The mastoid is a thick, cone-shaped piece of bone that juts out behind and below your ear. Formally called the mastoid process, it’s part of the temporal bone on each side of your skull. You can feel it yourself by pressing the bony bump right behind your earlobe. Despite being easy to overlook, this small structure plays a role in head movement, hearing, and middle ear health, and it’s the site of a well-known infection called mastoiditis.
Where the Mastoid Sits
The mastoid process is the lowest, most posterior portion of the temporal bone. It sits directly behind the ear canal and connects to the middle ear through a small passage. Unlike other hollow spaces in the skull (like the sinuses behind your nose and forehead), the air-filled chambers inside the mastoid drain into the middle ear rather than into the nasal cavity. That direct connection to the middle ear is what makes the mastoid relevant to hearing and ear infections.
What’s Inside: Mastoid Air Cells
The mastoid isn’t solid bone. Its interior is honeycombed with tiny, air-filled pockets called mastoid air cells. These cells act as a gas reserve for the middle ear, helping keep pressure stable on the inner side of the eardrum. A larger air cell system means the middle ear can go longer between pressure adjustments through the Eustachian tube, which is the narrow passage connecting the middle ear to the back of the throat. In practical terms, well-developed mastoid air cells help buffer against the kind of pressure imbalances you feel during altitude changes or when you have a cold.
The air cells don’t play a direct role in transmitting sound. That job belongs to the tiny bones (ossicles) in the middle ear cavity itself. But by stabilizing pressure in the space around those bones, the mastoid supports the conditions hearing depends on.
How the Mastoid Develops
The mastoid begins forming before birth. Around the 24th week of pregnancy, a single air cell (the mastoid antrum) becomes visible, lined by a thin layer of tissue that will eventually generate more air cells. After birth, these cells gradually enlarge and multiply. The process is mostly complete by around age 10, though the size and number of air cells varies widely from person to person. Babies and toddlers have a much less developed mastoid, which is one reason young children are more vulnerable to ear infections that spread into this area.
Muscles and Movement
The mastoid process serves as an anchor point for several muscles in the neck and head. The largest is the sternocleidomastoid, the thick muscle running from behind your ear down to your collarbone and breastbone. It’s the muscle you feel tighten when you turn your head to one side. The mastoid also has a groove on its inner surface (the mastoid notch) where a muscle involved in opening the jaw and swallowing attaches. Without a sturdy attachment point like the mastoid process, these movements would be weaker and less controlled.
Mastoiditis: When the Mastoid Gets Infected
Because the mastoid air cells connect directly to the middle ear, an ear infection can spread backward into the bone. This is mastoiditis, and it’s the most common medical problem involving the mastoid. The majority of cases occur in children under two, with a median age of 12 months. Children are especially susceptible because their immune systems are still developing and they get ear infections far more frequently than adults.
Before antibiotics existed, roughly 20% of middle ear infections progressed to mastoiditis, often with serious complications. Today, thanks to antibiotics and pneumococcal vaccines, only about 0.002% of childhood ear infections advance to this stage. Symptoms typically include pain, swelling, and redness behind the ear, along with fever and sometimes the ear being pushed forward by the swelling.
Untreated mastoiditis can lead to severe problems because of what surrounds the bone. The brain’s protective lining, major blood vessels, and the facial nerve all sit close to the mastoid. Infection that erodes through the bone can potentially cause meningitis, abscesses near the brain, blood clots in nearby veins, or facial nerve damage. These complications are rare today but are the reason mastoiditis is treated aggressively with antibiotics and, when necessary, surgery.
Mastoid Surgery
A mastoidectomy, the surgical removal of infected or damaged mastoid air cells, is performed when antibiotics alone can’t clear the infection or when a growth called a cholesteatoma (an abnormal skin cyst) develops in the middle ear and mastoid. The procedure is also sometimes used as a first step in operations to remove tumors near the base of the skull.
There are several variations. In the simplest form, the surgeon opens the outer layer of bone and clears out the infected air cells while leaving the ear canal intact. More extensive versions involve removing part of the ear canal wall, and the most aggressive type permanently opens the middle ear to the outside. The choice depends on how far infection or disease has spread.
Full recovery from a mastoidectomy takes about six to 12 weeks, though most people return to work or school within one to two weeks. The primary goal is eliminating infection, not restoring hearing. Some hearing loss is common after more extensive procedures where the ear canal wall is removed or the tiny middle ear bones can’t be preserved. For less invasive versions, hearing outcomes are generally better.
Why You Can Feel It
The mastoid process is one of the most easily palpable landmarks on the skull, which is why doctors check it during ear exams. Tenderness or swelling over the mastoid is a key sign of mastoiditis, especially in a child with a recent ear infection. If you’ve ever had a doctor press behind your ear and ask if it hurts, they were checking this bone. Its surface location also makes it a reference point for surgeons, audiologists fitting hearing devices, and physical therapists assessing neck muscle function.