Impact of modeling fluid-structure interaction in the computational analysis of aortic root biomechanics.

Numerical modeling can provide detailed and quantitative information on aortic root (AR) biomechanics, improving the understanding of AR complex pathophysiology and supporting the development of more effective clinical treatments. From this standpoint, fluid-structure interaction (FSI) models are currently the most exhaustive and potentially realistic computational tools. However, AR FSI modeling is extremely challenging and computationally expensive, due to the explicit simulation of coupled AR fluid dynamics and structural response, while accounting for complex morphological and mechanical features. We developed a novel FSI model of the physiological AR simulating its function throughout the entire cardiac cycle. The model includes an asymmetric MRI-based geometry, the description of aortic valve (AV) non-linear and anisotropic mechanical properties, and time-dependent blood pressures. By comparison to an equivalent finite element structural model, we quantified the balance between the extra information and the extra computational cost associated with the FSI approach. Tissue strains and stresses computed through the two approaches did not differ significantly. The FSI approach better captured the fast AV opening and closure, and its interplay with blood fluid dynamics within the Valsalva sinuses. It also reproduced the main features of in vivo AR fluid dynamics. However, the FSI simulation was ten times more computationally demanding than its structural counterpart. Hence, the FSI approach may be worth the extra computational cost when the tackled scenarios are strongly dependent on AV transient dynamics, Valsalva sinuses fluid dynamics in relation to coronary perfusion (e.g. sparing techniques), or AR fluid dynamic alterations (e.g. bicuspid AV).

Aortic valve repair via neo-chordae technique: mechanistic insight through numerical modelling.

Recently, the neo-chordae technique (NCT) was proposed to stabilize the surgical correction of isolated aortic valve (AV) prolapse. Neo-chordae are inserted into the corrected leaflet to drive its closure by minimal tensions and prevent relapses. In a previous in vitro study we analysed the NCT effects on healthy aortic roots (ARs). Here we extend that analysis via finite element models (FEMs). After successfully replicating the experimental conditions for validation purposes, we modified our AR FEM, obtaining a continent AV with minor isolated prolapse, thus representing a realistic clinical scenario. We then simulated the NCT, and systematically assessed the acute effects of changing neo-chordae length, opening angle, asymmetry and insertion on the aorta. In the baseline configuration the NCT restored physiological AV dynamics and coaptation, without inducing abnormal leaflet stresses. This outcome was notably sensitive only to neo-chordae length, suggesting that the NCT is a potentially easy-to-standardize technique. However, this parameter is crucial: major shortenings (6 mm) prevent coaptation and increase leaflet stresses by 359 kPa, beyond the yield limit. Minor shortenings (2-4 mm) only induce a negligible stress increase and mild leaflet tethering, which however may hamper the long-term surgical outcome.

Restricted cusp motion in right-left type of bicuspid aortic valves: a new risk marker for aortopathy.

OBJECTIVE: Bicuspid aortic valve disease is heterogeneous with respect to valve morphology and aortopathy risk. This study searched for early imaging predictors of aortopathy in patients with a bicuspid aortic valve with right-left coronary cusp fusion, the most common morphotype. METHODS: Time-resolved magnetic resonance imaging was performed in 36 subjects with nonstenotic, nonregurgitant bicuspid aortic valves and nondilated aortas and in 10 healthy controls with tricuspid aortic valves. Sinus dimensions (diameter, width, and height), ascending tract diameters, and wall strain were measured for each sinus/leaflet unit and corresponding ascending tract area to account for asymmetries. A novel parameter, "cusp opening angle," measured the degree of valve leaflet alignment to outflow axis in systole, quantifying cusp motility. Phase-contrast magnetic resonance imaging and computational fluid dynamic models assessed flow patterns. Aortic growth rate was estimated over a follow-up period ranging from 9 to 84 months. RESULTS: The expected restriction of bicuspid aortic valve opening (conjoint cusp opening angle, 62°±5° vs 76°±3° for nonfused leaflet and 75°±3° for tricuspid aortic valve cusps; P<.001) was confirmed, and the introduced parameter reproducibly quantified this phenomenon. Phase-contrast magnetic resonance imaging demonstrated systolic flow deflection toward the right, affecting the right anterolateral ascending wall. Computational models confirmed that restricted cusp motion alone is sufficient to cause the observed flow pattern. Ascending tract wall strain was not circumferentially homogeneous in bicuspid aortic valves. In multivariable analyses, the conjoint cusp opening angle independently predicted ascending aorta diameters and growth rate (P<.001). CONCLUSIONS: In the bicuspid aortic valve commonly defined as normofunctional by echocardiographic criteria, restricted systolic conjoint cusp motion causes flow deflection. The novel measurement introduced can quantify restricted cusp opening, possibly assuming prognostic importance.

Trends in biomedical engineering: focus on Patient Specific Modeling and Life Support Systems.

Over the last twenty years major advancements have taken place in the design of medical devices and personalized therapies. They have paralleled the impressive evolution of three-dimensional, non invasive, medical imaging techniques and have been continuously fuelled by increasing computing power and the emergence of novel and sophisticated software tools. This paper aims to showcase a number of major contributions to the advancements of modeling of surgical and interventional procedures and to the design of life support systems. The selected examples will span from pediatric cardiac surgery procedures to valve and ventricle repair techniques, from stent design and endovascular procedures to life support systems and innovative ventilation techniques.

Biomechanical implications of the congenital bicuspid aortic valve: a finite element study of aortic root function from in vivo data.

OBJECTIVE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

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

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