Abstract: A surgical instrument for operating percutaneously in a living body includes an elongated hollow handle, a shaft with a proximal portion extending within and secured, during use, to the handle and a distal portion extending distally of the handle, a cutting device, e.g., a surgical blade, fixedly supported during use by the distal end of the shaft, and a slidable tubular sheath having a distal portion extending distally of the handle, about the shaft, to the region of the blade. The sheath is sized and constructed for introduction into the living body through a puncture opening in its flesh. The instrument includes a mechanism, e.g., a sliding or rotating actuation knob, for moving the sheath axially relative to the handle between a first position in which the distal portion of the sheath surrounds the blade enabling safe positioning of the instrument in the body without cutting, and a second, retracted position exposing the blade.