An Annular Prosthesis for the Treatment of Functional Mitral Regurgitation: Finite Element Model Analysis of a Dog Bone–Shaped Ring Prosthesis

Computational Modeling of the Subject-Specific Effects of Annuloplasty Ring Sizing on the Mitral Valve to Repair Functional Mitral Regurgitation

Surgical repair of functional mitral regurgitation (FMR) that occurs in nearly 60% of heart failure (HF) patients is currently performed with undersizing mitral annuloplasty (UMA), which lacks short- and long-term durability. Heterogeneity in valve geometry makes tailoring this repair to each patient challenging, and predictive models that can help with planning this surgery are lacking. In this study, we present a 3D echo-derived computational model, to enable subject-specific, pre-surgical planning of the repair. Three computational models of the mitral valve were created from 3D echo data obtained in three pigs with HF and FMR. An annuloplasty ring model in seven sizes was created, each ring was deployed, and post-repair valve closure was simulated. The results indicate that large annuloplasty rings (> 32 mm) were not effective in eliminating regurgitant gaps nor in restoring leaflet coaptation or reducing leaflet stresses and chordal tension. Smaller rings (≤ 32 mm) restored better systolic valve closure in all investigated cases,but excessive valve tethering and restricted motion of the leaflets were still present. This computational study demonstrates that for effective correction of FMR, the extent of annular reduction differs between subjects, and overly reducing the annulus has deleterious effects on the valve.

Numerical modeling for efficiency and endurance assessment of an indirect mitral annuloplasty device

In recent years, several transcatheter systems have been introduced for treatment of common mitral regurgitation (MR). Such a system that is based on indirect mitral annuloplasty (IMA) is currently indicated for functional MR. Very few clinical studies have been performed to assess the efficiency and durability of such devices, despite their high risk of fracture resulting from ongoing exposure to large cyclic deformations. In this study, numerical models of moderate primary MR were created to test the implantation procedure of a customized IMA device and its sealing efficiency. The ability of the implanted device to reduce systolic leakage was evaluated and affirmed with a model of a more generic device. The long-term durability of the device was tested using a range of Nickel Titanium material properties. Our results demonstrated a considerable reduction in leakage for both the simplified generic device and the more detailed customized device models. The device met different fatigue criteria, confirming its resiliency and safety even after 10 years, even under the harsher conditions of primary MR. This is the first study to assess the performance and fatigue risk of IMA devices for the treatment of more complicated MR conditions. These findings may pave the way for further research to ultimately consider the device in selective cases of PMR.

Fluid structure computational model of simulating mitral valve motion in a contracting left ventricle

BACKGROUND

Fluid structure interaction simulations h hold promise in studying normal and abnormal cardiac function, including the effect of fluid dynamics on mitral valve (MV) leaflet motion. The goal of this study was to develop a 3D fluid structure interaction computational model to simulate bileaflet MV when interacting with blood motion in left ventricle (LV).

METHODS

The model consists of ideal geometric-shaped MV leaflets and the LV, with MV dimensions based on human anatomical measurements. An experimentally-based hyperelastic isotropic material was used to model the mechanical behaviour of the MV leaflets, with chordae tendineae and papillary muscle tips also incorporated. LV myocardial tissue was prescribed using a transverse isotropic hyperelastic formulation. Incompressible Navier-Stokes fluid formulations were used to govern the blood motion, and the Arbitrary Lagrangian Eulerian (ALE) method was employed to determine the mesh deformation of the fluid and solid domains due to trans-valvular pressure on MV boundaries and the resulting leaflet movement.

RESULTS

The LV-MV generic model was able to reproduce physiological MV leaflet opening and closing profiles resulting from the time-varying atrial and ventricular pressures, as well as simulating normal and prolapsed MV states. Additionally, the model was able to simulate blood flow patterns after insertion of a prosthetic MV with and without left ventricular outflow tract flow obstruction. In the MV-LV normal model, the regurgitant blood flow fraction was 10.1 %, with no abnormality in cardiac function according to the mitral regurgitation severity grades reported by the American Society of Echocardiography.

CONCLUSION

Our simulation approach provides insights into intraventricular fluid dynamics in a contracting LV with normal and prolapsed MV function, as well as aiding in the understanding of possible complications after transcatheter MV implantation prior to clinical trials.

Numerical biomechanics modelling of indirect mitral annuloplasty treatments for functional mitral regurgitation

Mitral valve regurgitation (MR) is a common valvular heart disease where an improper closure leads to leakage from the left ventricle into the left atrium. There is a need for less-invasive treatments such as percutaneous repairs for a large inoperable patient population. The aim of this study is to compare several indirect mitral annuloplasty (IMA) percutaneous repair techniques by finite-element analyses. Two types of generic IMA devices were considered, based on coronary sinus vein shortening (IMA-CS) to reduce the annulus perimeter and based on shortening of the anterior-posterior diameter (IMA-AP). The disease, its treatments, and the heart function post-repair were modelled by modifying the living heart human model (Dassault Systèmes). A functional MR pathology that represents ischaemic MR was generated and the IMA treatments were simulated in it, followed by heart function simulations with the devices and leakage quantification from blood flow simulations. All treatments were able to reduce leakage, the IMA-AP device achieved better sealing, and there was a correlation between the IMA-CS device length and the reduction in leakage. The results of this study can help in bringing IMA-AP to market, expanding the use of IMA devices, and optimizing future designs of such devices.

Numerical Biomechanics Models of the Interaction Between a Novel Transcatheter Mitral Valve Device and the Subvalvular Apparatus

Objective

Mitral valve regurgitation (MR) is a common valvular heart disease where improper closing causes leakage. Currently, no transcatheter mitral valve device is commercially available. Raanani (co-author) and colleagues have previously proposed a unique rotational implantation, ensuring anchoring by metallic arms that pull the chordae tendineae. This technique is now being implemented in a novel device design. The aim of this study is to quantify the rotational implantation effect on the mitral annulus kinematics and on the stresses in the chordae and papillary muscles.

Methods

Finite element analysis of the rotational step of the implantation in a whole heart model is employed to compare 5 arm designs with varying diameters (25.9 mm to 32.4 mm) and rotation angles (up to 140°). The arm rotation that grabs the chordae was modeled when the valve was in systolic configuration.

Results

An increase in the rotation angle results in reduced mitral annulus perimeters. Larger rotation angles led to higher chordae stresses with the 29.8 mm experiencing the maximum stresses. The calculated chordae stresses suggest that arm diameter should be <27.8 mm and the rotation angle <120°.

Conclusions

The upper limit of this diameter range is preferred in order to reduce the stresses in the papillary muscles while grabbing more chords. The findings of this study can help improving the design and performance of this unique device and procedural technique.

Effect of Edge-to-Edge Mitral Valve Repair on Chordal Strain: Fluid-Structure Interaction Simulations

Edge-to-edge repair for mitral valve regurgitation is being increasingly performed in high-surgical risk patients using minimally invasive mitral clipping devices. Known procedural complications include chordal rupture and mitral leaflet perforation. Hence, it is important to quantitatively evaluate the effect of edge-to-edge repair on chordal integrity. in this study, we employ a computational mitral valve model to simulate functional mitral regurgitation (FMR) by creating papillary muscle displacement. Edge-to-edge repair is then modeled by simulated coaptation of the mid portion of the mitral leaflets. in the setting of simulated FMR, edge-to-edge repair was shown to sustain low regurgitant orifice area, until a two fold increase in the inter-papillary muscle distance as compared to the normal mitral valve. Strain in the chordae was evaluated near the papillary muscles and the leaflets. Following edge-to-edge repair, strain near the papillary muscles did not significantly change relative to the unrepaired valve, while strain near the leaflets increased significantly relative to the unrepaired valve. These data demonstrate the potential for computational simulations to aid in the pre-procedural evaluation of possible complications such as chordal rupture and leaflet perforation following percutaneous edge-to-edge repair.

Fluid-Structure Interaction Analysis of Subject-Specific Mitral Valve Regurgitation Treatment with an Intra-Valvular Spacer

Mitral regurgitation imposes a significant symptomatic burden on patients who are not candidates for conventional surgery. For these patients, transcatheter repair and replacement devices are emerging as alternative options. One such device is an intravalvular balloon or spacer that is inserted between the mitral valve leaflets to fill the gap that gives rise to mitral regurgitation. In this study, we apply a large-deformation fluid-structure interaction analysis to a fully 3D subject-specific mitral valve model to assess the efficacy of the intra-valvular spacer for reducing mitral regurgitation severity. The model includes a topologically 3D subvalvular apparatus with unprecedented detail. Results show that device fixation and anchoring at the location of the subject’s regurgitant orifice is imperative for optimal reduction of mitral regurgitation.

New Insights into Valve Hemodynamics

Heart valve diseases are common disorders with five million annual diagnoses being made in the United States alone. All heart valve disorders alter cardiac hemodynamic performance; therefore, treatments aim to restore normal flow. This paper reviews the state-of-the-art clinical and engineering advancements in heart valve treatments with a focus on hemodynamics. We review engineering studies and clinical literature on the experience with devices for aortic valve treatment, as well as the latest advancements in mitral valve treatments and the pulmonic and tricuspid valves on the right side of the heart. Upcoming innovations will potentially revolutionize treatment of heart valve disorders. These advancements, and more gradual enhancements in the procedural techniques and imaging modalities, could improve the quality of life of patients suffering from valvular disease who currently cannot be treated.

Finite element analysis of annuloplasty and papillary muscle relocation on a patient-specific mitral regurgitation model

Objectives

Functional mitral regurgitation (FMR) is a significant complication of left ventricle (LV) dysfunction associated with poor prognosis and commonly treated by undersized ring annuloplasty. This study aimed to quantitatively simulate the treatment outcomes and mitral valve (MV) biomechanics following ring annulopalsty and papillary muscle relocation (PMR) procedures for a FMR patient.

Methods

We utilized a validated finite element model of the left heart for a patient with severe FMR and LV dilation from our previous study and simulated virtual ring annuloplasty procedures with various sizes of Edwards Classic and GeoForm annuloplasty rings. The model included detailed geometries of the left ventricle, mitral valve, and chordae tendineae, and incorporated age- and gender- matched nonlinear, anisotropic hyperelastic tissue material properties, and simulated chordal tethering at diastole due to LV dilation.

Results

Ring annuloplasty with either the Classic or GeoForm ring improved leaflet coaptation and increased the total leaflet closing force while increased posterior mitral leaflet (PML) stresses and strains. Classic rings resulted in larger coaptation forces and areas compared to GeoForm rings. The PMR procedure further improved the leaflet coaptation, decreased the PML stress and strain for both ring shapes and all sizes in this patient model.

Conclusions

This study demonstrated that a rigorously developed patient-specific computational model can provide useful insights into annuloplasty repair techniques for the treatment of FMR patients and could potentially serve as a tool to assist in pre-operative planning for MV repair surgical or interventional procedures.

A comprehensive pipeline for multi-resolution modeling of the mitral valve: Validation, computational efficiency, and predictive capability

Multiple studies have demonstrated that the pathological geometries unique to each patient can affect the durability of mitral valve (MV) repairs. While computational modeling of the MV is a promising approach to improve the surgical outcomes, the complex MV geometry precludes use of simplified models. Moreover, the lack of complete in vivo geometric information presents significant challenges in the development of patient-specific computational models. There is thus a need to determine the level of detail necessary for predictive MV models. To address this issue, we have developed a novel pipeline for building attribute-rich computational models of MV with varying fidelity directly from the in vitro imaging data. The approach combines high-resolution geometric information from loaded and unloaded states to achieve a high level of anatomic detail, followed by mapping and parametric embedding of tissue attributes to build a high-resolution, attribute-rich computational models. Subsequent lower resolution models were then developed and evaluated by comparing the displacements and surface strains to those extracted from the imaging data. We then identified the critical levels of fidelity for building predictive MV models in the dilated and repaired states. We demonstrated that a model with a feature size of about 5 mm and mesh size of about 1 mm was sufficient to predict the overall MV shape, stress, and strain distributions with high accuracy. However, we also noted that more detailed models were found to be needed to simulate microstructural events. We conclude that the developed pipeline enables sufficiently complex models for biomechanical simulations of MV in normal, dilated, repaired states.

Multi-resolution geometric modeling of the mitral heart valve leaflets

An essential element of cardiac function, the mitral valve (MV) ensures proper directional blood flow between the left heart chambers. Over the past two decades, computational simulations have made marked advancements toward providing powerful predictive tools to better understand valvular function and improve treatments for MV disease. However, challenges remain in the development of robust means for the quantification and representation of MV leaflet geometry. In this study, we present a novel modeling pipeline to quantitatively characterize and represent MV leaflet surface geometry. Our methodology utilized a two-part additive decomposition of the MV geometric features to decouple the macro-level general leaflet shape descriptors from the leaflet fine-scale features. First, the general shapes of five ovine MV leaflets were modeled using superquadric surfaces. Second, the finer-scale geometric details were captured, quantified, and reconstructed via a 2D Fourier analysis with an additional sparsity constraint. This spectral approach allowed us to easily control the level of geometric details in the reconstructed geometry. The results revealed that our methodology provided a robust and accurate approach to develop MV-specific models with an adjustable level of spatial resolution and geometric detail. Such fully customizable models provide the necessary means to perform computational simulations of the MV at a range of geometric accuracies in order to identify the level of complexity required to achieve predictive MV simulations.

Modelling mitral valvular dynamics-current trend and future directions

Dysfunction of mitral valve causes morbidity and premature mortality and remains a leading medical problem worldwide. Computational modelling aims to understand the biomechanics of human mitral valve and could lead to the development of new treatment, prevention and diagnosis of mitral valve diseases. Compared with the aortic valve, the mitral valve has been much less studied owing to its highly complex structure and strong interaction with the blood flow and the ventricles. However, the interest in mitral valve modelling is growing, and the sophistication level is increasing with the advanced development of computational technology and imaging tools. This review summarises the state-of-the-art modelling of the mitral valve, including static and dynamics models, models with fluid-structure interaction, and models with the left ventricle interaction. Challenges and future directions are also discussed.

Fluid-Structure Interaction Analysis of Ruptured Mitral Chordae Tendineae

The chordal structure is a part of mitral valve geometry that has been commonly neglected or simplified in computational modeling due to its complexity. However, these simplifications cannot be used when investigating the roles of individual chordae tendineae in mitral valve closure. For the first time, advancements in imaging, computational techniques, and hardware technology make it possible to create models of the mitral valve without simplifications to its complex geometry, and to quickly run validated computer simulations that more realistically capture its function. Such simulations can then be used for a detailed analysis of chordae-related diseases. In this work, a comprehensive model of a subject-specific mitral valve with detailed chordal structure is used to analyze the distinct role played by individual chordae in closure of the mitral valve leaflets. Mitral closure was simulated for 51 possible chordal rupture points. Resultant regurgitant orifice area and strain change in the chordae at the papillary muscle tips were then calculated to examine the role of each ruptured chorda in the mitral valve closure. For certain subclassifications of chordae, regurgitant orifice area was found to trend positively with ruptured chordal diameter, and strain changes correlated negatively with regurgitant orifice area. Further advancements in clinical imaging modalities, coupled with the next generation of computational techniques will enable more physiologically realistic simulations.

Finite Element Analysis of Patient-Specific Mitral Valve with Mitral Regurgitation

Functional mitral regurgitation (FMR) is a significant complication of left ventricular dysfunction and strongly associated with a poor prognosis. In this study, we developed a patient-specific finite element (FE) model of the mitral apparatus in a FMR patient which included: both leaflets with thickness, annulus, chordae tendineae, and chordae insertions on the leaflets and origins on the papillary muscles. The FE model incorporated human age- and gender-matched anisotropic hyperelastic material properties, and MV closure at systole was simulated. The model was validated by comparing the FE results from valve closure simulation with the in vivo geometry of the MV at systole. It was found that the FE model could not replicate the in vivo MV geometry without the application of tethering pre-tension force in the chordae at diastole. Upon applying the pre-tension force and performing model optimization by adjusting the chordal length, position, and leaflet length, a good agreement between the FE model and the in vivo model was established. Not only were the chordal forces high at both diastole and systole, but the tethering force on the anterior papillary muscle was higher than that of the posterior papillary muscle, which resulted in an asymmetrical gap with a larger orifice area at the anterolateral commissure resulting in MR. The analyses further show that high peak stress and strain were found at the chordal insertions where large chordal tethering forces were found. This study shows that the pre-tension tethering force plays an important role in accurately simulating the MV dynamics in this FMR patient, particularly in quantifying the degree of leaflet coaptation and stress distribution. Due to the complexity of the disease, the patient-specific computational modeling procedure of FMR patients presented should be further evaluated using a large patient cohort. However, this study provides useful insights into the MV biomechanics of a FMR patient, and could serve as a tool to assist in pre-operative planning for MV repair or replacement surgical or interventional procedures.

Heart Valve Biomechanics and Underlying Mechanobiology

Heart valves control unidirectional blood flow within the heart during the cardiac cycle. They have a remarkable ability to withstand the demanding mechanical environment of the heart, achieving lifetime durability by processes involving the ongoing remodeling of the extracellular matrix. The focus of this review is on heart valve functional physiology, with insights into the link between disease-induced alterations in valve geometry, tissue stress, and the subsequent cell mechanobiological responses and tissue remodeling. We begin with an overview of the fundamentals of heart valve physiology and the characteristics and functions of valve interstitial cells (VICs). We then provide an overview of current experimental and computational approaches that connect VIC mechanobiological response to organ- and tissue-level deformations and improve our understanding of the underlying functional physiology of heart valves. We conclude with a summary of future trends and offer an outlook for the future of heart valve mechanobiology, specifically, multiscale modeling approaches, and the potential directions and possible challenges of research development. © 2016 American Physiological Society. Compr Physiol 6:1743-1780, 2016.

Numerical modeling of heart valves using resistive Eulerian surfaces

The goal of this work is the development and numerical implementation of a mathematical model describing the functioning of heart valves. To couple the pulsatile blood flow with a highly deformable thin structure (the valve's leaflets), a resistive Eulerian surfaces framework is adopted. A lumped-parameter model helps to couple the movement of the leaflets with the blood dynamics. A reduced circulation model describes the systemic hemodynamics and provides a physiological pressure profile at the downstream boundary of the valve. The resulting model is relatively simple to describe for a healthy valve and pathological heart valve functioning while featuring an affordable computational burden. Efficient time and spatial discretizations are considered and implemented. We address in detail the main features of the proposed method, and we report several numerical experiments for both two-dimensional and three-dimensional cases with the aim of illustrating its accuracy. Copyright © 2015 John Wiley & Sons, Ltd.

High-resolution subject-specific mitral valve imaging and modeling: experimental and computational methods

The diversity of mitral valve (MV) geometries and multitude of surgical options for correction of MV diseases necessitates the use of computational modeling. Numerical simulations of the MV would allow surgeons and engineers to evaluate repairs, devices, procedures, and concepts before performing them and before moving on to more costly testing modalities. Constructing, tuning, and validating these models rely upon extensive in vitro characterization of valve structure, function, and response to change due to diseases. Micro-computed tomography ([Formula: see text]CT) allows for unmatched spatial resolution for soft tissue imaging. However, it is still technically challenging to obtain an accurate geometry of the diastolic MV. We discuss here the development of a novel technique for treating MV specimens with glutaraldehyde fixative in order to minimize geometric distortions in preparation for [Formula: see text]CT scanning. The technique provides a resulting MV geometry which is significantly more detailed in chordal structure, accurate in leaflet shape, and closer to its physiological diastolic geometry. In this paper, computational fluid-structure interaction (FSI) simulations are used to show the importance of more detailed subject-specific MV geometry with 3D chordal structure to simulate a proper closure validated against [Formula: see text]CT images of the closed valve. Two computational models, before and after use of the aforementioned technique, are used to simulate closure of the MV.

Finite Element Modeling of Mitral Valve Repair

The mitral valve is a complex structure regulating forward flow of blood between the left atrium and left ventricle (LV). Multiple disease processes can affect its proper function, and when these diseases cause severe mitral regurgitation (MR), optimal treatment is repair of the native valve. The mitral valve (MV) is a dynamic structure with multiple components that have complex interactions. Computational modeling through finite element (FE) analysis is a valuable tool to delineate the biomechanical properties of the mitral valve and understand its diseases and their repairs. In this review, we present an overview of relevant mitral valve diseases, and describe the evolution of FE models of surgical valve repair techniques.

Percutaneous Mitral Annuloplasty

Percutaneous mitral valve therapies are emerging as an alternative option for high-risk patients who are not good candidates for conventional open-heart surgery. Recently, multiple technologies and diversified approaches have been developed and are under clinical study or in preclinical development. This article on transcatheter mitral annuloplasty devices, describes the different technologies, and reports on the initial clinical and preclinical experiences.

Fluid-Structure Interaction Analysis of Papillary Muscle Forces Using a Comprehensive Mitral Valve Model with 3D Chordal Structure

Numerical models of native heart valves are being used to study valve biomechanics to aid design and development of repair procedures and replacement devices. These models have evolved from simple two-dimensional approximations to complex three-dimensional, fully coupled fluid-structure interaction (FSI) systems. Such simulations are useful for predicting the mechanical and hemodynamic loading on implanted valve devices. A current challenge for improving the accuracy of these predictions is choosing and implementing modeling boundary conditions. In order to address this challenge, we are utilizing an advanced in vitro system to validate FSI conditions for the mitral valve system. Explanted ovine mitral valves were mounted in an in vitro setup, and structural data for the mitral valve was acquired with [Formula: see text]CT. Experimental data from the in vitro ovine mitral valve system were used to validate the computational model. As the valve closes, the hemodynamic data, high speed leaflet dynamics, and force vectors from the in vitro system were compared to the results of the FSI simulation computational model. The total force of 2.6 N per papillary muscle is matched by the computational model. In vitro and in vivo force measurements enable validating and adjusting material parameters to improve the accuracy of computational models. The simulations can then be used to answer questions that are otherwise not possible to investigate experimentally. This work is important to maximize the validity of computational models of not just the mitral valve, but any biomechanical aspect using computational simulation in designing medical devices.

Effect of Congenital Anomalies of the Papillary Muscles on Mitral Valve Function

Parachute mitral valves (PMVs) and parachute-like asymmetric mitral valves (PLAMVs) are associated with congenital anomalies of the papillary muscles. Current imaging modalities cannot provide detailed biomechanical information. This study describes computational evaluation techniques based on three-dimensional (3D) echocardiographic data to determine the biomechanical and physiologic characteristics of PMVs and PLAMVs. The closing and opening mechanics of a normal mitral valve (MV), two types of PLAMV with different degrees of asymmetry, and a true PMV were investigated. MV geometric data in a patient with a normal MV was acquired from 3D echocardiography. The pathologic MVs were modeled by altering the configuration of the papillary muscles in the normal MV model. Dynamic finite element simulations of the normal MV, PLAMVs, and true PMV were performed. There was a strong correlation between the reduction of mitral orifice size and the degree of asymmetry of the papillary muscle location. The PLAMVs demonstrated decreased leaflet coaptation and tenting height. The true PMV revealed severely wrinkled leaflet deformation and narrowed interchordal spaces, leading to uneven leaflet coaptation. There were considerable decreases in leaflet coaptation and abnormal leaflet deformation corresponding to the anomalous location of the papillary muscle tips. This computational MV evaluation strategy provides a powerful tool to better understand biomechanical and pathophysiologic MV abnormalities.

A finite strain nonlinear human mitral valve model with fluid-structure interaction

A computational human mitral valve (MV) model under physiological pressure loading is developed using a hybrid finite element immersed boundary method, which incorporates experimentally-based constitutive laws in a three-dimensional fluid-structure interaction framework. A transversely isotropic material constitutive model is used to characterize the mechanical behaviour of the MV tissue based on recent mechanical tests of healthy human mitral leaflets. Our results show good agreement, in terms of the flow rate and the closing and opening configurations, with measurements from in vivo magnetic resonance images. The stresses in the anterior leaflet are found to be higher than those in the posterior leaflet and are concentrated around the annulus trigons and the belly of the leaflet. The results also show that the chordae play an important role in providing a secondary orifice for the flow when the valve opens. Although there are some discrepancies to be overcome in future work, our simulations show that the developed computational model is promising in mimicking the in vivo MV dynamics and providing important information that are not obtainable by in vivo measurements.

Mitral Valve Annuloplasty Rings: Review of Literature and Comparison of Functional Outcome and Ventricular Dimensions

In the past decades, more than 40 mitral valve annuloplasty rings of various shapes and consistency were marketed for mitral regurgitation (MR), although the effect of ring type on clinical outcome remains unclear. Our objective was to review the literature and apply a simplification method to make rings of different shapes and rigidity more comparable. We studied relevant literature from MEDLINE and EMBASE databases related to clinical studies as well as animal and finite element models. Annuloplasty rings were clustered into 3 groups as follows: rigid (R), flexible (F), and semirigid (S). Only clinical articles regarding degenerative (DEG) or ischemic/dilated cardiomyopathy (ICM) MR were included and stratified into these groups. A total of 37 rings were clustered into R, F, and S subgroups. Clinical studies with a mean follow-up of less than 1 year and a reported mean etiology of valve incompetence of less than 60% were excluded from the analysis. Forty-one publications were included. Preimplant and postimplant end points were New York Heart Association class, left ventricular ejection fraction (LVEF), left ventricular end-systolic dimension (LVESD), and left ventricular end-diastolic dimension (LVEDD). Statistical analysis included paired-samples t test and analysis of variance with post hoc Bonferroni correction. P < 0.05 indicated statistical difference. Mean ± SD follow-up was 38.6 ± 27 and 29.7 ± 13.2 months for DEG and ICM, respectively. In DEG, LVEF remained unchanged, and LVESD decreased in all subgroups. In our analysis, LVEDD decreased only in F and R, and S did not change; however, the 4 individual studies showed a significant decline. In ICM, New York Heart Association class improved in all subgroups, and LVEF increased. Moreover, LVESD and LVEDD decreased only in F and S; R was underpowered (1 study). No statistical difference among R, F, and S in either ICM or DEG could be detected for all end points. Overall, owing to underpowered data sets derived from limited available publications, major statistical differences in clinical outcome between ring types could not be substantiated. Essential end points such as recurrent MR and survival were incomparable. In conclusion, ring morphology and consistency do not seem to play a major clinical role in mitral valve repair based on the present literature. Hence, until demonstrated otherwise, surgeons may choose their ring upon their judgment, tailored to specific patient needs.

Novel Mitral Repair Device for the Treatment of Severe Mitral Regurgitation: Preclinical Ovine Acute and Chronic Implantation Model

Objective

A project is now underway to implement a novel percutaneous mitral repair system for severe mitral regurgitation (MR). The initial phase of the project consists of proof-of-concept by testing device characteristics using open surgical implantation. When surgical proof-of-concept of the intended percutaneous design is completed, a second phase of the project will consist of in vivo testing of the percutaneous transseptal system. The device is currently being designed to fold into a 17F catheter system and to unfold within the left atrium where attachment is accomplished using a reversible anchoring system. The purpose of this study was to show functionality of the device in elimination of MR using the open surgical method.

Methods

We have performed surgical prototype device implantation in 5 acute and 7 chronic sheep preparations. We created a P2-flail model of severe (4+) MR in the 12 sheep. Via a minimally invasive left thoracotomy incision and open repair on cardiopulmonary bypass, the device was implanted to determine efficacy of elimination of severe MR. Implantation was considered successful if 4+ regurgitation was converted to 1+ MR or lower. Left ventriculography and epicardial 2-dimensional/3-dimensional echocardiography were used to assess repair; serial 2-dimensional/3-dimensional transthoracic echocardiography was used to assess long-term mitral repair status.

Results

Twelve sheep had surgical creation of severe (4+) MR by cutting all chordae to the P2 scallop of the mitral valve; this preparation was tested and was found to produce 100% acute fatality without repair of the mitral valve. Five sheep had acute implantation of the device with elimination of regurgitation in 5/5 sheep. Seven sheep had chronic (1–7 month) implantation of the device. The device was tested in the chronic model for clinical status, residual regurgitation, thrombosis, and histopathology. All sheep had mitigation of MR and survived to the intended date of death.

Conclusions

Proof-of-concept of a novel percutaneous mitral repair device has been completed using an ovine P2-flail severe MR model. The device has characteristics that will allow its use in posterior leaflet degenerative disease and functional/secondary MR. Open, minimally invasive, and robotic surgical implantation of the device can also be developed as an alternative to the percutaneous approach.

Computational mitral valve evaluation and potential clinical applications

The mitral valve (MV) apparatus consists of the two asymmetric leaflets, the saddle-shaped annulus, the chordae tendineae, and the papillary muscles. MV function over the cardiac cycle involves complex interaction between the MV apparatus components for efficient blood circulation. Common diseases of the MV include valvular stenosis, regurgitation, and prolapse. MV repair is the most popular and most reliable surgical treatment for early MV pathology. One of the unsolved problems in MV repair is to predict the optimal repair strategy for each patient. Although experimental studies have provided valuable information to improve repair techniques, computational simulations are increasingly playing an important role in understanding the complex MV dynamics, particularly with the availability of patient-specific real-time imaging modalities. This work presents a review of computational simulation studies of MV function employing finite element structural analysis and fluid-structure interaction approach reported in the literature to date. More recent studies towards potential applications of computational simulation approaches in the assessment of valvular repair techniques and potential pre-surgical planning of repair strategies are also discussed. It is anticipated that further advancements in computational techniques combined with the next generations of clinical imaging modalities will enable physiologically more realistic simulations. Such advancement in imaging and computation will allow for patient-specific, disease-specific, and case-specific MV evaluation and virtual prediction of MV repair.

Computational modeling of cardiac valve function and intervention

In the past two decades, major advances have been made in the clinical evaluation and treatment of valvular heart disease owing to the advent of noninvasive cardiac imaging modalities. In clinical practice, valvular disease evaluation is typically performed on two-dimensional (2D) images, even though most imaging modalities offer three-dimensional (3D) volumetric, time-resolved data. Such 3D data offer researchers the possibility to reconstruct the 3D geometry of heart valves at a patient-specific level. When these data are integrated with computational models, native heart valve biomechanical function can be investigated, and preoperative planning tools can be developed. In this review, we outline the advances in valve geometry reconstruction, tissue property modeling, and loading and boundary definitions for the purpose of realistic computational structural analysis of cardiac valve function and intervention.

On the mechanics of growing thin biological membranes

Despite their seemingly delicate appearance, thin biological membranes fulfill various crucial roles in the human body and can sustain substantial mechanical loads. Unlike engineering structures, biological membranes are able to grow and adapt to changes in their mechanical environment. Finite element modeling of biological growth holds the potential to better understand the interplay of membrane form and function and to reliably predict the effects of disease or medical intervention. However, standard continuum elements typically fail to represent thin biological membranes efficiently, accurately, and robustly. Moreover, continuum models are typically cumbersome to generate from surface-based medical imaging data. Here we propose a computational model for finite membrane growth using a classical midsurface representation compatible with standard shell elements. By assuming elastic incompressibility and membrane-only growth, the model a priori satisfies the zero-normal stress condition. To demonstrate its modular nature, we implement the membrane growth model into the general-purpose non-linear finite element package Abaqus/Standard using the concept of user subroutines. To probe efficiently and robustness, we simulate selected benchmark examples of growing biological membranes under different loading conditions. To demonstrate the clinical potential, we simulate the functional adaptation of a heart valve leaflet in ischemic cardiomyopathy. We believe that our novel approach will be widely applicable to simulate the adaptive chronic growth of thin biological structures including skin membranes, mucous membranes, fetal membranes, tympanic membranes, corneoscleral membranes, and heart valve membranes. Ultimately, our model can be used to identify diseased states, predict disease evolution, and guide the design of interventional or pharmaceutic therapies to arrest or revert disease progression.

Three-dimensional valve repair-the better care? Midterm results of a saddle-shaped, rigid annuloplasty ring in patients with ischemic mitral regurgitation

OBJECTIVES

Undersized ring annuloplasty is the treatment of choice for functional mitral regurgitation. However, recurrence of mitral regurgitation within the first years is frequent. The aim of this study was to analyze the functional and clinical outcome after mitral valve repair with the 3-dimensional saddle-shaped Edwards GeoForm (Edwards Lifesciences LLC, Irvine, Calif) annuloplasty ring in patients with ischemic mitral regurgitation.

METHODS

Between November 2006 and November 2012, 70 patients (mean age, 68 ± 10 years; mean left ventricular ejection fraction, 40% ± 15%) with functional mitral regurgitation due to ischemic cardiomyopathy underwent mitral valve repair with the Edwards GeoForm annuloplasty ring. Concomitant procedures, such as coronary artery bypass grafting (75.7%), tricuspid valve repair (25.7%), aortic valve replacement (8.6%), and the Maze procedure (4.3%), were performed in 92.9% of patients. Follow-up is 97% complete (mean, 3.0 ± 1.7 years). Transthoracic echocardiography was obtained 2.4 ± 1.7 years postoperatively.

RESULTS

Thirty-day mortality was 5.9%. Overall survival at 5 years was 71.3% ± 6.9%. At 4 years, overall freedom from recurrence of mitral regurgitation grade 3+ or greater was 92.5% ± 3.6%, and freedom from recurrence of mitral regurgitation grade 2+ or greater was 71.0% ± 8.7%. Three patients required a mitral valve-related reoperation for ring dehiscence. New York Heart Association functional class improved from 3.6 ± 0.6 to 1.6 ± 0.6 during follow-up (P < .05). Mean mitral valve pressure gradient was 3.3 ± 1.8 mm Hg across all ring sizes at the time of follow-up.

CONCLUSIONS

Mitral valve repair with the 3-dimensional saddle-shaped Edwards GeoForm annuloplasty ring in case of ischemic mitral regurgitation shows a low rate of recurrent regurgitation at 4 years. Clinically relevant mitral stenosis was not detected. The importance of secure anchoring of the device in the mitral annulus has to be emphasized to prevent ring dehiscence.

Regional annular geometry in patients with mitral regurgitation: implications for annuloplasty ring selection

BACKGROUND

The saddle shape of the normal mitral annulus has been quantitatively described by several groups. There is strong evidence that this shape is important to valve function. A more complete understanding of regional annular geometry in diseased valves may provide a more educated approach to annuloplasty ring selection and design. We hypothesized that mitral annular shape is markedly distorted in patients with diseased valves.

METHODS

Real-time 3-dimensional echocardiography was performed in 20 patients with normal mitral valves, 10 with ischemic mitral regurgitation, and 20 with myxomatous mitral regurgitation (MMR). Thirty-six annular points were defined to generate a 3-dimensional model of the annulus. Regional annular parameters were measured from these renderings. Left ventricular inner diameter was obtained from 2-dimensional echocardiographic images.

RESULTS

Annular geometry was significantly different among the three groups. The annuli were larger in the MMR and in the ischemic mitral regurgitation groups. The annular enlargement was greater and more pervasive in the MMR group. Both diseases were associated with annular flattening, although though the regional distribution of that flattening was different between groups. Left ventricular inner diameter was increased in both groups. However, relative to the Left ventricular inner diameter, the annulus was disproportionately dilated in the MMR group.

CONCLUSIONS

Patients with MMR and ischemic mitral regurgitation have enlarged and flattened annuli. In the case of MMR, annular distortions may be the driving factor leading to valve incompetence. These data suggest that the goal of annuloplasty should be the restoration of normal annular saddle shape and that the use of flexible, partial, and flat rings may be ill advised.

The effect of patient-specific annular motion on dynamic simulation of mitral valve function

Most surgical procedures for patients with mitral regurgitation (MR) focus on optimization of annular dimension and shape utilizing ring annuloplasty to restore normal annular geometry, increase leaflet coaptation, and reduce regurgitation. Computational studies may provide insight on the effect of annular motion on mitral valve (MV) function through the incorporation of patient-specific MV apparatus geometry from clinical imaging modalities such as echocardiography. In the present study, we have developed a novel algorithm for modeling patient-specific annular motion across the cardiac cycle to further improve our virtual MV modeling and simulation strategy. The MV apparatus including the leaflets, annulus, and location of papillary muscle tips was identified using patient 3D echocardiography data at end diastole and peak systole and converted to virtual MV model. Dynamic annular motion was modeled by incorporating the ECG-gated time-varying scaled annular displacement across the cardiac cycle. We performed dynamic finite element (FE) simulation of two sets of patient data with respect to the presence of MR. Annular morphology, stress distribution across the leaflets and annulus, and contact stress distribution were determined to assess the effect of annular motion on MV function and leaflet coaptation. The effect of dynamic annular motion clearly demonstrated reduced regions with large stress values and provided an improved accuracy in determining the location of improper leaflet coaptation. This strategy has the potential to better quantitate the extent of pathologic MV and better evaluate functional restoration following MV repair.

Mechanics of the mitral valve: a critical review, an in vivo parameter identification, and the effect of prestrain

Alterations in mitral valve mechanics are classical indicators of valvular heart disease, such as mitral valve prolapse, mitral regurgitation, and mitral stenosis. Computational modeling is a powerful technique to quantify these alterations, to explore mitral valve physiology and pathology, and to classify the impact of novel treatment strategies. The selection of the appropriate constitutive model and the choice of its material parameters are paramount to the success of these models. However, the in vivo parameters values for these models are unknown. Here, we identify the in vivo material parameters for three common hyperelastic models for mitral valve tissue, an isotropic one and two anisotropic ones, using an inverse finite element approach. We demonstrate that the two anisotropic models provide an excellent fit to the in vivo data, with local displacement errors in the sub-millimeter range. In a complementary sensitivity analysis, we show that the identified parameter values are highly sensitive to prestrain, with some parameters varying up to four orders of magnitude. For the coupled anisotropic model, the stiffness varied from 119,021 kPa at 0 % prestrain via 36 kPa at 30 % prestrain to 9 kPa at 60 % prestrain. These results may, at least in part, explain the discrepancy between previously reported ex vivo and in vivo measurements of mitral leaflet stiffness. We believe that our study provides valuable guidelines for modeling mitral valve mechanics, selecting appropriate constitutive models, and choosing physiologically meaningful parameter values. Future studies will be necessary to experimentally and computationally investigate prestrain, to verify its existence, to quantify its magnitude, and to clarify its role in mitral valve mechanics.

Finite element modeling of mitral leaflet tissue using a layered shell approximation

The current study presents a finite element model of mitral leaflet tissue, which incorporates the anisotropic material response and approximates the layered structure. First, continuum mechanics and the theory of layered composites are used to develop an analytical representation of membrane stress in the leaflet material. This is done with an existing anisotropic constitutive law from literature. Then, the concept is implemented in a finite element (FE) model by overlapping and merging two layers of transversely isotropic membrane elements in LS-DYNA, which homogenizes the response. The FE model is then used to simulate various biaxial extension tests and out-of-plane pressure loading. Both the analytical and FE model show good agreement with experimental biaxial extension data, and show good mutual agreement. This confirms that the layered composite approximation presented in the current study is able to capture the exponential stiffening seen in both the circumferential and radial directions of mitral leaflets.

The effect of mitral annuloplasty shape in ischemic mitral regurgitation: a finite element simulation

BACKGROUND

Undersized mitral annuloplasty (MA) is the preferred surgical treatment for chronic ischemic mitral regurgitation. However, the preferred shape of undersized MA is unclear.

METHODS

A previously described finite element model of the left ventricle with mitral valve based on magnetic resonance images of a sheep with chronic ischemic mitral regurgitation after posterolateral myocardial infarction was used. Saddle-shape (Edwards Physio II) and asymmetric (IMR ETlogix) MA rings were digitized and meshed. Virtual annuloplasty was performed using virtual sutures to attach the MA ring. Left ventricular diastole and systole were performed before and after virtual MA of each type.

RESULTS

Both types of MA reduced the septolateral dimension of the mitral annulus and abolished mitral regurgitation. The asymmetric MA was associated with lower virtual suture force in the P2 region but higher force in P1 and P3 regions. Although both types of MA reduced fiber stress at the left ventricular base, fiber stress reduction after asymmetric MA was slightly greater. Neither type of MA affected fiber stress at the left ventricular equator or apex. Although both types of MA increased leaflet curvature and reduced leaflet stress, stress reduction with saddle-shape MA was slightly greater. Both MA types reduced stress on the mitral chordae.

CONCLUSIONS

The effects of saddle-shape and asymmetric MA rings are similar. Finite element simulations are a powerful tool that may reduce the need for animal and clinical trials.

Functional and structural effects of percutaneous edge-to-edge double-orifice repair under cardiac cycle in comparison with suture repair

Percutaneous procedures for double-orifice mitral valve repair using the MitraClip® device (clip) have been recently introduced as new treatment options as alternatives to medical management and open-heart surgery, especially for patients with high estimated operative risk. Similarly to the open-heart surgical technique, where suturing is used, the clip creates a double-orifice configuration that not only improves the closing function of the valve, but also significantly modifies its behaviour, particularly in the diastolic phase. While several clinical trials have been conducted, and are ongoing, in order to assess the safety and effectiveness of this technique, a deeper knowledge of the structural and functional effects on the valve, and of the cyclic loads transmitted to the clip itself, would allow a comparison with other repair techniques, and could serve as a foundation for possible further optimization of the clip design. The effects of the MitraClip® device developed by Evalve Inc. were studied by means of a finite element model of the mitral valve, specifically developed to study the structural effects of the original, suture-based, edge-to-edge technique. A second model was developed in order to simulate the effects of a suture with similar extension from the leaflet edge in a direction to the annulus, in order to compare the two repair techniques. The mitral valve area and transvalvular pressure gradient predicted by the models for the clip and the suture are quite similar. Similar leaflet cyclic stresses, both in value and in location, were noted for the two mechanisms of linking the leaflets, while minor differences were found in the load transmitted to the suture and the clip, with slightly higher values for the clip. The model satisfactorily allowed functional parameters (valve area and transvalvular pressure gradient) and structural parameters (load, leaflet stress) to be determined. Overall, the structural effects of the clip and the suture are quite similar under the cyclic loading conditions imposed by the cardiac cycle.

Response of two annular prostheses to functional mitral regurgitation main determinants: an in vitro evaluation

Functional mitral regurgitation (FMR) is usually treated through annuloplasty, i.e., the restriction of the mitral annulus by implanting an undersized prosthetic ring. We conceived a steady-state fluid-dynamic mock simulator that allows for controlling the main mechanic determinants of FMR: transmitral pressure and papillary muscle (PM) apical and lateral dislocation. We used our system to compare the FMR-specific Geoform ring with the general purpose Physio ring in the treatment of FMR. Each ring was implanted on 10 excised fresh porcine valves. Different transmitral pressures (40, 80, 120, 140, and 160 mm Hg) and symmetrical PM apical displacements (2.5-15 mm, step 2.5 mm) were imposed with submillimetric precision. In each configuration, the regurgitant flow through the valve was measured. For PM apical displacement ≥7.5 mm, the regurgitant flow was lower (p < 0.05) with the Geoform ring than with the Physio ring. Differences and their statistical significance increased as PM displacement or transmitral pressure increased. Regression analysis showed that this outcome did not depend on the morphology of the valves. The adopted approach proved itself simple and reliable and allowed to highlight the differences between the two examined annuloplasty devices in countering the two main determinants of FMR: high apical PM dislocation and transvalvular pressure.

First finite element model of the left ventricle with mitral valve: insights into ischemic mitral regurgitation

BACKGROUND

Left ventricular remodeling after posterobasal myocardial infarction can lead to ischemic mitral regurgitation. This occurs as a consequence of leaflet tethering due to posterior papillary muscle displacement.

METHODS

A finite element model of the left ventricle, mitral apparatus, and chordae tendineae was created from magnetic resonance images from a sheep that developed moderate mitral regurgitation after posterobasal myocardial infarction. Each region of the model was characterized by a specific constitutive law that captured the material response when subjected to physiologic pressure loading.

RESULTS

The model simulation produced a gap between the posterior and anterior leaflets, just above the infarcted posterior papillary muscle, which is indicative of mitral regurgitation. When the stiffness of the infarct region was reduced, this caused the wall to distend and the gap area between the leaflets to increase by 33%. Additionally, the stress in the leaflets increased around the chordal connection points near the gap.

CONCLUSIONS

The methodology outlined in this work will allow a finite element model of both the left ventricle and mitral valve to be generated using noninvasive techniques.

Effects of different annuloplasty rings on anterior mitral leaflet dimensions

OBJECTIVE

To assess the effects of annuloplasty rings on anterior mitral leaflet dimensions.

METHODS

Sixteen radiopaque markers were sutured evenly spaced over the surface of the anterior mitral leaflet in 57 sheep. The following rings were implanted in a releasable fashion: size 28-mm Cosgrove-Edwards band (Edwards Lifesciences, Irvine, Calif) (n = 11), rigid saddle-shaped annuloplasty ring (St Jude Medical Inc, St Paul, Minn) (n = 12), Carpentier-Edwards Physio (Edwards Lifesciences) (n = 12), IMR-ETlogix (Edwards Lifesciences) (n = 10), and GeoForm (Edwards Lifesciences) (n = 12). Under acute open chest conditions, 4-dimensional marker coordinates were measured using biplane videofluoroscopy with the annuloplasty ring inserted and after annuloplasty ring release. Septal-lateral and commissure-commissure dimensions were calculated from opposing marker pairs on the septal-lateral and commissure-commissure aspect of the anterior mitral leaflet at end diastole and end systole. To assess changes in anterior mitral leaflet shape, a "planarity index" was assessed by calculating the root mean square values as distances of the 16 anterior mitral leaflet markers to a best fit anterior mitral leaflet plane at end systole.

RESULTS

At end diastole, anterior mitral leaflet septal-lateral and commissure-commissure dimensions did not change with the Cosgrove ring compared with control, whereas the rigid saddle-shaped annuloplasty ring and Physio, IMR-ETlogix, and GeoForm rings reduced anterior mitral leaflet commissure-commissure but not septal-lateral anterior mitral leaflet dimensions. At end systole, the septal-lateral anterior mitral leaflet dimension was smaller with the IMR-ETlogix and GeoForm rings, but did not change with the Cosgrove ring, rigid saddle-shaped annuloplasty ring, and Physio ring. Anterior mitral leaflet shape was unchanged in all 5 groups.

CONCLUSION

With no changes in anterior mitral leaflet planarity, the 4 complete, rigid rings (rigid saddle-shaped annuloplasty ring, Physio, IMR-ETlogix, and GeoForm) reduced the anterior mitral leaflet commissure-commissure dimension at end diastole. The IMR-ETlogix and GeoForm rings decreased the septal-lateral anterior mitral leaflet dimension at end systole, probably as the result of inherent disproportionate downsizing. These changes in anterior mitral leaflet geometry could perturb the stress patterns, which in theory may affect repair durability.

Role of Computational Simulations in Heart Valve Dynamics and Design of Valvular Prostheses

Computational simulations are playing an increasingly important role in enhancing our understanding of the normal human physiological function, etiology of diseased states, surgical and interventional planning, and in the design and evaluation of artificial implants. Researchers are taking advantage of computational simulations to speed up the initial design of implantable devices before a prototype is developed and hence able to reduce animal experimentation for the functional evaluation of the devices under development. A review of the reported studies to date relevant to the simulation of the native and prosthetic heart valve dynamics is the subject of the present paper. Potential future directions toward multi-scale simulation studies for our further understanding of the physiology and pathophysiology of heart valve dynamics and valvular implants are also discussed.

How much septal-lateral mitral annular reduction do you get with new ischemic/functional mitral regurgitation annuloplasty rings?

OBJECTIVE

Disproportionate reduction of the mitral septal-lateral annular dimension is the goal in the surgical treatment of ischemic or functional mitral regurgitation and avoids the need for ring "downsizing." How much the new annuloplasty rings designed for patients with ischemic/functional mitral regurgitation reduce annular septal-lateral dimension, however, is proprietary information and debated.

METHODS

Outer and inner septal-lateral and commissure-commissure diameters of all available sizes of Edwards GeoForm, Edwards IMR ETlogix (both Edwards Lifesciences, Irvine, Calif), St Jude Medical Rigid Saddle Annuloplasty Ring (St Jude Medical, Inc, St Paul, Minn), and Medtronic Profile 3D (Medtronic, Minneapolis, Minn) annuloplasty rings with and without the fabric covering were measured with electronic calipers. These rings were compared with a Carpentier-Edwards Physio ring (Edwards Lifesciences) to assess the relative amount of septal-lateral and commissure-commissure dimension change. Average fractional changes (% +/-1 standard deviation) versus the Physio ring were calculated.

RESULTS

The GeoForm provided the greatest outer septal-lateral reduction relative to Physio ring (-24% +/- 2%), followed by the IMR ETlogix (-9% +/- 2%) and Profile 3D (-8% +/- 5%). The septal-lateral diameter of the Rigid Saddle Annuloplasty Ring was similar to that of the Physio ring (+1% +/- 3%). Although commissure-commissure outer diameters of the IMR ETlogix, Rigid Saddle Annuloplasty Ring, and Profile 3D were similar to that of the Physio ring (0% +/- 2%, +4% +/- 3%, and +3% +/- 4%, respectively), the GeoForm had a larger commissure-commissure dimension (+12% +/- 2%). The inner diameter septal-lateral reductions were even more pronounced.

CONCLUSIONS

Relative to the Physio ring, the GeoForm has the most outer and inner septal-lateral reduction but larger commissure-commissure dimension; the IMR ETlogix and Profile 3D provide a moderate degree of septal-lateral reduction without affecting commissure-commissure dimension, and Rigid Saddle Annuloplasty Ring septal-lateral and commissure-commissure diameters are similar to those of the Physio ring. Knowing the degree of disproportionate septal-lateral downsizing inherent in each ring type will help guide surgical decision making.

Mitral Valve Finite Element Modeling: Implications of Tissues’ Nonlinear Response and Annular Motion

Finite element modeling represents an established method for the comprehension of the mitral function and for the simulation of interesting clinical scenarios. However, current models still do not include all the key aspects of the real system. We implemented a new structural finite element model that considers (i) an accurate morphological description of the valve, (ii) a description of the tissues’ mechanical properties that accounts for anisotropy and nonlinearity, and (iii) dynamic boundary conditions that mimic annulus and papillary muscles’ contraction. The influence of such contraction on valve biomechanics was assessed by comparing the computed results with the ones obtained through an auxiliary model with fixed annulus and papillary muscles. At the systolic peak, the leaflets’ maximum principal stress contour showed peak values in the anterior leaflet at the strut chordae insertion zone (300 kPa) and near the annulus (200–250 kPa), while much lower values were detected in the posterior leaflet. Both leaflets underwent larger tensile strains in the longitudinal direction, while in the circumferential one the anterior leaflet experienced nominal tensile strains up to 18% and the posterior one experienced compressive strains up to 23% associated with the folding of commissures and paracommissures, consistently with tissue redundancy. The force exerted by papillary muscles at the systolic peak was equal to 4.11 N, mainly borne by marginal chordae (76% of the force). Local reaction forces up to 45 mN were calculated on the annulus, leading to tensions of 89 N/m and 54 N/m for its anterior and posterior tracts, respectively. The comparison with the results of the auxiliary model showed that annular contraction mainly affects the leaflets’ circumferential strains. When it was suppressed, no more compressive strains could be observed and peak strain values were located in the belly of the anterior leaflet. Computational results agree to a great extent with experimental data from literature. They provided insight into some of the features characterizing normal mitral function, such as annular contraction and leaflets’ tissue anisotropy and nonlinearity. Some of the computed results may be useful in the design of surgical devices and techniques. In particular, forces applied on the annulus by the surrounding tissues could be considered as an indication for annular prostheses design.

Mitral valve finite-element modelling from ultrasound data: a pilot study for a new approach to understand mitral function and clinical scenarios

In the current scientific literature, particular attention is dedicated to the study of the mitral valve and to comprehension of the mechanisms that lead to its normal function, as well as those that trigger possible pathological conditions. One of the adopted approaches consists of computational modelling, which allows quantitative analysis of the mechanical behaviour of the valve by means of continuum mechanics theory and numerical techniques. However, none of the currently available models realistically accounts for all of the aspects that characterize the function of the mitral valve. Here, a new computational model of the mitral valve has been developed from in vivo data, as a first step towards the development of patient-specific models for the evaluation of annuloplasty procedures. A structural finite-element model of the mitral valve has been developed to account for all of the main valvular substructures. In particular, it includes the real geometry and the movement of the annulus and papillary muscles, reconstructed from four-dimensional ultrasound data from a healthy human subject, and a realistic description of the complex mechanical properties of mitral tissues. Preliminary simulations allowed mitral valve closure to be realistically mimicked and the role of annulus and papillary muscle dynamics to be quantified.

Material properties of the ovine mitral valve anterior leaflet in vivo from inverse finite element analysis

We measured leaflet displacements and used inverse finite-element analysis to define, for the first time, the material properties of mitral valve (MV) leaflets in vivo. Sixteen miniature radiopaque markers were sewn to the MV annulus, 16 to the anterior MV leaflet, and 1 on each papillary muscle tip in 17 sheep. Four-dimensional coordinates were obtained from biplane videofluoroscopic marker images (60 frames/s) during three complete cardiac cycles. A finite-element model of the anterior MV leaflet was developed using marker coordinates at the end of isovolumic relaxation (IVR; when the pressure difference across the valve is approximately 0), as the minimum stress reference state. Leaflet displacements were simulated during IVR using measured left ventricular and atrial pressures. The leaflet shear modulus (G(circ-rad)) and elastic moduli in both the commisure-commisure (E(circ)) and radial (E(rad)) directions were obtained using the method of feasible directions to minimize the difference between simulated and measured displacements. Group mean (+/-SD) values (17 animals, 3 heartbeats each, i.e., 51 cardiac cycles) were as follows: G(circ-rad) = 121 +/- 22 N/mm2, E(circ) = 43 +/- 18 N/mm2, and E(rad) = 11 +/- 3 N/mm2 (E(circ) > E(rad), P < 0.01). These values, much greater than those previously reported from in vitro studies, may result from activated neurally controlled contractile tissue within the leaflet that is inactive in excised tissues. This could have important implications, not only to our understanding of mitral valve physiology in the beating heart but for providing additional information to aid the development of more durable tissue-engineered bioprosthetic valves.

Current surgical management of mitral regurgitation

From Walton Lillehei, who performed the first successful open mitral valve surgery in 1956, until the advent of robotic surgery in the 21st Century, only 50 years have passed. The introduction of the first heart valve prosthesis, in 1960, was the next major step forward. However, correction of mitral disease by valvuloplasty results in better survival and ventricular performance than mitral valve replacement. However, the European Heart Survey demonstrated that only 40% of the valves are repaired. The standard procedures (Carpentier's techniques and Alfieri's edge-to-edge suture) are the surgical basis for the new technical approaches. Minimally invasive surgery led to the development of video-assisted and robotic surgery and interventional cardiology is already making the first steps on endovascular procedures, using the classical concepts in highly differentiated approaches. Correction of mitral regurgitation is a complex field that is still growing, whereas classic surgery is still under debate as the new era arises.

Fluid-structure interaction models of the mitral valve: function in normal and pathological states

Successful mitral valve repair is dependent upon a full understanding of normal and abnormal mitral valve anatomy and function. Computational analysis is one such method that can be applied to simulate mitral valve function in order to analyse the roles of individual components and evaluate proposed surgical repair. We developed the first three-dimensional finite element computer model of the mitral valve including leaflets and chordae tendineae; however, one critical aspect that has been missing until the last few years was the evaluation of fluid flow, as coupled to the function of the mitral valve structure. We present here our latest results for normal function and specific pathological changes using a fluid-structure interaction model. Normal valve function was first assessed, followed by pathological material changes in collagen fibre volume fraction, fibre stiffness, fibre splay and isotropic stiffness. Leaflet and chordal stress and strain and papillary muscle force were determined. In addition, transmitral flow, time to leaflet closure and heart valve sound were assessed. Model predictions in the normal state agreed well with a wide range of available in vivo and in vitro data. Further, pathological material changes that preserved the anisotropy of the valve leaflets were found to preserve valve function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valve function. The addition of blood flow and an experimentally driven microstructural description of mitral tissue represent significant advances in computational studies of the mitral valve, which allow further insight to be gained. This work is another building block in the foundation of a computational framework to aid in the refinement and development of a truly non-invasive diagnostic evaluation of the mitral valve. Ultimately, it represents the basis for simulation of surgical repair of pathological valves in a clinical and educational setting.