A heart valve prosthesis (88, 128) having a plurality of leaflets (81, 20, 22, 138, 140 and 142). The valve (10, 130) is covered with a sheath (90, 172) of natural tissue. The sheath (90, 172) extending from an inlet end (12, 132) of the valve (10, 130) beyond the outlet end (14, 134) of the valve (10, 130) to define a plurality of lobes (98, 100, 102, 182, 184 186). annular substantially flexible ring (60, 162) can be positioned between the sheath (90, 172) and the heart valve (10, 132) to provide additional stability.
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