Modelling volumetric growth in a thick walled fibre reinforced artery

A novel framework for simulating growth and remodelling (G&R) of a fibre-reinforced artery, including volumetric adaption, is proposed. We show how to implement this model into a finite element framework and propose and examine two underlying assumptions for modelling growth, namely constant individual density (CID) or adaptive individual density (AID). Moreover, we formulate a novel approach which utilises a combination of both AID and CID to simulate volumetric G&R for a tissue composed of several different constituents. We consider a special case of the G&R of an artery subjected to prescribed elastin degradation and we theorise on the assumptions and suitability of CID, AID and the mixed approach for modelling arterial biology. For simulating the volumetric changes that occur during aneurysm enlargement, we observe that it is advantageous to describe the growth of collagen using CID whilst it is preferable to model the atrophy of elastin using AID

A chemo-mechano-biological modeling framework for cartilage evolving in health, disease, injury, and treatment

Background and Objective:Osteoarthritis (OA) is a pervasive and debilitating disease, wherein degeneration of cartilage features prominently. Despite extensive research, we do not yet understand the cause or progression of OA. Studies show biochemical, mechanical, and biological factors affect cartilage health. Mechanical loads influence synthesis of biochemical constituents which build and/or break down cartilage, and which in turn affect mechanical loads. OA-associated biochemical profiles activate cellular activity that disrupts homeostasis. To understand the complex interplay among mechanical stimuli, biochemical signaling, and cartilage function requires integrating vast research on experimental mechanics and mechanobiology—a task approachable only with computational models. At present, mechanical models of cartilage generally lack chemo-biological effects, and biochemical models lack coupled mechanics, let alone interactions over time. Methods:We establish a first-of-its kind virtual cartilage: a modeling framework that considers time-dependent, chemo-mechano-biologically induced turnover of key constituents resulting from biochemical, mechanical, and/or biological activity. We include the “minimally essential” yet complex chemical and mechanobiological mechanisms. Our 3-D framework integrates a constitutive model for the mechanics of cartilage with a novel model of homeostatic adaptation by chondrocytes to pathological mechanical stimuli, and a new application of anisotropic growth (loss) to simulate degradation clinically observed as cartilage thinning. Results:Using a single set of representative parameters, our simulations of immobilizing and overloading successfully captured loss of cartilage quantified experimentally. Simulations of immobilizing, overloading, and injuring cartilage predicted dose-dependent recovery of cartilage when treated with suramin, a proposed therapeutic for OA. The modeling framework prompted us to add growth factors to the suramin treatment, which predicted even better recovery. Conclusions:Our flexible framework is a first step toward computational investigations of how cartilage and chondrocytes mechanically and biochemically evolve in degeneration of OA and respond to pharmacological therapies. Our framework will enable future studies to link physical activity and resulting mechanical stimuli to progression of OA and loss of cartilage function, facilitating new fundamental understanding of the complex progression of OA and elucidating new perspectives on causes, treatments, and possible preventions

Optimization schemes for endovascular repair with parallel technique based on hemodynamic analyses

Endovascular repair with parallel stent-grafts (SG) is a challenging technique that reconstructs the luminal flow pathways by implanting parallel-placed SGs into the vessel. After treatment, occlusion and shifting of the parallel SGs are sometimes reported, which could be fatal and difficult to be re-operated. These issues are highly related to the local hemodynamic conditions in the stented region. In this study, a patient case treated by octopus endograft technique (a head-SG with three limb-SGs) and experienced limb-SG occlusion is studied. 3-D models are established based on CT-angiography datasets pre- and post-treatment as well as during follow-ups. Hemodynamic quantities such as pressure drop, wall shear stress-related parameters and flow division in limb-SGs and visceral arteries are quantitatively investigated. Optimizations on the length of the head-SG and diameter of the limb-SGs are analyzed based on various scenarios. The results indicate that when reconstructing the flow pathways via octopus stenting, it is important to ensure the flow distribution as physiological required with this new morphology. Position (or length) of the head-SG and diameter of the limb-SGs play an important role in controlling flow division, and high TAWSS around the head-SG acts as a main factor for graft immigration. This study, by proposing optimization suggestions with hemodynamic analyses for a specific case, implicates that pre-treatment SG scenarios may assist in wise selection and placement of the device and thus may improve long-term effectiveness of this kind of challenging endovascular repair techniques

The unexplained success of stentplasty vasospasm treatment

Background Cerebral vasospasm (CVS) following subarachnoid hemorrhage occurs in up to 70% of patients. Recently, stents have been used to successfully treat CVS. This implies that the force required to expand spastic vessels and resolve vasospasm is lower than previously thought. Objective We develop a mechanistic model of the spastic arterial wall to provide insight into CVS and predict the forces required to treat it. Material and Methods The arterial wall is modelled as a cylindrical membrane using a constrained mixture theory that accounts for the mechanical roles of elastin, collagen and vascular smooth muscle cells (VSMC). We model the pressure diameter curve prior to CVS and predict how it changes following CVS. We propose a stretch-based damage criterion for VSMC and evaluate if several commercially available stents are able to resolve vasospasm. Results The model predicts that dilatation of VSMCs beyond a threshold of mechanical failure is sufficient to resolve CVS without damage to the underlying extracellular matrix. Consistent with recent clinical observations, our model predicts that existing stents have the potential to provide sufficient outward force to successfully treat CVS and that success will be dependent on an appropriate match between stent and vessel. Conclusion Mathematical models of CVS can provide insights into biological mechanisms and explore treatment approaches. Improved understanding of the underlying mechanistic processes governing CVS and its mechanical treatment may assist in the development of dedicated stents

Computational fluid dynamics in aneurysm research: critical reflections, future directions

No abstract available

Abdominal aortic aneurysms - a new mathematical model

No abstract available

On the structure of probability functions in the natural world

The purpose of this paper is to describe the underlying insights and results obtained by the authors, and others, in a series of papers aimed at modeling the distribution of 'natural' probability functions, more precisely the probability functions on {0, 1}n which we encounter naturally in the real world as subjects for statistical inference, by identifying such functions with large, random, sentences of the propositional calculus. We explain how this approach produces a robust parameterized family of priors, Jn, with several of the properties we might have hoped for in the context, for example marginalisation, invariance under (weak) renaming, and an emphasis on multivariate probability functions exhibiting high interdependence between features

Dynamic behaviour of aortic and chorded mitral prostheses

We study the dynamic behaviour of aortic and chorded mitral prostheses using numerical and experimental methods. An Immersed Boundary (IB) code is developed for the numerical analysis. A left heart simulation rig is used to subject the valves to physiological pressure and flow conditions in a controlled environment. Pressure gradients are recorded and compared with our numerical simulations for four different cycling flow rates. Good agreement is found for both aortic and mitral valves. We conclude that the IB is a very effective tool to study the pressure gradients across heart valve prostheses