Non-linear finite element modelling of porcine bioprosthetic valves

Abstract

There are two major types of artificial heart valves in use, namely mechanical valves and bioprosthetic valves. Mechanical valves last longer but require the recipient to undergo long-term anticoagulant therapy. Bioprosthetic valves are more biocompatible than the mechanical valves and long-term anticoagulant theraphy is not necessary. The major mode of failure of mechanical valves is through fatigue. This type of valve failure is catastrohic. Death is certain if the failed valve is not replaced immediately. The bioprosthetic valves normally fail by tearing of the leaflet. Many workers in this field believe that the tearing of the leaflet is associated with accumulated calcium deposits on the leaflets which makes the leaflet less flexible. A bioprosthetic valve usually has a soft failure path, with the leaflet tearing gradually. As the valve becomes less effective, the heart weakens slowly and there is time for diagnosis, treatment and valve replacement. Since there is a strong relationship between areas of high engineering stresses and calcification, engineering stress analysis and design studies offer the possibility of increasing the lifetime of the valve. The aims of the current research are to identify the areas of high stressesin the leaflets of bioprosthetic porcine valves and the ways of reducing these stresses. The stress distribution on the leaflets varies significantly as the leaflets cycle between the open and the closed positions. Thus, a time-stepping finite element code which is capable of non-linear stress analysis was needed to analyse the leaflets. The finite element package currently in use is the OASYSDYNA3D package. It is capable of non-linear geometrical modelling. A non-linear material model for the porcine valve is currently under development. Once the OASYS-DYNA3D package is capable of both non-linear geometrical and material modelling, the porcine valve leaflets can be fully analysed and the areas of high stresses be identifiedso that the design of the valve can be improved.