An investigation into computational modelling strategies to predict the in vitro performance of aortic valve replacements

The development and design of effective aortic valve replacements (AVRs) presents significant challenges given the structural and hemodynamic performance requirements that must be achieved. Conventional design approaches require substantial amounts of experimental testing, involving complex pulsatile flow rigs that mimic the conditions of the cardiac cycle while allowing for accurate measurements of valve performance. While these experiments are extremely valuable, they are time consuming and require multiple iterations of physical prototypes during the testing phase. Computational simulations could overcome these challenges, streamlining the design process and optimising the structural and hydrodynamic performance of AVRs under development. The objective of this thesis is to investigate the potential of computational modelling to predict hemodynamic and structural performance of aortic valves through a combined experimental-computational approach. In particular, the thesis investigates both finite element and fluid-structure interaction based approaches, implemented through Abaqus commercial software, and assesses their potential to robustly predict both the structural and hydrodynamic performance of AVRs. In this thesis, a tri-leaflet polymeric AVR, which was developed at the University of Galway, formed the basis for both experimental and computational work. All devices were manufactured in-house through compression moulding and a series of experimental bench top studies were carried out using an in vitro pulsatile flow rig to investigate the hydrodynamic performance of different valve designs according to ISO 5840. In parallel, a finite element based computational framework was developed to predict the systolic and diastolic configurations from in vitro testing and a range of surrogate parameters were proposed to provide direct insight into the in vitro hydrodynamic performance. This approach was then used to examine the effects of asymmetric and regional calcification patterns on the stenosed hydrodynamics of the aortic valve. Finally, a fluid-structure interaction model was developed using the Abaqus Coupled Eulerian Lagrangian (CEL) approach to predict the structural and hydrodynamic performance of the AVRs and to investigate the potential to conduct in silico bench testing of valve devices. To enable this, a bench top rig was designed and manufactured to enable detailed measurements of leaflet deformation and serve as validation for the Fluid Structure Interaction (FSI) simulations conducted. Through in vitro testing and in silico modelling, it was found that three-dimensional finite element modelling could be used as a predictor of the in vitro hydrodynamic performance of tri-leaflet aortic valve implants. Specifically, several surrogate measures were identified through regression analysis, whereby leaflet coaptation area, geometric orifice area and opening pressure were found to be suitable indicators of experimental in vitro hydrodynamic parameters of regurgitant fraction, effective orifice area and transvalvular pressure drop performance, respectively. This finite element framework was used to show that asymmetric and non-uniform calcification of aortic valves had a distinct effect on the predicted hydrodynamic performance, measured parameters and the indicated in vitro hydrodynamic performance. In particular, it was found that asymmetric calcification coverage was highly detrimental to the systolic Geometric Orifice Area (GOA), while symmetric calcification was actually more detrimental to diastolic parameters of diastolic GOA and Leaflet Coaptation Area (LCA). Finally, it was found that the Abaqus/CEL fluid-structure interaction approach could accurately predict experimentally observed leaflet deformations under two-dimensional flow conditions. In predicting the three-dimensional performance of a tri-leaflet valve, it was found that the computational model could capture certain features of the experimental performance across both a bicuspid and tricuspid valve, including peak systolic GOA, but failed to accurately capture bulk measures of performance that were present over the loading cycle (e.g. Effective Orifice Area (EOA)). This thesis highlights the distinct challenges in validating FSI-based models of structural and hydrodynamic performance of AVRs, while provided much-needed experimental data to this community.

Publisher

NUI Galway

Rights

Attribution-NonCommercial-NoDerivs 3.0 Ireland
CC BY-NC-ND 3.0 IE

cbn

Collections

University of Galway Theses (PhD Theses)