Tissue valves can be classified as follow
The allograft valve replacement with an allograft valve gives certainty of freedom from thromboembolism and normal haemodynamic function if accurately inserted . (79)
Tissue valves have gained widespread reputation when used during the mid of 1970 where the first biological valves implanted were homografts with valves explanted from cadavers withing 48 hours of their death .Preservation of the valves include various techniques of sterilization such freezing , drying and immersing in antibiotic solution .(112)
Biological valves are made from biological tissues .
These prostheses are preferable in patients who do not tolerate anticoagulants , however bioprosthetic valves are prone to calcification and leaflet tear with an average lifetime of about two years before replacement is necessary .Two major disadvantages with the use of tissue valves are the need for life – long anticoagulation therapy and the accompanying of bleeding , for thermos , the hemodynamic function of even the best designed valves differs significantly from that of the natural healthy heart valve . (118) today biological valve is the most widely used in clinical practice.