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.

Aims: In recent years, radiation crosslinking has become an important processing step in the manufacture of ultra-high molecular weight polyethylene (PE) components of joint replacement prostheses due to its associated high wear resistance. Gamma or electron beam radiation treatment is usually followed by a heating step, either complete melting or annealing of PE close to but below the melting temperature for a specific time duration. The heat treatment is performed to decrease free radical concentration within the crystalline lamellae in order to make PE more oxidation resistant.

In this study, we hypothesized that high pressure processing of PE would be advantageous if it is performed only after irradiation and quenching of free radicals and that it would be detrimental if it preceded irradiation. We used accelerated oxidation, mechanical tests and wear tests to show

Methods: Ram-extruded rod stock of GUR 1050 PE (Ticona, Bayport, TX) was purchased from MediTECH Medical Polymers (Fort Wayne, IN) and machined into cylinders to snugly fit into a custom-built high-pressure cell. A Carver hydraulic press was used to apply a pressure of 500MPa to PE specimens preheated to various temperatures, slow cooled to room temperature followed by pressure release. The PE cylinders and untreated control PE were subjected to 50kGy gamma radiation, which is a dose sufficient for a high degree of crosslinking in PE. A Parr bomb reactor filled with oxygen gas and operating at 5atm pressure and 70_C temperature was used to oxidize PE for a period of 14 days, according to ASTM standard F2003–02, and later characterized using Fourier Transform Infrared Spectroscopy (FTIR). A second batch of PE was first irradiated, melted and then subjected to high pressure processing. ASTM standard tensile tests were conducted to determine whether there was any increase in mechanical properties. Scanning electron microscopy (SEM) and differential scanning calorimeter (DSC) were used to characterize the lamellar morphology.

Results: The morphological characterization techniques, SEM and DSC, showed that high pressure processing increased the crystallinity as well as lamellar thickness regardless of whether the process was conducted prior to or after irradiation. FTIR showed that there was a monotonic increase in oxidation with lamellar thickness if the irradiation was carried out after high pressure processing. Several mechanical properties such as modulus and yield stress of PE increased with increase in crystallinity, which is desirable for applications where PE is subjected to high stresses.

Conclusions: High-pressure processing benefits the mechanical properties of crosslinked PE when it is conducted after irradiation and melting. However, if it conducted prior to irradiation and is not followed by thermal treatment, it can lead to more trapped free radical and excessive oxidation. Therefore, it is important to employ this processing method after irradiation so that it improves the mechanical properties of crosslinked PE.

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.