Computer-assisted design of butterfly bileaflet valves for the mitral position

This paper describes the application of computer testing to a design study of butterfly bileaflet mitral prostheses having flat or curved leaflets. The curvature is in the plane normal to the pivot axes and is such that the convex sides of the leaflets face each other when the valve is open. The design parameters considered are the curvature of the leaflets and the location of the pivot points. In this study, stagnation is assessed by computing the smallest value (over the three openings of the valve) of the peak velocity, and hemodynamic performance is judged by a benefit/cost ratio: the net stroke volume divided by the mean transvalvular pressure difference. Unlike the case of a pivoting single-disc valve, the inclusion of a constraint on the maximum angle of opening of the leaflets is found to be essential for adequate, competent performance. Results are presented with both 85° and 90° constraints, since best performance is achieved with the opening-angle constraint in this range. Asymmetry of leaflet motion which is observed with flat leaflets in the mitral position is reduced with modest leaflet curvature. Leaflet curvature also ameliorates central orifice stagnation, which is observed with flat leaflets. Curvature of the valve produces the following improvements in comparison with the best flat valve when the opening-angle constraint is 85°: a 38 % increase in the minimum peak velocity and a 16% increase in the hemodynamic benefit/cost ratio. With a 90° constraint the corresponding improvements are 34% and 20% respectively.