A surgical instrument has an end effector mounted on the end of an elongate shaft extending from a handle. The end-effector is capable of 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 arranged to drive the blade assembly longitudinally within the elongate shaft. Specifically, the trigger mechanism is formed of a three pivot slider-crank mechanism comprising a component to which the blade assembly is connected. The component is contained within a housing, wherein the component is free to rotate concentrically within the housing, which in turn rotates the blade assembly. Therefore, the component and the blade assembly are free to rotate without interfering with the operation of the trigger mechanism. The blade assembly is able to move both rotationally and longitudinally.
