An implantable prosthetic device, comprising: a frame (12, 302) movable between a radially compressed and a radially expanded configuration, the frame (12, 302) comprising an inflow end portion and an outflow end portion, the frame (12, 302) comprising a plurality of struts; at least one expansion and locking mechanism comprising: a first member coupled to the frame at a first location, a second member coupled to the frame (12, 302) at a second location spaced apart from the first location, the second member extending at least partially into the first member, a third member having a first end portion and a second end portion, the first end portion extending at least partially into the first member and the second end portion comprising an engagement portion; wherein the plurality of struts includes a first strut and a second strut pivotably coupled to one another to form an apex, the first strut comprising a first flange and the second strut comprising a second flange; and wherein, when the frame (12, 302) is in the expanded configuration, advancement of the third member in a distal direction positions the engagement portion between the first and second flanges (322a, 322b) such that the first and second flanges (322a, 322b) engage the engagement member to resist pivoting of the first and second struts (304a, 304b) relative to one another in a first direction to resist radial compression of the frame (12, 302). a frame (12, 302) movable between a radially compressed and a radially expanded configuration, the frame (12, 302) comprising an inflow end portion and an outflow end portion, the frame (12, 302) comprising a plurality of struts; at least one expansion and locking mechanism comprising: a first member coupled to the frame at a first location, a second member coupled to the frame (12, 302) at a second location spaced apart from the first location, the second member extending at least partially into the first member, a third member having a first end portion and a second end portion, the first end portion extending at least partially into the first member and the second end portion comprising an engagement portion; a first member coupled to the frame at a first location, a second member coupled to the frame (12, 302) at a second location spaced apart from the first location, the second member extending at least partially into the first member, a third member having a first end portion and a second end portion, the first end portion extending at least partially into the first member and the second end portion comprising an engagement portion; wherein the plurality of struts includes a first strut and a second strut pivotably coupled to one another to form an apex, the first strut comprising a first flange and the second strut comprising a second flange; and wherein, when the frame (12, 302) is in the expanded configuration, advancement of the third member in a distal direction positions the engagement portion between the first and second flanges (322a, 322b) such that the first and second flanges (322a, 322b) engage the engagement member to resist pivoting of the first and second struts (304a, 304b) relative to one another in a first direction to resist radial compression of the frame (12, 302).