The maxillary antrum is the largest of the four pairs of paranasal sinuses, sitting inside each cheekbone just below the eye socket and alongside the nasal cavity. It is the first sinus to form during fetal development, is already present at birth, and continues growing until roughly age 18. Because of its size and location, the maxillary antrum plays a central role in conditions ranging from common sinus infections to dental complications and facial trauma. Understanding its shape, its neighbors, and the thin boundaries it shares with the mouth and the orbit helps explain why so many different medical and dental problems trace back to this single air-filled chamber.
Shape, Walls, and the Natural Opening
The maxillary antrum is often described as a pyramid lying on its side. The base faces the lateral wall of the nasal cavity, while the apex points toward the cheekbone. The roof is the orbital floor, the floor sits just above the roots of the upper back teeth, and the front wall lies beneath the skin of the cheek. Each of these walls is thin enough that disease on one side can easily affect the other, which is why an infected tooth can cause a sinus problem and a sinus tumor can push into the eye socket.
The sinus drains into the nose through a small natural opening called the ostium. This ostium does not sit at the bottom of the sinus, as you might expect for a drainage hole, but high up on the medial wall, which means the sinus relies on active mucus transport rather than simple gravity to stay clear. A study examining cadaveric specimens found that the ostium most commonly opens into the back third of a narrow groove in the nasal sidewall called the hiatus semilunaris. The same study noted that accessory openings were present in close to three-quarters of cases, providing a secondary route between the sinus and the nose.1PubMed Central. The location of maxillary sinus ostium and its clinical application Those accessory openings are clinically relevant because mucus can sometimes circulate out one hole and back in the other, creating a loop that prevents effective clearance.
Growth From Birth Through Adolescence
At birth, the maxillary antrum is roughly the size of a small pea. It expands in all three dimensions throughout childhood, with the most dramatic growth happening during the first eight years. By the end of age 16, the sinus has essentially reached its adult dimensions in height, width, and depth. Growth continues at a slower pace until about age 18, after which the sinus size remains largely stable. A study tracking sinus development from birth to age 18 found no meaningful difference between the left and right sides, but gender-related size differences appeared after age eight, with males tending toward slightly larger sinuses.2PubMed. Development of the maxillary sinus from birth to age 18. Postnatal growth pattern
This growth pattern matters clinically. In young children, the sinus is too small and too high relative to the teeth to cause the dental-sinus interactions that are common in adults. As the sinus floor drops lower through childhood and adolescence, the roots of the upper molars move closer to, and sometimes protrude into, the sinus cavity. That proximity sets the stage for many of the conditions discussed below.
Anatomic Variations That Affect Treatment
No two maxillary sinuses look exactly alike on a scan. A large systematic review and meta-analysis pooling data from multiple imaging studies found two particularly common variations. Septa, which are bony partitions that divide the sinus into compartments, appeared in about 39% of sinuses. Haller cells, small air cells that sit along the orbital floor near the natural ostium, had a pooled prevalence of about 30%.3PubMed Central. Anatomical variations and abnormalities of the maxillary region and clinical implications: A systematic review and metaanalysis
Both variations cause practical headaches for surgeons. Septa can block access during sinus-lift procedures for dental implants, requiring the surgeon to adjust technique or create multiple bone windows. Haller cells can narrow the drainage pathway, predisposing a person to recurrent sinus infections even without any underlying immune problem. These variations are usually discovered incidentally on CT scans done for other reasons, but knowing they exist helps clinicians plan procedures and anticipate complications.
The Lining Membrane and Mucociliary Clearance
The interior of the maxillary antrum is lined by a thin mucous membrane called the Schneiderian membrane. This membrane is covered in microscopic hair-like structures called cilia that beat in a coordinated wave, sweeping mucus and trapped debris toward the natural ostium. From there, the mucus drains into the nose and down the throat. The entire system works like a slow conveyor belt, and when it functions well, the sinus stays sterile despite being connected to a nasal cavity full of bacteria.
When the membrane becomes inflamed or thickened, the cilia stop working efficiently, mucus pools, and infection can set in. A cone-beam CT study found Schneiderian membrane thickening in about 43% of scans, with a significantly higher rate in people who had undergone root-canal treatment or who had infections at the tips of their tooth roots.4SpringerLink (Odontology). Schneiderian membrane thickening and its association with apical periodontitis: a cone-beam computed tomography analysis In other words, dental problems below the sinus floor can silently irritate the membrane above without ever producing obvious sinus symptoms.
How Close Are the Teeth?
The relationship between the upper back teeth and the sinus floor is one of the most clinically important features of the maxillary antrum. The second molar tends to sit closest to the sinus, followed by the first molar and then the premolars. In some people the roots actually poke through the bony floor and project into the sinus cavity, separated from the air space by nothing more than the thin Schneiderian membrane. One imaging study found that the mesiobuccal root of the second molar had the shortest distance to the sinus floor, while the palatal roots of the first and second molars were farthest away.5PubMed Central. Evaluation of the relationship between the maxillary sinus floor and the root apices of the maxillary posterior teeth using cone-beam computed tomographic scanning
A separate study confirmed this pattern and added that roughly a quarter of maxillary first and second molars were actually inside the sinus, and the likelihood of root penetration increased with age.6PubMed. Anatomic relationship between the root apex of maxillary molars and maxillary sinus in the presence of antagonist teeth: A Kwak’s method with divisions This is why pulling an upper molar sometimes creates a hole straight into the sinus, and why an abscess at a molar root tip can seed bacteria directly into the sinus lining.
Sinusitis and Its Microbiology
Maxillary sinusitis is the most common type of sinus infection, largely because the ostium sits high on the wall rather than at the sinus floor, making drainage inherently poor. Acute episodes are typically triggered by viral upper respiratory infections that cause swelling around the ostium, trapping bacteria inside the sinus. The classic culprits in acute sinusitis are familiar respiratory bacteria. One study tracking patients whose acute sinusitis failed initial antibiotic treatment found that the original infections were dominated by common respiratory species, but as therapy failed and the infection lingered, resistant anaerobic bacteria emerged in subsequent cultures.7PubMed. Microbiology of the transition from acute to chronic maxillary sinusitis
This shift in bacterial populations helps explain why chronic sinusitis is harder to treat. By the time the infection has been present for weeks, the microbial community has changed. A study comparing acute and chronic maxillary sinusitis of dental origin found that anaerobic bacteria dominated both, but chronic cases had a higher total bacterial load and a more diverse mix of species.8PubMed. Microbiology of acute and chronic maxillary sinusitis associated with an odontogenic origin When sinusitis follows a dental problem rather than a cold, the bacterial profile tends to lean more heavily anaerobic from the start, which is a clue that clinicians use to suspect a tooth-related cause.
Odontogenic Sinusitis
Somewhere between 10% and 40% of all maxillary sinusitis cases have a dental origin, depending on the study population and how carefully the dental connection is investigated. The condition is frequently missed because patients see an ENT specialist for sinus symptoms without anyone examining their teeth, or they see a dentist who does not think to check the sinus. A review of the literature identified the most common dental triggers as prior dentoalveolar procedures (extractions, implant placement, root canals), infections of the upper teeth, and dental trauma.9PubMed Central. Odontogenic sinusitis: A review of the current literature
The hallmark of odontogenic sinusitis is that it typically affects only one side, whereas viral or allergic sinusitis tends to be bilateral. A foul smell or taste is another red flag. Treatment requires addressing the dental source in addition to treating the sinus infection; antibiotics alone usually fail because the source of bacterial seeding remains active.
Oroantral Fistula
An oroantral fistula is a persistent, epithelialized channel between the mouth and the maxillary sinus. It typically starts as a simple communication created during a molar or premolar extraction. The bone between the tooth socket and the sinus floor is sometimes paper-thin or even absent, and removing the tooth creates a direct opening. Communications smaller than about 2 mm often heal on their own if left undisturbed.10PubMed Central. Management of Oro-antral Communication and Fistula: Various Surgical Options Larger ones that persist beyond three weeks become lined with tissue and turn into true fistulas, which will not close without surgery.11PubMed Central. Oro-Antral Fistulas and their Management: Our Experience
Patients with an oroantral fistula may notice air passing from the mouth into the nose, liquids leaking into the nose during drinking, or a persistent bad taste. The sinus on that side almost always becomes infected. Surgical repair involves clearing any sinus disease first and then closing the defect, often with a flap of tissue from the inner cheek. Newer techniques using materials like platelet-rich fibrin or bone grafts with resorbable membranes can avoid the need for a separate donor site.12PubMed Central. Surgical options in oroantral fistula management: a narrative review
Cysts and Benign Growths
Mucous retention cysts and pseudocysts are among the most common incidental findings on imaging of the maxillary sinus. A retention cyst forms when a mucus-producing gland in the membrane becomes blocked, creating a fluid-filled sac. A pseudocyst is not a true cyst at all; it is a dome-shaped collection of fluid that accumulates under the membrane, often in response to inflammation or allergic changes. Both types are almost always harmless. They are frequently discovered on dental panoramic X-rays or CT scans done for unrelated reasons, and most require no treatment.
When a cyst is small and a sinus augmentation procedure is planned for dental implants, the cyst can sometimes be left alone or aspirated during the procedure without complication. Larger cysts occupying a significant portion of the sinus may need to be removed first, with the sinus membrane allowed to heal before any bone grafting is attempted.13PubMed Central. Maxillary Sinus Augmentation and Implant Installation in Patients With Pseudocysts and Mucous Retention Cysts
Tumors of the Maxillary Antrum
True tumors of the maxillary sinus are uncommon but worth knowing about, because they are often diagnosed late. The sinus is a roomy cavity, so a mass can grow for a long time before it presses on surrounding structures and produces symptoms. The most common benign tumor in this region is the inverted papilloma, which grows inward from the sinus lining rather than outward like a typical polyp. Despite being benign, inverted papillomas recur frequently after removal and carry a recognized risk of transforming into squamous cell carcinoma.14PubMed Central. Sinonasal Inverted Papilloma and Squamous Cell Carcinoma: Contemporary Management and Patient Outcomes Other malignant transformations, such as into adenoid cystic carcinoma, are considerably rarer.15Polski Przegląd Otorynolaryngologiczny. Adenoid cystic carcinoma within inverted papilloma of the maxillary sinus – a case report
Symptoms that should prompt investigation include unilateral nasal obstruction, blood-tinged nasal discharge on one side, facial numbness, or a bulging cheek. Imaging followed by biopsy confirms the diagnosis. Complete surgical removal with clear margins remains the standard treatment, and long-term follow-up is essential because recurrences can appear years later.
Trauma and Orbital Floor Fractures
The roof of the maxillary antrum doubles as the floor of the eye socket, and it is one of the thinnest bones in the face. A blow to the eye can buckle this floor downward, creating what is known as an orbital blowout fracture. Soft tissue from the orbit, including fat and sometimes eye muscles, can herniate through the fracture into the sinus below. Patients typically present with pain, swelling around the eye, double vision (because a trapped eye muscle restricts movement), and sometimes numbness along the cheek from a damaged nerve.
In children and young adults, the bone tends to crack in a “trapdoor” pattern, where the fragment swings down and then snaps back, trapping orbital contents. One study classifying fracture types found that trapdoor fractures were the most common pattern, accounting for 20 of 30 cases, and that younger patients were more likely to have this pattern while older patients more often had open blowout fractures.16PubMed. Endoscopic transantral and transnasal repair of orbital floor fracture with the ballooning technique, and classification and characterization of orbital floor fractures In rare extreme cases, the eyeball itself can be displaced into the maxillary sinus, requiring emergency surgical repositioning.17PubMed Central. Closed head injury combined with orbital blowout fracture and displacement of the eyeball into the maxillary sinus in a 14-year-old boy: a case report
Silent Sinus Syndrome
Silent sinus syndrome is a rare and often baffling condition in which the maxillary sinus gradually collapses inward over months or years. The walls of the sinus retract, the sinus volume shrinks, and the orbital floor drops, causing the eye on that side to sit lower and sink backward. Patients often notice that one eye looks different from the other, or they develop persistent headaches. The condition is called “silent” because the sinus itself may produce no pain or drainage, even though it is severely compromised.
Although more commonly recognized in adults, silent sinus syndrome can occur in children, and the literature suggests that most pediatric cases involving sinus underdevelopment show the characteristic orbital floor changes.18PubMed. Silent sinus syndrome in children Treatment involves restoring sinus ventilation through an endoscopic procedure that widens the natural drainage pathway. Orbital floor reconstruction may be performed as a second stage if the eye position does not improve on its own.19PubMed. Silent sinus syndrome and maxillary sinus atelectasis in children
Imaging the Maxillary Sinus
Plain X-rays can show an opacified or cloudy sinus but offer limited detail. CT scanning, especially cone-beam CT (CBCT), is the workhorse for evaluating the maxillary antrum in both ENT and dental settings. CBCT provides detailed bony anatomy at radiation doses far lower than standard medical CT. One in-vitro comparison found that low-dose protocols for both standard multi-detector CT and CBCT delivered diagnostic-quality images using roughly 7 to 11 times less radiation than a standard scan.20PubMed Central. Evaluation of different low-dose multidetector CT and cone beam CT protocols in maxillary sinus imaging: part I—an in vitro study
MRI is less commonly used for routine sinus evaluation, but it excels at distinguishing soft tissue types, which is valuable when a tumor or fungal ball is suspected. A comparison of sinus measurements on CBCT and MRI found excellent agreement between the two modalities, suggesting that MRI could serve as a radiation-free alternative for sinus analysis when the clinical question centers on soft tissue rather than bone.21Loma Linda University Electronic Theses, Dissertations & Projects. Comparison of Maxillary Sinus Dimensions and Volumes on CBCT and 3T MR Images
Endoscopic and Surgical Approaches
The introduction of endoscopic sinus surgery in the 1980s transformed how the maxillary antrum is treated. Procedures that once required cutting through the upper gum and opening the front wall of the sinus can now be done through the nose using cameras and specialized instruments, preserving the sinus lining and its mucociliary function.22PubMed Central. Endoscopic Management of Maxillary Sinus Diseases of Dentoalveolar Origin
The size of the surgically created opening matters and is tailored to the severity of disease. For isolated or mild maxillary sinusitis, balloon sinuplasty can dilate the natural ostium without removing tissue. A standard antrostomy, created with cutting instruments, is better suited for advanced disease involving thickened mucosa or polyps because it allows the surgeon to see inside the sinus and enables topical medications to reach the lining. For patients with stubborn, recurrent disease or impaired mucociliary clearance, a “mega-antrostomy” provides a large enough opening for gravity-dependent drainage.23Current Opinion in Otolaryngology & Head and Neck Surgery. What is the optimal maxillary antrostomy size during sinus surgery?
Fungal balls, which are dense collections of fungal debris that accumulate in the sinus without invading tissue, are another common reason for endoscopic surgery. A study comparing a gauze-assisted removal technique to conventional methods found both were effective, but the gauze technique was significantly faster.24PubMed Central. “Gauze Technique” in the Treatment of the Fungus Ball of the Maxillary Sinus: A Technique as Simple as It Is Effective
Sinus Lift Procedures for Dental Implants
When a person loses upper back teeth, the bone beneath the sinus floor gradually resorbs, and the sinus expands downward into the space that used to hold tooth roots. This bone loss can leave too little height for a dental implant. A sinus lift, also called sinus augmentation, addresses the problem by elevating the Schneiderian membrane off the sinus floor and packing bone graft material into the space created.
Two main approaches exist. The direct technique enters through a window cut in the lateral wall of the sinus and can generate substantial bone gain, averaging about 8.5 mm in one comparison study. The indirect technique works from below, through the implant site itself, using instruments to gently push the sinus floor upward. It is less invasive but produces a smaller gain, averaging about 4.4 mm. The indirect approach is generally recommended when at least 6 mm of bone already exists and only a few extra millimeters are needed.25PubMed Central. Direct vs. indirect sinus lift procedure: A comparison Both techniques support successful implant placement at similar rates, so the choice usually comes down to how much bone is available to start with.
Getting Medications Into the Sinus
After sinus surgery, saline rinses are a mainstay of aftercare. They flush out crusts, thin mucus, and can deliver dissolved medications directly to the sinus lining. But getting the rinse to actually enter the maxillary sinus is harder than most patients realize. The natural ostium is narrow, and fluid can rush past it rather than flowing in. Studies in cadaveric models show that sinuses that have been surgically opened are far more likely to be penetrated by irrigation than intact ones. Logistic analysis in one study determined that there was a 95% chance of fluid penetrating a sinus when the ostial opening measured at least about 4 mm.26PubMed. Pre- and postoperative sinus penetration of nasal irrigation
Head position and technique also make a difference. Research using transparent sinus models found that irrigating through the lower nostril (so that fluid pools against gravity and presses against the ostium) produced significantly better and faster sinus filling than the conventional approach of pouring through the upper nostril.27PubMed Central. Topical irrigation against gravity may lead to better sinus penetration For patients using a squeeze bottle or neti pot at home, this means tilting the head so the affected sinus is on the lower side and irrigating through that same-side nostril can improve how much rinse actually reaches the sinus cavity. High-volume irrigation also outperforms nasal sprays and atomizers when the goal is maxillary sinus penetration.28PubMed Central. Sinus penetration of saline solution irrigation and atomizer in a cadaveric polyp and allergic fungal sinusitis model
The Evolutionary Puzzle of Sinus Size
People sometimes assume that the maxillary sinus must have gotten larger (or smaller) as human faces evolved to become flatter than those of other primates. A comparative study of sinus volume across living and fossil hominoids tested this idea directly and found no evidence that the relative size of the maxillary sinus has changed during the course of hominoid evolution. When body and cranial size are accounted for, sinus volume scales proportionally across species.29PubMed. Isometric scaling of maxillary sinus volume in hominoids The sinus appears to be a structural byproduct of facial growth rather than something that has been independently selected for. This result complicates older theories that tried to use sinus size as a marker for classifying fossil species, and it reinforces the idea that the maxillary sinus persists not because it serves an indispensable physiological function but because the developmental program that builds the face produces it as a side effect.