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