A medical instrument having an elongate frame with proximal and distal ends spaced in a lengthwise direction along a central axis. A working assembly at a distal region of the frame has a U-shaped portion defined by first and second legs meeting at a base. The U-shaped portion defines a cutting space. The working assembly further has a cutting assembly with a sharp edge in the cutting space. The working assembly and elongate frame are configured so that a tissue entryway is defined through which a tissue can be advanced to be moved into the cutting space and against the sharp edge. The medical instrument further has a first assembly configured to be operated to vary a configuration of the entryway.
