Damage evolution in acetabular replacements under long-term physiological loading conditions

Damage development in cemented acetabular replacements has been studied in bovine pelvic bones under long-term physiological loading conditions, including normal walking, stair climbing and a combined block loading with representative routine activities. The physiological loading conditions were achieved using a specially designed hip simulator for fixation endurance testing. Damage was detected and monitored using micro-CT scanning at regular intervals of the experiments, and verified by microscopic studies post testing. The results show that debonding at the bone–cement interface defined the failure of cement fixation in all cases, and debondings initiated near the dome of the acetabulum in the superior–posterior quadrant, consistent with the high-stress region identified from the finite element analysis of implanted acetabular models Zant, N.P., Heaton-Adegbile, P., Hussell, J.G., Tong, J., 2008b. In-vitro fatigue failure of cemented acetabular replacements—a hip simulator study. Journal of Biomechanical Engineering, Transactions of the ASME, 130, 021019-1–9]; [Tong, J., Zant, N.P., Wang, J-Y., Heaton-Adegbile, P., Hussell, J.G., 2008. Fatigue in cemented acetabulum. International Journal of Fatigue, 30(8), 1366–1375].     

Visualization of single receptor molecules bound to human rhinovirus under physiological conditions

Dynamic force microscopy (DFM) was used to image human rhinovirus HRV2 alone and complexed with single receptor molecules under near physiological conditions. Specific and site-directed immobilization of HRV2 on a model cell membrane resulted in a crystalline arrangement of virus particles with hexagonal symmetry and 35 nm spacing. High-resolution imaging of the virus capsid revealed about 20 resolvable structural features with 3 nm diameters; this finding is in agreement with protrusions seen by cryo-electron microscopy. Binding of receptor molecules to individual virus particles was observed after injection of soluble receptors into the liquid cell. Virus-receptor complexes with zero, one, two, or three attached receptor molecules were resolved. The number of receptor molecules associated to virions increased over time. Occasionally, dissociation of single receptor molecules from viral particles was also observed.     

The effect of muscle loading on the simulation of bone remodelling in the proximal femur

A large number of finite element analyses of the proximal femur rely on a simplified set of muscle and joint contact loads to represent the boundary conditions of the model. In the context of bone remodelling analysis around hip implants, muscle loading affects directly the spatial distribution of the remodelling signal. In the present study we performed a sensitivity analysis on the effect of different muscle loading configurations on the outcome of the bone remodelling simulation. An anatomical model of the femur with the implanted stem in place was constructed using the CT data of the Visible Human Project dataset of the National Institute of Health. The model was loaded with three muscle force configurations with increasing level of complexity. A strain adaptive remodelling rule was employed to simulate the post-operative bone changes around the implant stem and the results of the simulation were assessed quantitatively in terms of the bone mineral content changes in 18 periprosthetic regions of interest. The results showed considerable differences in the amount of bone loss predicted between the three cases. The simplified models generally predicted more pronounced bone loss. Although the overall remodelling patterns observed were similar, the bone conserving effect of additional muscle forces in the vicinity of their areas of attachment was clear. The results of this study suggest that the loading configuration of the FE model does play an important role in the outcome of the remodelling simulation.     

Resonance-based oscillations could describe human gait mechanics under various loading conditions

The oscillatory behavior of the center of mass (CoM) and the corresponding ground reaction force (GRF) of human gait for various gait speeds can be accurately described in terms of resonance using a spring–mass bipedal model. Resonance is a mechanical phenomenon that reflects the maximum responsiveness and energetic efficiency of a system. To use resonance to describe human gait, we need to investigate whether resonant mechanics is a common property under multiple walking conditions. Body mass and leg stiffness are determinants of resonance; thus, in this study, we investigated the following questions: (1) whether the estimated leg stiffness increased with inertia, (2) whether a resonance-based CoM oscillation could be sustained during a change in the stiffness, and (3) whether these relationships were consistently observed for different walking speeds. Seven healthy young subjects participated in over-ground walking trials at three different gait speeds with and without a 25-kg backpack. We measured the GRFs and the joint kinematics using three force platforms and a motion capture system. The leg stiffness was incorporated using a stiffness parameter in a compliant bipedal model that best fitted the empirical GRF data. The results showed that the leg stiffness increased with the load such that the resonance-based oscillatory behavior of the CoM was maintained for a given gait speed. The results imply that the resonance-based oscillation of the CoM is a consistent gait property and that resonant mechanics may be useful for modeling human gait.     

Calcium binding to rabbit skeletal myosin under physiological conditions

At a free Mg²⁺ concentration of 1.0 mM, myosin binds one Ca²⁺ per molecule when the Ca²⁺ concentration is 20 μM, a value in the concentration range expected during contraction of skeletal muscle. Mg²⁺ alters Ca²⁺ binding in a complex manner, not by simple competition. In the range from 20 to 100 μM Mg²⁺ it produces positive cooperativity between the high-affinity Ca²⁺ binding sites, in addition to shifting binding to higher Ca²⁺ concentrations. High-affinity Ca²⁺ binding is not significantly affected by the addition of ATP, increase in ionic strength to 0.1 and changes in temperature. Ca²⁺ binding did not increase actin-activated ATPase activity in the absence of regulatory proteins, but rather inhibited it.     

Nitric oxide binding to oxygenated hemoglobin under physiological conditions

We have added nitric oxide (NO) to hemoglobin in 0.1 M and 0.01 M phosphate buffers as well as to whole blood, all as a function of hemoglobin oxygen saturation. We found that in all these conditions, the amount of nitrosyl hemoglobin (HbNO) formed follows a model where the rates of HbNO formation and methemoglobin (metHb) formation (via hemoglobin oxidation) are independent of oxygen saturation. These results contradict those of an earlier report where, at least in 0.01 M phosphate, an elevated amount of HbNO was formed at high oxygen saturations. A radical rethink of the reaction of oxyhemoglobin with NO under physiological conditions was called for based on this previous proposition that the primary product is HbNO rather than metHb and nitrate. Our results indicate that no such radical rethink is called for.     

In vitro long-term culture of human bone under physiological load conditions]

There is evidence that mechanical loading is an important, if not the most important factor influencing bone mass and architecture. Investigations under in vivo conditions and cell culture methods, performed during the last years, helped to elucidate these mechanisms. However, the mechanisms by which load bearing acts on bone tissue are until now not completely understood. It is well accepted that weight-bearing exercise increases bone mass and on the other hand lower physical activity engenders bone loss. But neither a physiological threshold for bone loss or bone growth nor the character of the mechanical stimulus concerning amount, frequency and duration of the applied load are known. Even more speculative is the idea how this signal is transformed into the biological response of growing bone. Three-dimensional bone-culture-systems with simultaneous applied mechanical load enables to improve the knowledge of regulation of bone metabolism. We show the results of a long-term in vitro experiment with human cancellous bone under physiological load conditions.     

Cyanomet human hemoglobin crystallized under physiological conditions exhibits the Y quaternary structure

Cyanomet human hemoglobin has been crystallized at a chloride ion concentration and pH similar to physiological conditions. Molecular replacement calculations definitively show that the hemoglobin subunits are arranged in the Y quaternary form recently discovered in carbon monoxy hemoglobin Ypsilanti (99 Asp–Tyr), and subsequently observed in carbon monoxy normal human hemoglobin crystallized at low ionic strength and low pH. The structure has been refined at 2.09 Å resolution to an R-value of 0.232, and further refinement is currently underway. Although the refinement is not yet complete, our results are the first indication that the Y structure may represent an important quaternary form of liganded hemoglobin under physiological buffer conditions. These results suggest the need for a reexamination of structure–function correlations in the hemoglobin system. © 1994 John Wiley & Sons, Inc.     

Nitric oxide binding to oxygenated hemoglobin under physiological conditions.

We have added nitric oxide (NO) to hemoglobin in 0.1 M and 0.01 M phosphate buffers as well as to whole blood, all as a function of hemoglobin oxygen saturation. We found that in all these conditions, the amount of nitrosyl hemoglobin (HbNO) formed follows a model where the rates of HbNO formation and methemoglobin (metHb) formation (via hemoglobin oxidation) are independent of oxygen saturation. These results contradict those of an earlier report where, at least in 0.01 M phosphate, an elevated amount of HbNO was formed at high oxygen saturations. A radical rethink of the reaction of oxyhemoglobin with NO under physiological conditions was called for based on this previous proposition that the primary product is HbNO rather than metHb and nitrate. Our results indicate that no such radical rethink is called for.     

Automated multi-well device to measure transepithelial electrical resistances under physiological conditions

Measurement of transendothelial or transepithelial electrical resistances (TERs) is a straightforward in situ experimental approach to monitor the expression or modulation of barrier-forming cell-to-cell contacts (tight junctions) in cultured cells grown on porous filters. Although widely accepted, there is currently no device available to automatically measure the time course of TERs under ordinary cell culture conditions (37 degrees C, 5% or 10% CO2). This paper describes a development from our laboratory that is capable of following in parallel the TERs of several filter-grown cell layers with time and in an entirely computer-controlled fashion. The cell cultures can be followed even in long-term experiments without any manual assistance or opening of the incubator Besides reading TER values, this approach also returns the electrical capacitance of the cell layers, which is indicative of the expression of microvilli and other membrane extrusions. The device is based on reading the frequencydependent impedance of the cell layer, followed by equivalent circuit modeling to extract the cell-related parameters. It is compatible with several multi-well formats (up to 96 wells) and controlled by custom-designed software that reads, analyzes, and presents the data.     

Endothelial cell response to biomechanical forces under simulated vascular loading conditions

In vivo, endothelial cells (EC) are constantly exposed to the haemodynamic forces (HF) of pressure, wall shear stress and hoop stress. The main aim of this study was to design, create and validate a novel perfusion bioreactor capable of delivering shear stress and intravascular pressure to EC in vitro and to characterise their morphology, orientation and gene expression. Here we report the creation and validation of such a simulator and the dual application of pressure (120/60 mmHg) and low shear stress (5 dyn/cm(2)) to a monolayer of EC established on a non-compliant silicone tube. Under these conditions, EC elongated and realigned obliquely to the direction of applied shear stress in a time-dependent manner. Furthermore, randomly distributed F-actin microfilaments reorganised into long, dense stress fibres crossing the cells in a direction perpendicular to that of flow. Finally, combinatorial biomechanical conditioning of EC induced the expression of the inflammatory-associated E-selectin gene.     

A CT-based high-order finite element analysis of the human proximal femur compared to in-vitro experiments

The prediction of patient-specific proximal femur mechanical response to various load conditions is of major clinical importance in orthopaedics. This paper presents a novel, empirically validated high-order finite element method (FEM) for simulating the bone response to loads. A model of the bone geometry was constructed from a quantitative computerized tomography (QCT) scan using smooth surfaces for both the cortical and trabecular regions. Inhomogeneous isotropic elastic properties were assigned to the finite element model using distinct continuous spatial fields for each region. The Young's modulus was represented as a continuous function computed by a least mean squares method. p-FEMs were used to bound the simulation numerical error and to quantify the modeling assumptions. We validated the FE results with in-vitro experiments on a fresh-frozen femur loaded by a quasi-static force of up to 1500 N at four different angles. We measured the vertical displacement and strains at various locations and investigated the sensitivity of the simulation. Good agreement was found for the displacements, and a fair agreement found in the measured strain in some of the locations. The presented study is a first step toward a reliable p-FEM simulation of human femurs based on QCT data for clinical computer aided decision making.     

The contribution of the glenoid labrum to glenohumeral stability under physiological joint loading using finite element analysis

The labrum contributes to passive glenohumeral joint stability. Cadaveric studies have demonstrated that this has position and load dependency, which has not been quantified under physiological loads. This study aims to validate subject-specific finite element (FE) models against in vitro measurements of joint stability and to utilise the FE models to predict joint stability under physiological loads. The predicted stability values were within ± one standard deviation of experimental data and the FE models showed a reduction in stability of 10–15% with high, physiological, loads. The developed regression equations provide the first representation of passive glenohumeral stability and will aid surgical decision-making.     

Interaction of group A streptococci with human plasmin(ogen) under physiological conditions

A series of methods for analyzing the interaction of group A streptococci with the human plasminogen system are described. Examples of group A streptococcal isolates capable of assembling surface plasminogen activator activity when grown in human plasma are presented and the key requirements for this process are evaluated. The stabilities of cell-associated plasmin and plasminogen activator complexes are compared and a model for the interaction of group A streptococci with the plasminogen system in an infected host is presented.     

Structural rearrangement of human lymphotactin, a C chemokine, under physiological solution conditions

NMR spectra of human lymphotactin (hLtn), obtained under various solution conditions, have revealed that the protein undergoes a major conformational rearrangement dependent on temperature and salt concentration. At high salt (200 mm NaCl) and low temperature (10 degrees C), hLtn adopts a chemokine-like fold, which consists of a three-stranded antiparallel beta-sheet and a C-terminal alpha-helix (Kulo?lu, E. S., McCaslin, D. R., Kitabwalla, M., Pauza, C. D., Markley, J. L., and Volkman, B. F. (2001) Biochemistry 40, 12486-12496). We have used NMR spectroscopy, sedimentation equilibrium, and intrinsic fluorescence to monitor the reversible conformational change undergone by hLtn as a function of temperature and ionic strength. We have used two-, three- and four-dimensional NMR spectroscopy of isotopically enriched protein samples to determine structural properties of the conformational state stabilized at 45 degrees C and 0 mm NaCl. Patterns of NOEs and (1)H(alpha) and (13)C chemical shifts show that hLtn rearranges under these conditions to form a four-stranded, antiparallel beta-sheet with a pattern of hydrogen bonding that is completely different from that of the chemokine fold stabilized at 10 degrees C and 200 mm NaCl. The C-terminal alpha-helix observed at 10 degrees C and 200 mm NaCl, which is conserved in other chemokines, is absent at 45 degrees C and no salt, and the last 38 residues of the protein are completely disordered, as indicated by heteronuclear (15)N-(1)H NOEs. Temperature dependence of the tryptophan fluorescence of hLtn in low and high salt confirmed that the chemokine conformation is stabilized by increased ionic strength. Sedimentation equilibrium analytical ultracentrifugation showed that hLtn at 40 degrees C in the presence of 100 mm NaCl exists mainly as a dimer. Under near physiological conditions of temperature, pH, and ionic strength, both the chemokine-like and non-chemokine-like conformations of hLtn are significantly populated. The functional relevance of this structural interconversion remains to be elucidated.     

Specimen-specific predictions of contact stress under physiological loading in the human hip: validation and sensitivity studies

Hip osteoarthritis may be initiated and advanced by abnormal cartilage contact mechanics, and finite element (FE) modeling provides an approach with the potential to allow the study of this process. Previous FE models of the human hip have been limited by single specimen validation and the use of quasi-linear or linear elastic constitutive models of articular cartilage. The effects of the latter assumptions on model predictions are unknown, partially because data for the instantaneous behavior of healthy human hip cartilage are unavailable. The aims of this study were to develop and validate a series of specimen-specific FE models, to characterize the regional instantaneous response of healthy human hip cartilage in compression, and to assess the effects of material nonlinearity, inhomogeneity and specimen-specific material coefficients on FE predictions of cartilage contact stress and contact area. Five cadaveric specimens underwent experimental loading, cartilage material characterization and specimen-specific FE modeling. Cartilage in the FE models was represented by average neo-Hookean, average Veronda Westmann and specimen- and region-specific Veronda Westmann hyperelastic constitutive models. Experimental measurements and FE predictions compared well for all three cartilage representations, which was reflected in average RMS errors in contact stress of less than 25 %. The instantaneous material behavior of healthy human hip cartilage varied spatially, with stiffer acetabular cartilage than femoral cartilage and stiffer cartilage in lateral regions than in medial regions. The Veronda Westmann constitutive model with average material coefficients accurately predicted peak contact stress, average contact stress, contact area and contact patterns. The use of subject- and region-specific material coefficients did not increase the accuracy of FE model predictions. The neo-Hookean constitutive model underpredicted peak contact stress in areas of high stress. The results of this study support the use of average cartilage material coefficients in predictions of cartilage contact stress and contact area in the normal hip. The regional characterization of cartilage material behavior provides the necessary inputs for future computational studies, to investigate other mechanical parameters that may be correlated with OA and cartilage damage in the human hip. In the future, the results of this study can be applied to subject-specific models to better understand how abnormal hip contact stress and contact area contribute to OA.