Leaflet Stress Research Articles

Simulated transcatheter aortic valve deformation: A parametric study on the impact of leaflet geometry on valve peak stress.

In this study, we developed a computational framework to investigate the impact of leaflet geometry of a transcatheter aortic valve (TAV) on the leaflet stress distribution, aiming at optimizing TAV leaflet design to reduce its peak stress. Utilizing a generic TAV model developed previously [Li and Sun, Annals of Biomedical Engineering, 2010. 38(8): 2690-2701], we first parameterized the 2D leaflet geometry by mathematical equations, then by perturbing the parameters of the equations, we could automatically generate a new leaflet design, remesh the 2D leaflet model and build a 3D leaflet model from the 2D design via a Python script. Approximately 500 different leaflet designs were investigated by simulating TAV closure under the nominal circular deployment and physiological loading conditions. From the simulation results, we identified a new leaflet design that could reduce the previously reported valve peak stress by about 5%. The parametric analysis also revealed that increasing the free edge width had the highest overall impact on decreasing the peak stress. A similar computational analysis was further performed for a TAV deployed in an abnormal, asymmetric elliptical configuration. We found that a minimal free edge height of 0.46 mm should be adopted to prevent central backflow leakage. This increase of the free edge height resulted in an increase of the leaflet peak stress. Furthermore, the parametric study revealed a complex response surface for the impact of the leaflet geometric parameters on the peak stress, underscoring the importance of performing a numerical optimization to obtain the optimal TAV leaflet design. Copyright © 2016 John Wiley & Sons, Ltd.

Neochord placement versus triangular resection in mitral valve repair: A finite element model

BackgroundRecurrent mitral regurgitation after mitral valve repair is common, occurring in nearly 50% of patients within 10 years of surgery. Durability of repair is partly related to stress distribution over the mitral leaflets. We hypothesized that repair with neochords (NCs) results in lower stress than leaflet resection (LR). Materials and methodsMagnetic resonance imaging and 3D echocardiography were performed before surgical repair of P2 prolapse in a single patient. A finite element model of the left ventricle and mitral valve was created previously, and the modeling program LS-DYNA was used to calculate leaflet stress for the following repairs: Triangular LR; LR with ring annuloplasty (LR + RA); One NC; Two NCs; and 2NC + RA. Results(1) NC placement resulted in stable posterior leaflet stress: Baseline versus 2 NC at end diastole (ED), 12.1 versus 12.0 kPa, at end systole (ES) 20.3 versus 21.7 kPa. (2) In contrast, LR increased posterior leaflet stress: Baseline versus LR at ED 12.1 versus 40.8 kPa, at ES 20.3 versus 46.1 kPa. (3) All repair types reduced anterior leaflet stress: Baseline versus 2 NC versus LR 34.2 versus 25.8 versus 20.6 kPa at ED and 80.8 versus 76.8 versus 67.8 kPa at ES. (4) The addition of RA reduced leaflet stress relative to repair without RA. ConclusionsNeochord repair restored normal leaflet coaptation without creating excessive leaflet stress, whereas leaflet resection more than doubled stress across the posterior leaflet. The excess stress created by leaflet resection was partially, but not completely, mitigated by ring annuloplasty.

A lipoprotein/β-barrel complex monitors lipopolysaccharide integrity transducing information across the outer membrane.

Lipoprotein RcsF is the OM component of the Rcs envelope stress response. RcsF exists in complexes with β-barrel proteins (OMPs) allowing it to adopt a transmembrane orientation with a lipidated N-terminal domain on the cell surface and a periplasmic C-terminal domain. Here we report that mutations that remove BamE or alter a residue in the RcsF trans-lumen domain specifically prevent assembly of the interlocked complexes without inactivating either RcsF or the OMP. Using these mutations we demonstrate that these RcsF/OMP complexes are required for sensing OM outer leaflet stress. Using mutations that alter the positively charged surface-exposed domain, we show that RcsF monitors lateral interactions between lipopolysaccharide (LPS) molecules. When these interactions are disrupted by cationic antimicrobial peptides, or by the loss of negatively charged phosphate groups on the LPS molecule, this information is transduced to the RcsF C-terminal signaling domain located in the periplasm to activate the stress response.

Characterization of three-dimensional anisotropic heart valve tissue mechanical properties using inverse finite element analysis

Computational modeling has an important role in design and assessment of medical devices. In computational simulations, considering accurate constitutive models is of the utmost importance to capture mechanical response of soft tissue and biomedical materials under physiological loading conditions. Lack of comprehensive three-dimensional constitutive models for soft tissue limits the effectiveness of computational modeling in research and development of medical devices. The aim of this study was to use inverse finite element (FE) analysis to determine three-dimensional mechanical properties of bovine pericardial leaflets of a surgical bioprosthesis under dynamic loading condition. Using inverse parameter estimation, 3D anisotropic Fung model parameters were estimated for the leaflets. The FE simulations were validated using experimental in-vitro measurements, and the impact of different constitutive material models was investigated on leaflet stress distribution. The results of this study showed that the anisotropic Fung model accurately simulated the leaflet deformation and coaptation during valve opening and closing. During systole, the peak stress reached to 3.17MPa at the leaflet boundary while during diastole high stress regions were primarily observed in the commissures with the peak stress of 1.17MPa. In addition, the Rayleigh damping coefficient that was introduced to FE simulations to simulate viscous damping effects of surrounding fluid was determined.

Acute effect of MitraClip implantation on mitral valve geometry in patients with functional mitral regurgitation: insights from three-dimensional transoesophageal echocardiography.

Our aim was to evaluate the acute effects of transcatheter edge-to-edge mitral valve repair using the MitraClip device on mitral valve geometry in patients with functional mitral regurgitation (FMR). Forty-two patients (age 73 years [IQ range 66.1-78.0], 55% men, 62% ischaemic FMR) with moderate-to-severe and severe FMR treated with the MitraClip were included. Three-dimensional transoesophageal echocardiography was performed prior to and immediately after MitraClip implantation. Acute changes of mitral annular and leaflet geometry were assessed with dedicated mitral modelling software. FMR less than moderate grade was achieved in 36 (86%) patients. After MitraClip implantation, the mitral annulus became more elliptical (ellipticity from 122±17% to 129±18%; p=0.04) with a non-significant reduction in anteroposterior diameter (33±6 to 32±5 mm, p=0.08). The coaptation area increased from 350 mm2 (IQ range 289-493 mm2) to 434 mm2 (IQ range 328-523 mm2, p=0.008). In particular, a larger part of the anterior mitral leaflet was included in the coaptation, leaving a smaller exposed anterior leaflet length of the A2 segment (from 27±6 mm to 25±5 mm, p<0.05) while the exposed length of the posterior leaflet (P2 level) remained unchanged (12±4 mm pre- vs. 13±4 mm post-repair, p=0.15). There was no change in total leaflet area (1,811±582 mm2 pre- vs. 1,870±506 mm2 post-repair, p=0.18). Annular height to intercommissural width ratio and tenting volume remained unchanged, suggesting no increase in leaflet stress. The MitraClip device affects MV geometry in FMR patients by increasing mitral annular ellipticity and coaptation area.

A Parametric Computational Study of the Impact of Non-circular Configurations on Bioprosthetic Heart Valve Leaflet Deformations and Stresses: Possible Implications for Transcatheter Heart Valves.

Although generally manufactured as circular devices with symmetric leaflets, transcatheter heart valves can become non-circular post-implantation, the impact of which on the long-term durability of the device is unclear. We investigated the effects of five non-circular (EllipMajor, EllipMinor, D-Shape, TriVertex, TriSides) annular configurations on valve leaflet stresses and valve leaflet deformations through finite element analysis. The highest in-plane principal stresses and strains were observed under an elliptical configuration with an aspect ratio of 1.25 where one of the commissures was on the minor axis of the ellipse. In this elliptical configuration (EllipMinor), the maximum principal stress increased 218% and the maximum principal strain increased 80% as compared with those in the circular configuration, and occurred along the free edge of the leaflet whose commissures were not on the minor axis (i.e., the "stretched" leaflet). The D-Shape configuration was similar to this elliptical configuration, with the degree to which the leaflets were stretched or sagging being less than the EllipMinor configuration. The TriVertex and TriSides configurations had similar leaflet deformation patterns in all three leaflets and similar to the Circular configuration. In the D-Shape, TriVertex, and TriSides configurations, the maximum principal stress was located near the commissures similar to the Circular configuration. In the EllipMinor and EllipMajor configurations, the maximum principal stress occurred near the center of the free edge of the "stretched" leaflets. These results further affirm recommendations by the International Standards Organization (ISO) that pre-clinical testing should consider non-circular configurations for transcatheter valve durability testing.

HEMODYNAMIC AND STRUCTURAL STUDY OF VARIOUS BIOPROSTHETIC AORTIC HEART VALVE

Simulation of heart and particularly heart valves is amongst the most common and fundamental methods in diagnosis and treatment of heart disorders. Replacement of natural valves with prosthetic valves is a prevalent treatment to valvular damages. The idea of using tissue engineering heart valves is in progress in developing countries. This is because of their unique biocompatibility and biodegradability according to special characteristics and geometries of every single patient’s heart. By considering and changing specific parameters for optimization of valve design, one can obtain the most favorable dimensions of the scaffold. So far, there are various studies which have been focused on leaflet stress distribution and deformation, but considering the blood in these studies have been less taken into account. Excellence of considering fluid structure interaction is to provide real physiological condition for heart valve simulation. Simulation of bioprosthetic heart valve with 19[Formula: see text]mm diameter for various geometries of leaflets was performed. Hemodynamic parameters such as mean velocity, flow rate, transvalvular pressure drop, effective orifice area, wall shear stresses, and structural parameters like Von Mises were investigated. Results indicated that leaflet with 12[Formula: see text]mm height among other geometries is the best case for hemodynamic parameters and Von Mises stress.

Abstract 13643: Impact of Aortic Valve Repair and Valve-sparing Procedures on the Mitral Annular Geometry Assessed by 3-dimensional Transesophageal Echocardiography

Background: Annular non-planarity, the "saddle-shape" of the mitral valve (MV) annulus, plays a role in minimizing leaflet stress and preserving valve function. Aortic valve (AV) repair/sparing procedures are increasingly used to treat aortic regurgitation (AR). The impact of these procedures on MV geometry and function is unknown. Methods: 2D and 3D transesophageal echocardiography (TEE) of the MV was acquired pre-operatively and immediately after surgery in 30 patients with severe AR and/or aortic root dilatation (Group 1 (n=10): bicuspid AV undergoing AV repair and valve-sparing root replacement with AV reimplantation; Group 2 (n=10): tricuspid AV undergoing AV repair and AV reimplantation; Group 3 (n=10): isolated AV cusp repair) and in 10 controls (Group 4). MV morphology was assessed by dedicated quantification software (fig. 1). Results: Comorbidities were similar in all groups. Groups 1, 2 and 3 had increased LV end-diastolic volumes compared to group 4 (214±90, 193±56, 180±55 vs. 86± 28ml for groups 1, 2, 3 and 4, respectively; p&lt;0.001); LVEF was similar (59.5 ± 6.2%). The baseline dimensions of the ventriculo-aortic junction and sinuses of Valsalva were larger in groups 1 and 2 vs. groups 3 and 4 (p&lt;0.0001). AV and MV parameters are summarized (table). Pre-operative MV parameters did not differ from those of normal subjects. The annular height and annular height to commissural width ratio (AHCWR) significantly decreased after surgery in groups 1 and 2, as did the tenting area. Significant changes in the coaptation depth and mitro-aortic angle were only found in group 1. Conclusions: AV repair/sparing procedures alter the MV geometry, including the non-planarity of the annulus and the depth of coaptation of the leaflets, especially among patients with a bicuspid AV undergoing AV reimplantation. These alterations could have long-term implications on MV function.

Abstract 18319: Virtual Mitral Valve Repair for Improved Pre-surgical Planning: Quantitative Evaluation of Neochordoplasty versus Leaflet Resection

Background: Neochordoplasty and/or leaflet resection are reliable and reproducible mitral valve (MV) repair techniques for the treatment of chordal rupture with severe mitral regurgitation (MR). We developed a novel computational evaluation strategy to determine the biomechanical and physiologic characteristics of MV dynamics prior to and following potential MV repair techniques to optimize surgical planning for neochordoplasty or leaflet resection. Methods: Virtual MV models from patients with P2 chordal rupture and severe MR were created using 3D echocardiographic data (N=5). Virtual neochordoplasty was designed by adding six neochordae between the papillary muscles and the P2 scallop. Virtual resection was performed by removing a pre-defined quadrangular shaped leaflet portion in the P2 scallop and merging the excised leaflet edges. Computational simulations of MV function (pre- and post-repair) were performed using dynamic finite element methods. Coaptation lengths and leaflet stress distributions were evaluated. Results: The MVs with P2 chordal rupture demonstrated severe P2 prolapse, leaflet malcoaptation, and large stress concentrations. Both repair techniques markedly reduced posterior leaflet prolapse and restored sufficient leaflet coaptation. Virtual neochordoplasty showed larger coaptation lengths (A2-P2) compared to virtual resection. Excessive stress concentrations in the P2 scallop disappeared and peak stress values decreased by up to 85% following both repair techniques. Conclusion: We have quantitatively evaluated patient-specific MV function before and after potential MV repair using a novel virtual MV repair protocol. Both virtual neochordoplasty and leaflet resection techniques decreased posterior leaflet prolapse, lessened stress concentration, and restored leaflet coaptation. This virtual simulation strategy has the potential for improved pre-surgical planning to optimize post-repair MV function.

TCT-659 Impact of Balloon-Expandable Transcatheter Aortic Valve Size on Stent and Leaflet Stresses

As transcatheter aortic valve replacement (TAVR) is being considered for surgical patients of lower risk and younger age, concerns remain regarding TAVR durability. From a valve design perspective, durability decreases with increased stresses on the stent and leaflets. Patient annulus sizes can fall

IMPACT OF AORTIC VALVE REPAIR AND VALVE-SPARING PROCEDURES ON THE MITRAL ANNULAR GEOMETRY ASSESSED BY 3-DIMENSIONAL TRANSESOPHAGEAL ECHOCARDIOGRAPHY

Background: Annular non-planarity, the saddle-shape of the mitral valve (MV) annulus, plays a role in minimizing leaflet stress and preserving valve function. Aortic valve (AV) repair/sparing procedures are increasingly used to treat aortic regurgitation (AR). The impact of these procedures on MV geometry and function is unknown. Methods: 2D and 3D transesophageal echocardiography (TEE) of the MV was acquired pre-operatively and immediately after surgery in 30 patients with severe AR and/or aortic root dilatation (Group 1 (n=10): bicuspid AV undergoing AV repair and valve-sparing root replacement with AV reimplantation; Group 2 (n=10): tricuspid AV undergoing AV repair and AV reimplantation; Group 3 (n=10): isolated AV cusp repair) and in 10 controls (Group 4). MV morphology was assessed by dedicated quantification software (fig. 1). Results: Comorbidities were similar in all groups. Groups 1, 2 and 3 had increased LV end-diastolic volumes compared to group 4 (214±90, 193±56, 180±55 vs. 86± 28ml for groups 1, 2, 3 and 4, respectively; p<0.001); LVEF was similar (59.5 ± 6.2%). The baseline dimensions of the ventriculo-aortic junction and sinuses of Valsalva were larger in groups 1 and 2 vs. groups 3 and 4 (p<0.0001). AV and MV parameters are summarized (table). Pre-operative MV parameters did not differ from those of normal subjects. The annular height and annular height to commissural width ratio (AHCWR) significantly decreased after surgery in groups 1 and 2, as did the tenting area. Significant changes in the coaptation depth and mitro-aortic angle were only found in group 1. Conclusions: AV repair/sparing procedures alter the MV geometry, including the non-planarity of the annulus and the depth of coaptation of the leaflets, especially among patients with a bicuspid AV undergoing AV reimplantation. These alterations could have long-term implications on MV function.

MODELING AORTIC VALVE INSUFFICIENCY AND AORTIC VALVE REPAIR: APPLICATIONS FROM BENCH TO THE OPERATING ROOM

Aortic valve repair(AVr) has become an attractive alternative for the correction of aortic insufficiency(AI) as compared to aortic valve replacement; however, little information exists regarding the effects of acute AI and subsequent AV repair(AVr) on ventricular and valvular dynamics. The objective of this study was to measure and compare hemodynamic and echocardiographic properties in a novel, ex-vivo model of severe acute AI due to cusp pathology followed by valve repair. Porcine aortic roots with intact aortic valves were placed in a left heart simulator mounted with a high-speed camera for baseline hemodynamic assessment. 3-D echocardiography was used to evaluate valve function, AI, and leaflet coaptation (coaptation surface area [CoSA]). In the first model, an 8mm diameter circular perforation was created in the body of the non-coronary cusp (NCC) leaving an intact free margin(AI “lesion” assessment) followed by patch repair (AVr “treatment” assessment). In the second model, 75% of the NCC was excised followed by cusp replacement with a pericardial patch. Hemodynamic and echocardiographic data are expressed as medians with interquartile ranges. Finite element modeling (FEM) was performed in both models to understand stress characteristics. Wilcoxon-signed rank tests were used to determine significance between paired sample experiments. The cusp perforation model demonstrated a significant increase in the regurgitant fraction (RF) after AI was created (control= 5.2 [5,5.6] % versus AI= 50.1 [49,53] %; p=0.04) and returned to baseline “control” levels after AVr (AVr= 11.7 [7.1,11.8] %; p=0.14). The cusp excision and repair model demonstrated a significant increase in RF after AI was created (control= 4.8 [3.2,9.1]% versus AI = 69.6 [69.2,75.3]%; p=0.04) but returned to baseline with valve repair “treatment” (AVr= 10.4[9.7,11] %; p=0.5). Furthermore, CoSA was significantly reduced after creation of AI (1.54cm2 versus 1.14[0.75,1.18] cm2; p=0.04) which then returned to baseline levels after AVr (1.54cm2 versus 1.59cm2; p=0.68). Finally, after comparing all hemodynamic parameters after valve repair in both models, there was no significant difference. Figure 1 summarizes the findings with FEM. We successfully simulated acute AI and effective repair due to clinically relevant cusp pathology in an ex-vivo left heart simulator along with 3-D echocardiography. After valve repair, both models demonstrated similar leaflet stress changes (+24% increase) suggesting that no matter the intervention, the repaired cusp retains major stress; making it more vulnerable to failure. Further investigation using this reproducible ex-vivo protocol with FEM will help reveal optimal therapeutic options in more complex models of AI.

TCT-621 The Synergistic Impact of Eccentric and Incomplete Stent Deployment on Transcatheter Aortic Valve Leaflet Stress Distribution

TCT-621 The Synergistic Impact of Eccentric and Incomplete Stent Deployment on Transcatheter Aortic Valve Leaflet Stress Distribution

The Impact of Fluid Inertia on In Vivo Estimation of Mitral Valve Leaflet Constitutive Properties and Mechanics.

We examine the influence of the added mass effect (fluid inertia) on mitral valve leaflet stress during isovolumetric phases. To study this effect, oscillating flow is applied to a flexible membrane at various frequencies to control inertia. Resulting membrane strain is calculated through a three-dimensional reconstruction of markers from stereo images. To investigate the effect in vivo, the analysis is repeated on a published dataset for an ovine mitral valve (Journal of Biomechanics 42(16): 2697-2701). The membrane experiment demonstrates that the relationship between pressure and strain must be corrected with a fluid inertia term if the ratio of inertia to pressure differential approaches 1. In the mitral valve, this ratio reaches 0.7 during isovolumetric contraction for an acceleration of 6 m/s(2). Acceleration is reduced by 72% during isovolumetric relaxation. Fluid acceleration also varies along the leaflet during isovolumetric phases, resulting in spatial variations in stress. These results demonstrate that fluid inertia may be the source of the temporally and spatially varying stiffness measurements previously seen through inverse finite element analysis of in vivo data during isovolumetric phases. This study demonstrates that there is a need to account for added mass effects when analyzing in vivo constitutive relationships of heart valves.

Leaflet stress and strain distributions following incomplete transcatheter aortic valve expansion

Transcatheter aortic valve replacement (TAVR) is an established treatment alternative to surgical valve replacement in high-risk patients with severe symptomatic aortic stenosis. The current guidelines for TAVR are to upsize transcatheter aortic valve (TAV) relative to the native annulus to secure the device and minimize paravalvular leakage. Unlike surgical stented bioprosthetic valves where leaflets are attached to a rigid frame, TAVs must expand to fit within the native annulus. Fully-expanded circular TAVs have consistent leaflet kinematics; however, subtle variations in the degree of stent expansion may affect leaflet coaptation. The objective of this study was to determine the impact of incomplete TAV expansion on leaflet stress and strain distributions. In this study, we developed finite element models of a 23mm homemade TAV expanded to diameters ranging from 18 to 23mm in 1mm increments. Through dynamic finite element simulations, we found that leaflet stress and strain distributions were dependent on the diameter of the inflated TAV. After complete expansion of the TAV to 23mm, high stress and strain regions were observed primarily in the commissures during diastole. However, 2–3mm incomplete TAV stent expansion induced localized high stress regions within the TAV commissures, while 4–5mm incomplete stent expansion induced localized high stress regions within the belly of the TAV leaflets during the diastolic phase of the cardiac cycle. Increased mechanical stress and flexural deformation on TAV leaflets due to incomplete stent expansion may lead to accelerated tissue degeneration and diminished long-term valve durability.

Biomechanical drawbacks of different techniques of mitral neochordal implantation: When an apparently optimal repair can fail

ObjectivesIntraoperative assessment of the proper neochordal length during mitral plasty may be complex sometimes. Patient-specific finite element models were used to elucidate the biomechanical drawbacks underlying an apparently correct mitral repair for isolated posterior prolapse. MethodsPreoperative patient-specific models were derived from cardiac magnetic resonance images; integrated with intraoperative surgical details to assess the location and extent of the prolapsing region, including the number and type of diseased chordae; and complemented by the biomechanical properties of mitral leaflets, chordae tendineae, and artificial neochordae. We investigated postoperative mitral valve biomechanics in a wide spectrum of different techniques (single neochorda, double neochordae, and preconfigured neochordal loop), all reestablishing adequate valvular competence, but differing in suboptimal millimetric expanded polytetrafluoroethylene suture lengths in a range of ±2 mm, compared with the corresponding “ideal repair.” ResultsDespite the absence of residual regurgitation, alterations in chordal forces and leaflet stresses arose simulating suboptimal repairs; alterations were increasingly relevant as more complex prolapse anatomies were considered and were worst when simulating single neochorda implantation. Multiple chordae implantations were less sensitive to errors in neochordal length tuning, but associated postoperative biomechanics were hampered when asymmetric configurations were reproduced. Computational outcomes were consistent with the presence and entity of recurrent mitral regurgitation at midterm follow-up of simulated patients. ConclusionsSuboptimal suture length tuning significantly alters chordal forces and leaflet stresses, which may be key parameters in determining the long-term outcome of the repair. The comparison of the different simulated techniques suggests possible criteria for the selection and implementation of neochordae implantation techniques.

Material Selection and Performance Index for Polymeric Prosthetic Heart Valve Design1

The number of patients requiring replacement heart valves due to valvular heart disease is forecast to rise from 290,000 in 2003 to 850,000 in 2050. Currently, treatment of these patients is dominated by either surgically implanted mechanical or bioprosthetic valves. Unfortunately, none of these solutions are optimal, they simply replace native valvular disease with “prosthetic valvular disease”—mechanical valves require lifelong anticoagulation, and bioprostheses have a limited durability of approximately 15 yr. As such, since the 1950s, researchers have sought solutions which may overcome these drawbacks. Flexible leaflet polymeric prosthetic heart valves (PHVs) have been proposed as a potential solution—able to provide good longterm durability and function without the need for anticoagulation, however, no valve has been clinically successful, and in the past 40 years of development no valve has reached clinical trials [1]. As such, the process by which we design polymeric valves should be seriously considered. The design of polymeric valves may be split into a structural valve design and material selection problem. The use of material selection indices to aid the quantitative design of engineering components was formalized by Ashby et al. [2], and in this report, we propose the use of a novel material performance index (PI) to evaluate potential polymer materials for PHVs. We identify the leaflets as the critical component for the valves, and this report is focused on material selection for leaflets. We limit our search of the material “kingdom” to elastomeric materials whose mechanical properties are similar to the biological tissues. This allows us to produce quasi-physiological flow. We proceed by producing a shortlist of materials based upon hemocompatibility (minimum of inflammation and thrombogenicity) and biostability (resistance to oxidation and hydrolysis). A number of elastomers and coatings fulfill these criteria. In the realm of PHV, a number of “biocompatible” and “biostable” polymers have been proposed: polyhedral oligomeric silsesquioxane polycarbonate urethane, polystyrene (PS)-block-polyisobutyleneb-PS (and cross linked), Elasteon, cellulose, PS-b-polyethylenepropylene-b-PS, and polyethylene-hydraluron. Within a group such as this we seek to determine the most suitable polymer to fulfill the PHV role. Durability and calcification are the two failure routes which have plagued all flexible leaflet polymeric valves tested in vitro or in vivo. Durability is a function of material and valve design. Although various leaflet forms and dimensions can result in reduced leaflet stresses, the first order determinant of the stresses induced in a leaflet when loaded during diastole is a function of the leaflet’s thickness; thicker leaflets have lower stresses. Concurrently, we must be mindful of the hemodynamics of the valve. In general, the reduction in leaflet thickness leads to a lower mean pressure gradient [3]. Given these conflicting criteria regarding leaflet thickness, we must seek a compromise, leading to the use of a material PI [2]. We must define a relationship for hemodynamics and durability using leaflet thickness and appropriate material parameters. For a trileaflet valve to successfully function the curvature of each leaflet must be reversed, in theory, TPGmax / Et where TPGmax is the maximum transvalvular pressure gradient, E is the Young’s Modulus (or flexural modulus), and t is the leaflet thickness. Using this relation for hydrodynamics, Haworth [4] used tear strength as the parameter for prediction of lifetime. The use of tear strength failed in part because it describes failure at a stress state several magnitudes greater than those found in physiological scenarios. Parfeev et al. [5] improved upon this by comparing polymers based upon wear limits after 10 cycles at 22 Hz. We performed frequency sweep dynamic mechanical analysis of candidate polymers showing that they undergo a transition at 15–20 Hz, making testing at such a high frequency unacceptable for predicting low frequency behavior. Since the 1980s, fatigue testing and prediction of rubber component failure has received significant research in other fields. In particular, the use of crack growth models to predict long term behavior has been successfully employed in the cyclic straining of various rubber components [6]. We now propose a novel material PI based on cyclic fatigue theory, for the improved design of polymeric PHVs.

The Freedom SOLO bovine pericardial stentless valve

The Freedom SOLO bovine pericardial stentless valve Olaf Stanger, Hendrik Tevaearai, Thierry Carrel Clinic for Cardiovascular Surgery, University Hospital Berne, Switzerland Abstract: The third-generation bovine pericardium Freedom SOLO (FS) stentless valve emerged in 2004 as a modified version of the Pericarbon Freedom stentless valve and as a very attractive alternative to stented bioprostheses. The design, choice of tissue, and anticalcification treatment fulfill most, if not all, requirements for an ideal valve substitute. The FS combines the single-suture, subcoronary implantation technique with the latest-generation bovine pericardial tissue and novel anticalcification treatment. The design allows imitation of the native healthy valve through unrestricted adaption to the patient&#39;s anatomy, reproducing a normal valve/root complex. However, despite hemodynamic performance superior to stented valves, we are approaching a critical observation period as superior durability, freedom from structural valve deterioration, and nonstructural failure has not been proven as expected. However, optimal performance and freedom from structural valve deterioration depend on correct sizing and perfect symmetric implantation, to ensure low leaflet stress. Any malpositioning can lead to tissue fatigue over time. Furthermore, the potential for better outcomes depends on optimal patient selection and observance of the limitations for the use of stentless valves, particularly for the FS. Clearly, stentless valve implantation techniques are less reproducible and standardized, and require surgeon-dependent experience and skill. Regardless of whether or not stentless valve durability surpasses third-generation stented bioprostheses, they will continue to play a role in the surgical repertoire. This review intends to help practitioners avoid pitfalls, observe limitations, and improve patient selection for optimal long-term outcome with the attractive FS stentless valve. Keywords: aortic valve, bioprosthesis, cardiac surgery, aortic valve replacement, tissue valve, stentless aortic valve, hemodynamics, long-term results

Abstract 18607: Intra-ventricular Papillary Muscle Banding vs. Undersized Mitral Annuloplasty to Treat Ischemic Mitral Regurgitation in a Chronic Swine Model

Introduction: Intra-ventricular papillary muscle banding (PMB), is a new mitral repair technique for ischemic mitral regurgitation (IMR), which reduces lateral distance between the two muscles to relieve leaflet tethering (Fig A). In this study, we compared the hemodynamic efficacy of PMB against undersized mitral annuloplasty (UMA) and the combination of PMB and UMA, in a chronic swine IMR model. Methods: Six swine were induced with chronic IMR via percutaneous infarction of the postero-lateral wall of the left ventricle(LV), and followed to 6 weeks to develop &gt;3+ IMR. At 6 weeks, via a left atriotomy an adjustable PMB externalized through the LV wall, and an adjustable UMA suture externalized through the anterior annulus were implanted (Fig B1,B2). The animals were weaned from bypass, and physiological swine hemodynamics of 90/65 mm Hg were achieved at 75 bpm. PMB was done such that 50% reduction in the lateral distance between the muscles was achieved, and UMA reduced septal-lateral annular dimension to 24mm. Epicardial echocardiography was performed to assess mitral valve function, with IMR severity and tenting area recorded. Computational valve modeling was performed to quantify leaflet stresses and chordal forces, to assess and compare repair durability. Results: Table 1 summarizes valve function after each repair, with PMB reducing MR grade from 3.67±0.52 to 0.33±0.52 (p&lt;0.05), which was significantly better than the persistent MR grade of 1.5±0.55 after UMA (p&lt;0.05). PMB+UMA achieved the best results overall. Similar trends in tenting area were seen, with PMB reducing the tenting area better than UMA, but the combination demonstrating the best results. Table 2 summarizes the valve and chordal stresses, with PMB alone or with UMA reducing the overall leaflet peak stress and chordal peak forces. Conclusions: PMB alone or concomitant with UMA is an effective technique to repair IMR in this model, with good hemodynamic efficacy and better mechanics compared to UMA.

Structural analysis of a stented pericardial heart valve with leaflets mounted externally

Our aim was to understand the structural and functional behaviour of a pericardial heart valve with biological leaflets attached externally to a stent. To our knowledge, there is little if any literature concerning these kinds of bioprosthetic heart valves, while there is more concerning bioprosthetic heart valves with leaflets mounted internally. We studied the problem using a finite element approach considering leaflets and stent interaction, the influence of leaflet anisotropy and stent stiffness, by comparing quasi-static and dynamic loadings. Although we considered the problem to be symmetric and fluid-structure interaction was not implemented, we believe that our results could be a solid basis for valve optimization. We found regions of high stress concentration at the commissure near the stent tip and at the base of the leaflet cusp. The structural behaviour in the first region was complex, while the stress in the second region acted radially because of high bending. Although leaflet tissue anisotropy and stent stiffness exerted a significant influence on the structural and functional behaviours, they had a contrasting effect on leaflet stress state, coaptation and valve opening. Therefore, a good optimization should take into account both structural and functional requirements when tuning tissue properties and stent stiffness.