A radially-expandable stent (20) is shaped so as to define one or more tether interfaces (50), a lower-securement portion (56), and a higher-securement portion (64). The lower-securement portion (56) extends (a) along at least a contiguous lower-securement axial segment (58) of the stent (20) and (b) circumferentially around a contiguous lower-securement circumferential portion (60) of the stent (20), which lower-securement axial segment (58) and lower-securement circumferential portion (60) include the one or more tether interfaces (50). The higher-securement portion (64) extends (a) along at least a contiguous higher-securement axial segment (65) of the stent (20) and (b) circumferentially around between 215 and 330 degrees of a circumference (C), at all circumferential locations other than those of the lower-securement circumferential portion (60). The stent (20) is shaped so as to define a plurality of outward protrusions (70) at respective circumferential locations (72) around the higher-securement portion (64), and not around the lower-securement portion (56). One or more tethers (34) are coupled to the one or more tether interfaces (50), respectively, and to one or more tissue anchors (30), respectively.