Maxillary Sinus

What Is the Maxillary Sinus?

The maxillary sinus is a paired, air-filled cavity in the maxilla, the bone that forms the upper jaw, most of the cheek and part of the floor of the eye socket. It is the largest of the paranasal sinuses, the group of air spaces around the nasal cavity that also includes the frontal, ethmoid and sphenoid sinuses.

Its older name is the maxillary antrum, or antrum of Highmore, after the English physician Nathaniel Highmore, who described it in the seventeenth century. Antrum simply means cave. Like the other sinuses, it is lined by mucous membrane, is normally filled with air and connects to the nose through a narrow opening.

Explained simply: behind each cheek is a small hollow room filled with air. A tiny doorway near the top of that room leads into the side of your nose, and the mucus made inside the room leaves through that doorway.

Where Is the Maxillary Sinus Located?

The maxillary sinus is located in the cheek on each side of the nose, directly below the eye socket and above the roots of the upper back teeth. There is one on the right and one on the left, and each fills most of the body of its maxilla.

To find it on your own face, place a fingertip on the cheek just below the lower rim of the eye and beside the nose: the sinus lies under the bone there. Its inner wall is the outer wall of the nasal cavity, its roof is the floor of the orbit, and its floor sits over the tooth-bearing ridge of the upper jaw. Toward the outside it narrows into the zygomatic process of the maxilla, under the cheekbone.

The right and left maxillary sinuses are mirror images, though they are rarely exactly the same size. Each one drains into its own side of the nose, lateral to the nasal septum.

Maxillary Sinus Anatomy: Walls, Opening and Lining

The maxillary sinus has the shape of a pyramid lying on its side, with its base forming part of the lateral wall of the nasal cavity and its apex pointing toward the cheekbone. In an adult it holds roughly 15 milliliters on average, though size varies widely from person to person.

Walls

The sinus has a roof, a floor and front, back and inner walls, each facing a different neighbor, as the table below shows. The roof is thin and carries the infraorbital nerve and vessels in a groove or canal on their way to the face. The back wall separates the sinus from the infratemporal and pterygopalatine fossae, deep spaces that hold branches of the maxillary artery and nerve.

Opening

The natural opening, the maxillary ostium, lies high on the inner wall, close to the roof. It leads into a narrow channel called the ethmoidal infundibulum, which empties through the semilunar hiatus into the middle nasal meatus, under the middle nasal concha. Some people also have one or more small accessory openings.

Lining, vessels and nerves

The sinus is lined by a thin mucoperiosteum, often called the Schneiderian membrane, made of ciliated pseudostratified columnar respiratory epithelium with goblet cells. Blood comes from branches of the maxillary artery, including the infraorbital and superior alveolar arteries. Sensation travels in the infraorbital nerve and the superior alveolar nerves, all branches of the maxillary division of the trigeminal nerve.

Where Does the Maxillary Sinus Drain?

The maxillary sinus drains into the middle meatus of the nasal cavity, through the maxillary ostium, the ethmoidal infundibulum and the semilunar hiatus. Because the ostium sits near the roof rather than the floor, gravity cannot empty the sinus while you stand or sit upright.

Instead, the cilia of the lining beat in a coordinated pattern that moves the mucus blanket upward and inward, converging on the ostium from every wall. This is the mucociliary escalator at work. In the middle meatus the mucus joins secretions from the frontal sinus and the anterior ethmoid cells and is swept back toward the nasopharynx, where it is swallowed.

The narrow region where these three sinuses drain is called the ostiomeatal complex. Swelling there can block several sinuses at once, which helps explain why the maxillary sinus is so often involved when the sinuses become inflamed.

What Is the Function of the Maxillary Sinus?

The maxillary sinus shares the general jobs of all paranasal sinuses: it lightens the facial skeleton, adds resonance to the voice and supplies mucus that keeps the nasal lining moist. The full list, including theories still debated, is on the paranasal sinuses page; here are the points that apply most to this sinus.

• Lightening the midface. As the largest sinus, it accounts for much of the weight saved in the middle of the face.
• Shaping the face. It grows together with the upper teeth, helping give the cheeks and upper jaw their adult form.
• Moistening the airway. Its mucus flows into the busy middle meatus and adds to the film that conditions air on its way to the pharynx.
• Voice resonance. The air in the cheeks adds to the resonating spaces that color the sound of the voice, alongside the nose, mouth and throat.

Some researchers also describe the air-filled cheek as a crumple zone that absorbs force from a blow to the face and protects the eye, though this idea is harder to test.

How Is the Maxillary Sinus Related to the Upper Teeth?

The floor of the maxillary sinus lies just above the roots of the upper back teeth, usually the first and second molars and often the premolars. In many adults the root tips raise small bumps in the sinus floor, and sometimes only a thin plate of bone or the lining itself separates a root from the air space.

This closeness has well-known effects. The teeth and the sinus share the superior alveolar nerves, so pain from an inflamed sinus can feel like an upper toothache, and dental pain can feel like sinus pressure. Infection from an upper back tooth can spread upward into the sinus, and removing an upper molar occasionally leaves an opening between the mouth and the sinus, called an oroantral communication.

In adults the floor of the sinus usually lies lower than the floor of the nasal cavity. After upper back teeth are lost, the sinus can expand downward into the empty bone, a process called pneumatization.

Maxillary Sinus vs Sphenoid Sinus: How Do They Differ?

The maxillary sinus is the large sinus in the cheek at the front of the face, while the sphenoid sinus is a deep sinus in the center of the skull, behind the nasal cavity. They are the two sinuses farthest apart, and their neighbors are very different.

The maxillary sinus borders the eye socket and the upper teeth. The sphenoid sinus borders the pituitary gland, the optic nerves and the internal carotid arteries, and it drains into the sphenoethmoidal recess above the superior nasal concha rather than into the middle meatus. The maxillary sinus is present, though small, at birth, while the sphenoid sinus is only a tiny pocket at birth and hollows out its bone during childhood.

Common Conditions That Affect the Maxillary Sinus

The maxillary sinus is the sinus most often involved in sinus inflammation, and it can also be affected by dental problems, injuries and growths. These are listed for education only; the respiratory diseases page gives wider context.

• Maxillary sinusitis: inflammation of the lining, often after a cold, felt as pressure in the cheek. See sinusitis.
• Odontogenic sinusitis: sinus inflammation that starts from an upper tooth or a dental procedure.
• Retention cysts and polyps: small fluid-filled swellings of the lining are common and often found by chance on scans; an antrochoanal polyp grows from the maxillary sinus back toward the nasopharynx.
• Fractures: blows to the cheek can break the sinus walls, and a blowout fracture of the eye socket floor pushes into the sinus roof.
• Tumors: rare growths of the sinus lining, which can press on the eye, teeth or nose.

Lasting facial pain or swelling, or symptoms on only one side of the nose, are reasons to see a doctor.