The mastoid bone is the thick, rounded projection you can feel just behind and below each ear. It is technically not a separate bone but rather a prominent part of the temporal bone, one of the paired bones that form the sides and base of the skull. What makes the mastoid unusual is its interior: rather than solid bone, it is filled with a honeycomb of air-filled pockets called mastoid air cells, and this architecture gives it several functions ranging from middle ear pressure regulation to shock absorption during head trauma. The mastoid also serves as an anchor point for neck and head muscles, plays a role in clinical hearing tests, and even helps forensic scientists estimate biological sex from skeletal remains.
Where Exactly It Sits
If you reach behind your earlobe and press on the hard bump just behind the ear canal, you are touching the mastoid process. It juts downward and slightly forward from the lower rear portion of the temporal bone, which itself forms part of the side and floor of the skull. The mastoid sits behind the external ear canal, below the thin squamous part of the temporal bone, and just lateral to the opening where the spinal cord exits the skull. It is bordered internally by important structures: the sigmoid sinus (a large venous channel draining blood from the brain), the middle and inner ear, and the facial nerve, which threads through a bony canal inside the temporal bone on its way to the face muscles.
In humans, the mastoid process projects downward in a way that is distinct from other primates. Compared to the great apes, the human mastoid is positioned more laterally and projects more prominently below the skull base. This reconfiguration is tied to our upright posture and the changed geometry of head-and-neck muscle attachments that came with walking on two legs.
The Air Cell System Inside
The most striking feature of the mastoid is not visible from outside. Cut it open or scan it, and you find a network of air-filled cavities lined with mucous membrane. These mastoid air cells communicate with the middle ear through a space called the mastoid antrum, which in turn connects to the middle ear cavity. At birth, the antrum is already reasonably well developed, but the surrounding air cells are minimal. Over the first year of life, pneumatization (the process of air cells forming within bone) accelerates. Growth then proceeds in a roughly linear pattern from age one to about six, with air cell area expanding by roughly one square centimeter per year.
After age six the growth slows, then picks up again briefly during puberty before reaching adult size. The adult mastoid air cell system averages around 12 square centimeters of projected area, though individual variation is enormous. Some adults have extensively pneumatized mastoids with large, well-connected air cells; others have sclerotic (dense, poorly aerated) mastoids with very few air cells. This variation matters clinically, as we will see.
Beyond the visible air cells, micro-CT scanning has revealed a network of tiny channels threading through the compact bone of the mastoid, connecting the bone surface directly to the air cells and linking air cells to one another. These micro-channels average about 158 micrometers in diameter and appear to carry their own blood supply to the air cell lining, separate from the main mucosal blood supply of the middle ear.
Pressure Regulation in the Middle Ear
One of the mastoid’s most important jobs is serving as a gas reservoir for the middle ear. The middle ear is a small, sealed space behind the eardrum, and its pressure needs to stay close to atmospheric pressure for the eardrum and the tiny ossicle bones to vibrate properly. The main way pressure equalizes is through the Eustachian tube, the narrow passage connecting the middle ear to the back of the throat that opens briefly when you swallow or yawn. But between those openings, gas is constantly being absorbed by the mucous membranes lining the middle ear. If the middle ear were a tiny sealed box with no buffer, pressure would drop quickly and the eardrum would get sucked inward.
The mastoid air cells act as that buffer. Because the air cell system holds a relatively large volume of air connected to the middle ear, gas absorption happens more gradually. Research modeling the mastoid as a rate-limiter of pressure change found that larger air cell volumes slow down the rate at which middle ear pressure shifts due to gas exchange, meaning the Eustachian tube does not need to open as frequently to keep pressure near normal.
This reservoir function also matters after ear surgery. When surgeons reconstruct a damaged eardrum, the pneumatic spaces in the mastoid serve as an air reserve that can be drawn upon during periods when the Eustachian tube is not functioning well, buffering the middle ear against harmful negative pressure. One study on tissue-engineered regeneration of mastoid air cells found that when air cells regenerated well after surgery, Eustachian tube function improved significantly, likely because restored gas exchange in the mastoid released the negative pressure that had been locking the tube shut.
Shock Absorption During Head Trauma
The honeycomb structure of the mastoid does more than manage air pressure. The air cells appear to function as a built-in crumple zone. When the side of the head takes a direct blow, the mastoid’s network of thin bony walls and air spaces can absorb and disperse kinetic energy, reducing the chance that a fracture will propagate into deeper, more critical structures like the inner ear or the floor of the middle cranial fossa.
Imaging studies of patients with temporal bone fractures have supported this idea. Research published in the American Journal of Neuroradiology found that the mastoid portion of the temporal bone plays a role in absorbing and dispersing kinetic energy during lateral head trauma, reducing fracture incidence from direct impacts. A separate study using CT data similarly concluded that temporal bone pneumatization acts as a shock absorber, protecting surrounding vital structures during trauma.
The implication is somewhat counterintuitive: people with well-pneumatized mastoids (lots of air cells) may actually fare better after a blow to the side of the head than those with sclerotic, solid mastoids, because the air cells give the bone somewhere to deform without transmitting all the force inward. That said, researchers are careful to note this is a protective tendency, not a guarantee.
Muscle Attachment and Head Movement
The mastoid process is one of the skull’s major anchor points for muscles that move and stabilize the head. The sternocleidomastoid muscle, the thick band you can see and feel running diagonally along the side of your neck, inserts onto the mastoid. So does the splenius capitis, a deeper muscle that helps extend and rotate the head, and the longissimus capitis. The posterior belly of the digastric muscle, involved in opening the jaw and swallowing, also originates from a groove on the medial side of the mastoid.
The size and shape of the mastoid process in humans reflect these attachments. The process is large and projects downward partly because it needs surface area for these powerful muscles. Comparative anatomy work on large cats has shown how mastoid morphology correlates with predatory behavior. In big cats, the mastoid region anchors muscles originating from the atlas vertebra that flex the head powerfully downward, and in sabretoothed cats, an enlarged, forward-projecting mastoid process gave even greater leverage for head-flexing action used in prey capture. In humans the functional demands are different, centered on upright posture and head stabilization, but the principle is the same: the mastoid’s shape is sculpted by the muscles that pull on it.
The Mastoid in Hearing Tests
If you have ever had a hearing test, the clinician may have placed a small vibrating device on the bone behind your ear. That device is a bone conduction vibrator, and the spot it rests on is the mastoid process. The idea is straightforward: sound can reach the inner ear not just through the air (via the ear canal and eardrum) but also through vibration of the skull bones. By comparing how well you hear sounds delivered through the air versus sounds delivered through the bone, an audiologist can figure out whether a hearing problem is in the outer or middle ear (which air conduction depends on) or in the inner ear itself.
The mastoid is the standard placement site for bone conduction testing because it is close to the cochlea and accessible without interfering with the ear canal. But the process is not as simple as it sounds. The pathways by which vibration travels from the mastoid to the cochlea involve multiple routes, including direct skull bone vibration, compression of the inner ear fluids, and even sound radiated back out of the ear canal. Research on this topic has described how placing a bone vibrator on the mastoid triggers a wave along the basilar membrane inside the cochlea through at least two main categories of pathways. And because bone conduction can stimulate both ears, clinicians have to use masking noise in the non-test ear to avoid misleading results.
There are also practical challenges in calibrating bone conduction equipment. Studies have found problems estimating the force output of bone conduction transducers because the mechanical impedance of an artificial calibration device does not perfectly match the impedance of a real human mastoid, leading to a shift in resonance frequency.
Mastoiditis and Its Complications
Because the mastoid air cells connect directly to the middle ear, infections can spread from one space to the other. Acute mastoiditis, an infection of the mastoid air cells, is the most common complication of acute otitis media (middle ear infection). It occurs most frequently in children and typically presents with pain, swelling, and redness behind the ear, often pushing the outer ear forward.
Before antibiotics were available, mastoiditis was a leading cause of death in children. Even now, it demands prompt treatment because the infection sits dangerously close to the brain, the major venous sinuses, and the facial nerve. Complications can include abscess formation under the periosteum (the membrane covering the bone), deep neck abscess, facial nerve palsy, meningitis, encephalitis, venous sinus thrombosis, and seizures.
One particularly tricky presentation is “masked mastoiditis,” where the infection smolders without the dramatic external swelling people associate with the condition. In these cases, the course can be so insidious that the first sign of trouble is an intracranial complication. A case series described nine patients whose presenting symptoms were all vague and non-classical, yet seven already had complications like meningitis, facial paralysis, brain abscess, or papilledema (optic disc swelling) on admission. Two more had unsuspected epidural abscesses discovered during surgery. Masked mastoiditis is a reminder that the mastoid’s proximity to the brain makes any bone infection in this area potentially serious.
When imaging is needed, CT has traditionally been the standard for evaluating coalescent mastoiditis (the stage where bony walls between air cells break down). MRI, however, has shown promise. One study comparing the two found that MRI detected definite bone defects with perfect sensitivity for general coalescent mastoiditis, though specificity varied by anatomical subsite. A particular MRI measurement, the diffusion characteristics of material inside the mastoid, proved especially useful: when fluid in the mastoid was not diffusion-restricted and showed no intense enhancement, coalescent mastoiditis could be ruled out with high confidence.
Surgery on the Mastoid
Mastoidectomy, the surgical removal of some or all mastoid air cells, has a long history. Before the mid-nineteenth century, the operation was performed only sporadically, usually as a last-ditch effort to save lives during complicated ear infections. By the early twentieth century, draining acute mastoid abscesses became routine, particularly because there were no antibiotics to fall back on. Today, mastoidectomy is still performed for chronic ear infections that do not respond to medical treatment, for cholesteatoma (an abnormal skin growth in the middle ear), and to gain access for procedures like cochlear implantation.
The biggest surgical risk is injury to the facial nerve, which courses through a bony canal inside the temporal bone, passing through the mastoid segment on its way to the face. Accidental damage to this nerve during mastoidectomy can cause partial or complete facial paralysis on the affected side. A review of iatrogenic facial nerve injuries during mastoid surgery found that about 29 percent of injuries occurred in the mastoid segment of the nerve, with additional injuries at the junction of the tympanic segment and the second bend of the nerve’s course. The facial nerve’s path can vary between individuals, and in patients with middle ear disease, the nerve sometimes takes a more lateral course than usual, increasing the risk during certain surgical approaches.
Forensic Sex Estimation From the Mastoid
The mastoid process is one of the most sexually dimorphic features of the human skull, meaning it tends to differ consistently in size between males and females. Males generally have larger, more robust mastoid processes, and forensic anthropologists have long used this as one indicator when estimating the sex of unidentified skeletal remains.
Modern research has tried to quantify just how accurate the mastoid is as a sex predictor. A study of dry skulls from the North Indian population measured the so-called mastoid triangle, defined by three landmarks on the skull, and found that all measured parameters were significantly larger in males. The area of the mastoid triangle was the best single predictor, correctly classifying sex about 78 percent of the time. Similarly, a study of a Saudi population found that all three sides of the mastoid triangle were sexually dimorphic, though accuracy was somewhat lower, with the best single measurement correctly classifying sex about 69 percent of the time and all parameters combined reaching roughly 71 percent.
A more recent pilot study using cone-beam CT to measure mastoid volume and distances between landmarks in three dimensions found that the distance between the right and left mastoid processes had the greatest accuracy for estimating sex, at about 82 percent. When multiple measurements were combined in a statistical model, accuracy reached about 81 percent. These numbers are useful but not definitive on their own. In forensic practice, the mastoid is typically assessed alongside other skeletal features like the pelvis, brow ridges, and chin shape to build a more reliable profile. It is especially valuable in cases where the pelvis is missing or damaged, since the skull is often better preserved.
Why Mastoid Size Varies So Much Between People
If you compared CT scans of twenty adults, you would find a startling range of mastoid pneumatization. Some people have extensive air cell networks extending deep into the petrous part of the temporal bone and even into the tip of the mastoid, while others have dense, sclerotic mastoids with minimal aeration. Part of this variation is genetic, but a major factor appears to be the ear’s history during childhood. Repeated middle ear infections during the critical growth years from about age one to six, when air cells are expanding most rapidly, can impair normal pneumatization. The inflamed, thickened mucosa does not allow bone remodeling to proceed normally, and the result is an under-aerated mastoid.
This creates something of a vicious cycle. A poorly pneumatized mastoid has less gas reservoir capacity, which means middle ear pressure regulation is less efficient, which may make the ear more vulnerable to further problems like retraction of the eardrum or fluid buildup. Conversely, a well-pneumatized mastoid tends to correlate with a healthier middle ear history. Reviews of growth data have confirmed that bone expansion and air cell development share a similar growth pattern but that bone growth lags slightly behind the expansion of aeration, suggesting the air cells drive the process rather than following it.
Deep Learning and Mastoid Imaging
An emerging area of research involves using artificial intelligence to classify mastoid air cell patterns from CT scans. One team developed a deep learning model based on convolutional neural networks that categorized mastoid images into five groups: completely pneumatized, opacified within a pneumatized mastoid, partially pneumatized, opacified within a partially pneumatized mastoid, and non-pneumatized. The goal is to automate what radiologists currently do by eye, potentially speeding up diagnosis and making it more consistent, especially in settings where specialist radiologists are not readily available.
This kind of automated classification could eventually help surgeons plan mastoidectomies by mapping the extent of pneumatization and identifying areas of disease before the first incision. It could also assist in screening large populations of CT scans for incidental mastoid abnormalities that might otherwise be overlooked when the scan was ordered for a different reason, like evaluating a head injury or planning dental implant surgery.