A surgical instrument in the form of a crown-saw, for cutting out small pieces of bone, esp. from the skull.

An instrument for boring; in Surgery, a trephine, or the boring part of it; also, a miner's drill. Obsolete.

An improved form of trepan, with a transverse handle, and a removable or adjustable sharp steel centre-pin which is fixed upon the bone to steady…

Surgery. A trepan; (also) the crown of a trepan. Obsolete. rare.

Any of various types of trepan designed to prevent sudden penetration into the brain; (also) the crown of a trepan (see crown, n. 14).

Surgery. A trepan, or attachment for a trepan, used to remove bone in layers. rare. Now historical.