Simone Morganti

Simulation of transcatheter aortic valve implantation through patient-specific finite element analysis: Two clinical cases

Transcatheter aortic valve implantation (TAVI) is a minimally invasive procedure introduced to treat aortic valve stenosis in elder patients. Its clinical outcomes are strictly related to patient selection, operator skills, and dedicated pre-procedural planning based on accurate medical imaging analysis. The goal of this work is to define a finite element framework to realistically reproduce TAVI and evaluate the impact of aortic root anatomy on procedure outcomes starting from two real patient datasets. Patient-specific aortic root models including native leaflets, calcific plaques extracted from medical images, and an accurate stent geometry based on micro-tomography reconstruction are key aspects included in the present study. Through the proposed simulation strategy we observe that, in both patients, stent apposition significantly induces anatomical configuration changes, while it leads to different stress distributions on the aortic wall. Moreover, for one patient, a possible risk of paravalvular leakage has been found while an asymmetric coaptation occurs in both investigated cases. Post-operative clinical data, that have been analyzed to prove reliability of the performed simulations, show a good agreement with analysis results. The proposed work thus represents a further step towards the use of realistic computer-based simulations of TAVI procedures, aiming at improving the efficacy of the operation technique and supporting device optimization.

Simulation of transcatheter aortic valve implantation: a patient-specific finite element approach.

Until recently, heart valve failure has been treated adopting open-heart surgical techniques and cardiopulmonary bypass. However, over the last decade, minimally invasive procedures have been developed to avoid high risks associated with conventional open-chest valve replacement techniques. Such a recent and innovative procedure represents an optimal field for conducting investigations through virtual computer-based simulations: in fact, nowadays, computational engineering is widely used to unravel many problems in the biomedical field of cardiovascular mechanics and specifically, minimally invasive procedures. In this study, we investigate a balloon-expandable valve and we propose a novel simulation strategy to reproduce its implantation using computational tools. Focusing on the Edwards SAPIEN valve in particular, we simulate both stent crimping and deployment through balloon inflation. The developed procedure enabled us to obtain the entire prosthetic device virtually implanted in a patient-specific aortic root created by processing medical images; hence, it allows evaluation of postoperative prosthesis performance depending on different factors (e.g. device size and prosthesis placement site). Notably, prosthesis positioning in two different cases (distal and proximal) has been examined in terms of coaptation area, average stress on valve leaflets as well as impact on the aortic root wall. The coaptation area is significantly affected by the positioning strategy ( − 24%, moving from the proximal to distal) as well as the stress distribution on both the leaflets (+13.5%, from proximal to distal) and the aortic wall ( − 22%, from proximal to distal). No remarkable variations of the stress state on the stent struts have been obtained in the two investigated cases.

Prediction of patient-specific post-operative outcomes of TAVI procedure: The impact of the positioning strategy on valve performance

Prosthesis positioning in transcatheter aortic valve implantation procedures represents a crucial aspect for procedure success as demonstrated by many recent studies on this topic. Possible complications, device performance, and, consequently, also long-term durability are highly affected by the adopted prosthesis placement strategy. In the present work, we develop a computational finite element model able to predict device-specific and patient-specific replacement procedure outcomes, which may help medical operators to plan and choose the optimal implantation strategy. We focus in particular on the effects of prosthesis implantation depth and release angle. We start from a real clinical case undergoing Corevalve self-expanding device implantation. Our study confirms the crucial role of positioning in determining valve anchoring, replacement failure due to intra or para-valvular regurgitation, and post-operative device deformation.

Patient-specific simulation of a stentless aortic valve implant: the impact of fibres on leaflet performance

In some cases of aortic valve leaflet disease, the implant of a stentless biological prosthesis represents an excellent option for aortic valve replacement (AVR). In particular, if compared with the implant of mechanical valves, it provides a more physiological haemodynamic performance and a reduced thrombogeneticity, avoiding the use of anticoagulants. The clinical outcomes of AVR are strongly dependent on an appropriate choice of both prosthesis size and replacement technique, which is, at present, strictly related to surgeons experience and skill. This represents the motivation for patient-specific finite element analysis able to virtually reproduce stentless valve implantation. With the aim of performing reliable patient-specific simulations, we remark that, on the one hand, it is not well established in the literature whether bioprosthetic leaflet tissue is isotropic or anisotropic; on the other hand, it is of fundamental importance to incorporate an accurate material model to realistically predict post-operative performance. Within this framework, using a novel computational methodology to simulate stentless valve implantation, we test the impact of using different material models on both the stress pattern and post-operative coaptation parameters (i.e. coaptation area, length and height). As expected, the simulation results suggest that the material properties of the valve leaflets affect significantly the post-operative prosthesis performance.

Human dilated ascending aorta: Mechanical characterization via uniaxial tensile tests

Aneurysms of the ascending aorta (AsAA), i.e., a progressive and localized dilatation of the first part of the aorta, represent a severe life-threatening condition, often occurring with no symptom. AsAA formation is associated with a degeneration of the aortic wall tissue, which leads to changes in the tissue mechanical properties, and in particular to increased wall stress and/or decreased wall ultimate strength. Nowadays, the decision to surgically operate is usually based on the AsAA diameter, although such a criterion is not always predictive. The present study focuses on the mechanical characterization of the AsAA tissues. Specimens were cut from portions of dilated ascending aorta excised from 46 patients through open-heart surgery. Peak strain, peak stress, and maximum elastic modulus (i.e., tissue stiffness) were measured from uniaxial stress-strain curves. Such (ultimate) mechanical properties were collected for different regions of the aortic wall (anterior and posterior) as well as for different specimen orientations (circumferential and longitudinal). Relationships of ultimate mechanical properties with patient age and sex were also investigated. The obtained results highlighted a significant anisotropy of the AsAA tissue (as also observed for healthy aortic tissues), with higher value of strength and stiffness in the circumferential than in the longitudinal direction. Higher strength and stiffness were also found in the posterior region with respect to the anterior one for the circumferential orientation, whereas an opposite result was found for the longitudinal orientation. A decreasing trend of ultimate mechanical properties with aging was also highlighted. Finally, a significant difference in the strength between male and female was observed only in the circumferential direction.

Finite element analysis of aortic root dilation: a new procedure to reproduce pathology based on experimental data

Sinotubular junction dilation is one of the most frequent pathologies associated with aortic root incompetence. Hence, we create a finite element model considering the whole root geometry; then, starting from healthy valve models and referring to measures of pathological valves reported in the literature, we reproduce the pathology of the aortic root by imposing appropriate boundary conditions. After evaluating the virtual pathological process, we are able to correlate dimensions of non-functional valves with dimensions of competent valves. Such a relation could be helpful in recreating a competent aortic root and, in particular, it could provide useful information in advance in aortic valve sparing surgery.

Theoretical and Experimental Study of the Shape Memory Effect of Beams in Bending Conditions

In this study, the shape memory effect of SMA beams under complex stress conditions is studied by means of a finite element model. The 1D version of a well-established SMA constitutive model is utilized in the numerical computations and the required parameters are obtained experimentally starting from thermal cycling tests in tension under different constant loads. After being calibrated, the model is used to compute the deformation of beams loaded in bending and undergoing thermal cycling; three-point bending and cantilever configurations are considered in this stage. Finally, the response predicted by the model is compared to experimental results and model capabilities are discussed. In particular, insight of the stress and strain evolution in bending is provided.

Aortic root 3D parametric morphological model from 2D-echo images

The gold standard for the study of the macro-anatomy of the aortic root are multi-detector computed tomography (MDCT) and magnetic resonance (MR) imaging. Both technologies have major advantages and limitations. Although 4D echo is entering the study of the aortic root, 2D echo is the most commonly used diagnostic tool in daily practice. We designed and developed an algorithm for 3D modeling of the aortic root based on measures taken routinely at 2D echocardiography from 20 healthy individuals with normal aortic root. The tool was then translated in 12 patients who underwent both echo and MDCT. The results obtained with the 3D modeling program were quantitatively and qualitatively compared with 3D reconstruction from MDCT. Ad hoc ratios describing the morphology of the aortic root in MDCT and in the 3D model were used for comparison. In 12 patients with aortic root dilatation, the ratios obtained with our model are in good agreement with those from MDCT. Linear correlation for both long axis and short axis ratios was strong. The 3D modeling software can be easily adopted by cardiologists routinely involved in clinical evaluation of the pathology of the aortic root. The tool is easy to apply, does not require additional costs, and may be used to generate a set of data images for monitoring the evolution of the morphology and dimension of the aortic root, flanking the 3D MDCT and MR that remain the gold standard tools.

A framework for designing patient-specific bioprosthetic heart valves using immersogeometric fluid-structure interaction analysis

Numerous studies have suggested that medical image derived computational mechanics models could be developed to reduce mortality and morbidity due to cardiovascular diseases by allowing for patient-specific surgical planning and customized medical device design. In this work, we present a novel framework for designing prosthetic heart valves using a parametric design platform and immersogeometric fluid-structure interaction (FSI) analysis. We parameterize the leaflet geometry using several key design parameters. This allows for generating various perturbations of the leaflet design for the patient-specific aortic root reconstructed from the medical image data. Each design is analyzed using our hybrid arbitrary Lagrangian-Eulerian/immersogeometric FSI methodology, which allows us to efficiently simulate the coupling of the deforming aortic root, the parametrically designed prosthetic valves, and the surrounding blood flow under physiological conditions. A parametric study is performed to investigate the influence of the geometry on heart valve performance, indicated by the effective orifice area and the coaptation area. Finally, the FSI simulation result of a design that balances effective orifice area and coaptation area reasonably well is compared with patient-specific phase contrast magnetic resonance imaging data to demonstrate the qualitative similarity of the flow patterns in the ascending aorta.

Finite element analysis of TAVI: Impact of native aortic root computational modeling strategies on simulation outcomes

In the last few years, several studies, each with different aim and modeling detail, have been proposed to investigate transcatheter aortic valve implantation (TAVI) with finite elements. The present work focuses on the patient-specific finite element modeling of the aortic valve complex. In particular, we aim at investigating how different modeling strategies in terms of material models/properties and discretization procedures can impact analysis results. Four different choices both for the mesh size (from  20 k elements to  200 k elements) and for the material model (from rigid to hyperelastic anisotropic) are considered. Different approaches for modeling calcifications are also taken into account. Post-operative CT data of the real implant are used as reference solution with the aim of outlining a trade-off between computational model complexity and reliability of the results.