FSI analysis of a human trachea before and after prosthesis implantation

In this work we analyzed the response of a stenotic trachea after a stent implantation. An endotracheal stent is the common treatment for tracheal diseases such as stenosis, chronic cough, or dispnoea episodes. Medical treatment and surgical techniques are still challenging due to the difficulties in overcoming potential complications after prosthesis implantation. A finite element model of a diseased and stented trachea was developed starting from a patient specific computerized tomography (CT) scan. The tracheal wall was modeled as a fiber reinforced hyperelastic material in which we modeled the anisotropy due to the orientation of the collagen fibers. Deformations of the tracheal cartilage rings and of the muscular membrane, as well as the maximum principal stresses, are analyzed using a fluid solid interaction (FSI) approach. For this reason, as boundary conditions, impedance-based pressure waveforms were computed modeling the nonreconstructed vessels as a binary fractal network. The results showed that the presence of the stent prevents tracheal muscle deflections and indicated a local recirculatory flow on the stent top surface which may play a role in the process of mucous accumulation. The present work gives new insight into clinical procedures, predicting their mechanical consequences. This tool could be used in the future as preoperative planning software to help the thoracic surgeons in deciding the optimal prosthesis type as well as its size and positioning.     

Finite-element Analysis of a Stenotic Artery Revascularization Through a Stent Insertion

Short-term and long-term clinical follow-up data clearly indicate the superiority of stenting techniques within the family of mechanical treatments for percutaneous coronary revascularizations. However, restenosis phenomena are in general still present, representing the major drawback for this innovative non-invasive approach.

Experimental evidence indicates the mechanical interaction between the stent and the artery as a significant cause for the activation of stent-related restenosis. At the same time, the literature shows a significant lack of computational investigations within this field, possibly as consequence of the complexity of the problem.

According to these considerations, the aim of the present work is to study the bio-mechanical interaction between a balloon-expandable stent and a stenotic artery, highlighting considerations able to improve the general understanding of the problem.

In particular, given an initial stent design (J&J Palmaz-Schatz like), we show the presence of possible areas of artery injury during the stent deployment and areas of non-uniform contact pressure after the stent apposition, due to a non-uniform stent expansion. Since these concentrated mechanical actions can play an important role in the activation of restenosis mechanisms, we propose a modified stent design, which shows a more uniform expansion and for which typical stenting parameters (i.e., residual stenosis, elastic recoil, foreshortening) are computed and presented.     

On the effects of different strategies in modelling balloon-expandable stenting by means of finite element method

In recent years, computational structural analyses have emerged as important tools to investigate the mechanical response of stent placement into arterial walls. Although most coronary stents are expanded by inflating a polymeric balloon, realistic computational balloon models have been introduced only recently. In the present study, the finite element method is applied to simulate three different approaches to evaluate stent-free expansion and stent expansion inside an artery. Three different stent expansion modelling techniques were analysed by: (i) imposing a uniform pressure on the stent internal surface, (ii) a rigid cylindrical surface expanded with displacement control and (iii) modelling a polymeric deformable balloon. The computational results showed differences in the free and confined-stent expansions due to different expansion techniques. The modelling technique of the balloon seems essential to estimate the level of injury caused on arterial walls during stent expansion.     

Finite element modeling of a progressively expanding shape memory stent

Cardiovascular stents are small cylindrical devices introduced in stenosed arteries to reopen the lumen and restore blood flow. However, this treatment presents complications, including restenosis, which is the reclosing of the artery's diameter after the insertion of a stent. The structure of the prosthesis penetrates into and injures the walls of the patient's artery. There then follows a proliferation of cells and the formation of scar tissue around the injury, similar to the scarring of other organic tissues. This reaction to the trauma subjects the artery to close. The proposed solution is to develop a Nitinol stent with a progressive expansion device made of polyethylene, allowing smooth and gradual contact between the stent and the artery's wall by creep effect. The purpose of this paper is to describe the technology and methodology for the numerical study of this kind of stent through the finite element method. ANSYS 8.0 software is used to perform the analysis. The Nitinol is modeled with a superelastic law and the polyethylene with a yield hardening law. A first simulation determines the final geometry of the stent laser cut from a small tube. A second simulation examines the behavior of the prosthesis during surgery and over the 4 weeks following the operation. The results demonstrate that a compromise can be reached between a limited expansion prior the inflation of the expandable balloon and a significant expansion by creep of the polymer rings.     

Mechanical properties of coronary stents determined by using finite element analysis.

The mechanical function of a stent deployed in a damaged artery is to provide a metallic tubular mesh structure. The purpose of this study was to determine the exact mechanical characteristics of stents. In order to achieve this, we have used finite-element analysis to model two different type of stents: tubular stents (TS) and coil stents (CS). The two stents chosen for this modeling present the most extreme mechanical characteristics of the respective types. Seven mechanical properties were studied by mathematical modeling with determination of: (1) stent deployment pressure, (2) the intrinsic elastic recoil of the material used, (3) the resistance of the stent to external compressive forces, (4) the stent foreshortening, (5) the stent coverage area, (6) the stent flexibility, and (7) the stress maps. The pressure required for deployment of CS was significantly lower than that required for TS, over 2.8 times greater pressure was required for the tubular model. The elastic recoil of TS is higher than CS (5.4% and 2.6%, respectively). TS could be deformed by 10% at compressive pressures of between 0.7 and 1.3 atm whereas CS was only deformed at 0.2 and 0.7 atm. The degree of shortening observed increases with deployment diameter for TS. CS lengthen during deployment. The metal coverage area is two times greater for TS than for CS. The ratio between the stiffness of TS and that of CS varies from 2060 to 2858 depending on the direction in which the force is applied. TS are very rigid and CS are significantly more flexible. Stress mapping shows stress to be localized at link nodes. This series of finite-element analyses illustrates and quantifies the main mechanical characteristics of two different commonly used stents. In interventional cardiology, we need to understand their mechanisms of implantation and action.     

Mechanical behavior of coronary stents investigated through the finite element method

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow perfusion to the downstream tissues. The stent is mounted on a balloon catheter and delivered to the site of blockage. When the balloon is inflated, the stent expands and is pressed against the inner wall of the coronary artery. After the balloon is deflated and removed, the stent remains in place, keeping the artery open. Hence, the stent expansion defines the effectiveness of the surgical procedure: it depends on the stent geometry, it includes large displacements and deformations and material non-linearity.In this paper, the finite element method is applied (i) to understand the effects of different geometrical parameters (thickness, metal-to-artery surface ratio, longitudinal and radial cut lengths) of a typical diamond-shaped coronary stent on the device mechanical performance, (ii) to compare the response of different actual stent models when loaded by internal pressure and (iii) to collect suggestions for optimizing the device shape and performance.The stent expansion and partial recoil under balloon inflation and deflation were simulated. Results showed the influence of the geometry on the stent behavior: a stent with a low metal-to-artery surface ratio has a higher radial and longitudinal recoil, but a lower dogboning. The thickness influences the stent performance in terms of foreshortening, longitudinal recoil and dogboning.In conclusion, a finite element analysis similar to the one herewith proposed could help in designing new stents or analyzing actual stents to ensure ideal expansion and structural integrity, substituting in vitro experiments often difficult and unpractical.     

Load-dispersing design with twined-spring geometry of a distensible intracranial stent for cerebral aneurysms

Endovascular stents are increasingly being used to treat cerebral aneurysms. Mechanically, a cerebrovascular stent must have a low radial stiffness to prevent vessel dissection and rupture. To minimize these complications, we need to consider a stent design that has a low radial force and disperses the load within the stented artery. Therefore, highly distensible, load-dispersion stent designs are desirable for intracranial stenting. This study focused on closed-cell stent geometries and calculated the differences in stress within the artery because of the structure by using finite-element modeling. The results showed that the design with hexagonal cell geometry stretched in the circumferential direction had lower radial and circumferential stresses than did the other models. Comparing the maximum radial stress of our models, stress reduction of 35% was obtained with this design. Moreover, its radial stress was 47 kPa, which was similar to the critical stress of 42 kPa assumed in this study. This stent model was characterized by narrow strut spacing and a large surface area, which was dominated by the twined-spring geometry. It had low radial and circumferential stresses and a dispersed stress distribution compared with the other models. Therefore, this design is a desirable load-dispersing design for cerebrovascular treatment.     

Numerical investigations of the mechanical properties of a braided non-vascular stent design using finite element method

This paper discusses various issues relating to the mechanical properties of a braided non-vascular stent made of a Ni–Ti alloy. The design of the stent is a major factor which determines its reliability after implantation into a stenosed non-vascular cavity. This paper presents the effect of the main structural parameters on the mechanical properties of braided stents. A parametric analysis of a commercial stent model is developed using the commercial finite element code ANSYS. As a consequence of the analytical results that the pitch of wire has a greater effect than other structural parameters, a new design of a variable pitch stent is presented to improve mechanical properties of these braided stents. The effect of structural parameters on mechanical properties is compared for both stent models: constant and variable pitches. When the pitches of the left and right quarters of the stent are 50% larger and 100% larger than that of the central portion, respectively, the radial stiffness in the central portion increases by 10% and 38.8%, while the radial stiffness at the end portions decreases by 128% and 164.7%, the axial elongation by 25.6% and 56.6% and the bending deflection by 3.96% and 10.15%. It has been demonstrated by finite element analysis that the variable pitch stent can better meet the clinical requirements.     

Passive myocardial mechanical properties: meaning,measurement, models

Passive mechanical tissue properties are major determinants of myocardial contraction and relaxation and, thus, shape cardiac function. Tightly regulated, dynamically adapting throughout life, and affecting a host of cellular functions, passive tissue mechanics also contribute to cardiac dysfunction. Development of treatments and early identification of diseases requires better spatio-temporal characterisation of tissue mechanical properties and their underlying mechanisms. With this understanding, key regulators may be identified, providing pathways with potential to control and limit pathological development. Methodologies and models used to assess and mimic tissue mechanical properties are diverse, and available data are in part mutually contradictory. In this review, we define important concepts useful for characterising passive mechanical tissue properties, and compare a variety of in vitro and in vivo techniques that allow one to assess tissue mechanics. We give definitions of key terms, and summarise insight into determinants of myocardial stiffness in situ. We then provide an overview of common experimental models utilised to assess the role of environmental stiffness and composition, and its effects on cardiac cell and tissue function. Finally, promising future directions are outlined.     

Mechanical and hydrodynamic effects of stent expansion in tapered coronary vessels

Percutaneous coronary intervention (PCI) has become the primary treatment for patients with coronary heart disease because of its minimally invasive nature and high efficiency. Anatomical studies have shown that most coronary vessels gradually shrink, and the vessels gradually become thinner from the proximal to the distal end. In this paper, the effects of different stent expansion methods on the mechanical and hemodynamic behaviors of coronary vessels and stents were studied. To perform a structural-mechanical analysis of stent implantation, the coronary vessels with branching vessels and the coronary vessels with large bending curvature are selected. The two characteristic structures are implanted in equal diameter expansion mode and conical expansion mode, and the stress and mechanical behaviors of the coronary vessels and stents are analyzed. The results of the structural-mechanical analysis showed that the mechanical behaviors and fatigue performance of the cobalt-chromium alloy stent were good, and the different expansion modes of the stent had little effect on the fatigue performance of the stent. However, the equal diameter expansion mode increased distal coronary artery stress and the risk of vascular injury. The computational fluid dynamics analysis results showed that different stent expansion methods had varied effects on coronary vessel hemodynamics and that the wall shear stress distribution of conical stent expansion is more uniform compared with equal diameter expansion. Additionally, the vortex phenomenon is not apparent, the blood flow velocity is slightly increased, the hydrodynamic environment is more reasonable, and the risk of coronary artery injury is reduced.

     

Finite element comparison of performance related characteristics of balloon expandable stents

Cardiovascular stents are commonly made from 316L stainless steel and are deployed within stenosed arterial lesions using balloon expansion. Deployment involves inflating the balloon and plastically deforming the stent until the required diameter is obtained. This plastic deformation induces static stresses in the stent, which will remain for the lifetime of the device. In order to determine these stresses, finite element models of the unit cells of geometrically different, commercially available balloon expandable stents have been created, and deployment and elastic recoil have been simulated. In this work the residual stresses associated with deployment and recoil are compared for the various stent geometries, with a view to establishing appropriate initial stress states for fatigue loading for the stents. The maximum, minimum, and mean stresses induced in the stent due to systolic/diastolic pressure are evaluated, as are performance measures such as radial and longitudinal recoil.     

Simulation of a balloon expandable stent in a realistic coronary artery—Determination of the optimum modelling strategy

Computational models of stent deployment in arteries have been widely used to shed light on various aspects of stent design and optimisation. In this context, modelling of balloon expandable stents has proved challenging due to the complex mechanics of balloon–stent interaction and the difficulties involved in creating folded balloon geometries. In this study, a method to create a folded balloon model is presented and utilised to numerically model the accurate deployment of a stent in a realistic geometry of an atherosclerotic human coronary artery. Stent deployment is, however, commonly modelled by applying an increasing pressure to the stent, thereby neglecting the balloon. This method is compared to the realistic balloon expansion simulation to fully elucidate the limitations of this procedure. The results illustrate that inclusion of a realistic balloon model is essential for accurate modelling of stent deformation and stent stresses. An alternative balloon simulation procedure is presented however, which overcomes many of the limitations of the applied pressure approach by using elements which restrain the stent as the desired diameter is achieved. This study shows that direct application of pressure to the stent inner surface may be used as an optimal modelling strategy to estimate the stresses in the vessel wall using these restraining elements and hence offer a very efficient alternative approach to numerically modelling stent deployment within complex arterial geometries. The method is limited however, in that it can only predict final stresses in the stented vessel and not those occurring during stent expansion, in which case the balloon expansion model is required.     

Modeling of stents exhibiting negative Poisson's ratio effect

Intravascular stents are metallic scaffolding structures deployed in the stenotic arteries to restore the lumen for the blood flow to the down stream tissues. Most stents are balloon expandable and are deployed from its crimped state through a balloon catheter. The efficacy of the stenting procedure mainly depends on the way the stent is deployed. Both numerical and experimental evaluations show that almost all the present day stents undergo the most undesirable effects namely: (i) longitudinal foreshortening: the axial contraction in the length, and (ii) dogboning: flaring of the distal edges, during the radial expansion of the stents. Due to the foreshortening effect, clinicians are forced to select stents longer than the plaque. Still, the final length of the stent depends on the amount of radial expansion, which is subjective during the procedure. This paper introduces a new stent model called “Murugan”, which exhibits negative Poisson's ratio effect. That is, the stent may have zero axial contraction or can have extension when under radial expansion. The presence of hyperelastic balloon and the stent–balloon friction is also considered to study their effects in mechanical properties of the stents under consideration. Free expansion analysis is done using finite element method (FEM) to compare the new stent model with the present day stent geometries.     

Assessment of mechanical integrity for drug-eluting renal stent with micro-sized drug reservoirs

The drug-eluting stent (DES) has become the gold standard worldwide for the treatment of cardiovascular diseases. In recent years, an innovative variation of the DES with micro-sized drug reservoirs has been introduced. It allows programmable drug delivery with both spatial and temporal control and has several potential advantages over traditional DESs. However, creating such reservoirs on the stent struts may weaken the structure of the stent scaffolding and compromise its mechanical integrity. In this study, we propose to use this innovative stent concept in the renal indication for potential treatment of both renal artery stenosis (upstream) and its associated kidney diseases (downstream) at the same time. The effects of these micro-sized drug reservoirs on several key clinically relevant functional attributes of the drug-eluting renal stent were systematically and quantitatively investigated. Finite element models were developed to predict the mechanical integrity of a balloon-expandable stent at various stages. Results show that (1) creating drug reservoirs on a stent could impact the stent fatigue resistance to certain degrees; (2) drug reservoirs on the stent crowns lead to greater loss in all key stent attributes than reservoirs on either bar arms or connectors and (3) the proposed optimised depot stent was proven to be feasible and could triple drug capacity than the current DESs, with marginal trade-off in its key clinical attributes. These results can serve as the guidelines to help future stent designs to achieve the best combination of stent structural integrity and smart drug delivery in the future.     

The 3D structure of the tectorial membrane determined by second-harmonic imaging microscopy

The tectorial membrane (TM) is a highly hydrated non-cellular matrix situated over the sensory cells of the cochlea. It is widely accepted that the mechanical coupling, between the TM and outer hair cells stereocilia bundles, plays an important role in the cochlea energy transduction mechanism. Recently, we provided supporting evidence for the existence of mechanical coupling by demonstrating that the mechanical properties of the TM change along its longitudinal direction. Since the biochemical composition of the TM is similar throughout its entire length, it is likely that structural differences induce the observed material properties changes. Presently, however, the structure of the TM under physiological environments remains unknown. In this work, the 3D structure of native TM samples is shown by using two-photon second-harmonic imaging microscopy. We find that the collagen fibers at the basal region are arranged in a parallel orientation while being tilted in an angle with respect to the plane of the TM surface at the apical region. Moreover, we find an intensified marginal band at the basal OHC zone which forms a shell-like structure which engulfs the stereocilium imprints surface of the TM. In supports of our previous mechanical characterization, the analysis presented here provides a structural basis for the changes in TM's mechanical properties.     

Modeling of stents exhibiting negative Poisson's ratio effect

Intravascular stents are metallic scaffolding structures deployed in the stenotic arteries to restore the lumen for the blood flow to the down stream tissues. Most stents are balloon expandable and are deployed from its crimped state through a balloon catheter. The efficacy of the stenting procedure mainly depends on the way the stent is deployed. Both numerical and experimental evaluations show that almost all the present day stents undergo the most undesirable effects namely: (i) longitudinal foreshortening: the axial contraction in the length, and (ii) dogboning: flaring of the distal edges, during the radial expansion of the stents. Due to the foreshortening effect, clinicians are forced to select stents longer than the plaque. Still, the final length of the stent depends on the amount of radial expansion, which is subjective during the procedure. This paper introduces a new stent model called "Murugan", which exhibits negative Poisson's ratio effect. That is, the stent may have zero axial contraction or can have extension when under radial expansion. The presence of hyperelastic balloon and the stent-balloon friction is also considered to study their effects in mechanical properties of the stents under consideration. Free expansion analysis is done using finite element method (FEM) to compare the new stent model with the present day stent geometries.     

Changes in the mechanical environment of stenotic arteries during interaction with stents: computational assessment of parametric stent designs

Clinical studies have identified factors such as the stent design and the deployment technique that are one cause for the success or failure of angioplasty treatments. In addition, the success rate may also depend on the stenosis type. Hence, for a particular stenotic artery, the optimal intervention can only be identified by studying the influence of factors such as stent type, strut thickness, geometry of the stent cell, and stent-artery radial mismatch with the wall. We propose a methodology that allows a set of stent parameters to be varied, with the aim of evaluating the difference in the mechanical environment within the wall before and after angioplasty with stenting. Novel scalar quantities attempt to characterize the wall changes inform of the contact pressure caused by the stent struts, and the stresses within the individual components of the wall caused by the stent. These quantities are derived numerically and serve as indicators, which allow the determination of the correct size and type of the stent for each individual stenosis. In addition, the luminal change due to angioplasty may be computed as well. The methodology is demonstrated by using a full three-dimensional geometrical model of a postmortem specimen of a human iliac artery with a stenosis using imaging data. To describe the material behavior of the artery, we considered mechanical data of eight different vascular tissues, which formed the stenosis. The constitutive models for the tissue components capture the typical anisotropic, nonlinear and dissipative characteristics under supra-physiological loading conditions. Three-dimensional stent models were parametrized in such a way as to enable new designs to be generated simply with regard to variations in their geometric structure. For the three-dimensional stent-artery interaction we use a contact algorithm based on smooth contact surfaces of at least C-continuity, which prevents numerical problems known from standard facet-based contact algorithm. The proposed methodology has the potential to provide a scientific basis for optimizing treatment procedures and stent geometries and materials, to help stent designers examine new stent designs "virtually," and to assist clinicians in choosing the most suitable stent for a particular stenosis.     

Preventing mediastinal shift after pneumonectomy impairs regenerative alveolar tissue growth

To examine the effects of mechanical lung strain on regenerative growth of alveolar septal tissue after pneumonectomy (PNX), we replaced the right lungs of adult dogs with a custom-shaped inflatable silicone prosthesis. The prosthesis was either inflated (Inf) to maintain the mediastinum at the midline or deflated to allow mediastinal shift. The animals were euthanized approximately 15 mo later, and the lungs were fixed at a constant distending pressure. With the Inf prostheses, lung expansion, alveolar septal tissue volumes, surface areas, and diffusing capacity of the tissue-plasma barrier were significantly lower than with the deflated prostheses; the expected post-PNX tissue responses were impaired by 30-60%. Capillary blood volume was significantly higher with Inf prostheses, consistent with microvascular congestion. Measurements in the Inf group remained consistently and significantly higher than those expected for a normal left lung, indicating persistence of partial compensation. In one dog, delayed deflation of the prosthesis 9-10 mo after PNX led to vigorous lung expansion and septal tissue growth, particularly of type II epithelial cells. We conclude that mechanical lung strain is a major signal for regenerative lung growth; however, other signals are also implicated, accounting for a significant fraction of the compensatory response to PNX.     

Numerical study of the uniformity of balloon-expandable stent deployment

Stents are small tubelike structures, implanted in coronary and peripheral arteries to reopen narrowed vessel sections. This endovascular intervention remains suboptimal, as the success rate is limited by restenosis. This renarrowing of a stented vessel is related to the arterial injury caused by stent-artery and balloon-artery interactions, and a local subsequent inflammatory process. Therefore, efforts to optimize the stent deployment remain very meaningful. Several authors have studied with finite element modeling the mechanical behavior of balloon-expandable stents, but none of the proposed models incorporates the folding pattern of the balloon. We developed a numerical model in which the CYPHER stent is combined with a realistic trifolded balloon. In this paper, the impact of several parameters such as balloon length, folding pattern, and relative position of the stent with respect to the balloon catheter on the free stent expansion has been investigated. Quantitative validation of the modeling strategy shows excellent agreement with data provided by the manufacturer and, therefore, the model serves as a solid basis for further investigations. The parametric analyses showed that both the balloon length and the folding pattern have a considerable influence on the uniformity and symmetry of the transient stent expansion. Consequently, this approach can be used to select the most appropriate balloon length and folding pattern for a particular stent design in order to optimize the stent deployment. Furthermore, it was demonstrated that small positioning inaccuracies may change the expansion behavior of a stent. Therefore, the placement of the stent on the balloon catheter should be accurately carried out, again in order to decrease the endothelial damage.     

A multiscale model for red blood cell mechanics

The objective of this article is the derivation of a continuum model for mechanics of red blood cells via multiscale analysis. On the microscopic level, we consider realistic discrete models in terms of energy functionals defined on networks/lattices. Using concepts of Γ-convergence, convergence results as well as explicit homogenisation formulae are derived. Based on a characterisation via energy functionals, appropriate macroscopic stress–strain relationships (constitutive equations) can be determined. Further, mechanical moduli of the derived macroscopic continuum model are directly related to microscopic moduli. As a test case we consider optical tweezers experiments, one of the most common experiments to study mechanical properties of cells. Our simulations of the derived continuum model are based on finite element methods and account explicitly for membrane mechanics and its coupling with bulk mechanics. Since the discretisation of the continuum model can be chosen freely, rather than it is given by the topology of the microscopic cytoskeletal network, the approach allows a significant reduction of computational efforts. Our approach is highly flexible and can be generalised to many other cell models, also including biochemical control.