A medical implant (20) includes first and second ring members (22,24), eachincluding a resilient framework (26) having a generally cylindrical form. Atubular sleeve (28) is fixed to the first and second ring members so as tohold the ring members in mutual longitudinal alignment, thereby defining alumen (32) passing through the ring members. A constricting element (30) isfit around the sleeve at a location intermediate the first and second ringmembers so as to reduce a diameter of the lumen at the location.
