Each manufacturer used a different antioxidant or method of supplying the antioxidant. However, all of the antioxidant materials appeared to be effective at minimizing oxidation over the in vivo period of this study. The antioxidant materials prevented in vivo oxidation more effectively than both conventional gamma-sterilized and remelted HXL polyethylene, at least over the in vivo period represented. The toughness, or ability of the material to resist fatigue damage, decreased with increasing irradiation cross-linking dose (increasing TVI). The AO polyethylenes evaluated in this study had lower toughness than conventional gamma-sterilized polyethylene, but they avoided the loss of toughness due to remelting. Clinical relevance. Antioxidant polyethylene tibial retrievals showed superior oxidation resistance to conventional gamma-inert and remelted HXL inserts. Material toughness varied with the irradiation dose used to produce the material. Comparison of antioxidant retrieval tensile properties can be used as a guide for clinicians in choosing appropriate materials for the applications represented by their patients