The maxillary sinus is the largest of the four pairs of paranasal sinuses, hollow air-filled spaces in the bones of your face. Located in the cheekbone area on either side of your nose, each maxillary sinus holds roughly 15 mL of air in adults, about the size of a walnut. These cavities play a bigger role in your daily health than most people realize, from warming the air you breathe to explaining why a bad tooth can turn into a sinus infection.
Location and Structure
Each maxillary sinus sits inside the maxilla, the bone that forms your upper jaw and the middle portion of your face. The shape is roughly pyramidal, with the base sitting along the wall of your nasal cavity and the tip pointing outward toward your cheekbone. Six bony walls define the space: front, back, top, bottom, and two sides.
What makes the maxillary sinus clinically important is what surrounds it. The roof of the sinus is also the floor of your eye socket, which is why severe sinus infections can occasionally cause eye-related symptoms like swelling or pressure. The floor of the sinus is formed by the ridge of bone that holds your upper teeth. The roots of your upper molars and second premolars often sit remarkably close to, or even protrude into, this thin bony floor. That proximity creates a direct pathway for dental infections to spread into the sinus.
What the Maxillary Sinuses Do
Your sinuses aren’t just empty space. They serve several functions that affect how you breathe, speak, and carry your head.
- Humidifying and warming air. Your sinuses act as a built-in humidifier, transforming dry, cold air into warmer, moister air before it reaches your lungs. This protects your airways from irritation.
- Lightening your skull. Solid bone throughout your face would make your head significantly heavier. These air-filled cavities reduce that weight while maintaining structural strength.
- Shaping your voice. Sound waves bounce off the walls of your sinus cavities when you speak, making your voice sound fuller and more resonant than it would otherwise. This is why your voice sounds flat and nasal when your sinuses are congested.
The sinuses also produce mucus that traps dust, bacteria, and other particles. Tiny hair-like structures lining the sinus walls sweep this mucus toward a small opening called the ostium, which drains into the nasal cavity. The maxillary sinus ostium sits high on the inner wall of the sinus, which means gravity doesn’t help with drainage. This design quirk is one reason the maxillary sinus is particularly prone to infections: mucus has to travel upward to drain out.
How the Sinus Develops
The maxillary sinus starts forming before birth, but most of its growth happens afterward. In children, the sinus expands gradually as the face grows. Its width, depth, and volume reach near-adult dimensions by about age 12, though height continues to increase steadily until at least age 18. Some studies suggest the sinus may keep growing subtly into a person’s 20s or even 30s, which is why its final size varies from person to person.
This growth pattern matters for pediatric care. Young children have much smaller sinuses, which changes how infections present and how imaging is interpreted at different ages.
Sinusitis: When the Sinus Gets Infected
Maxillary sinusitis is one of the most common sinus infections. It typically starts when a cold or allergies cause swelling around the ostium, trapping mucus inside the sinus and creating a breeding ground for bacteria.
Doctors diagnose acute bacterial sinusitis when symptoms like thick, discolored nasal discharge combined with nasal congestion or facial pressure persist for at least 10 days without improvement, or when symptoms start to get better and then worsen again within that window. That “double worsening” pattern is a reliable signal that a bacterial infection has taken hold on top of a viral cold.
Chronic sinusitis is a different condition entirely. Rather than a single infection, it involves persistent inflammation lasting 12 weeks or longer. Confirming the diagnosis typically requires visual inspection of the nasal passages or a CT scan showing ongoing inflammation, not just symptoms alone. Mucosal thickening greater than 2 mm on a CT scan is generally considered a sign of sinus pathology, while thinner mucosal changes are often incidental findings that don’t indicate disease.
The Connection to Your Teeth
The maxillary sinus floor sits so close to your upper back teeth that dental problems are a surprisingly common cause of sinus infections. The roots of your upper molars, and to a lesser extent your second premolars, often come within millimeters of the sinus floor. In some people, only a paper-thin layer of bone separates tooth roots from the sinus cavity.
This means a deep cavity, a root canal infection, or even a tooth extraction can introduce bacteria directly into the sinus. These “odontogenic” sinus infections account for a meaningful share of maxillary sinusitis cases, and they don’t always respond to standard sinus treatments because the dental source needs to be addressed. If you have recurring sinus infections on one side only, a dental cause is worth investigating.
This proximity also matters for dental implant placement. When an upper molar is removed, the sinus floor can gradually drop lower as bone resorbs. Placing an implant may require a sinus lift procedure, where the sinus membrane is gently pushed upward to make room for new bone graft material. Age, gender, and which specific tooth is involved all influence how close the roots sit to the sinus, which is why imaging before any surgical procedure in this area is standard practice.
Cysts and Growths in the Sinus
Mucous retention cysts are one of the most common incidental findings when people get a CT scan or MRI for any reason. They appear as smooth, dome-shaped bumps rising from the sinus lining, most often along the floor of the maxillary sinus. These cysts form when a small mucus-producing gland gets blocked, or when fluid accumulates in the tissue beneath the lining. The fluid-filled variety are technically called pseudocysts because they lack a true cyst wall.
Most retention cysts are completely harmless and cause no symptoms. They’re discovered by accident and don’t need treatment. Occasionally, a cyst grows large enough to fill much of the sinus and push through the drainage opening into the nasal cavity. At that point it’s called an antrochoanal polyp and may cause one-sided nasal obstruction.
True nasal polyps are structurally different from pseudocysts. They contain more fibrous tissue, which makes them firmer and easier to remove surgically in one piece. Pseudocysts, being fluid-filled, tend to rupture during removal. Despite this difference, the two are frequently confused on imaging reports, which can lead to unnecessary concern when a routine cyst is labeled as a polyp.