Transient elastohydrodynamic lubrication analysis of metal-on-metal hip implant under simulated walking conditions

The transient elastohydrodynamic lubrication (EHL) analysis was performed in this study for a typical metal-on-metal bearing employing a polyethylene backing underneath a metallic cup inlay under dynamic operating conditions of load and speed representative of normal walking. A ball-in-socket configuration was adopted to represent the articulation between the femoral head and the acetabular cup. The governing Reynolds and elasticity equations were solved simultaneously by using both finite difference and finite element methods. The predicted transient film thickness from the present study was compared with the estimation based on the quasi-static analysis. It was found that the polyethylene backing employed in the typical metal-on-metal hip bearing, combined with dynamic squeeze-film action, significantly improved the transient lubricant film thickness under cyclic walking and consequently a fluid film lubrication regime was possible for smooth bearing surfaces with an average roughness less than 0.005 microm.     

Effect of bearing geometry and structure support on transient elastohydrodynamic lubrication of metal-on-metal hip implants

An effective lubrication can significantly reduce wear of metal-on-metal artificial hip joints. The improvement of the lubrication can be achieved through the optimisation of the bearing geometry in terms of a small clearance and/or the structural support such as a polyethylene backing underneath a metallic bearing in a sandwich acetabular cup form. The separate effects of these two factors on fluid film lubrication of 28 mm diameter metal-on-metal total hip joints under walking conditions were numerically investigated in this paper. The results show that a larger lubricant film due to the polyethylene backing can be significantly enhanced by the transient squeeze-film action, particularly during the stance phase, and a similar lubricant film can be developed for both the monolithic cup relying on the smaller clearance and the sandwich cup benefiting from the polyethylene backing. Both cup systems can function in a wide range of lubrication regimes, covering both mixed and fluid film, under the current design and manufacture conditions.     

Effect of swing phase load on metal-on-metal hip lubrication, friction and wear

There is renewed interest in metal-on-metal (MOM) total hip replacements (THRs), however, variable wear rates have been observed clinically. It is hypothesised that changes in soft tissue tensioning during surgery may alter loading of THRs during the swing phase of gait leading to changes in fluid film lubrication, friction and wear. This study aimed to assess the effect of swing phase load on the lubrication, friction and wear of MOM hip replacements. Theoretical lubrication modelling was carried out using elastohydrodynamic theory. All the governing equations were solved numerically for the lubricant film thickness between the articulating surfaces under the transient dynamic conditions with low and high swing phase loads. Friction testing was completed using a single axis pendulum simulator, simplified loading cycles were applied with low and high swing phase loads. MOM hip replacements were tested in a hip simulator, modified to provide different swing phase loading regimes; a low (100 N) and a high load (as per ISO 14242-1; 280 N). Results demonstrated that the performance of MOM bearings is highly dependent on swing phase load. Hence, changes in the tension of the tissues at surgery and variations in muscle forces may increase swing phase load, reduce lubrication, increase friction and accelerate wear. This may explain some of the variations that have been observed with clinical wear rates.     

Prediction of lubrication regimes in wrist implants with spherical bearing surfaces

The wrist joint is frequently affected by rheumatoid arthritis, resulting in wrist pain, deformity and ultimately loss of function. Artificial wrist implants have been introduced to treat the rheumatoid wrist, to attempt to alleviate pain and restore some function to the joint. The aim of this study was to predict the likely lubrication regimes that occur in wrist implants with spherical bearing surfaces. The implant was modelled as an equivalent ball-on-plane. Elastohydrodynamic lubrication theory was used to determine the minimum film thickness for the implant under different load, entraining velocity, lubricant viscosity, size of implant and material combinations. The results show that the highest film thickness is found in large implants, with high viscosity, high entraining velocity and low load. Hard-on-soft material combinations will operate with a boundary lubrication regime. Material combinations involving ceramic bearing surfaces have the potential to operate with a mixed lubrication regime.     

Detection of cobalt in synovial fluid from metal-on-metal hip prosthesis: correlation with the ion haematic level

Abstract

Hip replacement with large metal-on-metal (MoM) coupling has recently been under attention for its metal ions release, inducing several disorders. Since the blood level toxicity threshold for cobalt is known, the aim of this work is to determine whether Co serum and blood levels correlate with the synovial fluid level. Beside this, the synovial fluid Co from patients without prosthesis has also been measured. Co has been determined in 54 samples (32 unilateral MoM and 22 controls) with inductively coupled plasma mass spectrometry in the three matrices. In the prosthesized group, cobalt was significantly higher compared with controls and in the synovial fluid it strongly correlated with whole blood Co (p?=?0.847) and serum (p?=?0.855). Moreover, the Co levels into whole blood and serum were significantly different. In conclusion, haematic Co concentration correctly reflects Co local level.     

Synthesis and immunological evaluation of an antitumor neoglycopeptide vaccine bearing a novel homoserine Tn antigen

As part of our program on Tn-specific anti-tumor immunotherapy, our aim was to vary the nature of the aglyconic part of the tumor-associated Tn antigen (alpha-d-GalNAc-Ser/Thr). This report describes the synthesis of Fmoc-hSer-(alpha-d-GalNAc)-OH (4) in 19% overall yield from protected aspartic acid. The building block 4 was incorporated as trimeric clusters into a glycopeptide vaccine [MAG:Tn(hSer)3-PV], using solid-phase peptide synthesis. When injected in mice, the resulting MAG induces a strong antibody response, which recognizes native tumor-associated antigens (TAA) at the surface of human tumor cells. This approach may be extended to the use of other nonnatural TAA in order to improve half-life of synthetic anti-cancer vaccines.     

A surface plasmon resonance probe with a novel integrated reference sensor surface

A surface plasmon resonance (SPR) sensor probe with integrated reference surface is described. In order to fabricate the integrated reference surface, two dielectric layers with different thickness were deposited on the single gold SPR sensor surface via plasma polymerization of hexamethyldisiloxane. The working sensor surface was a 34 nm dielectric layer with immobilized bovine serum albumin (BSA) antigen and an adjacent thin 1 nm dielectric layer without BSA provided reference surface. A specific immunoreaction of anti-BSA antibody was detected after immersion of the SPR probe into sample solution. Simultaneous observation of reference and working surface response enabled determination of the immunoreaction without the need for the baseline measurement. Moreover, compensation of nonspecific adsorption could be confirmed using anti-human serum albumin antibody.     

Synthesis and properties of novel antisense oligonucleotides bearing an anthraquinone moiety at an internucleotide linkage.

Antisense oligonucleotides bearing an anthraquinone moiety at an internucleotide linkage were synthesized utilizing the stereoisomers of anthraquinone incorporated T-C dimer phosphoramidite derivatives. Some physicochemical properties of the anthraquinon bearing oligomers were investigated.     

Combining mechanism and drift in community ecology: a novel statistical mechanics approach

A key challenge for models of community ecology is to combine deterministic mechanism and stochastic drift in a systematic, transparent and tractable manner. Another challenge is to explain and unify different ecological patterns, hitherto modelled in isolation, within a single modelling framework. Here, we show that statistical mechanics provides an effective way to meet both challenges. We apply the statistical principle of maximum entropy (MaxEnt) to a simple resource-based, non-neutral model of a plant community. In contrast to previous ecological applications of MaxEnt, our use of MaxEnt emphasises its theoretical basis in the combinatorics of sampling frequencies, an approach that clarifies its ecological interpretation. In this approach, mechanism and drift are identified, respectively, with ecological resource constraints and entropy maximization. We obtain realistic predictions for species abundance distributions as well as contrasting stability-diversity relationships at community and population levels. The model also predicts critical behaviour that may provide a basis for understanding desertification and other ecological tipping points. Our results complement and extend previous ecological applications of MaxEnt to new areas of community ecology, and further illustrate MaxEnt as a powerful yet simple modelling tool for combining mechanism and drift in a way that unifies disparate ecological patterns.     

Sensitivity analysis of a cemented hip stem to implant position and cement mantle thickness

Patient-specific finite element models of the implanted proximal femur can be built from pre-operative computed tomography scans and post-operative X-rays. However, estimating three-dimensional positioning from two-dimensional radiographs introduces uncertainty in the implant position. Further, accurately measuring the thin cement mantle and the degree of cement–bone interdigitation from imaging data is challenging. To quantify the effect of these uncertainties in stem position and cement thickness, a sensitivity study was performed. A design-of-experiment study was implemented, simulating both gait and stair ascent. Cement mantle stresses and bone–implant interface strains were monitored. The results show that small variations in alignment affect the implant biomechanics, especially around the most proximal and most distal ends of the stem. The results suggest that implant position is more influential than cement thickness. Rotation around the medial–lateral axis is the dominant factor in the proximal zones and stem translations are the dominant factors around the distal tip.     

Adult rat spinal cord culture on an organosilane surface in a novel serum-free medium

Summary In this study, we have documented by morphological analysis, immunocytochemistry, and electrophysiology, the development of a culture system that promotes the growth and long-term survival of dissociated adult rat spinal cord neurons. This system comprises a patternable, nonbiological, cell growth-promoting organosilane substrate coated on a glass surface and an empirically derived novel serum-free medium, supplemented with specific growth factors (acidic fibroblast growth factor, heparin sulfate, neurotrophin-3, brain-derived neurotrophic factor, glial-derived neurotrophic factor, cardiotrophin-1, and vitronectin). Neurons were characterized by immunoreactivity for neurofilament 150, neuron-specific enolase, Islet-1 antibodies, electrophysiology, and the cultures were maintained for 4–6 wk. This culture system could be a useful tool for the study of adult mammalian spinal neurons in a functional in vitro system.     

A novel proteinaceous photolinker for simultaneous binding to an inert surface and a biomolecule

A simple and versatile method is developed for covalently binding a protein ligand onto a matrix irrespective of functional groups either on the ligand or the matrix. Prerequisite of the method is a novel proteinaceous photolinker having multiple light-activable functional groups. We have made photoreactive-BSA – a proteinaceous photolinker by the reaction of bovine serum albumin (BSA) with excess of 1-fluoro-2-nitro-4-azidobenzene (FNAB). When an enzyme is placed on an inert polystyrene matrix in presence of photoreactive-BSA and exposed to light the later forms highly reactive nitrenes some of which bind to the matrix and the rest to the ligand resulting simultaneous formation of covalent bonds with the matrix and the enzyme. The method is further exemplified by performing ELISA by covalent binding of antigen or antibody on a polystyrene microtiter plate in just 30 min using photoreactive-BSA. ELISA carried out in less than 3 h using photoreactive-BSA showed comparable results with that of conventional ELISA carried out in 18 h. Thus the method is potentially useful for rapid ELISA or covalent immobilization of ligands onto an inert surface without prior activation.     

A novel proteinaceous photolinker for simultaneous binding to an inert surface and a biomolecule

A simple and versatile method is developed for covalently binding a protein ligand onto a matrix irrespective of functional groups either on the ligand or the matrix. Prerequisite of the method is a novel proteinaceous photolinker having multiple light-activable functional groups. We have made photoreactive-BSA – a proteinaceous photolinker by the reaction of bovine serum albumin (BSA) with excess of 1-fluoro-2-nitro-4-azidobenzene (FNAB). When an enzyme is placed on an inert polystyrene matrix in presence of photoreactive-BSA and exposed to light the later forms highly reactive nitrenes some of which bind to the matrix and the rest to the ligand resulting simultaneous formation of covalent bonds with the matrix and the enzyme. The method is further exemplified by performing ELISA by covalent binding of antigen or antibody on a polystyrene microtiter plate in just 30 min using photoreactive-BSA. ELISA carried out in less than 3 h using photoreactive-BSA showed comparable results with that of conventional ELISA carried out in 18 h. Thus the method is potentially useful for rapid ELISA or covalent immobilization of ligands onto an inert surface without prior activation.     

A preliminary biomechanical assessment of a polymer composite hip implant using an infrared thermography technique validated by strain gage measurements

With the resurgence of composite materials in orthopaedic applications, a rigorous assessment of stress is needed to predict any failure of bone-implant systems. For current biomechanics research, strain gage measurements are employed to experimentally validate finite element models, which then characterize stress in the bone and implant. Our preliminary study experimentally validates a relatively new nondestructive testing technique for orthopaedic implants. Lock-in infrared (IR) thermography validated with strain gage measurements was used to investigate the stress and strain patterns in a novel composite hip implant made of carbon fiber reinforced polyamide 12 (CF/PA12). The hip implant was instrumented with strain gages and mechanically tested using average axial cyclic forces of 840 N, 1500 N, and 2100 N with the implant at an adduction angle of 15 deg to simulate the single-legged stance phase of walking gait. Three-dimensional surface stress maps were also obtained using an IR thermography camera. Results showed almost perfect agreement of IR thermography versus strain gage data with a Pearson correlation of R(2) = 0.96 and a slope = 1.01 for the line of best fit. IR thermography detected hip implant peak stresses on the inferior-medial side just distal to the neck region of 31.14 MPa (at 840 N), 72.16 MPa (at 1500 N), and 119.86 MPa (at 2100 N). There was strong correlation between IR thermography-measured stresses and force application level at key locations on the implant along the medial (R(2) = 0.99) and lateral (R(2) = 0.83 to 0.99) surface, as well as at the peak stress point (R(2) = 0.81 to 0.97). This is the first study to experimentally validate and demonstrate the use of lock-in IR thermography to obtain three-dimensional stress fields of an orthopaedic device manufactured from a composite material.     

Induced fit of an epitope peptide to a monoclonal antibody probed with a novel parallel surface plasmon resonance assay

Class II major histocompatibility complex proteins bind peptides for presentation to T-cells as part of the immune response process. Monoclonal antibody MEM-265 recognizes the peptide-free conformation of the major histocompatibility complex class II protein HLA-DR1 through specific binding to an epitope contained between residues 50-67 of the beta-chain. In previous work using alanine scanning (1), we identified residues Leu-53, Asp-57, Tyr-60, Trp-61, Ser-63, and Leu-67 as essential for specific recognition by MEM-265. The spacing of these residues approximates a 3.5-residue repeat, suggesting that MEM-265 may recognize the epitope in an alpha-helical conformation. In the folded, peptide-loaded DR1 structure, the beta-chain residues 50-67 contain a kinked alpha-helical segment spanning Glu-52-Ser-63 (2). However, the conformation of this segment in the peptide-free form is unknown. We have used a new surface plasmon resonance approach in a SpotMatrix format to compare the kinetic rates and affinities for 18 alanine scanning mutants comprising epitope residues 50-67. In addition to the six essential residues described previously, we found two additional residues, Glu-52 and Gln-64, that contribute by enhancing MEM-265 binding. By contrast, mutation of either Gly-54 or Pro-56 to an alanine actually improved binding to MEM-265. In essentially all cases peptide substitutions that either improve or reduce MEM-265 recognition could be traced to differences in the dissociation rate (k off). The kinetic details of the present study support the presence of a structural component in the antigenic epitope recognized by MEM-265 in the peptide-free form of major histocompatibility complex II DR1 beta-chain.