An advanced fluid structure interaction study of tri-leaflet aortic heart valve

Abstract

The main function of the heart valves is to maintain a unidirectional blood flow through the heart chambers. Various diseases could cause dysfunctionalities of one or more valves within the heart, which adversely affect the performance of the heart and subsequent failure or death of the patient. Out of the four valves within the heart, most affected is the aortic valve. The current practice, depending on the condition of the malfunctioned valve, is either to repair or to replace it with an artificial valve of mechanical types or tissue types. Currently, more than 250,000 valve replacement operations are performed worldwide, using various types of artificial heart valves in order to restore the normal function of the heart. Although significant developments have been made in designing methods and implanting techniques of Aortic Valve Replacement (AVR) in particular, over the past years, the exact solution for such a problem is still intensively challenging. Traditionally in-vitro experimentation is used in the designing and optimizing of artificial valves. However, the use of numerical techniques has gained considerable attention in bioengineering applications. Compared to in-vitro and in-vivo techniques, numerical methods are a very cost effective option, which have assisted researchers to propose more adequate designs for diverse bioengineering applications including various design of artificial valves. With the aid of numerical techniques various conceptual designs can be examined on a cost effective way with the aim of mimicking the natural performance of the valve. However, there are a number of limitations and complications in current numerical methods, specifically in the field of cardiovascular component design (multi domain problems). This needs to be addressed in order to develop an effective tool capable of analyzing the hemodynamics, the structure interactions, and the interdependent forces that have long and short term effects on the integrity and functionality of the proposed AVRs. The principal objective of this research is to develop and validate a fully coupled numerical scheme that is capable of determining all the hemodynamic and structure forces that are essential to assess the functionality of the “proposed tri-leaflet polymeric AVR model” or scaffold and its long and short term patency. This goal has been achieved with a systematic approach as described in the following tasks: Providing a brief overview of cardiovascular components, their natural function in human body, type of the disease, and treatment options with specific focus on heart valves; Performing an extensive review on parametrical and mathematical design methodology of AVR including the anatomical geometry of the aortic valves; Reviewing the current methods of extracting the mechanical properties of natural organ and the range of bio-compatible materials that could be used for AVR construction together with presenting the formulation for constitutive material model; Investigating the achievements and limitations of experimental and numerical design methods from the literature. The reviewed information has assisted this research to consolidate the basis of a new design while considering the key parameters affecting the performance of the natural and prosthetic valves. Appropriate numerical method is found to be a fully coupled Fluid Structure Interaction (FSI) where the current restrictions of implementing such method are examined. The utilization of the FSI approach in cardiovascular component design is initiated by conducting a numerical experimentation on Coronary Artery Bypass Grafting (CABG) system. The FSI method successfully predicted the critical regions in distal CABG in which long term patency rate of such an operation could be affected. Other hemodynamic results are also presented and discussed in terms of blood flow velocity profile in distal region together with spatial and temporal Wall Shear Stress (WSS) distributions. In addition, the importance of arterial wall compliance on accuracy of hemodynamical results is also presented and discussed and their effects on the short and long term patency of CABG system are also given. The application of FSI method is then extended to the main focus of this research which is concerned with a new conceptual design for AVR. The reviewed design methodology is carefully implemented while a new mathematical formulation is proposed for the leaflets and sinus of valsalva using hyperbolic curves. A modified grade of Polyurethane is used for construction of the valve where the presented non-linear stress-strain data are fitted to a third order Yeoh Model. Both solid and fluid domains are meshed separately where a set of boundary nodes are considered on their interface. The mesh independency results indicate an optimum discretization on both domains where the predictions for velocity and deformation fields have minimal variation. The major problem of rapid element degeneration in multi domain analysis is resolved by applying a dynamic mesh updating scheme and precise data interpolations. Subsequently, the physiological boundary conditions are applied to both domains where a pulsatile, Newtonian, and turbulent blood rheology is considered in fluid domain while spatial fixations are restricted the structural domain movements. Proper unstructured tetrahedron element types are applied for both Finite Element Analysis (FEA) and Finite Volume Analysis (FVA) with an extensive investigation on initial status of the contact regions. The governing equations are separately discussed and then the algorithm of solving these equations in fully synchronized scheme is completely tested. The simulation successfully predicted the hemodynamical and structural results with the targeted accuracy of less than 1E-5 as the residuals for most of the FEA/FVA equations. The proposed model is enabled to reproduce the natural performance of the dysfunctional organ with distinct enhancements on hemodynamical and structural parameters compared to existing prosthesis