Doctors have discovered a potential problem involving implanted heart valves that hundreds of thousands of people have received — they don't always open and close properly, possibly because a blood clot has formed that could raise the risk of stroke. Although the problem appears common, experts stressed that not enough is known about the situation to change practice now, and federal health officials say the valves still seem safe and well worth the risk. But it's bound to be unsettling for people with bioprosthetic aortic valves, ones made from cow, pig or human tissue.Read more on NewsOK.com