Valvular prostheses

Prosthetic heart valves may be mechanical or bioprosthetic. Mechanical valves, which are composed primarily of metal or carbon alloys, are classified according to their design as ball-caged, single-tilting-disc, or bileaflet-tilting-disc valves. In ball-cage valves, the occluder is a sphere which is contained by a metal ‘cage’ when the valve is in its open position, and fills the orifice when the valve is in its closed position. In single-tilting-valves, the occluder is a single circular disc which is constrained in its motion by a cage, a central strut, or a slanted slot in the valve ring, therefore it opens at an angle less than 90° to the sewing ring plane. In bileaflet-tilting-disc valves there two occluders, two semicircular discs that open forming three orifices, a central one and two lateral ones. Biological tissue valves prostheses may be heterografts, which are composed of porcine, bovine, or equine tissue (valvular or pericardial), or homografts, which are preserved human aortic valves. Heterografts include stented and stentless bioprostheses. In stented valves, the biological tissue of the valve is mounted on a rigid stent (plastic or metallic) covered with fabric. Conversely stentless bioprostheses use the patient’s native aortic root as the valve stent. The absence of a stent and sewing ring cuff make it possible to implant a larger valve for a given native annulus size, resulting in a larger effective orifice area (EOA).