A mastoidectomy is a surgery that removes diseased cells from the air-filled spaces in your mastoid bone. Your mastoid is the part of your skull that sits just behind your ear. Mastoidectomy is often used to treat cholesteatoma, or ear infections that have spread into your skull. It’s also used when placing cochlear implants. A mastoidectomy is a surgery that removes diseased mastoid air cells. These air cells form in the hollow spaces within your mastoid — a sponge-like, honeycomb-shaped bone that sits just behind your ear.