Mastoid cavity is the hollow bone behind the ear canal. In most patients the bone has large cells full of air. In some patients who have suffered from chronic ear disease the bone may not have many air cells in it. True infections of the mastoid cavity, known as mastoiditis, are very rare due to antibiotic use for management of ear infections. When the mastoid cavity is involved by disease, such as a cholesteatoma, the outer wall of this bone is removed to allow access to this hollow cavity. Mastoidectomy is usually performed in conjunction with tympanoplasty and is called tympanomastoidectomy. In modern era, a high speed drill and other highly specialized instruments are used for this surgery under the microscope.
Schematic view of the right ear with mastoid bone intact. Some of the important structures passing through the bone such as the venous system bringing blood back from the brain (deep blue), carotid artery taking blood to the brain (red), facial nerve (yellow) are seen.
The outer wall or cortex of the mastoid cavity is drilled away as well as all the air cells in the bone. The vital structures in the bone are protected by leaving a very thin layer of bone over them. The balance canals (light blue semi-circles) and facial nerve are identified and protected.
Example of a middle ear disease known as cholesteatoma or skin cyst (white) starting in the middle ear cavity and extending into the mastoid.
Images are courtesy of Dr. Rober Jackler, used with permission.