233 research outputs found
Methods for suspensions of passive and active filaments
Flexible filaments and fibres are essential components of important complex
fluids that appear in many biological and industrial settings. Direct
simulations of these systems that capture the motion and deformation of many
immersed filaments in suspension remain a formidable computational challenge
due to the complex, coupled fluid--structure interactions of all filaments, the
numerical stiffness associated with filament bending, and the various
constraints that must be maintained as the filaments deform. In this paper, we
address these challenges by describing filament kinematics using quaternions to
resolve both bending and twisting, applying implicit time-integration to
alleviate numerical stiffness, and using quasi-Newton methods to obtain
solutions to the resulting system of nonlinear equations. In particular, we
employ geometric time integration to ensure that the quaternions remain unit as
the filaments move. We also show that our framework can be used with a variety
of models and methods, including matrix-free fast methods, that resolve low
Reynolds number hydrodynamic interactions. We provide a series of tests and
example simulations to demonstrate the performance and possible applications of
our method. Finally, we provide a link to a MATLAB/Octave implementation of our
framework that can be used to learn more about our approach and as a tool for
filament simulation
Capturing nonlinear dynamics of two-fluid Couette flows with asymptotic models
The nonlinear stability of two-fluid Couette flows is studied using a novel evolution equation whose dynamics are validated by direct numerical simulations (DNS). The evolution equation incorporates inertial effects at arbitrary Reynolds numbers through a non-local term arising from the coupling between the two fluid regions, and is valid when one of the layers is thin. The equation predicts asymmetric solutions and exhibits bistability, features that are essential observations in the experiments of Barthelet et al. (1995). Related low-inertia models have been used in qualitative predictions rather than the direct comparisons carried out here, and ad hoc modifications appear to be necessary in order to predict asymmetry and bistability. Comparisons between model solutions and DNS show excellent agreement at Reynolds numbers of O(10³) found in the experiments. Direct comparisons are also made with the available experimental results of Barthelet et al. (1995) when the thin layer occupies 1/5 of the channel height. Pointwise comparisons of the travelling wave shapes are carried out and once again the agreement is very good
The associations between QCT-based vertebral bone measurements and prevalent vertebral fractures depend on the spinal locations of both bone measurement and fracture
Summary
We examined how spinal location affects the relationships between quantitative computed tomography (QCT)-based bone measurements and prevalent vertebral fractures. Upper spine (T4–T10) fractures appear to be more strongly related to bone measures than lower spine (T11–L4) fractures, while lower spine measurements are at least as strongly related to fractures as upper spine measurements.
Introduction
Vertebral fracture (VF), a common injury in older adults, is most prevalent in the mid-thoracic (T7–T8) and thoracolumbar (T12–L1) areas of the spine. However, measurements of bone mineral density (BMD) are typically made in the lumbar spine. It is not clear how the associations between bone measurements and VFs are affected by the spinal locations of both bone measurements and VF.
Methods
A community-based case–control study includes 40 cases with moderate or severe prevalent VF and 80 age- and sex-matched controls. Measures of vertebral BMD, strength (estimated by finite element analysis), and factor of risk (load:strength ratio) were determined based on QCT scans at the L3 and T10 vertebrae. Associations were determined between bone measures and prevalent VF occurring at any location, in the upper spine (T4–T10), or in the lower spine (T11–L4).
Results
Prevalent VF at any location was significantly associated with bone measures, with odds ratios (ORs) generally higher for measurements made at L3 (ORs = 1.9–3.9) than at T10 (ORs = 1.5–2.4). Upper spine fracture was associated with these measures at both T10 and L3 (ORs = 1.9–8.2), while lower spine fracture was less strongly associated (ORs = 1.0–2.4) and only reached significance for volumetric BMD measures at L3.
Conclusions
Closer proximity between the locations of bone measures and prevalent VF does not strengthen associations between bone measures and fracture. Furthermore, VF etiology may vary by region, with VFs in the upper spine more strongly related to skeletal fragility.National Institutes of Health (U.S.) (Grants R01AR053986, R01AR/AG041398, T32AG023480, and F31AG041629)National Heart, Lung, and Blood Institute. Framingham Heart Study (NIH/NHLBI Contract N01-HC-25195
Hydrodynamic Synchronisation of Model Microswimmers
We define a model microswimmer with a variable cycle time, thus allowing the
possibility of phase locking driven by hydrodynamic interactions between
swimmers. We find that, for extensile or contractile swimmers, phase locking
does occur, with the relative phase of the two swimmers being, in general,
close to 0 or pi, depending on their relative position and orientation. We show
that, as expected on grounds of symmetry, self T-dual swimmers, which are
time-reversal covariant, do not phase-lock. We also discuss the phase behaviour
of a line of tethered swimmers, or pumps. These show oscillations in their
relative phases reminiscent of the metachronal waves of cilia.Comment: 17 pages, 8 figure
An approximate model for cancellous bone screw fixation
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2013 Taylor & Francis.This paper presents a finite element (FE) model to identify parameters that affect the performance of an improved cancellous bone screw fixation technique, and hence potentially improve fracture treatment. In cancellous bone of low apparent density, it can be difficult to achieve adequate screw fixation and hence provide stable fracture fixation that enables bone healing. Data from predictive FE models indicate that cements can have a significant potential to improve screw holding power in cancellous bone. These FE models are used to demonstrate the key parameters that determine pull-out strength in a variety of screw, bone and cement set-ups, and to compare the effectiveness of different configurations. The paper concludes that significant advantages, up to an order of magnitude, in screw pull-out strength in cancellous bone might be gained by the appropriate use of a currently approved calcium phosphate cement
From tension to compression: Asymmetric mechanical behaviour of trabecular bone's organic phase
Characterisation of time-dependent mechanical behaviour of trabecular bone and its constituents
Trabecular bone is a porous composite material which consists of a mineral
phase (mainly hydroxyapatite), organic phase (mostly type I collagen) and water
assembled into a complex, hierarchical structure. In biomechanical modelling,
its mechanical response to loads is generally assumed to be instantaneous,
i.e. it is treated as a time-independent material. It is, however, recognised
that the response of trabecular bone to loads is time-dependent. Study
of this time-dependent behaviour is important in several contexts such as: to
understand energy dissipation ability of bone; to understand the age-related
non-traumatic fractures; to predict implant loosening due to cyclic loading; to
understand progressive vertebral deformity; and for pre-clinical evaluation of
total joint replacement.
To investigate time-dependent behaviour, bovine trabecular bone samples
were subjected to compressive loading, creep, unloading and recovery at multiple
load levels (corresponding to apparent strain of 2,000-25,000 με). The
results show that: the time-dependent behaviour of trabecular bone comprises
of both recoverable and irrecoverable strains; the strain response is nonlinearly
related to applied load levels; and the response is associated with bone volume
fraction. It was found that bone with low porosity demonstrates elastic
stiffening followed by elastic softening, while elastic softening is demonstrated
by porous bone at relatively low loads. Linear, nonlinear viscoelastic and nonlinear
viscoelastic-viscoplastic constitutive models were developed to predict
trabecular bone’s time-dependent behaviour. Nonlinear viscoelastic constitutive model was found to predict the recovery behaviour well, while nonlinear
viscoelastic-viscoplastic model predicts the full creep-recovery behaviour reasonably
well. Depending on the requirements all these models can be used to
incorporate time-dependent behaviour in finite element models.
To evaluate the contribution of the key constituents of trabecular bone and
its microstructure, tests were conducted on demineralised and deproteinised
samples. Reversed cyclic loading experiments (tension to compression) were
conducted on demineralised trabecular bone samples. It was found that demineralised
bone exhibits asymmetric mechanical response - elastic stiffening
in tension and softening in compression. This tension to compression transition
was found to be smooth. Tensile multiple-load-creep-unload-recovery experiments
on demineralised trabecular samples show irrecoverable strain (or
residual strain) even at the low stress levels. Demineralised trabecular bone
samples demonstrate elastic stiffening with increasing load levels in tension,
and their time-dependent behaviour is nonlinear with respect to applied loads .
Nonlinear viscoelastic constitutive model was developed which can predict its
recovery behaviour well. Experiments on deproteinised samples showed that
their modulus and strength are reasonably well related to bone volume fraction.
The study considers an application of time-dependent behaviour of trabecular
bone. Time-dependent properties are assigned to trabecular bone in a
bone-screw system, in which the screw is subjected to cyclic loading. It is
found that separation between bone and the screw at the interface can increase
with increasing number of cycles which can accentuate loosening. The
relative larger deformation occurs when this system to be loaded at the higher
loading frequency. The deformation at the bone-screw interface is related to
trabecular bone’s bone volume fraction; screws in a more porous bone are at
a higher risk of loosening
The Herts and Minds study: Evaluating the effectiveness of Mentalization-Based Treatment (MBT) as an intervention for children in foster care with emotional and/or behavioural problems: a phase II, feasibility, randomised controlled trial.
Trial registration at https://doi.org/10.1186/ISRCTN90349442 © The Authors 2017. Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Nick Midgley, Sarah Jane Besser, Helen Dye, Pasco Fearon, tim Gale, Kiri Jefferies-Sewell, Karen Irvine, Joyce Robinson, Solange Wyatt, David Wellsted and Sally Wood, 'The Herts and minds study: evaluating the effectiveness of mentalization-based treatment (MBT) as an intervention for children in foster care with emotional and/or behavioural problems: a phase II, feasibility, randomised controlled trial', Pilot and Feasibility Studies, Vol. 3(12, February 2017. The published version is available online at doi: 10.1186/s40814-017-0127-xBackground A significant proportion of children in the social care system in England present with mental health problems, with the majority experiencing some form of emotional and behavioural difficulties. The most effective treatments for these children are currently unknown, partly due to a lack of robust, controlled studies. Researchers have identified a number of obstacles to conducting well-designed research with this population, making the need to test the feasibility of a randomised controlled trial especially important. Methods/design This protocol outlines a two-arm, randomised control feasibility trial to explore the acceptability and credibility of mentalization-based treatment (MBT) as a treatment for reducing emotional and behavioural difficulties in looked after children and to test the possibility of addressing a number of methodological challenges to conducting high-quality research with this population. MBT is a relatively new intervention which, in the adaptation of the model tested here, includes many of the features of therapy identified in NICE guidelines as necessary to support children in care. The two arms are MBT and usual clinical care (UCC). The study will take place in Hertfordshire Partnership University NHS Foundation Trust with follow-up at 12 and 24 weeks. Discussion This study will aim to ascertain whether it is worthwhile and feasible to progress to testing the intervention in a full-scale definitive randomised controlled trial (RCT). This study therefore has the potential to improve our understanding of the obstacles to conducting high-quality research with this very vulnerable population, and in the medium term, could help to improve the stability of foster placements and the emotional well-being of children in care. Trial registration ISRCTN90349442Peer reviewe
Foam-like scaffolds for bone tissue engineering based on a novel couple of silicate-phosphate specular glasses: synthesis and properties
The Influence of Mineralization on Intratrabecular Stress and Strain Distribution in Developing Trabecular Bone
The load-transfer pathway in trabecular bone is largely determined by its architecture. However, the influence of variations
in mineralization is not known. The goal of this study was to examine the influence of inhomogeneously distributed degrees
of mineralization (DMB) on intratrabecular stresses and strains. Cubic mandibular condylar bone specimens from fetal and newborn
pigs were used. Finite element models were constructed, in which the element tissue moduli were scaled to the local DMB. Disregarding
the observed distribution of mineralization was associated with an overestimation of average equivalent strain and underestimation
of von Mises equivalent stress. From the surface of trabecular elements towards their core the strain decreased irrespective
of tissue stiffness distribution. This indicates that the trabecular elements were bent during the compression experiment.
Inhomogeneously distributed tissue stiffness resulted in a low stress at the surface that increased towards the core. In contrast,
disregarding this tissue stiffness distribution resulted in high stress at the surface which decreased towards the core. It
was concluded that the increased DMB, together with concurring alterations in architecture, during development leads to a
structure which is able to resist increasing loads without an increase in average deformation, which may lead to damage
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