"A shaft of metal with a hollow centre designed for overdrilling a surgical screw or screw shaft to expose it for its removal, with another instrument, when the screw head has become defective or detached. The device is commonly called a ""hollow mill"" or ""crown drill"", but may also be referred to as a ""hollow reamer"", and is designed as a cylindrical hollow shell with a thin wall that has saw teeth at the cutting edge. It is inserted into a surgical power tool for rotation. This device may also be used to obtain core bone biopsies. This is a reusable device."