The Relationship between the Composition and Mechanical Properties of Articular Cartilage

Articular cartilage (AC) is a soft connective tissue that lines the ends of synovial bones in joints. It is responsible for absorbing impact loads and resisting shear associated with joint articulation. Pathologies such as osteoarthritis have symptoms including degradation and sometimes complete loss of the articular cartilage, which can lead to poor load support and considerable pain. There has been considerable work in the field of cartilage mechanics. The transient deformation behaviour of loaded cartilage has been examined using several different experimental approaches, and complex models have been developed to accurately describe this behaviour. The microstructural response of AC to load is less well understood, however. The aim of this work was to investigate the anisotropy and heterogeneity of cartilage and its effect on the load bearing characteristics. Firstly, existing experimental data were used to create a layered finite element model of cartilage under load. Using this model, three sets of material parameters were evaluated for their suitability in reproducing experimentally observed strains, as well as minimising peak stresses. It was found that only by including the heterogeneity associated with collagen fibre orientation could the layer boundary deformations be exactly modelled, whilst preventing potentially damaging interfacial shear. Tensile testing of cartilage from the equine metacarpophalangeal joint, using samples obtained from each individual layer, was performed to validate findings from the model as well as to determine the variation in mechanical properties in regions of different weight bearing characteristics. It was found that the tensile stiffnesses varied with depth as predicted by the model, demonstrating an average value of 31.3 MPa at the surface and 9.4 MPa in the radial zone, although there was considerable variation. Polarised light microscopy was used to determine the preferential collagen orientation, as well as qualitatively assess the angular spread and other patterns in collagen organisation. It was found that the appearance of the collagen network varied both with depth and location on the joint, with high weight bearing regions showing more isotropic fibre distributions below the surface than low weight bearing regions. To directly probe the microstructural response of AC to load, the tensile loading rig was modified to allow simultaneous imaging of the sample using two photon fluorescence microscopy. This allowed the relative displacement of cells and elastin fibres, which are intrinsically fluorescent, to be observed at increasing levels of strain. From locations and orientations of these features, the strain field could be calculated at two length scales: in the vicinity of specific elastin fibres (microns) and intercellular strains averaged over whole stacks (hundreds of microns). The strains at the two different scales did not correlate, suggesting that the microscopic strain environment varies considerably. The elastin fibre network was also investigated, and it was found that fibres appear to interconnect both at pericellular matrices, as well as at 'nodes' in the extracellular matrix

Dark Matter Annihilation Signatures from Electroweak Bremsstrahlung

We examine observational signatures of dark matter annihilation in the Milky Way arising from electroweak bremsstrahlung contributions to the annihilation cross section. It has been known for some time that photon bremsstrahlung may significantly boost DM annihilation yields. Recently, we have shown that electroweak bremsstrahlung of W and Z gauge bosons can be the dominant annihilation channel in some popular models with helicity-suppressed 2 --> 2 annihilation. W/Z-bremsstrahlung is particularly interesting because the gauge bosons produced via annihilation subsequently decay to produce large correlated fluxes of electrons, positrons, neutrinos, hadrons (including antiprotons) and gamma rays, which are all of importance in indirect dark matter searches. Here we calculate the spectra of stable annihilation products produced via gamma/W/Z-bremsstrahlung. After modifying the fluxes to account for the propagation through the Galaxy, we set upper bounds on the annihilation cross section via a comparison with observational data. We show that stringent cosmic ray antiproton limits preclude a sizable dark matter contribution to observed cosmic ray positron fluxes in the class of models for which the bremsstrahlung processes dominate.Comment: 11 pages, 6 figures. Updated to match PRD versio

The micromechanics of the superficial zone of articular cartilage.

Journal ArticleOBJECTIVE: To investigate the relationships between the unique mechanical and structural properties of the superficial zone of articular cartilage on the microscopic scale. DESIGN: Fresh unstained equine metacarpophalangeal cartilage samples were mounted on tensile and compressive loading rigs on the stage of a multiphoton microscope. Sequential image stacks were acquired under incremental loads together with simultaneous measurements of the applied stress and strain. Second harmonic generation was used to visualise the collagen fibre network, while two photon fluorescence was used to visualise elastin fibres and cells. The changes visualised by each modality were tracked between successive loads. RESULTS: The deformation of the cartilage matrix was heterogeneous on the microscopic length scale. This was evident from local strain maps, which showed shearing between different regions of collagen under tensile strain, corrugations in the articular surface at higher tensile strains and a non-uniform distribution of compressive strain in the axial direction. Chondrocytes elongated and rotated under tensile strain and were compressed in the axial direction under compressive load. The magnitude of deformation varied between cells, indicating differences in either load transmission through the matrix or the mechanical properties of individual cells. Under tensile loading the reorganisation of the elastin network differed from a homogeneous elastic response, indicating that it forms a functional structure. CONCLUSIONS: This study highlights the complexity of superficial zone mechanics and demonstrates that the response of the collagen matrix, elastin fibres and chondrocytes are all heterogeneous on the microscopic scale.Arthritis Research U

Limited measurement dependence in multiple runs of a Bell test

The assumption of free will - the ability of an experimentalist to make random choices - is central to proving the indeterminism of quantum resources, the primary tool in quantum cryptography. Relaxing the assumption in a Bell test allows violation of the usual classical threshold by correlating the random number generators used to select measurements with the devices that perform them. In this paper, we examine not only these correlations, but those across multiple runs of the experiment. This enables an explicit exposition of the optimal cheating strategy and how the correlations manifest themselves within this strategy. Similar to other recent results, we prove that there remain Bell violations for a sufficiently high, yet non-maximal degree of free will which cannot be simulated by a classical attack, regardless of how many runs of the experiment those choices are correlated over.Comment: 8 pages, 3 figures. Version 2 reflects the published paper in Physical Review A, which among some minor additions/changes now includes Section V discussing the numerical approach to Bell inequalities other than CHS

Systematic problems with using dark matter simulations to model stellar halos

The limits of available computing power have forced models for the structure of stellar halos to adopt one or both of the following simplifying assumptions: (1) stellar mass can be "painted" onto dark matter (DM) particles in progenitor satellites; (2) pure DM simulations that do not form a luminous galaxy can be used. We estimate the magnitude of the systematic errors introduced by these assumptions using a controlled set of stellar halo models where we independently vary whether we look at star particles or painted DM particles, and whether we use a simulation in which a baryonic disk galaxy forms or a matching pure DM simulation that does not form a baryonic disk. We find that the "painting" simplification reduces the halo concentration and internal structure, predominantly because painted DM particles have different kinematics from star particles even when both are buried deep in the potential well of the satellite. The simplification of using pure DM simulations reduces the concentration further, but increases the internal structure, and results in a more prolate stellar halo. These differences can be a factor of 1.5-7 in concentration (as measured by the half-mass radius) and 2-7 in internal density structure. Given this level of systematic uncertainty, one should be wary of overinterpreting differences between observations and the current generation of stellar halo models based on DM-only simulations when such differences are less than an order of magnitude

Effect of stress and temperature on the optical properties of silicon nitride membranes at 1550 nm

Future gravitational-wave detectors operated at cryogenic temperatures are expected to be limited by thermal noise of the highly reflective mirror coatings. Silicon nitride is an interesting material for such coatings as it shows very low mechanical loss, a property related to low thermal noise, which is known to further decrease under stress. Low optical absorption is also required to maintain the low mirror temperature. Here, we investigate the effect of stress on the optical properties at 1,550 nm of silicon nitride membranes attached to a silicon frame. Our approach includes the measurement of the thermal expansion coefficient and the thermal conductivity of the membranes. The membrane and frame temperatures are varied, and translated into a change in stress using finite element modeling. The resulting product of the optical absorption and thermo-optic coefficient (dn/dT) is measured using photothermal common-path interferometry

Operational quasiprobabilities for qudits

We propose an operational quasiprobability function for qudits, enabling a comparison between quantum and hidden-variable theories. We show that the quasiprobability function becomes positive semidefinite if consecutive measurement results are described by a hidden-variable model with locality and noninvasive measurability assumed. Otherwise, it is negative valued. The negativity depends on the observables to be measured as well as a given state, as the quasiprobability function is operationally defined. We also propose a marginal quasiprobability function and show that it plays the role of an entanglement witness for two qudits. In addition, we discuss an optical experiment of a polarization qubit to demonstrate its nonclassicality in terms of the quasiprobability function.Comment: 10 pages, 4 figures, journal versio

The c(4×4)–a(1×3) surface reconstruction transition on InSb(001) : static versus dynamic conditions

The transition between the a(1 × 3) and c(4 × 4) surface reconstructions of InSb(0 0 1) has been carefully monitored by reflection high energy electron diffraction as a function of temperature and Sb2 flux, without incident In flux. Arrhenius-like behaviour is observed across the whole range of Sb2 fluxes and temperatures, allowing accurate internal calibration of substrate temperature. This behaviour is in contrast to aggregated data obtained under dynamic molecular beam epitaxy conditions, which show two regimes rather than a single Arrhenius-like phase boundary. The results are explained qualitatively by the atomistic kinetics in static versus dynamic conditions

Benthic megafauna on steep slopes at the Northern Mid-Atlantic Ridge

The role of small‐scale (<10 km) habitat availability in structuring deep‐sea hard substratum assemblages is poorly understood. Epibenthic megafauna and substratum availability were studied on steep slopes at the Mid‐Atlantic Ridge from May to July 2010 northwest, northeast, southwest and southeast of the Charlie‐Gibbs Fracture Zone (CGFZ; 48–54°N) at between 2095 and 2601 m depth. Megafauna were six times denser north of the CGFZ compared with the south and differences in density were almost entirely driven by sessile fauna. There was no significant difference in habitat availability amongst sites. Rocky substratum made up 48% of the total area surveyed, with individual transects having between 0% and 82% rock. Assemblage structures were different amongst all superstations. The north was dominated by demospongids and hexactinellids, whereas the southern superstations were dominated by anthozoans and hexactinellids. Differences in megafaunal assemblages north and south of the CGFZ primarily reflected variations in demospongid and anthozoan species composition. With 213–1825 individuals·ha−1, and 7–24 species per superstation, hexactinellids were the most species‐rich (36 species) and cosmopolitan taxa at the study site, supporting observations elsewhere along the ridge and in the CGFZ. The absence of significant differences in substrata availability suggested alternative drivers for density or percentage cover. The amount of hard substratum available only limited sessile megafauna density at one transect that was entirely covered with sediments. Species richness was highest for areas with intermediate values of substratum coverage (35–43% rock)