Anuj Bellare

Molecular rearrangements in ultra high molecular weight polyethylene after irradiation and long-term storage in air

Abstract Molecular rearrangements and alterations in supermolecular structure of ultra high molecular weight polyethylene (UHMWPE) due to cross-linking and oxidation-induced chain scission following irradiation and subsequent storage in air at room temperature have been studied over a period of 29 months. The techniques that were used are: equilibrium swelling in decalin, differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR) and electron spin resonance (ESR). The experimental results indicate that immediately after irradiation, cross-linking and an increase in crystallinity are the important processes. With time, chain scission induced by oxidation takes place resulting in a new phase of thinner crystallites in the amorphous region. In the absence of oxygen diffusion limitations, the free radicals survive for 30 months in the form of peroxides. The effect produced by higher doses and shorter aging times correspond to those produced by lower doses and longer aging times, thus, suggesting a superposition law between dose and aging time.

Quantification of the effect of cross-path motion on the wear rate of ultra-high molecular weight polyethylene

Abstract Ultra-high molecular weight polyethylene (UHMWPE) is used worldwide as a bearing material in orthopaedic implants. Multi-directional motion or “cross-shear” motion has been identified as one of the most significant factors affecting the wear rate of UHMWPE in total hip joint replacement prostheses. The loci or trajectory of motion at the point of contact between a femoral head and an acetabular cup takes a general quasi-elliptical or rectangular shape during a gait cycle. Due to variations in gait patterns, some patients have either more elongated or square motion patterns than others. It is postulated that differences in motion pattern affect the in vivo wear rates of UHMWPE cups in patients where other factors such as age, weight and body proportion are similar. In this study, UHMWPE pins were articulated against cobalt chromium disks in diluted calf serum using an OrthoPOD™ multi-directional wear tester under physiological loading conditions. Five different rectangular wear path geometries and linear tracking, all with identical path lengths per cycle, were tested. Gravimetric weight loss was converted into both volumetric wear data and wear factor values, k, in order to determine the effects of motion pattern on the wear rate of UHMWPE. The results supported the hypothesis that wear rate is dependent upon the wear path geometry that is in turn dependent upon gait cycle.

Wear behavior of bulk oriented and fiber reinforced UHMWPE

Abstract A study of the friction and wear properties of isotropic, fiber reinforced, and bulk oriented ultra-high molecular weight polyethylene (UHMWPE) was conducted on a reciprocating wear tester. The tests were carried out with a cobalt–chromium alloy cylinder sliding against a flat sample of UHMWPE in bovine serum at 1.5 Hz for over a million cycles. In the fiber reinforced material, layers of woven UHMWPE fibers were embedded in a UHMWPE matrix. Bulk orientation of UHMWPE was produced by channel die compression of standard UHMWPE. The fiber-reinforced samples were tested in one configuration and three different degrees of consolidation. The less consolidated samples failed catastrophically, and the more consolidated sample had a wear rate twice that of the unreinforced, standard UHMWPE. The channel die oriented samples were tested in various configurations and degrees of anisotropy. They all showed similar wear behavior as the standard, with a wear rate of approximately 1×10 −7 mm 3 /N m. The average values of the coefficient of friction of all the samples ranged from 0.08 to 0.11.

Morphology of rod stock and compression-moulded sheets of ultra-high-molecular-weight polyethylene used in orthopaedic implants.

Compression-moulded sheets and extruded rods of ultra-high-molecular-weight polyethylene (UHMWPE) are currently used in the production of joint replacement prostheses. Crystallographic texture present in rods and sheets of UHMWPE was measured using a combination of small-angle X-ray scattering and wide-angle X-ray diffraction. Crystallographic texture can induce anisotropy in macroscopic properties of polymers, such as modulus and yield stress. Both rods and sheets of UHMWPE revealed a low but discernible degree of preferred orientation of polyethylene chains within crystallites. There was a spatial variation in crystallographic orientation in extruded rods. The direction of chain alignment within crystallites located near the outer surface of rods was orthogonal to the radial direction, whereas the chain direction was orthogonal to the axial or extrusion direction in crystallites located near the centreline of extruded rods. Crystallographic texture was spatially uniform in compression-moulded sheets with the chain direction within crystallites aligned orthogonal to the moulding direction. In both cases the induced crystallographic texture can be explained in terms of crystallization from an oriented melt.

Development of texture in poly(ethylene terephthalate) by plane-strain compression

Abstract Morphological alterations induced by plane-strain compression of semicrystalline poly(ethylene terephthalate) (PET) at 190°C were studied using small-angle and wide-angle X-ray diffraction techniques, polarized light microscopy and differential scanning calorimetry. Based on the observations, a scheme of texture development is outlined. An initially spherulitic morphology transforms into one that comprises stacks of fragmented crystalline lamellae with lamellar normals oriented towards the flow direction. After some initial deformation by interlamellar sliding in the amorphous material (100)[001] chain slip operated throughout the remainder of the deformation. Up to a compression ratio of 2.6 the (100)[001] chain slip mechanism orients lamellar normals towards the compression axis. Further deformation caused fragmentation of the thinned-out lamellae and subsequent reorientation of lamellar normals towards the flow direction. Pole figure analysis indicated cooperative activity of the (100)[001] chain slip and (100)[010] transverse slip during the later stages of texture development. The (100)[010] transverse slip mechanism controlled the orientation of the crystal structure in a plane orthogonal to the flow direction. There was no evidence of (010)[001] chain slip during texture development. At a compression ratio of 3.3 pole figure analysis revealed the existence of a dual unit cell orientation that renders an overall orthotropic symmetry to plane-strain compressed PET.

Nanoparticulate fillers improve the mechanical strength of bone cement

Background and purpose Polymethylmethacrylate (PMMA-) based bone cement contains micrometer‐size barium sulfate or zirconium oxide particles to radiopacify the cement for radiographic monitoring during follow‐up. Considerable effort has been expended to improve the mechanical qualities of cements, largely through substitution of PMMA with new chemical structures. The introduction of these materials into clinical practice has been complicated by concerns over the unknown long‐term risk profile of these new structures in vivo. We investigated a new composite with the well characterized chemical composition of current cements, but with nanoparticles instead of the conventional, micrometer‐size barium sulfate radiopacifier. Methods In this study, we replaced the barium sulfate microparticles that are usually present in commercial PMMA cements with barium sulfate nanoparticles. The resultant “microcomposite” and “nanocomposite” cements were then characterized through morphological investigations such as ultra‐small angle X‐ray scattering (USAXS) and scanning electron microscopy (SEM). Mechanical characterization included compression, tensile, compact tension, and fatigue testing. Results SEM and USAXS showed excellent dispersion of nanoparticles. Substitution of nanoparticles for microparticles resulted in a 41% increase in tensile strain‐to‐failure (p = 0.002) and a 70% increase in tensile work‐of‐fracture (p = 0.005). The nanocomposite cement also showed a two‐fold increase in fatigue life compared to the conventional, microcomposite cement. Interpretation In summary, nanoparticulate substitution of radiopacifiers substantially improved the in vitro mechanical properties of PMMA bone cement without changing the known chemical composition.

Ultra-High Molecular Weight Polyethylene: Influence of the Chemical, Physical and Mechanical Properties on the Wear Behavior. A Review

Ultra-high molecular weight polyethylene (UHMWPE) is the most common bearing material in total joint arthroplasty due to its unique combination of superior mechanical properties and wear resistance over other polymers. A great deal of research in recent decades has focused on further improving its performances, in order to provide durable implants in young and active patients. From “historical”, gamma-air sterilized polyethylenes, to the so-called first and second generation of highly crosslinked materials, a variety of different formulations have progressively appeared in the market. This paper reviews the structure–properties relationship of these materials, with a particular emphasis on the in vitro and in vivo wear performances, through an analysis of the existing literature.

Tensile and tribological properties of high-crystallinity radiation crosslinked UHMWPE

Osteolysis due to particulate wear debris associated with ultrahigh molecular weight polyethylene (UHMWPE) components of total joint replacement prostheses has been a major factor determining their in vivo lifetime. In recent years, radiation crosslinking has been employed to decrease wear rates in PE components, especially in acetabular cups of total hip replacement prostheses. A drawback of radiation crosslinking is that it leads to a crosslinked PE (or XPE) with lower mechanical properties compared with uncrosslinked PE. In contrast, high-crystallinity PEs are known to have several mechanical properties higher than conventional PE. In this study, we hypothesized that increasing the crystallinity of radiation crosslinked and remelted XPE would result in an increase in tensile properties without compromising wear resistance. High-pressure crystallization was performed on PE and XPE and analyzed for the resulting morphological alterations using differential scanning calorimeter, low voltage scanning electron microscopy, and ultrasmall angle X-ray scattering. Uniaxial tensile tests showed that high-pressure crystallization increased the tensile modulus and yield stress in both PE and XPE, decreased the ultimate strain and ultimate stress in PE but had no significant effect on ultimate strain or ultimate stress in XPE. Multidirectional wear tests demonstrated that high-pressure crystallization decreased the wear resistance of PE but had no effect on the wear resistance of XPE. In conclusion, this study shows that high-pressure crystallization can be effectively used to increase the crystallinity and modulus of XPE without compromising its superior wear resistance compared with PE.

The relative effects of radiation crosslinking and type of counterface on the wear resistance of ultrahigh-molecular-weight polyethylene

The lifetime of total joint replacement prostheses utilizing ultrahigh-molecular-weight polyethylene (UHMWPE) components has historically been determined by their wear resistance. It has been discovered that radiation crosslinking of UHMWPE can substantially increase its wear resistance. However, it is also well recognized that there is a radiation-dose-dependent decrease in several important mechanical properties of UHMWPE, such as fracture toughness and resistance to fatigue crack propagation. In this study, the effect of radiation crosslinking (followed by remelting) on the morphology, tensile properties and wear resistance of UHMWPE was investigated. Wear tests were conducted against both the commonly used cobalt-chromium counterface polished to implant grade smoothness as well as a smoother ceramic (alumina) counterface. The results showed that 50kGy dose radiation crosslinking increased the wear resistance of UHMWPE against the cobalt-chromium counterface 7-fold, but the coupling of remelted, crosslinked UHMWPE against the smoother alumina counterface led to a 20-fold increase in wear resistance. This study shows that the use of an alumina counterface would circumvent the need to use a high radiation dose in crosslinking UHMWPE, associated with poor mechanical properties, without compromising wear resistance.

Influence of crystallization conditions on the tensile properties of radiation crosslinked, vitamin E stabilized UHMWPE.

Radiation crosslinking for ultra-high molecular weight polyethylene results in improved wear resistance but a reduction in mechanical properties. Incorporation of vitamin E has been known to decrease the rate of oxidative degradation occurring through radiation crosslinking and prevents the need for post-irradiation melting with subsequent loss of crystallinity. In this study, we aimed to determine the effect of thermal treatments prior to crosslinking on the morphology and tensile properties of vitamin-E-containing polyethylene. Vitamin-E-blended polyethylene was melted and subsequently quenched in ice water in order to induce high rate crystallization. A second group was additionally annealed at 126°C following quenching and all samples were irradiated using electron beam radiation to a dose of 100kGy. The morphology of control, quenched and quench-annealed polyethylene was characterized using small angle x-ray scattering and differential scanning calorimetry. Tensile properties of these polyethylenes were measured before and after radiation crosslinking with equilibrium swelling experiments performed to assess the crosslink density of irradiated samples. This study shows how the tensile properties of polyethylene can be enhanced by varying thermal treatments prior to crosslinking; and thus how it may be possible to offset the reduction in tensile properties afforded by the crosslinking process.