A COMPUTATIONAL STUDY OF PATCH IMPLANTATION AND MITRAL VALVE MECHANICS

Myocardial infarction (i.e., a heart attack) is the most common heart disease in the United States. Mitral valve regurgitation, or the backflow of blood into the atrium from the left ventricle, is one of the complications associated with myocardial infarction. In this dissertation, a validated model of a sheep heart that has suffered myocardial infarction has been employed to study mitral valve regurgitation. The model was rebuilt with the knowledge of geometrical changes captured with MRI technique and is assigned with anisotropic, inhomogeneous, nearly incompressible and highly non-linear material properties. Patch augmentation was performed on its anterior leaflet, using a simplified approach, and its posterior leaflet, using a more realistic approach. In this finite element simulation, we virtually installed an elliptical patch within the central portion of the posterior leaflet. To the best of the author’s knowledge, this type of simulation has not been performed previously. In another simulation, the effect of patch within the anterior leaflet was simulated. The results from the two different surgical simulations show that patch implantation helps the free edges of the leaflets come close to one another, which leads to improved coaptation. Additionally, the changes in chordal force distributions are also reported. Finally, this study answers a few questions regarding mitral valve patch augmentation surgeries and emphasizes the importance of further investigations on the influence of patch positioning and material properties on key outcomes. The ultimate goal is to use the proposed techniques to assess human models that are patient-specific

Finite element modeling of mitral leaflet tissue using a layered shell approximation.

The current study presents a finite element model of mitral leaflet tissue, which incorporates the anisotropic material response and approximates the layered structure. First, continuum mechanics and the theory of layered composites are used to develop an analytical representation of membrane stress in the leaflet material. This is done with an existing anisotropic constitutive law from literature. Then, the concept is implemented in a finite element (FE) model by overlapping and merging two layers of transversely isotropic membrane elements in LS-DYNA, which homogenizes the response. The FE model is then used to simulate various biaxial extension tests and out-of-plane pressure loading. Both the analytical and FE model show good agreement with experimental biaxial extension data, and show good mutual agreement. This confirms that the layered composite approximation presented in the current study is able to capture the exponential stiffening seen in both the circumferential and radial directions of mitral leaflets

Finite element modeling of mitral leaflet tissue using a layered shell approximation.

The current study presents a finite element model of mitral leaflet tissue, which incorporates the anisotropic material response and approximates the layered structure. First, continuum mechanics and the theory of layered composites are used to develop an analytical representation of membrane stress in the leaflet material. This is done with an existing anisotropic constitutive law from literature. Then, the concept is implemented in a finite element (FE) model by overlapping and merging two layers of transversely isotropic membrane elements in LS-DYNA, which homogenizes the response. The FE model is then used to simulate various biaxial extension tests and out-of-plane pressure loading. Both the analytical and FE model show good agreement with experimental biaxial extension data, and show good mutual agreement. This confirms that the layered composite approximation presented in the current study is able to capture the exponential stiffening seen in both the circumferential and radial directions of mitral leaflets

Finite element modeling of mitral leaflet tissue using a layered shell approximation.

The current study presents a finite element model of mitral leaflet tissue, which incorporates the anisotropic material response and approximates the layered structure. First, continuum mechanics and the theory of layered composites are used to develop an analytical representation of membrane stress in the leaflet material. This is done with an existing anisotropic constitutive law from literature. Then, the concept is implemented in a finite element (FE) model by overlapping and merging two layers of transversely isotropic membrane elements in LS-DYNA, which homogenizes the response. The FE model is then used to simulate various biaxial extension tests and out-of-plane pressure loading. Both the analytical and FE model show good agreement with experimental biaxial extension data, and show good mutual agreement. This confirms that the layered composite approximation presented in the current study is able to capture the exponential stiffening seen in both the circumferential and radial directions of mitral leaflets

Finite element modeling of mitral leaflet tissue using a layered shell approximation

The current study presents a finite element model of mitral leaflet tissue, which incorporates the anisotropic material response and approximates the layered structure. First, continuum mechanics and the theory of layered composites are used to develop an analytical representation of membrane stress in the leaflet material. This is done with an existing anisotropic constitutive law from literature. Then, the concept is implemented in a finite element (FE) model by overlapping and merging two layers of transversely isotropic membrane elements in LS-DYNA, which homogenizes the response. The FE model is then used to simulate various biaxial extension tests and out-of-plane pressure loading. Both the analytical and FE model show good agreement with experimental biaxial extension data, and show good mutual agreement. This confirms that the layered composite approximation presented in the current study is able to capture the exponential stiffening seen in both the circumferential and radial directions of mitral leaflets

Investigations of the Tricuspid Heart Valve Function: An Integrated Computational-Experimental Approach

The objective of this research is to employ both in silico modeling and in vitro experimental characterization methods to enhance the understanding of the biomechanical function of the tricuspid heart valve. A finite element (FE)-based computational model of the tricuspid valve (TV) is first developed. Specifically, the geometry used in this computational model is based on parametric representations of the TV leaflets from porcine and ovine hearts and a parametric representation of the chordae tendineae. A nonlinear, isotropic constitutive model is used to describe of the mechanical behaviors of the TV leaflets, while the TV chordae tendineae are modeled as a nonlinear, elastic solid. The developed FE model of the TV apparatus is then used to simulate various pathological states including: (i) pulmonary hypertension, (ii) TV annulus dilation, (iii) papillary muscle displacement associated with right ventricular enlargement, (iv) flattening of the TV annulus, and (v) the rupture of the TV chordae tendineae. Numerical results from this study, as compared to available clinical observations, suggest that the TV annulus dilation and papillary muscle displacement resulting from right ventricular enlargement are key contributors to TV regurgitation. On the other hand, pulmonary hypertension resulted in the largest increase in TV leaflet stress (+65%) indicating pulmonary hypertension may be a key contributor to the adverse remodeling of the leaflet and myocardium tissues. In addition, the simulations of the chordae rupture scenarios reveal that those chordae tendineae attached to the TV anterior and septal leaflets may be more important to preventing TV leaflet prolapse. Extensive biaxial mechanical testing of the TV leaflets is conducted to expand on the limited number of mechanical characterizations of the TV leaflets. These experimental efforts include: (i) a quantification of the TV leaflets’ biaxial mechanical responses, (ii) an investigation of the loading-rate and temperature effects on the TV leaflet tissue mechanics, (iii) an examination of the influence of species and aging on the TV leaflet’s mechanical properties, (iv) an evaluation of the spatial variations of the TV leaflet’s tissue mechanics, and (v) a determination of the contribution of the glycosaminoglycans (GAGs) to the TV leaflet’s mechanical responses. These in vitro experimental results suggest that (i) the TV leaflets are more extensible than the mitral valve leaflets, (ii) the TV leaflets’ responses depend slightly on the loading rate and temperature, (iii) the mechanical responses of the TV leaflets become stiffer with aging (+3.5%-6.1%), (iv) the TV leaflets exhibit spatial variance in the mechanical properties, and (v) the removal of the GAGs leads to an increased extensibility of the TV leaflets (+4.7%-7.6%). Finally, a constitutive modeling framework, based on the hyperelasticity theory, is formulated to describe the mechanical behaviors of the heart valve leaflets from the acquired biaxial mechanical data. Through the differential evolution optimization, model parameters of two strain energy density functions commonly adopted in the soft tissue biomechanics society are estimated by fitting to the representative biaxial mechanical testing data. Results from this numerical study suggest that a refined strain energy density function may be warranted, as part of the future extensions, to fully capture the complex mechanical responses of the heart valve leaflet, especially under combined tensile and compressive loading

Revisión de ensayos biaxiales para la evaluación del fallo transversal en laminados de material compuesto

El presente TFM tiene 2 objetivos fundamentales: Realización de una revisión detallada del estado del arte actual del problema en estudio en esta LI, en concreto de los ensayos biaxiales para la evaluación del fallo transversal en laminados de material compuesto. Desarrollo de una BBDD con las publicaciones científicas existentes, en la cual se recoja toda la información encontrada y posteriormente, sea sencillo analizar dicha información y extraer comparativas de datos mediante gráficos dinámicos. Para alcanzar dichos objetivos, ha sido necesario: La realización de una BBDD en Access, en la cual se han definido distintas tablas, formularios, consultas e informes para conseguir una recogida limpia y sencilla de datos en ella mediante un interfaz amigable al usuario. La búsqueda de referencias científicas mediante el uso de BBDD existentes revisadas por pares. La evaluación de las referencias encontradas para incluirlas en la BBDD creada en este TFM con cierto rigor y utilidad, consiguiendo así una base sólida para extraer datos y realizar comparativas ante las necesidades planteadas a los componentes del GERM.Universidad de Sevilla. Máster en Ingeniería Industria