Выражение встречается в следующем контексте:
Corneal transplant surgery:
The corneal button is removed and the anterior chamber filled with the preparation until it is level with the surface of the cornea.

Заранее спасибо

см.:
A cylindrical, cookie-cutter type instrument called a trephine is used to remove a round, button-shaped portion of the diseased or damaged cornea. A "button" of similar size is cutr from the donor cornea and placed within the round opening.

www.eye-surgical.com/CTrans.asp