Trepan, and Trephine (Gr. τρυπάν to perforate), two surgical instruments used for removing portions of bone from the skull or other parts of the bony structure. The first was an instrument like a gimlet, to which was attached a crown or cylinder with saw teeth on its lower edge, and which was worked by a rotatory motion till it perforated the bone. Several sizes of these cylinders were furnished. The trephine is of later invention. It has a cylindrical saw, but no gimlet. A sharp steel point called a centre pin, which can be pressed into the bone until the saw has made a groove for itself, passes down the centre of the instrument, and is removable by the operator as soon as the groove is made. The cutting is accomplished, not as in the trepan by a rotatory movement, but by semi-rotation, as in boring with an awl. The method of trephining is as follows: A crucial incision is made down to the bone, and the periosteum being dissected up, the trephine is applied, the centre pin being removed as soon as the track of the instrument is fixed, and the instrument itself raised every few strokes in order to see that it is not cutting through on either side, upon the tissues below.

The greatest danger is when the circular piece is nearly separated; and some operators raise the cut portion by means of the elevator, rather than permit the instrument to divide it completely. The spicula of bone which may remain around the orifice are carefully removed by means of forceps. Trephining has been considered as indicated when there is a fracture of a portion of the skull, from a fall or blow with a blunt instrument, in order to elevate the depressed portion; in some cases of concussion, where there is reason to believe that the inner table of the skull is fractured at the opposite side of the head, and is producing irritation of the brain; in cases where extravasation of blood has taken place under the meninges of the brain from injuries or disease, or where purulent matter has accumulated under the meninges; in caries affecting the bones of the skull, the sternum or breast bone, or the tibia; and in some cases of a collection of purulent matter under the sternum. During the middle ages, in the "heroic" period of surgical practice, trephining was one of the most common operations of surgery. Of late years the operation is but seldom practised.

Hey's saw, with a shaft and handle like a common steel fork, and having a plate of steel 1½ in. in breadth and perhaps 1¾ in. long attached to it, one edge of which is a straight and the other a convex saw, has almost entirely superseded the trephine for most of those injuries of the skull which were formerly thought to require its use. By this instrument the depressed portion is itself removed, instead of that which was not fractured, and the injuries to the skull can be remedied with far less loss of bony structure than under the old system.