Introduction. Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the most common bearing surface used in total joint arthroplasty due to its excellent wear resistance. While radiation cross-linking is currently used, cross-linking using a cross-linking agent such as a peroxide can also be effective with improved oxidative stability, which can be achived by an antioxidant such as vitamin E. The peroxide cross-linking behavior of UHMWPE in the presence of vitamin E was unknown. We investigated the cross-linking behavior and the clinically relevant mechanical and wear properties of peroxide cross-linked, vitamin E-blended UHMWPE. Materials and Methods. Medical grade UHMWPE (GUR1050) was blended with vitamin E and the peroxide (2,5-Dimethyl-2,5-di(t-butylperoxy)hexyne-3 or P130) before compression molding. Various vitamin E (0.1, 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0 wt%) and peroxide concentrations (0.5, 1 and 1.5 wt%) were studied. 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. Oxidative stability was determined using oxidation induction testing (Braithwaite 2010). Double-notching and IZOD impact testing was performed according to ASTM D256. Samples prepared with vitamin E concentrations of 0.3 wt% and above and P130 concentrations of 0.5 and 1 wt% were also terminally gamma sterilized. Controls were 150-kGy irradiated vitamin E blends of UHMWPE. Results and Discussion. The cross-link density of peroxide cross-linked UHMWPEs were higher than the irradiated controls at a given vitamin E concentration (For example 250, 301 and 355 mol/dm3 for 0.5, 1 and 1.5 wt% peroxide cross-linked UHMWPE compared to 217 mol/dm3 for 150 kGy irradiated UHMWPE; Figure 1). The cross-link density dependence of wear was similar to radiation cross-linked UHMWPE, resulting in clinically relevant wear rates of 0.5 to 1.5 mg/MC. While the cross-link density of radiation cross-linked UHMWPE became saturated at vitamin E concentrations above 0.3 wt% (Oral 2008), this was not observed in peroxide cross-linked UHMWPE (Figure 2), suggesting more efficient cross-linking in the presence of the antioxidant. The impact strength was 30% higher for the peroxide cross-linked UHMWPEs at the comparable wear rate compared to irradiated controls (72 vs. 56 kJ/m2). The oxidation induction time of all peroxide cross-linked UHMWPEs (up to 57 min) was higher than that of the 0.1 wt% vitamin E-blended, 150-kGy irradiated UHMWPE (6 min). Gamma sterilization of peroxide cross-linked vitamin E blends decreased wear (0.5 wt% peroxide in Figure 3). Thus, peroxide concentration for cross-linking can be reduced if terminal sterilization is used. The mechanical properties and the oxidative stability of the material were not significantly affected by gamma sterilization. Significance. Peroxide cross-linking enabled good wear resistance for high vitamin E concentration blends of UHMWPE (>0.3 wt%), previously not possible by irradiation. Peroxide cross-linking of vitamin E-blended UHMWPE can provide a one-step, cost-effective method to manufacture wear resistant total joint implants with improved oxidative stability

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

Introduction:. Irradiated ultra-high molecular weight polyethylene (UHMWPE), used in the fabrication of joint implants, has increased wear resistance [1]. But, increased crosslinking decreases the mechanical strength of the polymer [2], thus limiting the crosslinking to the surface is desirable. Here, we usedelectron beam irradiation with low energy electrons to limit the penetration of the radiation exposure and achieve surface cross-linking. Methods:. Medical grade 0.1 wt% vitamin E blended UHMWPE (GUR1050) was consolidated and irradiated using an electron beam at 0.8 and 3 MeV to 150 kGy. Fourier Transform Infrared Spectroscopy (FTIR) was used from the surface along the depth at an average of 32 scans and a resolution of 4 cm. −1. A transvinylene index (TVI) was calculated by normalizing the absorbance at 965 cm. −1. (950–980 cm. −1. ) against 1895 cm. −1. (1850–1985 cm. −1. ). TVI in irradiated UHMWPE is linearly correlated with the radiation received [3]. Vitamin E indices were calculated as the ratio of the area under 1265 cm. −1. (1245–1275 cm. −1. ) normalized by the same. Pin-on-disc (POD) wear testing was conducted on cylindrical pins (9 mm dia., 13 mm length, n = 3) as previously described at 2 Hz [4] for 1.2 million cycles (MC). Wear rate was measured as the linear regression of gravimetric weight change vs. number of cycles from 0.5 to 1.2 MC. Double notched IZOD impact testing was performed (63.5 × 12.7 × 6.35 mm) in accordance with ASTM F648. Cubes (1 cm) from 0.1 wt% blended and 150 kGy irradiated pucks (0.8 MeV) were soaked in vitamin E at 110°C for 1 hour followed by homogenization at 130°C for 48 hours. Results:. The penetration of the electron beam for cross-linking was limited at low beam energy and cross-linking of the surface 2 mm was achieved (Fig 1). The wear rate of samples irradiated at 0.8 and 3 MeV was 1.12 ± 0.15, and 0.98 ± 0.11, respectively (p > 0.5). In addition, the wear rate of the surface (0.8 MeV) irradiated UHMWPE was 0.33 ± 0.02 mg/MC 1 mm below the surface. The impact strength of UHMWPE irradiated at 0.8 MeV was 73 kJ/m. 2. and 54.2 kJ/m. 2. for that irradiated at 3 MeV (p = 0.001). Doping with vitamin E and homogenization increased the surface vitamin E concentration from undetectable levels to 0.11 ± 0.01. Discussion:. The wear rate of this surface cross-linked UHMWPE was comparable to uniformly cross-linked UHMWPEs irradiated at higher electron beam energies. Even lower wear rate subsurface suggested the feasibility of machining 1 mm from the surface in implant fabrication. Limiting cross-linking to the surface resulted in higher impact strength compared to a uniformly cross-linked UHMWPE. Vitamin E was optionally replenished by additional doping after cross-linking; an advantage of this method may be increased oxidation resistance. Significance: Low energy irradiation of vitamin E blended UHMWPE is feasible to fabricate total joint implants with high wear resistance and impact strength

Introduction. Radiation cross-linked UHMWPE is preferred in total hip replacements due to its wear resistance [1]. In total knees, where stresses are higher, there is concern of fatigue damage [2]. Antioxidant stabilization of radiation cross-linked UHMWPE by blending vitamin E into the polymer powder was recently introduced [3]. Vitamin E greatly hinders radiation cross-linking in UHMWPE [4]. In contrast peroxide cross-linking of UHMWPE is less sensitive to vitamin E concentration [5]. In addition, exposing UHMWPE to around 300°C, increases its toughness by inducing controlled chain scission and enhanced intergranular diffusion of chains, simultaneously [6]. We present a chemically cross-linked UHMWPE with high vitamin E content and improved toughness by high temperature melting. Methods and Materials. Medical grade GUR1050 UHMWPE was blended with vitamin E and with 2,5-Di(tert-butylperoxy)-2,5-dimethyl-3-hexyne or P130 (0.5% Vitamin-E and 0.9% P130). The mixed powder was consolidated into pucks. The pucks were melted for 5 hours in nitrogen at 300, 310 and 320°C. One set of pucks melted at 310°C was accelerated aged at 70°C at 5 atm. oxygen for 2 weeks. Tensile mechanical properties were determined using ASTM D638. Izod impact toughness was determined using ASTM D256 and F648. Wear rate was determined using a bidirectional pin-on-disc (POD) tester with cylindrical pins of UHMWPE against polished CoCr discs in undiluted, preserved bovine serum. Results. The vinyl index increased as a function of temperature (Fig 1a). Cross-link density steadily decreased and impact strength increased with increasing vinyl index (Fig 1b). The ultimate tensile strength (UTS) was not affected by HTM (Table 2). Impact strength was significantly improved for all treatment temperatures (P<0.05) and wear was significantly increased only for the sample melted at 320°C (Table 2). Discussion. High temperature melting (HTM) was shown to increase toughness of UHMWPEs presumably due to controlled chain scissioning and increased intergranular diffusion of chains [6]. For radiation cross-linked UHMWPE, it was shown that an increase in elongation-at-break and impact strength could be obtained without sacrificing wear resistance up to an elongation of about 500% [7]. This vitamin E-blended, peroxide cross-linked, high temperature melted UHMWPE has very high oxidation resistance due to its high antioxidant content, high wear resistance due to cross-linking and much improved toughness, representing an optimum joint replacement surface. For figures/tables, please contact authors directly.

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

Artificial knee joints are continuously loaded by higher contact stress than artificial hip joints due to a less conformity and much smaller contact area between the femoral and tibial surfaces. The higher contact stress causes severe surface damage such as pitting or delamination of polyethylene (PE) tibial inserts. To decrease the risks of these surface damages, the oxidation degradation of cross-linked polyethylene (PE) induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization should be prevented. Vitamin E (VE), as an antioxidant, blended PE (PE(VE)) has been used to solve the problems. In addition, osteolysis induced by PE wear particles, bone cement and metallic debris is recognized as one of the important problems for total knee arthroplasty (TKA). To decrease the generation of PE wear particles, we have developed the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface having high wear resistance. In this study, we have evaluated the surface, mechanical under severe oxidative condition, and wear properties of PMPC-grafted cross-linked PE(VE) (PMPC-CLPE(VE)) material for artificial knee joints. Untreated and PMPC-grafted 0.1 mass% VE-blended PE (GUR1020E resin) with a gamma-ray irradiation of 100 kGy for cross-linking and 25 kGy for sterilization were prepared (CLPE(VE) and PMPC-CLPE(VE), respectively). Surface properties were evaluated by Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscope (TEM) observations. Surface wettability and frictional property were measured by static water contact angle measurement and ball-on-plate friction test. To evaluate the oxidation degradation resistance, mechanical and physical properties such tensile test, izod impact test, small punch test and cross-link density measurement before and after accelerated aging were measured. Wear properties of the tibial inserts were examined by using knee simulator in the combination of Co-Cr-Mo femoral components according to ISO14243-3. Gravimetric wear, volumetric penetration and the number of generated wear particles were measured. By the FT-IR measurements and TEM observation, P–O peaks attributed to MPC unit and uniform PMPC layer with 100–200 nm thick was observed only on PMPC-CLPE(VE) surface. Static water contact angle of CLPE(VE) was almost 100 degree, while that of PMPC-CLPE(VE) decreased significantly to almost 35 degree. There was no significant difference in the mechanical and physical properties between CLPE(VE) and PMPC-CLPE(VE). Moreover, both the CLPE(VE) and PMPC-CLPE(VE) maintained these properties even after the accelerated aging of 12 weeks [Fig. 1]. Blended VE in CLPE would act as radical scavengers to prevent oxidation degradation. In the knee simulator wear test, the PMPC-CLPE(VE) tibial inserts showed about a half gravimetric wear compared to the CLPE(VE) tibial inserts [Fig. 2]. This would be due to the significant differences observed in wettability of the surface. Water thin film formed on the hydrated PMPC graft layer, would act as significantly efficient lubricant. From these results, the PMPC-CLPE(VE) is expected to be one of the great bearing materials not only preventing surface damages due to higher contact stress and oxidation degradation but also improving wear resistance, and to provide much more lifelong artificial knee joints. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Cellular therapies play an important role in tendon tissue engineering with tenocytes being described as the most prominent cell population if available in large numbers. However, in vitro expansion of tenocytes in standard culture leads to phenotypic drift and cellular senescence. Recent work suggests that maintenance of tenogenic phenotype in vitro can be achieved by recapitulating different aspects of the native tendon microenvironment. One approach used to modulate the in vitro microenvironment and enhance extracellular matrix (ECM) deposition is macromolecular crowding (MMC). MMC is based on the addition of inert macromolecules to the culture media mimicking the dense extracellular matrix. In addition, as tendon has been described to be a relatively avascular and hypoxic tissue and low oxygen tension can stimulate collagen synthesis and cross-linking, we venture to assess the synergistic effect of MMC and low oxygen tension on human tenocyte phenotype maintenance by enhancing synthesis and deposition of tissue-specific ECM. Human tendons were kindly provided from University Hospital Galway, after obtaining appropriate licenses, ethical approvals and patient consent. Afterwards, tenocytes were extracted using the migration method. Experiments were conducted at passage three. Optimization of MMC conditions was assessed using 50 to 500 μg/ml carrageenan (Sigma Aldrich, UK). For variable oxygen tension cultures, tenocytes were incubated in a Coy Lab (USA) hypoxia chamber. ECM synthesis and deposition were assessed using SDS-PAGE (BioRad, UK) and immunocytochemistry (ABCAM, UK) analysis. Protein analysis for Scleraxis (ABCAM, UK) was performed using western blot. Gene analysis was conducted using a gene array (Roche, Ireland). Cell morphology was assessed using bright-field microscopy. All experiments were performed at least in triplicate. MINITAB (version 16, Minitab, Inc.) was used for statistical analysis. Two-sample t-test for pairwise comparisons and ANOVA for multiple comparisons were conducted. SDS-PAGE and immunocytochemistry analysis demonstrated that human tenocytes treated with the optimal MMC concentration at 2% oxygen tension showed increased synthesis and deposition of collagen type I, the major component of tendon ECM. Moreover, immunocytochemistry for the tendon-specific ECM proteins collagen type III, V, VI and fibronectin illustrated enhanced deposition when cells were treated with MMC at 2% oxygen tension. In addition, protein analysis revealed elevated dexpression of the tendon-specific protein Sclearaxis, while a detailed gene analysis revealed upregulation of tendon-related genes and downregulation of trans-differentiation markers again when cells cultured with MMC at 2% oxygen tension. Finally, low oxygen tension and MMC did not affect the metabolic activity, proliferation and viability of human tenocytes. Collectively, results suggest that the synergistic effect of MMC and low oxygen tension can accelerate the formation of ECM-rich substitutes, which stimulates tenogenic phenotype maintenance. Currently, the addition of substrate aligned topography together with MMC and hypoxia is being investigated in this multifactorial study for the development of an implantable device for tendon regeneration

Adherent cells are known to respond to physical characteristics of their surrounding microenvironment, adapting their cytoskeleton and initiating signaling cascades specific to the type of cue encountered. Scaffolds mimicking native biophysical cues have proven to differentiate stem cells towards tissue-specific lineages and to maintain the phenotype of somatic cells for longer periods of culture time. Although the characteristic anisotropy of tendon tissue is commonly replicated in scaffolds, relevant physical cues such as tendon rigidity or mechanical loading are often neglected. The objective of this study is to use tendons' main extracellular matrix component, collagen type I, to create scaffolds with an anisotropic surface topography and controlled rigidity, in an effort to engineer functional tendon tissue equivalents, with native organization and strength. Porcine collagen type I in solution was treated with one of the following cross-linkers: glutaraldehyde, genipin or 4-arm polyethylene glycol (4SP). The resulting mixture was poured on micro-grooved (2×2×2 μm) or planar polydimethylsiloxane (PDMS) molds and dried in a laminar flow hood to obtain 5 mg/ml collagen films. Surface topography and elastic modulus of the final scaffolds were analyzed using SEM/AFM and rheometry, respectively. Human tendon cells were isolated from adult tendon tissue and cultured on micro-grooved/planar scaffolds for 4, 7 and 10 days. Cell morphology, collagen III and tenascin C expression were analyzed by immunocytochemistry. Among the different cross-linkers used, only the treatment with 4SP resulted in scaffolds with a recognizable micro-grooved surface topography. Precise control over the micro-grooved topography and the rigidity of the scaffolds was achieved by cross-linking the collagen with varying concentrations of 4SP at low pH and temperature. The elastic modulus of the scaffolds cross-linked with the highest concentration of 4SP matched the physiological values reported in developing tendons (∼15 kPa). Around eighty percent of the human tendon cells cultured on the cross-linked collagen films aligned in the direction of the anisotropy for 10 days in culture. At 4 days, tenoyctes cultured on micro-grooved substrates presented a significant higher nuclei aspect ratio than tenocytes cultured on planar substrates for all the 4SP concentrations. Synthesis, deposition and alignment of collagen III and tenascin C, two important tenogenic markers, were up regulated selectively in the rigid micro-grooved scaffolds after 7 days in culture. These results highlight the synergistic effect of matrix rigidity and cell alignment on tenogenic cell lineage commitment. Collectively, this study provides new insights into how collagen can be modulated to create scaffolds with precise imprinted topographies and controlled rigidities. Gene expression analysis and a replicate study with hBMSCs will be carried out to support the first results and to further identify the optimal biophysical conditions for tenogenic cell lineage commitment. This potentially leads to the design of smart implants that not only restore immediate tendon functionality but also provide microscopic cues that drive cellular synthesis of organized tissue-specific matrix

Introduction. Vitamin-E (VE)-blended UHMWPE has been developed as a bearing-surface material due to the antioxidant ability of VE and has demonstrated a low wear rate in knee simulator [1]. Additionally, in vitro biological response testing has revealed that wear particles from VE blended UHMWPE induce the secretion of inflammatory cytokines at significantly lower levels compared to conventional UHMWPE [2]. However, as the joint kinematics are different between the knee and the hip, it is not guaranteed that these improvements will be repeated in the hip. In this study, the wear resistance of VE-blended UHMWPE was evaluated in knee and hip simulator tests and the effects of VE concentration and electron-beam irradiation were investigated. Materials and Methods. VE blended samples (GUR_VE xx%) were manufactured via direct compression molding following the blending of UHMWPE resin powder with VE at several concentrations (0, 0.1, 0.3, 1.0%). Cross-linking for the VE samples was achieved by 10 MeV electron beam at several irradiance doses (30, 90, 300 kGy) and annealed below the melting point of UHMWPE. Knee and hip simulator testing were carried out according to ISO 14243 and ISO 14242, respectively, and the volumetric wear was calculated. The gel fraction was determined by measuring the weight of the samples before and after soaking in decahydronaphthalene at 150°C. The oxidative resistance of the material was determined by measuring the Oxidation Index (OI) following ASTM F2102 before and after compulsory aging (ASTM2003). Radical measurements were made using high-sensitivity X-band ESR. Results & Discussion. In knee simulator testing, the volumetric wear for GUR_VE was lower than that for non-blended UHMWPE (GUR), with this difference amplified by aging. Conversely, in hip simulator testing, the wear rates were the same for the GUR and GUR_VE samples, while the GUR_XL samples showed almost no wear both before and after aging. Additionally, the wear for the non-XL samples (GUR and GUR_VE) actually decreased after aging. Treatment utilizing electron-beam irradiation and annealing below the melting point achieved a high degree of cross-linking in VE-blended UHMWPE, while also preserving the antioxidant ability of VE. These results suggest that electron-beam irradiated VE blended UHMWPE can be used as a bearing surface material for hip prostheses. However, the longevity of prosthesis is determined not only by its wear performance, but also by its biocompatibility. ESR measurements have revealed that VE radicals are formed during radiation crosslinking of VE blended UHMWPE, and it is thought that the VE radicals may reduce the anti-inflammatory effects of UHMWPE particles containing VE. We are now developing a multidirectional lift-off type aseptic wear simulation device and procedures to measure the biological response to wear particles produced therein [3]. Acknowledgement. A part of this study was supported by the “Super special consortia” for supporting the development of cutting edge medical care (Cabinet office, Government of Japan) and Innovation promotion program (NEDO)

Introduction. Periprosthetic osteolysis is considered the main problem limiting the longevity and clinical success of artificial hip joints. Aiming at the reduction of the wear particles and the elimination of periprosthetic osteolysis, we have recently developed a novel articular cartilage-inspired technology for surface modification (Aquala® technology) with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) grafting (100–150 nm in thickness) for an acetabular liner in an artificial hip joint. Our previous study on the mechanical and biological effects of PMPC revealed that the grafting decreased the production of wear particles and the bone resorptive responses. However, as well as wear-resistance, oxidation is an important indicator of the clinical performance of acetabular liners. The incorporation of the antioxidant vitamin E has been proposed recently as an alternative to post melting treatment after gamma-ray irradiation to avoid oxidation. The purpose of this study is to investigate the effects of substrate materials, vitamin E-blended cross-linked polyethylene (CLPE), on the oxidative stability and wear resistance of the PMPC-grafted CLPE liner for artificial hip joints. Materials & Methods. Vitamin E-blended (0.1 mass%) PE sheet stock was irradiated with a high dose of gamma-rays (100–150 kGy) and annealed for cross-linking (HD–CLPE+E). PMPC grafting onto the HD–CLPE+E liners was performed by a photoinduced polymerization technique. Then, the PMPC-grafted HD-CLPE+E was sterilized by gamma-ray with a dose of 25 kGy. A CLPE with 50 kGy gamma-ray irradiation and 25 kGy gamma-ray sterilization was used as control. Surface properties and oxidative properties of the liners were examined. The wear test was performed using a 12-station hip joint simulator according to the ISO 14242-3. A 26-mm Co-Cr-Mo alloy femoral head component was used for the tests. Results. After PMPC grafting, the peaks ascribed to the MPC unit were clearly observed in both Fourier-transform infrared and X-ray photoelectron spectroscopy spectra. Furthermore, PMPC-grafted CLPE and HD-CLPE+E surface became wettable drastically. Oxidation-induction time of PMPC-grafted HD-CLPE+E was significantly longer compared with non-additive CLPE. After 5.0 million cycles of the simulator test, PMPC-grafted HD–CLPE+E were found to show extremely low and stable wear. Substantially fewer wear particles isolated from lubricants were found for both PMPC-grafted liners than for untreated CLPE liners. Discussion. In this study, we confirmed that the PMPC-grafted layer was successfully fabricated on the HD-CLPE+E surface, and the PMPC-grafted HD-CLPE+E brought high oxidation and wear-resistances. When the surface is modified by PMPC grafting, the PMPC-grafted layer leads to a significant reduction in the sliding friction between the surfaces which are grafted because water thin films formed can act as extremely efficient lubricants. Based on clinical trials and other related evidence, the Japanese government (Ministry of Health, Labour and Welfare) approved the clinical use of PMPC-grafted CLPE without vitamin E acetabular liners in April 2011. Furthermore, and in spite of high-dose gamma-ray irradiation for cross-linking, the substrate modified by vitamin E-blending maintains high oxidation-resistance. Indeed vitamin E is an extremely efficient radical scavenger. Conclusion. In conclusion, the PMPC-grafted HD–CLPE+E provides not only high wear resistance but also high oxidation stability, i.e., life-long durability

INTRODUCTION. Since the early 2000s, highly cross-linked (HXL) UHMWPE's have become a popular option with multiple experimental and clinical studies showing that gamma or electron radiation doses between 50–100kGY reduce wear and potentially extend the bearing life of UHMWPE. However, the increased wear resistance came at a compromise to mechanical properties due to the cross-linking process. Vitamin E has been added to some HXL UHMWPE materials to offer a solution to the compromise by increasing oxidation resistance and maintaining sufficient fatigue strength. However, limited data is available on the effect of the fabrication process, especially the method of irradiation, on the properties of the Vitamin E blended HXL UHMWPE. The purpose of this study was to evaluate the effects of adding the antioxidant vitamin E to highly crosslinked UHMWPE on wear rates. METHODS. Wear testing was performed on six highly crosslinked UHMWPE acetabular liners containing vitamin E (0.1% wt. alpha tocopherol) fabricated using the Cold Irradiation Mechanically Annealed (CIMA) process, initially cross-linked with approximately 100 kGy gamma irradiation, and terminally gamma sterilized. The liners were paired with three 40mm CoCr femoral heads and 40mm three ceramic femoral heads. Testing was completed per ASTM F1714 and ISO 14242 on an orbital hip joint wear simulator (Shore Western, California) and lubricated with 90% bovine calf serum, 20mM EDTA, 0.2% wt. NaN. 3. and DI water. A 1.1Hz Paul-type loading waveform with a peak of 2kN was used for a total of 5E6 wear cycles. Three loaded soak controls were used in parallel to adjust for fluid absorption. Samples were weighed every 5E5 wear cycles. RESULTS. The wear rates for the HXL blended vitamin-E liners were calculated using the slope of the linear regression over the steady state and resulted in a wear rate of 0.49mg/Mc. This is a decrease of approximately 95% compared to the 9.54 mg/Mc 28mm ID conventional UHMWPE wear rates as well as a notable difference for the other HXL UHMWPE liner wear rates discussed in the review. DISCUSSION. HXL blended vitamin-E 40mm liners demonstrated an approximate 95% reduction in wear rates compared to a 28mm ID conventional UHMWPE. The reduced wear rate confirmed the design expectation that a higher irradiation dose in the fabrication process resulted in an increased amount of polymer crosslinking. Additionally, the wear rate of the HXL blended vitamin-E liners studied was well below 20mg/Mc, which was shown by Dowd et al. to be the threshold of osteolysis in THA. SIGNIFICANCE. The HXL vitamin E blended UHMWPE liner tested in this study demonstrated reduced wear rates by approximately 95% compared to conventional polyethylene. Osteolysis-causing wear debris is reduced while maintaining other mechanical properties. This liner material and manufacturing process is a promising alternative to conventional polyethylene, but long-term clinical results are warranted

The aim of our project is to develop resorbable nanostructured composite layer with controlled elution of antibiotics for implants survival rate enhancement. The nanostructured layers are expected to be used especially in the case of known systemic or local (joint) inflammation. This layer can provide a bone tissue/implant (titanium alloy) bioactive interface improving the physiological healing process and eliminating the risk of bacterial orthopedic infections. The main aim of this study was to verify whether the local concentration of released vancomycin exceeded the minimum inhibitory concentration (MIC) for vancomycin-resistant Staphylococcus aureus (VRSA, >16 mg/l). The layer is composed of collagen (type I, isolated form calf skin), hydroxyapatite nanoparticles and vancomycin hydrochloride (10 wt%). The stability of collagen was enhanced by EDC/NHS cross-linking. The in vitro release of vancomycin and crystalline degradation products from optimally cross-linked layers was investigated. An elution method and a high performance liquid chromatographic assay were employed to characterize the in vitro release rates of the vancomycin and its crystalline degradation antibacterial inactive products over a 21-day period. During the whole experimental period, the level of released vancomycin was high above the MIC for VRSA. The maximum average concentration was obtained between day 4 and day 8 and it reached 265 mg/l. At the end of the experiment (day 21), an average concentration of 104 mg/l was detected. Our study confirmed the prophylactic effects of studied vancomycin-loaded nanostructured layers

Introduction. Radiation cross-linking of ultrahigh molecular weight polyethylene (UHMWPE) has reduced the in vivo wear and osteolysis associated with bearing surface wear (1), significantly reducing revisions associated with this complication (2). Currently, one of the major and most morbid complications of joint arthroplasty is peri-prosthetic infection (3). In this presentation, we will present the guiding principles in using the UHMWPE bearing surface as a delivery device for therapeutic agents and specifically antibiotics. We will also demonstrate efficacy in a clinically relevant intra-articular model. Materials and Methods. Medical grade UHMWPE was molded together with vancomycin at 2, 4, 6, 8, 10 and 14 wt%. Tensile mechanical testing and impact testing were performed to determine the effect of drug content on mechanical properties. Elution of the drug was performed in phosphate buffered saline (PBS) for up to 8 weeks and the detection of the drug in PBS was done by UV-Vis spectroscopy. A combination of vancomycin and rifampin in UHMWPE was developed to address chronic infection and layered construct containing 1 mm-thick drug-containing UHMWPE in the non-load bearing regions was developed for delivery. In a lapine (rabbit) intra-articular model (n=6 each), two plug of the layered UHMWPE construct were placed in the trochlear grove of the rabbit femoral surface and a porous titanium rod with a pre-grown biofilm of bioluminescent S. Aureus was implanted in the tibia. Bioluminescent imaging was employed to visualize and quantify the presence of the bacteria up to 3 weeks. Results and Discussion. Increasing drug content decreased both the ultimate tensile strength (UTS) and the impact toughness of vancomycin-containing UHMWPE (Figure 1). Elution data and structural analysis suggested that a percolation threshold was reached at above 6 wt% drug in UHMWPE, which resulted in sustained drug delivery above the minimum inhibitory concentration (MIC; 1 mg/ml) for up to 8 weeks (Figure 2). The layered constructs implanted in rabbits were able to eradicate all detectable bacteria from the biofilm on the titanium surfaces implanted on the counterface (Figure 3), suggesting clinically relevant efficacy. Significance. To our knowledge, this is the first study showing the design and efficacy of an antibiotic-eluting UHMWPE bearing surface. Such a device has the potential of reducing all two-stage revisions to single-stage treatment with load-bearing components, enhancing the mobility and quality of life for the patients and reducing the cost of infection treatment in arthroplasty. For figures/tables, please contact authors directly.

Introduction. Revision for instability has supplanted revision for aseptic loosening and revision for osteolysis since the advent of improved polyethylene inserts with changes in both sterilization techniques and cross-linking. Having the ability to judiciously choose a higher level of constraint may be beneficial in complex primary total knee arthroplasty (TKA) scenarios which can not be balanced through traditional surgical methods. The purpose of this work was to investigate short term outcomes and survivorship in cases where a greater stabilizing insert was used with a posterior stabalizing (PS) femur to address instability in flexion or extension. Methods. Two high volume TKA centers retrospectively reviewed cases in which a greater stabilizer insert was used with a primary PS knee system. The studied insert had +/− 2 degrees of varus-valgus coronal restraint as opposed the standard with no coronal constraint. The study inserts had 7 degrees of transverse plane rotational freedom. The inserts were used when extension balance was not achieved despite the usual soft tissue releases and a thicker insert resulted in a flexion contracture statically during the procedure. This situation typically occurred in the following patient groups: valgus knees with medial collateral (MCL) stretching, iatrogenic MCL injury, varus knees with lateral ligament complex stretching, the “double-varus” knee, and patients with a previous high tibial osteotomy. Intra-operatively patients were taken through a range of motion and trial implants were then placed. A cruciate retaining trial insert was then used to assess stability so that a true assessment could be made of ligament balance. Bone cuts were checked before ligament release. The usual releases were then performed to achieve balance including subperiosteal releases medially and laterally and pie-crusting when indicated. Repeat trial reductions were then performed once the final implants were cemented in place again using the cruciate retaining insert. If the soft tissue releases did not achieve balance and a thicker insert resulted in a flexion contracture then the greater stabilizer insert was selected over the PS insert. Knee Society Score and plain radiographs were collected at pre-op, 2 year and 5 year follow-up. Results. One hundred seventy two cases with 2 year minimum follow-up and 41 cases with 5 year minimum follow up were assessed. All patients had good to excellent Knee Society Scores with good range of motion and pain relief. There were no aseptic revisions of the TKA's over this period. Specifically, there were no revisions for loosening, osteolysis, instability, or post breakage. Conclusions. A more stabilized tibial post insert which provides valgus-varus constraint but permits rotational freedom may provide needed stability in select primary situations without predisposing to early post failure or implant loosening or lysis

Introduction. We have previously demonstrated that peroxide crosslinked vitamin E-blended UHMWPE maintains its clinically-required wear and mechanical properties [1]. This material can potentially be used as an irradiation-free bearing surface for TJA. However, using organic peroxides in medical devices requires a thorough examination of tissues in contact with the implant. For this study we crosslinked polyethylene using five times the needed concentration of peroxide (2,5-Dimethyl-2,5-di(t-butylperoxy)-hexyne-3 or P130), followed by implantation to determine implant biocompatibility, and pre and post implant peroxide residual contents. Methods. The study was performed after institutional approval following ISO standard 10993–6. Study groups: not crosslinked (0.2 (1050) VE), crosslinked (0.2 VE (1050)/5% P130) and crosslinked-high temperature melted (HTM) (0.2 VE (1050)/5% P130). Materials were blended and consolidated, machined (2.5 diameter × 2.5 cm height), sterilized and implanted in the dorsum New Zealand white rabbits. Pre and post implantation FTIR was performed. Two samples were implanted in each rabbit; n=6 samples were included for each group. After 4 weeks, samples were explanted, analyzed using FTIR, and subcutaneous tissues processed for histological analysis. Results. FTIR absorbances at 914cm. −1. , 1169cm. −1. , and the OH absorbance at 3450cm. −1. showed differences between materials (Fig 1A). There was a significant increase in the absorbance at 914 for the non-crosslinked and crosslinked samples after explantation (p = 2.77E–17, p = 4.22E–23, Fig 1B). There was a significant decrease in all peroxide related absorbances after explantation for the crosslinked and HTM samples (p < 0.05, Fig 1B). Before implantation, these absorbances were significantly higher in the crosslinked and crosslinked/HTM samples than those in the non crosslinked sample (p<0.05, Fig 2A). Peroxide related absorbances of the crosslinked sample were also significantly higher than those of the crosslinked/HTM sample (p<0.05, Fig 2A). After explantation, the crosslinked samples had significantly higher absorbances than both the non crosslinked and crosslinked/HTM samples (p < 0.05, Fig 2A). All peroxide related absorbances of the crosslinked/HTM samples were significantly higher than those of the non crosslinked sample (p < 0.05, Fig 2A). The non crosslinked sample showed no significant differential between these absorbances at implantation and after retrieval. The crosslinked sample had the largest differential between the total peak absorbances before implantation and retrieval at 914cm. −1. The crosslinked/HTM samples had the largest differential between the total peak absorbances before and after implantation for both 1169cm. −1. and the OH absorbances (Fig 2B). All explants were recovered after four weeks in vivo (Fig 3A). No difference was found in the histological analysis of the tissue characterized by a synovial-like lining with signs of fibrosis around the implants (Fig 3B). Discussion. The main challenge of this study was identifying pre and postoperative implant peroxide residual peaks via FTIR. We wanted to ensure that peroxide was present in implants before implantation, to ensure their elution into tissues. Conclusions. Peroxide crosslinked polyethylene stabilized with vitamin E can potentially be used as an alternate bearing surface. Irradiation-free processing could result in cost-effectiveness and more accurate cross-linking of polyethylene implants

Introduction. Vitamin-E (VE, dl-α-tocopherol) is a powerful antioxidant for highly cross-linked polyethylene (XLPE). It was previously reported that VE-stabilized XLPE succeeded in retaining no measurable oxidation even after accelerated aging tests combined with cyclic loading or lipid absorption. Thus, VE-stabilized XLPE is nowadays recognized worldwide as one of the new standard materials in total hip arthroplasty (THA). However, the effects of such VE addition on physical behavior of polyethylene remain to be fully elucidated by contrast to the clear statement of its chemical role (i.e., the enhanced oxidation resistance) in the published literature. In this presentation, we shall attempt to provide those missing notations and to explore the microstructural and biomechanical role of VE in XLPE acetabular liner on the molecular scale. Methods. The two different types of XLPE acetabular liners, VE-blended and VE-free (no VE-blended) component (n=3 for each sample), were investigated by means of laser-scanning confocal polarized micro-Raman spectroscopy. In both components, the cross-linking was achieved by electron-beam irradiation with a total dose of 300kGy in vacuum. Raman spectroscopy offers non-destructive, contactless, and high-resolution analyses of polymer morphologies. In this study, we performed an in-depth profiling of crystalline and non-crystalline phase (i.e., amorphous and intermediate phase between crystalline and amorphous regions) percentages and degree of molecular orientation in the above two liners before and after introducing the 10% plastic deformation via uniaxial compression loading at room temperature. These results were also compared to the morphological analyses under the same compression conditions performed on the virgin conventional polyethylene (Virgin liner) without radiation crosslinking as well as VE blending. Results. In the deformed state, Virgin and VE-blended liner showed a pronounced development of the surface crystalline texture. On the other hand, deformation-induced texturing occurred at much less extent in VE-free liner. According to the results of phase percentages, there was no crystallinity change in VE-blended liner by contrast to the marked increase of crystallinity in Virgin and VE-free liner after compression deformation. Alternatively, amorphous-to-intermediate phase transition was confirmed in VE-blended liner. Discussion/Conclusion. We found molecular rearrangement and phase transitions in crystalline and non-crystalline phase as a reconstruction process after plastic deformation in the investigated samples, which can be deeply related to their wear and mechanical properties. The morphological comparisons between Virgin and VE-free liner suggested that the intermolecular cross-linked networks in polyethylene highly restricted the molecular chain mobility as evidenced by few texture evolutions in VE-free liner. On the other hand, the comparisons between VE-free/-blended liner indicated that the presence of VE might promote molecular chain mobility even in the cross-linked structure, resulting in the significant surface texturing. These physical and structural aspects of VE blending would imply the possibility of the increased micromechanical wear through the strain-softening and weakening phenomena due to the molecular reorientation during in-vivo service. However, in other words, wear resistance of VE-blended liner might be further maximized by the more rigid control of molecular movements

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

To prevent aseptic loosening resulting from osteolysis induced by polyethylene (PE) wear particles in THA, it is necessary to develop a high wear-resistance bearing material. We have investigated the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface. High wear-resistance of PMPC-grafted surface has been revealed in the hip simulator wear test of 20 million cycles. Additionaly, in THA, oxidation degradation induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization is also regarded as serious issue. Recently, gas plasma (GP) sterilization has been used as a less residual radical sterilization method. In this study, we ask a question: the GP sterilization would affect to PMPC surface and/or PE substrate? Hence, we investigated surface chemical, wear, mechanical, physical and oxidation properties of GP sterilized PMPC-grafted highly cross-linked PE (CLPE). GP-sterilized CLPE and PMPC-grafted CLPE (CLPE (GP) and PMPC-CLPE (GP), respectively; GUR 1020 resin, 75 kGy irradiation), and 25 kGy-gamma-sterilized PMPC-grafted CLPE (PMPC-CLPE (g); GUR 1020 resin, 50 kGy irradiation) were evaluated. Surface property of PMPC layer was evaluated by X-ray photoelectron spectroscopy (XPS), fourier-transform infrared (FT-IR) spectroscopy, fluorescence microscope and cross-sectional transmission electron microscope (TEM) observations. Wettability and lubrication of the PMPC-CLPE surface were evaluated by static water contact angle measurement and ball-on-plate friction test, respectively. Wear properties of the acetabular cups were examined by using hip simulator in the combination with Co-Cr-Mo femoral heads. To evaluate the GP sterilization effect to the CLPE substrate, tensile test, izod impact test, small punch test, gel content, residual radical concentration and oxidation degradation were conducted. Oxidation degradation was evaluated as oxidation index by using a FT-IR spectroscopy. By the XPS and FT-IR measurements, phosphorus peak and P-O peak attributed to grafted PMPC were observed, respectively. Uniform PMPC layer (100–200 nm thick) was observed on both surfaces of PMPC-CLPE (g) and PMPC-CLPE (GP) [Fig. 1]. Water contact angle of CLPE (GP) was almost 100 degree, while those for PMPC-CLPE (g) and PMPC-CLPE (GP) decreased dramatically to almost 10 degree. Dynamic coefficient of friction of PMPC-CLPE (g) and PMPC-CLPE (GP) was lower than that for CLPE (GP). In the hip simulator wear test, PMPC-CLPE (g) and PMPC-CLPE (GP) cups showed significantly lower amount of wear than that of CLPE (GP) [Fig. 2]. The number of the wear particles was extremely less in PMPC-CLPE (g) and PMPC-CLPE (GP), though the size was not different of all cases. Water thin film might be formed at the grafted PMPC layer, which acted as significantly efficient lubricant. There was no difference in the mechanical and physical properties among three groups. Oxidation index for PMPC-CLPE (GP) after acceleration of aging was lower than that of PMPC-CLPE (g). The GP sterilization might affect only to the PMPC-grafted surface, whereas gamma irradiation affects also to the PE substrate. From these results, the PMPC-CLPE (GP) is expected to be one of the great bearing materials having not only high-wear resistance but also high-oxidation resistance, which could give further longevity of implantation

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

Introduction. Osteolysis caused by wear of the ultrahigh molecular weight polyethylene (UHMWPE) often leads to failure. Cross-linking improves wear, but also produces residual free radicals that decrease oxidative stability. In vitro studies have shown that the anti-oxidative properties of vitamin E UHMWPE stabilize free radicals while retaining the physical and chemical properties of UHMWPE. The porous surface of the Regenerex™ shell was developed for improved bone in-growth fixation. The increased porosity of the Regenerex™ shell promotes early bony in-growth with the goal of greater long-term stability. The purpose of this study was to evaluate vitamin E infused polyethylene (VEPE) wear and stability of acetabular and femoral components using RSA. Methods. 58 patients (64 observed hips), all with osteoarthritis, gave informed consent to participate in a 5 year RSA study. Each patient received a VEPE liner, a Regenerex™ acetabular shell, and an uncemented stem with either a 32mm or 36 mm cobalt chrome femoral head. Tantalum beads were inserted into the VEPE, the pelvic and the femoral bone to measure head penetration into the polyethylene, and shell and stem stability over time, using RSA. RSA radiographs were scheduled immediately postoperatively (up to 6 weeks) and 6 months, 1, 2, 3, and 5 years after surgery. The Wilcoxon signed-ranks nonparametric test was used to determine if changes in penetration or migration were significant over time at p≤0.05. Results. 58 hips have been followed for 6 months, 55 at 1 year, 52 at 2 years, 47 at 3 years and 18 at 5 years. The 36mm cohort data is not reported at this time due to insufficient follow-up. The median± standard error (SE) superior head penetration into the polyethylene was 0.05±0.01mm at 2 years, 0.05±0.01mm at 3 years, and 0.05±0.02 mm at 5 years. The acetabular components had a median± SE cup translation in the proximal direction of 0.09±0.03mm at 2 years, 0.04±0.04mm at 3 years, and 0.06±0.06mm at 5 years. The median± SE cup rotation was −0.09±0.16 degrees at 2 years, −0.02±0.15 degrees at 3 years, and 0.30±0.20 degrees at 5 years. There was a statistically significant difference in cup rotation between the 6 month and 1 year intervals (p=0.007), but no significant differences in translation or head penetration. The median± SE stem distal migration was 0.08±0.07mm at 2 years, 0.05±0.23mm at 3 years, and 0.02±0.17mm at 5 years, with a significant difference between the 6 month and 3 year intervals (p=0.029). Discussion. The VEPE liners show low head penetration at 5 years. The early head penetration, probably due to creep, is substantially lower relative to that reported for non-vitamin E stabilized UHMWPE measured by similar techniques. At 5 years, all acetabular components were stable, with the early significant difference in rotation at 1 year likely due to early settling of the cup. This study documents the longest-term evaluation of in vivo wear performance of vitamin E stabilized UHMWPE. The stability of the Regenerex™ shell and femoral stem shows promise for long-term survivorship