Quantification of Biomechanical Interaction of Transcatheter Aortic Valve Stent Deployed in Porcine and Ovine Hearts

Success of the deployment and function in transcatheter aortic valve replacement is heavily reliant on the tissue–stent interaction. The present study quantified important tissue–stent contact variables of self-expanding transcatheter aortic valve stents when deployed into ovine and porcine aortic roots, such as the stent radial expansion force, stent pullout force, the annulus deformation response and the coefficient of friction on the tissue–stent contact interface. Braided Nitinol stents were developed, tested to determine stent crimped diameter vs. stent radial force from a stent crimp experiment, and deployed in vitro to quantify stent pullout, aortic annulus deformation, and the coefficient of friction between the stent and the aortic tissue from an aortic root–stent interaction experiment. The results indicated that when crimped at body temperature from 26 mm to 19, 21 and 23 mm stent radial forces were approximately 30–40% higher than those crimped at room temperature. Coefficients of friction leveled to approximately 0.10 ± 0.01 as stent wire diameter increased and annulus size decreased from 23 to 19 mm. Regardless of aortic annulus size and species tested, it appeared that a minimum of about 2.5 mm in annular dilatation, caused by about 60 N of radial force from stent expansion, was needed to anchor the stent against a pullout into the left ventricle. The study of the contact biomechanics in animal aortic tissues may help us better understand characteristics of tissue–stent interactions and quantify the baseline responses of non-calcified aortic tissues.     

An In Vitro Evaluation of the Impact of Eccentric Deployment on Transcatheter Aortic Valve Hemodynamics

Patients with aortic stenosis present with calcium deposits on the native aortic valve, which can result in non-concentric expansion of Transcatheter Aortic Valve Replacement (TAVR) stents. The objective of this study is to evaluate whether eccentric deployment of TAVRs lead to turbulent blood flow and blood cell damage. Particle Image Velocimetry was used to quantitatively characterize fluid velocity fields, shear stress and turbulent kinetic energy downstream of TAVRs deployed in circular and eccentric orifices representative of deployed TAVRs in vivo. Effective orifice area (EOA) and mean transvalvular pressure gradient (TVG) values did not differ substantially in circular and eccentric deployed valves, with only a minor decrease in EOA observed in the eccentric valve (2.0 cm² for circular, 1.9 cm² for eccentric). Eccentric deployed TAVR lead to asymmetric systolic jet formation, with increased shear stresses (circular = 97 N/m² vs. eccentric = 119 N/m²) and regions of turbulence intensity (circular = 180 N/m² vs. eccentric = 230 N/m²) downstream that was not present in the circular deployed TAVR. The results of this study indicate that eccentric deployment of TAVRs can lead to altered flow characteristics and may potentially increase the hemolytic potential of the valve, which were not captured through hemodynamic evaluation alone.     

Stress Variations in the Human Aortic Root and Valve: The Role of Anatomic Asymmetry

The asymmetry of the aortic valve and aortic root may influence their biomechanics, yet was not considered in previous valve models. This study developed an anatomically representative model to evaluate the regional stresses of the valve within the root environment. A finite-element model was created from magnetic-resonance images of nine human valve–root specimens, carefully preserving their asymmetry. Regional thicknesses and anisotropic material properties were assigned to higher-order elastic shell elements representing the valve and root. After diastolic pressurization, peak principal stresses were evaluated for the right, left, and noncoronary leaflets and root walls. Valve stresses were highest in the noncoronary leaflet (538 kPa vs right 473 kPa vs left 410 kPa); peak stresses were located at the free margin and belly near the coaptation surfaces (averages 537 and 482 kPa for all leaflets, respectively). Right and noncoronary sinus stresses were 21% and 10% greater than the left sinus. In all sinuses, stresses near the annulus were higher than near the sinotubular junction. Stresses vary across the valve and root, likely due to their inherent morphologic asymmetry and stress sharing. These factors may influence bioprosthetic valve durability and the incidence of isolated sinus dilatation. © 1998 Biomedical Engineering Society. PAC98: 8745Bp, 8710+e, 0270Dh     

Finite Element Investigation of Stentless Pericardial Aortic Valves: Relevance of Leaflet Geometry

Recent developments in aortic valve replacement include the truly stentless pericardial bioprostheses with single point attached commissures (SPAC) implantation technique. The leaflet geometry available for the SPAC valves can either be a simple tubular or a complex three-dimensional structure molded using specially designed molds. Our main objective was to compare these two leaflet designs, the tubular vs. the molded, by dynamic finite element simulation. Time-varying physiological pressure loadings over a full cardiac cycle were simulated using ABAQUS. Dynamic leaflet behavior, leaflet coaptation parameters, and stress distribution were compared. The maximum effective valve orifice area during systole is 633.5 mm² in the molded valve vs. 400.6 mm² in the tubular valve, and the leaflet coaptation height during diastole is 4.5 mm in the former, in contrast to 1.6 mm in the latter. Computed compressive stress indicates high magnitudes at the commissures and inter-leaflet margins of the tubular valve, the highest being 3.83 MPa, more than twice greater than 1.80 MPa in the molded valve. The molded leaflet design which resembles the native valve exerts a positive influence on the mechanical performance of the SPAC pericardial valves compared with the simple tubular design. This may suggest enhanced valve efficacy and durability.     

Transcatheter Valve Underexpansion Limits Leaflet Durability: Implications for Valve-in-Valve Procedures

Transcatheter aortic valve (TAV) implantation within a failed bioprosthetic valve is a growing trend for high-risk patients. The non-compliant stent of the previous prosthesis may prevent full expansion of the TAV, which has been shown to distort the leaflet configuration, and has been hypothesized to adversely affect durability. In this study, TAV leaflet fatigue damage under cyclic pressurization in the setting of stent underexpansion by 0 (fully expanded), 1, 2 and 3 mm was simulated using finite element analysis to test this hypothesis. In the 2 and 3 mm underexpanded devices, the TAV leaflets exhibited severe pin-wheeling during valve closure, which increased leaflet stresses dramatically, and resulted in accelerated fatigue damage of the leaflets. The leaflet fatigue damage in the 1 mm underexpanded case was similar to that in the fully expanded case. Clinically a range of 10–15% underexpansion is generally considered acceptable; however, it was observed in this study that ≥2 mm (≥9.1%) underexpansion, will significantly impact device durability. Further study is necessary to determine the impact of various deployment conditions, i.e. non-uniform and non-circular deployments and different implantation heights, on differing TAV devices, but it is clear that the normal TAV leaflet configuration must be preserved in order to preserve durability.     

Dynamic finite element analysis of the aortic root from MRI-derived parameters

An understanding of aortic root biomechanics is pivotal for the optimisation of surgical procedures aimed at restoring normal root function in pathological subjects. For this purpose, computational models can provide important information, as long as they realistically capture the main anatomical and functional features of the aortic root.Here we present a novel and realistic finite element (FE) model of the physiological aortic root, which simulates its function during the entire cardiac cycle. Its geometry is based on magnetic resonance imaging (MRI) data obtained from 10 healthy subjects and accounts for the geometrical differences between the leaflet-sinus units. Morphological realism is combined with the modelling of the leaflets’ non-linear and anisotropic mechanical response, in conjunction with dynamic boundary conditions.The results show that anatomical differences between leaflet-sinus units cause differences in stress and strain patterns. These are notably higher for the leaflets and smaller for the sinuses. For the maximum transvalvular pressure value, maximum principal stresses on the leaflets are equal to 759, 613 and 603 kPa on the non-coronary, right and left leaflet, respectively. For the maximum aortic pressure, average maximum principal stresses values are equal to 118, 112 and 111 kPa on the right, non-coronary and left sinus, respectively.Although liable of further improvements, the model seems to reliably reproduce the behaviour of the real aortic root: the model's leaflet stretches, leaflet coaptation lengths and commissure motions, as well as the timings of aortic leaflet closures and openings, all matched with the experimental findings reported in the literature.     

A possible relation between pressure loading and thickened leaflets of the aortic valve: a model simulation

A much smaller percentage of thickened leaflets of the aortic valve have been found in the right or left coronary leaflet than in the noncoronary leaflet. This study investigated the pressure loading transferring to the leaflets of the aortic valve and their effects on the valvular thickening. A simple ascending aorta model was established, and a simulation was made. The pressure loading in the coronary and noncoronary leaflets then were estimated. The simulation results showed that 5.8% to 17.% percentage of pressure loading to the coronary leaflet may be decreased by the coronary perfusion in diastole. The coronary arteries play an important role on pressures in the sinuses of Valsalva. The smaller pressure loading transferring to the coronary leaflet than that to the noncoronary leaflet is one reasonable explanation related to the thickened leaflets of the aortic valve.     

Design considerations and quantitative assessment for the development of percutaneous mitral valve stent

Percutaneous heart valve replacement is gaining popularity, as more positive reports of satisfactory early clinical experiences are published. However this technique is mostly used for the replacement of pulmonary and aortic valves and less often for the repair and replacement of atrioventricular valves mainly due to their anatomical complexity. While the challenges posed by the complexity of the mitral annulus anatomy cannot be mitigated, it is possible to design mitral stents that could offer good anchorage and support to the valve prosthesis. This paper describes four new Nitinol based mitral valve designs with specific features intended to address migration and paravalvular leaks associated with mitral valve designs. The paper also describes maximum possible crimpability assessment of these mitral stent designs using a crimpability index formulation based on the various stent design parameters. The actual crimpability of the designs was further evaluated using finite element analysis (FEA). Furthermore, fatigue modeling and analysis was also done on these designs. One of the models was then coated with polytetrafluoroethylene (PTFE) with leaflets sutured and put to: (i) leaflet functional tests to check for proper coaptation of the leaflet and regurgitation leakages on a phantom model and (ii) anchorage test where the stented valve was deployed in an explanted pig heart. Simulations results showed that all the stents designs could be crimped to 18F without mechanical failure. Leaflet functional test results showed that the valve leaflets in the fabricated stented valve coapted properly and the regurgitation leakage being within acceptable limits. Deployment of the stented valve in the explanted heart showed that it anchors well in the mitral annulus. Based on these promising results of the one design tested, the other stent models proposed here were also considered to be promising for percutaneous replacement of mitral valves for the treatment of mitral regurgitation, by virtue of their key features as well as effective crimping. These models will be fabricated and put to all the aforementioned tests before being taken for animal trials.     

Simulated Thin Pericardial Bioprosthetic Valve Leaflet Deformation Under Static Pressure-Only Loading Conditions: Implications for Percutaneous Valves

Percutaneous aortic valve (PAV) replacement is currently being investigated as an endovascular alternative to conventional open-chest valve surgery for patients with severe aortic stenosis. The results of multi-center clinical trials of PAV devices have been encouraging. However, there are serious adverse events associated with this procedure. Furthermore, long-term durability and safety of PAV need to be studied carefully. In this study, we developed a thin pericardial bioprosthetic valve model, which has similar design features of PAV. We utilized this model to investigate PAV deformation under static, pressure-only loading conditions using Finite Element method. Mechanical properties of PAV leaflet were obtained from planar biaxial testing of glutaraldehyde treated thin bovine pericardium (BP) and porcine pericardium (PP), and characterized by the Fung-elastic model. Simulations were performed to examine the effects of tissue thickness and anisotropy on the valve deformation and stress distribution. The results indicated peak stress and strain occurred in the vicinity of commissures. The peak maximum principal stresses (MPS) were reduced with the increase of leaflet tissue thickness, by 36% and 59% from the mean thickness to 0.35 mm for BP and PP, respectively. The PAV with BP leaflet had a lower peak MPS than that with PP leaflet. Moreover, leaflet material orientation had a significant influence on the peak MPS of PAV.     

起旋式人工主动脉瓣构型设计及血流动力学评价

目的 设计一种附起旋功能的双叶机械瓣,通过改善其血流状态预防术后并发症。 方法 基于导流片式局部起旋器结构,将瓣叶作为导流叶片,并定义瓣叶包角以探究具有较优血流动力学特性的瓣膜构型。 应用有限元分析软件,对心缩期峰值流量状态下的主动脉流场进行仿真,螺旋性、壁面切应力分布等血流动力学特征。 结果 相较于对照瓣膜,起旋瓣具有更大的有效开口面积与更小的跨瓣压差,一定瓣叶包角范围内的起旋瓣能促进右手螺旋流的生成,并使血流趋向流道中心;起旋瓣壁面切应力分布也更加均匀,具有较少的低应力区与高应力区,壁面切应力峰值也相对较小。 针对研究中的主动脉模型,具有最优血流动力学特性的瓣叶包角为 15° ~ 20°。 结论 该新型人工主动脉瓣能调节主动脉内的血流特征,降低主动脉瓣置换术引起主动脉扩张与主动脉瘤的风险,对未来机械瓣构型设计具有指导意义。     

Aortic root performance after valve sparing procedure: a comparative finite element analysis

David and Yacoub sparing techniques are the most common procedures adopted for the surgical correction of aortic root aneurysms. These surgical procedures entail the replacement of the sinuses of Valsalva with a synthetic graft, inside which the cusps are re-suspended. Root replacement by a synthetic graft may result in altered valve behaviour both in terms of coaptation and stress distribution, thus leading to the failure of the correction. A finite element approach was used to investigate this phenomenon; four 3D models of the aortic root were developed to simulate the root in physiological, pathological and post-operative conditions after the two different surgical procedures. The physiological 3D geometrical model was developed on the basis of anatomical data obtained from echocardiographic images; it was then modified to obtain the pathological and post-operative models. The effectiveness of both techniques was assessed by comparison with the first two simulated conditions, in terms of stresses acting on the root, leaflet coaptation and interaction between leaflets and the graft during valve opening. Results show that both sparing techniques are able to restore aortic valve coaptation and to reduce stresses induced by the initial root dilation. Nonetheless, both techniques lead to altered leaflet kinematics, with more evident alterations after David repair.     

Flow changes in the aorta associated with the deployment of a AAA stent graft

The purpose of this study was to investigate the hemodynamic implications of a proximal shift in the aortic bifurcation that results from abdominal aortic aneurysm (AAA) stent graft deployment. A flow model was constructed in which an anatomically accurate model of the aorta was subjected to physiologic pulsatile flow. The model included the celiac, superior mesenteric, left and right renal arteries. The aortic bifurcation, leading to the right and left iliac arteries was included, as well as the lumbar curvature. Flow simulations were performed under resting and mild exercise conditions with and without a Cordis AAA stent graft deployed. Flow patterns were visualized with dye injection and recorded onto video. The flow rates through the iliac and renal arteries were continuously monitored using ultrasonic flowmeters. Flow visualization revealed that flow disturbances at the level of the renal arteries were slightly increased with the deployment of the stent graft. The orientation of the endolegs within the aorta had no perceptible effect on these disturbances. Under mild exercise conditions, very little flow disturbance was observed. In conclusion, there are slight changes in flow disturbance near the renal arteries due to stent graft deployment, but these changes would not be expected to have significant clinical implications.     

On the opening mechanism of the aortic valve: some observations from simulations

Viewed from the standpoint of mechanical engineering design, the aortic valve is impressive. However, our understanding of its mechanics is limited by our inability to study its in vivo function closely and in detail. Computer simulation methods offer an alternative approach and a first step towards the construction of a more complete cardiac model is described. The model includes the aortic valve, its leaflets and their supporting root, and the sinuses modelled as nonlinear materials. An explicit finite element code has been used to examine the time-varying displacements of the structure that was subject to pressure distributions, which included left ventricular, aortic and thoracic pressures. It was shown that the leaflets of the valve open by a combination of root expansion in a radial direction and leaflet movement in the direction of blood flow. This was compared to a model in which the aortic root was stiffened significantly, and it was found that this modified valve opened by leaflet folding to give a much smaller orifice. These findings, concerning the importance of root expansion, are in agreement with earlier experimental observations.     

Optical methods for the nondestructive evaluation of collagen morphology in bioprosthetic heart valves

The aim of the present study was to assess the suitability of nondestructive optical methods as a means of evaluating collagen morphology in bioprosthetic heart valve leaflets. The results of this study demonstrate that transmitted polarized light and incident polarized light optics facilitate the imaging of the inherent birefringence of valvular collagen fibers. Polarized light optics readily document the different patterns of collagen orientation and configuration in porcine aortic valvular (PAV) and bovine pericardial valvular (BPV) bioprostheses. Incident polarized light optics also provide information on leaflet surface morphology. Verification that the birefringence observed by polarized ligh optics represents leaflet collagen was provided by conventional histologic and transmission electron microscopic methods. Quantitative determinations of the spacing of collagen bundle waves gave similar values in intact and in sectioned BPV leaflets. Potential applications of polarized light optics in the assessment of bioprosthetic valve collagen are as follows: the selection of the desired orientation of collagen bundles within pericardium intended to be configured into bioprosthetic leaflets; evaluation of the effects of mechanical stresses and leaflet motion on collagen morphology in bioprosthetic valve leaflets; and initial screening of leaflet specimens and selection of the desired collagen orientation for embedding and sectioning of samples for conventional morphologic studies.     

Transcatheter Aortic Valve Replacement versus Sutureless Aortic Valve Replacement: A Single Center Retrospective Cohort Study

PurposeThis study sought to compare clinical outcomes between transcatheter aortic valve replacement (TAVR) and sutureless aortic valve replacement (SU-AVR).Materials and MethodsIn total, 320 patients with symptomatic severe aortic stenosis who underwent TAVR (n=254) or SU-AVR (n=66) at Severance Cardiovascular Hospital between July 2011 and September 2019 were included for analysis. Propensity score matching and inverse probability weighted adjustment were performed to adjust for confounding baseline characteristics. Outcomes defined by the Valve Academic Research Consortium-2 in 62 patients pairs were compared.ResultsDevice success (79.0% vs. 79.0%, p>0.999) and 30-day mortality (4.8% vs. 0.0%, p=0.244) did not differ between the TAVR and SU-AVR groups. The TAVR group developed more frequent mild or moderate paravalvular leakage (59.7% vs. 8.1%, p<0.001), whereas SU-AVR was associated with higher rates of major or life-threatening bleeding (9.7% vs. 22.6%, p=0.040), acute kidney injury (8.1% vs. 21.0%, p=0.041), and new-onset atrial fibrillation (4.8% vs. 32.3%. p<0.001) at 30 days, along with longer stays in the intensive care unit (ICU) (1.9±1.6 days vs. 5.9±9.2 days, p=0.009) and hospital (7.1±7.9 days vs. 13.1±8.8 days, p<0.001). The TAVR group showed a trend towards a higher 1-year all-cause mortality, compared with the SU-AVR group (7.0% vs 1.7%, p=0.149). Cardiovascular mortality, however, did not differ significantly (1.6% vs 1.7%, p=0.960).ConclusionTAVR achieved a similar 1-year survival rate free from cardiovascular mortality as SU-AVR and was associated with a lower incidence of complications, except for paravalvular leakage, and shorter stays in the ICU and hospital.     

Assessment of the influence of the compliant aortic root on aortic valve mechanics by means of a geometrical model

In recent years several researchers have suggested that the changes in the geometry and angular dimensions of the aortic root which occur during the cardiac cycle are functional to the optimisation of aortic valve function, both in terms of diminishing leaflet stresses and of fluid-dynamic behaviour. The paper presents an analytical parametric model of the aortic valve which includes the aortic root movement. The indexes used to evaluate the valve behaviour are: the circumferential membrane stress and the stress at the free edge of the leaflet, the index of bending strain, the bending of the leaflet at the line attachment in the radial and circumferential directions and the shape of the conduit formed by the leaflets during systole. In order to evaluate the role of geometric changes in valve performance, two control cases were considered, with different reference geometric configuration, where the movement of the aortic root was ignored. The results obtained appear consistent with physiological data, especially with regard to the late diastolic phase and the early ejection phase, and put in evidence the role of the aortic root movement in the improvement of valve behaviour.     

CT-scan images preprocessing and segmentation to improve bioprosthesis leaflets morphological analysis

The visualization of bioprosthesis leaflet morphology might help to better understand the underlying mechanism of dysfunction in degenerated aortic bioprosthesis. Because today such visualization of bioprosthesis leaflet morphology is intricate to impossible with other imaging techniques, we hypothesized that the processing of multi-detector CT images would allow better visualization of the prosthetic valve leaflets after biological aortic valve replacement. The purpose of our study was to prospectively evaluate patients with a degenerated aortic bioprosthesis, waiting for reoperation, by using 64-slice CT to evaluate prosthetic leaflets morphology. A semi-automatic segmentation of pre-operative tomodensitometric images was conducted, using 2 different implementations of the region growing algorithm. Here we report all segmentation steps (selection of the region of interest, filtering, segmentation). Studied degenerated aortic bioprostheses were represented by two Carpentier-Edwards Supra Annular Valve (porcine leaflets), one Edwards Perimount (pericardial leaflets) and one Medtronic Mosaic (porcine leaflets). Both segmentation methods (Isotropic Region Growing and Stick Region Growing) allowed a semi-automatic segmentation with 3D reconstruction of all bioprosthetic components (stent, leaflets, degeneration/calcifications). Explanted bioprosthesis CT images were also processed and used as reference. Segmentation results were compared by means of quantitative criteria. Semi-automatic segmentation using region growing algorithm seems to provide an interesting approach for the morphological characterization of degenerated aortic bioprostheses. We believe that in the next future CT scan images segmentation may play an important role to better understand the mechanism of dysfunction in failing aortic bioprostheses. Moreover, bioprostheses 3D reconstructions could be integrated into preoperative planning tools to optimize valve-in-valve procedure.     

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.     

High-Resolution Fluid–Structure Interaction Simulations of Flow Through a Bi-Leaflet Mechanical Heart Valve in an Anatomic Aorta

We have performed high-resolution fluid–structure interaction simulations of physiologic pulsatile flow through a bi-leaflet mechanical heart valve (BMHV) in an anatomically realistic aorta. The results are compared with numerical simulations of the flow through an identical BMHV implanted in a straight aorta. The comparisons show that although some of the salient features of the flow remain the same, the aorta geometry can have a major effect on both the flow patterns and the motion of the valve leaflets. For the studied configuration, for instance, the BMHV leaflets in the anatomic aorta open much faster and undergo a greater rebound during closing than the same valve in the straight axisymmetric aorta. Even though the characteristic triple-jet structure does emerge downstream of the leaflets for both cases, for the anatomic case the leaflet jets spread laterally and diffuse much faster than in the straight aorta due to the aortic curvature and complex shape of the anatomic sinus. Consequently the leaflet shear layers in the anatomic case remain laminar and organized for a larger portion of the accelerating phase as compared to the shear layers in the straight aorta, which begin to undergo laminar instabilities well before peak systole is reached. For both cases, however, the flow undergoes a very similar explosive transition to the small-scale, turbulent-like state just prior to reaching peak systole. The local maximum shear stress is used as a metric to characterize the mechanical environment experienced by blood cells. Pockets of high local maximum shear are found to be significantly more widespread in the anatomic aorta than in the straight aorta throughout the cardiac cycle. Pockets of high local maximum shear were located near the leaflets and in the aortic arc region. This work clearly demonstrates the importance of the aortic geometry on the flow phenomena in a BMHV and demonstrates the potential of our computational method to carry out image-based patient-specific simulations for clinically relevant studies of heart valve hemodynamics.     

支架作用下主动脉血管壁应力分析

目的 利用有限元方法模拟主动脉覆膜支架与考虑残余应力的主动脉血管壁的耦合作用,研究支架对主动脉血管壁应力分布的影响.方法 采用应力驱动的主动脉各向异性生长模型,降低轴向和周向跨壁应力梯度,从而在理想化健康人双层主动脉模型中生成三维残余应力场;建立覆膜支架模型实现其虚拟植入;对覆膜支架作用下的主动脉血管壁进行应力分析.结...