A trephine () is a surgical instrument with a cylindrical blade. It can be of one of several dimensions and designs depending on what it is going to be used for. They may be specially designed for obtaining a cylindrically shaped core of bone that can be used for tests, cutting holes in bones (i.e. the skull) or for cutting out a round piece of the cornea for eye surgery.A cylindrically shaped core of bone (or bone biopsy) obtained with the 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 lies in the bone, undisturbed, and is a useful diagnostic tool in certain circumstances.