Biomechanical implications of the bicuspid aortic valve: A finite element study from in vivo data

Department of Bioengineering, Politecnico di Milano, Milan, Italy.
The Journal of thoracic and cardiovascular surgery (Impact Factor: 4.17). 04/2010; 140(4):890-6, 896.e1-2. DOI: 10.1016/j.jtcvs.2010.01.016
Source: PubMed


Congenital bicuspid aortic valves frequently cause aortic stenosis or regurgitation. Improved understanding of valve and root biomechanics is needed to achieve advancements in surgical repair techniques. By using imaging-derived data, finite element models were developed to quantify aortic valve and root biomechanical alterations associated with bicuspid geometry.
A dynamic 3-dimensional finite element model of the aortic root with a bicuspid aortic valve (type 1 right/left) was developed. The model's geometry was based on measurements from 2-dimensional magnetic resonance images acquired in 8 normotensive and otherwise healthy subjects with echocardiographically normal function of their bicuspid aortic valves. Numeric results were compared with those obtained from our previous model representing the normal root with a tricuspid aortic valve. The effects of raphe thickening on valve kinematics and stresses were also evaluated.
During systole, the bicuspid valve opened asymmetrically compared with the normal valve, resulting in an elliptic shape of its orifice. During diastole, the conjoint cusp occluded a larger proportion of the valve orifice and leaflet bending was altered, although competence was preserved. The bicuspid model presented higher stresses compared with the tricuspid model, particularly in the central basal region of the conjoint cusp (+800%). The presence of a raphe partially reduced stress in this region but increased stress in the other cusp.
Aortic valve function is altered in clinically normally functioning bicuspid aortic valves. Bicuspid geometry per se entails abnormal leaflet stress. The stress location suggests that leaflet stress may play a role in tissue remodeling at the raphe region and in early leaflet degeneration.

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    • "In particular, was the risk of aortic dissection in BAV patients determined before or after aortic valve replacement (AVR) surgery? Considering the recently published evidence on BAV function and effects of transvalvular flow [4] [5] [6] [7] [8] [9] [10], AVR surgery may be a key factor in the natural history of BAV-associated aortopathy with considerable influence on the risk of future aortic events. "
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    ABSTRACT: The risk of acute aortic events in patients with bicuspid aortic valve (BAV) disease is a controversial issue. The real risk of aortic dissection in patients with BAV disease is unknown. An indirect assessment of this risk, however, could be gained with a more detailed understanding of the pathogenesis of BAV aortopathy. There are two major issues that should be clarified before one addresses the question of aortic dissection risk in BAV patients. The first issue, when analysing the data from previous BAV cohorts, is to determine what stage of BAV disease was present in the described patient population. In particular, was the risk of aortic dissection in BAV patients determined before or after aortic valve replacement (AVR) surgery? The second issue to consider is the functional state of the pathological valve within the observed population. In particular, did patients predominantly suffer from BAV stenosis or BAV insufficiency? Unfortunately, the vast majority of published reports do not separate between the different BAV phenotypes, thereby complicating interpretation of the results. Considering these two important clinical variables (i.e. the stage of BAV disease and the functional phenotype), we herein aim to explain the inconsistency of the published data with regard to the risk of aortic dissection in patients with BAV disease.
    Interactive Cardiovascular and Thoracic Surgery 12/2013; 18(3). DOI:10.1093/icvts/ivt518 · 1.16 Impact Factor
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    • "Therefore, the minimum possible area of 0.165 cm 2 is found when the valve is fully closed because of the cusp thickness. In both models, peak systole was at 86 ms after the beginning of the opening, where it is defined as the first time the valve reached its maximum opening state [18] [19]. A similar opening area is found in both models at this stage of the cardiac cycle, 2.76 and 2.69 cm 2 for the symmetric and mapped models, respectively. "
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    ABSTRACT: Background: Native aortic valve cusps are composed of collagen fibers embedded in their layers. Each valve cusp has its own distinctive fiber alignment with varying orientations and sizes of its fiber bundles. However, prior mechanical behavior models have not been able to account for the valve-specific collagen fiber networks (CFN) or for their differences between the cusps. Method of approach: This study investigates the influence of this asymmetry on the hemodynamics by employing two fully coupled fluid-structure interaction (FSI) models, one with asymmetric-mapped CFN from measurements of porcine valve and the other with simplified-symmetric CFN. The FSI models are based on coupled structural and fluid dynamic solvers. The partitioned solver has non-conformal meshes and the flow is modeled by employing the Eulerian approach. The collagen in the CFNs, the surrounding elastin matrix and the aortic sinus tissues, have hyperelastic mechanical behavior. The coaptation is modeled with a master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries for both models. Results: The mapped case showed highly asymmetric valve kinematics and hemodynamics even though there were only small differences between the opening areas and cardiac outputs of the two cases. The regions with a less dense fiber network are subjected to higher principal stress in the tissues and a higher level of flow shear stress. Conclusions: The asymmetric flow leeward the valve might damage not only the valve itself, but also the ascending aorta. The regions with a less dense fiber network are more prone to damage since. Keywords: numerical model, fluid-structure interaction, heart valve, asymmetry.
    Journal of Biomechanical Engineering 06/2013; DOI:10.1115/1.4024824 · 1.78 Impact Factor
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    • "There are no simple explanations for such a difference, but our findings suggest that the presence of BAV and the associated tensile stress in its leaflets accelerates valve degeneration, representing an additive factor to age, as previously suggested. The geometry of the bicuspid valve entails abnormal leaflet stress, which is responsible for tissue remodeling at the raphe region and early leaflet degeneration and dysfunction 17, 18. In contrast, aortic regurgitation (greater than or equal to moderate), although very frequent in BAV patients, was rarely an indication for surgery in the absence of superimposed ailments, and, consequently, it was not a predictive factor of future surgery. "
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    ABSTRACT: Background and Aim: Bicuspid aortic valve (BAV) increases the risk of aortic valve dysfunction and ascending aorta aneurysm and, consequently, the need for aortic valve replacement and/or aortic repair. However, there is no universal consensus about the surgical criteria and the predictors for surgery. The aim of this study was to investigate related factors to the need for surgery in the setting of a strict long-term follow-up with relatively conservative surgical criteria. Methods: We prospectively followed 120 patients after the diagnosis of BAV. Predisposing factors for a future need for aortic valve replacement and ascending aorta repair were assessed. Aortic surgery was indicated when the ascending aorta diameter was ≥55 mm and was recommended based on patient characteristics and in the presence of a severe aortic valve dysfunction with an aortic diameter ≥50 mm. Results: During follow-up (mean, 86 months), 34 patients (28%) (mean age, 56±12 years) were surgically treated. Aortic valve dysfunction (n=22; 64%) and ascending aorta dilatation (n=12; 36%) were the indications for surgery. Aortic regurgitation was the most frequent valve dysfunction at the time of diagnosis for BAV, but aortic stenosis was the most frequent indication for surgery. The presence at surgery of either aortic regurgitation or stenosis was clearly related to age, with regurgitation predominating in patients under 55 years, and aortic stenosis in older patients. Multivariate Cox analysis showed that aortic stenosis (hazard ratio 4.1, p=0.001), indexed ascending aorta dilatation (hazard ratio 3.0, p=0.03) and left ventricular end-diastolic diameter ≥60 mm (hazard ratio=4.0, p=0.01) at diagnosis were factors associated with future surgery. Aortic dissection was not observed in patients that did not undergo surgery. Conclusions: A relatively conservative approach for the indication of ascending aortic surgery in BAV is safe. In this setting, the presence of aortic or left ventricle dilatation and aortic stenosis at diagnosis of BAV were predictive of the need for surgery in the follow-up.
    International journal of medical sciences 01/2013; 10(2):176-182. DOI:10.7150/ijms.5399 · 2.00 Impact Factor
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