Influence of Microarchitecture on the Mechanical Fatigue Behaviour of Equine Subchondral Bone

Fractures of the equine metacarpophalangeal (MCP) joint are among the most common and fatal injuries experienced by racehorses. These bone injuries are a direct result of repetitive, high intensity loading of the skeleton during racing and training and there is consensus that they represent a mechanical fatigue phenomenon. Existing work has found the fatigue life of bone to be strongly determined by bone microarchitecture and the resulting stressed volume (i.e., the volume of bone stressed above yield). The purpose of this study was to quantify the influence of bone microarchitecture on the mechanical fatigue behaviour of equine subchondral bone from the MCP joint. Forty-eight subchondral bone samples were prepared from the third metacarpal (MC3) and proximal phalanx (P1) and subsequently imaged using high resolution micro-computed tomography (μCT) to quantify microarchitectural features of interest, including bone volume fraction, tissue mineral density, pore size, pore spacing, and pore number. Samples were cyclically loaded in compression to a stress of 70 MPa, and fatigue life was defined as the number of cycles until failure. Finite element models were created from the μCT images and used to quantify the stressed volume. Based on the expected log point-wise predictive density (ELPD), stressed volume was a strong predictor of fatigue life in both the MC3 and P1. Normalized stress (i.e., initial nominal strain) was also a strong predictor of fatigue life in samples from the MC3, but not for samples from the P1. This disparity can be attributed to differences in microstructure homogeneity. A regional analysis indicated fatigue life was more strongly associated with bone volume fraction in the superficial (r2 = 0.32, p < 0.001) and middle (r2 = 0.70, p < 0.001) regions of the subchondral bone, indicating that the cortical plate plays a more prominent role in the fatigue resistance of subchondral bone. By improving our understanding of the variance in fatigue life measurements, this research helps begin to clarify the underlying mechanisms of the mechanical fatigue process and provide a basic understanding of subchondral bone injuries in the equine fetlock joint

Kinetic energy spectrum of horizontal motions in middle-atmosphere models

Data from several middle-atmosphere general circulation models are used to calculate kinetic energy spectra as a function of total horizontal wavenumber n. The horizontal and vertical resolution between models varies but all have upper. boundaries at heights greater than or similar to 80 km. Tropospheric spectra show power-law behavior with slopes slightly shallower than -3 for wavenumbers n greater than or similar to 10 (horizontal wavelengths less than or similar to 4000 km) and are dominated by the rotational part of the flow. These spectra agree well with those calculated using data obtained from a global assimilation model and with the results of previous observational studies. Stratospheric spectra have larger amplitudes than tropospheric ones at planetary scales and smaller amplitudes at smaller scales. Mesospheric spectra are characterized by enhanced spectral amplitudes at all wavenumbers compared to the stratosphere and spectral slopes in the wavenumber range n greater than or similar to 10 are generally shallower. Stratospheric and mesospheric spectra include approximately equal contributions from the rotational and divergent parts of the flow for n greater than or similar to 20 in all models. These features appear to be independent of model resolution. The divergent part of the flow, presumably associated with explicitly resolved inertiogravity waves in the models, increases more rapidly with height above the lower stratosphere than the rotational part. The divergent part is fairly insensitive to season, whereas the rotational part changes considerably between January and July in the middle-atmosphere region. Spectral amplitudes and vertical growth rates of both parts vary widely between models for a given season. The horizontal diffusion schemes used by the models are compared in an attempt to explain some of these differences

On the nature of large-scale mixing in the stratosphere and mesosphere

Studies of tracer transport in the stratosphere have shown that adiabatic quasi-horizontal tracer evolution is controlled primarily by the large-scale low-frequency component of the flow. This behavior is consistent with the concept of chaotic advection, wherein the Eulerian velocity field is spatially coherent and temporally quasi-regular on timescales over which the Lagrangian evolution is chaotic. In this study, winds from a middle atmosphere general circulation model (the Canadian Middle Atmosphere Model) are used to compare and contrast the nature of tracer evolution in the stratosphere and mesosphere. It is found that the concept of chaotic advection is relevant in the stratosphere but not in the mesosphere. The explanation for this behavior is the increased strength of gravity wave activity in the mesosphere as compared with the stratosphere, which leads to shallower kinetic energy spectra on synoptic scales and a much shorter Eulerian correlation time. The shallower kinetic energy spectra imply that tracer evolution in the mesosphere is spectrally local, in contrast with the spectrally nonlocal regime that prevails in the stratosphere. This means that tracer advection calculations in the mesosphere are controlled primarily by the gravity wave spectrum and are intrinsically resolution dependent

Model-measurement comparison of mesospheric temperature inversions, and a simple theory for their occurrence

Mesospheric temperature inversions are well established observed phenomena, yet their properties remain the subject of ongoing research. Comparisons between Rayleigh-scatter lidar temperature measurements obtained by the University of Western Ontario's Purple Crow Lidar (42.9°N, 81.4°W) and the Canadian Middle Atmosphere Model are used to quantify the statistics of inversions. In both model and measurements, inversions occur most frequently in the winter and exhibit an average amplitude of ∼10 K. The model exhibits virtually no inversions in the summer, while the measurements show a strongly reduced frequency of occurrence with an amplitude about half that in the winter. A simple theory of mesospheric inversions based on wave saturation is developed, with no adjustable parameters. It predicts that the environmental lapse rate must be less than half the adiabatic lapse rate for an inversion to form, and it predicts the ratio of the inversion amplitude and thickness as a function of environmental lapse rate. Comparison of this prediction to the actual amplitude/thickness ratio using the lidar measurements shows good agreement between theory and measurements

The troposphere-to-stratosphere transition in kinetic energy spectra and nonlinear spectral fluxes as seen in ECMWF analyses

Global horizontal wavenumber kinetic energy spectra and spectral fluxes of rotational kinetic energy and enstrophy are computed for a range of vertical levels using a T799 ECMWF operational analysis. Above 250 hPa, the kinetic energy spectra exhibit a distinct break between steep and shallow spectral ranges, reminiscent of dual power-law spectra seen in aircraft data and high-resolution general circulation models. The break separates a large-scale ‘‘balanced’’ regime in which rotational flow strongly dominates divergent flow and a mesoscale ‘‘unbalanced’’ regime where divergent energy is comparable to or larger than rotational energy. Between 230 and 100 hPa, the spectral break shifts to larger scales (from n 5 60 to n 5 20, where n is spherical harmonic index) as the balanced component of the flow preferentially decays. The location of the break remains fairly stable throughout the stratosphere. The spectral break in the analysis occurs at somewhat larger scales than the break seen in aircraft data. Nonlinear spectral fluxes defined for the rotational component of the flow maximize between about 300 and 200 hPa. Large-scale turbulence thus centers on the extratropical tropopause region, within which there are two distinct mechanisms of upscale energy transfer: eddy–eddy interactions sourcing the transient energy peak in synoptic scales, and zonal mean–eddy interactions forcing the zonal flow. A well-defined downscale enstrophy flux is clearly evident at these altitudes. In the stratosphere, the transient energy peak moves to planetary scales and zonal mean–eddy interactions become dominant

The impact of war on cytopathological practice in Ukraine

Aim: To investigate the impact of Russian invasion on Ukrainian healthcare services related to cytopathology screening and diagnostics. Methods: The workload and output of the cytopathological laboratory located in Kyiv before and during 8 months of Russian invasion were analyzed. The laboratory performance was assessed by the total number of cytologic cases, the turn-around time (TAT) index, and the percentage of timely reported test results. The geographic distribution of cases was also analyzed. Results: There was a significant decline in the workload of the laboratory during the first two months of the war, and the numbers of cytopathology specimens fell to less than one third of prewar levels, and the TAT was prolonged. Since May 2022, the efficiency and quality of cytopathologic testing has been largely restored in most parts of Ukraine, only to be affected again by increased Russian bombardment toward the end of summer 2022. The number of cytology specimens was reduced and the source of cytology specimens changed from the prewar conditions reflecting the relocation of the CSD Lab to western Ukraine. Conclusion: Cytolopathological screening and diagnostics were seriously disrupted in Ukraine during the Russian invasion resulting in a decreased volume of cytology specimens received in the CSD Lab during the early months of the war. By adapting to the war conditions and reorganizing the cytology services, CSD Lab has continued providing cytology services at a level of efficiency similar to those of the prewar period. However, the volume of cytology specimens remains much smaller than before war, indicating that the cytology services have been adversely affected by the war

Sponge layer feedbacks in middle-atmosphere models

Middle-atmosphere models commonly employ a sponge layer in the upper portion of their domain. It is shown that the relaxational nature of the sponge allows it to couple to the dynamics at lower levels in an artificial manner. In particular, the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating is shown to induce a drag force in the sponge that modifies the response expected from the “downward control” arguments of Haynes et al. [1991]. In the case of an imposed local force the sponge acts to divert a fraction of the mean meridional mass flux upward, which for realistic parameter values is approximately equal to exp(−Δz/H), where Δz is the distance between the forcing region and the sponge layer and H is the density scale height. This sponge-induced upper cell causes temperature changes that, just below the sponge layer, are of comparable magnitude to those just below the forcing region. In the case of an imposed local diabatic heating, the sponge induces a meridional circulation extending through the entire depth of the atmosphere. This circulation causes temperature changes that, just below the sponge layer, are of opposite sign and comparable in magnitude to those at the heating region. In both cases, the sponge-induced temperature changes are essentially independent of the height of the imposed force or diabatic heating, provided the latter is located outside the sponge, but decrease exponentially as one moves down from the sponge. Thus the effect of the sponge can be made arbitrarily small at a given altitude by placing the sponge sufficiently high; e.g., its effect on temperatures two scale heights below is roughly at the 10% level, provided the imposed force or diabatic heating is located outside the sponge. When, however, an imposed force is applied within the sponge layer (a highly plausible situation for parameterized mesospheric gravity-wave drag), its effect is almost entirely nullified by the sponge-layer feedback and its expected impact on temperatures below largely fails to materialize. Simulations using a middle-atmosphere general circulation model are described, which demonstrate that this sponge-layer feedback can be a significant effect in parameter regimes of physical interest. Zonally symmetric (two dimensional) middle-atmosphere models commonly employ a Rayleigh drag throughout the model domain. It is shown that the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating, in this case, is noticeably modified from that expected from downward control, even for a very weak drag coefficien