trephine trephine a trephine (/trɪˈfaɪn/; from greek trypanon, meaning an instrument for boring) 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 (e.g., the skull) or for cutting out a round piece of the cornea for eye surgery. a trephine with a center pin can be seen on the left. dr. john clarke trepanning a skull, ca. 1664, in one of the earliest american portraits. clarke has a trephine in his right hand. the painting is in harvard medical school. a cylindrically shaped core of bone (or bone biopsy) obtained with a bone marrow trephine is usually examined in the histopathology department of a hospital under a microscope. it shows the pattern and cellularity of the bone marrow as it lay in the bone and is a useful diagnostic tool in certain circumstances such as bone marrow cancer and leukemia.