Trephination comes from the Greek and means "auger" or "borer." More to the point, it involves an opening made by a circular saw of any type.
As a surgical procedure, trephination of the skull has been around since early
civilization. No one knows exactly why, but it is thought that it was done to allow spirits to come or go. It may have been done for headaches, fractures, infections, insanity, or for convulsions.
Some think it was done to get the disks of bone to use for charms, amulets, or talismans.
Today, trephination is sometimes used on damaged meniscus to stimulate blood flow by opening channels for blood to pass through. Many holes or shavings are made in the torn part of the meniscus. The idea is to promote healing with better blood flow to the area.
You can see a drawing of this procedure at
http://www.steadman-hawkins.com/meniscus/treat.asp and an animated video at http://www.emedx.com/emedx/diagnosis_information/knee_disorders/meniscus_trephination_sur