Neural caliper

From Memory Alpha, the free Star Trek reference.

The neural caliper is a small device often used as part of the preparation for surgery. It is placed on the patients forehead and when activated it causes the patient to lose consciousness.

In 2365, a neural caliper was used on Captain Jean-Luc Picard during surgery to have his artificial heart replaced on Starbase 515. (TNG: "Samaritan Snare")