A surgical instrument may include a knife moveable from a stowed position completely within said surgical instrument to a cutting position in which at least part of the knife is positioned outside the surgical instrument. The surgical instrument may also include at least two cam slots, a cam pin extending from the knife into the cam slots, at least two guide slots that are substantially linear and are spaced apart from said cam slots, and a guide pin extending from the knife into the guide slots. In some embodiments, the cam slots are shaped to cause the knife to move from the stowed position to the cutting position as the knife is urged proximally.