Mastoidectomy is an operation that allows the exposure of the mastoid air cells, middle ear space and ossicles. It is useful in eradicating chronic infections of the ear and the removal of cholesteatomas. This operation is also useful in exposing the facial nerve and in certain approaches to the inner ear structures.
The picture above shows the external auditory canal (EAC) and the mastoid antrum (the largest air cell in the mastoid). The sigmoid sinus is a large vein that drains blood from the brain. The tegmen is the bone that separates the mastoid from the brain.