A surgical instrument having an end effector such as a retractable blade controlled by a trigger mechanism on the handle. The trigger mechanism comprises a trigger arm 24 pivotable about a first axis A and a frame 346 connected to the trigger arm 24 at a second pivot axis B and to a collar 352 at a third pivot axis C. Movement of the trigger arm 24 is translated into longitudinal movement of the collar 352 which causes movement of the end effector. The trigger arm and collar may snap-fit to the frame, which may have an H-shape. A spring 350 may bias the trigger arm 24 into a first position. The trigger mechanism may non-monotonically or variably apply force to the end effector when a constant force is applied to the trigger.
