Tests of biocompatibility of α-tocopherol with respect to the use as a stabilizer in ultrahigh molecular weight polyethylene for articulating surfaces in joint endoprostheses

To inhibit the oxidation in vivo of hip-cups made of ultrahigh molecular weight polyethylene (UHMW-PE), the natural antioxidant -tocopherol was added to the polymer. The added -tocopherol may however undergo chemical transformations during manufacturing and sterilization by -irradiation of hip-cups which may differ from human metabolism. Therefore, the question of the biocompatibility of the respective transformation products was investigated on test samples, which were prepared under the same conditions as applied for the production and sterilization of hip-cups. Thin plates (25×18×2 mm³) were fabricated out of test samples to investigate the cytotoxic activity according to EN 30993-5. In cytotoxicity testing, proliferation, mitochondrial activity and membrane integrity were not influenced by the material. In contrast, cell adhesion and cell spreading were diminished as shown with hemalum staining. In order to investigate the genotoxicity, the -tocopherol and its transformation products were extracted from test specimens by n-heptane at 185 °C under nitrogen atmosphere. Then the n-heptane was evaporated in vacuo and the remaining -tocopherol and its transformation products were dissolved in DMSO. The genotoxicity of this extract was then tested by the Ames-test according to DIN UA 12 (1995), which showed no indication for genotoxic activity.     

Animal experiments with ultra-high molecular weight polyethylene (UHMW-PE) stabilised with α-tocopherol used for articulating surfaces in joint endoprostheses

Numerous investigations proved the impressive suitability of α-tocopherol as a stabilizer for ultra-high molecular weight polyethylene (UHMW-PE) used for endoprostheses. Regarding the biocompatibility of this new biomaterial, in-vitro celltoxicity tests gave no hint for a cyto- or genotoxic activity. In this study, animal experiments are carried out to further ensure the biocompatibility of this biomaterial. Thin UHMW-PE-films (20 × 6 × 0.23 mm³) were implanted subcutaneously into rats. Morphology and reactivity of surrounding connective tissue against either pure UHMW-PE material or UHMW-PE containing α-tocopherol were studied at timed intervals (2 weeks, 3 month, and 6 month after operation) in 3 groups of animals, each group comprising 10 animals. Parallel to these tests, material changes were investigated in these 3 groups of 10 animals at the same intervals after operation with the help of infrared spectroscopy (FTIR). Within the implantation time, no noteworthy oxidative degradation could be observed. The amount of lost α-tocopherol due to diffusion is low enough to ensure a lifetime stabilisation of the UHMW-PE. The implants were all well tolerated and definitely encapsulated already 2 weeks after operation. Presence or absence of α-tocopherol in the implants did not evince morphological differences. Therefore, negative consequences were not manifest in the presence of α-tocopherol. In partial fulfillment of a Ph.D. (Dr.mont.) thesis at the University of Leoben     

Biocompatibility of ultra-high molecular weight polyethylene (UHMW-PE) stabilized with α-tocopherol used for joint endoprostheses assessed in vitro

Adding the natural antioxidant α-tocopherol to ultra-high molecular weight polyethylene (UHMW-PE) can remarkably delay the oxidation of hip cups made thereof. However, α-tocopherol is likely to undergo different chemical transformations during manufacturing and sterilization of hip cups than in human metabolism. Therefore, the biocompatibility of the putative transformation products has to be investigated. In-vitro tests with L929 mice fibroblast-cells gave no evidence for cytotoxicity. To further ensure the biocompatibility, in-vitro tests with human cells were carried out in this study. Two different human cell lines, one adherent cell line, HF-SAR, and one suspension culture, GSJO, were tested on UHMW-PE-tablets (diameter: 15 mm; thickness: 2 mm; processed according to standard procedures for artificial hip-cups) with and without α-tocopherol with respect to cell viability, proliferation and morphology by means of cell counting, WSt-1 proliferation assay and scanning electron microscopy. Similar proliferation rates were found with both polyethylene samples. Further, we found intact morphology in light and electron microscopy on each substrate. The morphologic characteristics of skin fibroblasts were not changed by any material. Normal adherence and spreading of the fibroblasts was found on controls of glass, as well as on polystyrene and on stabilized and unstabilized polyethylene. The characteristic behaviour as suspension of the GSJO cells remained unchanged. The mitochondrial activity, as studied by WST-1 cell proliferation reagent, was identical on each substrate during the whole observation period of 7 days. Christian Wolf—in partial fulfillment of a Ph.D. (Dr.mont.) thesis at the University of Leoben     

Stabilisation of crosslinked ultra-high molecular weight polyethylene (UHMW-PE)-acetabular components with α-tocopherol

A stabilisation of crosslinked ultra-high molecular weight polyethylene (UHMW-PE) with α-tocopherol (vitamin E) used for endoprostheses can increase its resistance against oxidative degradation remarkably. However, the method used for conventional UHMW-PE of adding α-tocopherol to the UHMW-PE powder before processing can not be applied for crosslinked UHMW-PE, since the α-tocopherol hinders the crosslinking process, which would be accompanied by a heavy degradation of this vitamin. The α-tocopherol has therefore to be added after the crosslinking process. This paper presents two methods for a stabilisation of finished products with α-tocopherol. In method 1, UHMW-PE-cubes (20 × 20 × 20 mm³) were stored in pure α-tocopherol under inert atmosphere at temperatures from 100^∘C to 150^∘C resulting in a high mass fraction of α-tocopherol in the edge zones. For further homogenisation, the cubes were stored in inert atmosphere at temperatures from 160^∘C to 200^∘C. In method 2, supercritical CO₂ was used to incorporate the vitamin into the UHMW-PE. In an autoclave vessel, the cubes were treated with α-tocopherol dissolved in supercritical CO₂ for several hours at temperatures from 100^∘C to 170^∘C. In both cases, the mass fraction of α-tocopherol was detected with the help of a FTIR-microscope. Both methods are well suited to stabilise crosslinked UHMW-PE with α-tocopherol. A stabilisation of the sensitive edge layer as well as a nearly homogenous distribution with varying α-tocopherol content may be realised by varying the process parameters. Using method 2, standard hip cups were stabilized nearly homogeneously with varying mass fraction of α-tocopherol. No oxidation of the UHMW-PE could be detected by infrared spectroscopy (FTIR) and HPLC studies showed a very low degradation of the α-tocopherol for both processes. In partial fulfilment of a Ph.D. (Dr.mont.) thesis at the University of Leoben In partial fulfilment of a diploma thesis at the University of Leoben     

Accelerated ageing experiments with crosslinked and conventional ultra-high molecular weight polyethylene (UHMW-PE) stabilised with α-tocopherol for total joint arthroplasty

Samples of untreated ultra-high molecular weight polyethylene (UHMW-PE), UHMW-PE sterilized with γ-rays in nitrogen atmosphere (conventional UHMW-PE, widely used for articulating surfaces in endoprostheses) and UHMW-PE, which has been crosslinked by electron beam irradiation and annealed subsequently, were stabilized with α-tocopherol and aged in air at 120^∘C as well as in 10% aqueous hydrogenperoxide with 0.04 mg/ml FeCl₃ as catalyst at 50^∘C. The oxidative degradation was monitored with the help of infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), solubility measurements and size exclusion chromatography (SEC) and were compared to unstabilized samples. When aged in air at 120^∘C, the crosslinked UHMW-PE showed a slightly slower increase of the carbonyl (CO)-number (according to DIN 53383) in FTIR than conventional UHMW-PE. A stabilisation with 0.4% w/w α-tocopherol resulted in an increase of lifetime by a factor of approx. 40 for all samples. Ageing in 10% aqueous H₂O₂ at 50^∘C yielded similar results for all three unstabilised samples. The addition of the natural antioxidant α-tocopherol led to a prolongation of lifetime by a factor of approx. 2.5. A linear loss of α-tocopherol was detected during ageing. An increase of crystallinity as well as lamella thickness during ageing was observed with the help of DSC. The two-phase structure of crosslinked UHMW-PE with two melting endotherms at 114^∘C and 137^∘C was replaced very quickly by a single melting point at 130^∘C. This effect was delayed with the stabilized samples. In the solubility and SEC measurements, a severe molecular degradation and drop of molar mass of all materials could be observed after ageing in H₂O₂, leading to a complete destruction and, in case of crosslinked UHMW-PE, to a serious damage of the molecular network, respectively. in partial fulfilment of a Ph.D. (Dr.mont.) thesis at the University of Leoben     

Studies on the effect of electron beam radiation on the molecular structure of ultra-high molecular weight polyethylene under the influence of α-tocopherol with respect to its application in medical implants

Ultra-high molecular weight polyethylene (UHMW-PE) is being used successfully for articulating surfaces in joint endoprostheses, especially for cups of total hip endoprostheses. Sintered specimens containing various amounts of -tocopherol (vitamin E) as a biocompatible stabilizer, were irradiated in nitrogen atmosphere as well as in air with various dosages of electron beam radiation. Size exclusion chromatography (SEC) was used to analyze the soluble fractions of the UHMW-PE samples according to their molecular weight distribution prior to and after irradiation. In nitrogen atmosphere the radiation-induced crosslinking showed to be dependent on the added amount of -tocopherol in the sintered specimens. With an increasing content of -tocopherol, the stabilizer acted as a scavenger for free radicals. Thus, the crosslinking was more and more hindered. The same effect was observed on the samples irradiated in air, where, in addition to the crosslinking process, oxidative molecular degradation occurred. The highest extent of crosslinked material was yielded with unstabilized samples in nitrogen atmosphere.     

Probing the effects of cone potential in the electrospray ion source: consequences for the determination of molecular weight distributions of synthetic polymers

Shifts in the relative intensities of oligomer ions are found to accompany changes in the cone potential in the electrospray ion source, which introduce uncertainties into average molecular weight determinations for polymer distributions. Similar shifts with changes in cone potential have long been recognized in the multiple-charge distributions of proteins and other biomolecules. In the case of multiple-charge distributions of a single, or small number of, species there are no major consequences for calculation of molecular weight; however, mass distributions and the averages thereof, are of major concern with synthetic polymers and understanding the shifts in relative intensities becomes critically important. We report here an evaluation of the effects of cone potentials on the molecular weight distributions of synthetic polymers, which we compare with the effects on charge-state distributions of peptides. The effects of cone potential have been modeled mathematically, from which we conclude that cone potentials exert a focusing effect dependent on the mass-to-charge ratios of ions. It is largely this focusing effect that determines the dependence of oligomer ion intensities upon cone potential in the ESI mass spectra of polymers. The influence of cone potential on molecular weight determinations of polymers of varying polydispersities (P(o)) is compared and discussed. For polymers with low polydispersities (e.g., narrow molecular weight poly(ethyleneglycol) standards with P(o) < 1.5), the variation in molecular weight determinations tends to be small (typically <5%), whereas with synthetic polymers with polydispersities greater than 2, variations in cone potential can influence molecular weight determinations significantly (by 100% or even more).     

Changes to the morphology,structure and properties as a consequence of polyethylene working in a polymer–metal kinematic pair

A change is presented of the orientation of lamellar structure, degree of crystallinity, the degree of the spatial arrangement of the structure, micromechanical properties, and the surface morphology and thickness of a plastically deformed upper layer. These changes are the effect of work in a polymer–metal kinematic pair, which have occurred as a result of plastic deformation of polyethylene during its service. It has been shown that, as a result of selecting proper parameters of UHMW polyethylene via the initial draft and electron-beam irradiation, such a structure of the polymer can be obtained, which will enable the above-mentioned changes in morphology and structure to take place during service. This in turn, will allow a reduction of the susceptibility of the polymer to permanent deformation by 3–6 times, and its wear by more than 5 times, compared to the initial material.     

What is the tensile strength of a ceramic to be used in numerical models for predicting crack initiation?

Criteria for predicting initiation of cracks in brittle materials like ceramics are based on two parameters: the material fracture toughness and the tensile strength. Standardized experiments exist to estimate the former. However, the tensile strength is often taken from experiments (mainly uniaxial bending) on specimens with various geometries and surface finish, usually tested under ambient conditions at a given loading rate. The reported strength is commonly the Weibull characteristic strength, which scatters due to the critical defect size distribution on the tested specimen. In this work, we propose a definition of the “inherent” or “intrinsic” tensile strength to be used in numerical models, making a distinction between extrinsic defects due to manufacturing and intrinsic ones relying on the microstructure. Our approach is based on the Finite Fracture Mechanics theory and the Coupled Criterion applied to small surface flaws and its influence on the measured (extrinsic) strength. Numerical results are compared with experiments on alumina reported in the literature. In addition, a model for the Petch law (strength vs. grain size) in polycrystalline materials is proposed using the Coupled Criterion, which predicts an initial crack length of increasing numbers of grains as the grain size decreases.     

Onset of Superconductivity and Hysteresis in Magnetic Field for a Long Cylinder as Obtained from a Self-Consistent Solution of the Ginzburg–Landau Equations

Based on the self-consistent solution of a nonlinear system of one-dimensional GL-equations, the onset and destruction of superconductivity, the phase transitions and hysteresis phenomena are discussed for a cylinder (radius R) in the axial magnetic field (H) for arbitrary R, , H, m ( is the GL-parameter, m is the total vorticity of the system). The edge-suppressed solutions (which correspond to the jumps of the magnetization in the states with fixed vorticity m), the depressed solutions (which are responsible for the hysteresis in type-II superconductors), and the precursor solutions (which describe the onset of superconductivity in type-I superconductors) are also studied. The limits of applicability of the linear equation approximation are discussed.