Trepanning
2 of the 7 encyclopedias on this shelf carry an entry for Trepanning. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the operation of cutting a circular opening into the skull by means of a surgical instrument called a trepan or trephine. This consists of a handle, to which is fixed a small hollow steel cylinder, of about 1/2 to 1 inch in diameter, having teeth cut on its lower edge so as to form a circular saw. The operation of trepanning is resorted to for the purpose of relieving the brain from pressure; such pressure may be caused by the depression of a portion of the cranium, or it may be produced by an extravasation of blood, or by the lodgment of matter betwixt the skull and the dura mater, occasioned by a blow upon the head, or the inflammation of the membranes of the brain. TREPOMONADIDÆ, in zoology, a family of pantostomatous Flagellata, with a single genus, Trepomonas. Animalcules naked, free-swimming, asymmetrical; two flagella separately inserted; no distinct oral aperture.
Aiton's Encyclopedia (1910)
a well known surgical operation. It consists in the removal of a portion of the skull to relieve the brain from pressure. The operation is performed usually with an instrument known as the trepan, crown saw, or trephine. The crown saw consists essentially of a cylinder fastened on a central shaft or pin. The end of the cylinder which is to perform the work is provided with sharp, saw-like teeth. The central pin holds the instrument in position. When twirled between the fingers or driven by power it cuts out a circular piece of bone neatly, making an orifice varying according to the instrument from the size of a dime to that of a silver dollar. If performed with care the wound heals again and no evil results follow. The term is to be distinguished from trapan, sometimes, but less properly, spelled trepan, to take in a trap or snare.