The Role of Scapular Dyskinesis in Rotator Cuff and Biceps Tendon Pathology

Shoulder tendon injuries including impingement, rotator cuff disease, and biceps tendon pathology are common clinical conditions and are a significant source of joint pain, instability, and dysfunction. These injuries may progress into partial tears then to complete tendon ruptures, which have limited healing capacity even when surgically repaired. These injuries are frequently seen in the presence of abnormal scapulothoracic joint kinematics (termed scapular dyskinesis). However, the cause and effect relationship between scapular dyskinesis and shoulder injury has not been directly defined. Additionally, while the incidence of shoulder injuries and recurrent failure of repairs is well-documented, the mechanisms behind them are not well-established, making optimal clinical management difficult. Therefore, the objectives of this study were to examine the effect of scapular dyskinesis on the initiation and progression of pathological changes in the rotator cuff and biceps tendon and to define the mechanical processes that lead to these changes. Unfortunately, clinical and cadaveric studies are unable to address the underlying causes of injury and cannot evaluate the injury process over time. Therefore, a rat model of scapular dyskinesis (created by denervating the trapezius and serratus anterior) was developed and used, both alone and in combination with overuse, to investigate the cause and effect relationships between changes in joint loading and alterations in tendon mechanical, histological, organizational, and biological properties. We hypothesized that scapular dyskinesis would result in altered joint loading conditions that would lead to degeneration of the rotator cuff and long head of the biceps. We found that scapular dyskinesis diminished joint function and passive joint mechanics and significantly reduced tendon properties. We also investigated the effect of overuse on tendon properties and found that overuse activity in the presence of scapular dyskinesis resulted in significantly more structural and biological adaptations than scapular dyskinesis alone. We also investigated the effect of scapular dyskinesis on supraspinatus tendon healing and found that scapular dyskinesis was detrimental to tendon properties. These results indicate that scapular dyskinesis is a causative mechanical mechanism of shoulder tendon injury. Identification of scapular dyskinesis as a mechanism of pathological changes will help inform and guide clinicians in developing optimal prevention and long-term rehabilitation strategies

Z2 vortices in the ground states of classical Kitaev Heisenberg models

The classical nearest neighbor Kitaev Heisenberg model on the triangular lattice is known to host Z2 spin vortices, forming a crystalline superstructure in the ground state. The Z2 vortices in this system can be understood as distortions of the local 120 degree N el parent order of the Heisenberg only Hamiltonian. Here, we explore possibilities of stabilizing further types of Z2 vortex phases in Kitaev Heisenberg models, including those which rely on more complicated types of noncollinear parent orders such as tetrahedral states. We perform extensive scans through large classes of Kitaev Heisenberg models on different lattices employing a two step methodology which first involves a mean field analysis followed by a stochastic iterative minimization approach. When allowing for longer range Kitaev couplings, we identify several Z2 vortex phases such as a state based on the 120 degree N el order on the triangular lattice which shows a coexistence of different Z2 vortex types. Furthermore, perturbing the tetrahedral order on the triangular lattice with a suitable combination of first and second neighbor Kitaev interactions, we find that a kagomelike superstructure of Z2 vortices may be stabilized, where vortices feature a counter rotating winding of spins on different sublattices. This last phase may also be extended to honeycomb lattices where it is related to cubic types of parent orders. In total, this analysis shows that Z2 vortex phases appear in much wider contexts than the 120 degree N el ordered systems previously studie

Discrete exterior calculus (DEC) for the surface Navier-Stokes equation

We consider a numerical approach for the incompressible surface Navier-Stokes equation. The approach is based on the covariant form and uses discrete exterior calculus (DEC) in space and a semi-implicit discretization in time. The discretization is described in detail and related to finite difference schemes on staggered grids in flat space for which we demonstrate second order convergence. We compare computational results with a vorticity-stream function approach for surfaces with genus 0 and demonstrate the interplay between topology, geometry and flow properties. Our discretization also allows to handle harmonic vector fields, which we demonstrate on a torus.Comment: 21 pages, 9 figure

Backbone conformational flexibility of the lipid modified membrane anchor of the human N-Ras protein investigated by solid-state NMR and molecular dynamics simulation

AbstractThe lipid modified human N-Ras protein, implicated in human cancer development, is of particular interest due to its membrane anchor that determines the activity and subcellular location of the protein. Previous solid-state NMR investigations indicated that this membrane anchor is highly dynamic, which may be indicative of backbone conformational flexibility. This article aims to address if a dynamic exchange between three structural models exist that had been determined previously. We applied a combination of solid-state nuclear magnetic resonance (NMR) methods and replica exchange molecular dynamics (MD) simulations using a Ras peptide that represents the terminal seven amino acids of the human N-Ras protein. Analysis of correlations between the conformations of individual amino acids revealed that Cys 181 and Met 182 undergo collective conformational exchange. Two major structures constituting about 60% of all conformations could be identified. The two conformations found in the simulation are in rapid exchange, which gives rise to low backbone order parameters and nuclear spin relaxation as measured by experimental NMR methods. These parameters were also determined from two 300 ns conventional MD simulations, providing very good agreement with the experimental data

Ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model on the triangular lattice: A variational study based on entangled-plaquette states

We study, on the basis of the general entangled-plaquette variational ansatz, the ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model on the triangular lattice. Our numerical estimates are in good agreement with available exact results and comparable, for large system sizes, to those computed via the best alternative numerical approaches, or by means of variational schemes based on specific (i.e., incorporating problem dependent terms) trial wave functions. The extrapolation to the thermodynamic limit of our results for lattices comprising up to N=324 spins yields an upper bound of the ground-state energy per site (in units of the exchange coupling) of $-0.5458(2)$ [$-0.4074(1)$ for the XX model], while the estimated infinite-lattice order parameter is $0.3178(5)$ (i.e., approximately 64% of the classical value).Comment: 8 pages, 3 tables, 2 figure

BEATRIX: The international breeder materials exchange

The BEATRIX experiment is an IEA-sponsored effort that involves the exchange of solid breeder materials and shared irradiation testing among research groups in several countries. The materials will be tested in both closed capsules (to evaluate material lifetime) and opened capsules (to evaluate purge-flow tritium recovery). Pre- and post-irradiation measurement of thermophysical and mechanical properties will also be carried out

Anisotropic susceptibilities in the honeycomb Kitaev system α−RuCl3

The magnetic insulator α−RuCl3 is a promising candidate to realize Kitaev interactions on a quasi-two-dimensional honeycomb lattice. We perform extensive susceptibility measurements on single crystals of α−RuCl3, including angle dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-Γ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal Γ spin exchange. The Γ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data