Internal Plateau Research Articles

Fractura de platillo tibial interno tras artroplastia unicompartimental de rodilla

The aim of the article is to report the existence of internal tibial plateau fractures as a possible complication in certain circumstances following a unicompartmental knee replacement. We report a patient who suffered a fracture of the internal tibial plateau after unicompartmental knee arthroplasty (UKA) that required revision surgery with a bone and mesh allograft. Peri-prosthetic fractures after a unicompartmental knee arthroplasty are rare but with the increasing indications for this procedure in recent years there are more and more cases with complications that can be a challenge for the orthopaedic surgeon. When there is a periprosthetic fracture in a unicompartmental knee arthroplasty, revision total joint replacement with bone and mesh allograft can provide good results, maintaining the functionality and avoiding long periods of immobilization.

Internal tibial plateau fractures following a unicompartmental knee arthroplasty

The aim of the article is to report the existence of internal tibial plateau fractures as a possible complication in certain circumstances following a unicompartmental knee replacement. We report a patient who suffered a fracture of the internal tibial plateau after unicompartmental knee arthroplasty (UKA) that required revision surgery with a bone and mesh allograft. Peri-prosthetic fractures after a unicompartmental knee arthroplasty are rare but with the increasing indications for this procedure in recent years there are more and more cases with complications that can be a challenge for the orthopaedic surgeon. When there is a periprosthetic fracture in a unicompartmental knee arthroplasty, revision total joint replacement with bone and mesh allograft can provide good results, maintaining the functionality and avoiding long periods of immobilization.

Mesoscale optical turbulence simulations above Dome C, Dome A and South Pole

In two recent papers the mesoscale model Meso-NH, joint with the Astro-Meso-NH package, has been validated at Dome C, Antarctica, for the characterization of the optical turbulence. It has been shown that the meteorological parameters (temperature and wind speed, from which the optical turbulence depends on) as well as the Cn2 profiles above Dome C were correctly statistically reproduced. The three most important derived parameters that characterize the optical turbulence above the internal antarctic plateau: the surface layer thickness, the seeing in the free-atmosphere and in the total atmosphere showed to be in a very good agreement with observations. Validation of Cn2 has been performed using all the measurements of the optical turbulence vertical distribution obtained in winter so far. In this paper, in order to investigate the ability of the model to discriminate between different turbulence conditions for site testing, we extend the study to two other potential astronomical sites in Antarctica: Dome A and South Pole, which we expect to be characterized by different turbulence conditions. The optical turbulence has been calculated above these two sites for the same 15 nights studied for Dome C and a comparison between the three sites has been performed.

Mesoscale optical turbulence simulations at Dome C: refinements

In a recent paper, the authors presented an extended study aimed at simulating the classical meteorological parameters and optical turbulence at Dome C during the winter with the atmospheric mesoscale model Meso-NH. The goal of that paper was to validate the model above Dome C with the support of measurements and to use it afterwards above the Internal Antarctic Plateau to discriminate between the qualities of different potential astronomical sites on the plateau. A statistical analysis has been presented and the conclusions of that paper were very promising. Wind speed and temperature fields (important for the computations of the optical turbulence parameters) were revealed to be reconstructed very well by the Meso-NH model, with better performances than achieved with the European Centre for Medium-Range Weather Forecasts (ECMWF) global model, especially near the surface. All results were revealed to be resolution-dependent and it has been proved that a grid-nesting configuration (three domains) with a high horizontal resolution (ΔX= 1 km) for the innermost domain is necessary to reconstruct all the optical turbulence features with a good correlation to measurements. High-resolution simulations provided an averaged surface-layer thickness just ∼14 m higher than estimated by measurements, and the seeing in the free atmosphere showed a dispersion from the observed one of just a few hundredths of an arcsec (Δɛ∼ 0.05 arcsec). The unique limitation of the previous study was that the optical turbulence in the surface layer appeared to be overestimated by the model in both low- and high-resolution modes. In this study we present the results obtained with an improved numerical configuration. The same 15 nights have been simulated, and we show that the model results now match the observations almost perfectly in all their features: the surface thickness, the seeing in the free atmosphere and in the surface layer. This result now permits us to investigate other Antarctic sites using a robust numerical model well adapted to the extreme polar conditions (Meso-NH).

Numerical simulations of Optical Turbulence at low and high horizontal resolution in Antarctica with a mesoscale meteorological model

It has already been demonstrated that a mesoscale meteorological model such as Meso-NH is highly reliable in reproducing 3D maps of optical turbulence. Preliminary measurements above the Antarctic Plateau have so far indicated a pretty good value for the seeing: around 0.3" at Dome C. However some uncertainties remain. That's why our group is focusing on a detailed study of the atmospheric flow and turbulence in the internal Antarctic Plateau. Our intention is to use the Meso-NH model to do predictions of the atmospheric flow and the corresponding optical turbulence in the internal plateau. The use of this model has another huge advantage: we have access to informations inside an entire 3D volume which is not the case with observations only. Two different configurations have been used: a low horizontal resolution (with a mesh-size of 100 km) and a high horizontal resolution with the grid-nesting interactive technique (with a mesh-size of 1 km in the innermost domain centered above the area of interest). We present here the turbulence distribution reconstructed by Meso-NH for 16 nights monitored in winter time 2005, looking at the the seeing and the surface layer thickness.

Optical Turbulence above the Internal Antarctic Plateau

The internal antarctic plateau revealed in the last years to be a site with interesting potentialities for the astronomical applications due to the extreme dryness and low temperatures, the typical high altitude of the plateau, the weak level of turbulence in the free atmosphere down to a just few tens of meters from the ground and the thin optical turbulence layer developed at the ground. The main goal of a site testing assessment above the internal antarctic plateau is to characterize the site (optical turbulence and classical meteorological parameters) and to quantify which is the gain we might obtain with respect to equivalent astronomical observations done above mid-latitude sites to support plans for future astronomical facilities. Our group is involved, since a few years, in studies related to the assessment of this site for astronomical applications that include the characterization of the meteorological parameters and optical turbulence provided by general circulation models as well as mesoscale atmospherical models and the quantification of the performances of Adaptive Optics (AO) systems. In this talk I will draw the status of art of this site assessment putting our studies in the context of the wide international site testing activity that has been done in Antarctica. I will focus on the site assessment relevant for astronomical applications to be done in the visible up to the near infrared ranges, i.e. those ranges for which the optical turbulence represents a perturbing element for the quality of the images and the AO techniques an efficient tool to correct these wavefront perturbations.

Study of the winter 2005 Antarctica polar vortex

During winter and springtime, the flow above Antarctica at high altitude (upper troposphere and stratosphere) is dominated by the presence of a vortex centered above the continent. It lasts typically from August to November. This vortex is characterized by a strong cyclonic jet centered above the polar high. In a recent study of our group (Hagelin et al., 2008) of four different sites in the Antarctic internal plateau (South Pole, Dome C, Dome A and Dome F), it was made the hypothesis that the wind speed strength in the upper atmosphere should be related to the distance of the site to the center of the Antarctic polar vortex. This high altitude wind is very important from an astronomical point of view since it might trigger the onset of the optical turbulence and strongly affect other optical turbulence parameters. What we are interested in here is to localize the position of the minimum value of the wind speed at high altitude in order to confirm the hypothesis of Hagelin et al. (2008).

Can X-ray emission powered by a spinning-down magnetar explain some gamma-ray burst light-curve features?

Long-duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetized outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetized millisecond pulsar or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray light curves of all GRBs observed by the Swift satellite, we identified a subset of bursts which have a feature in their light curves which we call an internal plateau – unusually constant emission followed by a steep decay – which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength, and find that they are consistent with the most extreme predicted values for magnetars.

Mesoscale optical turbulence simulations at Dome C

These last years ground-based astronomy has been looking towards Antarctica, especially its summits and the internal continental plateau where the optical turbulence (OT) appears to be confined in a shallow layer close to the surface. Preliminary measurements have so far indicated pretty good value for the seeing above 30-35 m: 0.36" (Agabi et al. 2006), 0.27" (Lawrence et al. 2004) and 0.3" (Trinquet et al. 2008) at Dome C. Site testing campaigns are however extremely expensive, instruments provide only local measurements and atmospheric modeling might represent a step ahead towards the search and selection of astronomical sites thanks to the possibility to reconstruct 3D Cn2 maps over a surface of several kilometers. The Antarctic Plateau represents therefore an important benchmark test to evaluate the possibility to discriminate sites on the same plateau. Our group (Hagelin et al. 2008) has proven that the analyses from the ECMWF global model do not describe with the required accuracy the antarctic boundary and surface layers in the plateau. A better description could be obtained with a mesoscale model. The Meso-Nh model has proven to be reliable in reproducing 3D maps of OT above mid-latitude astronomical sites (Masciadri et al. 1999ab, 2004, Masciadri and Jabouille 2005). In this paper we study the ability of the Meso-Nh model in reconstructing the meteorological parameters as well as the OT above Dome C with different model configurations. We concentrate our attention on the model abilities in reproducing the OT surface layer thickness Hsl and the integral of the Cn2 in the free atmosphere and in the surface layer. It is worth to highlight that these are the first estimates ever done so far with a mesoscale model of the optical turbulence above the internal Antarctic Plateau.

Comparison of the atmosphere above the South Pole, Dome C and Dome A: first attempt

The atmospheric properties above three sites (Dome C, Dome A and the South Pole) on the Internal Antarctic Plateau are investigated for astronomical applications using the monthly median of the analyses from ECMWF (the European Centre for Medium-Range Weather Forecasts). Radiosoundings extended on a yearly time scale at the South Pole and Dome C are used to quantify the reliability of the ECMWF analyses in the free atmosphere as well as in the boundary and surface layers, and to characterize the median wind speed in the first 100 m above the two sites. Thermodynamic instability properties in the free atmosphere above the three sites are quantified with monthly median values of the Richardson number. We find that the probability to trigger thermodynamic instabilities above 100 m is smaller on the Internal Antarctic Plateau than on mid-latitude sites. In spite of the generally more stable atmospheric conditions of the Antarctic sites compared to mid-latitude sites, Dome C shows worse thermodynamic instability conditions than those predicted above the South Pole and Dome A above 100 m. A rank of the Antarctic sites done with respect to the strength of the wind speed in the free atmosphere (ECMWF analyses) as well as the wind shear in the surface layer (radiosoundings) is presented.

Symmetry-breaking transitions and dissociation of two-dimensional Plateau borders

We have experimentally studied the dissociation/coalescence of internal Plateau borders (PBs) in simple monolayer bubble clusters, as a result of changing the liquid fraction. At large liquid content, the clusters consist of n bubbles of the same size, symmetrically placed around an internal n-sided PB (n-PB). On decreasing the liquid fraction we observed symmetry-breaking transitions in the 4- and 5-bubble clusters (but not in the 3-bubble cluster), followed by dissociation of the PBs. We used the Surface Evolver to determine the various equilibrium configurations of the corresponding two-dimensional wet clusters and their surface energies. The sequence of 4-bubble cluster configurations observed on varying the liquid fraction correlates qualitatively with that predicted on the basis of Surface Evolver calculations. The same is not true of the 5-bubble cluster.

Damping capacity of layered materials

Extensive measurements of the internal friction in a variety of layered materials such as graphite, metal dichalcogenides (MoS2, TiS2, NbSe2, TaS2, TaSe2, and TaTe2), and montmorillonite clays are reported. The internal friction of these layered materials exhibits features similar to those observed in graphite. In particular, a high temperature internal friction plateau develops at a temperature that is characteristic of the material. The physical mechanism for this plateau is discussed.

The Marcus-Wake Seamounts and Guyots as paleofracture indicators and their relation to the Dutton Ridge

The Dutton Ridge is on the oldest extant oceanic crust (the Jurassic) in the northwestern Pacific and lies on the Pacific plate. Because of internal alignments and different summit plateau break depths in line on a secondary axis, the ridge was probably once more contiguous, in the form of a plateau. The direction of movement of the Pacific plate is known to have been northwesterly for the past 43 m.y. and almost due north from then into the Cretaceous. However, there is a southwesterly azimuth through the region from the Hawaiian Emperor elbow to the subduction zone where the Dutton Ridge presently resides. This portion of the Pacific plate is overprinted by the Marcus-Wake Seamounts which formed during the Cretaceous. Evidence of the SW-striking features including fracture zones, flank rift zones, ridge alignments, and tectonic spreading fabric is shown. The time frame of the change in spreading direction and consequent fracture zone formation is suggested to have been between the formation of the Dutton Plateau and the Marcus-Wake Group, possibly from Late Jurassic to Early Cretaceous. The breakup of the Dutton Plateau to form the Dutton Ridge is suggested to have been caused by fracturing.

Ring furrows: Inversion of topography in Martian highland terrains

Ring furrows are flat-floored trenches, circular in plan view, surrounding a central, flat-topped, mesa or plateau. The internal plateau is about the same elevation or lower than the plain outside the ring. The outer wall is often breached by valley drainage or opened to low, degraded surfaces. Related landforms range from ring furrows with fractured central plateaus to those with isolated circular mesas without depressed rings. Ring furrows are superposed on many types of materials, but they are most common on cratered plateau-type materials which are interpreted as volcanic flows overlying ancient cratered terrain. Most rings occur in or near regions of fretted terrain. Ring furrows are formed by preferential removal of the exposed rims of partially buried craters. Ground ice decay and sapping followed by fluvial erosion are proposed for removal of the least resistant rim materials. Thus, differential erosion has caused an inversion of topography in which the originally elevated rim is reduced to negative relief.