An assembly can comprise a radially expandable and compressible annular frame, at least one linear actuator assembly coupled to the frame and at least one locking mechanism coupled to the frame. The linear actuator can be configured to apply a distally directed force and/or a proximally directed force to the frame to radially expand or compress the frame. The locking mechanism can comprise a first sleeve member connected to the frame at a first location, a second sleeve member having internal threads and being connected to the frame at a second location, and a first screw configured to engage the internal threads of the second sleeve member to retain the frame in a radially expanded state.
