Early in the history of mitral valve surgery, replacement of the valve was the most commonly performed treatment. It is still performed today, and the valve replacements fall into two general categories, mechanical valves and bioprosthetic ("tissue") valves. While mechanical valves have a long lifespan, patients will need to be treated with blood-thinners (i.e. coumadin) for the rest of their lives. Bioprosthetic valves can last 10-15 years, but patients do not need any treatment with blood-thinners. Mitral valve replacement surgery is generally indicated in those patients with advanced rheumatic valvular disease or significantly calcified valves, for whom mitral valve repair surgery is not possible.