A trephine (/trɪˈfaɪn/; from Greek trypanon, meaning an instrument for boring)[2] is a surgical instrument with a cylindrical blade. It can be of one of several dimensions and designs depending on what it is meant to be used for. They may be specially designed for obtaining a cylindrically shaped core of bone that can be used for tests and bone studies, cutting holes in bones (i.e., the skull) or for cutting out a round piece of the cornea for eye surgery.