A more recent type of mechanical prosthesis, bileaflet valves pivot upon hinges to completely open or close, depending upong the pressure placed on it. They have the least resistance, allowing the best blood flow of the the three mechanical valves. However, they allow for back-flow which is highly damaging.
Caged Ball Valves
Prothetic Ball Valves (more commonly called Caged Ball Valves) were the first mechanical valve prosthesis on the market, beginning mass production in 1960. The design, patented by Edwards and Starr, utilizies the pressure built up in a previous chamber of the heart to push the ball back into the cage, releasing the blood (and pressure) into the next section of the heart. The first surgery implementing a caged heart valve was accomplished in 1952, although this model could only be placed in the aortic arch, and not the heart itself.
Tilting Disc
A precursor to the bileaflet valve, the first tilting-disc valves were introduced to the market in 1971, to correct for the lack central blood flow found in the Caged-Ball design. It open and closes according to the same principles as the Caged-Ball Design. This first model was called the Bjork-Shiley valve.