Summary

Prosthetic UHMWPE added with vitamin E and crosslinked UHMWPE are able to decrease significantly the adhesion of various bacterial and fungal strains limiting biomaterial associated infection and consequent implant failure.

Recent findings about UHMWPE oxidation from in vivo stresses lead to the need of a better understanding of which anti-oxidant additivation method is the best option for the use in orthopaedic field. A GUR 1050 crosslinked Vitamin E - blended UHMWPE has been investigated, to provide an accurate outline of its properties.

DSC and FTIR measurements, together with ageing and tensile tests were performed on compression moulded blocks, as well as biocompatibility tests, including implantation on rabbits. Moreover, wear simulations on finished components (Delta acetabular liners) have been completed.

All the test procedures have been repeated for a reference material, a GUR 1050 crosslinked and remelted standard UHMWPE (commercial name UHMWPE X-Lima), and the outcomes have been compared to the crosslinked Vitamin E - blended UHMWPE ones.

On the additivated UHMWPE, we found a ultimate tensile strength of 43 MPa, a yield strength value of 25 MPa, and an elongation to breakage equal to 320%. The degree of cristallinity was 45 ± 2%, and no signal of creation of oxidation products was detected up to 2000 h of permanence in oxidant ambient after the ageing test. The reference material showed comparable mechanical resistance values (∗ = 40 MPa, y = 20 MPa, 350% elongation), a cristallinity of 46 ± 2%, and the creation of oxidation products starting from 700 h in oxidant ambient. The biocompatibility tests indicate that the additivated material is biocompatible, as the reference X-Lima UHMWPE.

Wear tests gave a wear rate of 5,09 mg/million cycles against 6,13 mg/million cycles of the reference material, and no sign of run in wear rate.

Our results indicate that there is no change in mechanical properties in respect to the reference material. This is confirmed by DSC measurements, that show no change in cristallinity. The blend between polymer and additive assures an uniform concentration of Vitamin E across the whole thickness of the moulded block, and ageing test results on additivated UHMWPE have shown that the material possess a superior resistance to degradation phenomena.

Biocompatibility assess that the presence of Vitamin E is not detrimental for the in vivo use of the material, and wear results indicate a better wear resistance of the material, especially in the first stages of the wear process.

From these considerations, it can be concluded that the material, in respect to the standard UHMWPE, is highly resistant to oxidation phenomena, therefore it is expected to have superior in vivo endurance performance.

The performance of ultra-high molecular weight polyethylene (UHMWPE) used in total joint replacement prosthesis depends on its wear resistance, oxidation resistance and mechanical properties. Several studies have now established that radiation crosslinking by applying a dose of 50–100 kGy gamma or electron beam radiation followed by remelting to quench free radicals fulfils the criterion of high wear resistance as well as oxidation resistance. However, post-irradiation remelting leads to a decrease in several mechanical properties of UHMWPE including fracture toughness and resistance to fatigue crack propagation, which are deemed important for components in joints where they are subjected to high stresses, such as in tibial components.

In this study, we used uniaxial compression and high-pressure crystallization to disentangle UHMWPE, expecting that this would assist in increasing its crystallinity since disentangled polymer chains would be more readily incorporated into crystalline lamellae, thereby increasing overall crystallinity. This could then result in an increase in some mechanical properties of irradiated, remelted UHMWPE since high crystallinity is associated with high modulus and yield stress. Uniaxial compression of irradiated, remelted GUR 1050 UHMWPE at 130C to a compression ratio up to 2.5 followed by remelting to recover crystallographic orientation showed no statistically significant increase in crystallinity (p> 0.05, ANOVA). High-pressure crystallization at 500 MPa and temperatures in a range of 130-220C also did not show statistically significant increase in crystallinity of irradiated, remelted UHMWPE. However high-pressure crystallization at 500MPa pressure and 240C, where crystallization occurs via the hexagonal phase, increased the crystallinity from 46.2% to 56.4% (p< 0.05, ANOVA).

We conclude that high-pressure crystallization via the hexagonal phase is more effective than uniaxial compression followed by strain recovery or high-pressure crystallization via the orthorhombic phase in increasing the crystallinity of irradiated, remelted UHMWPE, with potential to recover some mechanical properties.

Aim: Wear of the UHMWPE component is responsible for many TJR failures. It is now well known that oxidation of UHMWPE, induced by radiation sterilisation in the presence of oxygen, dramatically increases the wear rate. Vitamin E is already used as biocompatible antioxidant in a number of applications, thus it has been suggested as a suitable stabiliser for orthopaedic UHMWPE. This work investigates the role of the vitamin E on the oxidation process of a gamma-sterilized material.

Methods: GUR 1050 (Meditech, Fort Wayne, IN) resins were blended with 0.05 wt% to 0.5wt% vitamin E and compression molded into billets. Gamma irradiation to 30 and 100 kGy was carried out in an industrial plant. This material was then sectioned using a microtome into 180 micron-thick specimens in preparation for the accelerated ageing.

The samples were treated in an air circulating oven at 90°C. Every 20 hours they were analyzed with FTIR and the carbonyl concentration was recorded.

The CL-imaging measurements were run at 180°C under oxygen in a Differential Scanning Calorimetry (DSC) coupled to a CCD camera. The Oxidation Induction Time (OIT) has been measured as the starting time of oxidation, extrapolated from the CL curve in the function of time.

Results: The results obtained from both techniques clearly evidence that 0.05 wt % of Vitamin E it is enough to stabilize even the material irradiated to the highest dose (100 kGy). Irradiation of UHMWPE leads to the formation of alkyl radicals. When irradiation is carried out in air, macroalkyl radicals can react with oxygen to form hydroperoxides, which in turn decompose giving other oxidation species, mainly ketones and acids. The overall result of irradiation in air is the formation of oxidation products and a decrease in the molecular mass, due to chain fragmentation.

The ability of α-tocopherol as a free radicals scavenger during gamma irradiation prevents the reaction of polymer radicals with oxygen. While performing this role, α-tocopherol is consumed and transformed into a variety of by-products. Nevertheless, higher OIT for the doped and irradiated specimens compared to the control (0% Vit.E, 0 kGy) suggest even a stabilising effect of these by-products.

Conclusions: Vitamin E has been shown to be highly efficient against radiation-induced oxidation and therefore it should be recommended as biocompatible stabilizer for orthopaedic UHMWPE, in order to preserve good mechanical properties.

Introduction: Packaging of Muscolo-Skeletal Tissues (MST) stored at −80°C must assure safety and sterility in order to minimize any risk of bacterial contamination and/or mechanical failure. Polymeric bags demonstrated problem of integrity at −80°C; gamma ray sterilisation induce oxidation decreasing mechanical properties, whereas Ethylene Oxide (EtO) does not. Antioxidant biocompatible additive, as Vitamin E, could improve mechanical resistance.

Objectives: Based on a previous paper presented at EATB 2005 congress, to analyse mechanico-chemical properties of plastic bags routinely used in MST Banks and new samples in order to identify and solve possible problems arising from the chemical composition and/or sterilisation.

Materials and Methods: Five different polymeric sterile bags used in three International Banks (three gamma and two EtO sterilised) and four experimental sample, manufactured on purpose from a Linear Low Density PolyEthylene (LLDPE) 150 microns thick films (EtO and e-beam sterilised), two added of Vitamin E, were analysed. Impact resistance was evaluated both on frozen and unfrozen material (in oven at 37°C); results were related to chemical composition, Tg, sterilisation and Fourier Transformed InfraRed Spectroscopy (FTIR).

Results: Three samples routinely used (one gamma and one EtO) showed severe macroscopic modification (glassy behaviour) at frozen temperature with no resistance to any mechanical stress Two samples (EtO) did not resist to mechanical tests at frozen state. The four experimental LLDPE, EtO and e-beam sterilised, resisted to mechanical tests. FTIR analysis confirmed the chemical composition declared by the commercial film: pure LLDPE, without any toxic additive and LLDPE with vitamin E.

Discussion: Packaging must use polymers with adeguate glass transition temperature (Tg) in order to maintain at −80° the rubbery state, not stiff nor fragile (not to pass to glassy state). High energy radiation oxidize polymer and decrease their mechanical resistance. LLDPE combine low Tg of the amorphous phase and low crystallinity, resulting in good mechanical properties at working temperature and at −80°C. Addition of Vitamin E protect against oxidation. EtO sterilisation does not modify the structure.

Conclusion: A LLDPE added of Vitamin E, sterilised by EtO and e-beam could improved packaging and storage of tissues at −80°C, with increased resistance to oxidation.

During the last 15 years we have had the opportunity of analysing more than 700 UHMWPE prosthetic components (hip, knee and shoulder). Among them, about 500 were retrieved during revision surgery, while the remaining were new, ready-to implant, variably shelf-aged samples. The analysis of such a large, representative sample provided several important insights into the variables which influence the behaviour of UHMWPE in vivo; moreover, a long period of observation gave us the opportunity to follow changes and improvements in the field over time. All samples dated back to the nineties or before and sterilized with high energy radiation, either shelf-aged or retrieved, showed variable, but generally high, oxidation levels. Starting from the observation of these samples and with the aid of specimens irradiated on purpose under controlled conditions, some improvement has been achieved in the knowledge of radiation-induced oxidation process. The importance of the determination of hydroperoxides on the oxidation potential has been highlighted and the influence of variables such as sterilisation atmosphere, packaging, temperature and dose rate on the oxidation process has been clarified. The need for a suitable stabilizer to minimize oxidation arises during these studies.

We also had the opportunity of analysing a large number of EtO-sterilised samples, both new and retrieved. A small amount of them, all manufactured in the nineties, showed some bulk-oxidation which has been related to the presence of calcium stearate into the pristine resin. None of the newly produced, calcium stearate-free samples showed any oxidation and this group allowed to explore the behaviour of undegraded UHMWPE in vivo and in the shelf. Diffusion of polar compounds from the synovial fluid into polyethylene was observed in the majority of the retrieved samples. The nature of these products have been investigated along with their possible influence on the mechanical properties of the polymer.

In the last five years, we had the opportunity to study a significant number of crosslinked polyethylenes, both new and retrieved. The results of this study indicate that the variables of the crosslinking process can greatly influence final material properties and that not all cross-linked polyethylenes are the same.

Introduction. Wear of the UHMWPE component is responsible for many TJR failures. It is now well known that oxidation of UHMWPE, induced by radiation sterilisation in air, dramatically increases the wear rate. ASTM regulations for orthopaedic UHMWPE forbids the addiction of any antioxidant to the polymer powder or to fabricated forms. Vitamin E is widely employed as a biocompatible stabiliser in the food and cosmetic industry. Aim of the present study is to evaluate the efficiency of Vitamin E as a stabiliser for prosthetic UHMWPE.

Materials. Virgin UHMWPE samples were obtained from compression moulded slabs (GUR 1020, Perplas). In addiction, compression moulded slabs of GUR 1020 mixed with 500 and 1000 ppm of Vitamin E respectively were also studied. Electron beam irradiation was performed with doses ranging from 50 to 225 kGy, in air, at room temperature. Slices of controlled thickness (0,1–0,3 mm) were microtomed from the blocks and accelerated ageing was carried out in a ventilated oven at 90°C. FTIR spectroscopy were used to monitor changes in the polymer structure after irradiation and ageing. Mechanical properties were evaluated using the small punch test, as described in ASTM F2183-02.

Results. FTIR measurements on the aged samples showed that the addiction of Vitamin E induces a substantial increase in the oxidative stability of UHMWPE. The overall work to failure of original UHMWPE irradiated at 100 kGy was halved after 160 hours of accelerated ageing, due to the developed oxidation. On the other hand, the work to failure of samples with Vitamin E was constant up to 1800 hours of ageing under the same conditions.

Discussion Irradiation of UHMWPE induces C-C and C-H bond scissions, leading to the formation of alkyl radicals. When irradiation is carried out in air, macroal-kyl radicals can react with oxygen to form hydroperox-ides, which in turn decompose giving other oxidation species, mainly ketones and acids, which decrease the molecular mass. Oxidation of the polymer has been found to cause a dramatic deterioration of its mechanical properties. Vitamin E has been shown to be highly efficient against radiation-induced oxidation and therefore it should be recommended as biocompatible stabilizer for orthopaedic UHMWPE, in order to preserve good mechanical properties.

Introduction Sterilisation of UHMWPE prosthetic components by high-energy radiation in air induces an oxidative degradation of the polymer, which may compromise the mechanical performances of the whole implant. Many manufacturers shifted to gas sterilization with EtO and gas plasma or to radiation sterilization in inert atmosphere and with barrier packaging. Aim of the present study was to investigate the relationship between sterilisation method, packaging, oxidation and mechanical properties of current orthopaedic UHMWPE.

Materials 100 sterilised UHMWPE hip, knee, and shoulder components by 19 orthopedic manufacturers were studied. The components were cut in half and sectioned using a microtome into slices of controlled thickness (0,1–0,3mm) which were analysed by FTIR. The UHMWPE packaging was also evaluated by FTIR, in order to correlate the extent of oxidation to the storage conditions. Mechanical properties were evaluated using the small punch test, as described in ASTM F2183-02.

Results The UHMWPE packaging was classified, when possible, into one of the following types; A: PET blister(s) with Tyvek® gas-permeable cover; B: packaging involving a polymeric multilayer bag; C: packaging involving at least one Aluminium foil. Using Type A, air permeable packaging for radiation-sterilized UHMWPE is the equivalent to irradiation in air. Many radiation sterilized implants packaged using Type A materials were severely oxidized. In the case of packaging type B, we detected moderately low oxygen index (OI) in the majority of samples, but an average high hydroperoxide level, even though type B packaging has well-documented oxygen barrier properties. UHMWPE components contained in packaging type C exhibit low OI and hydroperoxide level, due to the impermeable Al foil. The small punch test measurements showed that the oxidised sample exhibit generally diminished mechanical properties. Reductions in the yield load (up to 15%), in the ultimate load (up to 33%) and in the ultimate displacement (up to 21%), compared to the original or EtO sterilised material, have been measured on the majority of the oxidised samples.

Discussion The present results confirm that oxidation has a negative effect on the mechanical properties of UHMWPE. It remains difficult to generalise about the overall effectiveness of barrier packaging at protecting UHMWPE from oxidation, but it is our opinion that a complete absence of sterilisation-induced oxidation can only be guaranteed by gas sterilisation.

Aims: to investigate the mechanical properties of a new nanocomposite bone cement radiopacified with Barium Sulfate (BaSu) nanoparticles added at different concentrations, compared to a control cement with the classical BaSu microparticles.

Methods: the starting material was Endurance (J& J/ DePuy, USA) bone cement without BaSu; the radi-opacifier particles have been mixed into the cement powder in several different concentrations of 5, 10, 20, 30, 40% of the weight respectively. Two groups were studied: controls, with classical medical grade BaSu particles (average size 1000 nm) and nanocomposites, with nanoparticles (av. size 100 nm). In accordance with the ASTM, an Instron 4201 machine tested a minimum of 6 specimens for each concentration. Tensile tests were performed at cross-head speeds of 1mm/sec, while compression tests were performed at 25,4 mm/sec. Results were statistically analysed.

Results: nanocomposites had higher compressive Yield strength in all groups except 30 and 40% and lower compressive Modulus in all but 5% group (no significant difference). Nanocomposites had higher tensile values in 5%, 10%, and 40% concentrations for Strain-to-failure, yield stress, and Work-of-Fracture, and no significant differences in the other concentrations. Tensile modulus had not statistically significant variations. Higher BaSu concentrations give increases in tensile modulus and decreases in the other tensile properties for both the groups. The nanocomposite outperformed the control in the 5, 10, and 20% groups, while the 30 and 40% groups had no significant differences; all the results were above ASTM requirements.

Conclusions: bone cement has several uses, like joint replacement, filling defects in tumour or revision surgery, and more recently vertebroplasty. These applications require different properties and would have benefits from the possibility to change viscosity, radiopacity, time of polymerisation, mechanical features. Previous studies have demonstrated the improved performances of the new nanocomposite cement at the clinical used concentration of 10%. This study investigated the possibility to augment the concentration of BaSu and therefore the radiopacity and their relative effect on the mechanical properties; the results demonstrated the good compliance of the nanoparticles cement in this field. This would be useful in particular for specific applications such vertebroplasty. Further studies are needed to investigate and determine the ideal fatigue, handling and mixing properties, viscosity and radiopacity.

Aims: To understand why during routine analyses of the physico-chemical properties of retrieved UHMWPE prosthetic components (Pes), it was noticed that some cups, which were directly in contact with bone, evidence a material loss in correspondence with the area adjacent to bone.

Methods: PEs retrieved during revision surgery and stored in formalin prior to observation, have been analysed by Fourier Transform InfraRed (FTIR) spectroscopy and Scanning Electron Microscopy (SEM).

Results: The results of the FTIR analyses did not display appreciable differences compared to those of the majority of the retrieved prostheses. Oxidation of UHMWPE was detected, but it is known to be due to sterilisation with high energy radiation in air, under uncontrolled conditions.

The SEM analyses indicated that the PEs surface which was directly in contact with bone shows an anomalous degradation. The surface looks as it has been corroded or “bitten” and its morphology is significantly different from that of surfaces abraded either in vivo or in vitro.

Conclusions: The results so far obtained seem to indicate that the surfaces, which have been in contact with bone during the implant time, undergo a selective biodegradation process, facilitated by gamma in air sterilisation, and influenced by the biological reactivity of the patient (such as osteclast activation).

Introduction – Polyethylene (PE), or better Ultra High Molecular Weight Polyethylene (UHMWPE) wear was demonstrated to be the main cause of Total Knee Replacement failure during the ‘90ties years. Wear, that occurred during the in vivo service, was related to the implant biomechanics, both the prosthetic design (constrained - non constrained, PCL sacrifice…) and the implant technique with rotational and alignment defects. In all these studies, retrieved PE inserts and wear particles were supposed to be UHMWPE, with the same chemical and physical characteristics of the original certified polymer. Unfortunately, degradation of UHMWPE, that is the modification of the chemical and physical structure, may occur during the preparation of prosthetic components; in particular, gamma irradiation in air is responsible for superficial and deep, unpredictable, inhomogeneous oxidative degradation of the polymeric biomaterial (1–6). Therefore, new PE components sterilised by in air gamma irradiation and ready for implant can be supposed to be UHMWPE, but they could not be. Sterilisation with ethylene oxide (EtO) does not modify the chemical and physical properties of the original PE. Furthermore, during the service in vivo cholesterol and other components of the synovial fluid diffuse in the PE components and modify the mechanical properties of the polymer (7).

Aim of the study - To characterise new PE components (hereafter called PEs in this paper) ready for implants and retrieved PEs obtained from failed total knee replacements in order to evaluate the wear, oxidation level and, in the retrieved ones, the diffusion products after service in vivo. Only after this characterisation some mechanical considerations and therefore wear in vivo could be discussed.

Materials - 24 new and 75 retrieved PEs were analysed. New PEs were produced by 9 different firms, 18 were sterilised by gamma irradiation in air, 1 in inert atmosphere or in vacuum, 5 by EtO. Surgical revisions were performed after an average time of 5 years (min 3 months, max 15 years) because of aseptic loosening (51 cases), septic failure (16 cases), PE severe wear (4 cases), other causes (4 cases). The retrieved PEs had been produced by 15 different firms; 74 were sterilised by gamma irradiation, while only 1 by EtO. The mean age at revision was 70 years (range 57–82 years).

Methods – At the surgical revision, PEs were photographed; wear area score according to Collier and wear severity score according to Plante-Bordeneuve and Freeman were evaluated. Prior to the analyses, PEs were stored in the dark in formaldehyde 4%. New and retrieved PEs were cut perpendicularly to the articular surface. A series of slices of controlled thickness (from 100 to 300 microns) were recovered from the cross-section using a Poly Cuts Microtome (Reichert-Jung) at 20 mms⁻¹ in air at room temperature. A FTIR Microscope (Perkin Elmer System 2000) equipped with a x-y motorised micropositioning stage was used to identify and map the distribution and level of oxidation. Identification of oxidised species was carried out by derivatisation and IR analysis. Soxhlet extraction in boiling cyclohexane for 20 hours was performed to extract low molecular weight substances which diffused into PEs.

Results – All new PEs sterilised by gamma irradiation in air presented surface and bulk oxidation, variable in severity and distribution. Wear of retrieved PEs sterilised by gamma irradiation in air was extended for more than 50% of the articular surface (score 3) in 60% of cases and was severe (score 5–8) in 47% of them. In most of the gamma irradiated in air retrieved plateaux a “crown zone” at a depth of 1–2 mm from the surface was observed. This zone has been found to correspond to the maximum of the oxidation profile, measured by FTIR mapping. Diffusion of cholesterol and its esters with fatty acids has been observed in many of the samples, in variable amount depending both on the clinical situation of the patient and on the implant time.

Conclusions – These results show that significant modifications of the physical properties of UHMWPE can be introduced by the sterilisation treatment. The chain reactions that follow gamma ray sterilisation and lead to oxidation tends to decrease the molecular weight of UHMWPE. The presence of the subsurface “crown zone” is the macroscopic evidence of an extremely high level of oxidation, responsible for delamination and wear of the tibial plateaux. Critical mechanical phenomena can be related with oxidation due to gamma irradiation and in vivo degradation. Diffusion in the PEs of cholesterol and other synovial fluid components may affect the mechanical resistance in vivo. All these results emphasise that discussion about mechanical behaviour in vivo of different prostheses, particularly for total knee replacement where the biomechanics is complicate, must be proceeded by accurate control of physicochemical properties of the ready-to-implant prosthesis and of the retrieved components. New tests must be introduced to control the integrity of the ready-to-implant components, besides that of the row material. Furthermore, suitable in vitro tests might give a prediction on the effects of diffusion on the material performances.