Gil Marom

Cusp height in aortic valves

OBJECTIVES Successful aortic valve repair must normalize cusp and root dimensions. Limited information is available on the normal dimensions of human cusps, in particular the cusp height. METHODS The cusp height was measured intraoperatively in 621 patients during aortic valve repair procedures. A tricuspid anatomy was present in 329 patients and bicuspid in 286 patients. In addition, patient age, gender, height, weight, preoperative degree of aortic regurgitation, and aortic dimensions were recorded. The data were analyzed for possible interrelation between the cusp height and clinical variables. RESULTS In the bicuspid valves, the geometric height of the nonfused cusp ranged from 15 to 30 mm (mean, 23.8 ± 2.0). Significant correlations were found between the cusp height and all clinical variables. In the tricuspid valves, the height of the noncoronary cusp ranged from 14 to 28 mm (mean, 20.7 ± 2.2). The height of the left coronary cusp varied from 12 to 25 mm (mean, 20.0 ± 2.1) and that of the right coronary cusp from 12 to 25 mm (mean, 20.0 ± 2.1). The noncoronary cusp was significantly greater than the left and the right coronary cusp (P = .000). No difference was found between the left and right cusps (P = .513). Significant correlations between the geometric height and clinical parameters were found for most clinical variables, excluding the degree of aortic regurgitation. CONCLUSIONS We found the cusp height was larger than previously published. It shows marked variability and correlates with the clinical variables. These data might serve as the basis for decision making in aortic valve repair.

A fluid-structure interaction model of the aortic valve with coaptation and compliant aortic root

While aortic valve root compliance and leaflet coaptation have significant influence on valve closure, their implications have not yet been fully evaluated. The present study developed a full fluid–structure interaction (FSI) model that is able to cope with arbitrary coaptation between the leaflets of the aortic valve during the closing phase. Two simplifications were also evaluated for the simulation of the closing phase only. One employs an FSI model with a rigid root and the other uses a “dry” (without flow) model. Numerical tests were performed to verify the model. New metrics were defined to process the results in terms of leaflet coaptation area and contact pressure. The axial displacement of the leaflets, closure time and coaptation parameters were similar in the two FSI models, whereas the dry model, with imposed uniform load on the leaflets, produced larger coaptation area and contact pressure, larger axial displacement and faster closure time compared with the FSI model. The differences were up to 30% in the coaptation area, 55% in the contact pressure and 170% in the closure time. Consequently, an FSI model should be used to accurately resolve the kinematics of the aortic valve and leaflet coaptation details during the end-closing stage.

Aortic root numeric model: Annulus diameter prediction of effective height and coaptation in post–aortic valve repair

OBJECTIVE The aim of the present study was to determine the influence of the aortic annulus (AA) diameter in order to examine the performance metrics, such as maximum principal stress, strain energy density, coaptation area, and effective height in the aortic valve. METHODS Six cases of aortic roots with an AA diameter of 20 and 30 mm were numerically modeled. The coaptation height and area were calculated from 3-dimensional fluid structure interaction models of the aortic valve and root. The structural model included flexible cusps in a compliant aortic root with material properties similar to the physiologic values. The fluid dynamics model included blood hemodynamics under physiologic diastolic pressures of the left ventricle and ascending aorta. Furthermore, zero flow was assumed for effective height calculations, similar to clinical measurements. In these no-flow models, the cusps were loaded with a transvalvular pressure decrease. All other parameters were identical to the fluid structure interaction models. RESULTS The aortic valve models with an AA diameter range of 20 to 26 mm were fully closed, and those with an AA diameter range of 28 to 30 mm were only partially closed. Increasing the AA diameter from 20 to 30 mm decreased the averaged coaptation height and normalized cusp coaptation area from 3.3 to 0.3 mm and from 27% to 2.8%, respectively. Increasing the AA diameter from 20 to 30 mm decreased the effective height from 10.9 to 8.0 mm. CONCLUSIONS A decreased AA diameter increased the coaptation height and area, thereby improving the effective height during procedures, which could lead to increased coaptation and better valve performance.

A general three-dimensional parametric geometry of the native aortic valve and root for biomechanical modeling.

The complex three-dimensional (3D) geometry of the native tricuspid aortic valve (AV) is represented by select parametric curves allowing for a general construction and representation of the 3D-AV structure including the cusps, commissures and sinuses. The proposed general mathematical description is performed by using three independent parametric curves, two for the cusp and one for the sinuses. These curves are used to generate different surfaces that form the structure of the AV. Additional dependent curves are also generated and utilized in this process, such as the joint curve between the cusps and the sinuses. The models feasibility to generate patient-specific parametric geometry is examined against 3D-transesophageal echocardiogram (3D-TEE) measurements from a non-pathological AV. Computational finite-element (FE) mesh can then be easily constructed from these surfaces. Examples are given for constructing several 3D-AV geometries by estimating the needed parameters from echocardiographic measurements. The average distance (error) between the calculated geometry and the 3D-TEE measurements was only 0.78±0.63mm. The proposed general 3D parametric method is very effective in quantitatively representing a wide range of native AV structures, with and without pathology. It can also facilitate a methodical quantitative investigation over the effect of pathology and mechanical loading on these major AV parameters.

Fluid-Structure Interaction Model of Aortic Valve With Porcine-Specific Collagen Fiber Alignment in the Cusps

Native aortic valve cusps are composed of collagen fibers embedded in their layers. Each valve cusp has its own distinctive fiber alignment with varying orientations and sizes of its fiber bundles. However, prior mechanical behavior models have not been able to account for the valve-specific collagen fiber networks (CFN) or for their differences between the cusps. This study investigates the influence of this asymmetry on the hemodynamics by employing two fully coupled fluid-structure interaction (FSI) models, one with asymmetric-mapped CFN from measurements of porcine valve and the other with simplified-symmetric CFN. The FSI models are based on coupled structural and fluid dynamic solvers. The partitioned solver has nonconformal meshes and the flow is modeled by employing the Eulerian approach. The collagen in the CFNs, the surrounding elastin matrix, and the aortic sinus tissues have hyperelastic mechanical behavior. The coaptation is modeled with a master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries for both models. The mapped case showed highly asymmetric valve kinematics and hemodynamics even though there were only small differences between the opening areas and cardiac outputs of the two cases. The regions with a less dense fiber network are more prone to damage since they are subjected to higher principal stress in the tissues and a higher level of flow shear stress. This asymmetric flow leeward of the valve might damage not only the valve itself but also the ascending aorta.

Numerical model of the aortic root and valve: Optimization of graft size and sinotubular junction to annulus ratio

OBJECTIVE The aim of this study was to determine the influence of aortic annulus (AA) diameter and the ratio of the sinotubular junction (STJ) diameter to the AA diameter on aortic valve hemodynamics and tissue mechanics and to suggest optimal values. METHODS Sixteen cases of aortic roots with AA diameters between 22 and 28 mm and an STJ/AA diameter ratio between 0.8 and 1.4 were numerically modeled. Average coaptation height and mechanical stresses were calculated from 3-dimensional finite element analysis of the aortic valve and root. Five additional fluid structure interaction (FSI) models with an AA diameter of 24 mm and an STJ/AA ratio between 0.6 and 1.4 were also constructed. The material properties of the tissues were from porcine valves and boundary conditions were physiologic and normal blood pressures. RESULTS In most cases, average coaptation height decreased with an increase in the STJ/AA ratio. Those cases with AA diameters between 24 and 26 mm and an STJ/AA ratio between 0.8 and 1.0 had a relatively large average coaptation height (>3 mm) and similar stress distribution during diastole. The flow shear stress values on the cusps at peak systole increased at the same time as the STJ/AA ratio decreased, similar to the opening area. CONCLUSIONS Relatively large coaptation, low stress in the tissues during diastole, and low flow shear stress during systole is the best combination for cases of AA diameter between 24 and 26 mm with identical STJ diameter. Valve-sparing procedures that prevent AA expansion are preferable.

Progressive aortic valve calcification: Three-dimensional visualization and biomechanical analysis

Calcific aortic valve disease (CAVD) is a progressive pathology characterized by calcification mainly within the cusps of the aortic valve (AV). As CAVD advances, the blood flow and associated hemodynamics are severely altered, thus influencing the mechanical performance of the AV. This study proposes a new method, termed reverse calcification technique (RCT) capable of re-creating the different calcification growth stages. The RCT is based on three-dimensional (3D) spatial computed tomography (CT) distributions of the calcification density from patient-specific scans. By repeatedly subtracting the calcification voxels with the lowest Hounsfield unit (HU), only high calcification density volume is presented. RCT posits that this volume re-creation represents earlier calcification stages and may help identify CAVD initiation sites. The technique has been applied to scans from 12 patients (36 cusps) with severe aortic stenosis who underwent CT before transcatheter aortic valve implantation (TAVI). Four typical calcification geometries and growth patterns were identified. Finite elements (FE) analysis was applied to compare healthy AV structural response with two selected CAVD-RCT configurations. The orifice area decreased from 2.9cm(2) for the healthy valve to 1.4cm(2) for the moderate stenosis case. Local maximum strain magnitude of 0.24 was found on the edges of the calcification compared to 0.17 in the healthy AV, suggesting a direct relation between strain concentration and calcification geometries. The RCT may help predict CAVD progression in patients at early stages of the disease. The RCT allows a realistic FE mechanical simulation and performance of calcified AVs.

Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid-structure interaction approach

Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid-structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our hemodynamic shearing device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization.

Effect of Balloon-Expandable Transcatheter Aortic Valve Replacement Positioning: A Patient-Specific Numerical Model

Transcatheter aortic valve replacement (TAVR) has emerged as a life-saving and effective alternative to surgical valve replacement in high-risk, elderly patients with severe calcific aortic stenosis. Despite its early promise, certain limitations and adverse events, such as suboptimal placement and valve migration, have been reported. In the present study, it was aimed to evaluate the effect of various TAVR deployment locations on the procedural outcome by assessing the risk for valve migration. The deployment of a balloon-expandable Edwards SAPIEN valve was simulated via finite element analysis in a patient-specific calcified aortic root, which was reconstructed from CT scans of a retrospective case of valve migration. The deployment location was parametrized in three configurations and the anchorage was quantitatively assessed based on the contact between the stent and the native valve during the deployment and recoil phases. The proximal deployment led to lower contact area between the native leaflets and the stent which poses higher risk for valve migration. The distal and midway positions resulted in comparable outcomes, with the former providing a slightly better anchorage. The approach presented might be used as a predictive tool for procedural planning in order to prevent prosthesis migration and achieve better clinical outcomes.

Aortic root numeric model: Correlation between intraoperative effective height and diastolic coaptation

Effective height (hE) is a geometric parameter that can be measured during procedures and has been suggested as a predictor of valve performance. Few studies have suggested that the hE can be increased only by cusp intervention. In a previous study, we used 3dimensional numeric models of compliant aortic valves and roots and studied the influence of aortic annulus diameter. Although we found a correlation between the hE and the coaptation height (hC), we did not check the influence of the cusp size on this correlation. In this communication, we expand on our previous study and determine the correlation between hE and hC on the ba-