Biological Valves :  Biological valves are available for all positions. For mitral position they are usually mounted on a stent (stent mounted) whereas for aortic position they may be stentless or stented. The different varieties that have been tried are allograft (homograft) aortic valves, xenograft (procine or bovine) aortic valves, pericardial, fascia lata or durarnater valves. The latter two types have been discontinued because of structural failure.