It is known that the stresses acting in the leaflets of a heart valve prosthesis, during the opening and closing phase, are responsible for most of their mechanical failure.
We postulate that bending and tensile stresses in the closed leaflets, can be significantly reduced by making a new type of synthetic valve prosthesis with fiber-reinforced leaflets, such that the fibers transmit the load from the leaflets to the aortic walls, similar to the natural valve. The fibers can be laid down with different patterns, reinforcing the structure in the areas where the highest stresses occur, such as in the commisures during the maximal systolic valve opening. Therefore it is very important to optimize the fiber layout in order to minimize the stresses.
In our laboratory we produce two types of valve prototypes: stented, where a rigid stent support the three leaflets, and stentless, where the leaflets are made within a piece of the aorta, which is flexible.
Using a finite element package (MARC), we simulated the opening and closing behavior of the fiber-reinforced valve prostheses, both stented and stentless. Only 1/6 of the whole valve is modelled, as the synthetic valve is symmetric. The leaflet was assumed to be of uniform thickness with an orthotropic linear-elastic behavior for the composite material, which closely follow that found in experiments. The geometry of the models is based on measurements on prototypes. The mesh consists of four-node thick shell elements, to include the bending stiffness of the leaflet. A contact algorithm is used to model the coaptation of two leaflets.
From a mechanical point of view, the opening and closing of the leaflets, which is coupled with the so-called `snap through' behavior, is difficult to simulate. As it cannot be solved with a fixed loading step procedure, we must use variable load stepping, based upon the Riks arc length method. Full Newton-Raphson iteration and a large displacement procedure are used, the latter requiring the use of the total Lagrange method.
For different fiber layouts the resulting stresses are analyzed. The results show that in the fiber-reinforced structure the stresses are reduced with respect to the same structure without fibers by up to 60%. Moreover, the flexible leaflet attachment, in the stentless valve, reduces the stresses by up to 65%, with respect to a stented valve with the same type of reinforcement.
This is joint work with J. de Hart, G.W.M. Peters, P.J.G. Schreurs and F.P.T. Baaijens.