Effect of Geometry on the Leaflet Stresses in Simulated Models of Congenital Bicuspid Aortic Valves

Abstract

The aim of the study was to assess the effect of geometric variations on the stresses developed in the leaflets of congenital bicuspid aortic valves (CBAV). We developed a model for the human tri-leaflet aortic valve based on the geometry and dimensions published in the literature. We also developed simulated CBAV geometry based on the most common geometry present in patients with CBAV that is published in the literature. We employed a constitutive relationship for the leaflet material from the previously published experimental data of fresh porcine aortic valve leaflet specimens for the analysis. We performed dynamic finite element (FE) structural analysis of the valves in the aortic position in order to compute the strain and stress distribution on the leaflets of the tri-leaflet valve and the CBAV models. Our results showed that large changes in the computed in-plane leaflet strain and stress occurred with variations in the geometry of the simulated CBAV whereas changes due to alterations in material constants were correspondingly less. The valve orifice area in the fully open position was significantly reduced in CBAV compared to that for the tri-leaflet valve. The changes in geometry of CBAV resulted in large changes in in-plane strain and stress and our results suggest that geometrical variations may be a potential risk factor inducing calcific aortic stenosis frequently present in patients with CBAV.

FIGURE 1

Uniaxial stress–strain curves in the circumferential and radial directions derived from the material model employed in the simulation.

FIGURE 2

(a) Aortic valve models created and used in this study: tri-leaflet valve and CBAV: Type I, three malformed leaflets, half of a purely bicuspid valve and two leaflets smaller in size than normal; Type II, purely bicuspid; Type III, single fusion of leaflets lacking raphe; and Type IV, most common CBAV morphology with raphe included through the addition of stiffer material to the section highlighted in red; (b) leaflet height and radius for the leaflets are shown on the left. Three-dimensional representation of the model used for normal tri-leaflet geometry is shown on the right.

FIGURE 3

Pressure load specified on the ventricular surface of the leaflet. Values represent the difference between ventricular and aortic pressure throughout the complete cardiac cycle of normal healthy humans. The leaflet stress distribution is compared for the various simulated geometries with the valve in the fully open position and the fully closed position as indicated in the plot.

FIGURE 4

In-plane principal stress contours for the tri-leaflet aortic valve and simulated CBAVs with the leaflets in the fully open configuration approximately 90 ms into the cardiac cycle. Units for stress are in Pascal (N/m²). The CBAV leaflets shown were chosen for having the largest stress magnitude.

FIGURE 5

In-plane principal stress contours for the tri-leaflet aortic valve and simulated CBAVs with the leaflets in the fully closed configuration approximately 300 ms into the cardiac cycle. Units for stress are in Pascal (N/m²). The CBAV leaflets shown were chosen for having the largest stress magnitude.

FIGURE 6

The comparison of the valve orifice area in the fully open position for the tri-leaflet valve and CBAV: Type I; Type II; and Type III. The addition of a raphe did not affect the fully open geometry and hence valve Type IV has the same open orifice as that of Type III.