Abstract: A surgical knife includes a blade secured to the knife body and having a cutting edge projecting transversely beyond the body profile. A slotted ski member is movable axially relative to the cutting edge to selectively expose the cutting edge by means of plural thread segments which cooperate to convert rotary motion to said axially controlled axial motion. The position of the ski and blade beyond the body profile enhances visibility of the incision. In the preferred embodiment, the ski is part of a shaft which is threadedly received in a spindle, which, in turn, is threadedly received in the same sense within the knife body. The ski shaft is procluded from rotary motion relative to the knife body and its threads have a greater pitch than the body threads; consequently, axial displacement of the spindle in one direction produces axial displacement of the ski in the opposite direction.