Embodiments of the present invention provide an improved surgical instrument having an end effector mounted on the end of an elongate shaft extending from a handle. The end-effector is capable of several different operations, including grasping, cutting, and sealing and/or coagulating tissue, and one of the operations is controlled by a trigger mechanism contained within the handle. In some embodiments, the end effector includes a blade assembly, the trigger mechanism being formed of a three pivot slider-crank mechanism arranged to drive the blade assembly longitudinally along a rotatable drive shaft contained within the elongate shaft. As such, the mechanism is configured so as to drive the blade assembly with enough force that the cutting blade is able to cut through tissue without the user needing to apply any additional force to the trigger. Such an arrangement is particularly advantageous when the blade is being used to cut thick tissue.
